Page 135 - Compression Machinery for Oil and Gas

122 SECTION II Types of Equipment

ideal installation for a compressor is to use a “down nozzle” arrangement with
inlet and discharge lines coming up from the bottom and a clear aisle all around
the equipment string. In this type of arrangement, an overhead crane can be
made available ranging the entire footprint of the equipment for maintenance
purposes without concern of hitting any piping. A further advantage of the down
nozzle arrangement is that the top half of a horizontally split compressor can be
easily removed without requiring any piping disassembly. If the compressor is
vertically split, sufficient axial space to remove the inner bundle from the com-
pressor is a necessity. In many cases, ideal arrangements are not possible, and
some sacrifices are required such as having an up nozzle arrangement or having
reduced space around the equipment.
Per design, most compressor maintenance only requires rudimentary disas-
sembly. Alignment can be performed by adjustments at the compressor feet.
Couplings are accessible by simply removing the coupling guard, and from
there can be easily replaced. Journal and thrust bearings are accessible through
their own covers without requiring large-scale disassembly. Bearings are hor-
izontally split to allow for easy replacement. End seals tend to require a higher
degree of disassembly, but here too, they are designed to be accessible from the
ends of the compressor without removing either the top half lid of a horizontally
split compressors or inner bundle of a vertically split compressor.
Critical auxiliaries such as the lubrication system and DGS support system
are also designed for easy maintenance. Most maintenance activities can be
performed without taking the compressor offline. The lubrication systems are
equipped with duplex filters and twin coolers that can be switched over and
isolated via continuous-flow transfer valves. Filters can then be replaced and
cooler bundles can be removed and cleaned. The lube oil supply pumps are
also spared and can be switched over as well. Likewise, the DGS systems
are capable of draining and replacing filter elements concurrent with compres-
sor operation.
Maintenance practices can be divided into corrective maintenance, predic-
tive maintenance, and preventive maintenance. Corrective maintenance, also
known as breakdown maintenance, occurs due to an unplanned event such as
a trip that requires the equipment being taken offline in a forced or unplanned
outage to correct a problem. Predictive maintenance, also known as condition-
based maintenance, occurs due to monitoring and trending activities that indi-
cate a potential problem that allows for a planned action to occur. Preventive
maintenance occurs at scheduled maintenance intervals such as every 7years
for example. All three types of maintenance have taken important steps forward
due to monitoring and inspection technology.
Regardless of the maintenance practice, critical spare parts are normally
kept on hand including couplings, journal bearings, thrust bearings, seals,
and a spare rotor. Spare filters are normally kept on hand for lubrication systems
and seal systems.