Level Switch Displacer Chamber's Vent

Yes, except maybe in some unusual configurations.

Thanks Tornado for answering my query,

By saying yes you meant that we still need the vent connection, right? as a newbie, i still could not understand why we need to have the vent on a pressurized vessel, i thought i will be overkill to have the optional vent connection. This is because most manufacturer state vent connection is only optional. I try to google for the purpose but still to no avail. Is it to prime the chamber to avoid airlock?

Dear Tornado,

I'm sorry, i didn't explain the matter completely. The level switch will be installed side-side to a bridle/standpipe which also attached side-side to a horizontal vessel. the level switch itself has a displacer chamber by which the displacer moves freely up-down based on level inside the vessel.

As per shell std dwg S38.056-D, it depicts a typical displacer chamber with a 3/4" nozzle connection on the upper side before the top flange connecting to the switch enclosure. My doubt is, if the vessel is pressurized, and we intend to drain the content inside the displacer chamber for maintenance purpose......we don't need to vent first right? provided the diplacer level switch comes with isolation valve at the connecting nozzle, just isolate the valve and open the drain....the liquid will come out....will not get stuck (as per drinking straw case, dip into wwater, close the mouth lid, lift the straw...the water will not go out until we release the mouth lid).....

sorry, if i explained in silly way.....(indeed i'm silly asking this kind of question). I really need to know the purpose of the vent, whether overkill or not if installed on a pressurized vessel.

To give an accurate reading whatever the pressure, a displacer level gauge needs two connections. That's why all proprietary gauges have two connections.

Where is the vent connection?

Do you have a drawing?

Somewhere in here maybe:

A drawing says more than 1000 words!

PWSlack, i'm sorry i didn't mentioned it earlier......the chamber that i said comes with upper and lower nozzle connection. you are correct, we must have at least two (upper/lower) connection to get accurate result....but this is another connection slightly opposite to the upper nozzle which told by my superior for vent purpose.

Yes, exactly IdeaSmith....that's the drawing which similar to s38.056-D.....thus my doubt is, since the vent connection(depicted as Y, usually plugged) in the catalog is optional, what is exactly the purpose of the vent connection? is it to prime the chamber?

Without seeing a vessel drawing, the difference between an upper level nozzle and a vent nozzle is abstruse.

Follow the "for construction" P&ID as it will have been through a HazOp Study for review by all concerned.

It's a vent nozzle. The purpose is in the words. If it isn't needed, fit a blank. If it may be needed at some point in the future, fit a valve and a blank. If it has some other purpose, then the "for construction" P&ID, which cannot be seen from here, will show it.

<unsubscribes>

Why the need to vent a stand pipe chamber? as newbie, since there are upper and lower nozzles, pressure difference between gas and liquid i think won't give a difference between the level in the vessel and in the chamber. therefore, i don't see any need to vent the chamber. Please correct me if i'm wrong.....i seek those who have experience with this matter to share their knowledge

...which would, of course, have come out during the HazOp Study for the plant. GA

thanks tony and PWSlack for responding....

Yes Tony, in my actual case we do have block valves at both upper/lower nozzle. "wet" test is kinda functional test of the switch, only to check whether it still triggers or not, right?

if we want to sure that the switch triggers at the right level, we must isolate, disconnect the whole chamber and connect a level gauge in parallel right?then we fill lliquid into the chamber via that particular vent nozzle.....then only we know the actual trigger level of the switch right? again correct me if i'm wrong.

It's all in the postings above.

Good luck with the project.

<unsubscribes again>

Thank you PWSlack.

To the rest who still interested, im sorry im still not satisfied. If we test the level switch by flooding the chamber, it will not indicate anything about setpoint accuracy but only saying that the switch is functioning OK. doubt arises whther it triggers at correct setpoint or not. thus, since there are already block valves and we can easily remove the switch without depressurizing the vessel, i think bench calibration is better and having the vent nozzle in my opinion would be overkill. I invite you to throw your opinion on this.

Obviously our opinion does not matter.

Not all level chambers come with vent nozzle.

See this

http://www.ge-energy.com/products_and_services/products/levels/masoneilan_model_12800_pneumatic_liquid_level_controller.jsp

yes srini, not all come with vent nozzle and in manufacturer's catalog, it stated "optional" to have vent nozzle.

this came to my attention since i've proposed a chamber without vent initially, and suddenly required to add the vent nozzle just to comply with S38.056-D, whereas it is only optional in vendor's catalog. can anyone explain on what condition/installation do we need to have the vent connection?

When complying with S38.056-D, for one example. When the customer's or designers specs call for it, for another.

which concludes there's no right or wrong by not having the vent connection......

thanks all.

It concludes nothing of the kind.