A new control valve will execute control operations at peak performance. But to keep the same performance year after year, and decade after decade, a regular and necessary maintenance plan is needed.
For control valves, the best plan is based on diagnostics during valve operation and predictive maintenance.
Control valve Diagnostics
New microprocessor-based diagnostic tools permit technicians to test and diagnose valves on plant operation sites. This is equivalent to an auto service technician connecting your car to a computer to run diagnostics, rather than dissembling the engine to examine each component piece by piece.
With these advanced diagnostics control valve technicians can pinpoint issues such as Dead band and friction. Not only dead band and friction as well as can perform root cause analysis of their entire control systems.
In other words, technicians can find any problem and its cause without having to shut down the plant operation. If the valve needs to be taken out of service for repair, the technician will already know exactly what is wrong and how to resolve it. It means that the valve can be repaired within a short time and is ready for operation.
Beyond diagnosing current running problems, new microprocessor-based diagnostic tools allow technicians to predict where problems may take place and take necessary steps to protect against them.
This is called predictive maintenance and can reduce downtime considerably and cost related to maintenance work also.
The predictive maintenance tool also helps technicians to develop more accurate maintenance schedules
For example, a technician may test a valve multiple times to establish a performance baseline. Then, when the annual maintenance time comes, the technician will test the valve again. If the valve still operating within the baseline range, no service may be required. By doing this, you may be able to extend your maintenance schedule so that certain valves are serviced only every two to four years, as long as they are working well.
This is a more efficient procedure than the traditional maintenance model. It also means you won’t waste time or money servicing valves that are still like new.
Control valve repair
Technicians are called to job sites for four main reasons. In particular, there are times when a repair is necessary.
1. Internal leak (valve passing)
2. External leak
3. Valve sticks
4. Control valve Total failure.
If you notice a control valve leaking internally, first check the Maximum Allowable Leakage (MAL) specified by ANSI/FCI 70-2. Not all control valves are designed to be tight.
If a valve is leaking above the maximum allowable limit (MAL), internal components may be worn or there may be a problem with the actuator, positioner, or controller.
This is possibly the biggest problem with control valves, at least from an environmental point of view.
Control valves are by far the worst offenders of fugitive emissions. Approximately 50% to 60% of plant fugitive emissions come from valves. And 70% of that amount can be directly attributed to the control valve.
Most of the time, the problems have to do with the valve packing, with causes ranging from improper maintenance to material incompatibilities.
The valve sticks
There are a few situations we must have come across and identified that can cause valves to stick in place.
Here is another common one-over tightening the valve packing in an attempt to reduce leakage. This may seem like a useless situation. If the packing is too loose, the control valve will leak.
If it is too tight, the valve will not work.
But it does not have to be that way, you don’t have to choose between the environment and your production line. You can prevent both problems from occurring by selecting the correct valve for your specific application to begin with.
Occasionally control valves just stop working altogether. This can happen for many reasons, but the underlying cause of most of them is that the valve was not designed for the service conditions in which is being used.
Control valve repair and maintenance techniques.
It is hard to make too many general statements about control valve service and repair because every situations technician themselves in is unique.
These are some of the most common maintenance and repair procedures performed on the control valve. Hydrostatic tests, testing positioners.
The correct positioner can also extend the life of the valve. For example, you can configure a digital positioner to disallow conditions that are difficult for the valve, such as when there is a large inlet pressure but a small outlet pressure. This can prevent you from having to take the valve apart to repair it every time there is a cut.
The biggest consequence of all control valve problems is instability in the control circuit. This usually takes the form of time out. The result is that during the delay between the controller sending the signal and the valve moving to the desired position, the level of the process variable will be either too high or too low.
The trick to avoiding downtime, and its consequences, is to select the right valve for your application, make sure the valve is installed correctly, and follow a regular maintenance and repair schedule.
Author: Abraham Moses