5 replies to this topic

[jojeecares]

We have a compressor (two stage doube acting)that has been sized for a Suction Pressure=250 psig, Discharge Pressure=1250 psig and a Gas Volume Handling Capacity=10 MMSCFD. This compressor is to be installed as Front End Compressor to boost gas pressure (coming from wellhead) to the desired discharge pressure in order to maintain facilities operating pressure. The compressor shall be installed downstream of a 3-phase separator to ensure single phase entry into the compressor machine.

 

The query is that the upstream pressure at the inlet of compressor is high initially i.e. in the range of 700-750 psig. Now we have two ways of controlling this pressure and ensuring optimum machine performance.

 

1. One way to do this is to install a PCV (pressure control valve) and limit the outlet pressure of PCV to 250 psig to operate the compressor. Then the machine inherent controls like VVCP and RPM can be brought into play to adjust the required flow alongwith recycling if required.

 

2. The second way is to use unloader valves to blow by the 1st stage completely and introduce the high pressure directly into the 2nd stage.

 

Although both options are theoretically possible and have been confirmed by the vendor/packager as doable, I personally believe that putting a valve upstream of compressor with valve sensing to maintain a constant suction pressure is more doable and practical rather than opting for option 2. 

 

I would like to have inputs by senior members of the forum and what options are practical and doable and what is the industry norm in this matter. Any references in the matter shall also be appreciated.

[saeed.s]

Dear jojeecare

 

If vendor confirmed that first stage of compressor dosnt have any problem with 700-750 psig, then I prefer second way.

Disadvantage of first way : 1-need more capital cost to install PCV   2- lower efficiency : doping pressure with pcv and again increasing pressure with compressor.

[S.AHMAD]

Hi Behnam.

My assumption is that you need to control the pressure because the piping system design is below the expected pressure.

From energy saving point of view it is better to control the pressure using control valve at the suction. However, 

A better option is to upgrade the piping system provided the compressor casing pressure design is within the maximum possible pressure.

 

s.ahmad

[saeed.s]

Dear S.AHMAD

 

Thanks for your comment. As you mentioned one of considerations is suction piping design pressure but as  jojeecares said: vendor confirmed second way and I assumed they considered all of the condition like :

 

1- suction piping & first stage design pressure

2- O-ring & sealing system in first stage 

3- shaft  force balance & vibration possibility after by passing 1 stage

4- .........

[Bobby Strain]

Never assume anything! Verify.

 

Bobby

[jojeecares]

I appreciate all the inputs given by professionals on my thread. The thing is that the compressor OEM has confirmed that the compressor can run with high pressure going into the 1st stage and we have reviewed and checked all the performance runs of machine to ensure the machine does not undergo vibration when exposed to higher pressures. I would like to put the query in a different way that although Option-2 is doable, is Option-1 (putting a PCV to control the pressure upstream of compressor) doable and practical and is it being exercised elsewhere. Although the pressure is high but shall last only 2 months and gradually drop and the concept it to put PCV to control upstream pressure from day one. However, our Operations Team is reluctant that this will not work whereas I believe it is very much workable so that's why looking for references.