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The following are some of the causes of reduction in the reliability of auxiliary system control and instrumentation. The reliability can be boosted by considering the below factors.
Control valve instability can be the result of many factors. To name a few; improper valve sizing, improper valve actuators, air in hydraulic lines or water in pneumatic lines. This is an excerpt from best practices presented by William E Forsthoffer and Michael Sean Forsthoffer at the 2016 Asia Turbomachinery Symposium.
Control valve sensing lines should always be supplied with bleeders to assure that liquid in pneumatic lines or air in hydraulic lines is not present. 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. Attention is drawn to instruction books to insure that proper settings are maintained. Frequently direct acting control valves exhibit instabilities (hunting on transient system changes). If checks for 5 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 assure 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 valve 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. Observation of valve travel periodically during operation of the unit will indicate any significant flow changes.
Control valve sensing linesFrequently, 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 assure 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 modesAuxiliary 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 on line maintenance, an operator should be constantly present to monitor and modulate the control valve manual bypass as required.
Accumulator considerationsConcerning accumulators, checks should be made when unit is shut down for accumulator bladder condition if supplied with bladders. One area which can cause significant problems in auxiliary systems are accumulators supplied with a continuous charge. That is, charge lines (nitrogen or air) that come directly from a plant utility system. Any rupture of a diaphragm will provide a means for entrance of charge gas directly into the lube system. Most plant utility lines contain pipe scale that could easily 6 plug systems and cause significant critical equipment damage. In addition, the following reliability factors should be noted:
■ Be sure to install a check valve upstream of the accumulators to assure all accumulator oil is delivered to the desired components.
■ Accumulators should be checked periodically (monthly) for proper pre-charge and bladder condition by isolating and draining the accumulator. Note that the accumulator pre-charge pressure cannot be determined while on line
■ When refilling the accumulators, care must be taken not to suddenly open the supply valve. Best practice is to install an orifaced bypass valve to be used for filling the accumulator.
■ Best practice is also to install two (2) full size accumulators to assure that one accumulator is always on line during monthly checks.