Page 125 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Compressor Best Practices Be st Practice 3.1
thesuction valve.Thepressure differenceacrossthe suction Centrifugal single stage (low ratio)
valve causes the suction valve to open and admit gas into the A typical single stage centrifugal low ratio compressor is shown
cylinder. The gas flows into the cylinder until the piston in Figure 3.1.9. These types are known as single stage overhung
reaches the end of its stroke. The compressor stroke starts compressors, since the impeller is outboard of the radial bear-
when the piston starts its return movement. When the pres- ings the cases are radially split.
sure in the cylinder increases above the pressure on the op-
posite side of the suction valve, the suction valve closes to trap Centrifugal single stage integral gear
the gas inside the cylinder. As the piston continues to move Figure 3.1.10 shows a “Sundyne” single stage integral gear
toward the end of the cylinder, the volume of the gas in the compressor section. This type of compressor is an in-line type
cylinder decreases, and the pressure of the gas increases. (similar to a pump), usually driven by motor through an in-
When thepressure inside thecylinderreaches thedesign tegrally mounted gear box (not shown). These compressors are
pressure of the stage, the discharge valve opens and discharges used for low flow, high energy (head) applications, and are now
the contents of the cylinder to the suction of the second stage. used in many applications that were previously serviced by
The second stage takes a suction on the discharge of the first positive displacement compressors. These compressors operate
stage, further compresses the gas and discharges to the third at high speeds (8,000 e 34,000 rpm), and are limited to
stage. The third stage takes a suction on the discharge of the approximately 300 kW (400 horsepower).
second stage and compresses the gas to the final discharge
pressure. Centrifugal multi-stage horizontal split
Figure 3.1.7 shows a balanced opposed, four throw re- A typical, multi-stage, horizontally split, centrifugal compressor
ciprocating compressor typical of the type that would be used in is shown in Figure 3.1.11. The casing is divided into upper and
refinery hydrogen make-up service. lower halves along the horizontal centerline. This allows access
to the internal components of the compressor without
disturbing the rotor to casing clearances or bearing alignment. If
Dynamic compressors possible, piping nozzles should be mounted on the lower half of
the compressor casing, to allow disassembly of the compressor
Dynamic compressors are used wherever possible because of without removal of the process piping.
their low maintenance requirements. The single stage integral
gear centrifugal compressor allows the use of a dynamic com- Centrifugal multi-stage with side loads
pressor in many applications where positive displacement This type of compressor is used exclusively for refrigeration
compressors have previously been used. The two types of dy- services. The only difference from the previous type is that gas is
namic compressors are centrifugal and axial. induced or removed from the compressor via side load nozzles.
A typical refrigeration compressor is shown in Figure 3.1.12.
Note that this type of compressor can be either horizontally or
Centrifugal compressors e general principles
radially split.
of operation
Centrifugal compressors increase the energy of a gas by in- Centrifugal multi-stage (barrel)
creasing the tangential velocity (V T ) of a gas, as shown in A typical multi-stage, radially-split, centrifugal compressor is
Figure 3.1.8. The principle of operation of a centrifugal shown in Figure 3.1.13. The compressor casing is constructed as
compressor is very similar to that of a centrifugal pump. The a complete cylinder, with one end of the compressor being re-
gasentersthrough theinlet nozzle e which is proportioned so movable to allow access to the internal components. Multi-
that the gas enters the impeller with minimum shock or tur- stage, radially-slit centrifugal compressors are commonly called
bulence. The impeller, which consists of a hub and blades, is barrel compressors.
mounted on a rotating shaft. This receives the gas from the Barrel compressors are used for the same types of applica-
inlet nozzle and dynamically compresses it, by increasing the tions as the multi-stage, horizontally-split centrifugal compres-
energy of gas proportional to the product of the blade tip sors. Because of the barrel design, however, they are normally
speed velocity (U T ) and the gas tangential velocity change used for higher pressure applications or certain low molecular
(V T ) in the impeller. V R represents the velocity of the gas weight gas compositions (hydrogen gas mixtures).
relative to the blade. The resultant velocity (V) is the vector
sum of the relative velocity (V R ) and the blade tip speed Centrifugal multi-stage integral gear
velocity (U T ). A typical four-stage, integrally-geared, centrifugal compressor is
A diffuser surrounds the impeller, and it gradually reduces shown in Figure 3.1.14. Integrally-geared, centrifugal com-
the velocity of the gas as it leaves the impeller. The diffuser pressors have a low-speed (bull) gear that drives two or more
converts the velocity energy to a higher pressure level. In a single high-speed gears (pinions). Impellers are mounted at one or
stage compressor, the gas exits the diffuser through a volute both ends of each pinion. Each impeller has its own casing that is
casing that surrounds the diffuser. The volute casing collects the bolted to the gear casing. The gear casing may be horizontally
gas to further reduce its velocity, and to recover additional ve- split to allow access to the gears.
locity energy. The gas exits through the discharge nozzle. In The gas enters the compressor through the first stage inlet
a multi-stage compressor, the gas exits the diffuser and enters nozzle to the impeller. The impeller receives the gas from the
return vanes, which direct the gas into the impeller of the next inlet nozzle, dynamically compresses it, and discharges to the
stage. diffuser. The diffuser surrounds the impeller, which gradually

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