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Stacytk21
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Join Date: May 2013
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Measuring the Level of a Closed Vessel in a Process Under Vacuum

05/16/2013 4:16 AM

Dear All

I kindly request that you assist me in the challenges I have when it comes to measuring the level of steam condensate in a vessel that is under vacuum. The vessel is under vacuum pressure of 92 kpa and the tank has a height of 2m and using a differential pressure transmitter with a wet leg connected on the LP side and the HP side which is a flange mount type is connected to the base of the tank.

Thank you
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PWSlack
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#1

Re: measuring the level of a closed vessel in a process under vacuum

05/16/2013 4:24 AM

Perhaps the condensate is boiling in the vessel, or even the pressure transmitter legs, in which case consider a different measuring technique, such as radar.

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PWSlack
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#2

Re: measuring the level of a closed vessel in a process under vacuum

05/16/2013 4:48 AM

Consider replacing the vessel and any electrical condensate pump with a steam-operated Ogden Pump Set (usual disclaimer) that does not need separate level instrumentation in order to operate automatically.

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Vulcan
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#3

Re: measuring the level of a closed vessel in a process under vacuum

05/16/2013 8:11 AM

We have several vessels similar to yours (containing steam condensate under a vacuum). For all of them we use differential pressure transmitters with two diaphragms. We don't have any problems with them as long as you calibrate them regularly.

regards,

Vulcan

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bigg
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#4

Re: Measuring the Level of a Closed Vessel in a Process Under Vacuum

05/16/2013 8:52 AM

If you have a bottom mounted transmitter and are, indeed, able to maintain a full wet-leg to the top of the drum, it will have a higher level, i.e. higher head/pressure than the bottom connection. Therefore, the wet leg should be connected to the HP side of your transmitter. Also, the reading will be reversed, i.e. more differential will be seen when the level is low. The differential will be zero when the level in the drum equals the height of the wet leg.
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Stacytk21
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#7
In reply to #4

Re: Measuring the Level of a Closed Vessel in a Process Under Vacuum

05/17/2013 3:43 AM

Thanks biggs, the transmitter uses a negative range because the wet leg is always filled with water. The range of the transmitter is: lower range value = -29.0 Kpa and an upper range value of -10.0 kpa.
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eltech
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#5

Re: Measuring the Level of a Closed Vessel in a Process Under Vacuum

05/16/2013 5:30 PM

Agree with Bigg, It is logical to have your HP to your wet leg, cause that is going to be your High pressure, but it will still work exactly the same if you have the connections in reverse.More important is that you set your zero and span accordingly.On your current set up you are going to have negative values which may cause some confusion to the maintenance crew.
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Iris
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#6

Re: Measuring the Level of a Closed Vessel in a Process Under Vacuum

05/16/2013 9:20 PM

Condensate temperatures can exceed what the meter body can tolerate, so it not unusual to find drop leg impulse tubing (H2) on the high side port (low level tap) that isolates the transmitter from the condensate's temperature (which is what the wet leg also does on the other side), as shown in the Foxboro graphic below:

Here's Rosemount's example for configuration of DP level with a wet leg:
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bigg
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#8
In reply to #6

Re: Measuring the Level of a Closed Vessel in a Process Under Vacuum

05/17/2013 8:19 AM

I knew you'd check in with the best GA

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Anonymous Poster #1
#9

Re: Measuring the Level of a Closed Vessel in a Process Under Vacuum

05/17/2013 3:28 PM

So , just to be clear ---

this is what we have ( ? )
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Users who posted comments:

Anonymous Poster (1); bigg (2); eltech (1); Iris (1); PWSlack (2); Stacytk21 (1); Vulcan (1)

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