Page 131 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Compressor Best Practices Be st Practice 3.2

Best Practice 3.2Practice 3.2Practice 3.2Practice 3.2
Best
Best
Best
Define all process conditions and specific requirements on One recent example was a lubricated screw compressor installation
the compressor data sheet. where the compressor could not operate for more than 24 continuous
Confirm all process conditions on the data sheet, while accurately hours due to the actual field gas analysis (sour) being different from the
defining the gas analysis, the highest gas head required and all data sheet gas analysis (sweet).
expected operating points (head and volume flows).
Meet with the EP&C and/or process licensor process engineers to Benchmarks
ensure that all operating and upset process conditions are accurately Since the mid-1970s I have met with process engineering and senior
defined. operators in the process design phase to ensure that all conditions
were defined. This best practice has resulted in compressor reliability
Lessons Learned of the highest levels (greater than 99.7%).
Inaccurate process conditions have led to scope changes
during the project and increased delivery time by 6 months
or longer.

B.P. 3.2. Supporting Material Satisfying the objective
In this section we will cover the relationships that the com- The objective of the end user is to deliver a specified amount of
pressor vendor uses to determine the head produced, efficiency, a given gas. Refer to Figure 3.2.1 and note that his objective can
horsepower required and overall design for a particular com- best be stated by the relationship:
pressor application. The end user’s or the purchaser’s objective
is to deliver a specified amount of a given gas to the process. Gas Flow Produced ¼ Gas Flow Delivered:
Therefore, the data that the compressor vendor obtains is re- This, incidentally, is the reason why most process control
quired mass flow, inlet pressure, temperature conditions and gas systems monitor pressure in the process system and install
composition. With this data a compressor manufacturer will a controller to either modulate flow via a control valve (change
calculate actual flow, the ideal energy and the horsepower that is the head required by the process) or vary the speed of the
required to achieve that objective. The calculation for horse- compressor (change the head produced by the compressor).
power will require a specific compressor efficiency, as well as The vendor then determines the head required by the pro-
compressor mechanical losses; i.e., friction losses from bearings cess, on the basis of the parameters given by the contractor and
and discs, and seal losses. end user on the equipment data sheet. It is very important to
Gas characteristics are defined in this chapter. and useful note that all possible sources should be used to confirm that the
relationships are presented to enable the reader to calculate conditions stated on the data sheet are correct and realistic. This
various compressor requirements. Once the vendor obtains the fact is especially true for dynamic compressors, since erroneous
data, the gas head can be calculated. Once the head and required process conditions will impact the throughput of the
flow are known, the impeller can be selected. compressor.
The principle of impeller design is chiefly based on that of
specific speed. This is defined as the ratio of speed times the Gas characteristics
square root of the actual flow divided by head raised to the three
quarters power. It can be shown that increasing values of specific
Figure 3.2.2 presents the relationships which are used to cal-
speed will result in increasing impeller efficiencies. Therefore,
having been given the required flow and energy (head), the only culate the design parameters for the compressor. Note that the
same relationships are used regardless of the type of compressor
source of obtaining higher specific speed for the vendor is to
increase the compressor speed. This fact is very significant, (positive displacement or dynamic) under consideration.
because while compressors have increased in efficiency over the The gas characteristics that are used in the determination of
years, their mechanical requirements have also increased design parameters are defined in Table 3.2.1.
significantly, i.e., higher bore impeller stresses, etc. resulting in Table 3.2.2 shows some useful relationships that are used in
potential reliability problems. Therefore, the design of the compressor calculations, together with the definitions of the
impeller is a very fine balance between the performance constants used.
requirements and the mechanical constraints of the components
used in the compressor design. Compression head
Efficiency is presented as a ratio of ideal to actual energy, as
depicted on a typical Mollier Diagram. In addition, the Fan Laws The ideal gas head equations are again defined in Figure 3.2.3.
are presented, showing how increased impeller energy can be As previously stated, polytropic head is the usual choice among
obtained via speed change in a compressor application. compressor vendors.

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