Always follow the control valve manufacturer’s maintenance manuals. Typical maintenance topics are summarized here together with recommendation to extend the life of the control valves. Optimization of control valve assets depends on an effective maintenance philosophy and program. Three of the most basic approaches are:
Reactive – Action is taken after an event has occurred. Wait for something to happen to a valve and then repair or replace it.
Preventive – Action is taken on a timetable based on history; that is, try to prevent something bad from happening.
Predictive – Action is taken based on field input using state-of-the-art, non-intrusive diagnostic test and evaluation devices or using smart instrumentation. Although both reactive and preventive programs work, they do not optimize valve potential. Following are some of the disadvantages of each approach.
Reactive maintenance allows subtle deficiencies to go unnoticed and untreated, simply because there is no clear indication of a problem. Even critical valves might be neglected until they leak excessively or fail to stroke properly. In some cases, feedback from production helps maintenance react before serious problems develop, but valves might be removed unnecessarily on the suspicion of malfunction. Large valves or those welded in-line can require a day or longer for removal, disassembly, inspection, and re-installation. Time and resources could be wasted without solving the problem if the symptoms are actually caused by some other part of the system.
Preventive maintenance generally represents a significant improvement. However, because maintenance schedules have been able to obtain little information on valves that are operating, many plants simply overhaul all control valves on a rotating schedule. Such programs result in servicing some valves that need no repair or adjustment and leaving others in the system long after they have stopped operating efficiently.
Today, plant operators often extend the time between turnarounds to three or four years, and even longer, in order to maximize plant up-time. These extended run times offer less opportunity for traditional, out-of-service valve diagnostics.
The traditional maintenance process consists of four distinct modes:
Fault Detection – A majority of valve maintenance effort is spent in monitoring valves while in service to detect the occurrence of a fault. When a fault is identified, the maintenance process transitions to fault discrimination.
Fault Discrimination – During this mode, valve assets are evaluated to determine the cause of the fault and to establish a course of corrective action.
Process Recovery – Corrective action is taken to fix the source of the defect.
Validation – In this final mode, valve assets are evaluated relative to either as-new condition or the last established baseline condition. Once validated, the maintenance process returns to fault detection status.
Control Valve Diagnostics and Maintenance Practices
Instrument Air Leakage
Air mass flow diagnostics measure instrument air flow through the actuator assembly. Because of multiple sensors, this diagnostic can detect both positive (supply) and negative (exhaust) air mass flow from the digital valve controller. This diagnostic not only detects leaks in the actuator or related tubing, but also much more difficult problems. For example, in piston actuators, the air mass flow diagnostic can detect leaking piston seals or damaged o-rings.
The supply pressure diagnostic detects control valve problems related to supply pressure. This in-service diagnostic will detect both low and high supply pressure readings. In addition to checking for adequate supply pressure, this diagnostic can be used to detect and quantify droop in the air supply during large travel excursions. This is particularly helpful in identifying supply line restrictions.
Travel Deviation and Relay Adjustment
The travel deviation diagnostic is used to monitor actuator pressure and travel deviation from setpoint. This diagnostic is useful in identifying a stuck control valve, active interlocks, low supply pressure, or shifts in travel calibration.
The relay adjustment diagnostic is used to monitor crossover pressure on double-acting actuators. If the crossover pressure is too low, the actuator loses stiffness, making the valve plug position susceptible to being overcome by process forces. If the crossover pressure is set too high, both chambers will be near supply, the pneumatic forces will be roughly equal, the spring force will be dominant and the actuator will move to its spring-fail position.
Instrument Air Quality
The I/P and relay monitoring diagnostic can identify problems such as plugging in the I/P primary or in the I/P nozzle, instrument diaphragm failures, I/P instrument o-ring failures, and I/P calibration shifts. This diagnostic is particularly useful in identifying problems from contaminants in the air supply and from temperature extremes.
In-Service Friction and Friction Trending
The in-service friction and dead-band diagnostic determines friction in the valve assembly as it is controlled by the control system. Friction diagnostics data is collected and trended to detect valve changes that affect process control.
In-service custom diagnostics can be configured to collect and graph any measured variable of a smart valve. Custom diagnostics can locate and discriminate faults not detectable by other means. Often, these faults are complicated and require outside expertise. In such cases, data is collected by local maintenance personnel and is then sent to a valve condition monitoring specialist for further analysis. Therefore, avoiding the costs and delays associated with an on-site visit.
Recommendation to extend the life of the control valves
Utilize smart, microprocessor-based valve instrumentation that evaluates the operating health of the control valve assembly while the valve is in service. Data is collected without intruding on normal process operations. The instrumentation analyzes the information in real-time and provides maintenance recommendations for each valve operating problem that it identifies.
Recommended Spare Parts
Most companies will identify parts to be a recommended spare and provide a list of these parts for a given valve or actuator. The user should consider stocking the recommended spare parts and be available to service the valve when required. Since this could be a large investment and a considerable number of parts, several factors should be considered when deciding what to stock. The first thing to consider is the criticality of the valve or asset. Second, consider the risk of not having the unit function as intended for any given amount of time the asset is down. Then third, what is the availability of the parts needed, are they on-site or available from the manufacturer quickly.
The manufacturer may be able to provide a consolidated spares list where they have identified parts that could be used in multiple valves. Should you order parts to change the original construction due to process condition changes or other factors, let your Original Equipment Manufacturers (OEM) know which serial number you are changing. This will allow the manufacturers to update the serial card to reflect the new changes and help to ensure that the next time parts are ordered you will receive the correct parts and eliminate additional downtime.
Consider Upgrades for the Valve Trim
During the maintenance cycle, it is always important to examine parts and consider the need to upgraded. Considerations could be valve noise when operating, excessive trim damage, or if system operating parameters have changed from the original design. Many times changes to the valve trim can address these issues.
* Sourced from Emerson.