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DIFFERENTIAL PRESSURE FLOWMETERS
Differential pressure flowmeters use various methods of obstructing flow to create a pressure drop across a section of pipe. The pressure drop is proportional to the square of the fluid velocity.

VARIABLE-AREA
Variable-area flowmeters are among the most popular flowmeters because of their straightforward, direct reading. VARIABLE-AREA Variable-area flowmeters are among the most popular flowmeters because of their straightforward, direct reading.



GENERAL INFORMATION
In variable-area meters, fluid flowing through a slightly tapered tube causes a float (as in the animation below left), piston, or vane to change position inside the tube, adjusting the area available for fluid to pass through. The fluid velocity determines the final float position.

EQUIPMENT DESIGN
Rotameters such as the one shown above right contain a float inside a tapered tube to indicate flowrate. The taper is so slight it's very difficult to see. When a balance is reached between the upward force of the flowing stream and the float's weight, the float is stable. The scaled or direct flowrate is read from the markings on the tube. The higher the float, the higher the flowrate. A number of tube and float shapes are used to compensate for various flow conditions.

Piston meters have a spring-loaded piston mounted on a tapered shaft in place of a float, as shown below. Fluid flow pushes the piston against the spring until the pressure forces are balanced such that the annular area between the piston and the shaft is proportional to the flow rate.

Vane meters are also spring-loaded. Fluid flow causes the vane to rotate against the spring to increase the orifice area in a chamber through which the fluid flows, as shown below.

Piston meters and vane meters can either be outfitted with electrical sensing equipment, or have a pointer attached to the shaft of the spring mechanism to indicate the flowrate. In some piston meters, such as rotameters, flowrate can be read directly from tube markings. USAGE EXAMPLES The vane meter to the left monitors high pressure water flows. The rotameter to the right is made of plastic, making it capable of handling strong chemicals and corrosive materials. The piston meters shown here are similar in design, but calibrated to handle different materials. The one on the left meters water while the one on the right meters oil. Variable-area meters such as these can also be used to indicate flow of concrete additives, anesthesia delivery, suction in portable halogen leak detectors, and flow of pressurized dry air used to keep moisture out of telephone cables. Return to Top   ORIFICES GENERAL INFORMATION/EQUIPMENT DESIGN Orifices, also called orifice plates, constrict fluid flow using a flat metal disc with a circular hole in the center, such as the one in the center. This constriction causes a pressure drop across the plate. Pressure taps on both sides of the orifice measure the differential. The left picture shows an orifice plate installed between flanges in a pipe. Different flow conditions are accommodated by changing the location of the orifice in the plate and the way the edge is bored, as to the right. USAGE EXAMPLES Most gas, water, steam, and air applications are easily metered by orifice plates. Orifices such as the one left, made of bronze, and right, made of bronze and cast iron, can be used to meter lube oil, cooling water systems, and compressed air flow. The two orifice meters below are made of stainless steel and are specially designed to handle corrosive materials, such as strong acids and bases. Return to Top   FLOW NOZZLES GENERAL INFORMATION/EQUIPMENT DESIGN Flow nozzles are similar to orifice plates in that they contain a circular hole, smaller than and concentric with the pipe diameter, that constricts fluid flow, causing a pressure differential. In flow nozzles, however, the nozzle follows the hole, reducing the pressure differential across the device. Flow nozzles are typically used to meter steam and gases or liquids with a small degree of suspended solids. Return to Top   VENTURI & FLOW TUBES GENERAL INFORMATION/EQUIPMENT DESIGN Venturi tubes are specially shaped pipe sections with tapered inlets and outlets, and straight throats. As the pipe diameter decreases, the fluid velocity increases, causing a pressure drop proportional to the flowrate across the venturi tube's throat. Flow tubes operate on the same principle as venturi tubes, but differ in shape. Flow tubes have no tapered entrance, but they do have a tapered throat as well as a long tapered exit. USAGE EXAMPLES Venturi tubes such as the one to the left are common in water and wastewater treatment facilities, power plants, and chemical and petrochemical processing operations. The flow tube at right is designed to meter fluids such as cooling water, steam, and combustion air. Return to Top</a> <hr align="left" width="200"> <a name="PITOT TUBES"></a> PITOT TUBES GENERAL INFORMATION/EQUIPMENT DESIGN Pitot tubes measure the difference between impact pressure in a flowing stream and static pressure at the pipe wall. The impact tube's end is bent at a right angle to face the flow and has a hole in the tip, as shown in the animation. Fluid flows into the hole and up the length of the tube. The static tube is straight and has one or several holes along its length, as shown in the picture. The height of the liquid in the tube indicates the pressure. For more information about how pitot tubes work, see the Pressure Measurement section of this encyclopedia. USAGE EXAMPLES Pitot tubes are used industrially to meter clean liquids, gases and steam. They are also frequently used to measure air velocity, as in the plane to the left and NASA's X-31 fighter jet to the right. The jet travels at speeds of 1000 mph at an altitude of 40,000 ft. Most jets can only travel at 250-500 mph. Click in the red box for a closer look at the jet's pitot tube. <a href="#top">Return to Top</a> <hr align="left" width="200"> <a name="TARGET"></a> TARGET GENERAL INFORMATION/EQUIPMENT DESIGN Target meters measure the force of a flowing stream on a target, usually a vane or disc, suspended in the stream. The force on the target, measured by a strain gauge, is directly proportional to the stream's velocity. The animation (above right) shows a target flow meter being used for solid particles. The gray plate measures the force the particles exert as they flow through the meter. The magnitude of the force depends on the velocity, angle of impact, mass, and energy absorbing tendencies of the solids. The force is proportional to the flowrate. USAGE EXAMPLES Target meters are among the very few flowmeters capable of measuring solids flow. The pictures show this type of flowmeter. The one on the left is used to meter pulverized coal and coke. The one on the right meters plastic pellets. In addition to metering industrial liquid, gas, and steam flows, target meters can be used in the food and beverage, dairy, and pharmaceutical industries. <a href="#top">Return to Top</a> <hr align="left" width="200"> <a name="V-CONE"></a> V-CONE GENERAL INFORMATION/EQUIPMENT DESIGN V-cone flowmeters work in much the same manner as other differential pressure flowmeters. A cone inserted into the path of flow causes a pressure drop proportional to the flowrate. Pressure taps are located slightly upstream and in the downstream face of the cone. The shape of the cone is designed to smooth the velocity profile so that an accurate measurement can be made regardless of the fluid's flow characteristics. V-cone flowmeters are often used in applications where static mixing is desired due to the formation of vortices at the base of the cone, as shown in the schematic below. USAGE EXAMPLES V-cone flowmeters are often used in chemical and pharmaceutical processing, the food and beverage industry, the pulp and paper industry, and in water and wastewater treatment operations. They are also commonly used to meter air flow for fans and air motors, and leakage from airplane cabins and home windows. <a href="#top">Return to Top</a> <hr align="left" width="200"> <a name="ELBOW METERS"></a> ELBOW METERS GENERAL INFORMATION/EQUIPMENT DESIGN When fluid flows through an elbow, it exerts centrifugal force on the inside of the pipe wall, creating a pressure differential between the inner and outer radii of the elbow. Pressure taps in the elbow measure this differential, which is proportional to the elbow's radius. In most cases, one tap at the 45o point is enough, as in the schematic, but an additional tap at the 22.5o point may also be used. Elbow meters are used when lack of space prohibits use of most other flowmeters, which require long, straight pipe. <a href="#top">Return to Top</a> <hr align="left" width="200"> FLOWMETERS <a href="../DifferentialPressure/DifferentialPressure.html"> DIFFERENTIAL PRESSURE </a> <a href="../Mass/Mass.html">MASS</a> <a href="../OpenChannel/OpenChannel.html">OPEN CHANNEL</a> <a href="../PositiveDisplacement/PositiveDisplacement.html">POSITIVE DISPLACEMENT</a> <a href="../Velocity/Velocity.html">VELOCITY</a> DIFFERENTIAL PRESSURE FLOWMETERS <a href="#VARIABLE-AREA">VARIABLE-AREA</a> <a href="#ORIFICES">ORIFICES</a> <a href="#FLOW NOZZLES">FLOW NOZZLES</a> <a href="#VENTURI & FLOW TUBES">VENTURI & FLOW TUBES</a> <a href="#PITOT TUBES">PITOT TUBES</a> <a href="#TARGET">TARGET</a> <a href="#V-CONE">V-CONE</a> <a href="#ELBOW METERS">ELBOW METERS</a> <a href="javascript:history.back"><img src="../../../Images/backbutton.png"></a> <a href="../../Main.html"><img src="../../../Images/homebutton.png"></a> <a href="../../Index.html"><img src="../../../Images/indexbutton.png"></a>