Look Beyond Orifice Plates

Consider other types of elements for differential pressure measurement.

By By Greg Livelli and Steve Pagano, ABB Instrumentation

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Vendors can make nozzles from any machinable material, such as aluminum, fiber glass, stainless steel and chrome-alloy steel. The bevel on the discharge side of the nozzle is a critical point of manufacture. The throat should be perfectly round with no taper. The standard surface finish is 16 roughage measurement system (RMS). The standard nozzle coefficient is 0.9962, which users can adjust for actual beta ratio and throat Re.

A special variation known as a venturi nozzle combines the 1932 ISA nozzle inlet profile with the divergent cone of a venturi tube. Taps in the throat transmit the lower DP pressure. This nozzle can serve as a secondary flow-rate standard when experiencing “choked” (sonic velocity) flow. It’s also commonly used to test steam turbines.

The wedge element consists of a V-shaped restriction welded into the top of the meter body (Figure 3). In profile the wedge looks like a segmental orifice plate to the incoming fluid; particulate matter or entrained gases easily pass through. This basic meter has been on the market for more than 40 years and has proven its ability to handle tough, dirty fluids. The slanted faces of the wedge provide self-scouring action and minimize damage from impact with secondary phases. The fixed-body design provides a constant discharge coefficient over a wide range, 8:1, which is relatively high for a DP element. Accuracies to ± 0.5% of full scale are possible.

wedge element flow meter
Figure 3. Wedge -- The shape of the restriction allows
solids to pass easily, while its ruggedness resists damage.

The wedge meter is popular for oil and gas applications, especially in production fields. For difficult fluids it can be equipped with a pair of remote seals that effectively isolate the metered fluid from the DP transmitter without affecting accuracy while keeping the flowing fluid contained within the pipe.

Wedge meters are characterized by an H/D ratio (analogous to the orifice’s beta ratio), where H is the height of the restricted opening and D is the unrestricted inside diameter of the wedge element (Figure 3). The ability of the wedge to produce varying DP ranges depends on the H/D ratio selected. Elements come with ratios in fixed steps (0.2, 0.3, 0.4, etc.) that allow a wide range of element sizing for a given pipe size. The meter coefficient is established at the time of factory calibration (in water).

Some wedge meters can handle flows with Re as low as 500, which is very helpful in metering slurries and liquids with high viscosities, and also allow measuring flows bidirectionally with the same degree of accuracy.
Wedge elements can be supplied with remote seals having large diameter diaphragms, affording more sensitive response to DP changes while eliminating plugging of impulse lines. These wafer-type seal connections are raised off the meter body and may be preferable for more aggressive and erosive applications. A second design permits use of isolating seals that can seat with diaphragms flush to the meter body. This seal arrangement keeps process fluid contained within the wedge element, with no dead zones under the seals where sludge and waste can build up. If the element is sized to have enough fluid velocity, a natural washing action will occur over the seal diaphragms and restriction, keeping the meter clean and sustaining maximum performance.

Like the nozzle, the wedge doesn’t rely on a restriction with sharp edges or machined bores. As a result, it will perform for long periods of service without the need for maintenance and repairs. Particulates and solid debris easily pass under the V-shaped wedge and the inherent ruggedness of the restriction resists damage to its measuring edge, sustaining the initial calibrated accuracy.
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