Page 191 - Compression Machinery for Oil and Gas

180 SECTION II Types of Equipment

Frame and Skid Flexibility
Designing a skid to be adequately stiff can be a challenge for higher power
reciprocating compressors, above about 1500kW. Up to 1500kW it is relatively
easy to make a skid stiff so that alignment of the compressor to the driver can be
maintained during transportation. But even then, alignment is always checked
as part of the commissioning activity and will often require adjustment.
For powers above 1500kW alignment of the compressor to the driver will
always require adjustment during commissioning.

Testing Options
All reciprocating compressors used in the oil and gas industry undergo a
mechanical run test as outlined in API Standard 618. Each compressor manu-
facturer will perform the test a little differently but the fundamental API 618
requirement is for a 4h no-load mechanical run test. “No-load” means that
no gas is compressed as the suction and discharge valves are not installed in
the cylinders during the test. The basic idea is to prove the machine is mechan-
ically sound and the running gear and cylinder lubrication systems are working
properly. More can be learned about testing in Chapter 12.
Some users ask that an inspection be done after the test that requires some
dismantling of the compressor. Many manufacturers balk at this request because
they hesitate to dismantle a compressor that has passed a successful run test and
reintroduce an element of human error during reassembly.
It is not common that the manufacturer or packager performs a performance
test either in the shop or in the field. Shop performance tests for reciprocating
compressors (commonly referred to as “string tests” with centrifugal machin-
ery) can be very expensive to setup and perform and are extremely rare. How-
ever, the performance of reciprocating machinery is usually confirmed in-situ
during commissioning of the equipment in the field.

Vibration Concerns

Reciprocating compressors impose heavy dynamic forces on the foundation.
These forces are of two types:
Global forces apply due to net unbalance inertia forces from the rotating and
reciprocating parts plus unbalance pulsation forces in the pulsation bottles and
in the cylinder nozzles. These are typically the only forces reported and act on
the entire foundation.
Local forces are internally balanced within the compressor and so local
forces on foundation result from the differential elastic stretch of the compres-
sor. These include the gas force acting on the piston and head, the inertia forces
of the rotating and reciprocating parts acting on the frame and vertically at the