Compressor Discharge PCV

[Khansahib]

Gentlemen,
We are in the process of installing gas compressors for Gas injection into wells at a pressure of 3000 PSI. In the design there was no backpressure PCV on the compressor discharge line, which compresses gas from 1400 psig to 3500 psig. In the HAZOP it was recommended that a PCV is necessary to avoid any problem due to well pressure fluctuation. Now the question has risen that should this PCV be Fail to Close or Fail to Open.

In my opinion it should be Fail to Close whereas my colleagues are of the view that it should be Fail to Open. My argument is based on global (Instrument air compressor failure) failure of air whereas my colleague opinion is based on localized (Tubing rupture or clogging) Failure of air.

Appreciate your expert opinion.

 

[zdas04]

I like to look at this sort of problem with regard to the process being controlled instead of with regard to the controlling stream.

Where a discharge pressure control valve is required (rarely, but this sounds like it might be a legitimate use) then it should always be fail open and the high pressure kill should be set below the process MAWP. The reason for Fail Open is that the credible failure scenario (i.e., loss of control air) is independent of the injection well condition. If the well is in a down cycle (i.e., injection pressure close to the 1400 psi minimum) opening the PCV will reduce the hp load and may cause blow-through of the last stage of compression, but it won't hurt anything. If the well is at the high end of the cycle then it doesn't matter if the PCV is there at all because the discharge check will be held shut by the well pressure (assuming that the compressor has controls to prevent discharging above 3,000 psig). There is no downside to having the PCV fail open on loss of control gas (either global or local failure).

On the other hand, having it fail closed will immediately take the compressor towards the high discharge kill for no good reason. Recycles can be set up to automatically prevent reaching the kill, but that only saves you until the increasing inlet temp forces you toward a high Temp kill.

David

 

[Khansahib]

Thanks David, the detail is convincing...appreciated

 

[dcasto]

I see no reason for a control valve, any upside from the well problems being filtered by such a valve is offset hugely by wasting energy and dealing with failures.

I'd look at a VFD monitoring the discharge pressure if electrict drive or speed control on an engine driver

 

[Khansahib]

dcasto
Despite of the fact that the valve in not necessarily required but operation insisted to have a back pressure PCV for steady operation of the compressor and I obliged, as a mater of fact the leasing contractor of rental compressor agreed to install.

 

[dcasto]

I'd still let the VP operations know that they will be losing millions in fuel and when the greenies come marching in his office, he'll have to explain why HE is adding to global warming.

I am bound by my oath to present all the relevent issues, doesn't matter what someone wants.

 

[Khansahib]

dcasto,
In the part of world that I work, there is no concept of energy saving, climate warming of marching of Greenies. To give you an example a few months ago we were flaring 70 mmscfd from one platform and I have to increase the flaring capacity to 100,000 mmscfd to increase the oil production. This is first time the Govt has put her foot to reduce flaring and I think we are the first to implement.

 

[MikeClay]

The PCV may not be needed during normal operation. In a current gas lift installation, the discharge PCV was set below expected pressure. It would only close if the pressure dropped enough to overload the compressor. As long as the wells operate as expected, the only lost energy is the pressure drop across the open valve.

--Mike--

 

[maddocks]

It depends on your blowdown strategy. Do you have the ability to blowdown the piping downstream of the PCV? If so, then I would make the valve fail-close. If you need an integrated blowdown, then the valve should be fail-open. Is the compressor equipped with automated ESD valves on suction and discharge?