We have the following Hyrogen compressors in our Diesel Hydrotreater. The op.pressure of reactor is of the range of 115 - 120 kg/cm2g.

Compressor desing pressures are...

[1] Make-up Gas Compressor MUGC - 2 Stage Reciprocating.
With Nitrogen->
1st STAGE :: Suction P=6.0 kg/cm2 g; Disch P=14.0 kg/cm2 g
2nd STAGE :: Suction P=13.5 kg/cm2 g; Disch P=33.7 kg/cm2 g
And
With Hydrogen->
1st STAGE :: Suction P=20.0 kg/cm2 g; Disch P=47.0 kg/cm2 g
2nd STAGE :: Suction P=45.4 kg/cm2 g; Disch P=104.0 kg/cm2 g

[2] Recycle Gas Compressor RGC - Single Stage Reciprocating.
With Nitrogen->
Suction P = 32.5 kg/cm2 g
Disch P = 43.0 kg/cm2 g
And
With Hydrogen->
Suction P = 104.0 kg/cm2 g
Disch P = 126.0 kg/cm2 g


We have to keep reactor pressure below 30 kg/cm2 g pressure during start-up before we reach pre-defined Min Pressurisation Temp(MPT).
So, we want to run Recycle Gas compressor with suction of 20-23 kg/cm2 g pressure with minimum possible discharge pressure. This are the conditions during Reactor dryout with Nitrogen.

Make up gas compressor can fulfill the requirement BUT it does not have configuration to have re-circulation of flow from reactor and also, the capacity is also low. So we have to run Recycle Gas Compressor. But compressor vendor is recommending not to start the compressor with low suction in any case.

I want to know if someone can explain me why is it not possible to start with low suction with same compression ratio.

During commissioning, plant start-up procedure also requires to run recycle gas compressor with 20 kg/cm2 g suction pressure of Hydrogen. The circulate hydrogen through reactor, increase temperature of the reactor catalyst bed and then start pressurising gradually with Make-up gas compressor upto operating pressure. But under the circumstances, we CAN NOT start recycle gas compressor.

What could be the feasible solution???

thanx in adv.