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Active Contributor

Join Date: Jul 2017
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Reason for Both Legs Connected in Wet Leg Level Measurement?

09/23/2017 2:48 AM

Dear all, As you can see below, thank you.
upload a photo on internet

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Guru
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#1

Re: what is the reason in Wet leg level measurement, both leg is connected?

09/23/2017 4:36 AM

Because if the tank has pressure, reference liquid will splatter all over the floor from the top of the wet leg.

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#2

Re: Reason for Both Legs Connected in Wet Leg Level Measurement?

09/23/2017 2:27 PM

adpabd,

Bro, both legs connected is not just due wet legs, remember that if you have some positive or negative pressure on a closed tank, that pressure would be necessary be applied on both ports of the transmitter in order to be cancelled its effect, if not you will be measuring static pressure inside the tank, so, to measure level you will need to measure hydrostatic pressure produced by the liquid.

Wet Leg is used when some gaseous products could be condensate inside the impulse lines producing offsets on the measurement.

The key is here: Dp = High - Low, hydrostatic pressure produced by liquid, pressure exerted by any gas on the superfice at closed tanks, fluid inside of capiliaries and so on...

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#3
In reply to #2

Re: Reason for Both Legs Connected in Wet Leg Level Measurement?

09/28/2017 5:16 AM

Thank you for the reply Sir,

so the reason that there is a tubing connected (red line on picture) on both leg is to moved the relative reference of pressure producing independent pressure measurement.

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Power-User

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#4

Re: Reason for Both Legs Connected in Wet Leg Level Measurement?

10/03/2017 9:50 PM

No, not quite.

Below is a revised drawing without the silly red line that means nothing.

The reason for the low side leg is to provide the reference pressure to the pressure transmitter.

That reference pressure can be either a dry leg (empty, no liquid fluid, gas pressure only) or a wet leg (liquid fluid filled to the top).

The reasons for a using a wet leg are explained well by Instruments-guy-SAV.

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Active Contributor

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#5
In reply to #4

Re: Reason for Both Legs Connected in Wet Leg Level Measurement?

10/15/2017 5:53 AM

the red line is there because thats how the tubing constructed in our plant.

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Power-User

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#6

Re: Reason for Both Legs Connected in Wet Leg Level Measurement?

10/15/2017 6:59 PM

There is no purpose for the red line pipe/tube. It adds nothing needed for a level measurement.

Take the red pipe/tube away and the level measurement remains the same: the wet leg provides the (reference pressure + Pgas pressure) and the high side provides the (hydrostatic liquid level head pressure + Pgas pressure).

The liquid level in the red pipe/tube will be at the same level as in the tank/vessel/
The P(gas) pressure is the same in the red pipe/tube as in the tank/vessel and the wet leg pipe/tube.

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