Page 181 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Compressor Best Practices Best Practice 3 .13
which multistage compressor configurations have on the overall energy production at a given flow point is essentially fixed, we
compressor performance curve. can see that pressure rise will change as gas composition
The stage curve is defined as the curve representing the changes. It must be remembered that each gas composition
performance of one stage which consists of the impeller, the requires a different amount of head (energy) to increase its
stage seals, the diffuser, the cross over and return passage. Each pressure level to a given amount. An impeller or blade is
compressor blade row or impeller stage has a specific perfor- designed for only one gas composition and therefore only
mance curve that is plotted as actual volume flow vs. head aspecific amount of energy is designed in a blade or impeller
(energy) and actual volume flow vs. efficiency. for a given application at a specific flow rate. In a multistage
The amount of impeller stage head is limited by both me- compressor, the effect of gas composition change can have
chanical and aerodynamic factors. Mechanical factors include asignificant effect on the overall compressor curve. If, for
impeller stresses and blade natural frequencies. Aerodynamic instance, a gas of higher molecular weight were to be handled,
factors are tip speed, optimum efficiency, Mach number and the pressure produced in the first stage of a multistage com-
flow range considerations. pressor section would increase. Since the gas is compressible,
A compressor section is defined as the number of stages be- this would result in a reduced volume to the second stage.
tween turbo-compressor casing nozzles. A section can contain This impeller was initially designed for a higher volume but
one or more stages. Its performance is defined by a section now will handle a lower volume. Also, a dynamic blade or
curve. The number of stages per section is limited by discharge impeller will produce higher energy at a reduced flow,
temperature, process gas characteristics, casing and configura- therefore compounding the effect of the molecular weight
tion, and rotor stiffness considerations. increase with increased blade energy capability resulting in
We will examine stage and section performance for an ideal a further reduction in flow rate to the succeeding stage. This
designed compressor case, a case with fouled impellers, and fact will result in a shifting of the section curve from the
a case with varying molecular weight. design point towards the surge point and could result in surge
Any multistage compressor is designed on the basis that of a compressor with no significant change in system
succeeding impellers will compress a lower volume flow resistance.
resulting from the compressibility of the specific gas handled.
Remembering that each application is designed for only one The stage curve
operating point, the compressor designer matches each succes-
sive impeller as closely as possible to achieve operation at the Most of the dynamic compressors used today are multistage
individual impeller design point, such that each individual im- because the head required by most processes are in excess of the
peller will operate at its best efficiency point. It is important to head produced by one stage of a dynamic compressor. Typical
understand that in many applications, operating at the com- values for one stage of dynamic compression are:
pressor section rated point does not mean operating at each
individual compressor impeller best efficiency (design point). m-kg f ft-lb f
Many multistage compressors of older design were built using - Centrifugal closed impeller 3,050 kg M or 10,000 lb M
specific impeller designs. For any given application, the
succeeding impeller may not be at its optimum efficiency point m-kg f
(design point) but may be significantly far from that operating - Centrifugal closed impeller 4,575e7,625 kg M or
point. A good rule of thumb in selecting a multistage compressor 15,000e25,000 ft-lb f
is to ensure that when operating at the compressor section rated lb M
point, each impeller operates at approximately þ / 10% of its
design point. Today (2010) some compressor manufacturers are - Axial blade row 915 m-kg f or 3,000 ft-lb f
using computer (CAD/CAM) design to manufacture each im- kg M lb M
peller individually, so that each operates at its design point,
giving a minimum of mismatch between impellers. Therefore, it Figure 3.13.12 shows the components associated with one
can be seen that the overall performance section curve is the stage of a centrifugal closed impeller. For each impeller or blade
composite of the operation of each individual impeller perfor- stage, there exists a specific head vs. flow performance curve. In
mance curve. order to achieve maximum efficiency and flow range in a mul-
If the performance of one or more of the impeller stages in tistage dynamic compressor, each stage should be operating at
a compressor section were to deteriorate, for instance as a result its maximum efficiency point when the compressor is at its rated
of fouling or increased labyrinth seal clearances, the overall (guaranteed) point. This is not true for many older designs,
performance curve will be affected. The amount of this effect where only a limited number of compressor impeller or blade
will depend on the performance deterioration of the individual designs were available to select from. The amount of head
impeller stages. We have shown a case where the first and produced by one stage is limited by certain mechanical and
second impellers of a three stage compressor section become aerodynamic factors. These factors are presented in
fouled. The resulting section operating curve can deteriorate Figure 3.13.13.
significantly in the terms of head (energy), flow range and As an example, open impellers can produce as much as 250%
efficiency. greater head than a closed impeller because the impeller stresses
Another case to consider is the change in gas molecular at higher speeds are significantly reduced by omitting the im-
weight. Remembering that once an impeller is designed, its peller cover or side plate.

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