Page 139 - Compression Machinery for Oil and Gas

126 SECTION II Types of Equipment

Equipment availability is standardized as the ability of an item to be in a
state to perform a required function under given conditions at a given instant
of time, or in average over a given time interval, assuming that the required
external resources are provided [32]. For more information on availability
and optimization in oil and gas operation, Brissaud et al. [33] should be
consulted.

Construction Options for Maintainability and
Upgrades—Optimization and Restage
Gas compressors are typically sized in a high-efficiency zone to meet the initial
pressure, temperature, gas composition, flow, and other initial conditions. How-
ever, the changing of operating conditions, such as gas field depletion and nat-
ural gas demand increasing, is the nature for either production or pipeline
compressor applications. While centrifugal compressors provide a tremendous
flexibility, it is often economic to restage the existing compressor to optimize
for new process conditions, gains in compressor efficiency, flow capability, or
reduction in fuel or electricity consumption.

Principles of Gas Compressor Restage
Typical compressor has to adopt a wide range of operation. If conditions, inlet/
discharge temperature/pressure, flow, speed, and gas properties, make compres-
sor performing the best efficiency point, no restaging is needed. When condi-
tions change in one direction away from the best efficiency point, compressor
restaging should be considered (Fig. 3.84).
The change of temperature, pressure, and gas composition mainly moves the
operating point in the T (speed Topping) or D (speed Decreasing) direction.
When suction temperature is increased, the head is increased for the same pres-
sure ratio and higher speed is required to move the flow point to the T direction.
Higher temperature also tends to tilt the map in the counterclockwise direction.
Suction pressure has similar effect too. For example, in a declining field, the
suction pressure reduces overtime. The pressure ratio increases as the same dis-
charge pressure typically is kept the same. More pressure ratio requires higher
speed. Meanwhile, as filed pressure drops, the density drops, thus more volume
flow passes through the compressor. The operating point moves in the T direc-
tion. In the opposite, there are also cases where the suction pressure is increased.
The design point moves to the D direction, as the required head reduces. For
discharge pressure, when it increases, the pressure ratio increases. The head
is increased. The flow point moves in the T direction. When discharge pressure
deceases and the pressure ratio decreases, the flow point moves in the D
direction.
For pipeline transmission, gas composition stays relatively constant. But for
gas production applications, gas composition changes. Heavier gas requires less
power to reach the same pressure ratio, the compressor speed needs to be