In a previous article, we identified five common control valve problems. This article takes a look at the four most common reasons our technicians are called out to job sites for control valve repair.
When You Need to Repair Your Control Valve
1. Control valve leaks internally
Control valves may leak internally for several reasons.
The first is that they aren’t actually designed to shut off 100%.
ANSI/FCI Standard 70-2 divides control valves into six different leak rate classifications and defines the maximum allowable leakage (MAL) for each. So, even if your valve is leaking, as long as it’s within the allowable range, your valve probably doesn’t require repair quite yet.
If your valve is leaking more than the MAL, here are some possible causes:
- The internal components are worn out.
- The actuator isn’t set up properly.
- The positioner isn’t calibrated correctly.
- The distributed control system (DCS) isn’t configured correctly.
A technician can identify all of these problems using a diagnostic tool such as ValScope.
2. Control valve leaks externally
Control valves are some of the worst culprits for external leaks, which release harmful fugitive emissions into the atmosphere.
Most of the time, especially in the refining world, external leaks result from a problem with the valve packing.
Here are some examples of circumstances that cause packing problems from the book Control Valves, published by the ISA:
- Improper maintenance
- Excessive packing box temperatures
- Vibration of the valve assembly
- Material incompatibilities
- Thermal cycling
3. Control valve sticks
When a valve leaks, it’s common for an operator to try to stop the leak by tightening the packing. Often, however, this leads to a different problem — the valve sticks in place.
A valve is an engineered product that is sized for a particular application with a specific packing torque value. If you overtorque the valve packing, it will stop working properly.
Positioner issues also commonly manifest as sticky valves. This is especially true for pneumatic valves, which comprise the majority of control valves in use. So, if you have a valve that’s sticking, be sure to check the positioner.
4. Control valve just doesn’t work anymore
Sometimes a control valve simply stops working. This could be because the pneumatic diaphragm wears out or because the valve is being used in a service condition that it wasn’t designed for.
It’s also possible that the actuator isn’t the right size or the positioner isn’t calibrated properly. This is a problem our technicians encounter frequently.
When you purchase a control valve or any other part of the control valve assembly, it’s absolutely essential that you have all of the information you need to size and select the components properly. This includes details about the process conditions, like flow, media, and temperature.
Without this information, you run the risk of purchasing the wrong equipment. The consequence of this is that you’ll find yourself repairing the same valve assembly year after year. This is a very common problem — and one that can easily be avoided by doing your due diligence in the sizing and selection process.
Need control valve support? We’re here to help. Contact an Allied Valve control valve expert.