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Sometimes, un-comprehensive review of process optimization, fails to recognize the effect of changing set point around compressor. Below are examples how this problem happens.

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Centrifugal Compressor – Part 4

Sometimes, un-comprehensive review of process optimization, fails to recognize the effect of changing set point around compressor. Below are examples how this problem happens.

One of problem in refrigeration system was, how to ensure that there is no liquid carry over to gas phase after condensed liquid is collected and separated at Low Temperature Separator (LTS). The simple solution, is increasing operating pressure in order to reduce actual gas velocity across vessel.

How high is high? Well, this is not a simple question. Because increasing pressure in surface facilities means increasing barrier for gas flowing from subsurface. However, too low in setting pressure at separator has a significant impact in liquid carry over, especially if the system has foaming tendency.

Let we try to increase the suction pressure. How to do it? Please refer to the Process Flow Diagram (PFD) attached.

The control of compressor can be simplified below.

The basic control philosophy is “suction pressure control with high discharge pressure override”. If the compressor suction pressure goes below the set point, the turbine speed decreases in order to build up the suction pressure. If the suction pressure goes above the set point, the turbine speed increases in order to take in more gas and there by, reduce the suction pressure. However, during this increase in suction pressure, if the discharge pressure goes above its set point, there will not be any further increase in the turbine speed and this would result in the anti-surge or recycle valve opening.

Surge in compressor is caused due to compressor receiving insufficient flow at the operating pressure. The surge in compressor may be avoided by either decreasing the differential pressure across the compressor, or by increasing the flow rate through the compressor. In some case, opening anti-surge valve may cause process not stable, since when this valve opening, usually direct wide open. (In order to bring back compressor operations away from surge).

Recycle system was designed to solve this potential problem. The input from suction and discharge side is compared to get the lowest signal then the output is proceed to the another selector to compare the number with signal from anti-surge controller. So, the system is designed to open the recycle valve first prior anti-surge valve takes action. This hopefully will minimize the plant from transient condition, e.g. to minimize opening anti-surge valve. The recycle system also provides, as total plant outlet controls in case the buyer demand is low below of normal compressor capacity.

Understanding this control principle, then we go to our intention, increase the suction pressure.

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