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Questions on DP Transmitter Wet Leg Compensating Impulse Line

Calculate the appropriate LRV and URV pressures for this hydrostatic level measurement system.

Assuming the process liquid has a weight density of 44 pounds per cubic foot at a temperature of 23 degrees Celsius, and that the “wet leg” compensating impulse line is filled with a glycerin/water mix (S.G. = 1.13).

Assume a static vessel pressure of 126 PSI:

Questions on DP Transmitter Wet Leg Compensating Impulse Line

Question 1:

Express your answers in units of inches water column (”W.C. or ”H2O).

Answer:

LRV = +173.7 ”H2O

URV = +94.76 ”H2O

The temperature and static pressure values are extraneous information, included for the purpose of challenging students to identify whether or not information is relevant to solving a particular problem.

Question 2:

As liquid level increases in this vessel, will the transmitter’s signal increase or decrease (i.e. is this level transmitter direct or reverse acting?

Question 3:

If the process liquid heats up and becomes less dense (but the actual level remains the same), will the transmitter’s 4-20 mA signal increase, decrease, or remain the same?

Question 4:

If the wet leg is filled with pure glycerin instead of a glycerin/water mixture (but the process liquid level remains the same), will the transmitter’s 4-20 mA signal increase, decrease, or remain the same?

Question 5:

How would this system respond if someone closed the upper nozzle block valve, isolating the compensating leg impulse line from the process vessel?

Question 6:

Describe a step-by-step procedure for re-filling the wet leg with glycerin, assuming a pressurized vessel.

Do you find any mistake in these questions?

Share with us through below comments section.

Credits: Tony Kuphaldt

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2 comments

hayder azeez January 19, 2021 at 10:42 pm

Answer2: decrease
Answer3:remain the same
Answer4:remain the same
Answer5: signal is stay constant with last case before close nozzle
Answer6:
The column will be affected by the pressure of the tank, whether it is an increase or decrease, and thus the reading of the transmitter will change if there is a change in the liquid level
All that by imposing a closed air pressure equalization valve

-I hope I understand the questions well

best regards

Reply
Ahmed January 27, 2021 at 7:25 pm

How possible the low side of the DP be connected to the bottom of the Process VESSEL.

Answer for URV and LRV for the above question is not correct , I’ve got (LRV = -206.79” H2O, URV= -128.79 )

Formula I used

LRV= Lmin * SGp – (H1+H2) * SGf

URV= Lmax* SGp – (H1+H2) * SGf

Reply

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Questions on DP Transmitter Wet Leg Compensating Impulse Line

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