Page 239 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Compressor Best Practices Best Practice 3 .26

B.P. 3.26. Supporting Material System design considerations

Accepting the fact that reduced system resistance is the most
System objectives
cost effective way to eliminate surge damage, a number of surge
system design considerations should be mentioned. These con-
Thesurge system objectives aretoensurethatthe gas ve- siderations fall into two categories; compressor considerations
locity in each impeller stage in any dynamic compressor ex-
ceeds the critical velocity that will cause stall. Refer to and process system considerations. The considerations are
presented in Figure 3.26.3.
Figure 3.26.1.

Compressor considerations
Ensure the relative gas velocity in each impeller stage exceeds
Fast system response
the critical velocity that will cause stall
Control line automatic adjustment
Ability to adjust control line to gas density and operating point
changes
Fig 3.26.1 The surge system objective
Recycle stream cooling
Check valve location
Process system considerations
Available options Gradual process flow changes
Flow reversals
In order to ensure that impeller velocity always exceeds the Start-up procedure
stated critical velocity, a number of options are available. These Shutdown procedure
options mainly fall into three (3) categories. Refer to
Figure 3.26.2. Fig 3.26.3 System design considerations

A history of system types
Shutdown unit before critical flow is reached
Increase the relative gas velocity:
By impeller speed increase Figure 3.26.4 presents a general chronological listing of surge
By adjusting inlet gas angle system history.
Decrease system resistance:
Process system decrease
Direct gas flow through blow off or recycle line
‘Surgeless’ applications
Surge alarm systems
Fig 3.26.2 Options to achieve the objective Single parameter systems – flow
Biased systems – flow and differential pressure
Modern systems
1. Shut down the unit before the critical flow is reached. Compensation
Naturally, when one considers that most dynamic compressors System back-ups
are in critical service, that is un-spared, this option is not Controller output options
possible.
2. Increase the relative gas velocity. Fig 3.26.4 Surge system history
This objective can be achieved by either increasing the speed
of the impellers or by adjusting the inlet angle of the gas
relative to the impeller vanes. Both alternatives result in the In my experience, many so called ‘surgeless’ applications have
increase of turbo-compressor energy and should provide the been observed. These are applications where the process designer
higher velocity through the impeller. Note, however, that this assesses the system and concludes that there is no possibility to
alternative may not be effective under certain system surge the machine. Often, this assessment is based on system
resistance characteristics. resistance alone, the conclusion being that, since there are no
variable resistance points in the process system (coolers, reactors
3. Decrease the system resistance.
This can be achieved by either directly reducing process system that can foul or control valves), the turbo-compressor cannot
surge. If the molecular weight and gas inlet temperature are
resistance, fully opening a throttle valve, or by directing gas flow
through a blow off or a recycle line. Of the two alternatives constant, and the turbo-compressor will not be subjected to
available, the latter is more efficient since it does not affect the mechanical damage or fouling, this assessment is true. If any of
energy in the process system. the above mentioned possibilities can occur, however, the turbo-
compressor in question can be forced into surge as a result of its
Given the applications in which turbo-compressors are inability to produce the energy required for throughput flow. As
employed, it can be readily observed that option three (3) is the previously discussed, the operating point of any compressor is the
most cost-effective way to protect turbo-compressors against equilibrium between required process system energy and the
surge and corresponding mechanical damage. compressor produced energy. Both can change, and do.

213