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In my DHDT (Diesel hydro treater unit), anti surge of recycle gas compressor remain open 20-25% always. Could anyone explain whether it is instrument fault or process problem? How can I rectify it?
 
Answers
07/03/2009 A: Virendra Kapoor, Petroleum Refining Consultants, vkkapoor9@yahoo.com
Surge protection is necessary to save centrifugal compressors from damage. Antisurge devices are provided for the purpose. Machine suppliers or compressor experts may help you to identify cause and save appreciable energy due to spill back.
04/03/2009 A: Virendra Kapoor, Petroleum Refining Consultants, vkkapoor9@yahoo.com
Surge is one of the very bad restrictive characteristics of centrifugal compressors. Below a specified suction flow compressor may fall into unstable operating range. Compressor suppliers provide the curve for discharge pressure v/s suction volume and the surge point. Under the surge conditions rotor slides back and forth damaging the bearings and machine damage occur too. Knowledge of polytropic head is important for surge control Machines must be provided with antisurge devices. Surging conditions may differ for each machine. Higher energy is spent if you have high opening. The help from compressor supplier is required to work for most optimum energy consumption
02/03/2009 A: keith bowers, B and B Consulting, kebowers47@gmail.com
The purpose of the anti-surge control bypass is to keep the compressor from entering an aerodynamic stall in one or more sections of the compressor. When in this operating state, there is no net forward flow of gas, and all the driver energy input is going into vibration and heat generation.
The manufacturer can provide you (and you should already have) a 'surge-line' plotted on a 'Flow vs Head' for a family of RPM's graph for the molecular weight of the recycle gas.
It is important to stay a reasonable amount on the safe side of the surge line to ensure transient conditions do not cause surge to occur. Some compressors and gear boxes can be badly damaged with even brief periods of operating in 'surge' Some can even be severely damaged on a trip if the anti-surge controller and valve system is not fast enough.
You will have to determine the present operating conditions and plot on the compressor's family of curves to see where you are presently operating. It may be the system is operating as-intended, or it may not. Running 20% spill-back is wasting energy of course. There are numerous methods to reduce the wastage, but they are totally dependent on your system details and operating condition
A compressor specialist, perhaps the manufacturer of the compressor, should be consulted and their expert advice sought before making any adjustment to the present settings.