Putting a squeeze on process
By Thomas Collins
In today's industrial facilities, knowing the flow rate of a process fluid, or gas, is crucial to production. When problems with flow occur, companies can end up losing millions of dollars. To avoid this situation, you must measure flow precisely.
The most common devices for inferring an actual flow value in industry today are orifice plates, also known as restrictors. Their precision and inexpensive nature cause them to have a predominant influence in industry in all parts of the world.
These mechanical flow measuring instruments are fairly simple when it comes to their theory of operation. Basically an obstruction, in this case an orifice plate (a thin, concentric square-edged reducer inside a pipe), is placed between two flanges and allowed to contact the process fluid. For a better understanding, imagine when you place your thumb over the end of a water hose. What is actually happening is the velocity of the fluid increases because the area at the end of the hose decreases. This causes the pressure inside the line to be greater than the pressure on the other side of your thumb. The measurement aspect is just as easy to grasp.
The principal operation for these devices comes from Bernoulli's principle, in essence, the Ohm's law for the flow of fluids. It states Q  AV, where A is the size of the restrictor; V equals the velocity of the fluid, and Q is the flow rate of the fluid.
This formula states when the area a fluid is flowing through decreases, its velocity increases. Furthermore, when you place a constrictor in the line, not only will the velocity increase, but a significant pressure drop forms across obstruction. This pressure drop can be directly related to percent of flow. The reasoning behind this phenomenon is simple; static pressure, which is the force measured from a stationary point, decreases whenever atoms travel in a more uniform manner. In other words, the atoms flow. Most people might think because the velocity increases, the static pressure would increase as well. But it is not static pressure that increases; rather Ram pressure.
Ram pressure is pressure related to atoms in motion, or the force or momentum of atoms while flowing. Think of a fan with different settings-high, medium, and low. The Ram pressure would be far greater on the high setting versus the low setting. This illustrates another part of Bernoulli's principle, which states Ram pressure is directly proportional to flow; therefore when velocity increases, the momentum of the fluid increases as well. Ram pressure should not be totally neglected, but it is not a great deal of concern in these head-type measuring devices because the velocities of most process fluids usually hold between 8-10 feet per static pressure exerted on the fluids container.
The construction and use of a differential pressure producer is also fairly simple. You would machine holes in the flanges on each side of the orifice plate, and place taps inside these holds. On the downstream side, you would take a pressure reading, and subtract from the upstream side (hence the name differential pressure) by taking the square root of this value (√∆) and inferring the percent of flow through the orifice. If the upstream side of the transmitter reads 100 inches of water, and the downstream reads 50 inches of water, you simply take the square root of the difference, which is in percent. This would infer a flow value of 70.70% of flow (√ (50/100) = 70.70%).
ABOUT THE AUTHOR
Thomas Collins (firstname.lastname@example.org) is studying instrumentation and electrical systems at Texas State Technical College in Waco, Tex. He is also president of the ISA student section in Waco.
Differential pressure: Orifice plates
Orifices are easy to install. One differential pressure (dp) transmitter will apply for any pipe size, and quite a few materials are available to meet process requirements. Type 316 stainless steel is the most common material used in orifice plates unless the process conditions require material of higher quality. Orifice plates have no moving parts, have been researched extensively, and their application data has been well documented.
However, the disadvantages include process fluid in the impulse line, allowing a potential for freezing and plugging (unless you use chemical seals). Changes in density, viscosity, and temperature affect their accuracy, and they require frequent calibration.
The orifice plate typically has a drain hole located at the bottom for steam and gas applications (to drain condensables) and a vent hole on the top for liquid applications (to let gas bubbles through).
A square-edged orifice plate consists of a flat piece of metal in which you would bore a concentric or eccentric hole. Fluid flow creates a dp across the plate. The square root of the dp is proportional to flow. A common value that sees use in orifice plate measurement is the beta ratio, which is equal to the inner diameter of the orifice divided by the inner diameter of the pipe. Typically, the beta ratio should be within 0.3 to 0.7 and the dp between 25 and 200"WC (600 and 5,000 mmWC).
SOURCE: The Condensed Handbook of Measurement and Control, 3rd Edition, by N.E. Battikha (2007, ISA Press).
∆P: Change in pressure P1-P2
RAM pressure: Force exerted from a fluid when it is in motion
Static pressure: Pressure measured from a stationary point, at steady conditions