Valves on the inlet of a PSV 3

[keyvagirl]

From AMSE Section VIII Division 1

“UG-135 (b)(1) The opening through all pipe, fittings, and nonreclosing pressure relief devices (if installed) between a pressure vessel and its pressure relief valve shall have at LEAST THE AREA OF THE PRESSURE RELIEF VALVE INLET.”

“M-5.6 Stop Valve(s) Provided Upstream or Downstream of the Pressure Relief Device Exclusively for Maintenance of That Device FULL AREA STOP VALVE(S)”

"M-6 (a) The nominal pipe size of all piping, valves and fittings, and vessel components between a pressure vessel and its safety, safety relief, or pilot operated pressure relief valves shall be AT LEAST AS LARGE AS THE NOMINAL SIZE OF THE DEVICE INLET"

The PSV we are using has an inlet cross sectional area which is much smaller than the cross section of the nominal inlet size flange because of a sleeve which fits into the inlet flange.

The reduce port valve for the same nominal size as the inlet flange has a cross sectional area greater than the PSV but smaller than the nominal pipe cross sectional diameter.

We are having difficulty getting a full port valve of the same nominal diameter as the valve.

An argument was made that the valve has a cross sectional area which meets the requirement of UG-135 since actual area is larger than the actual area of PSV inlet area (since UG-135 says nothing about nominal pipe size)

A suggestion was also made to get a reduce port valve of the next size up and run a larger size of piping and valve up to the PSV flange. The valve would be reduced port but the cross sectional area would be larger than both the inlet of the PSV and the nominal pipe size of the PSV flange.

Does the valve have to be a full port valve even if it has a larger diameter than the inlet of the PSV?

Does the valve have to be a full port valve even if it has a larger diameter than the nominal diameter size of the PSV (because the reduced port valve is larger than the nominal size of the PSV?

Does the valve have to be a full port valve no matter what?

We can meet the 3% rule for all cases – reduce port same size as the PSV inlet, full port valve same size as inlet, and reduced port next size larger than the inlet

Thank you

 

[KVVS]

As far as normal engineering practices valve should not put at inlet of PSV, but if it is required for maintnance purpose than put Locked open valve. now come to the question , as per my observation valve should have full port same as nominal dia of pipe.

 

[zdas04]

KVVS,
"Normal" is such a slippery concept. I ALWAYS put a block valve and an entry vent under a PSV so that I can test the valve without removing it. I think that that is a "good Engineering practice". I also specify that the valve will be locked open as required by code.

keyvagirl,
This is a bit fuzzy. You have sleeved the PSV and then done the calcs and it is adequate for your most severe credible scenario. To my thinking, the sleeve ID now defines the "nominal pipe size", and if a reduced port valve is that big or larger then I would classify it as "adequate".

On the other hand, why do you want to use reduced port valves? When I've priced reduced port vs. full port flanged valves in sizes bigger than 3-inch in the last few years, reduced port has been more expensive than full port (and the difference has rarely been very much money anyway).

Further, if you go with reduced port, you will write a justification for the selection (the writing of which will cost more than the price difference if reduced port happens to be less expensive this week), put it in a file that no one will ever read, and have to answer "reduced port valves are not allowed in this service" questions for the whole time the vessel is in service. Seems like a false economy to me.

David

 

[keyvagirl]

It is fairly normal practice to put car seal open valves under PSV for maintenance purposes and it is allowed under appendix M of the ASME code.

For clarification we have not sleeved the PSV. It is common manufacturing practice for PSVs now to insert a sleeve into a standard inlet flange rather than machine down inside the valve. Or at least the manufacturing cutaways show this. Check out

http://webywebyweby.wordpress.com/2007/10/03/psv-selection-for-beginner/

The issue is, from what we have found, the reduced port valves are available in a reasonable delivery schedule. (i.e. we can get them before our Oct outage) and the full port valves are not. These are made of a high alloy which may be making the difference. I have been asked if we can install the reduced port alves and still be in compliance with ASME code. That is what I am trying to figure out.

 

[don1980]

keyvagirl,

The part you refer to as a sleeve is actually an integral part of an API 526 PSV. That is the the valve "nozzle".

If your inlet pipe is one or more sizes larger than the nominal inlet size of the PSV, then it's OK to use a standard (reduced port) gate valve under the PSV. Other types of valves are OK too but check to be sure the x-sec area is at least as large as the nominal inlet x-sec of the PSV. Be aware that plug valves and globe valves have greatly reduce port areas, and should generally never be used under PSVs. Butterfly valves should be avoided too, because a failure of the disk-to-stem attachment can cause the valve disk to be forced into the closed position.

 

[tickle]

If you used a larger block valve with a reduced bore equal to nominal size of the PSV, that would meet the code.

i.e. PSV = 4"x6" then on the inlet use a 6" reduced bore of 4" and on the outlet use a 8" with a reduced bore of 6".

 

[zdas04]

What valve technology are you looking at? I've gotten into the Oil & Gas mindset that a "block valve" is synonymous with "Ball Valve". When I think about it, there are a lot of services where a gate valve would be perfectly suitable for less money.

I'm with Don in saying avoid any valves that are not straight though (like a globe valve), avoid plug valves for every service, and don't put a bluff body in the flow upstream of the PSV (i.e., no butterfly valves either).

David