Soft  braking  solution  for  the  conveyor  and  MH  equipment:  Nalco  case  study  Ganesh  Babu  Kumaran,  National  Sales  Manager,  Svendborg  Brakes  India  Pvt.  Ltd,  India    

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

©  Ganesh  Babu  Kumaran  

Soft Braking solution for the Conveyor and Material handling equipment : A case study of 15 km long

NALCO Downhill cable belt conveyor Abstract

Conveyors are the most efficient equipment in factories and other industrial mines and metallurgical plants. But conveyors, like other heavy equipment, pose several hazards to employees and also safety of the equipment in particular to the Long Distance Down Hill ( DH ) Conveyor, where the operation are mostly unmanned to a large extend due to its distance, terrain and profiles.

Generally Long distance conveyors move material uphill or uphill followed by downhill and along the surface of the earth LDC, conveyor designers when exposed to this type of design face real challenge. The challenge in designing the conveyors when different section combinations are loaded/empty, causing natural instability.

With increase in tonnages and high negative lift conveyors of several hundred

meters of drop, it is imperative to develop an intelligent soft braking system to address the starting and stopping requirements under various load conditions.

This paper describes the critical design criteria on one of the largest regenerative

conveyors overcoming disadvances in mechanical braking systems and control technologies of the past.

Long distance special belt conveyor demands gradual application and release of brakes. The gradual application of brakes reduces the forces in the main belt member as well as Longitudinal vibration along conveyor. As the conveyor is a high inertia machine, the situation is more demanding when high inertial mass is resting on elastic tension

members (belt). Therefore, Hydraulic Operated Soft Braking Disc Brakes are more suitable and are effectively used for long-distance, Downhill, high speed and regenerative conveyors.

These Disc brakes are capable of creating braking torque to full value gradually

depending on need, which is adjustable thru Program, with multiple deceleration braking ramps. It also provides an option of rollback control for the uni-direction conveyor and most importantly for Uphill / Downhill regenerative conveyor the opening of brake also can be controlled in a gradual programmable way by continuous monitoring of conveyor speed by means of encoder and controlling the braking ramp action in closed loop PID control.

In the case of NALCO Damanjodi Downhill Conveyor (Asia Longest downhill conveyor),supplied by CABLE BELT UK, is in operation for more than 25 years. Initially this belt conveyor was supplied with 2 x 1050kw sync motor with Dynamic resistor Bank to generate braking in regenerative mode and with a battery backup - grid resistor bank for breaking during power failure. Only Hydro pneumatic parking brake was provided, which was used only during stop and maintenance condition.

New Braking system requirement for NALCO:

Over the period of operation the demand and need of stopping the conveyor in

running condition had arised on many occasions, which made the task difficult, in particular during emergency and power failure conditions. This challenge was overcome in 2008 by review of site conditions, health of existing equipments and replacement of old Sync motor control to NEW 4 QUADRENT VFD , and new SVEDENBORG –DISC BRAKE with INTELEGENT PROGRAMMABLE SOFT BRAKING MODULE in Service and Emergency braking needs.

It was a great challenge to execute the requirement considering the existing

equipment condition and still to give a solution that can fulfill the requirement of full Torque requirement of 2500 kw at 21 rpm with a braking torque requirement of 1700 KNm at factor of safety 1.5, during service, emergency and power failure condition. This project was Engineered and Supplied, Implemented in Feb 2009 and working satisfactorily since then.

This paper explains in brief about the importance of Intelligent Soft braking solution

in Conveyor, considering various load conditions in a Downhill Cable belt conveyor. This will give a broad insight in understanding the various considerations, implication of such solutions. The overall operational flexibility, productivity was improved and importantly safety of men and machine was protected to a world class level, with implementation of this new braking technology

NALCO DAMANJODI, ORISSA STATE, INDIA, MINE DOWNHILL CABLE BELT CONVEYOR

Damanjodi Mine is situated at Koraput district of State Orissa in the east coast of India.

The conveyor was designed and installed by Cable Belt UK, in early of 1980’s with a single distance of 14.7 km, and a deepest single downfall of 530 mtrs. This conveyor plays a vital role as life line to the alumina plant ore supply which was built and commissioned in early 80’s. The drive selected was 2x1250 kw, combined with single helical parallel gearbox coupled to Koppe Wheel drive ,operating at a speed of 4.5 m/sec ,supported by endless Steel rope cable. CONVEYED MATERIAL SPECIFICATIONS: 1. Material conveyed Bauxite ore 2. Design tonnage 1800 tph 3. Single distance of conveyor 14.7 Km 4. Environmental condition DUST / DIRTY 5. Maintenance condition POOR 6. Hours in service per day 16+

Key Words:- Downhill conveyor,

Disc Brake, Soft Braking option, Braking torque, Ramp

CABLE BELT DOWNHILL BELT CONVEYORS- THE STARTING AND SPEED CONTROL THERE OF – Nalco Refurbishment case

INTRODUCTION

Generally, most conveyors move material uphill, or along the surface of the earth, conveyor designers have been exposed to this type of design which has become reasonably simple. But a real challenge exists when considering downhill or regenerative conveyors.

This becomes a challenge to the design engineers due to the downhill conveyors natural instability under load. With increased tonnages , velocity and more drop, it has become necessary to develop an intelligent braking system to address their starting and stopping requirements under various load conditions refer Fig 1 below.

This paper describes the critical brake design criteria of large regenerative conveyors and some of the advances in mechanical braking systems control technology.

DESIGN REQUIREMENTS

This NALCO DH conveyor was originally designed by Cable belt UK ( now Metso Minerals ) as one of the long DH conveyor in 80’s. The conveyor serves a long way to the ore feeding requirement of Nalco. The new increased operational requirement, Outdated technology of the existing system and technical support for maintenance, spares was a challenge to keep the system up to full capacity.

After a long technical review of various technologies available and due consideration the team of Nalco has considered to engage EIL as consultant for up gradation or refurbishment requirement. Under this refurbishment project the following Major changes are considered.

Removing of existing Synchronized motor and replacement with VFD grade induction motor and regenerative Drive panel.

Introduction of Fail Safe Disc Brake with Soft braking system Introduction of PLC based control and SCADA.

Fig 1 for different loading conditions

As this belt conveyor was in serving condition and the only source to convey the raw material from Mines to alumina plant, it was the lifeline of NALCO. Hence a due weightage on the technical and workable system was in search to ensure the success of up gradation and reassurance of its fullest capacity with improved operation ease, safety and maintenance of Cable belt conveyor.

Brief details of the conveyors

MATERIAL SPECIFICATIONS

1. Material conveyed Bauxite 2. Design tonnage 1800 t/h 3. Conveyor length 14.7 km

4. Environmental condition. DUST / DIRTY 5. Maintenance condition... LOW 6. Hours in service per day 16+hrs 7. Speed Max 4.7 m/s

BELT SPECIFICATIONS

1. Motor nameplate Kilowatts 2 x 1100 2. Motor rpm 1470 Max Running rpm 1470 3. Gear Box output RPM 21 4. Koppe Wheel Dia 4450 mm 5. Drive Max Kw in worst load condition Kilo watts 1800

The basic conveyor design data was available with Nalco and since there is no change in mechanical, operational and layout of cable belt, the same Motor drive kw at various load conditions are taken and considered for brake selection.

Empty belt @ 4.5 m/s the motor power is close to 725 kw

With full loaded 1800 TPH @ 4.5 m/s the motor power shall be -125 kw

At full loaded @ 4.5 m/s in the down side of conveyor shall be of – 870 kw

At full loaded @ 4.5 m/s in up side of conveyor shall be of 1700 kw

With their dynamics of varying power condition according to material loading condition, leaves with the following challenges

control of belt during start, running, Over speed and Worst case

power failure or black out and Emergency are the challenge.

As the mechanical braking system is the ultimate device to ensure that the conveyor does not run away in either direction. In both cases, these are safety devices not only to avoid damage to the conveyor but to personnel as well. Therefore it is imperative to make absolutely sure that the braking system for the downhill conveyor should be 100% secure and should be fail safe in the event of a worst case, power failure and emergency.

BRAKE SYSTEMS SELECTION

The proper design and selection of the drive and brake systems is extremely important. This directly influences the operation during starting and stopping controls as they have a direct impact on the ability to control dynamic instabilities. Thorough understanding of Brake response time, holding torque , control on ramp, reliability, and failure modes will be required to design and program the intelligent braking system ( SOBO ).

The drive and brake selection have to take into account the technical requirements, reliability, serviceability and finally the cost. The clients requirement of low, ease maintenance of brake system, considering available site resources.

In case of starting, if the material is loaded uniformly along the belt and maximum load on uphill portion creates a roll back condition. This creates high tension in the rope and results in undue lift-up conditions from support rollers. In such condition the Sobo monitor and takes corrective action to apply brake and signals to Main PLC or SCADA so that operator can react to the operation parameter and manage the situations.

It is very important to realize that the mechanical brake is the only means of stopping a loaded downhill conveyor in case of motor, VFD Panel, coupling or other mechanical components failure. This is true even when the selected drive can provide retarding torque during healthy power supply. As a result, the brake system is one of the most critical safety components in a downhill conveyor.

The brake and its control system must always be designed to safely stop the fully loaded conveyors from full speed to zero and under any failure condition without damaging the conveyor components, stress or fatigue to conveyor and safe to men in work.

As is often the case in engineering design, the final choice is usually a tradeoff between cost, simplicity and performances. The best choice is the simplest, most reliable system which will do the job satisfactorily.

HYDRAULIC DISC BRAKES

The "intelligent" braking system must have a controller requiring multiple feedback to a PLC, through the monitoring of the belt speed feedback, the brake torque can be controlled using a simple hydraulic valve to regulate the pressure. With this in mind, at no stage must the brake be oversized / undersized, as this will lead to over-tensioning the belt and possible disaster. Loop stability must be installed with control relating to valve response time, time delay in brake application and maximum rate of change in drive torque.

Hydraulic disc brake systems are available in all sizes and in many different configurations for this case we had selected the Hydraulic pump with Accumulator and poppet valve operated manifold to control the brake line pressure ( for details refer the fig 2 ).

Fig 2: hydraulic circuit SLD

Here a huge Koppe wheel of 4450 mm – 2 nos are well available and fit to use it as brake disc. We had considered to use the koppe wheel as disc and provide the required special mounting bracket arrangement to fit the brake caliber for 240 mm width of koppe wheel. For the required control of torque we have to mount 4 x BSFI 3120 brake on each koppe wheel, such that a total of 8 BSFI 3120 brake are used. For details of brake calculation please refer below details.

Brake Caliber Selection Chart:-

To brake calibers

BRAKE SELECTION CALCULATION:- Mb=a*Fc*µ*(Do 0,2) = 1,651,200 Nm Fc=Merf./(a*µ*(Do 0,2)) = 89,234 N

Loss of force by 0 mm pad wear New brake pads

Mvor=Mb (Verschl.*Verl. in %) = 1,651,200 Nm > 1,227,857 Nm SF = 1.34

Loss of force by 1 mm pad wear Used brake pads

Mvor=Mb (Verschl.*Verl. in %) = 1,502,592 Nm > 1,227,857 Nm SF = 1.22

Loss of force by 2 mm pad wear Used brake pads

Mvor=Mb (Verschl.*Verl. in %) = 1,353,984 Nm > 1,227,857 Nm SF = 1.10

Where as :- Mb = Braking Torque in Nm . a = No. of brake Caliber Fc = Clamping force µ = Co. efficient of friction = 0.4 Do = Disc outer dia

Use of brake at low speed side has main advantages are increased safety, high thermal capacity due to larger disc sizes, and increased pad sweep area. Low speed brakes are a natural choice on downhill conveyors where their increased safety and large thermal capacity are essential.

Spring applied calipers are used whenever a fail-safe design is required. In these brakes, the pad pressure is supplied by sets of mechanical springs, and hydraulic pressure is used to release the pads. This ensures that the maximum braking torque is always available, even during power failure. Spring applied calipers offer down to 125 millisecond response time and good torque control.

Fundamental operation of Fail safe Brake:-

Intelligent SOBO soft braking option system

Low speed hydraulic fail safe spring applied disc brake incorporating speed feedback linked to a Sobo PLC to control braking sequence.

Fig 3:-The main parts in the SOBO system

The aim of the SOBO system is to prevent the high braking torque even for normal braking with full torque applied lead to stress to conveyor belt, structural and other mechanical components.

The normal approach to solving this problem would often be to control the braking torque regulating the pressure in the brakes using a proportional valve. This is however quite an expensive, cumbersome design with high power consumption and

development of high oil temperature leading to high maintenance and running cost.

In the SOBO system this is obtained by using a relatively simple hydraulic regulation unit, based on directional poppet valves, throttling and a dedicated Programmable Logic

controller. During braking the actual rpm is measured and compared with an ideal sequence expressed as rpm versus time. If the speed is below that required, the brake torque is reduced, and vice versa.

If over-speed is detected the system is designed to react according to specifications, i.e. a conveyor could be brought down to the nominal rpm by controlled braking or bring to full stop. More rpm limits with different reaction patterns can be built into the system.

The SOBO system ensures:

A specified braking time is achieved, almost independently of the applied load.

The braking torque during soft braking will not reach the maximum level (Sudden braking).

The overshoot and oscillations at the end of the braking sequence are minimised or eliminated.

To ensure a very fast response in case of emergency braking, one of the directional valves is drained directly to the tank, bypassing any throttle and accumulator that may slow down the system.

Sample trend of test done on Nalco DH Belt

Type of brake Ramp in use:-

Normal Braking Ramp 1:- least priority

In normal operation the VFD Drive take the stop ramp up to 90% of full speed and the rest will be with Sobo disc brake following same ramp till full stop or parking. As in Nalco case we had set a Constant deceleration ramp with 100 % speed to 120 sec of stop time

Conveyor

RPMBreak

Pressure

Constant Deceleration Mode:- Constant deceleration is used in order to achieve the same deceleration independent of speed. Braking time in a constant deceleration function is increased or decreased depending on speed at the time the brake command is given. The deceleration curve will remain the same under all braking sequences

Worst case Braking Ramp 2:- next least priority

In Worst case the conveyor shall be taken to 100% mechanical braking control from it actual running rpm to its full stop or parking. As in Nalco case we had set a Constant deceleration ramp with 100 % speed to 120 sec of stop time

Emergency Braking Ramp 3:- Top / Hi priority

In Emergency the conveyor shall be taken to 100% mechanical braking control from it actual running rpm to its full stop or parking. As in Nalco case we had set a Constant deceleration ramp with 100 % speed to 20 sec of stop time

Later this parameter was fine tuned based on the various operation and equipment behavioral condition a site. This is one of the major advantage of Intelligent soft braking system, which are flexible to program and achieve various braking time based on changing needs at site itself without call for a change in system design or replacement of components.

The SOBO system was supplied including a encoder pick-up for speed generation feedback to the PLC (fitted to low speed side for safety). A uninterrupted power supply (UPS) was also supplied to have brake control in case of a power failure.

SUMMARY

With the implementation of new Soft Braking disc brake system in NALCO DH Conveyor,

1. The belt stretch was drastically reduced during emergency, Worst case and Normal stopping. This was concluded by observing the take up counter weight lifting by less than 2 meter as against general lift of 8-10 meters.

2. Also the each stop of conveyor was in full control of operation to achieve the same stop time of belt leading to uninterrupted sequence operation of material feed.

3. On many occasions the Sobo had to detect a roll-back and applied brake in control with a feedback to operator leading to control. This was not possible without a huge backstop arrangement in GB.

4. In short the belt is ensured of uniform loading or no empty patch along the conveyor due to free run of conveyor for more than 20 min compared to earlier non control braking and during power failure.

5. The new brake control avoided manual unloading of material in the uphill potion before starting, which was created due to uncontrolled run out of conveyor without control braking.

6. The overall control and easy maintenance of conveyor with the help of new Sobo braking solution makes the NALCO to upgrade the DH conveyor with latest technology at least investment.

13. ACKNOWLEDGEMENTS

To NALCO Damanjodi Mine, Orissa for their assistance and valuable information. To every person that helped along with the production of this paper.

14. REFERENCES

PARTICULARS OF AUTHOR Full Name: Ganesh Babu K Position: National Manager – Svendborg Brakes India Pvt. Ltd. Main activity of the company: Manufacture and supply of Hydraulic disc brake solutions

Education: Diploma Engineering, State Board of Technical education, Tamil Nadu India, 1990;

BBA, Pondicherry University, 2000

Master in Marketing, Pondicherry University, 2002 (PhD ) in International Marketing from Centre for Latin American Studies, Goa university

Contact Details:

kgb@svendborg-brakes.com, +91 9380580419, www.svendborg-brakes.com