Hi All, Can someone tell me as t 2

[starrproe]

Hi All,

Can someone tell me as to what type of block valve (ball / gate) should be used upstream and downstream of relief valves in process piping?

Thanks in advance.

 

[pennpiper]

Normally block valves are not allowed up-stream and down stream of relief valves. Unless, and I do mead unless there is another relief valve that would be available to provide the safety for the vessel and system.

You should look at the P&ID and only install block valves if and when the P&ID says they are to be installed.

Then, as to the type of valve, you should only install the type of valve that is designated as the primary block valve for use in the piping line class (Piping Specification) for the applicable (pressure, temperature, commodity) system.

Go talk to a Piper.

 

[BigInch]

neither one

http://virtualpipeline.spaces.msn.com

 

[dcasto]

You should use a full port valve. A ball valve is cheaper and easier to operate. YOU must car seal these valves open.Block valves under and after relief valves are common in the world of long runtime plants.

 

[mbeychok]

.
I totally agree with pennpiper's response that:

Normally block valves are not allowed up-stream and down stream of relief valves. Unless, and I do mean unless there is another relief valve that would be available to provide the safety for the vessel and system.

The primary reason for isolating a relief valve by providing block valves before and after the relief valve is so that a faulty relief valve can be repaired or replaced while the plant system is still in operation. When that situation occurs, there must be another installed and available relief valve to protect the plant system.


That applies whether the plant system has a short runtime or a long runtime.

Milton Beychok
(Visit me at www.air-dispersion.com)
.

 

[BigInch]

OK if you must. Although they can be somewhat of a hazard, they do exist. If they only have a minimal pressure drop, they could be installed for "practical reasons". Therefore to provide that minimal pressure drop, they should be full port. AS decasto says.. go CSO, and keep your fingers crossed. I've been to many a compressor station where a number of those valves have been closed, and prefer not to use them given my choice. I would rather let ops and maintenance modify the piping if they want to install them.

http://virtualpipeline.spaces.msn.com

 

[MortenA]

Offshore tw PSV with interlocked block valves are very common. There are systems where its an integrated block where you only have one spindel so that one PSV is allways un-blocked.

I dont givbe that much for CSO and would prefer LO. But operators hate the keys i know.

Best regards

Morten

 

[SeanB]

As practice, with any new PSV installation we always install a block valve upstream and downstream of the PSV. If you don't, you have to wait for a unit shutdown to be able to perform a "pop" test on the PSV. I don't know if it is a code thing, but here we are required to "pop" test our PSV's every 36 months. Without isolation, that can't be done. Which is why is some of the older units here we have PSV's that haven't been tested in 10 years. Without the isolation you have to isolate the flare header from the PSV and then remove the PSV under fresh air. It's not like the old days where an operator would throw on a scot air pack and remove the PSV with flare gas blowing out in his face.

 

[dcasto]

ASME requires an inspection / test every five years on relief valves. We use a risk management "gradeing" system to set a testing time period anywhere from 6 months to five years. Shutting down plants every 6 months to test a single relif valve is not logical, so a valve is placed under it. We remove the valve and replace it with a tested valve and go. That section of the plant is stabil and a person is poised to manually monitor and control in case of an upset in the 30 minute period where the relief valve is out of service.

Since compressors are taken down for simple repairs like hot valves, oil changes on the engines, we inspect relief valves then. On pumps with 100% spares, no block valves too. On storage tanks, block valves under the relief valve, we are not going to empty a tank to inspect a relief valve.

 

[jte]

I'm totally with dcasto on this one. It is unusual for me to see a pressure relief valve which does not have a means of isolating it. Check with your local jurisdiction, but it is totally legal within VIII-1. See UG-135: (d) There shall be no intervening stop valves between the vessel and its pressure relief device or devices, or between the pressure relief device or devices and the point of discharge, except: (1) when these stop valves are so constructed or positively controlled that the closing of the maximum number of block valves possible at one time will not reduce the pressure relieving capacity provided by the unaffected pressure relief devices below the required relieving capacity; or (2) under conditions set forth in Appendix M. Appendix M has too much to copy the whole thing, but one selective quote is: Full area stop valve(s) may be provided upstream and/ or downstream of the pressure relieving device for the purpose of inspection, testing, and repair of the pressure relieving device or discharge header isolation, provided that, as a minimum, the following requirements are complied with...

Within California, block valves to isolate PRV's are allowed in refineries in accordance with the Petroleum Safety Orders

http://www.dir.ca.gov/title8/6858.html:

(3) Stop valves may be installed between a pressure relief device and the operating vessel for the purposes of inspection, repair and/or replacement of the pressure relief device. Where stop valves are installed, the employer shall develop, implement, and maintain a written plan containing all of the following:

So they are clearly allowed by ASME VIII-1, and may be allowed depending on the jurisdiction.

Now, to answer starrproe's question, I've usually seen gate valves in this service, with the stems in the horizontal position so that a dropped gate will not unintentionally block the path. This is in line with various company standards, and also with the California Petroleum Safety Orders: Valves as permitted above shall not be gate valves installed with the stems pointing above the horizontal plane of the valve body or globe valves installed so that the disk will open against pressure. This limitation need not apply to stop valves closing the vessel from its source of pressure. Other provisions acceptable to the Division may be used to assure that the valve is in the open position.

I don't see an advantage to ball valves, except perhaps for small pipe sizes.

jt

 

[Ashereng]

I think a full port ball or a full port gate are pretty much the same thing. They both minimise pressure drop.

The selection will probably be based on price, if there is a difference, and site preference, if there is a preference.



"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[timbones]

I would be careful with the blanket statement that these valves must be car-sealed. In some cases, car-sealing is definately not the way to go. The pipeline company I used to work for locked open all of the PSV isolation valves because it made the valves tamper proof unless you had the key (CSO is not tamper proof). This was particularly important for remote unmaned compressor stations.


See section 322.6.1 of ASME B31.3 for more info on adding block valves in pressure relief piping.

Tim

 

[Ashereng]

Tim,

A car seal is more documenation and tracking. A physical lock is a physical lock.

One can do both.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[dcasto]

timbones, at some point locks become a problem too. Piant, keys, frozen, and finally, a lock keeps honest people honest... If you don't trust your employees, fire them...

At remote B31.8 sites, we just put a little wire on our 3/4" valves connected to our overpressure protection transmitters and SCADA. The regulators only got excited when the saw no chain and lock on our mainline valves. We locked the clutch handle so the handwheel would turn but not the valve, that really got them scratching their heads.