While a company facing a challenge to produce more gas, operation is requested to increase gas rate with the existing facilities. The first common question is: Can our centrifugal compressor deliver more gas than current operation?

While a company facing a challenge to produce more gas, operation is requested
to increase gas rate with the existing facilities. The first common question
is: Can our centrifugal compressor deliver more gas than current operation?

Do not rush to answer that question! We must do carefully verify what effect
if we have to increase compressor rate. Suppose gas wells are still strong enough
to against current setting of suction compressor. So, the simple thing to increase
the capacity of the compressor is raise up the suction pressure of the compressor.
The first step is, we have to review the present power available of the compressor.
Do we still have power spare to compress more gas? If yes, second question may
be: How far current operation close to T5 of turbine? Sometimes even though
power is still available, T5 is close with shutdown setting limit. It means
you must do something, detergent wash the turbine is a common solution.

Figure above represents a common configuration of centrifugal compressor installation.

Increasing the setting pressure of suction side does increasing suction pressure
of the compressor. According to the gas rule’s, by increasing the pressure,
actual volume of the gas decreases, means the compressor will compress more
actual volume of gases. Refer to the compressor equation, increasing suction
pressure, which is one of denominators of the equation, causes less power requirement
for the same rate at lower suction pressure, or for the same power input to
the compressor, the gas compressed increase. If power input to compressor increased
again, more gas compressed.

Increasing setting point on PT automatically will decrease compressor speed
so rate decreases. So how increasing the suction pressure can increase the gas
rate? If the condition happened, it means the gas flow to the compressor is
not enough! Remember that gas rates from wells are still slightly reduced because
we increase "the resistance" at suction compressor. The only way to
increase the speed is, increase the gas rate. This is the time to more open
choke valve or possibly, open stand-by wells. This is our manager want!

In papers, it seems good, but there are still some considerations to be aware.
Increasing PT setting should be communicated to entire system of centrifugal
compressor. The centrifugal compressor systems are not just compressor and driver,
but also cover suction scrubber, and inlet separator, and off course, process/safety
instrumentation (e.g.: PCV, LCV, PSV, etc). Let’s explore more deeply.

Increasing the setting point sometime gives the new number too close with the
PCV flare (PCV-1). If it is, so increase the setting of PCV. Okay, but how if
the new setting of the PCV is too close with PSHH setting. Then we increase
the setting of the PSHH. How if the new setting of PSHH also is too close with
PSV of inlet separator (PSV-1)? Then we increase the setting. Wait the minute!
We touch safety issue here. We can’t set the PSV higher than 1.1 of MAWP’s separator
as per API RP-520, except the PSV is fire-based. (Figure before showed PSV-1
sized based on block discharge case). Sometimes, even though the PSV’s new setting
still acceptable, we may have to buy a new spring for the PSV. Be prepared…

Let’s decide on that case, our PSV setting is far enough from PSHH so still
acceptable. What we should worries now? I think is settling out pressure of
the compressor. What is that? It is the equilibrium pressure when compressor
shutdown and anti-surge valve open. The equilibrium pressure value is between
suction and discharge pressure of the compressor. By increasing the suction
pressure, settling out pressure increases. What we should worry? Your setting
pressure of PSV-2, located on suction side of compressor. Normally, this PSV
is fire-based and set at value, which is still far enough from settling out
pressure. If you forgot to review this PSV, it could cause a problem by frequently
popping up during compressor shutdown.

All have been reviewed completely? I think yes, except if your systems have
special features that need more consideration.

Can I increase the gas rate by reducing the setting of suction pressure of
compressor? Off course. Do we need to consider another system that may impact
due to lowering the suction pressure? Off course. All details will be explained
someday.