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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The Rosemount conditioning plates DO work, and I have had several installations immediately downstream from pump/check valve assemblies (6 D, 2D) which checked well against a turbine meter - minimum-flow installation within a piping layout that forgot the meter runs!

Permanent loss is close to that of a single orifice with the same nominal beta and dp. The sizing equation gives this.

Ian H. Gibson
gibs0108@optusnet.com.au

A: Use the Darcy Equation (an approximation only, because velocity immediately at the outlet of the orifice is unknown. Normal calculation of outlet velocity is based on mature velocity profile, such as at 10D downstream of orifice):

[(2.gc.144.ΔP) / ρ] = 1.5 v2
where v is the inlet velocity in feet/second
P is pressure in psi
ΔP is pressure drop in psi
p is density in lbm/ft3
gc is universal gravitational constant, 32.2 (lbm ft)/(lbf.s2)
1.5 is the loss coefficient of the orifice plate with 4 holes. (Reference to Crane Technical Publication 410, inlet loss coefficient of sudden contraction is 0.5; outlet loss coefficient of sudden expansion is 1).

These four holes are in parallel. Therefore, their effective loss coefficient equals to that of one orifice (of equivalent hydraulic radius due to the four holes).

The loss coefficient due to line loss is usually negligible compared to the loss coefficient of the orifice.

Additionally, note that the orifice plate with four holes was not in any approved standard.

I hope to see experiments showing 4D is all that is required for establishing mature velocity profile for the orifice plate with 4 holes.

Gerald Liu, P. Eng.
gerald.liu@shaw.ca

A: To connect the DP transmitter to the orifice plate for all liquids, the connection must be under the midpoint of the pipe to avoid air/gas entrapment in the sensing line.

All meters will require some upstream and downstream pipe sizing. With flow conditioners, you can reduce the upstream/downstream lengths to minimal lengths, but still some.

The orifice arrangement by Emerson/Rosemount has a flow conditioner in the orifice plate holder arrangement. The accuracy in such a flow transmitter will be dependent on the stability of the flow as well as having as little variation as possible of the process variables. The conditioner compensates for upstream and downstream lengths in the conditioning section, which is generally much less than the standard upstream and downstream pipe lengths.

One thing to take into consideration is that such a conditioning element, be it a flow conditioner, flow vanes, etc., will make the transmitter/orifice plate arrangement more expensive, but with your constraints of dimensions it might be worth it.

Alejandro Varga
vargaalex@yahoo.com

A: If you use a DP with remote diaphragm seals/capillaries and flanged tappings on the orifice flanges, you can mount above the orifice. You would have the tappings coming off the sides.

Discuss with the vendor, who would have the sizing program. This would be a specialist sizing program developed by the vendor and is not covered by industry standards or a generalized sizing program.

Simon Lucchini
Simon.Lucchini@Fluor.com

A: I have not personally had an issue with not having the pipe tap below the pipe, but we once had a major issue with the lack of free pipe downstream. We had 4-5 D straight pipe downstream and then a 90° bend. The D/P cell told us we had reverse flow of water, which could not happen in a 500-PSI pressurized system. The eddy currents at the elbow really messed up the flow measurements.

Bruce Land
Bruce.Land@jhuapl.edu

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