British Standard

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BRITISH STANDARD BS 8005-2:1987

Sewerage

Part 2: Guide to pumping stations and pumping mains

UDC 628.213

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BS 8005-2:1987

This British Standard, having been prepared under the direction of the Civil Engineering and Building Structures Standards Committee, was published under the authority of the Board of BSI and comes into effect on31 December 1987

BSI 02-2000

First published as CP 2005 December 1968First Part revision as BS 8005-2 December 1987

The following BSI references relate to the work on this standard:Committee reference CSB/5Draft for comment 84/11182 DC

ISBN 0 580 15992 2

Committees responsible for this British Standard

The preparation of this British Standard was entrusted by the Civil Engineering and Building Structures Standards Committee (CSB/-) to Technical Committee CSB/5, upon which the following bodies were represented:

Association of Consulting EngineersAssociation of County CouncilsAssociation of District CouncilsBritish Ceramic Research Ltd.British CoalBritish Plastics FederationBritish Precast Concrete Federation Ltd.British Tunnelling SocietyClay Pipe Development Association LimitedConcrete Pipe AssociationConstruction Industry Research and Information AssociationConvention of Scottish Local AuthoritiesCounty Surveyors SocietyDepartment of the Environment (Property Services Agency)Department of Transport (Highways)Federation of Civil Engineering ContractorsFibre Cement Manufacturers Association LimitedHealth and Safety ExecutiveHydraulics Research Station Ltd.Institute of Water Pollution ControlInstitution of Civil EngineersInstitution of Environmental Health OfficersInstitution of Public Health EngineersInstitution of Structural EngineersInstitution of Water Engineers and ScientistsRoyal Institute of British ArchitectsScottish Development DepartmentTrades Union CongressWater Authorities AssociationWater Research Centre

The following bodies were also represented in the drafting of the standard, through subcommittees and panels:

British Effluent and Water AssociationBritish Pump Manufacturers Association

Amendments issued since publication

Amd. No. Date of issue Comments

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Contents

PageCommittees responsible Inside front coverForeword ii

Section 1. General1 Scope 12 Definitions 13 Abbreviations 1

Section 2. Range of components and appliances4 General 25 Pumps 26 Prime movers and drives 37 Controls and electrical equipment 58 Pipework and valves 79 Miscellaneous 8

Section 3. Design of pumping stations10 General 911 Health, safety and welfare design features 912 Maximum and minimum pumping rates 913 Pumping heads 914 Number and size of pumpsets 1015 Layout of pumpsets, pipework, control equipment

and ancillary plant 1116 Substructure design 1217 Wet wells 1218 Ventilation, smell and noise 1319 Lifting facilities 1320 Superstructure 1421 Environment and access 14

Section 4. Design of pumping mains22 Velocities of flow 1523 Diameter 1524 Number of mains 1525 Pressures 1526 Valves 1627 Profiles 1628 Discharge arrangements 1629 Anchorages 1630 Control of septicity 17

Publications referred to Inside back cover

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Foreword

This British Standard has been prepared under the direction of the Civil Engineering and Building Structures Standards Committee and is directed at general engineering practitioners who may either be embarking on a career in sewerage or be dealing with a particular aspect for the first time. It is not intended to be exhaustive in any field but sets out to present guidance on basic principles and good practice, indicating where a more detailed and comprehensive study may be made. BS 8005 supersedes and enhances CP 2005:1968, which is withdrawn, although some of the material incorporated is a restatement or a revision of the earlier text.BS 8005 gives guidance on the planning, design, construction, operation and maintenance of works to convey sewage, including storm sewage, surface water and trade effluents to a sewage treatment works, tidal waters or other final place of disposal. Recommendations are given for the repair, renovation and replacement of sewers.Many end users of this British Standard, such as governments, public authorities, sewerage authorities and consultants, issue their own recommendations and specifications for sewerage which BS 8005 is intended to complement rather than replace.BS 8005-0 directs the reader to sources of more detailed information, particularly on important and specialized fields such as health and safety. It should be regarded as supplying essential background information for the other Parts of BS 8005.BS 8005 is to be published in six separate Parts, as follows.

Part 0, Introduction and guide to data sources and documentation; Part 1, Guide to new sewerage construction; Part 2, Guide to pumping stations and pumping mains; Part 3, Guide to sewers in tunnel1); Part 4, Guide to design and construction of outfalls; Part 5, Guide to rehabilitation of sewers1).

It has been noted that substantial one-part codes and guides take a long time to revise and if they are reviewed at infrequent intervals, they tend to become out of date quickly, especially in a field where technological development is rapid. It is intended therefore to keep a constant watch on new developments and to update BS 8005, Part by Part, as soon as the work can be justified.BS 8301 sets out recommendations for building drainage and, while it relates generally to smaller pipelines, there is some overlap between it and BS 8005. BS 6297 gives recommendations for the design and installation of small sewage treatment works and cesspools.Apart from Part 0, which is directed more specifically at the UK sewerage field, BS 8005 is for use both in the UK and, in appropriate circumstances, overseas.Suggestions for the improvement of any Part of BS 8005 will be welcomed by the Secretary of CSB/5 at 2 Park Street, London W1A 2BS.A British Standard dose not purport to include all the necessary provision of a contract. Users of British Standards are responsible for their correct application.Compliance with a British Standard does not of itself confer immunity from legal obligations.

Summary of pages This document comprises a front cover, an inside front cover, pages i and ii, pages 1 to 18, an inside back cover and a back cover.This standard has been updated (see copyright date) and may have had amendments incorporated. This will be indicated in the amendment table on the inside front cover.

1) In preparation.

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Section 1. General

1 ScopeThis Part of BS 8005 provides guidance on the components, appliances and design of pumping stations and pumping mains.NOTE The titles of the publications referred to in this standard are listed on the inside back cover.The titles of British Standards not referred to in this Part of BS 8005 but of interest as dealing with closely associated subjects are listed in Appendix A of BS 8005-1:1987.Other publications that may be of interest are listed in Appendix B of BS 8005-1:1987.

2 DefinitionsFor the purposes of this Part of BS 8005 the definitions given in BS 8005-0 apply.

3 AbbreviationsFor the purposes of this Part of BS 8005 the abbreviations given in BS 8005-1 apply.

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Section 2. Range of components and appliances

4 GeneralThe type and size of sewage pumping stations and equipment depend upon their duties, location and any special operational requirements, such as remote or automatic control. This section covers the main range of components, whilst methods of installation and housing are dealt with in section 3.Components and appliances are required to be reliable, robust, easy to maintain and appropriate for pumping water and other liquids. In addition, the aggressive nature of sewage, with its variable solid content and possibility of toxicity and explosive gases, calls for a high degree of caution and the adoption of the latest safeguards to meet all possible hazards.

5 Pumps5.1 General

Pumps for handling sewage should be unchokeable and wear resisting. They may be divided broadly into four groups: rotodynamic; reciprocating; pneumatic and Archimedean screw. (See also BS 6297.)

5.2 Rotodynamic pumps

Rotodynamic pumps are relatively cheap to buy, of small overall dimensions in relation to capacity, light in weight and can be arranged vertically or horizontally. They may vary from moderate to high efficiency according to the size of the pump, type of impeller and the head/quantity characteristic of the duty to be performed. All types of rotodynamic pumps afford a high degree of flexibility. Both quantity and head can be varied by changing the speed and/or diameter of the impeller.When two or more pumps are required to discharge in parallel to a common rising main thehead/quantity characteristics should be studied in order to obtain stable conditions and a good overall efficiency. This important group of pumps is divided into three types.

a) The centrifugal pump. The capacity of traditional dry well centrifugal pumps for reasonably economic working may vary from a minimum of about 7 L/s up to 700 L/s and more, with heads varying from 3 m to about 45 m. With small to medium capacities the pump should be of the unchokeable type wherein any solid, up to a maximum of about a 100 mm sphere, that may enter the pump suction will be passed through the pump.

The recessed impeller type of centrifugal pump (also called vortex or torque flow pumps), although of lower efficiency, is less likely to be affected by fibrous material and can be easier to open up for maintenance.Submersible centrifugal sewage pumps are available for a similar range of duties, either as stationary submerged units or as transportable submersible installations. They are self-priming with both pump and motor totally submersible and are accordingly suited for use in wet wells or in dry wells where there is a flooding risk.Cooling is a problem in a dry well and special design precautions may be necessary. The discharge connection of the pump is adaptable for either a flexible hose or static pipework, and the electric motors are available certified for use in a hazardous area in accordance with BS 5345.For wet well installations submersible centrifugal pump units are available which will slide down guides and seat automatically on the permanent discharge connection. The weight of the pump forces the mating flanges into contact thus providing a seal on the discharge side.Centrifugal disintegrator pumps may be used to assist the treatment of the sewage. Running and maintenance costs are higher, especially if they are on a combined sewerage system where there is a high content of grit in the sewage.b) The mixed flow pump. The mixed flow pump is more efficient where the volume of sewage to be pumped is large and where the head lies in the range of 6 m to 18 m.c) The axial flow or propeller-type pump. The axial flow pump is suitable where large volumes of sewage have to be pumped against low heads.

The above pump classifications are generalized and, in particular, the mixed flow design of impeller overlaps the head ranges of both axial and straight centrifugal pumps. Many modern design unchokeable pumps have mixed flow impellers but are of relatively low efficiency. High efficiency centrifugal pumps, mixed flow pumps and axial flow pumps should only be installed in association with preliminary screening or the reduction of the coarser suspended solids.For very small flows a small high efficiency centrifugal pump can be used as part of a sewage diverter where the coarser solids are prevented from passing through the pump.

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5.3 Reciprocating pumps

The reciprocating pump is heavy and of large dimensions in relation to its capacity. It is reliable, efficient when first installed, and is capable of operating with a high suction lift and of discharging against very high heads. It is susceptible to choking, heavy wear and tear, and loss of efficiency through wear and valve jamming. The reciprocating pump may be either of single-acting or double-acting type. Reciprocating pumps are more expensive in first cost than other types. They are expensive to maintain and therefore are rarely used for pumping crude sewage. Preliminary screening of sewage to remove large solids is advisable.As a safeguard against excessive pressure a bypass is often provided between delivery and suction. This should be combined with a pressure relief valve. An alternative is to relieve the discharge to the suction well.A reciprocating pump requires a slow speed drive; this is provided through gearing between the prime mover and the pump. A steady rated discharge is maintained over a wide variation in head. Discharge is altered by varying the speed.

5.4 Pneumatic pumps

The pneumatic ejector, whether of the automatically filled vessel or the air lift type, is suitable where reliability and ease of maintenance are of greater importance than overall efficiency, and where a small quantity of sewage is to be pumped against a relatively small head.The installation usually comprises an ejector together with an automatic self-starting air compressor, with provision for air storage. In special situations two ejectors should be provided to facilitate repairs; where breakdown would have serious results, a second air compressor should be provided. It is possible to serve several ejector stations from one central air compressing station if the distances are not too great.

5.5 Archimedean screw pumps

Archimedean screw pumps are basically screws revolving at a fixed speed. They provide a steady rate of pumping and high efficiency over a wide range of flows and are also effective in pumping varying flows. They are suitable for lifting large volumes of unscreened sewage or storm water against low heads.The actual volume lifted for any particular diameter is dependent on the speed of rotation and on the angle of inclination; the greater the angle the less the rate of discharge. The angle of inclination varies from a minimum of 27 to the horizontal to a maximum of 40. The preferred angle is 38.

Archimedean screw pumps are in two main groups, namely open-screw and encased-screw. Neither group requires a deep sump. While the open-screw types are virtually unchokeable, in certain applications it is advantageous to install a coarse bar screen at the inlet to prevent large objects, such as baulks of timber, from entering the screw. A higher degree of protection is required for the encased-screw pumps.Capacities of Archimedean screw pumps cover a very wide range varying, depending on diameter and inclination, from 7 L/s up to about 10 000 L/s.

6 Prime movers and drives6.1 General

The prime movers normally employed for driving sewage pumping plant are electric motors and internal combustion engines (diesel, dual fuel or petrol). They should be suitable for the types of pumps selected and rated for operational conditions. The choice may depend upon the availability of electricity or a fuel supply. Due consideration should be given to capital, running and maintenance costs in selecting either electricity or fuel, together with the effect of possible interruption of supply from outside influences such as shortage, mechanical breakdown and supply difficulties.Where necessary, explosion-proof units should be used. Fire detection and alarm systems in all buildings should comply with BS 5839-1.Electricity is normally adopted as the cleanest and most convenient form of motive power. In the UK 415 V 3-phase supply is normal for motors up to about 150 kW to 200 kW, whilst 3.3 kV or higher voltage is often used for larger motors. Direct current (d.c.) drive is occasionally adopted, either by rectifying from the a.c. grid supply or by local generation from internal combustion engines.Standby electricity supply in case of breakdown is frequently provided by a second feeder from a different substation or by switching to diesel generating plant situated locally or mounted on a vehicle.

6.2 Electric motors

The electric motor is a convenient, cheap and reliable prime mover for all types of sewage pumping. Varieties of electric motor are available to suit the particular conditions of duty to be performed.As automatic controls have been developed to a high degree of reliability, an electric motor is particularly suitable for an unattended automatically operated station.

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The following are the more usual types of electric motor used for sewage pumping.

a) Squirrel cage induction motor. This type of motor is the simplest and most robust in design for use on alternating current (a.c.) supplies. It is commonly used for single speed applications but can be wound for multi-speed operation, dual speed windings being fairly common. Other motor speed variations are available.Starting the motor direct on line demands a high starting current from the supply, and other methods of starting such as star-delta, autotransformer, or electronic soft start, may have to be considered in order to reduce the high starting current from the supply (and its cost) and satisfy the requirements of the supply authority.Care should be taken when selecting reduced current methods of starting that the motor accelerating torque matches the pump characteristics.This type of motor is suitable for electronic methods of speed control.Many types of power electronic drive equipment are available to enable the squirrel cage induction motor to be considered as an alternative to motors under items b), c)and d).The main types are as follows:

1) voltage controlled invertors;2) current controlled invertors;3) pulse width modulated invertors;4) variable voltage at constant frequency.

b) Wound rotor induction motor. This type of motor has a wound rotor and can have a lower starting current than the squirrel cage motor. It is suitable for speed control by means of external resistors, usually contained in the control equipment. It is normally only used where the speed control is small and applied for short periods. The motor efficiency is less at reduced speeds.c) Synchronous induction motor. This type of motor runs at a fixed speed independent of the load, the speed being determined by the frequency of the supply and the number of poles in the motor.Normally it has good efficiency and power factor and may attract favourable terms from the supply authority. However, a separate d.c. supply is required for exciting windings and the starting performance is poor.

d) Direct current motor. The d.c. motor may be used with advantage for variable speed applications. Starting methods are simple and it has a good starting performance and high efficiency over a wide range of duties. However, maintenance and prime cost are more expensive than with a.c. motors.

Where a wide range of pumping duties is required, it may be more economical in respect of capitaland/or energy costs to use motors with two (or possibly three) speeds or a larger number of constant speed pumping units controlled in an optimum sequence depending on flow or level control strategy.

6.3 Internal combustion engine

The following are the more usual types of internal combustion engine.

a) Diesel engine. The diesel engine is a reliable, efficient type of prime mover. The medium and slow speed units generally have longer lives and are heavy; they require heavy foundations and relatively more space. The high speed units are efficient, compact and light but generally have a shorter life; they are not so expensive (in capital costs), nor do they require heavy foundations. High speed units are, however, often noisy.The slow and medium speed units can be operated automatically but they usually need the regular attendance of a skilled staff. High speed units are suitable for automatic operation, but need more highly skilled maintenance to ensure reliability.b) Dual fuel engine. Sewage gas, a by-product of sewage purification, is an economical fuel for dual fuel engines. These engines can also be operated efficiently on diesel.c) Petrol engine. The petrol engine is rarely adopted as a form of prime mover at a permanent sewage pumping station owing to the comparatively high cost of fuel and maintenance. Portable pumps are sometimes powered by petrol engines.

Suitable arrangements should be made for the safe handling of flammable liquids and for the safe ventilation of combustion products, particularly where mobile plant is involved.

6.4 Drives

Electric motors (either horizontally or vertically mounted) and internal combustion engines can be arranged to drive most types of pumps, by one of the following means.

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a) Direct coupling. Direct coupling of the pump to the prime mover through a suitable flexible coupling, or occasionally through a clutch, is normal for horizontal shaft pumps and vertically mounted pumps.b) Geared drive. A gear box may be inserted to reduce or increase the pump speed in relation to the prime mover speed, or to change the direction of the drive from horizontal to vertical.c) Belt drive. Belt drive, with a flat, V-section or toothed belt, can be used in place of geared drive, where space permits. This may be less costly.d) Direct drives. Electro-submersible pumps usually have the impeller mounted directly on to an extended shaft of the electric motor. They may be mounted vertically or horizontally. Very large units may, however, have an intermediate gear box between the motor shaft and the impeller drive shaft.e) Intermediate shafting. Shafting, either horizontal or vertical, connecting the prime mover or gear box to the pump, normally needs to be supported by intermediate bearings (efficiently lubricated). Shafting and bearings need to be stiff enough to ensure steady running without whip, and flexible couplings should be provided to allow for any misalignment.

The prime mover, the gearbox (if required) and the pump can sometimes be mounted on a combined base unit. This arrangement reduces the installation time, and maintenance can be economical.Control equipment for the drive units should always be placed above ground. When equipment is placed below ground, the dampness will shorten its life, make it unreliable in operation, and in certain stations, especially where flammable gas may be present, make it a hazard and is likely to make it a serious source of ignition leading to explosion.

7 Controls and electrical equipment7.1 General

Most electrically driven pumps are controlled automatically; manual control is now exceptional. Diesel driven pumps can be controlled automatically but this may be unnecessary if they are at a station which is always attended. The design policy on control equipment should be agreed with the user, who may wish to have similar equipment at several stations in one operating area. The equipment should, wherever possible, be capable of adjustment after operational experience.

For all types of automatically controlled pumps it is essential that an anti-roll-back device is incorporated in the drive arrangement. The lower fixed bearing is invariably under water and a return oil lubrication coupled with three seals is recommended to minimize wear on the lower bearing.

7.2 Controls

Usually the control of pumps is based on a liquid level, the operation of a pump starter being activated by the closing of electrical control circuits. Various devices are used, e.g.:

a) floats;b) electrodes;c) air pressure discharge bubblers;d) ultrasonic beams;e) photoelectric light beams;f) flow rate detectors (not necessarily related to liquid level);g) pressure transducers.

The selection of a suitable system for a particular station is a combination of suitability of proprietary equipment and the experience and preferences of a designer. It is important to design for easy accessibility and maintenance of the equipment. Standby equipment should be considered and also alarms to indicate failure. The use of telephone and radio alarm and information systems at remote unattended stations is often justified.A control system for an installation of electric motor driven pumps usually automatically operates the motor starter to a pre-determined sequence. The system should provide for the sequence to be varied either automatically or manually, e.g. so that one pump can be the duty pump for a period, and then another. It can be simple, such as a float which directly closes and opens a switch in a starter, or complex, such as a series of detectors with relays and a mini-computer to deal with a range of variable speed pumps. The alternatives that are available provide variations in flexibility, together with safeguards and economy of operation. A selector switch should select automatic or manual operation.Except at small stations, it may be desirable to include time delay equipment in the control scheme to ensure sequential starting of pumps after a power failure. This avoids an excessive momentary electrical load that might otherwise arise in this exceptional circumstance.

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7.3 Electrical equipment

Each motor starter needs to be suitable for frequent duty and should be appropriately rated to the motor it controls. It should incorporate a suitably rated externally operated means of isolation, mechanically interlocked with its door. It should have adjustable overload releases that lock-out the equipment, with time lags, and an inherent no-volt release which should be such that the motor willre-start automatically on the resumption of voltage following power failure. In the event of the motor tripping out on overload, consideration should be given to having a reset button for manual reset. Automatic reset of the overloads is not considered desirable. Other features are high rupturing capacity back-up fuses of suitable rating on each phase, control circuit fuses and contacts for operation by the control gear. It is usual to provide indicating lights showing supply on and motor energized. Emergency stop push buttons should be provided at the starter and at points of possible danger such as near the motor and near the pump. Pump starters should be linked to a flow monitor, load monitor or non-return valve, through a time delay, to shut down a pump in the event of blockage.Anti-condensation heaters may be provided in starter enclosures, possibly thermostatically controlled, and also in the motors. In larger motors thermal devices may be justified to open the starter controls in the event of overheating.Individual starters and other electrical items can be wall mounted in their own enclosures. However, at medium and large stations it may be more satisfactory to provide a floor mounted panel for all the electrical gear, including the incoming supply circuit breakers, electrical meters and distribution equipment. A composite panel should be arranged so that an individual unit can be isolated for maintenance while the other units remain live and in operation. It is essential that each isolation switch be capable of being padlocked in the off position.

7.4 Telemetry

The purpose of any telemetry system is to provide operational and management data to a remote management centre and, in selected cases, to provide the facility for override control of the plant from the management centre.Telemetry systems usually cover a sewage treatment works and any pumping stations within its catchment.

The minimum data required is such that, at the management centre, decisions can be made, especially during out of normal hours periods, whether or not to commit limited manpower resources for immediate attendance at the pumping station to rectify operational problems or plant breakdowns.Information required from each pumping station can range from the transmission of a single high level alarm for very small automatic pumping stations, to full monitoring and individual alarms such as those detailed for each of the following items.

a) Pumps: Running, Failed, Stopped, Off auto.b) Electricity supply: Mains failed, Phase failed.c) Standby generator (where installed): Running, Failed, Stopped, Low fuel alarm.d) Wet well: High level alarm.e) Dry well: Flooding alarm.f) Screen: Blockage alarm, Failure, High differential level.g) Storm overflow: Operating.h) Intruder/fire: Alarms.

Depending on the application the following may also be monitored.

1) Works inlet flow/discharge flow rates and associated integrated totals.2) Other qualitative and quantitative information.

The means of data transmission can be via public service telephone lines, own dedicated lines, radio or other media (e.g. fibre optics, laser links) dependent on system requirements and availability. Consideration should be given in selecting the transmission media to system requirements (e.g. update times/scan rates), operating and maintenance costs, data security and reliability, expansion capability and the display and archiving of operational and management information.The telemetry equipment can be enhanced by local automatic programmable control facilities of various levels of sophistication, so as to provide data logging and data processing functions required for system optimization. Automatic or manual override control through this equipment is possible from the management centre.Information on telemetry and computer control of sewerage operations is obtainable from the Water Authorities Association (WAA) and the Water Research Centre (WRc).

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8 Pipework and valves8.1 Pumping station pipes and joints

Pipework within pumping stations is usually of ductile iron but grey iron, steel or plastics pipework can be used. The pipework should be able to withstand possible distortion due to jointing and vibration and surge pressures.Pipe joints within pumping stations are mostly flanged. Detachable flexible couplings should be interposed where they will facilitate dismantling and accommodate vibration, but these do not usually hold end thrust and long tie bolts may be needed for bracing.A closely spaced pair of flexible joints should be provided on a pumping main immediately outside a station to accommodate possible differential settlement.Provision of tappings on suction and delivery pipework for permanent or temporary pressure gauges should be considered.

8.2 Pumping main pipes and joints

The materials of pipes used for pumping mains include ductile iron, steel, asbestos cement, GRP, polyethylene, polybutylene, unplasticized PVC and, occasionally, concrete. The class(es) of pipework should be selected to withstand the maximum hydraulic pressures (including surge pressures) and, where applicable, external loadings. When necessary pipelines should be protected against corrosion, internally and externally. (See BS 8005-1 regarding materials and BS 1710 regarding colour coding of services.)Pipe joints for use below ground should preferably be of the flexible type. If flanges are used on buried pipes the fastenings should be specially protected. Protection should be provided by galvanizing, by wrapping with waterproof tape or by enclosing in bitumen.

8.3 Valves

Pumps normally discharge through their ownnon-return valves. One or more isolating valves should be included in the installation. A non-return valve may also be required on a pumping main. For isolating and maintenance purposes the isolating valve should be positioned downstream from the non-return valve.

The non-return valve, sometimes called a reflux or a check valve, complying with BS 5153 prevents backflow when pumping ceases. It should give a clear flow when the flap is open to avoid accumulation of rags. For this reason valves with multiple flaps are not usually satisfactory. Seatings for the flap and the hinge pin should be renewable as they are subject to severe wear. An external lever may be provided on the hinge pin so that the flap can be opened manually, either in attempting to clear a blockage or for backwashing or drainage. The lever also gives a visual indication of the extent to which the flap is open during pumping; this can be adjusted by the addition of a counter-weight to the lever. Slamming of non-return valves may take place when a reversal of flow occurs before the flap closes. Partial closing as the forward velocity of the sewage diminishes and before the flow is reversed can be assisted by the external lever arm and counterweight.A non-return valve should be easy to open for maintenance. The casing should have an arrow cast on to indicate direction of flow.Isolating valves are normally sluice (or gate) valves which are also used as washouts. An extensive range is available. Wedge pattern gates with copper alloy facings, or resilient seal gates, are usually preferred. Some valves have inside non-rising screws and some have rising screws which give a clear indication whether the valve is open or closed. Diaphragm isolating valves and ball and plug valves are suitable for pumping installations handling raw sewage but butterfly valves should be avoided as rags in the sewage may cause blockages.Sluice valves should preferably be sited with the operating spindle either vertical or inclined at an angle above the horizontal. If unavoidable, or if chain operation is to be used, the spindle may be horizontal but it should never be inclined below the horizontal as solids can enter the bonnet and interfere with operation.The most economical operating arrangement is for the handwheel of the isolating valve to be fixed on the protruding stem of the valve. If access would then be difficult the spindle can then be extended (and cranked through gearing or universal joints if necessary) to a conveniently situated headstock. Valves usually close clockwise but this is not universal and the direction of opening and closing should be marked on each handwheel. A designation label is also useful. Where valves may need to be left in a partly opened state there should be a position indicator.

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Large valves may have a water jetting attachment to allow for cleaning the bottom of the seating of accumulated grit. Large valves on storm water systems which may normally be closed can be provided with electric, hydraulic or pneumatic actuators for power opening and closing. A power operated isolating valve can be used on a large delivery main instead of separate non-return and isolating valves. In the event of mains failure, power operated valves will not close to stop backflow unless alternative energy sources and additional equipment are fitted.Air release valves should be of a type suitable for sewage, with adequate capacity for the passing of air and gas produced during operation.Rarely used valves should be operated at regular (or biannual) intervals over their full distance of travel.

9 Miscellaneous9.1 Pump protection

Sewage pumps are designed to handle solids and consequently they are less efficient than pumps for clean water. The solids vary in character and include unexpected items which get into sewers and become potential hazards. Rags are a frequent source of trouble; grit may be a problem after storms. Proprietary refinements in certain sewage pumps are directed to reducing the risk of pump blockage by solids.In some circumstances special pumps or plant may be provided to reduce the risk of failure of the pumps. Special plant is usually unnecessary at a small pumping station which is served by small sewers. It should be considered for larger stations where it is vital to maintain uninterrupted pumping capacity. The character of the incoming sewage may be a factor if it is known to have an unusual solids content.Coarse screens can be used to prevent large objects and some solids from entering pumps. They always collect rags and this, with the large objects, causes a build-up which may restrict the sewage flow. The screenings need to be removed, either manually or by a machine, and either macerated and returned to the flow or otherwise disposed of. Although the plant can be automatic it will need attention and maintenance. The selection of screening plant is a matter of experience and judgement as there are several basic types and refinements. If there is doubt as to the need for a coarse screen, provision can be made for its installation later after a period of operational experience.

Comminutors or macerators are occasionally provided to protect pumps.At outlying pumping stations, grit removal plant for pump protection is an exception. If provided, however, then some attendance will be required.If grit deposits are likely to occur, sufficient to cause problems with the pumping system, their build up may be prevented by using water jets to lift the grit into suspension to be pumped away with the sewage.With submersible or submerged pumping units, additional equipment may be provided on the pump which will allow re-circulation within the sump to take place prior to the commencement of the pumping cycle. This re-circulation will also assist in putting the grit into suspension and will work automatically without additional attendance.In general, screening and grit removal is best carried out at a sewage treatment plant and it should be avoided at an outlying pumping station unless essential. (See CIRIA Technical Note 119: Screens and Grit in Sewage: Record, treatment and disposal2).)

9.2 Overflow

Every pumping station should have an emergency overflow system which will operate if there is a complete failure of the pumping plant. The system can be on the incoming sewer or at the pumping station.An emergency overflow of sewage could cause nuisance, pollution, damage or flooding. On no account should the dry well of a pumping station be liable to flooding. The design of the station and its overflow system should be such that repairs can always be made to any plant that has failed.

9.3 Flow measurement

Flow measurement is rarely justified at a pumping station but its absence may mean that it is not possible to know the actual discharge rates or quantities of sewage that are handled. The importance of this information should therefore be considered. All measurement devices require stable hydraulic flow conditions and this is not usually possible near pumps. The flow measuring apparatus may therefore need to be some distance from the pumping station.Some pump control systems can be associated with incoming flow measurement (e.g. at a flume). Flow meters can be incorporated in pumping mains.

2) Obtainable from CIRIA, 6 Storeys Gate, London SW1P 3AU.

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Section 3. Design of pumping stations

10 GeneralThe type and size of pumping stations and pumps depends on the pumping duties, the location, whether the station will be attended, and the preferences of the user and designer.The conventional pumping station has a dry well for pumps and other plant and a separate wet well, usually housing some of the control equipment. The roof of the dry well, which may extend partly or wholly over the wet well, should be above ground (and flood) level and serves as the floor of the superstructure building for motors and electrical equipment.The building can include facilities for operators such as a toilet, messroom and store. At large stations it can also have a workshop and garage. Because of possible smell and noise problems it is not usually advisable to locate offices or amenity buildings at pumping stations.It may be possible and necessary to construct a pumping station partly or wholly underground, for instance to deter vandalism, but this calls for special precautions in designing the substructure and in observing health and safety requirements.A screw pumping station is used to discharge into a channel or gravity sewer and not into a pumping main. The motors and control gear should be housed and, if the screws are in the open, they should be provided with removable safety covers.Small pumping devices, such as ejectors, may have an integral reception chamber and can therefore be installed in basements rather than in separate structures.

11 Health, safety and welfare design featuresIt is essential when designing sewage pumping stations and pumping mains to incorporate necessary health, safety and welfare features to comply with statutory requirements. In addition, relevant recommendations from authoritative bodies such as the Health and Safety Executive, the British Standards Institution, the Trades Union Congress and water supply industry codes of practice should be carefully studied.The scale of provision will depend on the numbers of staff and the frequency of visits to the station.Typical hazards are as follows:

a) falls of persons from heights, and into liquids or on to moving machinery;b) tripping or slipping on stairways, walkways or other means of access;c) falling or other travelling objects;

d) inadequate levels of ventilation, particularly in confined spaces (see clause 18);e) combustion and explosion of flammable gases;f) electrical shocks and burns;g) faults in the installation and guarding of machinery (see BS 5304);h) excessive noise, vibration or fumes (see clause 18).

The following equipment should be provided, as appropriate:

1) first aid and rescue equipment;2) emergency equipment and alarms;3) telephone and/or radio communication;4) toilet and washing facilities;5) facilities for the changing and storage of clothes and for the storage of tools and equipment;6) meal and office facilities.

For small pumping stations the design could provide for the use of a specially equipped vehicle to incorporate some of the above facilities.

12 Maximum and minimum pumping ratesThe maximum discharge rate from a pumping station, when all the duty pumps and pumping mains are in use, should be equal to, or preferably greater than, the maximum design rate of flow to the station. The minimum pumping rate should achieve a self-cleansing rate of flow in the pumping main(s). At a large station the minimum pumping rate may be governed by an assumed minimum flow to the station.For a small station, with one constant speed duty pump, the pumping rate will be intermittent and may be unrelated to the rate of flow to the station.Pumping will also be intermittent at multi-pump stations whenever the flow to the station is less than the minimum pumping rate.At medium and large stations, the station discharge can be kept approximately equal to the rate of flow of the incoming sewage by the adoption of variable speed pumps. This is not possible at small stations with constant speed pumps.

13 Pumping headsFor a selected pumping rate the total pumping head (or pressure) comprises the static lift, the friction in the pumping main, the friction through the pumps and station pipework and valves and the entry and exit head losses.

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14 Number and size of pumpsetsThe selection of the type of pumps, and their sizes and numbers depends, among other things, on the desired maximum and minimum pumping rates and on the need, or otherwise, to control the variations in the rate of discharge from the station.A station with one constant speed duty pump should normally have a second pump to provide 100 % standby. This may be the most economical arrangement as far as pumping plant and electrical power is concerned, but it will result in intermittent discharge.If the pumping main velocities are satisfactory, a station can have one variable or one two-speed duty pump and a similar standby pump. This would reduce the flow fluctuation but the electrical plant would cost more; it would be less efficient electrically than a constant speed installation. An alternative is to have two constant speed duty pumps discharging to the one pumping main, with a similar pump as standby; this installation can be further refined by the provision of variable ortwo-speed motors.To maintain acceptable velocities and reasonable friction losses, the individual suction and delivery pipe legs are, in many cases, larger than the pumps. The taper piece required on the delivery side should be included immediately at the pump branch before the non-return valve. Tapering on the suction side should be fitted between the sluice valve and the pump and should be of level soffit pattern. Tapers should be selected to give good velocity profiles particularly at inlet, and any bends should, where possible, be of long radius.If the friction in a pumping main is significant, no more than two similar pumps should discharge simultaneously into a single pumping main. The additional output from a third pump into the same main could be quite small. If greater flexibility of discharge is desired, two sets of two duty pumps and one standby, each with its own delivery pumping main, might be appropriate. When the amount of storm water is significant one set might use larger pumps than the other.When two pumps discharge to a pumping main where the friction head is significant the maximum duty is their combined discharge. When one pump is operating (at the same speed) it will deliver more than half of this discharge. Hence head/output calculations (using the pump characteristic curves) are needed before the duty of a single pump can be assessed.

Commercially available pumps should be selected, and familiarity with the range of duties of typical pumps is therefore necessary. The friction head calculations involve several assumptions and cannot be precise. The selected installation may have a capacity that is different (often greater) from that intended. If this is likely to be a serious impediment to the scheme, arrangements can be made for adjusting the pump impellers after a trial period of operation.Even in very small stations it is usually prudent to provide standby plant to operate automatically when a duty pump fails. Standby pumps can also be used during maintenance and repair of other pumps.The number of standby units which should be provided will depend on the station layout and the possible consequences of pumps failing at a time of maximum incoming flow. It should not be overlooked that one pumpset may be undergoing maintenance when this situation arises.At small stations a portable pump is sometimes used as a standby. A branch to the pumping main should then be provided for connection of the portable pump.Provision of an emergency pumping inlet at any station is always a safeguard against mains failure, especially if the failure affects a wide area and there are insufficient mobile generators to serve all stations. Permanent provision also eliminates the hardest and most accident prone task, of inserting the suction pipes.The need for standby electricity supply depends on the importance of continued operation during a possible period of electricity failure.

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15 Layout of pumpsets, pipework, control equipment and ancillary plantMany small and medium sized wet and dry well pumping stations with rotodynamic pumps have comparable layouts. The pumps should be in a line with their vertical spindles passing through the roof slab to the motors on the floor above. Where universal joints are provided in the shafts, there should be slight misalignment so that the bearings do not track on the same path and cause failure. Although this is referred to as a vertical pumpset, the impeller is revolving horizontally in its volute. The weight of the pump assembly should be taken on a support (stool) below the pump casing and the motor and the drive shaft supported by the floor above. The pump suction pipeline consists of a vertical 90 bend below the central inlet of each pump casing, followed by a horizontal pipeline (with its isolating valve) which passes through the wall between the wet and dry wells and terminates as a bell-mouth inlet.Submersible pumps can be used in a dry well situation. It is essential that only pumps with adequate cooling arrangements be used and the manufacturers approval should be obtained for the proposed application.Pumps situated in dry wells need to be primed before they are started. This is normally achieved by siting the pumps below the desired start water levels in the wet well and by providing a small air release pipe from the top of the pump casing. The level of the suction pipeline should also be coordinated with the details of the wet well. Siting of a rotodynamic pump above the start water level should be avoided if possible due to the problems and additional maintenance which inevitably result. A special priming method will be required, either using an additional automatic extractor pump or by using liquid from the pumping main or by vacuum priming.The outlet from the pump casing of a vertical pumpset in a dry well is horizontal. The pump delivery pipeline connects to it and should first include a non-return valve, which should be in a horizontal attitude, and then an isolating valve to enable the non-return valve to be readily isolated in the event of its requiring attention, e.g. to clear a clogged seating. The suction and delivery sluice valves should preferably not be rotated through more than 45 to the vertical. The delivery pipelines from the pumps combine into a header main (bus main or manifold) at the commencement of the pumping main. Connections should preferably be horizontal and as short as possible to minimize problems caused by silt and other debris.

The delivery pipework should be located above the suction pipework and between the pumps and the wall separating the wet and dry wells. This leaves the space between the pumps and the opposite wall clear for access. This arrangement usually means that the header main is directed to one of the end walls.As the only reliable dimensions of pipe fittings can be along their centrelines, the detailed levels and positions of the pipework should refer to centreline levels and not to invert levels.The pipe bends in the arrangement described above facilitate possible dismantling but it is also prudent to introduce flexible couplings on the suction pipeline for this purpose. A flexible coupling should be provided on the delivery pipeline if possible; this may not be practicable due to the anchoring arrangements. It is important to allow for the whole assembly of suction pipelines, pumps and delivery pipelines to be erected before the pipelines are built into the walls. It may be practicable to isolate the whole assembly for pressure testing and in most cases the delivery pipework can be tested at pump closed valve head when setting to work.Other matters which should be considered are as follows.

a) Drainage facilities for emptying isolated pumps and pipework before they are dismantled, and air/gas release arrangements at high points (which should be avoided if possible).b) Cross connections and valves to enable suction lines to be back flushed either with another pump or by using the contents of the pumping main.c) Inspection and rodding openings strategically located on various items in the composite assembly.d) The need to provide intermediate bearings and flexible couplings on drive spindles between pumps and motors.e) The need to collaborate with the pump supplier. The detailed designs should be acceptable to the pump supplier if he is to be responsible for the operational efficiencies of the pumpsets.f) The dry well should be adequately ventilated by extraction from low level to prevent the build-up of heavier-than-air gases and at high level to prevent the build-up of methane.

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Some pump control equipment is usually in the wet well and its function is normally associated with the sewage level. It should be arranged so that its operation is not impeded by disturbed liquid surfaces, or by fat, rags or other extraneous matter. Facilities should be provided for the vertical and positional adjustment after initial operational experience. The remainder of the control equipment should be grouped with electrical equipment.If a floor-mounted cubicle is used, it can also house motor starters and control equipment (see 6.2 and clause 7). It is important to provide generous facilities for cables and other connections and to allow ample working space around the cubicle.

16 Substructure designThe form of substructure should suit the types and layout of the pumps and other plant. If alternatives are being considered it will probably be found that submerged pumpsets require the smallest substructures, vertical pumpsets the next larger and horizontal pumpsets the largest.The following guidelines apply to all pumping stations including the very small, and to ejector stations.

a) Adequate access openings should be provided for all operational and safety items that will have to be introduced into the station and which may have to be removed from it.b) There should be liberal dimensional tolerances in level and location for all installed items so that they can be conveniently fitted together and fixed to the structure.c) Pipework is normally anchored where it is built into the walls of the station and at these locations some designers provide cast or welded-on puddle flanges. Elsewhere the pipes and fittings should be supported to avoid excessive strain on the joints. Large valves should have individual supports. Vertical pipe runs can be supported at the base on duckfoot bends and horizontal runs on reinforced supports with detachable metal straps. The supports and anchorages may need to withstand both test and surge pressures. They should not impede dismantling.d) Reasonable access facilities and working space should be available for operation and maintenance.e) Floor drainage for a dry well should be generous as it will be needed during construction and also when pumps and pipework are emptied. It is usually better to add the floor surfacing after installation of the major items of plant and pipework. A sump pump should normally be included.

f) Facilities or provision should be included for emptying a wet well.g) Adequate lighting should be provided in a wet well, and electrical apparatus should be certified for use in a hazardous area in compliance with BS 5345. Provision may be required for emergency lighting.h) Electrical power points for portable lights and tools should be provided above ground for use with portable low voltage output transformers complying with BS 3535.i) Hosing facilities may be justified for cleaning the wet well and its control equipment.j) Structural recommendations are as follows.

1) Information about the subsoil of the site, and the groundwater, should be obtained in advance of detailed design.2) The dividing wall between wet and dry wells should be considered as water retaining in accordance with the recommendations of BS 8007.3) As with all buried structures, the substructure should be designed so as not to suffer movement because of a high external water table. A risk of subsidence or flotation may also affect the design and should always be allowed for in the pipelines entering and leaving the station.4) Protection of the concrete should be considered if there are aggressive soils or a risk of septic sewage or corrosive industrial discharges. The surfaces above sewage level may be vulnerable if hydrogen sulphide is liberated. In exceptional locations sulphates in the soil and groundwater may be significant.

k) The environment of a pumping station substructure is inevitably always humid and steel and ironwork are rapidly corroded unless effectively protected.

17 Wet wells17.1 Capacity

The size of a wet well should be related to the pumping rates as, except at large stations, it provides storage for intermittent pumping. At large stations the incoming sewers can provide some of the wet well capacity.For small and medium stations the size of the wet well should be such that the pumps will not start and stop too frequently (six to 12 starts per hour is a guide).

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17.2 Design

The lower part of the wet well is the sump, which should be shaped to suit the pump suctions. An inefficient arrangement can result in a significant reduction in pump output due to air entrainment. It should also be shaped to prevent deposition of grit and sewage solids which rapidly occurs when the sewage ceases to move.It is advisable to provide means of stopping the inflow to the wet well for maintenance purposes.Incoming sewers and sump design should be arranged to avoid sewage dropping into the wet well, as this can also cause air entrainment into the pump suctions. The sewers can backdrop externally into the wet well. If possible, the pump cut-in levels should be below the level of the incoming sewers, to prevent backing up except at large stations (see 17.1). Backdrops cause problems of:

a) turbulence as a result of their discharging below normal water level introducing air direct to the suction pipe; andb) blockage in the backdrops themselves.

Sump design should attempt to prevent air entrainment and subsequent cavitation in a pump.As the efficient operation of a station will depend on both the pumps and the design of the sump, the pump supplier should approve the design of the sump and the suction pipework. At small stations it is usually sufficient if the pump suctions are not physically restricted and are well submerged when pumping commences, but for large stations it may be prudent to have hydraulic model tests to achieve an efficient design for the composite arrangement. There is considerable divergence of views on the detailed design of suction pipework.

17.3 Operation

A build-up of scum and grease at the sewage surface in a wet well can affect the operation of control equipment and access should be provided for cleaning the control equipment and, if necessary, for removing the scum. The part of the wet well in which the pump control equipment is located should have a sewage surface which is always reasonably tranquil.Adequate ventilation should be provided as a safeguard against the accumulation of dangerous gases or vapours.

18 Ventilation, smell and noiseCareful consideration should be given to the question of adequate and safe ventilation of the buildings, and of any confined spaces. Toxic and flammable gases can arise during the handling and processing of sewage, and the system should be so designed and operated that any air or gas discharged is vented to a safe place in the open air. Where this is impracticable, comprehensive tests should be made to ascertain the nature of any contaminants which might enter the system or be generated within it, and appropriate precautions should be incorporated in the design and operating procedures to deal with them. (See Table 5 of BS 8005-1:1987.)It is difficult to avoid smell nuisances when pumping sewage particularly in the circumstances of overflow or pump failure. Care can be taken when siting buildings to take advantage of prevailing winds and by covering over outside tanks containing sewage. It is possible to install filters and odour removal equipment to deal with certain types of noxious gases.Particular attention should be paid to the prompt disposal of screenings at pumping stations. The disposal of screenings may be subject to the statutory requirements of the Control of Pollution Act 1974. (See BS 8005-0.) Excessive noise can be damaging as well as unpleasant to operatives and their neighbours. It can be reduced by the careful design, selection and installation of machinery. Noise levels within the building can be reduced by the use of sound absorbent linings. Transmission between compartments and from the building can be reduced by the use of heavy imperforate building materials or of discontinuous construction. The use ofdouble-door vestibules and double glazing with a large air gap is also effective. Outside the building, the use of baffles and vegetation will also absorb and disperse escaping noise.

19 Lifting facilitiesAt every pumping station appropriate and suitable lifting equipment should be provided, maintained in a serviceable condition and used. This could take the form of a simple pulley block, or in a large station an overhead gantry crane.

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The type, rating and range of operation of cranes and other lifting equipment will vary widely depending on the pumps and ancillary equipment which have to be installed and maintained. For larger installations, permanently installed gantry cranes covering the whole area of the pumphouse are convenient. Multi-purpose lifting appliances such as lorry mounted cranes, fork-lift trucks and small hydraulic excavators are in common use in the vicinity of pumping stations. Particularly for mobile plant, consideration should be given to the question of adequate headroom, the proximity of overhead power cables, turning circle and surface wheel bearing capacity.Slings, chains, ropes and other lifting gear should be suitable for the particular lifting operation.The general statutory standard for the construction and use of all lifting equipment is contained in Section 2 of the Health and Safety at Work etc. Act 1974, and even if the Factories Act 1961 does not apply, Sections 26 and 27 of the Factories Act 1961 may be taken as guidelines to detailed testing, inspections and certification. Advice on the application of the Factories Act to any particular installation may be obtained from the local office of the Health and Safety Executive.The high incidence of back injury among pumping station operatives in particular justifies the provision of suitable mechanical devices for theoff-loading of plant and materials from transport vehicles.

20 SuperstructureThe superstructure of a sewage pumping station will have to suit the substructure in providing accommodation for pumping units, equipment and operators. The design of the actual building requires special consideration in respect of size, type and appearance.Buildings should be substantial, well-proportioned and with a choice of materials suitable to operational and climatic conditions. This includes provisions such as damp-proofing, insulation,air-conditioning and protection against the weather.Pumping stations should also be protected against vandalism and unlawful entry by fitting adequate locking devices to windows and doors. For remote stations and in high-risk areas, alarm systems can be fitted in addition. If resort is to be made to underground stations for security purposes, the risk of flooding, fire and explosion should be seriously taken into account.

21 Environment and accessSewage pumping stations are normally situated in the outskirts of residential and industrial areas or in the rural countryside. Good access is essential for vehicles and plant for maintenance and emergency circumstances, whatever the weather conditions.Fencing and warning signs are advisable in hazardous or vulnerable locations.Access roads and parking areas should be designed with suitability, durability and maintenance requirements in mind. Similar consideration should be taken in deciding areas to be grassed and trees and hedges (or fences) to be provided. Landscaping can be hastened by the use of quick-growing trees and shrubs, but this involves extra trimming and the risk of excessive root growth entering into sewer tanks and pipes, and undermining foundations.Power failures and flooding due to weather or burst pipes present hazards to be met, particularly in riverside and remote areas. These can lead to inconvenience to the public from flooding, pollution and smell unless such emergencies are taken into account in general environmental considerations.

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Section 4. Design of pumping mains

22 Velocities of flowTo avoid sedimentation, the minimum recommended velocity in pumping mains is about 0.75 m/s, but if there is a velocity of about 1.2 m/s for several hours each day, the minimum velocity can be as low as 0.5 m/s. The maximum velocity should normally not be above 3 m/s. Power considerations usually impose this limit.

23 DiameterThe diameter of a pumping main should be determined by an economic analysis of the pipeline and pumping costs and by an assessment of the engineering factors which may sometimes override the economic analysis. Alternative diameters should be examined which produce minimum and maximum velocities within the acceptable limits, and pumping costs should be estimated taking account of the normal rate of pumping (not necessarily the peak rate). The most economical scheme will be the one which involves the lowest overall annual cost, including repayment of capital cost, running and maintenance.If septicity of the sewage is likely to be a problem the retention period in the pumping main should be reduced by adopting a smaller diameter and accepting a higher velocity of flow, even though this may mean higher power consumption.The minimum diameter of a main is usually 100 mm but sometimes smaller mains may be considered to maintain a minimum velocity and avoid septicity. Mains as small as 50 mm can be used but it is then necessary to install a macerator in the system to reduce the size of solids.

24 Number of mainsDuplication of pumping mains should be considered in the following circumstances.

a) To provide a standby in the event of the other main being temporarily out of action. Duplication may be provided for the whole length or for part of the length, e.g. at crossings of watercourses, canals and railways.b) To accommodate storm sewage flows which could not be carried in a single main within the acceptable limits of velocity.c) To permit parallel working of the centrifugal pumps where their characteristics do not lend themselves to combined working through a single main.

Arrangements should be made to ensure frequent use of both mains to prevent septicity. Automatic pump changeover is a convenient method.In certain circumstances it may be advisable to provide a connection to a pumping main adjacent to a pumping station for a mobile pump to enable the station to be bypassed.

25 PressuresThe maximum friction head (pressure) arises at the maximum velocity. It should be calculated by one of the recognized hydraulic systems for friction losses in a pipeline flowing full. It should be remembered that friction factors and viscosity of the liquid are likely to change when air or oxygen injection is employed to control septicity (see BS 8005-1).The possibility of positive and negative pressures due to surge (water hammer) should be considered. They are more likely to be significant in long mains or where high velocities arise. Surge analysis is complicated and is usually only undertaken when surge pressures are expected to be important. There are now many computer programmes available to assist in this analysis.Surge pressures can be alleviated by various means as follows:

a) a suitably designed regulating or non-return valve in the main;b) a pressure regulating or surge chamber on the main;c) flywheels on the pumps to avoid sudden shut down;d) a standpipe, close to the pumping station;e) double-acting surge relief valves on system.

Other methods can be included such as stageshut-down, or variable speed drives to reducelong-term fatigue failure of pipelines due to the effect of surge pressures. Surge pressures influence the selection of material and class of pipe of a pumping main. Recommendations are given in CP 312-2 and CP 2010-2, CP 2010-3, CP 2010-4 and CP 2010-53).The pumping main should be so designed as to be capable of withstanding a hydraulic test pressure of not less than 1.5 times the maximum working pressure or not less than 1.5 times the maximum surge pressure, whichever is the greater, subject to the recommendations in the above codes.

3) Under revision

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26 ValvesThe arrangement and location of isolating, air release, washout and non-return valves should be planned together. Preferably a vehicle should be able to reach each location, but this needs to be reconciled with the importance of a valve and the possible interference with land usage.On long mains, valves should be included to allow for sections to be isolated and emptied within a reasonable time. Special consideration should be given to crossings of major roads, railways, watercourses and hazardous locations, but otherwise the sections might be up to about 1 km long. An isolating valve should be provided just inside or outside a pumping station so that the station pipework can be dismantled without emptying the main. Twin mains should be cross connected, usually at the points selected for isolation and emptying. Where a section of a main is to be emptied through a washout valve, provision should be made for the removal of the contents.Air release valves suitable for sewerage systems should be provided at summits:

a) to release air when the main is being filled;b) to release air and gas which arise during normal pumping;c) to mitigate the effects of surge;d) to permit air to enter when the main is being emptied,

In the vicinity of an air release valve a main should rise to the valve at a gradient preferably not flatter than 1 in 500 and fall away at a gradient not flatter than 1 in 300 for a significant length each side of the summit.Air release valves can make considerable noise when operating and they should be regularly maintained. If a chamber is provided it should be adequately ventilated to release the volume of air from the main and to prevent the accumulation of malodorous, toxic or flammable gases. In some situations air can adequately be released manually through a vertical pipe with a cock.Care should be taken not to exacerbate possible surge problems (see clause 25) by the siting or the use of incorrect types of air release valves. In certain situations it may be necessary to restrict the rate at which a pumping main is filled.

Non-return valves are used to prevent backflow after pumping has stopped and should be provided at the pumps. In special situations they may have to be sited on a pumping main; they should have extended spindles and lever arms so that they can be manually opened for emptying the main but the size of the non-return valve and static head dictate whether this is practicable. A bypass may be necessary in some cases.

27 ProfilesWhere possible, a pumping main should be laid with a continuous uphill grade and with gentle curves on both horizontal and vertical planes. When a continuous uphill grade is not possible, air release valves should be incorporated at high points and as the profile of the main dictates. Washout valves should be installed at low points.

28 Discharge arrangementsThe discharge of a pumping main should be arranged to avoid turbulence or splashing. It is preferable to avoid a vertical drop pipe and to arrange that the end of the pumping main is always full. If this is not possible, and the sewage may be septic, the surfaces of the structure at the discharge should be protected against corrosion. Chambers into which pumping mains discharge should be well ventilated. (See clause 11.)

29 AnchoragesPumping mains require anchorages to resist the thrusts developed at changes of direction, tapers, tees, valves and blank ends. Anchorages should not impede flexibility or expansion and, as far as possible, they should allow for possible replacement of fittings in the pipeline. The maximum thrusts usually occur when the pipelines are being tested.In situ concrete blocks should be provided for buried pipelines. For horizontal mains they should take the form of a cradle wedged against the undisturbed trench side; the design should be based on the safe bearing pressure of the ground. Vertical or inclined fittings should be clamped with metal straps to concrete blocks beneath them. Inclined pipelines, steeper than 1 in 6 should be anchored by concrete blocks cast across the pipes and set into undisturbed ground.

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In situations where it may be impractical to provide an anchor block to resist thrust, the use of flanged joints, self-restraining flexible couplings, or special harness assemblies across joints, may be considered. These should transfer the thrust along the pipeline until either it is possible to provide a concrete anchor block, or sufficient frictional resistance is developed between the pipes and the refilled ground to overcome the thrust.

30 Control of septicitySepticity in pumping mains can be prevented or controlled by the addition of oxidants to the sewage either in the form of oxygen or oxidizing chemicals such as hydrogen peroxide. Chlorine injection, with appropriate safeguards, may also be considered.The injection of gaseous oxygen, either pure or as air, causes complications which should be taken into account when designing the pipeline. Automatic air release valves at peaks may be incompatible with the process and it may be better to have air release cocks for occasional purging. Materials in the pipeline, fittings, valves, etc., should be compatible with the oxygen/chlorine/sewage mixture.The use of air instead of oxygen is not advised as the quantity needs to be five times greater. Oxygen absorption from air is less efficient than from undiluted oxygen and it may be necessary to remove a considerable volume of nitrogen.

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BS 8005-2:1987

BSI 02-2000

Publications referred to

BS 1710, Specification for identification of pipelines and services.BS 3535, Specification for safety isolating transformers for industrial and domestic purposes.BS 5153, Specification for cast iron check valves for general purposes.BS 5304, Code of practice. Safeguarding of machinery.BS 5345, Code of practice for the selection, installation and maintenance of electrical apparatus for use in potentially explosive atmospheres (other than mining applications or explosive processing and manufacture).BS 5839, Fire detection and alarm systems in buildings.BS 5839-1, Code of practice for installation and servicing.BS 6297, Code of practice for design and installation of small sewage treatment works and cesspools.BS 8005, Sewerage.BS 8005-0, Introduction and guide to data sources and documentation.BS 8005-1, Guide to new sewerage construction.BS 8005-3, Guide to sewers in tunnel4).BS 8005-4 Guide to design and construction of outfalls.BS 8005-5, Guide to rehabilitation of sewers4).BS 8007, Code of practice for design of concrete structures for retaining aqueous liquids.BS 8301, Code of practice for building drainage5).CP 312, Code of practice for plastics pipework (thermoplastics material).CP 312-2, Unplasticized PVC pipework for the conveyance of liquids under pressure.CP2010, Code of practice for pipelines6).CP2010-2, Design and construction of steel pipelines in land.CP2010-3, Design and construction of iron pipelines in land.CP2010-4, Design and construction of asbestos cement pipelines in land.CP2010-5, Design and construction of prestressed concrete pressure pipelines in land.CIRIA Technical Note 119: Screens and Grit in Sewage: Record, treatment and disposal.

4) In preparation.5) Referred to in the foreword only.6) Under revision as BS 8010.

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