Flow Meter: Remember the Old Reliable Orifice Plate

Sept. 11, 2006
The cost-effective dP flow meter is ideal for utility surveys, Senior Editor Dirk Willard says in this month's Chemical Processing Field Notes column.

It sounded like sand caught in a vacuum cleaner! I approached the gas pipe cautiously. My interest was captured by a set of flanges and copper tubing with a chart recorder in a glass case. The tubing vibrated with the rattle of the pipe: it was a differential pressure (dP) orifice flow meter.

I was a young engineer, full of curiosity, so I talked to my supervisor after I finished doing my rounds of the oil wells. Bud Sweeney handed me an old Singer-American Meter manual. This is the bible on orifice plates, which allows sizing from a series of empirical tables and charts. With some reluctance, he indulged my questions. The sound was sand. An orifice meter was chosen because it required no electricity, except the chart recorder, and because it was tough — sand would not destroy it, at least not quickly.

Since devouring the Singer manual I have learned much more about dP flow meters.

To size an orifice plate, you must know the ß (orifice diameter/pipe ID), the pressure drop at 100% span (typically, 50 to100 inches of water column (IWC) at 68°F) and the physical properties of the liquid or gas. Properties include: density, for liquid; for a gas, molecular weight, Cp, heat capacity, and the critical properties (Pc, Tc, w). The initial temperature and pressure also are required. The span, or differential, should be sufficient for accurate measurement without a high pressure drop. For a gas with a ß of 0.3, 85% of the differential pressure will be lost; for a ß of 0.7, 50% of the differential will be lost. 0.3 to 0.7 represents a good practical range for measurement with orifice plates. The sizing equation presented in Singer contains almost a dozen coefficients and is too complex to present here (several references are available, e.g., ASME MFC-14M-2001).

With the increased demand for cutting utility costs, the advantages of the orifice meter often outweigh its disadvantages. Compared to other flow meters, such as a vortex meter, it is inexpensive, the straight-pipe required is about the same (15-dia. upstream, 10-dia. downstream), plates are available at any pipe size, unlike other flow meters, in a wide range of materials, including cladding, and flanges are so cheap that they can be installed in-place for future measurements. Now, for the disadvantages: rangeability, or turndown is limited to about 1:5 for reasonable accuracy; the fluid should be clean and non-condensing; pressure drop can be significant for some systems, such as fans or blowers (limiting dP to ~ 5 IWC); flange tubing can leak, plug or corrode more easily than pipe; orifices can be eroded with time; the slow responsiveness of the diaphragm and fill means that sudden changes in flowrate cannot be measured (time < 0.1 sec.); detailed engineering calculations are needed for accurate measurement — compared to some flow meters, the relationship between flow rate and the parameter being measured, pressure, is non-linear and complex, a 1-2% is the best accuracy possible because of knowledge of physical properties and conditions, turbulent flow is required at the orifice (Re > 10,000) or significant corrections come to bear which reduce accuracy and the fluid viscosity should be less than 50 cP.

Orifice meters can be used for condensing vapors but special consideration is required. Drain holes can be drilled into the bottom of the plates to prevent condensate from interfering with measurement. While there is no hard and fast rule, a plate that has a hole for drainage can tolerate a maximum of about 1% (by mass) condensation; sometimes a vent hole is added for liquid service for two-phase flow.

The meter and tubing can be insulated to prevent excessive condensate from building up. Care must be taken with insulated tubing because insulation makes leak detection difficult. Regardless of the improvements, accuracy will suffer when orifices are used to measure a condensing vapor. In one case, I showed an error of up to 10% in measuring the chlorine flow from a vaporizer. I developed programming changes to incorporate a value of Z, the gas compressibility factor, in the measurement.

Many recent improvements in orifice meters have enhanced their capabilities. Modern transmitters can measure changes in pressure and temperature across the plate, compensating for these errors and providing accuracies and turndowns competitive with vortex meters, but not quite as good as Coriolis meters. This measurement of temperature and pressure (density) changes allows determination of mass flow. All of these changes come with a cost, which, in my opinion, isn’t worth it for utility measurement.

The usefulness of the orifice dP meter in defining utility cost is unparalleled. In one study, I watched an engineer install orifices throughout a nitrogen supply network to identify where use could be cut. We applied a similar approach to define refrigeration capacity in a chiller system. The durability of the orifice flow meter in the marketplace is testament to its low cost, ruggedness and adaptability.

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