Relief Valve Blowdown Rings; D/P and Straight Runs

Readers Ask: When It Comes to Safety Relief Valves, What Is Best? Using Two Blowdown Rings or Ones? and Can a Transmitter Be Installed Above Pipe Taps?

By Bela Liptak

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Secondly, we do not have the ISO 5167 straight-run requirements up and downstream. Is there any flowmeter which will work without that?

Lastly, Rosemount has an new model orifice plate (Figure 2) that needs only four pipe diameters upstream and downstream, but I can't find information about its pressure drop? Is there a way I calculate its pressure drop?

Hassan Mommadzadeh

A: A horizontal takeoff from one side will work adequately—a slight slope (1:10) up to the tapping points will ensure any air bubbles can be displaced.

The Rosemount conditioning plates do work, and I have had several installations immediately downstream from pump/checkvalve assemblies (6D, 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

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

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 four holes (Figure 2).

(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 that of one orifice of equivalent hydraulic radius. The line loss coefficient is usually negligible compared to the loss coefficient of the orifice. 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 four holes.

Gerald Liu, P. Eng.

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 (Figure 2).

The orifice arrangement by Emerson/Rosemount has a flow conditioner in the orifice plate holder, the accuracy of which will be dependent on the stability of the flow and the process variables. The conditioner compensates for upstream and downstream lengths. The conditioning element will make the system more expensive, but with your constraints, it might be worth it.

Alejandro Varga

Figure 3
Figure 3: On right: preferred, on left: alternate connection.

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. Another option is a Coriolis Mass flow meter.

Simon Lucchini

A: I have not personally had 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-degree bend.  he dP 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

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