Pad Type D/P Level Installation; Orifice Plate Location

How Reliable is a Pad-Type Flange? Can It Withstand Pressure and Leakage? Also, Is There Any Way to Locate a D/P Transmitter above the Pipe Tapping Point?

By Bela Liptak

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Q: My current project uses a pad-type flange connection for Level DP on tanks and exchangers. My concern is the reliability of a pad-type flange to withstand pressure and leakage. Will this affect the installation of my transmitter? And does it require additional consideration?

Glai Mendoza

A: I had no leakage problems with the many flat or extended diaphragm d/p transmitters, which I have used on viscous, sludge, slurry or other plugging services (Figure 1). If the low-pressure side of the d/p cell also required protection from plugging I also often used dual chemical seals (Figure 2), which are convenient if one wants to locate the transmitter above or away from the tank.

Also Read: Guided Wave Radar vs. Differential Pressure Transmitters for Liquid Level

Béla Lipták

A: I see two questions. The first is the structural one. Designed and fabricated per the codes and standards, pads have been used for a long time and are considered reliable and safe. The only disadvantage I see is that the threaded holes for the studs could be damaged and are difficult to repair.

The instrument issue is that without a nozzle to accumulate "stuff" the measurement might be somewhat more reliable, especially if the pressure connection is a remote diaphragm.

Beyond that, I see no problems unless there are other significant aspects not mentioned in the question.

Cullen Langford

A: By your mail, I understand that you have a chemical seal capsule and capillary connected transmitters. If so, we have used these without any problem of leakages or extra maintenance issues. However you may need a block-and-bleed manifold to release the pressure before you open the flange, for sake of safety.

H S Gambhir

Q: Straight pipe run for conditioning orifices

I am supposed to install an orifice plate to measure the water flow in a pipe, which is 0.3 m above ground. Therefore we do not have room to install the d/p transmitter below the pipe. Is there any way to locate a d/p transmitter above the pipe tapping point?

As per ISO 5167 we should provide straight up and downstream pipe run, but we don’t have enough space for this either. What should we do? Rosemount has a new model orifice plate that needs only 4 pipe diameters total upstream and downstream, but I can’t find anything about its pressure drop. Is there a way I calculate its pressure drop?

Also Read: Sealing an Oil Well; Orifice Sizing--OSO versus AGA

Hassan Mommadzadeh

A: The Rosemount compact orifice meter design (Figure 3) allows one to locate the d/p cell above the pipe. The manufacturer claims that the inaccuracy of this flowmeter is under 1% of actual flow over a flow range of 8:1 and that it requires only two straight pipe diameters up and downstream when installed. I emphasize that these capabilities are only claimed by the supplier, and I have not seen any reports from any independent testing laboratories that supported these claims. Similarly, the claim that two pipe diameters of straight pipe are sufficient both up and downstream is hard to believe, and I would like to see that substantiated by independent testing when 90º elbows are up and downstream.

My understanding is that the up- and downstream pressures are measured by corner taps right on the two sides of the orifice plate, and therefore, the performance is likely to be similar to corner tap installations (Figure 4).

As shown in Figure 5, the transmitter has to be installed 50° below the horizontal (to protect against gas accumulation). You probably have enough room to do that, but if your water is not clean, I would not use this flowmeter design, because of the plugging of the pressure taps.

As to pressure drop, it is similar to that of the traditional orifice at the same beta ratios.

Béla Lipták

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