A friction bound control valve (right and/or oil sludge in packing) can cause a unit trip during a main oil pump trip if sufficient system oil (because the valve does not immediately close) is not available during the transient event to prevent a low oil pressure unit trip. A failed control valve diaphragm, which can be replaced on-line, exposes the plant to shutdown, since improper adjustment of the manual bypass valve around the control valve and/or taking out or putting the affected control valve back into service can cause a low oil pressure trip.
A plugged control valve sensing line pulsation suppression device can cause an oil actuated control valve to go unstable (hunting), which can also cause a unit low pressure shutdown.
Preventive maintenance (PM) of the above items during a turnaround is good insurance for a trouble-free run of the highest reliability. To ensure optimum control valve reliability, there should be preventive maintenance of control valves and pulsation suppression devices at every turnaround by changing diaphragms, checking valve stem freedom of movement and cleaning pulsation suppression devices. Note that most plants today (2010) are targeting six year uninterrupted runs of critical equipment.
A number of reliability considerations are worthy of mention concerning auxiliary system control and instrumentation.
Control valve instability
Control valve instability can be the result of many factors, such as improper valve sizing, improper valve actuators, air in hydraulic lines or water in pneumatic lines. Control valve sensing lines should always be supplied with bleeders to ensure that no liquid is present in pneumatic lines or air in hydraulic lines. The presence of these fluids will usually cause instability in the system.
Control valve hunting is usually a result of improper controller setting on systems with pneumatic actuators. Consulting instruction books will ensure that proper settings are maintained. Direct-acting control valves frequently exhibit instabilities (hunting on transient system changes). If checks for air prove inconclusive, it is recommended that a snubber device (mentioned previously) be incorporated in the system to prevent instabilities.
Some manufacturers install orifices which sufficiently dampen the system. If systems suddenly act up where problems previously did not exist, any snubber device or orifice installed in the sensor line should be checked immediately for plugging.
Excessive valve stem friction
Control valves should be stroked as frequently as possible, to ensure minimum valve stem friction. Excessive valve stem friction can cause control valve instabilities or unit trips.
Control valve excessive noise or unit trips
Squealing noises suddenly produced from control valves may indicate operation at low travel (Cv) conditions. Valves installed in bypass functions that exhibit this characteristic may be signaling excessive flow to the unit. Remember the concept of control valves being crude flow meters. Periodic observation of valve travel during operation of the unit will indicate any significant flow changes.
Control valve sensing lines
Frequently, plugged or closed control valve sensing lines can be a root cause of auxiliary system problems. If a sensing line that is dead ended is plugged or closed at its source, a bypass valve will not respond to system flow changes and could cause a unit shutdown.
Conversely, if a valve sensing line has a bleed orifice back to the reservoir (to ensure proper oil viscosity in low temperature regions), plugging or closing the supply line will cause a bypass valve to fully close rendering it inoperable and may force open the relief valve in a positive displacement pump system.
Valve actuator failure modes
Auxiliary system control valve failure modes should be designed to prevent critical equipment shutdown in case of actuator failure. Operators should observe valve stem travel and pressure gauges to confirm valve actuator condition. In the event of actuator failure, the control valve should be designed for isolation and bypass while on line.
This design will permit valve or actuator change-out without shutting down the critical equipment. During control valve online maintenance, an operator should be constantly present to monitor and modulate the control valve manual bypass as required.
Plants frequently experience unscheduled unit trips caused by control valve issues, it is good practice to PM the control valve systems during a turnaround. This best practice has been used since the mid-1980s. It has been recommended to all clients since that time, and has resulted in trouble-free control valve operation during steady state and transient conditions.