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There is always concern about the transient behavior of a lubrication oil system for any turbomachinery. Two of the most critical issues are the oil pump changeover and the oil accumulator.
Cutting off the oil flow to turbomachinery could result in catastrophic failure of lubricated bearings in a few seconds. The lubrication oil pump changeover in various emergency situations is an important topic in any turbomachinery train. But for critical equipment, API 614 requirements should be properly satisfied to ensure an effective changeover of oil pumps in the event of a fall in the oil header pressure.
The addition of a pressure switch on the lubrication oil header and relay logic (thereby providing a pressure signal directly to the standby lubrication oil pump) can usually achieve a rapid start of the standby oil pump. This is the most simple and cost-effective way to solve the oil pump changeover issue.
Some vendors may offer a one or two second delay in the turbomachinery train trip to meet a transient case challenge (such as a voltage dip ride through a VSD-driven turbocompressor). However, this could mean the operation of oil-fed hydrodynamic bearings without oil flow for around two seconds. Usually, though, it is not acceptable to apply any time delay to the critical lubrication oil pressure low level trip.
At least two fully sized lubrication oil pumps should be employed in any turbomachinery train with lubricated bearings. Sometimes the following items should also be studied:
• The third emergency Uninterruptible Power Supply (UPS) DC electric motor driven lubrication oil pump
• The oil accumulator with 4+1 second supply capacity (4 seconds as per API 614 and 1 second extra as margin)
One of the most common causes of unscheduled downtime could be the inability of the lubrication system to ride through a transient event. For some hydrodynamic bearings, even a one or two second cut in oil flow during a transient situation could cause damage. The same is true for many control oil systems (such as turbine control or hydraulic oil systems).
Therefore, an oil accumulator should be provided to maintain the oil pressure and flow while the standby oil pump accelerates from an idle condition to the operating speed or has to deal with other transient situations. Oil pressure should be maintained above the equipment manufacturer’s minimum specified supply pressure for all normal, abnormal or transient operating conditions. An adequate size and design of the oil accumulator is necessary because:
1. Over time, new consumers may add to the plant electrical network and various degradations could occur. Available voltage to the electric motor of a standby oil pump will decrease considerably. This voltage drop can impose some serious limitations on the start-up time of the standby oil pump.
2. Inaccuracies in theoretical assumptions, the model and in analytical calculations have to be allowed for.
A bladder-type accumulator is always recommended. An oil accumulator is usually constructed from a 300-series stainless steel shell and usually has a “Nitrile” bladder (or a bladder made from some other suitable material), which forms a barrier between the inert gas and the oil.
An oil accumulator is usually installed vertically. Too often, the inert gas valve is on the top end and the oil port is on the bottom end. Nitrogen is the best inert gas for this application because of its availability, as well as its chemical and physical properties. Air should never be used because of its corrosive or explosive properties. As a rough indication, the inert gas pre-charge is usually maintained around 80% of the working pressure of the oil system.
Realize, too, that the high flexibility of the bladder can cause a quick response time, which is an important factor considering the likelihood of a fast pressure drop in the oil system in an emergency. Various pressure points and particularly differential pressures (between the oil accumulator and the oil consumers) should be anticipated carefully based on detailed calculations and operational experiences.
For example, the oil accumulator’s minimum pressure is usually considered to be around 20% above the oil system’s low pressure alarm threshold. A smaller differential pressure (between the oil accumulator and the oil consumption point) can result in a relatively slow reaction time.
A properly selected and large enough differential pressure offers a fast response. An oil accumulator should also provide the oil flow fast enough and maintain it for the necessary period at a sufficient pressure.
Further, a special calculation is required for an oil accumulator which is completely different from other process or water accumulators. An oil accumulator is supposed to push oil to the system once the minimum pressure (the low pressure alarm + a margin) is reached.
The required oil should be maintained for enough time (ideally 4+1 seconds), while the mid-pressure (pressure between the maximum and the minimum) is retained. This usually results in two to four times more volume for an oil accumulator compared with a conventional water (or process) accumulator.
Amin Almasi is a Chartered Professional Engineer in Australia, Queensland and U.K. (M.Sc. and B.Sc. in mechanical engineering). He is a senior consultant specializing in rotating equipment, condition monitoring and reliability.