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Orifice Plate Calculation

Orifice Plate Calculation

Orifice Plate Calculation

I designed and installed an orifice plate for 1.4MMSCFD on a 80bar 2" SCH80 natural gas line. I calculated the orifice with a beta of 0.61 and I used a 25"H2O differential pressure transmitter. When I did the test the DP transmitter saturated. I check the calculation and I did not find any mistake on the DP range. Any suggestion on what could be wrong?

RE: Orifice Plate Calculation

Things I've seen that produced a false DP high value reading:

- DP xmtr configured for 0-2.50"wc range, not 0-25.0"wc

- 3 valve manifold low side valve blocked (scale, crud) or not opened, xmtr sees only high side pressure

- Low side impulse tube crimped closed

- Site fabricated orifice flange union tap not drilled all the way through on the low side (yes, I've seen it)

- Xmtr mounted beneath the pipe, high side impulse tube filled with liquid

- Saturation is a fault-high condition (21mA) indicating a failed transmitter

- Honeywell DP - ST3000, failed terminal block

RE: Orifice Plate Calculation

Thank you Dan for the checking list. I will go through it in the next test I will do this Friday.

The strange thing is that the xmtr started to provide some readings while we were opening the block valve downstream the orifice plate, but it saturated in less than 10% of opening. Therefore I guess this leave out of the picture item 2,3 and 4. Item 5 was ok (xmtr process connections were connected to the top orifices of the orifice flanges), item 6 and 7 were OK (it was a hart xmtr and there was no error messages). I am double checking item 1, the instrument technician swear that the xmtr range was OK.

Keith I am attaching the Excel sheet I used for sizing the xmtr DP range.

Thank you

RE: Orifice Plate Calculation

You say it saturated at 10% of the block valve opening so obviously the DP range is not enough.
Do a simple test by isolating the transmitter and pressurizing the HP side measuring at the same time with a "U" tube water manometer made from a couple of feet of plastic tube, you should be able to blow hard enough to reach full scale, if not connect to the LP connection and suck it down.

I wound up with egg on my face once measuring oxygen flow, it turned out I was doing the square root in the DCS and also in the transmitter, check you aren't doing that also.

RE: Orifice Plate Calculation

Good idea for using U tube water manometer, it's quick and easy.

Double square rooting provides false high flow rates, but will not saturate the output because the square root is calculated on the normalized percentage of span and the square root of 1 = 1 no matter how many times you do it. The one point on the curve that is always the same is 100% of flow rate.

RE: Orifice Plate Calculation

Edited post posting (brain fart): mmH2O, mbar, mmHg engineering units will all read false highs and saturate the output for a 0-25.0 range

RE: Orifice Plate Calculation


you might check you design flow, I come up with less than 11 fps mass flow velocity for the meter at 25"wc, that is remarkably low

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