Rank: New forum user 
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3 boliers producing steam at 100 psi. Need to isolate one for inspection. what is the minimum recomended level of steam isolation. For example 1 hand operated valve locked shut! or 2 valves. Does a none return check valve count as an isolation valve?
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Rank: Super forum user
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NRVs are not sufficiently positive means of isolation.
You probably need Level 1 Isolation physical disconnection or 'double block and bleed' isolation
See HSG 253
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Rank: Super forum user
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Mark
Non return valve should never be taken as an isolation. Are the 3 boilers feeding onto one common line or into a steam receiver?
Normally isolation will require 2 stop valves locked shut, then the pressure released / vented, then open the steam trap fully and lock open to makes sure any residual hot water is drained and doesn't flash off again.
Probably don't need to remind you regards the electrical / mechanical / water feed isolation's?!
Jonty
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Rank: Super forum user
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As others have said, non-return valve will not count as an isolation.
I would say the important thing will be being able to prove and secure the isolation. A single valve may be acceptable if you can prove its integrity.
There was a fatal accident on the QE2 ship caused by a poor isolation on a steam system. Report is available at http://www.maib.gov.uk/p...03/queen_elizabeth_2.cfm
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