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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.

Variable-Area Flowmeter

(Picture courtesy of Krohne Anerica Inc., Peabody, MA)

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.

<embed src="../../../Movies/RotoMtr.mov" border="1" width="200" height="330" autoplay="false" controller="true" loop="true" align="center" title="Variable-Area Meter" bgcolor="#F2ECE1"></embed>	<img src="../../../Images/Flow-diff-var-equ1.JPG" align="baseline" border="1">
Variable-Area Flowmeter	Rotameter
	(Picture courtesy of Krohne Anerica Inc., Peabody, MA)

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.

<embed src="../../../Movies/PistnMtr.mov" border="1" width="200" height="330" autoplay="false" controller="true" loop="true" align="center" title="Piston meter" bgcolor="#F2ECE1"></embed>	<embed src="../../../Movies/VaneMtr.mov" border="1" width="250" height="240" autoplay="false" controller="true" loop="true" align="bottom" title="Vane meter" bgcolor="#F2ECE1"></embed>
Piston Meter	Vane Meter

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.

<img src="../../../Images/Flow-diff-var-use1.JPG" align="baseline" border="1">	<img src="../../../Images/Flow-diff-var-use2.JPG" align="baseline" border="1">
Vane Meters
(Picture courtesy of Universal Flow Monitors, Inc., Hazel Park, Mi)	(Picture courtesy of KOBOLD Instruments Inc., Pittsburg, PA)

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.

<img src="../../../Images/Flow-diff-var-use3.JPG" border="1" align="baseline">	<img src="../../../Images/Flow-diff-var-use4.JPG" border="1" align="baseline">
Piston Meters
(Pictures courtesy of KOBOLD Instruments Inc., Pittsburg, PA)

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.

ADVANTAGES

DISADVANTAGES

Low pressure drop, which remains mostly constant across the operating range of the meter.
Easily read, installed, and maintained.
May be constructed from a variety of materials and cover a wide range of temperatures and pressures.
Easily converted to handle flows of different fluids.
Can be installed immediately before or after pipe fittings.
Self-cleaning.
Piston meters and vane meters can be mounted in any position.
Low cost.

Requires relatively clean fluid.
Rotameters must be vertically mounted.
Usually only provides direct readings for air and water, calculations must be made for other fluids.
Rotameters are adversely affected by moderate to high viscosities.

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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.

<img src="../../../Images/Flow-diff-ori-gen1.JPG" align="baseline" border="1">	<img src="../../../Images/Flow-diff-ori-gen2.JPG" align="baseline" border="1">	<img src="../../../Images/Flow-diff-ori-gen3.JPG" align="baseline" border="1">
Orifice	Circular Hole in an Orifice	Orifice
(Pictures courtesy of Badger Meter, Inc., Tulsa, OK)		(Picture courtesy of KOBOLD Instruments Inc., Pittsburg, PA)

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.

<img src="../../../Images/Flow-diff-ori-use1.JPG" align="baseline" border="1">	<img src="../../../Images/Flow-diff-ori-use2.JPG" align="baseline" border="1">
Bronze Orifice	Steel and Cast Iron Orifice
(Pictures courtesy of KOBOLD Instruments Inc., Pittsburg, PA)

The two orifice meters below are made of stainless steel and are specially designed to handle corrosive materials, such as strong acids and bases.

<img src="../../../Images/Flow-diff-ori-use3.JPG" align="baseline" border="1">	<img src="../../../Images/Flow-diff-ori-use4.JPG" align="baseline" border="1">
Stainless Steel Orifices
(Pictures courtesy of KOBOLD Instruments Inc., Pittsburg, PA)

ADVANTAGES	DISADVANTAGES
No moving parts, low maintenance. Low cost. Small.	Low accuracy. Pressure drops can be large. Requires lengths of straight pipe upstream and downstream. Contraction of flow reduces accuracy of downstream pressure measurement.

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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.

Flow Nozzle
<img src="../../../Images/Flow-diff-flonoz-gen1.JPG" align="baseline" border="1">
(Picture courtesy of Badger Meter, Inc., Tulsa, OK)

ADVANTAGES	DISADVANTAGES
Can handle suspended solids. For same flow rate, has significantly lower pressure drop than orifice plate. May be mounted horizontally or vertically. Less susceptible to erosion and corrosion than an orifice plate.	Unsuitable for viscous flow and sticky materials. Requires consistently smooth flow profile, can't handle eddies or vortices. Requires straight pipe runs up- and downstream.

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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.

<img src="../../../Images/Flow-diff-ven&flo-gen1.JPG" align="baseline" border="1">	<img src="../../../Images/Flow-diff-ven&flo-gen2.JPG" align="baseline" border="1">
Venturi Tube	Flow Tube
(Pictures courtesy of Badger Meter, Inc., Tulsa, OK)

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.

<img src="../../../Images/Flow-diff-ven&flo-use1.JPG" align="baseline" border="1">	<img src="../../../Images/Flow-diff-ven&flo-use2.JPG" align="baseline" border="1">
Venturi Tube	Flow Tube
(Pictures courtesy of Badger Meter, Inc., Tulsa, OK)

ADVANTAGES	DISADVANTAGES
Handle large flow volumes at low pressure drops. No moving parts, low maintenance. Accommodate liquids with high solids content. Shape is inherently self-cleaning. May be mounted horizontally or vertically.	Require four or more pressure taps to accurately measure pressure differential. Require lengths of straight pipe upstream and downstream. Accuracy depends on consistent flow profile. Less accurate at low flowrates.

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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.

<embed src="../../../Movies/Pitot9.mov" border="1" width="195" height="235" autoplay="false" controller="true" loop="true" align="baseline" title="Pitot tube" bgcolor="#F2ECE1"></embed>	<img src="../../../Images/Flow-diff-pit-gen1.JPG" align="baseline" border="1">
Pitot Tubes
	(Copyright 1999 OMEGA Engineering Inc. All rights reserved. Reproduced courtesy of OMEGA Engineering Inc., Stanford CT 06907)

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.

<img src="../../../Images/Flow-diff-pit-use1.JPG" align="baseline" border="1">	<img src="../../../Images/Flow-diff-pit-use2.JPG" align="baseline" border="1">
Plane	NASA's X-31 Fighter Jet
(Picture courtesy of Dr. Susan Montgomery, University of Michigan, Ann Arbor, Mi)	(Picture courtesy of NASA-Dryden, Edwards, CA)

ADVANTAGES	DISADVANTAGES
Suitable for measuring steam. Low pressure drop. No moving parts, low maintenance. Easy to install and ideal for retrofitting.	Subject to clogging. Limited to point measurements.

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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.

<img src="../../../Images/Flow-diff-tar-gen1.JPG" align="baseline" border="1">	<embed src="../../../Movies/tflow2.mov" border="1" width="165" height="210" autoplay="false" controller="true" loop="true" align="baseline" title="Target flow meter" bgcolor="#F2ECE1"></embed>
Target Meter	Target Flow Meter
(Picture courtesy of KOBOLD Instruments Inc., Pittsburg, PA)	(Animation based on a schematic courtesy of Miltronics, Peterborough, ON, Canada)

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.

<img src="../../../Images/Flow-diff-tar-use1.JPG" align="baseline" border="1">	<img src="../../../Images/Flow-diff-tar-use2.JPG" align="baseline" border="1">
Pulverized Coal and Coke Target Meter	Plastic Pellet Target Meter
(Pictures courtesy of Milltronics, Peterborough, ON, Canada)

In addition to metering industrial liquid, gas, and steam flows, target meters can be used in the food and beverage, dairy, and pharmaceutical industries.

ADVANTAGES	DISADVANTAGES
Particularly useful for steam, corrosive materials and liquid flows with suspended solids. One of the only methods of measuring solids flow. Low maintenance.	Reduced accuracy in gas and liquid metering compared to other types of flowmeters.

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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.

V-Cone Flowmeter

(Picture courtesy of McCrometer, Hemet, CA)

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.

V-Cone Flowmeter

(Picture courtesy of McCrometer, Hemet, CA)

ADVANTAGES	DISADVANTAGES
Don't require straight pipe runs, can be located closer to upstream disturbances than other differential pressure meters. Lower pressure drop than orifice plates. Low cost. Vortex formation immediately behind the cone encourages mixing, prevents buildup.	Reduced accuracy at lower flowrates.

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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.

Elbow Meter

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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>

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