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Video tutorial: Differential pressure flowmeters

Author : Peter Welander, process industries editor, Control Engineering

23 February 2010

Using differential pressure to calculate flow is a very basic and common method. Here's how it works.

Measuring flow with differential pressure
Measuring flow with differential pressure

One of the most basic ways to measure fluid flow is by using a differential pressure measurement of fluid pressure up- and down-stream of an obstruction that creates a pressure drop. The pressure drop will be proportional to the amount of fluid flowing through the pipe, so the greater the flow rate, the greater will be the pressure difference. (The concept works with liquid or gas, but having a compressible fluid makes it somewhat more complicated. For purposes of this discussion, we'll stick with liquids.)

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A simple way to introduce such an obstruction is a plain orifice disk, where a device is inserted in the pipe with an opening that is somewhat smaller than the normal cross sectional area of the pipe. Liquid flowing through the orifice loses some pressure. That loss can be measured using a differential pressure device comparing readings above and below the obstruction.

This approach is very common commercially, and companies that have refined the technology have found sophisticated ways to optimise orifice size and shape, select the most desirable pressure measurement points, and otherwise create the highest accuracy and turn-down ratio with the least actual pressure loss.

There are many advantages to this approach:
• Simple sensor that can be compact with no moving parts;
• Bi-directional flow is possible;
• Scalable over a very wide range of sizes and flow rates; and
• Relatively inexpensive.
Of course there are drawbacks and limitations too:
• Limited accuracy and turndown range-If high precision is your primary objective, there are probably better technologies;
• Causes a pressure drop-Compensation for this may require higher pumping energy;
• Reduces the pipe's free passage-As sensor technologies go, this is one of the more invasive, but the extent varies among manufacturers;
• Accuracy and turn-down are related to pressure drop-In other words, the more accuracy and range you need, the greater pressure drop you will have to tolerate; and
• Orifice is prone to wear-While there are no moving parts, the orifice itself is often a wear-prone point due to the high liquid velocity. If the orifice gradually enlarges with wear, the flow measurement will read low. On the other hand, if it becomes partly obstructed with debris, the measurement will read higher. The extent of this also depends on the configuration.

The same application and installation guidelines for good pressure sensor practice apply here. Anything that interferes with the pressure readings, such as clogged impulse lines, will interfere with a true flow calculation. Additionally, changes in fluid characteristics, including viscosity, density, multiphase flow, and even temperature, can affect readings. Of course these are generalities, so discuss your specific application needs with any prospective suppliers.

The video shows a demonstration of a home-made sensor that illustrates the basic concepts.


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