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2 Out Of 3 Voting Level Switch

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#1 paruzekk


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Posted 01 August 2018 - 04:10 AM

Hello to everyone, I have a question related to installation of level switches on existing vessel. Client likes to add pump dry protection from low-low level in suction vessel. The system is existing. They also want to have 2 out of three voting on this level switch which means installation of 3 independent transmitters. The question is if it is allowed to have these transmitters installed on common standpipe on vessel to avoid drilling 3 sets of nozzles for each transmitter. If the standpipe is allowed is there any rule for minimum size of standpipe and standpipe connection to vessel if the connection to individual transmitter is 2"?


Edited by paruzekk, 01 August 2018 - 04:12 AM.

#2 gegio1960


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Posted 01 August 2018 - 05:11 AM

the common standpipe is generally allowed but the reliability of the protection is less compared to 3 completely indipendent pieces.

the failure probability can be calculated for the two solutions and compared to the minimum requirement for the protection of the pump.

in order to improve the safety is also possible to install a completely different cut-out system (like a temperature switch) in parallel to the level transmitters.

#3 thorium90


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Posted 01 August 2018 - 05:52 AM

Im not sure what tank you have or what you are measuring, but if you need to install new level transmitters or switches, perhaps you can consider installing radar level transmitters that are mounted outside the tank. This could save you some trouble of drilling more nozzles.

The pictures below are from Vega level transmitters.





If you can make nozzles on the tank, might it be possible to put a level chamber at the side of the tank? Then you can mount magnetic non contact level switches around the outside of the chamber. Refer to attached picture.

Attached Files

Edited by thorium90, 01 August 2018 - 06:10 AM.

#4 Technical Bard

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Posted 12 August 2018 - 10:42 PM

In order to obtain the best SIL rating, you need three separate connections to the vessel, and three different types of measurement.  Do NOT use switches - use transmitters that report a level in the vessel.  Using switches is dangerous because you can't know if they have failed until you need them (q.v. Buncefield incident).

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