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Join Date: Feb 2013
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DP Capillary Across Filter Pad Cells at Different Heights. Cannot Zero Tx

02/26/2013 8:57 AM

Hi.

I have a DP Tx with Hi and Lo capillary across a filter. Pad cells are separated by approx 12m in height. Both capillarys are full extended and the Tx is mounted approx in the middle of the 2 pad cells. Therefore I cannot move the mounting of the Tx. The Tx range is -60 to 60Kpa. The Tx is reading 44Kpa and is out of range of a zero trim, it will not let me zero. The Hi alarm is set at 21Kpa therefore it is always showing alarm and will not give permissive to start the equipment in this state. My question is are there any things I can do to get a zero on this Tx other than ordering a Tx with longer capillarys allowing me to remount the Tx at the same height as the Hi pad cell.

Any thoughts? Appreciate all answers.

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#1

Re: DP Capillary Across Filter Pad Cells at Different Heights. Cannot Zero Tx

02/26/2013 7:42 PM

What is the specific gravity of the fluid in the pipe?

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#2
In reply to #1

Re: DP Capillary Across Filter Pad Cells at Different Heights. Cannot Zero Tx

02/26/2013 7:56 PM

Hi.

The application is across a sweet gas cooler. Measuring the DP of said cooler. There is nothing passing through the cooler right now.

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#3

Re: DP Capillary Across Filter Pad Cells at Different Heights. Cannot Zero Tx

02/26/2013 10:53 PM

1) The reason that the transmitter sees a large value at zero applied pressure is that its range is incorrect.

The LRV and URV range for the transmitter does not take into account the effect of gravity on the seals' fill fluid which introduces substantial zero offset.

The low side fill pushes and applies a positive pressure to the low side; the high side fill pulls away from the diaphragm, applying a negative pressure to the high side.

Since the DP calculation is high side minus low side, with no process pressure applied, the low side positive value is subtracted from the high side negative value to get a fairly large negative value for an LRV.

The calculation of the zero and span is similar to that of closed tank, with the exception that
- the span of the 'liquid level' in the tank becomes the DP range you expect to see across the filter.
- the static head pressure at zero flow has to be accounted for, but with gas flow, the 12m of static head pressure of gas is probably small enough to ignore.

2) Moving the transmitter up or down in elevation with respect to the pad cells will not change the reading because of the positive-push-on-the-low side and negative-pull-on-the-high-side effect that the fill fluid has on the sensing diaphragms. It always adds up to the total elevation difference, as a negative value.

As paradoxical as it may seem, a DP transmitter can be mounted below, at or above the lower datum (lower pad cell) and the LRV and URV values remain the same (as long as there are equal length capillaries).

3) Any error in the difference in elevation from your "approx 12m in height" will show up as an DP offset error. Hence, precise measurement, center-to-center, of the seal mounting flanges (pad cells) difference in elevation is advised.

4) The example below shows a calculation of LRV and URV for a static tank liquid level application, where one measures the liquid level which is the liquid column's head pressure. The calculation accounts for the push-pull gravity effect of the fill fluid on the sensing diaphragms.

- 12.00 meters is used as the elevation difference between the pad cells (in units of inches)

- the medium used is water, with SG = 1.00 to keep the measuring units in inches w.c. units. This only effects the span value (URV) value.

- the DP transmitter is assumed to be somewhere between the lower and upper pad cells, although any change of this elevation does NOT alter the LRV or URV or readings (with equal length capillary tubing).

- I haven't a clue what your assumed range of DP across the filter so I arbitrarily selected a high span of 60Kpa because the range of your transmitter has that a span value.
expected DP = 60 kPa = 8.70 psi = 241 in. w.c.

- the example uses DC-200 fill fluid's 0.94 specific gravity

- the program uses only "inches water column" units, so
472 inches water column (in. w.c.) = 12.0m = (difference in elevation of pad cells) = 117.7 kPa
expected DP is 241 in. w.c. = 60.0 kPa

The calculations:
- the LRV (lower range value) is -443.7 in w.c. or -110.5 Kpa
- the URV (upper range value) is -202.7 in w.c. or -50.5 Kpa
- the 4-20mA output span would be 0 to 60.0 Kpa (241 in w.c.).

You need to re-range the transmitter to an LRV that accounts for the fill fluid elevation effect and a URV that is a range for an maximum expected DP across the filter.

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#4
In reply to #3

Re: DP Capillary Across Filter Pad Cells at Different Heights. Cannot Zero Tx

02/27/2013 2:01 PM

Hi Iris.

Thank you very much for your reply.

The calibration range of of the Tx is 0 to 60Kpa as per spec. The problem I have is the instrument range is -62.3Kpa to 62.3Kpa (LSL, USL)

The operating DP shall be 21Kpa and max 50Kpa.

note; The fill fluid for the capillaries is Silicon Oil.

I am suspecting maybe damage to one of the pad cells now......

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#5

Re: DP Capillary Across Filter Pad Cells at Different Heights. Cannot Zero Tx

02/27/2013 5:09 PM

DC200, DC704, DC705, and Sytherm XLT from a major transmitter vendor are all silicone oils, but they all have different specific gravities, ranging from 0.85 to 1.09.

But I doubt your problem is a fill fluid other than DC200, or pad damage.

When the capillaries are hanging vertically over 12 meters of elevation, the fill fluid must impart 12m of weight on the sensing diaphragms. That weight is substantially more than the range limit of the transmitter.

The LRV has to be the negative value of the weight of the fill fluid over its elevation (for upwards flow with the high side at the bottom, low side at the top).

The 60kPa of working DP range has to be 60kPa ABOVE the LRV. That's what the example values show: 110.5 - 50.5 = 60.0kPa

Ranging it 0-60 kPa only works when there is zero DP in a static state, but a static state for your installation is 12m of fill fluid weight.

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#6
In reply to #5

Re: DP Capillary Across Filter Pad Cells at Different Heights. Cannot Zero Tx

02/27/2013 7:43 PM

Iris

Thanks again for your replies, appreciate your time.

I checked the pad cell, correct no damage.

I need to order a transmitter with larger range limits.

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