PRESSURE SAFETY VALVES

TABLE OF CONTENTS

S. No. TITLE PAGE No.

1 Introduction 1

2 PSV Classification 3

3 Assembly of Conventional Spring Loaded PSV 7

4 Codes, Standards and Recommended Practices 10

5 Activities Observed 11

6 Appendix 12

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INTRODUCTION Refineries deal very sensitive and volatile process fluids and extreme care must be taken

when it comes to handle them, but even then due to equipment failure, human error or some other external event such as tempest, earthquake etc. may trigger pressurization of

process equipment beyond its safety limits which may result in Relief Event.

A Relief Event is defined as any incident occurred during a process that causes loss or

damage to the Works and/or the Facilities, such as fire, over heating of pumps and compressors, liquid expansion in pipe or surge. These incidents can be avoided to a large

extent by the application Active Controls in the form of Safety Systems.

A Safety System includes a safety device along with associated piping and necessary

process equipments for safe handling of the material being ejected.

Pressure safety valve (PSV) or pressure relief valve (PRV) is commonly used to protect a

pressure containment part i.e. vessel, column, etc from overpressure. It is one of the code approved type of overpressure protection devices. This type of device is reclosing type

where the mechanism of devices is designed such that it will stop relief when the pressure is reduced to it reseat pressure. Besides PSV, rupture disc and rupture pin are also code

approved type of overpressure devices. This type of device is non-reclosing type where it will continue to relief until all inventory is completely evacuated from the system or with

operator intervention.

Safety valves should be installed wherever the maximum allowable working pressure

(MAWP) of a system or pressure-containing vessel is likely to be exceeded. In steam systems, safety valves are typically used for boiler overpressure protection and other

applications such as downstream of pressure reducing controls. Although their primary role is for safety, safety valves are also used in process operations to prevent product

damage due to excess pressure. Pressure excess can be generated in a number of different situations, including:

An imbalance of fluid flow rate caused by inadvertently closed or opened isolation valves on a process vessel.

Failure of a cooling system, which allows vapor or fluid to expand.

Compressed air or electrical power failure to control instrumentation.

Transient pressure surges.

Heat exchanger tube failure.

Uncontrollable exothermic reactions in chemical plants.

Ambient temperature changes.

PRESSURE SAFETY VALVES AND PRESSURE RELIEF

VALVES:

The names “safety” and “relief” are frequently used interchangeably but they should not be. Safety valves are for the compressible fluids: steam and other gases. This

compressibility demands quick over pressure relief. So safety valves have pop seats and

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plugs which open rapidly on overpressure, relieving at full flow. They may discharge steam to atmosphere or direct a gas back to the system.

Relief valves are for the non-compressible fluids-liquids such as water and oil. Immediate full-flow discharge is not needed since a very small flow significantly reduces

overpressure, so plug and seat open and close very slowly, discharging back to some low pressure point in the system to conserve the liquid.

Following API Standard 520 application and working principle of PRV, PSV and PSRV are:

a. A relief valve is a spring loaded pressure relief valve actuated by the static pressure upstream of the valve. The valve opens normally in proportion to the

pressure increase over the opening pressure. A relief valve is used primarily with incompressible fluids.

b. A safety valve is a spring loaded pressure relief valve actuated by the static pressure upstream of the valve and characterized by rapid opening or pop action.

A safety valve is normally used with compressible fluids.

c. A safety relief valve is a spring loaded pressure relief valve that may be used as

either a safety or relief valve depending on the application.

Figure 1: Typical safety valves

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PSV CLASSIFICATION There are generally two types of PSV

1. Direct Acting PSV

2. Pilot Operated PSV

1. DIRECT ACTING PSV

The oldest and the most commonly used type of PSV is the Direct Acting type. They

are designed as direct acting because the force element that keeps the valve closed is either a weight or a spring or a combination of both. The process to be relieved acts

directly on disc which is held closed by the spring force opposing the lifting process pressure.

SPRING LOADED DESIGN

The image on the right shows the construction of a conventional spring loaded PSV.

The valve consists of a valve inlet or nozzle mounted on the pressurized system, a disc held

against the nozzle to prevent flow under normal system operating conditions, a spring to hold the

disc closed, and a body and bonnet to contain the operating elements. The spring load is adjustable

to vary the pressure at which the valve will open.

When PSV begins to lift, the spring force

increases. Thus system pressure must increase if lift is to continue. For this reason pressure relief

valves are allowed an overpressure allowance to reach full lift. This allowable overpressure is

generally 10% for valves on unfired systems. This margin is relatively small and some means must

be provided to assist in the lift effort.

Most pressure relief valves, therefore, have a secondary control chamber or huddling chamber to enhance lift. As the disc begins to lift, fluid

enters the control chamber exposing a larger area of the disc to system pressure.

Figure 2: conventional spring

loaded PSV

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Figure 3: Typical disc arrangement in safety valves

Figure 4: Operation of a conventional safety valve

This causes an incremental change in force which overcompensates for the increase in

spring force and causes the valve to open at a rapid rate. At the same time, the direction of the fluid flow is reversed and the momentum effect resulting from the

change in flow direction further enhances lift. These effects combine to allow the valve to achieve maximum lift and maximum flow within the allowable overpressure

limits. Because of the larger disc area exposed to system pressure after the valve achieves lift, the valve will not close until system pressure has been reduced to some

level below the set pressure. The design of the control chamber determines where the closing point will occur.

RESEATING Once normal operating conditions have been restored, the valve is required to close again, but since the larger area of the disc is still exposed to the fluid, the valve will

not close until the pressure has dropped below the original set pressure. For compressible fluids, the blowdown is usually less than 10%, and for liquids, it can be

up to 20%.

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Figure 5: Relationship between pressure and lift for a typical safety valve

The design of the shroud must be such that it offers both rapid opening and relatively small blowdown, so that as soon as a potentially hazardous situation is reached, any

overpressure is relieved, but excessive quantities of the fluid are prevented from being discharged. At the same time, it is necessary to ensure that the system pressure

is reduced sufficiently to prevent immediate reopening.

BACK PRESSURE CONSIDERATIONS

Pressure relief valves on clean non-toxic, non-corrosive systems may be vented directly to atmosphere. Pressure relief valves on corrosive, toxic or valuable

recoverable fluids are vented into closed systems. For valves installed in a closed system, or when a long vent pipe is used, there is a possibility of developing high

back pressure. The back pressure on a pressure relief valve must always be evaluated and its effect on valve performance and relieving capacity must be considered.

CONVENTIONAL VALVES

Superimposed back pressure may be a result of the valve outlet being connected to a normally pressurized system or may be caused by other pressure relief valves venting

into a common header. Compensation for superimposed back pressure which is constant may be provided by reducing the spring force. Under this condition the force

of the spring plus back pressure acting on the disc would equal the force of the inlet set pressure acting to open the disc. It must be remembered, however, that the value

of the set pressure will vary directly with any change in back pressure.

BALANCED BELLOWS PSV

When superimposed back pressure is variable, a balanced bellows or balanced piston design is recommended. A typical balanced bellow is shown on the right. The bellows

or piston is designed with an effective pressure area equal to the seat area of the disc. The bonnet is vented to ensure that the pressure area of the bellows or piston will

always be exposed to atmospheric pressure and to provide a telltale sign should the

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bellows or piston begin to leak. Variations in back pressure, therefore, will have no effect on set

pressure. Back pressure may, however, affect flow.

In addition to offsetting the effects of variable back

pressure, the bellows or piston acts to seal process fluid from escaping to atmosphere and isolates the

spring, bonnet and guiding surfaces from contacting the process fluid. This is especially important for

corrosive services.

2. PILOT OPERATED PSV

A pilot operated pressure relief valve is a pressure relief valve in which the major relieving device is combined

with and is controlled by a self-actuated auxiliary pressure relief valve.

MATERIALS OF CONSTRUCTION

Compatibility with the process fluid is achieved by careful selection of materials of

construction. Materials must be chosen with sufficient strength to withstand the pressure and temperature of the system fluid. Materials must also resist chemical attack by the

process fluid and the local environment to ensure valve function is not impaired over long periods of exposure. Bearing properties are carefully evaluated for parts with guiding

surfaces. The ability to achieve a fine finish on the seating surfaces of the disc and nozzle is required for tight shut off. Rate of expansion caused by temperature of mating parts is

another design factor. Generally PSVs are constructed from the following materials

Carbon Steel SA216 WCB

Chromium Molybdenum Steel SA217 WC6

Austenitic Stainless Steel SA351 CF8M

Nickel Copper Alloy SA494 M35-1

Alloy 20 SA351 CN7M

Figure 6

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ASSEMBLY OF CONVENTIONAL SPRING LOADED

PSV

Figure 7

Following are the components of Conventional PSV that were observed during the turn

around of Lube - I refinery:

Cap:

Top most portion of the PSV that covers the adjustment screw. It is either threaded or

bolted to the bonnet.

Bonnet

It is the middle part of the PSV assembly that houses stem, spring, spring buttons and a guide plate. It may be open type (open yoke) or close type.

Body

It is the bottom most portion that encloses disc holder, disc, nozzle, blowdown ring

and locking nut.

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Adjustment Screw

It is used for the calibration of PSV to the specified set pressure.

Stem

It properly aligns the components and transmits the power of the spring to the disc assembly. Transmission of this power must occur in a uniform manner that allows

even distribution of the spring force to the valve seats.

Spring and spring buttons

Spring is used to keep the valve closed while spring buttons uniformly distribute the spring force

Guide

A guide or guiding system is incorporated in all PSVs as part of their basic design. Guiding systems can be classified as top-guided or bottom-guided, spindle-guided, or

disc/disc-holder-guided. Top-guided has the guiding system incorporated above the nozzle seat. Bottom-guided has the guiding system below the nozzle seat. The guide

is defined as the mechanism used to concentrically locate the disc/disc holder so as to create positive seating interface between the disc and nozzle seat.

Guides are susceptible to galling, corrosion, out of round and steam cuts.

Disc and Disc Holder

It is a plate that is directly subjected to process pressure and is vulnerable to galling, erosion, pitting, and any metal distortion in the contact areas

Machining and Lapping: The severity of damage combined with allowable disc tolerance determines whether machining can be performed before lapping. Most

manufacturers have machining tolerances and minimum tolerances. In most valves, disc inserts cannot be machined and only the seat should be lapped.

Nozzle

It is the entrance through which the process fluid enters into the valve. Nozzle designs

vary from one manufacturer to the next. Basic designs of nozzles are as follows:

• Full, threaded and removable

• Semi, threaded and removable

• Semi, welded in valve body

• Semi, pressed and removable

Full-nozzles are usually incorporated in safety valves designed for process and high

pressure applications, especially when the fluid is corrosive.

Conversely, the semi-nozzle design consists of a seating ring fitted into the body, the top of which forms the seat of the valve. The advantage of this arrangement is that the

seat can easily be replaced, without replacing the whole inlet.

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Blow Down or Notching Ring

It is used for the adjustment of blowdown. After removal, the nozzle ring should be cleaned (especially threads) so that a proper inspection can be made. The nozzle ring

should be examined for missing teeth or notches, steam cuts and thread damage.

Locking Nut

It keeps blowdown ring in its place.

Lift Lever

For manual pressure relief.

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CODES AND STANDARDS AND RECOMMENDED

PRACTICES

Standards relevant to safety valves vary quite considerably in format around the world, and many are sections within codes relevant to Boilers or Pressure Containing Vessels.

Some will only outline performance requirements, tolerances and essential constructional detail, but give no guidance on dimensions, orifice sizes etc. Others will be related to

installation and application. It is quite common within many markets to use several in conjunction with each other.

Table 1: Standards relating to safety valves

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ACTIVITIES OBSERVED

During the Turnaround of Lube – I refinery at NRL following activities were performed for the maintenance of Pressure Safety Valves:

DISMOUNTING

After steaming and flushing of the system, PSVs were removed from the node and a blind

flange is placed in place of it. It is also made sure that proper tagging is done to avoid any misplacement. The PSVs are then shifted to PSV workshop.

DISMANTLING

After visual inspection, PSV is then disassembled into its components whose visual

inspection is then carried out based on which decision is made whether to service, repair or replace certain component.

SERVICING

It involves washing of components with Rustolene oil to remove contaminants and rust.

Also lapping of nozzle lips and disc is done to improve their surface finish with the help of abrasive made of silicon carbide.

REASSEMBLING

All the components are put together after servicing.

CALIBRATION

It involves following steps

1. Clamping of PSV on Test Bench with the aid of mechanical jaws powered by air pressure.

2. Pressurization of PSV is done with the help nitrogen gas supplied from cylinders.

3. Pressure reading is observed on either low pressure dial or high pressure dial.

4. If specified CDTP is not obtained then PSV is depressurized and adjustment screw is either tightened or loosened.

5. The procedure is repeated again until the desired CDTP is achieved.

SEALING

An external seal is placed on PSV to show that it is ready.

PAINTING

Heat resistant paint is applied on PSV.

REINSTALLATION

The PSV is then installed at the node from where it was removed by matching its tag.

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APPENDIX

OPERATIONAL CHARACTERISTICS OF PRESSURE

RELIEF DEVICES

Maximum operating pressure - is the maximum pressure expected during normal

system operation.

Maximum allowable working pressure (MAWP) - is the maximum gauge pressure

permissible at the top of a completed vessel in its normal operating position at the designated temperature. The pressure is based on calculations for each element of the

vessel using actual nominal thickness, exclusive of additional metal thickness allowed for corrosion and loadings other than pressure. The maximum allowable working pressure is

the basis for the pressure setting of the pressure relief devices that protect the vessel.

Design pressure - of the vessel along with the design temperature is used to determine

the minimum permissible thickness or physical characteristic of each vessel component as determined by the vessel design rules. The design pressure is selected by the user to

provide a suitable margin above the most severe pressure expected during normal operation at a coincident temperature. It is the pressure specified on the purchase order.

This pressure may be used in place of the maximum allowable working pressure in all cases where the MAWP has not been established. The design pressure is equal to or less

than the MAWP.

Accumulation - is the pressure increase over the maximum allowable working pressure

of the vessel allowed during discharge through the pressure relief device, expressed in pressure units or as a percentage of MAWP or design pressure. Maximum allowable

accumulations are established by applicable codes for emergency operating and fire contingencies.

DEVICE PRESSURES

Set pressure - is the inlet gauge pressure at which the pressure relief device is set to open under service conditions.

Cold differential test pressure (CDTP) - is the pressure at which a pressure relief valve is adjusted to open on the test stand. The cold differential test pressure includes

corrections for the service conditions of back pressure or temperature or both.

Back pressure - is the pressure that exists at the outlet of a pressure relief device as a

result of the pressure in the discharge system. It is the sum of the superimposed and built-up back pressures.

Built-up back pressure - is the increase in pressure at the outlet of a pressure relief device that develops as a result of flow after the pressure relief device opens.

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Superimposed back pressure - is the static pressure that exists at the outlet of a pressure relief device at the time the device is required to operate. It is the result of pressure in the

discharge system coming from other sources and may be constant or variable.

Blowdown - the difference between actual popping pressure of a pressure relief valve

and actual reseating pressure expressed as a percentage of set pressure or in pressure units

Opening pressure - is the value of increasing inlet static pressure at which there is a

measurable lift of the disc or at which discharge of the fluid becomes continuous, as determined by seeing, feeling or hearing.

Closing / Reseating pressure - the value of decreasing inlet static pressure at which the valve disc reestablishes contact with the seat or at which lift becomes zero

Simmer - is the audible or visible escape of compressible fluid between the seat and disc which may occur at an inlet static pressure below the set pressure prior to opening.

Overpressure - a pressure increase over the set pressure of a pressure relief valve, usually expressed as a percentage of set pressure

Resealing / seal-off pressure - is the value of decreasing inlet static pressure at which no further leakage is detected after closing. The method of detection may be a specified

water seal on the outlet or other means appropriate for this application.