Control valve diagnostics

The control valve is also known as the final control element and its performance will have direct impact on the process. The process loop can be improved with advanced process control and real time optimization. However, the full potential can only be realized when the performance of the final element is optimized.

Control valve performance deterioration is difficult to detect, and if left unchecked, impact process control. In extreme cases, this may lead to unscheduled plant shutdown. Traditionally, evaluating control valve condition had to be accomplished with the valve bypassed or with the process shut down.

However with plant turnarounds becoming fewer and far apart, it is necessary to devise a methodology of determining control valve performance deterioration without shutting the plant or stopping the process.

There is a need for evaluating the health of a control valve assembly while the valve is in service. With the advancements in microprocessor-based valve instrumentation and sensor technology, data can be collected without interrupting the process. This data can be analyzed real-time providing maintenance recommendations specific to the problem at hand. In-service diagnostics can detect problems of instrument air leakage, valve assembly friction and dead band, instrument air quality, loose connections, supply pressure restrictions, and valve assembly calibration.

Therefore, it will be economical if valves that needed maintenance are identified early without disassembling.

Offline diagnostics

At startup, the valve assembly should be properly tuned to ensure stability and responsiveness. Step Response diagnostic can be used to check the dynamic response of the control valve assembly. A typical step response diagnostic is shown in Figure 1. The blue line represents

the set point and the red line is the actual valve response and the green line is the supply pressure. The supply pressure indicates the pressure demand when the valve tries to follow the set point, hence a potential low supply pressure can be detected. The actual valve response (red line) indicates the aggressiveness of the valve when it tries to follow the set point.

The Valve Signature graph (Figure 2) is unique for each valve assembly and is considered a blueprint for each valve. Accordingly, it will reflect the health and performance of the control valve assembly. The gap between the two outer lines represents the valve friction and the gradual and uniform slopes of these graph lines mean that the valve is stroking smoothly. Besides friction, the graph can also reveal calibration problems, improper valve and actuator set-up, improper sizing of the control valve, seating integrity, shutoff forces, valve sticking, pneumatic supply pressure problems, excessive packing friction and incomplete valve stroking. The summary of control valve characteristics such as seat load, required seat load, average friction and dynamic error band are provided in the “Analyzed” tab (Figure 3).

These diagnostics will help to identify potential problems during commissioning as well as provide a benchmark for a good control valve performance.

The Signature Analyzer (Figure 4)allows the user to set boundaries around the onward and return strokes and the software provides the user with a “Pass – Fail” recommendation.

Performance diagnostics

Traditionally, many plants are scheduled to shut down for maintenance every year, or once every two years. However, current competitive markets have driven plants to extend their turnaround periods to 5 years or longer; therefore it is important to sustain control valve performance until the next scheduled shutdown. Without any mechanism to monitor the valve’s performance, its health can deteriorate while it is still in-service which can lead to an undesirable shut down causing an interruption to production, and incurring a higher cost of getting the plant up and running again.

Today, smart valve diagnostics make it possible to detect potential problems while the process is in service. The information is collected passively while the process is running with no interruption to the process. The data is then correlated and an analysis is performed to establish possible causes.

When a problem is identified, the severity is reported, a possible cause identified, and a course of action recommended. Inservice diagnostics can detect problems of instrument air leakage, valve assembly friction and dead band, instrument air quality, loose connections, supply pressure restrictions, and valve assembly calibration. Presently, almost 200 different faults can be identified with in-service diagnostics in some digital positioners. Below are some online (in-service) diagnostics examples.

Friction Value & Friction Trending (Figure 5): the values of valve friction & dead band can be determined and displayed graphically while the valve is in operation.The on-line friction values can be compared with the friction value obtained from off-line diagnostics in Figure 3. The friction trend provides an indication of the valve packing changes.

A paraxylene plant in Thailand found that after two years of operation, one of their critical valves was fluctuating around ±1% despite a stable signal from the control system. It did not affect final production, but the operator was concerned about the erratic operation of the control valve. A service engineer ran an in-service friction diagnostic and found that the valve friction value had reduced by 30% over the span of two years due to packing wear. It was concluded that the reduction of valve friction value was the main cause of the control valve fluctuation.

The original positioner tuning values were too high for the new friction value. The valve was tuned and adjustments were made to the boosters in order to compensate for the decrease in friction value. As a result, an unnecessary shutdown was avoided. In this case, the end user estimated that a shutdown could have resulted in loss of revenue of approximately US$200,000.

Supply Pressure Diagnostic (Figure 6): As the name implies, this is for detecting problems with supply pressure, especially low supply pressure.

In a corn-processing facility in the US, the in-service supply pressure diagnostic identified that more than 10 percent of critical control valve assemblies had inadequate supply pressure, and control was being compromised. Although initial out-of-service tests found no valve problems, the dynamic, in-service diagnostic testing identified underlying air supply issues, which allowed for performance improvements in the overall process.

Air Mass Flow Diagnostic (Figure 7): By measuring the airflow through the control valve assembly, an external leak through the actuator diaphragm, tubing, loose connection or piston O-ring can be determined.

Travel Deviation and Relay Adjustment Diagnostics

(Figure 8): The travel deviation diagnostic is used to track the amount of deviation of the actual valve travel in comparison to its set point. Relay Adjustment diagnostic is only applicable for double acting actuators and is used to measure the crossover pressure. Crossover pressure is defined as the average stroke, and return pressures are expressed as a percent of supply.

At a syngas process plant, a turbine inlet control valve was found to swing in one direction although the signal was constant. As an interim solution, the customer locked the valve in the required position to stabilize it. An on-line Relay Adjustment diagnostic was run and found that the actuator air was not balanced correctly during the calibration. As a result, recalibration of the valve was required. I/P & Relay Integrity Diagnostics (Figure 9): is useful for identifying dirty supply air.

At a combined cycle power plant in the US, a routine I/P and relay integrity diagnostic on a main steam valve identified a previously undetected partial plugging in the I/P primary from contaminants in the air supply. The diagnostic detected the fault, identified the cause as partial plugging of the I/P primary, and recommended that the I/P be removed and cleaned. Maintenance personnel were able to take proactive corrective action and avoid an unplanned shutdown.

Diagnostics inside

With advancements in microprocessor based technology, now it is possible to incorporate all the on-line diagnostics in the positioner microprocessor. By doing this, the in-service diagnostics is continuously run 24/7 and alerts will be generated when the conditions are breached.

Currently this technology is available with some digital fieldbus positioners like the DVC6000f. Figure 10 shows some of the possible problems that can be detected: (a) I/P plug due to dirty supply air, (b) I/P calibration drift, (c) supply pressure problem, (d) external leak, (e) relay out of adjustment for double acting actuator and (f) travel calibration shift.

Conclusion

A digital positioner mounted on a control valve has become a powerful diagnostic tool. The positioner can help the maintenance personnel monitor and diagnose the condition of the control valve, and take proactive action when potential problems arise. This would help to sustain control valve performance and extend the time between shutdowns, hence increasing production up time.

The off-line diagnostics complement the on-line diagnostics because it can provide a detail analysis of the condition of the control valve assembly (seat profile and seating forces).

References:

Rinehart N. and Ingram D. (2004), Smart Control Valve Diagnostics: Predictive Maintenance And Beyond, Valve Magazine, Vol. 16, No. 4 H.K. Kung can be reached at hunkoy.kung@ap.emersonprocess.com