Specialty Item Flow Measuring Devices

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Flow-measuring devices are used in a piping system to determine accurate flow rates, where balancing valves may not be accurate enough for the desired results. Flow-measuring devices may also be part of the operating system specification because a specific flow rate may be critical for proper operation of the process.

There are two configurations for flow-measuring devices: linear scale and square-root scale. The difference between the two configurations lies in the construction of the device which dictates the relationship between the pressure drop across the device and the flow rate. There are two types of linear-scale, flow-measuring devices: the variable area flowmeter (rotameters) and the turbine flowmeter. For linear-scale devices, the relationship between pressure drop and flow rate are directly proportional.

There are five types of square-root-scale flow-measuring devices: orifice plate, venturi, pitot tube, flow nozzle, and annular flow tube. For square-root-scale devices, the relationship between pressure drop and flow rate is the square root of the pressure drop.

Linear-Scale, Flow-Measuring Devices
Rotameter or floatmeter — The most popular linear-scale flowmeter is the rotameter or floatmeter. This arrangement consists of a section of see-through pipe that has a float inside. The meter must be mounted vertically to allow the float to move unobstructed in the pipe. When fluid flows past this section of pipe, the float will rise in the pipe. The weight and drag of the float will cause it to stop traveling through the length of the pipe. The see-through pipe will have a scale, usually in gallons per minute (gpm), to determine the flow rate. The rotameter is calibrated to the particular fluid that is in the pipe. If the fluid type is changed, the meter will require recalibration.

Other linear-scale, float-measuring devices — Other linear scale flowmeters are turbine flowmeters and electromagnetic flow devices, which use electricity to create a voltage across the pipe that is proportional to the velocity of the fluid flowing through the pipe. These types do not have to be mounted vertically. This type of measuring device is well-suited to applications where the fluid is toxic or corrosive. The pressure drop through this device is not severe.

Square-Root-Scale, Flow-Measuring Devices
Orifice-Plate, Square-Root-Scale, Flow-Measuring Device — An orifice plate is a simple device that consists of a thin plate with a hole through its middle which is clamped between two pipe flanges. Since the hole in the plate is smaller than the pipe diameter, a pressure drop occurs across the device. Each side of the orifice plate must have a pressure tap; some orifice plates may have this as part of the assembly. Flow rate is determined by taking a reading of the pressure drop across the orifice and applying the Cv coefficient. The Cv coefficient is the valve-flow rate in gallons per minute (gpm) that will cause a pressure drop of 1 psi across the device. The orifice plate is calibrated and tagged by the manufacturer stating the valve’s Cv coefficient, or the proper place to find additional information.

Abrupt changes in flow will cause a buildup of impurities to occur on the inlet side. If the fluid is very contaminated, flow characteristics may change the properties of the orifice plate. The orifice-plate, square-root-scale, flow-measuring device is inexpensive, however, the high pressure drop across the orifice is a disadvantage.

Venturi-Meter, Square-Root-Scale, Flow-Measuring Device — A venturi meter operates on the same principal as an orifice plate but it is shaped in such a way as to avoid the great pressure drop of an orifice plate. To accomplish this, the venturi shape is on the order of a modified hour glass. This shape gradually decreases the diameter of the pipe section, thus creating an increase in the velocity, which will create a change in pressure.

One pressure tap will be located right before the venturi (upstream) and the other tap will be at the narrowest part of the venturi. Pressure changes are measured to determine the flow rate. The shape also makes the device self-cleaning. This device will also be calibrated and tagged by the manufacturer with a Cv coefficient. This device has to be milled to close tolerances for reliable service. Due to these close tolerances, this device is more expensive than an orifice-plate measuring device.

Pitot-Tube, Square-Root-Scale, Flow-Measuring Device — The last type of fluid-measuring device is a pitot tube. This device operates on the principles of an air-flow pitot tube. The only difference is that fluid is in the pipe instead of air in the duct. The pitot has two pressure taps: the first measures total pressure and the second measures static pressure. By subtracting the static pressure from the total pressure, the velocity pressure is found. Depending on pitot construction, it may be an averaging pitot or a pitot that has to be traversed across the pipe. The averaging pitot does exactly what its name states; it will be calibrated to give the average velocity pressure in the pipe.

The traverse pitot has to be moved manually to obtain a pressure drop profile across the pipe, then the pressure drops have to be converted to an average pressure drop. This is time-consuming and requires additional labor to take the readings, but the device has a moderate cost and will give reasonable results. The disadvantage of this device is that if the fluid is contaminated or has some suspended particles, the holes in the pitot may become blocked and greatly affect the pressure-drop reading. Thus, the device will require some maintenance and cleaning. There is no Cv rating for this device since the velocity pressure is being measured directly.

Flow-Measuring Device Pressure Fittings
All flow-measuring devices, except the linear flowmeter, require some form of pressure-tap fittings. Several pressure tap fittings are being used such as Schrader valves, Pete’s plugs, and others. The flow-measuring device manufacturer may also have installed a type of pressure-tap fitting. Pressure-tap fittings affect the type of hose connections. The operator taking the readings must determine the type used before beginning the tests.

Flow-Measuring Device Installation
When installing flow-measuring devices, the type of working fluid must be known in order to have the proper Cv ratings for that particular fluid. For example, there will be a significant difference between water and oil for these values. This information must be given so that the manufacturer can best recommend the proper device and Cv coefficients.

Flow measuring devices work best when the fluid is moving through the pipe smoothly in a laminar and symmetrical fashion. To accomplish laminar and symmetrical flow, none of these devices should be installed immediately following any fitting or device that may affect the flow, such as an elbow, reducer, or valve. The manufacturer may request that the device be installed in a certain configuration to produce the proper pressure readings. 

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