Page 70 - Compression Machinery for Oil and Gas

Centrifugal Compressors Chapter 3 61

Adjustable Inlet Vanes
Modifying the swirl of the flow into the impeller allows to modify the operating
characteristics of the stage (Fig. 3.29). This can be accomplished by adjustable
vanes upstream of the impeller. Increasing the swirl against the rotation of the
impeller increases the head and flow through the stage. Increasing the swirl with
the rotation of the impeller reduces the head and flow through the stage. This is
very effective to increase the range for a single stage. In multistage compres-
sors, the range increase is limited if only the first stage has adjustable vanes.
The technical difficulty for high-pressure compressors lies in the fact that com-
plicated mechanical linkage has to be actuated from outside the pressure
containing body.

Adjustable Diffusor Vanes
Vaned diffusers tend to limit the operating range of the compressor because the
vanes are subject to increased incidence at off-design conditions, thus eventu-
ally causing stall. Adjustable diffuser vanes allow adjustments for the changing
flow conditions, thus effectively allowing for operation at much lower flows by
delaying the onset of diffuser stall (Fig. 3.29). They will not increase the head or
flow capability of the stage. In multistage compressors, the range increase is
limited if only one stage has adjustable vanes. The technical difficulty for
high-pressure compressors lies in the fact that complicated mechanical linkage
has to be actuated from outside the pressure containing body. Another issue is
that for the vanes to operate, small gaps between the vanes and the diffuser walls
have to exist. Ubiquitous leakage through these gaps causes efficiency and
range penalties, in particular in machines with narrow diffusers.

Throttling (Suction and Discharge)
A throttle valve on the suction or discharge side of the compressor increases the
pressure ratio the compressor sees, and therefore, moves the operating point to
lower flows on the constant speed map. It is a very effective, but inefficient way
of controlling compressors (Fig. 3.29).

Recycling
A controlled recycle loop allows a certain amount of the process flow to go from
the compressor discharge back to compressor suction. The compressor there-
fore sees a flow that is higher than the process flow. This is a very effective,
but inefficient way to allow the compressor system to operate at a low flow.

Process Control With Centrifugal Compressors Driven
by Two-Shaft Gas Turbines
The following is a description of a typical control scenario, in this particular case
for compressors with a gas turbine driver that can operate at variable speeds.