Control valves in a vapor recovery system rarely fail dramatically; they tend to lose performance gradually, and the effect shows up as a slow drift in the way the unit runs. Because the duty is unusual compared with most process plant, choosing valves on general purpose criteria is a common source of trouble. The two questions that matter most are how fast the valve must respond, and how much of its life it will spend moving.
What makes vapor recovery duty different
The medium is a light hydrocarbon vapor rather than a liquid, flows can be intermittent, and the valve may sit in one position for long periods and then be asked to move quickly. Some duties involve a continuous modulating action, while others are essentially open or closed. Assuming the wrong one leads either to an oversized actuator or to a valve that cannot hold position.
Matching response to duty
| Duty | Typical requirement | Selection focus |
|---|---|---|
| Open or closed isolation | Reliable full stroke, infrequent movement | Sealing and long term freedom to move |
| Modulating flow control | Smooth response through the range | Characteristic, resolution and actuator stability |
| Pressure or vacuum regulation | Stable control at low differential | Sensitivity, and behaviour at small openings |
| Emergency shut off | Fast, reliable closing on demand | Stroke time and fail position |
Actuator considerations
- Confirm the failure position required by the design: open, closed or last position;
- Confirm the available supply pressure or voltage, and that it is available even during a shutdown;
- Check that the stroke time matches the process requirement, not simply the actuator default;
- Allow for the range of differential pressures the valve will actually meet;
- Keep the position indication visible so a problem is apparent during a walk round.
Issues that appear after commissioning
Valves that will not fully seat, actuators that hunt or oscillate around the set point, and valves that become stiff after a period of inactivity are all reported regularly. Stiffness after inactivity is particularly common where a valve sits in one position for months and is then required to stroke; periodic exercising as part of the maintenance routine addresses it.
Frequently asked questions
Should control valves be exercised during routine maintenance?
Where the design allows it, periodic stroking helps confirm the valve can still move through its full range and gives early warning of stiffness or a developing actuator problem.
What causes a valve to oscillate around the set point?
Often a mismatch between the valve characteristic, the actuator response and the controller settings. Resolving it is easier when the valve duty and the process requirement are both documented.
How important is the fail position?
It is one of the decisions that most affects safety, and it should come from the system design rather than from what happens to be available. The reason behind the chosen position should be recorded so it is not reversed later by accident.
Luoyang Wohong Petrochemical Equipment Co., Ltd. designs and builds vapor recovery units, membrane separation skids and condenser based recovery packages for fuel terminals, depots and service stations. Contact us to discuss a system selection or upgrade study.