Flow meters based on differential pressure (DP) represent a popular choice in the chemical processing industries — in fact, they account for a large plurality of installations. DP meters are used for liquid, gas and steam metering in both normal and extreme high temperatures and pressures.

All DP primary elements restrict the flow in some way. A restriction in a pipe results in an increase in the fluid velocity, according to Bernoulli’s law of conservation of energy. The ensuing conversion to kinetic energy reduces the static pressure. This pressure drop, the measured DP, is proportional to the square root of the flow rate and thus provides a means to measure flow.

The elements have no moving parts and can be fabricated in a wide selection of materials. Their purchase price is relatively low, even for large pipe sizes. Accuracy is moderate, ranging from 1% to 5%; compensation techniques can improve these values to better than 1%. Extensive research over decades has led to optimization of the flow elements and development of standards. DP meters are generally easy to select for a specific application.

Disadvantages revolve around rangeability, installation costs, density and flow profiles. The square-root relationship limits the range of flow rates that realistically can be measured in a particular application; the typical rangeability is 4:1 or slightly higher. Installation requires a DP transmitter, manifold, valving and impulse lines. The impulse lines leading to the DP transmitter can become plugged unless remote seals and filled capillaries transfer the pressures to the transmitter. The measured value varies with fluid density for both volumetric and mass flow. Additionally, flow elements tend to be sensitive to flow profiles within the pipe, requiring long upstream pipe runs or flow-straightening devices.

While the orifice is the most common restriction associated with DP measurement, several others — venturi tubes, flow nozzles, wedges and flow tubes — have found a solid place in process applications. Users should look at these designs, too, to come up with the optimum DP meter for the particular operating conditions and requirements. An important consideration is the pressure drop, which as a rule of thumb should be as small as possible. Technologies such as venturi tubes, flow nozzles, wedges and flow tubes feature very small pressure drops, leading to reduced energy loss and pumping requirements. So, in this article, we’ll look more closely at these elements.

Figure 1. Classic Venturi Tube -- The outlet cone gradually diverges, permitting recovery of nearly all the initial pressure loss.

Venturi tubes
The classic venturi tube (Figure 1) is a robust flow element that’s useful for applications requiring low loss of line pressure. A venturi tube essentially is a section of pipe with a converging conical entrance (about 20°), a straight cylindrical throat section and a diverging conical exit with a smooth, gradually increasing (about 7°) diameter. Unlike the orifice, the interior surfaces always remain in contact with the fluid. Additionally, because dirt won’t build up as it passes through the contoured sections (as it does in the front of an orifice), this differential producer can serve in dirty flow applications. For the same differential pressure, the classic venturi can pass about 60% more flow than an orifice plate. Years of test data have documented and validated the flow coefficients for various sizes and fluids.