Two valve Isolation rule 1

[jaxon02]

I'm looking for which spec or code states that when work is carried out on equipment on an operational plant that the piece of equipment is isolated by two valves. Any one remember where this is written down.

thanks

 

[metengr]

I have seen and heard this question asked many times in other forums. The code and most standards only provide minimum requirements, and are not intended to specify or regulate company safety programs. Safety programs require common sense and must be controlled by the organization that owns/operates or repairs hazardous equipment.

With that said, in certain applications - for example separate boilers (not twin furnace boilers) that feed into a common steam outlet header the ASME B&PV code requires a stop and check valve as a form of double isolation to assure that steam from one boiler does not back feed into the other boiler. This is part safety, and to protect isolated equipment from damage.

Regarding non-boiler applications, double isolation is a good thing, regardless. Safety is most important, and if you need to provide a zone of isolation using two valves to assure personal safety so be it. You might want to check OSHA requirements for more information.

 

[jte]

jaxon-

Presuming you are governed by US OSHA (not a more stringent State version or some other country), you need to read up on 29 CFR 1910. Make sure you read 1910.146 and 1910.147 available at

http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=STANDARDS&p_id=9797

and

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

What you seem to be referring to is a double block and bleed (DBB) setup (but that is most often three valves or one specialty valve and a bleeder valve). A DBB is one means of energy isolation which is acceptable to OSHA. Many will argue that it is not the best means of isolation, and I'll tend to agree that blinding or disaligning pipe spools is preferrable.

See also:
thread408-94849
thread408-122807

jt