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.

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.

Increasing suction pressure is done via increasing pressure setting on speed
PC. Then, by automatically, compressor’s speed decreases. Let we assume that
the wells are still strong enough so it will not make feed to compressor decreases.
We also carefully resetting all pressure control valve to flare system as well
as verify all pressure switches high-high (PSHHs).

We identified in this case, raising suction pressure did not cause problem
on settle out pressure or jeopardize suction scrubber PSV or cause compressor’s
suction PSHH activated. Everything is OK.

Increasing suction pressure is done. Then suddenly compressor shutdown with
notice cool down shutdown alarm……..Anti-surge valve opens and speed
of compressor decrease slowly???

Why? Why anti-surge valve open while we are still far from surge region?

Yes, we are still far from surge region, but increasing in suction pressure
for given same gas feed means decreasing driver engine speed.

Driver for compressor, usage gas turbine. Gas Turbine operation follows Brayton’s
cycle. Air is feed to air compressor, that usually have multistage stages and
axial type. In combustion chamber, air is mixed with fuel and burn to produce
energy heat. This energy is converted to flow energy then create force that
able to rotate turbine plus power turbine. Power turbine drives compressor to
rotate (work).

Air compressor, also had protection from surge. The normal surge control was
bleed off air from its casing via 2 bleed off valves.

As suction pressure increases, the speed of driver decreases. At that time,
axial compressor operating region close to its surge region and commands the
driver system to allow him opening bleed valves. The further action automatically
followed by cool down shutdown.

All above give us a lesson learnt to verify minimum of driver speed (NGP=gas
producer speed) as one of parameter that we have to consider during do plant
optimization. The simple solution is increasing feed to compressor.

We learnt this today….

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