Natural Gas Pipeline Suction Scrubber Over Pressure Protection

[stevehance]

I manage a combined cycle power plant that owns and operates a small section of natural gas pipeline and a compressor station. We connect to a pipeline that has an MAOP of 400 psi, and our pipeline has an MAOP of 549 psi up to the compressor building. Inside the compressor building we have 3 compressors each with suction scrubber vessels with an MAOP of 450 psi. Connected to the common header on the incoming line we have installed a PSV set at 450 psi, but between the the vessels themselves and the PSV there are several isolation valves that are locked open. The question I have is if this configuration is allowed, or do each of the suction scrubber vessels need their own PSV, and what code applies in this case?

 

[Chance17]

The locked open valves are OK but should be controlled with written administrative procedures.

The tough question is whether the scrubbers normally contain liquid.
If yes, my response would be that individual scrubber safety valves are needed (for external fire for times when the valve is closed).
If no, then no safety valve.
I am familiar with organizations that use a 5% RULE.
No safety valve if the vessel has less than 5% liquid volume.
No safety valve if there is liquid in the vessel less than 5% of the time.
You will not find the 5% rule written as a general Indusry standard.

 

[BigInch]

The scrubbers would have to have individual relief valves, or a continuous path to a relief valve. Without individual reliefs, the scrubber should be deactivated whenever all paths to exterior relief valves were closed at any given time.

It's not uncommon to see separate reliefs on both the vessels and the piping, for self-protection of the vessel when it could become isolated from a path to a common relief on the piping, and for self protection of the pipe, when the vessel is being bypassed and the rest of the facility remains open.

If its not possible to run one without running the other, you could have one relief, but the safe use of locked open/close valves should be carefully evaluated whenever the potential need, or desire, to run separately might exist. It wouldn't be the first time that paths to relief valves somehow became closed off. Murphy's law for relief valves is, If they're possible to lock off, they will be locked off when you need them the most.

"The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward X-CEO BP
"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.live.com

 

[DLite30]

DOT 192.199.h h) Except for a valve that will isolate the system under protection from its source of pressure, be designed to prevent unauthorized operation of any stop valve that will make the pressure relief valve or pressure limiting device inoperative.


My interpretation of is that by having the valves locked-open on the suction side, and having a relief valve protecting the 450 psig MAWP of the suction scrubbers, you satisfy the "prevent unauthorized operation" of the valves. But I've seen some DOT/PHMSA guys interpret things wildly differently.

Do you have a discharge relief valve, and is it set at some percentage above 400 psig? I'm sure you do. Then you're protected if the unit is isolated.

If I were a PHMSA inspector, I would ask you the following, and you need to be prepared to answer:

-What controls do you employ to control suction pressure to below 450 psig?
-What sizing criteria do you use for the suction side relief valve?

 

[stevehance]

We recently had a PHMSA inspector on site and they were fine with the system as it stands now and they stated that our current arrangment is similiar to what they have seen at other locations. The original design didn't even have the relief on the common header that we have recently installed and they were ok with that design as well.

The CalOSHA pressure vessel division appears to have a different opinion though, and that is what we are trying to clarify. I'm fairly up to date with pressure vessel code, but not so much with B31.8, and didn't know if vessels such as the suction scrubbers are treated differently.

The suction scrubbers are blown down manually every 4 hours, but they occationally have small amounts of liquids in the bottom.

In the event that a single scrubber would be tagged out of service and isolated from its relief path,our procedure requires us to open the vent, so that there is still protection in the event of a fire, or atmospheric changes in temperature.

We do have PSVs on the discharge of the compressors set to lift at 750psi to protect the 800 psi MAOP on that side of the system.

Controls on the suction side are provided by the upstream pipeline company that we connect into. Their pipe has an MAOP of 400 psi, and they have 2 in series regulators set at 380 psi.

The criteria that was used in sizing of the relief that was installed on the suction side was based on our discharge of our compressors. I'm not sure this is right since the discharge volume from our compressors really have little bearing on the pressure and volume of gas that could be coming down the pipeline on the suction side, but I'm told by an outside consultant that this is how its done??? I'm open to any suggestions if you have them.

 

[BigInch]

You are correct to worry about suction reliefs sized based on discharge conditions; normally that would be an error, but I see these as being sized correctly.

You state that the suction reliefs are designed based on, what I presume, is the VOLUMETRIC capacity of your station, which at standard conditions, which is the same for suction as it is for discharge conditions. So if the reliefs are designed for volumetric capacity at standard conditions, that would have been corrected to the actual volumetric capacity at their set pressure, here 450 psig, when their sizing calculations were done, so that would have been perfectly fine. It would be an error if they were designed for station volumetric capacity at the discharge pressure of 549 psig, but you have said its 450 psig, so I think that is not the case.

I would imagine that the defined volumetric capacity of your compressor station is also the defined maximum volumetric capacity of the upstream connecting pipeline, all defined at standard conditions that were corrected to actual relief volume at set pressure when sized. Volume couldn't be higher than that, pressures couldn't be higher than that, and therefore no problems as I see it.

"The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward X-CEO BP
"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.live.com

 

[DLite30]

Suction Relief Valve sizing:
if the upstream company's pipeline only has a lower MAOP than the suction side of your station, then you're protected from overpressure just by having the higher pressure capability...so the sizing criteria for your suction relief is kind of dubious, becasue their pipe would have to be way overpressured to have an effect on your system. So I think that the discharge sizing criteria you used is okay.

As for your main question about the scrubbers and relief valves:

1)If there is any way to isolate the scrubber from the process piping such that there's is no over pressure protection for it, then you need a thermal relief.

or

2)If the scrubbers are part of the unit process piping (which I imagine it is), you are protected by the unit discharge relief valve, therefore no additional relief is needed.