To positively prevent unit low oil pressure trips during transient events, it is necessary that dual SS accumulators are installed in critical equipment lube oil systems.
Even a properly designed lube oil system will eventually experience trips during transient events due to the following facts:
Installation of two stainless steel accumulators, each sized for four seconds of oil supply, will prevent unit low pressure trips and allow plant personnel to check accumulator pre-charge and bladder condition periodically (every three months) without taking the accumulator out of service.
It is also recommended that an orifice bypass line with a globe value be installed around (in parallel to) the accumulator supply line for personnel use to ensure that the accumulator is put back into service slowly to prevent a decrease in oil pressure. Oil systems can be easily modified for installation of an accumulator during a turnaround.
An accumulator is simply a vessel which compensates for rapid short term flow disturbances in the auxiliary system. Most accumulators contain blades. It is important to remember that transient disturbances are often of the order of microseconds, and usually less than five seconds in duration. The pre-charge pressure is set at the pressure that the volume of the accumulator flow is required in the system. (This value is usually around 60-70 percent of the normal header pressure in which the accumulator is installed.) The quantity of oil available from a pre-charged accumulator is extremely low.
Accumulators are often improperly sized because of the misconception that its stated size is in fact the capacity contained therein. The actual capacity in any accumulator is equal to the internal volume minus the gas volume over the liquid volume. Typically, these values are 50 percent of the stated capacity or less.
System reliability considerations
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 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. Please consult instruction books to 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 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.
Excessive noise or unit trips
Squealing noises suddenly produced from control valves may indicate valve operation at low travel 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 glow 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 changed 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, so 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 failure, the control valve should be designed for isolation and bypass while on-line.
Concerning 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 is accumulators that are supplied with a continuous charge – that is, charge lines (nitrogen or air) which come directly from a plant utility system. Any rupture of a diaphragm will provide a means for entry of charge gas directly into the lube system. Most plant utility lines contain pipe scale that could easily plug systems and cause significant critical equipment damage.
In addition, the following reliability factors should be noted:
Installing two accumulators
Best practice is also to install two full size accumulators to ensure that one accumulator is always on line during monthly checks. Lube oil systems installed without accumulators will eventually cause critical (un-spared) unit trips that will expose the user to significant revenue losses. Clients with critical lube oil systems without accumulators will often install them eventually, after experiencing unit trips that can easily justify the modification costs.
This best practice has been used since the 1990s when FAI performed numerous field audits for auxiliary systems. Installed accumulators immediately increased critical unit MTBFs and increased unit reliability significantly.