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 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.
(Read more in the Sept/Oct issue of Turbomachinery International)