DP transmitters on LPG bullets

S

Sumeet Chimalkar

I have a DP transmitter on a LPG bullet pressurized to between 3 bar to a maximum of 6 barg. The bullet diameter is 4 metres and
the transmitter is a smart 143DP Foxboro-Eckhardt model. The problem is, the original installation had the HP & LP ports connected to the Bullet liquid & vapour tappings respectively through impulse tubing. The transmitter reading used to drift in a +/- 10% range. This was apparently due to LPG vapours condensing in the LP leg.

Now I have added a seal pot with water in the LP leg and the reading does not drift but i am not sure about calculating the lower and upper range values. Can somebody please advise me on how to calculate these? Has anybody tried heating the LP leg instead of using a seal pot?

Sumeet

R

R J $$BOB$$ PAWLEY

Hi Sumeet

Try filling the LP leg with unity oil. This has a specific gravity of 1.0 so the calculation to discount this pressure is exactly the same as water. Oil will not condense or allow condensation to enter the leg so your problem is solved permanently. The downside, as there always is, is a little more care is needed during maintenance.

Bob Pawley

B

Brian T. Smith

Sumeet

> I have a DP transmiter on a LPG bullet pressurized to between 3 bar
> to a maximum of 6 barg. The bullet diameter is 4 metres and the
> transmitter is a smart 143DP Foxboro-Eckhardt model. The problem is,
> the original installation had the HP & LP ports connected to the
> Bullet liquid & vapour tappings respectively through impulse tubing.
> The transmitter reading used to drift in a +/-
> 10% range. This was apparently due to LPG vapours condensing
> in the LP leg.

[Brian T.Smith] Could it also be due to changes in the fluids density? Ambient temperature changes and sunshine on the vessel or leg lines could cause the temperature to change resulting in a density change. The measured level is the product of measurement height x Fluid Specific Gravity.

> Now I have added a seal pot with water in the LP leg and the
> reading does not drift but i am not sure about calculating the lower
> and upper range values. Can somebody please advise me on how to
> calculate these? Has anybody tried heating the LP leg instead of
> using a seal pot?

[Brian T.Smith] Assuming the transmitter is located at the Zero % height, The Lower Range Value (LRV) would be the head in the LP leg line.
That is the height from Transmitter to the 100% height or upper tap height (whichever is lower) in Inches x Specific Gravity of seal fluid x (-1). Multiply by -1 because the head pressure is on the LP connection.

If the transmitter is mounted below the 0% height you will also have to add to the previous LRV the positive head pressure of the difference between the transmitter and 0% height x Specific
Gravity of Process Fluid.

The URV is: (measurement height in Inches x Specific Gravity of process fluid) - (LRV).
Since Water SG > LPG SG, both URV and LRV will be negative numbers.

Yes I have heated LP legs to vaporize the liquid.

Brian T. Smith, CET, email: [email protected]
(519)332-1212 x7920, Fax519)339-7301.

S

santhiraj

hello:
your problem is similar to drum level measurement in a steam boiler.
in drum level measurement, a wet leg is used as described by you using water.
you can consult boiler handbook for solution.
Your LPG level will vary and it will be compared against a constant level in the wet leg and you can a DP. You have to map the DP with level. I think level and dp will exhibit "inverse" relationship. A bit of Physics will help you to derive the relationship.
Best of luck

santhiraj

J

jmGiraud

Millions of installations (similar to yours) do work perfectly, i.e: on condesable gas XTR below level and wet leg.
Your problem is one of calibration.
Forget that garbage zero suppression, zero elevation.
For an immediate solution, apply on both sides of the XTR your hi/lo leg, then zero the reading. Then increase the hi by the process value and adjust the span.
Note: always check an intermediate point.
Your XTR may be defectuous (I have seen many of them).

R

RPS Saini

use transmitter with seal on low side. dont juggle.
dont heat. it blasts only once in a lifetime.
regards

T

Tony Cantu

I recommend using a transmitter with capillaries.

Using tubing and seals and all is more trouble than it's worth.

Using tubing you never know when you have a huge error.

Capillaries are much more accurate.

Tony Cantu
Control Works, Inc.
www.cwinc.us
281-493-1212