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pradeepdhoni
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Pressure Transmitter Impulse Line Tapping

02/21/2015 5:36 AM

We have process line with steam and we are measuring the pressure at two ends of the line with the first one impulse line tapping at 4.2metre from ground and the second one with 6.2 metre from the ground. The steam flow is from first one part to the secdond one part. The approximate presure in the line is 4MPA.

In this case, the first pressue transmitter from the first impulse line is showing 4MPA and the second transmitter from the second impulse line showing 4.01MPA. I cannot understand why there is pressure increase between the two. Is there because both tapping at different elevation?
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JohnDG
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#1

Re: Pressure transmitter impulse line tapping

02/21/2015 5:56 AM

0.25% is a very small difference; I suspect it's less than the measuring capabilities of the instruments. What do you measure if you exchange the two instruments?

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lyn
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#2

Re: Pressure transmitter impulse line tapping

02/21/2015 9:26 AM

I agree with JohnDG.
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Iris
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#3

Re: Pressure transmitter impulse line tapping

02/21/2015 10:48 AM

1) If the transmitters are analog 4-20mA, D/A trim error at the transmitter or at the analog input could easily be 0.25% or more.

When were the transmitters and analog inputs last calibrated?

2) The 4.01-4.00 MPa difference is approx a difference of 1 meter of water column.

The difference in elevation of the taps could affect the installation of transmitter and the associated impulse lines.

With 40 bar/580 psi steam the transmitters are presumably isolated from the process with a wet leg of condensate, and maybe a condensate pot.

The height of the water column in the wet leg adds to the measurement. If this static wet leg pressure (at zero line pressure, atmosphere) is not "zeroed out", it becomes part of the indicated measurement (the output of the transmitter).

Have both transmitters been "zeroed" to eliminate the effect of the static wet leg ?

3). If the difference of 0.25% is the true magnitude of the error (are these readings at no flow conditions where you expect the same pressure reading?), then the accuracy of the devices has to examined, to see if the error falls within or outside the specified accuracy.

The very low accuracy/uncertainty values cited on a vendor's spec sheet is at "reference" conditions (25 Deg C), and at some restricted range of turndown.

Manufacturers typically provide a formula for calculating accuracy/uncertainty at operating conditions and actual turndown.

Note that this model's accuracy spec goes to 0.141% at an operating turndown of 5:1 and an operating temperature of 53 Deg C.

What is the operational accuracy of your transmitters?
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pradeepdhoni
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#4
In reply to #3

Re: Pressure transmitter impulse line tapping

02/22/2015 1:27 PM

Mr.Iris,

The accuracy is 0.065% of span. The stability is 0.1%. Make is yokogawa with model EJA430A. The static pressure in wet leg is not "zeroed out". The impulse lenght will be approximately 3 to 4 metres with two bends. Can you please explain how to calculate the static wet leg pressure and to make compensation for the same in transmitter?
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PWSlack
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#5
In reply to #4

Re: Pressure transmitter impulse line tapping

02/23/2015 3:54 AM

The static pressure at any point in a liquid is the height below the free surface multiplied by the density of the liquid and the acceleration due to gravity.

Compensation may be made by a suitable adjustment on-board in the transmitter or at the receiving device. In either option, the relevant equipment manual will give full details.

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Iris
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#6

Re: Pressure Transmitter Impulse Line Tapping

02/23/2015 8:38 PM

To clarify PWSlack's explanation, there's no calculation needed to determine the offset caused by a condensate wet leg. The elevation is the vertical distance between the tap and the port on the transmitter. Head does not care about turns, horizontal segments, angular segments or up/thendown traverses. Head is elevation or vertical distance. The fluid is steam condensate; water with a density of 1.0, so no density correction is involved.

But you don't need a calculation, you can zero the transmitters with the zero adjustment screw where the transmitter calls whatever head pressure is applied to it at that moment: "zero".

It is critically important to not have any process pressure applied during the zeroing operation. The transmitter needs to be isolated from the process, the impulse line needs to be momentarily vented in order to release any trapped process pressure on the wet leg. And the wet leg impulse tubing needs to be full.

Once the wet leg is isolated and the process pressure has been vented, use the zero adjustment screw on the outside of the transmitter to adjust bring the wet leg pressure down to zero:

or

Note that the EJA HART User's Manual is available at this link:http://cdn2.us.yokogawa.com/IM01C22T01-01EN_008.pdf4). The HART communicator
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