Sean Raymond, technical marketing manager for Fisher Instrumentation with Emerson Automation Solutions, recently published an article in the April 2021 issue of Power magazine. The article describes troubleshooting techniques for control valve problems and is titled Troubleshooting Issues and Eliminating Headaches Related to Control Valves, and it is summarized below.
“That new control valve is acting up again!” Similar words have been uttered by thousands of control room operators in power plants all over the world. The plant is not running well, and operators are quick to identify the culprit—a recently installed, misbehaving control valve. It might be cycling, it might be squealing, it might sound like it has rocks going through it, but it is definitely the cause.
Or is it?
Sean describes the situation.
When troubleshooting control issues, it is important to keep an open mind and look beyond the obvious. It is human nature to blame the “last thing changed” for any new problem that occurs. While erratic control valve behavior might be the apparent source of concern, the true cause is often located elsewhere.
Here are some application examples to illustrate his point.
Squealing control valve
A high-pressure spray valve was squealing after a few months of service. The valve was pulled, checked, and appeared to be functioning normally. When returned to service, the squealing resumed, and the plant demanded the “defective valve” be replaced. A little checking indicated the valve was being cycled by the control system between 0 and 10% open at a rate of 250,000 times a year. A little loop tuning and backpressure on the valve stopped the cycling and eliminated the squeals.
Jumpy valve response
A boiler feedwater pump recycle valve was sticking in the seat at startup. When the valve would first come off the seat, it would jump to the open position, creating control upsets (Figure 1).
Figure 1: A boiler feedwater pump recycle valve would jump 15 to 20% open (shown by the green line) when it was first activated, creating major issues due to surging flows.
Diagnostics were run and the air supply pressure was found to be set well above specification, four times higher than required for adequate seating. The high pressure damaged the seat and seal, causing the valve to hang.
Strange control valve behavior
A new control valve acted erratically while in operation, with flows surging and the valve swinging. Online diagnostics were run—and adjustments were made to the positioner tuning, supply air pressure, and pneumatic relays—but the problem remained. Ultimately, the source of the behavior was found to be a leaking bypass valve.
Identifying the Issue
Sean describes how to identify valve problems.
When troubleshooting a control issue, the team should start by fully understanding and identifying the problem. Often, operators will fixate on a single effect (such as erratic flow control, control valve behaving oddly, or noisy equipment) and complain about that.
One has to pose the right questions to obtain a more complete view of the problem. Probing questions often expose contributing conditions and take the investigation in an entirely different direction.
Troubleshooting tools, if available, are also helpful. Historical data and process trends might expose contributing factors such as surging pressures, swinging flows, or other process variations that could be causing the problem. Digital positioner diagnostic tools can shed light on how the valve is performing (Figure 3).
Figure 2: Diagnostics can be performed on valves to pinpoint problems.
But be careful as data can be misleading. Poor resolution or slow update times on a trend may mask problems or imply issues that may not exist. Also, valve diagnostics have their limitations since positioner data is only as good as the position information it receives (Figure 3). If the linkage is loose or worn, the indicated position will not match the true position.
Figure 3. Worn linkage components can create problems for a valve positioner. The worn valve coupler on the left will mislead the positioner into indicating the valve is moving when it is not.
Systematically eliminate causes
When armed with a full understanding of the issues and relevant data, one can begin to track the root cause. Start by identifying all possible culprits and then systematically eliminating them through further testing. Sean concludes:
When faced with a control problem, go in with open eyes and an open mind. Look beyond the obvious source of the issue. As Sherlock Holmes said: “It is a capital mistake to theorize before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts.”