Potential catastrophic failure of pressure-balanced cage-guided control valves and chokes

Introduction

1. This notice reminds dutyholders of the need to consider the risk from any process operation which may result in solids being deposited onto the internal surface of process equipment. It is particularly important that pressure balance ports on control valves and chokes remain free from blockage. In this Notice the terms choke and valve are used interchangeably when describing pressure-balanced equipment, as the design principles are the same.

Background

2. A recent incident occurred where an actuator was ejected from a choke following manual opening of the choke. The ejection of the actuator resulted in serious injury to the equipment operator.

3. The root cause of this incident was blockage of the choke pressure-balance ports, through the formation of calcium carbonate scale. In addition, a non-return valve (NRV) downstream of the choke was seized in the closed position due to scale formation, which caused a rapid build up of pressure downstream of the choke. This allowed full line pressure to be applied over the choke plug. A cross-section drawing of a pressure-balanced choke is shown in Figure 3.

Figure 1: Photograph of choke plug showing scale and blocked pressure-balance ports

Figure 2: Photograph of scaled NRV

4. The resulting thrust on the choke stem caused failure of the stem thrust-retention mechanism, and the screws securing the actuator to the choke. This resulted in the actuator being ejected from the choke with considerable force.

5. Cage-guided control valves and chokes are used in high-pressure service. The cage design aligns the valve plug to the seat ring, absorbs lateral forces due to pressure drop and minimises lateral vibration.

6. Pressure balance is the application of process pressure to both sides of the plug within the valve chamber. This is achieved by providing one or more ports through the valve plug. The result of this is that process pressure effectively only acts over the area of the stem, reducing the forces applied to the stem. This allows smaller stem diameters and a considerable reduction in the size of actuator required to operate. A cross-section drawing of this arrangement is shown in Figure 3.

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Figure 3: Cross-section drawing of a pressure-balanced choke

7. The actuation systems provided with these valves are therefore designed only for pressure-balanced operation. In the event of the pressure-balance ports becoming blocked, the pressure on the bonnet side of the plug can be significantly lower than the pressure on the process side. In this case, the process pressure will act across the total plug area resulting in stem thrusts that can significantly exceed the design stem loads. The same effect can occur if momentary pressure surges occur on the process side of the plug, resulting from fluid hammer for example.

8. The application of excessive thrust loads on the valve stem may result in failure of the stem thrust-retention mechanism. In Figure 3 the stem drive bushing is held in a screwed housing, and the housing retainer is fixed to the valve bonnet by capscrews. Excessive thrust load could force the stem drive bushing out of the threads retaining it in its housing, and result in failure of the screws which fix the housing retainer to the bonnet. This would result in the valve being forced open and cause separation of the actuator from the valve.

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Actions required

9. Dutyholders should consider the risks from any process operation which may result in solids from the process fluid being deposited on the internal surfaces of process equipment. The potential effects include block age of pipe work and seizure of process valves (NRV's, control valves, pressure release valves (PRV's) and, in particular, safety-critical shutdown valves). Particular attention should be given to the possibility of unexpected stem loads on pressure-balanced valves and chokes potentially leading to actuator/valve separation.

10. Deposits that can interfere with the operation of process equipment should not be allowed to accumulate on the equipment internal surfaces. It is particularly important that pressure-balance ports on control valves and chokes remain free from blockage.

11. A mitigation measure against such failure is to remotely operate the chokes and ensure personnel are not in the vicinity. Difficulties with actuated operation should require a check and fix of the actuator and not enforced manual operation. If the valve cannot be actuated, and actuator operation has been verified, the line should be isolated and the valve safely removed for investigation.

Relevant legal requirements

• the Provision and use of Work Equipment Regulations 1998 SI 1998/2306 The Stationery Office 1998 ISBN 0 11 085625 2; and
• the Management of Health and Safety at Work Regulations 1999 SI 1999/3242 The Stationery Office 1999 ISBN 0 11 025051 6.

These are legal duties on both the owner/operator of the plant and on those who carry out the repair and/or modification.

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Further information

Any queries relating to this notice should be addressed to:

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