*

La soupape est en service ou au repos.

Standby is often 'en restreint' for a person but here I'd have said 'en veille'. I don't feel that au repos really reflects that the valve is ready to activate itself to garantee a continuous supply of the chemical in question

en cas de panne, cela assure un relais

une suggestion....

dans ce cas..

12 Volt Standby Sump Pump
The ESP15 is a battery operated back-up sump pump. It does not replace a regular pump
:
Pompe De Puisard De Réserve 12V
Le modèle ESP15 est une pompe de puisard de sauvegarde actionnée par piles. Elle ne remplace pas une pompe régulière.
http://www.northerntool.com/downloads/manuals/10808.pdf

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Note added at 1 day13 hrs (2011-05-14 10:47:35 GMT)
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After return form my business trip I consulted my technical dictionaries at home:

stan-by battera / batterie f de réserve
~ condition / état m d'attente
~ converter (comp) / convertisseur m de réserve
~ cooling (reacteur) / refroidissement m de secours
~ lighting / éclairage m provisoire
~ loss (electr) / pertes f pl en attente
~ machine / f machine de réserve
~ unit (electr) / groupe électrogène de secours
(Ernst, Comprehensive Dictionary of Engineerign and Technology)

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Note added at 8 days (2011-05-21 11:24:06 GMT)
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The Standby Pump substitutes for either the process pump or the tower/evaporator pump on occasions when service and/or maintenance is required on these pumps.
http://www.advantageengineering.com/fyi/242/advantageFYI242....


Use
The use of change-over valves allows essential maintenance work to be carried out on a safety valve, without interruption of the plant operation, simply by changing over to the relevant stand by valve, thus protecting the plant against excessiv overpressure.
Plants are protected against undue excess pressure by switching over to the relevant standby safety valves. Full blow off capacity is maintained when the change-over-valve switches over from one side to the other.
http://wtavalves.com/index.php?option=com_content&task=view&...

We have had several of these problems in the past and have tried various things and materials on the trims, but we eventually found the best solution is to keep a complete hot standby valve and just replace it and then overhaul the damaged valve again and keep it as a hot standby again.
http://www.control.com/thread/1279272228

When you have decided to replace the existing safety relief valve with a new one, please make sure that you have a standby valve for the intermediately period. As this valve is associated with safety, therefore always select the valves, which can operate beyond the operating pressure range. Setting up of the valve is important, particularly when you are dealing with liquid and its mixture. As this valve handles the vapor pressure only, therefore keep the valve level over the liquid level, and make sure that the liquid cannot touch the valve.
http://www.safetyvalve.org/safety_relief_valve.html

See also http://www.tycovalves-na.com/ld/ANGMC-0241-US.pdf or http://www.aps.anl.gov/APS_Engineering_Support_Division/Mech...
(impossible to copy)