Working downstream of a rupture disc. 1

[scorer]

From a safety point of view, how do we best manage working on (breaking into) the vent line downstream of a rupture disc or pressure safety valve. The application in question is on storage of a toxic liquid. Multiple tanks have safety devices piped to a common abatement device (wet scrubber), which also handles normal venting operations.
Is it acceptable to work on the line/equipment when only protected from the process by the safety device? Draining down and washing out all tanks is no small or particularly safe job. I imagine that there is a similar issue when changing the safety device on a part full tank.

 

[MikeClay]

You cannot rely on the relief device to isolate the vessel from the relief header. What is to prevent the relief devices from performing their duty and filling the header with vapor while maintenance is being performed.

While the individual relief devices can have isolation valves (and/or blinds), the leads to all sorts of problems. Once all of the isolation valves are closed, none of the vessels will have pressure relief.

 

[Montemayor]

Scorer:

The only safe way to working on (breaking into) the vent line downstream of a rupture disc or pressure safety valve is to isolate the system from the possibility of any activation of these pressure relief devices. There is no alternative I know of under this situation.

That having been said, of course, means that you must either block off the source to the PSVs and substitue other PSVs for the tank(s) or simply empty the tanks and "safe" them. The majority of LPG bullets have multiple PSVs for the purpose of doing what you propose. This is not a recent necessity - or invention. In the case of toxic chemicals this is often the norm in order to maintain steady production and operation.

Working downstream of a rupture disc is inviting a disaster. MikeClay has stated it succinctly: You simply can't rely on the procedure.

Don't do it.

 

[jte]

Scorer-

Besides the common sense Mike and Montemayor put in their notes above, consider your legal standing. If you're in the US, you most likely fall under OSHA. Read

http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=9804

From the scope of that document: This standard covers the servicing and maintenance of machines and equipment in which the unexpected energization or start up of the machines or equipment, or release of stored energy could cause injury to employees. This standard establishes minimum performance requirements for the control of such hazardous energy.

A PSV will not quite fit the definition of "energy isolating device."

jt

 

[scorer]

Thank you for the replies. In this case we have diverted the relief discharge line to a different area - away from the working area. It doesn’t go to abatement, but it’s better than directing it to an area where there are sure to be people working.

Perhaps you could help me from a design point of view. In certain places we have relief to a single (no standby) abatement device. Should we be looking to install tee pieces after the relief valve/bursting disc so that the line to the abatement device can be spaded, and the relief sent out via the tee (plus extra pipework?). I’m not a fan of valves in vent lines, and there aren’t always enough nozzles to provide duplicate reliefs.

 

[SeanB]

I am probably just re-stating what everyone else has already said here but anytime we have to work on a line anything entering that system has to be blinded. You cannot rely on a valve for positive isolation.
We follow this philosophy:
1. Isolate
2. De-energize
3. Purge
4. Test

 

[Ashereng]

scorere,

I agree that using a valve for positive shutoff is not reliable. There have been many instances of valves leaking, resulting in major catastrophes. Phillips 66 back in the early 80's come to mind.

If you need to shut something off, the best way is to shut the process down. Next, close the valve, and either blind or plug behind the valve.

This is the best way, regardless of what the vavle is (DB&B, bubbble tight, etc).

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