Configuring lubrication systems

Centrifugal compressors have lubrication systems designed as per API 614 consisting of an oil reservoir, pumps, filter, cooler and oil lines. The programmable logic controller (PLC) starts the auxiliary AC power-driven lubricating oil pump.Pressure at the oil header creates starts the drive.If the header pressure is low, the standby oil pump starts and maintains oil pressure during operation. Oil is pumped to the lubricated parts (radial and thrust bearings). If a speed increaser gear is present, they are lubricated with the same oil as the bearings (Figure 1).

Figure 1: Generic lube oil system diagram for a centrifugal compressor[/caption]

Some end users require two pumps that have different types of drive or power sources. When both are driven electrically, they have to be connected to separate supply feeders in order to maintain constant and uninterrupted operation should a feeder fail.However, it is not required for oil coolers to have such redundancy.

In some compressors with step-up gearboxes, the main oil pump is required to be driven mechanically from the gearbox, and the auxiliary pump operates during the start-up and run-down phases of the compressor train. As API 614 calls for the oil reservoir to be away from the skid with a slope requirement, shaft-driven mechanical pump performance may be hampered by net positive suction head-related (NPSH) air bubble issues (Figure 2).

Figure 2: API 614 compliant lubricating oil system[/caption]

Sometime a screw pump connected to a gearbox can cause rotor-dynamic problems due to high overhung weight. Shaft-driven gear pumps are mostly used to maintain oil supply during coast down in the case of an electrical power outage as compressors need lubrication until their rotating shafts come to a complete stop.

A conventional approach is to maintain lubrication for a fixed period of time following a shutdown request. Based upon an estimate of the time for a rotating shaft to stop,this approach, does not always ensure that the shaft receives enough lubrication. Therefore, a rotating shaft and its corresponding bearings may be physically damaged if it rotates after that fixed period. To avoid possible issues,therefore, a well sized run-down tank is a better alternative.

Another big lubrication challenge is avoiding bearing damage during reverse rotation. This can occur during coast down, gas blow down, or short circuiting of high-pressure gas into a standby compressor.Most of the time,radial bearing pads have an offset pivot design while thrustbearings have a tapered land-thrust design that has a significantly lower load-carrying capacity in the reverse direction. The compressor designer can specify center-pivoted pads in bearings and bi-directional seals. But regardless of the design, rotor train inertia must be higher than gas inertia.This cannot be guaranteed for a high-pressure compressor if it has a low rotor weight.In that case, due diligence is required to overdesign the overhead tank OHT to meet the duration of reverse rotation,and to create a short circuit between the suction and discharge lines with a class IV sealing valve (open when the compressor is not in operation) or a fast-acting recycle valve.

If the compressor is a single operating unit with no standby, it is advisable to start the lube oil pump first before beginning blow down of the compressor loop. The duration of operation of the lube oil pump is dictated by API 521 as regards the blow down rate of loop. A non-slam check valve at the discharge side is usually installed although a clause of API 521 states that a single check valve cannot be regarded as a reliable layer of protection against reverse flow.

Another option is to provide a reverse rotation detector which can give a command to start a DC pump or the installed auxiliary AC motor driven pump.The reverse rotation detector can be set to arm a digital tachometer connected to a microprocessor or PLC.The tachometer should be suitable for low RPM detection. The RPM signal from the tachometer is transmitted to the PLC which can the start the auxiliary or DC pump.All of the above points must be taken into account while placing an order for a turbo-compressor package.

Mantosh Bhattacharya is a rotating equipment specialist based in the United Arab Emirates.