Page 121 - Compression Machinery for Oil and Gas

108 SECTION II Types of Equipment

The economics of which system makes the most sense often depends on the
application. When multiple compressors are installed at the same site, the com-
plexity of operation increases greatly as well, leading to transient startup and
shutdown challenges that can result in surge, stall, or excess vibration in
adjacent machines.

HP Inline Centrifugal Compressor Applications
Rotordynamic Implications
Seal Considerations
The balance piston seal type and geometry can have a significant impact on the
rotordynamic stability of a centrifugal compressor. Labyrinth seals are generally
preferred from a compressor performance standpoint, due to the tighter clear-
ances(possiblebecauseoftheuseofabradablematerials)andlowerleakagerates
involved. However, the destabilizing cross-coupling generated by these seals
(and a general lack of damping) can be detrimental. As such, when low stability
margins are evident, hole-pattern seals are often incorporated at the balance pis-
ton. These can provide significant damping, thus resulting in a strong benefit to
rotordynamic stability. However, care must be taken to ensure that the seal clear-
ance is carefully managed, since the behavior of these seals has been shown to be
very sensitive to both mean clearance and the taper of the clearance (difference
between inlet and exit clearance). In some cases, these seals have been known to
bedestabilizingiftheclearanceisnotproperlymanaged.Thisisespeciallytruein
high-pressure back-to-back machines, where significant division wall deflection
is possible. The effects of differential pressure, tolerance stack-up, and temper-
aturegradients should alsobewell understood insuchcases. Aninitialcoldclear-
ance offset may be necessary to ensure that the seal experiences an optimal
clearance environment during the planned operating conditions.

Typical Compressor Types and Their Market Space

Once an oil or gas reservoir is discovered and assessed, the task is to maximize
the amount of oil or gas that can ultimately be recovered. Oil and gas are con-
tained in the pore spaces of the reservoir rock. Some types of reservoirs allow
the oil and gas to move freely, making it easier to recover. Other reservoirs
restrict the flow of oil and gas and require special techniques to move the oil
or gas from the pores to a producing well. Very tight rock may even require
fracking to yield economical amounts of oil or gas. To prepare the well for pro-
duction, the bore hole is stabilized with a casing (lengths of pipe cemented in
place), and a small-diameter tubing string is centered in the wellbore and held in
place with packers. This tubing will carry the hydrocarbons from the reservoir
to the surface. Many oil and gas wells are on the ocean floor, and production
requires an offshore platform.