Upkeep and maintenance of safety valves

Valves are amongst the most important components of

Basics of safety valves

• Why a Safety Valve?

The Safety Valves ensure the proper function of pressurised systems and processes like steam boilers, vessels and pumps. Thereby the primary purpose is fulfilled to protect life, property and the environment against undesired overpressure.

• What is a Safety Valve?

A Safety Valve is characterised by rapid opening or pop action, or by opening in proportion to the increase in pressure over the opening pressure.

• Function of a Safety Valve

The downward force through the spring is applied to the disc and it keeps the safety valve closed. This is the normal and safe operating conditions. When the force below the disc exceeds the downward force (i.e., when working pressure crosses set pressure) the safety valve will open. It opens fully within permitted overpressure and discharges rated capacity. As the flow takes place, there is a pressure drop and the valve would close down slowly.

Types of safety valves

• The safety valves are generally of “Full nozzle” type and sizes ranging from 1 inlet to 8” inlet.
• The low-capacity relief valves are of “Modified nozzle” type and are generally of smaller sizes i.e., ¾” x 1”.

Applications in the boiler industry

• The package boiler is provided with two safety valves on the drum.
• The high-pressure/capacity boiler is provided with two valves on the drum and one on the Superheater. There would be one relief valve of min. 2” inlet on the economiser.
• The pressure-reducing station would have one safety valve installed on the downstream side (next to the first isolation valve).
• There would be safety valves installed on the process side and also relief valves on the hot/cooling water line.
• The other applications could be for installation on the Soot blower, Deaerator, Turbine exhaust, shell & tube side of the heaters etc.

IBR regulations applicable

• Regulation 290 – Chest (Valve Body)
• Regulation 291 – Safety Valves in General
• Regulation 292 – Types of safety valves (Ordinary, high and full lift)
• Regulation 293 – sizing and selection of safety valves for saturated steam (Eqn. 78) and superheated steam (Eqn. 79).
• Regulation 294 – Overpressure for safety valves
• Regulation 295 – Pressure drop (blowdown)
• Regulation 296 – Mountings of safety valves
• Regulations 297 to 314 cover – Openings in shell, Discharge Passage, design, construction, materials and dimensions safety valves.
• Appendix-L of IBR 1950 – Type test certification for computation of C constant under the witness of IBR Inspecting authority.

Installation guidelines

General Notes

Safety Valves are high-quality devices which should be handled with great care. To ensure proper performance, all parts are made with precision. Casual handling of valves in workshops, stores, during transportation or installation could cause leakage or possibly permanent damage.

The seating surfaces are machined, lapped and polished with high precision to ensure required tightness. Even though the surfaces are extremely hard, the seat can still be damaged. By all means, care should be taken to prevent dust, foreign particles etc. from entering the valve during transportation, installation and operation.

All safety valves are thoroughly tested and sealed. The purpose of sealing is to ensure that the pressure setting is not disturbed by any unauthorised persons. For new installations, if there is any performance-related issues, they should be reported to the manufacturer so that an immediate solution or to depute service person can be considered. It is highly recommended that the seal be kept intact.

When installing safety valves with threaded connections use only gaskets or metal seal washers. Sealing materials such as PTFE tape should not be used as this type of material can break off and enter the valve causing it to leak.

If valves with open bonnets and/or levers are to be re-painted after dispatch from the factory, care must be taken to protect sliding parts. Otherwise, a correct operation may be affected.

Transport Protection

The inlet and outlet of safety valves are protected during transportation with plastic caps. These caps should only be removed just before installing the valve.

Installation / Assembly

Safety valves should be installed with bonnet VERTICALLY UPRIGHT. Furthermore, it should be mounted in such a way that no inadmissible static, dynamic or thermal loads can be transmitted to the valve due to up and downstream pipe work.

Draining of Condensate

To prevent dirt and all kinds of impurities from the safety valve, the drainage of the discharge pipe must be done via a discharge pipe. Therefore, LESER safety valves are generally not provided with drain holes. According to the rules, a drain hole of sufficient size must be incorporated at the lowest point of pipe work. In all cases, the discharge pipe must first slope in a downward direction and be fitted with a suitable size drain hole before any bends are connected (refer to the sketch below)

Exception: In special cases, an optional drain hole may be recommended in valve body as it may be that pipe work drainage cannot be guaranteed at a lower point than the valve. The standard drain hole which will then be supplied by the manufacturer is with ¼” BSP or ½” BSP threads depending on valve size.

Insulation

In case insulation is provided for the pressure relief valve, the bonnet must be kept free to avoid unacceptable heating up of the spring.

Inlet Pipe

The inlet pipe for safety valves should be as short as possible and should be so arranged that when the valve is in its fully open position, the pressure drop must not exceed 3% of the set pressure. If the calculation results in a pressure drop higher than 3%, then the inlet pipe size must be enlarged.

Dismantling and Assembly Instructions

1. Loosen the existing lead seal.
2. Press the lever (43) towards the middle until it reaches the stop so that the lifting fork (44) no longer holds the spindle cap (46).
3. Loosen and remove the lever cover (40).
4. Loosen the spindle cap (46) from the spindle (12), and remove the securing ring (91) and pin (74).
5. Loosen the lock nut (19) of the adjusting screw (18).
6. Turn the adjusting screw (18) anticlockwise to remove the spring tension.
7. Remove the hexagonal nuts (56) from the flange of the bonnet (9).
8. Lift off the bonnet (9).
9. Remove the upper spring plate (16).
10. Lift off the spring (54) and remove the lower spring plate (17) and split ring (14).
11. Remove spindle (12) with a guide (8) and disc (7).
12. Carefully clean the seat (5) and disc (7), and if required body internals.
13. Refit spindle (12) with a guide (8) and disc (7).
14. Fit the split ring (14) into the spindle groove and retain it with the securing ring (59); slip on the lower spring plate (17) to locate the split ring (14).
15. Replace spring (54).
16. Slip on the upper spring plate (16) onto the spindle (12).
17. Align the adjusting screw (18), and bonnet (9), over the spindle (12) and refit.
18. Fit and tighten the hexagonal nuts (56).
19. Load the spring (54) to obtain the required set pressure. The clockwise rotation of the adjusting screw (18) increases pressure. Anticlockwise rotation of screw (18) reduces pressure.
20. Tighten the lock nut (19) onto the adjusting screw (18).
21. Refit and secure the spindle cap (46) by pin (74) and securing ring (91).
22. Screw on the lever cover (40).
23. Pull the lever (43) towards the middle so that the lifting fork (44) is pushed under the spindle cap (46).
24. The test spindle will lift correctly by pulling a lever.
25. Seal the valve.

Trouble Shooting

Before shipment, all safety valves go through a quality check for functional parameters at the manufacturing facility as per customer specifications. Valves which pass the test as per internal procedure, as well as applicable code and standards, will be further moved to shipment else the valves will be retested to achieve the specific result.

The possible complaints related to safety valves are as follows:

A. Safety Valve opens before or after the specified pressure:

All safety valves are passed for their functional parameters prior to shipment. This specified problem may occur due to various reasons such as

1. 1. Disturbance of setting due to misalignment during transportation.
   2. Disturbance of setting due to improper handling in transportation or at site.
   3. High impact on safety valve due to mishandling.
   4. Galling effect between the seats of the nozzle & disc due to the long storage period.

For any of the complaints above, the valve needs to be opened with the help of a manual lifting lever once or twice above the specified pressure. This is to restore valve realignment. Then the testing can be conducted to observe the actual opening of the safety valve. If the complaint is still not resolved by the above method, then it shall be reported to the manufacturer.

B. Safety Valve Leakage

All safety valves have been tested for their tightness at the manufacturer’s end. The testing shall be according to API 527 standard. The specific problem may occur due to various reasons such as:

1. 1. Presence of dust particles due to improper storage or removal of caps.
   2. Damage to the seating surface due to foreign particles present inside the piping.
   3. Damage of seat due to initial flushing of the system after installation of safety valve.

This complaint may be analysed by dismantling the safety valve with the help of the manufacturer. Generally, the nozzle and disc may need lapping and polishing in order to achieve the required flatness. At the same time, the guiding surface of the guide and spindle may also be cleaned with soft tissue paper to remove dust/foreign particles.

C. Safety Valve excess vibration during discharge

Safety valve may show signs of vibration during discharge and it would be mainly due to:

1. 1. Selection of oversized valve (higher discharge capacity)
   2. Inappropriate outlet piping construction

If there is a reasonably higher gap between steam discharge and valve flow capacity, there is a possibility of chattering which can result in vibration. Then the selection parameter needs to be cross-verified or referred to the manufacturer to suggest a suitable size valve to eliminate vibration.

If the installed discharge piping construction is inappropriate (i.e., too long with multiple bends), it can create resistance to the discharged medium and develop back pressure. This can result in vibration. To avoid such complaints, care to be taken during the design and installation of the discharge pipe construction.

D. Boiler pressure building up even during Safety valve full discharge:

The complaint could be that the safety valve discharges fully to its rated capacity; however, pressure continues to rise in the boiler. This situation may occur if the installed valve is not in line with capacity requirements. In such cases, the valve selection needs to be first looked in to. Then check the discharge capacity shown in Attachment 8 of IBR Form IIIC and compare it with boiler capacity. If it matches, then the issue may be taken up with the manufacturer for their reconfirmation of valve performance.

E. The safety valve is not reclosing within the specified limits of IBR 1950:

There are seldom complaints about valves not reclosing within permitted limits below set pressure. In such cases, the valve size and flow diameter need to be checked to correlate with IBR regulation requirements. IBR does permit 10% blowdown for safety valves having a flow diameter ≤ 32mm. If the installed valve is within the above range, up to 10% blowdown shall be acceptable. If the valve size has having flow diameter ≥ 32mm, the permitted blowdown shall be 5%. In such cases, it shall be referred to the manufacturer.

Author

Rajesh Palkar

Fainger Leser
AGM – Product Management