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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Four or more high pressure taps in an annular chamber leading to the straight throat section average the lower pressure reading. Initially designed for large line size (>6 in.) water and wastewater applications, the venturi today ranges in line sizes from 2 in. to 48 in.; installation possibilities include flanged, welded and threaded-end fittings. Manufacturers generally machine the smaller sizes from solid rods, while fabricating the larger sizes from rolled plate. The lengths of the elements typically run five pipe diameters.

Because most of the pressure recovers, the venturi is a good choice for large flows where the velocity is higher and Reynolds number, Re, is in the turbulent flow regime. And while it has a relatively long length, the venturi tube requires minimal upstream flow profiling, as its interior shape helps to condition the flow. Rangeability, while better than that of orifice plates, is less than 6:1, with typical accuracies of ±1% to 2% of full scale.

Variations of the classic venturi are available. Shorter versions increase the angle of the outlet cone with some sacrifice in pressure recovery. Eccentric inlet and outlet cones can handle mixed phases or build-up of heavy materials. Forms with a rectangular cross-section often serve in ductwork for gaseous flows.

Flow nozzles
Like the orifice and venturi tube, these are standard DP elements with extensive generic testing and documentation. Because of their rigidity, flow nozzles are dimensionally more stable at higher temperatures and velocities than an orifice plate. They typically measure fast flows that otherwise might damage an orifice plate from cavitation or erosion. Applications include high velocity steam or fluids with entrained solids.

Nozzles aren’t recommended for slurries or dirty fluids that might foul pressure taps. In contrast to an orifice plate, flow nozzles have no sharp edges that might wear over time and degrade performance — so they maintain long term accuracy and offer reduced possibility of distortion. Throat Re should exceed 10,000, although data are available down to about 6,000.

The initial cost of a flow nozzle is substantially higher than that of an orifice plate but lower than that of a venturi. However, the permanent pressure loss is significantly greater than that of a venturi. A flow nozzle will pass about 60% more flow than an orifice plate of the same diameter and DP. Because of the nozzle’s streamlined interior, unrecoverable pressure loss is slightly less than that of an orifice but still can range to 40% or more of the DP.

The standards for nozzles include ones for the so-called 1932 ISA nozzle, which is uncommon in the U.S., as well as the long radius nozzle.

flow nozzles
Figure 2. Flow Nozzles -- Low (right drawing) and high
beta ratio long-radius nozzles feature elliptically
shaped inlets.

The long-radius flow nozzle predominates in the U.S. It has a converging section that is a quarter ellipse followed by a cylindrical throat section, and comes in two design variations — one with a low beta ratio (throat/inlet-diameter ratios between 0.20 and 0.50) and the other with a high beta ratio (throat/inlet-diameter ratios between 0.45 and 0.80). The difference in geometry is a flattening of the elliptically shaped inlet in the high-beta-ratio version (Figure 2). The American Society of Mechanical Engineers (ASME) has developed standards for the nozzle geometries based on the beta ratio desired for the application.

Users either may weld the nozzles into the pipeline or mount them with a holding ring between flanges. Where inspections are required, the flange mounting provides accessibility. In the U.S., the DP taps are commonly found one pipe diameter upstream and one-half pipe diameter downstream from the inlet. Flow nozzles may be installed in any position. Vertical downward flows better suit wet steam or gases and liquids with suspended solids. Upstream and downstream piping requirements for flow conditioning are similar to those for orifices.
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