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Differential Pressure Flowmeters
The calculation of fluid flow rate by reading the pressure loss across a pipe restriction is perhaps the most commonly used flow measurement technique in industrial applications (Figure 2-1). The pressure drops generated by a wide variety of geometrical restrictions have been well characterized over the years, and, as compared in Table 2, these primary or "head" flow elements come in a wide variety of configurations, each with specific application strengths and weaknesses. Variations on the theme of differential pressure (d/p) flow measurement
In the 18th century, Bernoulli first established the relationship between static and kinetic energy in a flowing stream. As a fluid passes through a restriction, it accelerates, and the energy for this acceleration is obtained from the fluid's static pressure. Consequently, the line pressure drops at the point of constriction (Figure 2-1). Part of the pressure drop is recovered as the flow returns to the unrestricted pipe. The pressure differential (h) developed by the flow element is measured, and the velocity (V), the volumetric flow (Q) and the mass flow (W) can all be calculated using the following generalized formulas:
V = k (h/D)0.5
k is the discharge coefficient of the element (which also reflects the units of measurement), A is the cross-sectional area of the pipe's opening, and D is the density of the flowing fluid. The discharge coefficient k is influenced by the Reynolds number (see Figure 1-5) and by the "beta ratio," the ratio between the bore diameter of the flow restriction and the inside diameter of the pipe. Additional parameters or correction factors can be used in the derivation of k, depending on the type of flow element used. These parameters can be computed from equations or read from graphs and tables available from the American National Standards Institute (ANSI), the American Petroleum Institute (API), the American Society of Mechanical Engineers (ASME), and the American Gas Association (AGA), and are included in many of the works listed as references at the end of this chapter. The discharge coefficients of primary elements are determined by laboratory tests that reproduce the geometry of the installation. Published values generally represent the average value for that geometry over a minimum of 30 calibration runs. The uncertainties of these published values vary from 0.5% to 3%. By using such published discharge coefficients, it is possible to obtain reasonably accurate flow measurements without in-place calibration. In-place calibration is required if testing laboratories are not available or if better accuracy is desired than that provided by the uncertainty range noted above. The relationship between flow and pressure drop varies with the velocity profile, which can be laminar or turbulent (Figure 2-1) as a function of the Reynolds number (Re), which for liquid flows can be calculated using the relationship:
Re = 3160(SG)(Q)/(ID)µ
where ID is the inside diameter of the pipe in inches, Q is the volumetric liquid flow in gallons/minute, SG is the fluid specific gravity at 60°F, and µ is the viscosity in centipoises. At low Reynolds numbers (generally under Re = 2,000), the flow is laminar and the velocity profile is parabolic. At high Reynolds numbers (well over Re = 3,000), the flow becomes fully turbulent, and the resulting mixing action produces a uniform axial velocity across the pipe. As shown in Figure 1-5, the transition between laminar and turbulent flows can cover a wide range of Reynolds numbers; the relationship with the discharge coefficient is a function of the particular primary element. Today, many engineering societies and organizations and most primary
Accuracy & Rangeability The performance of a head-type flowmeter installation is a function of the precision of the flow element and of the accuracy of the d/p cell. Flow element precision is typically reported in percentage of actual reading (AR) terms, whereas d/p cell accuracy is a percentage of calibrated span (CS). A d/p cell usually provides accuracy of ±0.2% of the calibrated span (CS). This means that, at the low end of a 10:1 flow range (at 10% flow), corresponding to a differential pressure range of 100:1, the flowmeter would have an error of ±20% AR. For this reason, differential producing flowmeters have historically been limited to use within a 3:1 or 4:1 range. Flowmeter rangeability can be further increased without adverse effect on accuracy by operating several d/p flowmeters in parallel runs. Only as many runs are opened at a time as are needed to keep the flow in the active ones at around 75-90% of range. Another option is to stack two or more transmitters in parallel onto the same element, one for 1-10%, the other for 10-100% of full scale (FS) d/p produced. Both of these techniques are cumbersome and expensive. Intelligent transmitters offer a better option. The accuracy of intelligent transmitters is usually stated as ±0.1% CS, which includes only errors due to hysteresis, rangeability and linearity. Potential errors due to drift, temperature, humidity, vibration, overrange, radio frequency interference and power supply variation are all excluded. If one includes them, inaccuracy is about 0.2% CS. Because
In most flowmetering applications, density is not measured directly. Rather, it is assumed to have some normal value. If density deviates from this assumed value, error results. Density error can be corrected if it is measured directly or indirectly by measuring pressure in gases or temperature in liquids. Flow computing packages are also available that accept the inputs of the d/p transmitter and the other sensors and can simultaneously calculate mass and volumetric flow. To minimize error (and the need for density correction) when dealing with compressible fluids, the ratio of differential pressure (h) divided by upstream pressure (P) should not exceed 0.25 (measured in the same engineering units). Metering errors due to incorrect installation of the primary element can be substantial (up to 10%). Causes of such errors can be the condition of the mating pipe sections, insufficient straight pipe runs, and pressure tap and lead line design errors. Under turbulent flow conditions, as much as 10% of the d/p signal can be noise caused by disturbances from valves and fittings, both up- and downstream of the element, and by the element itself. In the majority of applications, the damping provided in d/p cells is sufficient to filter out the noise. Severe noise can be reduced by the use of two or more pressure taps connected in parallel on both sides of the d/p cell. Pulsating flow can be caused by reciprocating pumps or compressors. This pulsation can be reduced by moving the flowmeter away from the source of the pulse, or downstream of filters or other dampening devices. Pulsation dampening hardware can also be installed at the pressure taps, or dampening software can applied to the d/p cell output signal. One such filter is the inverse derivative algorithm, which blocks any rate of change occurring more quickly than the rate at which the process flow can change. Piping, Installation, & Maintenance Installation guidelines are published by various professional organizations (ISA, ANSI, API, ASME, AGA) and by manufacturers of proprietary designs. These guidelines include such recommendations as: When, in addition to measuring the flow, the process temperature or pressure is also to be measured, the pressure transmitter should not be installed in the process pipe, but should be connected to the appropriate lead line of the flow element via a tee. Similarly, the thermowell used for temperature measurement should be installed at least 10 diameters downstream of the flow element, to prevent velocity profile distortions. Welds should be ground smooth and gaskets trimmed so that no protrusion can be detected by physical inspection. In order for the velocity profile to fully develop (and the pressure drop to be predictable), straight pipe runs are required both up- and downstream of the d/p element. The amount of straight run required depends on both the beta ratio of the installation and on the nature of the upstream components in the pipeline. For example, when a single 90° elbow precedes an orifice plate, the straight-pipe requirement ranges from 6 to 20 pipe diameters as the diameter ratio is increased from 0.2 to 0.8. In order to reduce the straight run requirement, flow straighteners (Figure 2-2) such as tube bundles, perforated plates, or internal tabs can be installed upstream of the primary element. The size and orientation of the pressure taps are a function of both the pipe size and the type of process fluid. The recommended maximum diameter of pressure tap holes through the pipe or flange is 1/4" for pipes under 2" in diameter, 3/8" for 2" and 3" pipes, 1/2" for 4 to 8" and 3/4" for larger pipes. Both taps should be of the same diameter, and, where the hole breaks through the inside pipe surface, it should be square with no roughness, burrs, or wire edges. Connections to pressure holes should be made by nipples, couplings, or adaptors welded to the outside surface of the pipe. On services where the process fluid can plug the pressure taps or might gel or freeze in the lead lines, chemical seal protectors can be used. Connection sizes are usually larger (seal elements can also be provided with diaphragm extensions), and, because of the space requirement, they are usually installed at "radius tap" or "pipe tap" locations, as shown in Figure 2-3. When chemical seals are used, it is important that the two connecting capillaries, as they are routed to the d/p cell, experience the same temperature and are kept shielded from sunlight. The d/p transmitter should be located as close to the primary element as possible. Lead lines should be as short as possible and of the same diameter. In clean liquid service, the minimum diameter is G", while in condensable vapor service, the minimum diameter is 0.4". In steam service, the horizontal lead lines should be kept as short as possible and be tilted (with a minimum gradient of 1 in/ft with respect to the piping) towards the tap, so that condensate can drain back into the pipe. Again, both lead lines should be exposed to the same ambient conditions and be shielded from sunlight. In clean liquid or gas service, the lead lines can be purged through the d/p
Seal pots are on the wet leg in d/p cell installations with small ranges (under 10 in H2O) in order to minimize the level variation in the legs. In steam applications, filling tees are recommended to ensure equal height condensate legs on both sides of the d/p cell. If for some reason the two legs are not of equal height, the d/p cell can be biased to zero out the difference, as long as that difference does not change. If the process temperature exceeds the maximum temperature limitation of the d/p cell, either chemical seals have to be used or the lead lines need to be long enough to cool the fluid. If a large temperature drop is required, a coiled section of tubing (pigtail) can be installed in the lead lines to cool the process fluids. The frequency of inspection or replacement of a primary element depends on the erosive and corrosive nature of the process and on the overall accuracy required. If there is no previous experience, the orifice plate can be removed for inspection during the first three, six, and 12 months of its operation. Based on visual inspection of the plate, a reasonable maintenance cycle can be extrapolated from the findings. Orifices used for material balance calculations should be on the same maintenance cycle.
Sizing the Orifice Plate The orifice plate is commonly used in clean liquid, gas, and steam service. It is available for all pipe sizes, and if the pressure drop it requires is free, it is very cost-effective for measuring flows in larger pipes (over 6" diameter). The orifice plate is also approved by many standards organizations for the custody transfer of liquids and gases. The orifice flow equations used today still differ from one another, although the various standards organizations are working to adopt a single, universally accepted orifice flow equation. Orifice sizing programs usually allow the user to select the flow equation desired from among several. The orifice plate can be made of any material, although stainless steel is the most common. The thickness of the plate used ( 1/8-1/2") is a function of the line size, the process temperature, the pressure, and the differential pressure. The traditional orifice is a thin circular plate (with a tab for handling and for data), inserted
The concentric orifice plate (Figure 2-4A) has a sharp (square-edged) concentric bore that provides an almost pure line contact between the plate and the fluid, with negligible friction drag at the boundary. The beta (or diameter) ratios of concentric orifice plates range from 0.25 to 0.75. The maximum velocity and minimum static pressure occurs at some 0.35 to 0.85 pipe diameters downstream from the orifice plate. That point is called the vena contracta. Measuring the differential pressure at a location close to the orifice plate minimizes the effect of pipe roughness, since friction has an effect on the fluid and the pipe wall. Flange taps are predominantly used in the United States and are located 1 inch from the orifice plate's surfaces (Figure 2-3). They are not recommended for use on pipelines under 2 inches in diameter. Corner taps are predominant in Europe for all sizes of pipe, and are used in the United States for pipes under 2 inches (Figure 2-3). With corner taps, the relatively small clearances represent a potential maintenance problem. Vena contracta taps (which are close to the radius taps, Figure 2-4) are located one pipe diameter upstream from the plate, and downstream at the point of vena contracta. This location varies (with beta ratio and Reynolds number) from 0.35D to 0.8D. The vena contracta taps provide the maximum pressure differential, but also the most noise. Additionally, if the plate is changed, it may require a change in the tap location. Also, in small pipes, the vena contracta might lie under a flange. Therefore, vena contracta taps normally are used only in pipe sizes exceeding six inches. Radius taps are similar to vena contracta taps, except the downstream tap is fixed at 0.5D from the orifice plate (Figure 2-3). Pipe taps are located 2.5 pipe diameters upstream and 8 diameters downstream from the orifice (Figure 2-3). They detect the smallest pressure difference and, because of the tap distance from the orifice, the effects of pipe roughness, dimensional inconsistencies, and, therefore, measurement errors are the greatest.
Orifice Types & Selection The concentric orifice plate is recommended for clean liquids, gases, and steam flows when Reynolds numbers range from 20,000 to 107 in pipes under six inches. Because the basic orifice flow equations assume that flow velocities are well below sonic, a different theoretical and computational approach is required if sonic velocities are expected. The
Because of this minimum Reynolds number consideration, square-edged orifices are seldom used on viscous fluids. Quadrant-edged and conical orifice plates (Figure 2-5) are recommended when the Reynolds number is under 10,000. Flange taps, corner, and radius taps can all be used with quadrant-edged orifices, but only corner taps should be used with a conical orifice. Concentric orifice plates can be provided with drain holes to prevent buildup of entrained liquids in gas streams, or with vent holes for venting entrained gases from liquids (Figure 2-4A). The unmeasured flow passing through the vent or drain hole is usually less than 1% of the total flow if the hole diameter is less than 10% of the orifice bore. The effectiveness of vent/drain holes is limited, however, because they often plug up. Concentric orifice plates are not recommended for multi-phase fluids in horizontal lines because the secondary phase can build up around the upstream edge of the plate. In extreme cases, this can clog the opening, or it can change the flow pattern, creating measurement error. Eccentric and segmental orifice plates are better suited for such applications. Concentric orifices are still preferred for multi-phase flows in vertical lines because accumulation of material is less likely and the sizing data for these plates is more reliable. The eccentric orifice (Figure 2-4B) is similar to the concentric except that the opening is offset from the pipe's centerline. The opening of the segmental orifice (Figure 2-4C) is a segment of a circle. If the secondary phase is a gas, the opening of an eccentric orifice will be located towards the top of the pipe. If the secondary phase is a liquid in a gas or a slurry in a liquid stream, the opening should be at the bottom of the pipe. The drainage area of the segmental orifice is greater than that of the eccentric orifice, and, therefore, it is preferred in applications with high proportions of the secondary phase. These plates are usually used in pipe sizes exceeding four inches in diameter, and must be carefully installed to make sure that no portion of the flange or gasket interferes with the opening. Flange taps are used with both types of plates, and are located in the quadrant opposite the opening for the eccentric orifice, in line with the maximum dam height for the segmental orifice. For the measurement of low flow rates, a d/p cell with an integral orifice may be the best choice. In this design, the total process flow passes through the d/p cell, eliminating the need for lead lines. These are proprietary devices with little published data on their performance; their flow coefficients are based on actual laboratory calibrations. They are recommended for clean, single-phase fluids only because even small amounts of build-up will create significant measurement errors or will clog the unit. Restriction orifices are installed to remove excess pressure and usually operate at sonic velocities with very small beta ratios. The pressure drop across a single restriction orifice should not exceed 500 psid because of plugging or galling. In multi-element restriction orifice installations, the plates are placed approximately one pipe diameter from one another in order to prevent pressure recovery between the plates.
Orifice Performance Although it is a simple device, the orifice plate is, in principle, a precision instrument. Under ideal conditions, the inaccuracy of an orifice plate can be in the range of 0.75-1.5% AR. Orifice plates are, however, quite
The typical custody-transfer grade orifice meter is more accurate because it can be calibrated in a testing laboratory and is provided with honed pipe sections, flow straighteners, senior orifice fittings, and temperature controlled enclosures.
Venturi & Flowtubes Venturi tubes are available in sizes up to 72", and can pass 25 to 50% more flow than an orifice with the same pressure drop. Furthermore, the total unrecovered head loss rarely exceeds 10% of measured d/p (Figure 2-6). The initial cost of venturi tubes is high, so they are primarily used on larger flows or on more difficult or demanding flow applications. Venturis are insensitive to velocity profile effects and therefore require less straight pipe run than an orifice. Their contoured nature, combined with the self- scouring action of the flow through the tube, makes the device immune to corrosion, erosion, and internal scale build up. In spite of its high initial cost, the total cost of ownership can still be favorable because of savings in installation and operating and maintenance costs. The classical Herschel venturi has a very long flow element characterized by a tapered inlet and a diverging outlet. Inlet pressure is measured at the entrance, and static pressure in the throat section. The pressure taps feed into a common annular chamber, providing an average pressure reading over the entire circumference of the element. The classical venturi is limited in its application to clean, non-corrosive liquids and gases. In the short form venturi, the entrance angle is increased and the annular chambers are replaced by pipe taps (Figure 2-7A). The short-form venturi maintains many of the advantages of the classical venturi, but at a reduced initial cost, shorter length and reduced weight. Pressure taps are located 1/4 to 1/2 pipe diameter upstream of the inlet cone, and in
There are several proprietary flowtube designs which provide even better pressure recovery than the classical venturi. The best known of these proprietary designs is the universal venturi (Figure 2-7B). The various flowtube designs vary in their contours, tap locations, generated
Flowtube performance is much affected by calibration. The inaccuracy of the discharge coefficient in a universal venturi, at Reynolds numbers exceeding 75,000, is 0.5%. The inaccuracy of a classical venturi at Re > 200,000 is between 0.7 and 1.5%. Flowtubes are often supplied with discharge coefficient graphs because the discharge coefficient changes as the Reynolds number drops. The variation in the discharge coefficient of a venturi caused by pipe roughness is less than 1% because there is continuous contact between the fluid and the internal pipe surface. The high turbulence and the lack of cavities in which material can accumulate make flow tubes well suited for slurry and sludge services. However, maintenance costs can be high if air purging cannot prevent plugging of the pressure taps and lead lines. Plunger-like devices (vent cleaners) can be installed to periodically remove buildup from interior openings, even while the meter is online. Lead lines can also be replaced with button-type seal elements hydraulically coupled to the d/p transmitter using filled capillaries. Overall measurement accuracy can drop if the chemical seal is small, its diaphragm is stiff, or if the capillary system is not temperature-compensated or not shielded from direct sunlight.
Flow Nozzles The flow nozzle is dimensionally more stable than the orifice plate, particularly in high temperature and high velocity services. It has often been used to measure high flowrates of superheated steam. The flow nozzle, like the venturi, has a greater flow capacity than the orifice plate and requires a lower initial investment than a venturi tube, but also provides less pressure recovery (Figure 2-6). A major disadvantage of the nozzle is that it is more difficult to replace than the orifice unless it can be removed as part of a spool section. The ASME pipe tap flow nozzle is predominant in the United States (Figure 2-7C). The downstream end of a nozzle is a short tube having the same diameter as the vena contracta of an equivalent orifice plate. The low-beta designs range in diameter ratios from 0.2 to 0.5, while the high beta-ratio designs vary between 0.45 and 0.8. The nozzle should always be centered in the pipe, and the downstream pressure tap should be inside the nozzle exit. The throat taper should always decrease the diameter toward the exit. Flow nozzles are not recommended for slurries or dirty fluids. The most common flow nozzle is the flange type. Taps are commonly located one pipe diameter upstream and 1/2 pipe diameter downstream from the inlet face. Flow nozzle accuracy is typically
Nozzles can be installed in any position, although horizontal orientation is preferred. Vertical downflow is preferred for wet steam, gases, or liquids containing solids. The straight pipe run requirements are similar to those of orifice plates.
Segmental Wedge Elements The segmental wedge element (Figure 2-8A) is a proprietary device designed for use in slurry, corrosive, erosive, viscous, or high-temperature applications. It is relatively expensive and is used mostly on difficult fluids, where the dramatic savings in maintenance can justify the initial cost. The unique flow restriction is designed to last the life of the installation without deterioration. Wedge elements are used with 3-in diameter chemical seals, eliminating both the lead lines and any dead-ended cavities. The seals attach to the meter body immediately upstream and downstream of the restriction. They rarely require cleaning, even in services like dewatered sludge, black liquor, coal slurry, fly ash slurry, taconite, and crude oil. The minimum Reynolds number is only 500, and the meter requires only five diameters of upstream straight pipe run. The segmental wedge has a V-shaped restriction characterized by the H/D ratio, where H is the height of the opening below the restriction and D is the diameter. The H/D ratio can be varied to match the flow range and to produce the desired d/p. The oncoming flow creates a sweeping action through the meter. This provides a scouring effect on both faces of the restriction, helping to keep it clean and free of buildup. Segmental wedges can measure flow in both directions, but the d/p transmitter must be calibrated for a split range, or the flow element must be provided with two sets of connections for two d/p transmitters (one for forward and one for reverse flow). An uncalibrated wedge element can be expected to have a 2% to 5% AR inaccuracy over a 3:1 range. A calibrated wedge element can reduce that to 0.5% AR if the fluid density is constant. If slurry density is variable and/or unmeasured, error rises.
Venturi-Cone Element The venturi-cone (V-cone) element (Figure 2-8B) is another proprietary design that promises consistent performance at low Reynolds numbers and is insensitive to velocity profile distortion or swirl effects. Again, however, it is relatively expensive. The V-cone restriction has a unique geometry
that minimizes accuracy degradation due to wear, making it a good choice for high velocity flows and erosive/corrosive applications. The V-cone creates a controlled turbulence region that flattens the incoming irregular velocity profile and induces a stable differential pressure that is sensed by a downstream tap. The beta ratio of a V-cone is so defined that an orifice and a V-cone with equal beta ratios will have equal opening areas.
Beta ratio = (D2 - d2).05/D
where d is the cone diameter and D is the inside diameter of the pipe. With this design, the beta ratio can exceed 0.75. For example, a 3-in meter with a beta ratio of 0.3 can have a 0 to 75 gpm range. Published test results on liquid and gas flows place the system accuracy between 0.25 and 1.2% AR.
Although the pitot tube is one of the simplest flow sensors, it is used in a wide range of flow measurement applications such as air speed in racing cars and Air Force fighter jets. In industrial applications, pitot tubes are used to measure air flow in pipes, ducts, and stacks, and liquid flow in pipes, weirs, and open channels. While accuracy and rangeability are relatively low, pitot tubes are simple, reliable, inexpensive, and suited for a variety of environmental conditions, including extremely high temperatures and a wide range of pressures. The pitot tube is an inexpensive alternative to an orifice plate. Accuracy ranges from 0.5% to 5% FS, which is comparable to that of an orifice. Its flow rangeability of 3:1 (some operate at 4:1) is also similar to the capability of the orifice plate. The main difference is that, while an orifice measures the full flowstream, the pitot tube detects the flow velocity at only one point in the flowstream. An advantage of the slender pitot tube is that it can be inserted into existing and pressurized pipelines (called hot-tapping) without requiring a shutdown.
Theory of Operation Pitot tubes were invented by Henri Pitot in 1732 to measure the flowing velocity of fluids. Basically a differential pressure (d/p) flowmeter, a pitot tube measures two pressures: the static and the total impact pressure. The static pressure is the operating pressure in the pipe, duct, or the environment, upstream to the pitot tube. It is measured at right angles to the flow direction, preferably in a low turbulence location (Figure 2-9). The total impact pressure (PT) is the sum of the static and kinetic pressures and is detected as the flowing stream impacts on the pitot opening. To measure impact pressure, most pitot tubes use a small,
Vp = C(PT - P)½/D
When the flowrate is obtained by multiplying the point velocity (VP) by the cross-sectional area of the pipe or duct, it is critical that the velocity measurement be made at an insertion depth which corresponds to the average velocity. As the flow velocity rises, the velocity profile in the pipe changes from elongated (laminar) to more flat (turbulent). This changes the point of average velocity and requires an adjustment of the insertion depth. Pitot tubes are recommended only for highly turbulent flows (Reynolds Numbers > 20,000) and, under these conditions, the velocity profile tends to be flat enough so that the insertion depth is not critical. In 1797, G.B. Venturi developed a short tube with a throat-like passage that increases flow velocity and reduces the permanent pressure drop. Special pitot designs are available that, instead of providing just an impact hole for opening, add a single or double venturi to the impact opening of the pitot tube. The venturi version generates a higher differential pressure than does a regular pitot tube.
Static Pressure Measurement In jacketed (dual-walled) pitot-tube designs, the impact pressure port faces forward into the flow, while static ports do not, but are, instead, spaced around the outer tube. Both pressure signals (PT and P) are routed by tubing to a d/p indicator or transmitter. In industrial applications, the static pressure (P) can be measured in three ways: 1) through taps in the pipe wall; 2) by static probes inserted in the process stream; or 3) by small openings located on the pitot tube itself or on a separate aerodynamic element. Wall taps can measure static pressures at flow velocities up to 200 ft/sec. A static probe (resembling an L-shaped pitot tube) can have four holes of 0.04 inches in diameter, spaced 90° apart. Aerodynamic bodies can be cylinders or wedges, with two or more sensing ports. Errors in detecting static pressure arise from fluid viscosity, velocity, and fluid compressibility. The key to accurate static pressure detection is to minimize the kinetic component in the pressure measurement.
Single-Port Pitot Tubes A single-port pitot tube can measure the flow velocity at only a single point in the cross-section of a flowing stream (Figure 2-10). The probe must be inserted to a point in the flowing stream where the flow velocity is the average of the veloc
If the pressure differential generated by the venturi is too low for accurate detection, the conventional pitot tube can be replaced by a pitot venturi or a double venturi sensor. This will produce a higher pressure differential. A calibrated, clean and properly inserted single-port pitot tube can provide ±1% of full scale flow accuracy over a flow range of 3:1; and, with some loss of accuracy, it can even measure over a range of 4:1. Its advantages are low cost, no moving parts, simplicity, and the fact that it causes very little pressure loss in the flowing stream. Its main limitations include the errors resulting from velocity profile changes or from plugging of the pressure ports. Pitot tubes are generally used for flow measurements of secondary importance, where cost is a major concern, and/or when the pipe or duct diameter is large (up to 72 inches or more). Specially designed pitot probes have been developed for use with pulsating flows. One design uses a pitot probe filled with silicone oil to transmit the process pressures to the d/p cell. At high frequency pulsating applications, the oil serves as a pulsation dampening and pressure-averaging medium. Pitot tubes also can be used in square, rectangular or circular air ducts. Typically, the pitot tube fits through a 5/16-in diameter hole in the duct. Mounting can be by a flange or gland. The tube is usually provided with an external indicator, so that its impact port can be accurately rotated to face directly into the flow. In addition, the tube can be designed for detecting the full velocity profile by making rapid and consistent traverses across the duct. In some applications, such as EPA-mandated stack particulate sampling, it is necessary to traverse a pitot sampler across a stack or duct. In these applications, at each point noted in Figure 2-11, a temperature and flow measurement is made in addition to taking a gas sample, which data are then combined and taken to a laboratory for analysis. In such applications, a single probe contains a pitot tube, a thermocouple, and a sampling nozzle. A pitot tube also can be used to measure water velocity in open channels, at drops, chutes, or over fall crests. At the low flow velocities typical of laminar conditions, pitot tubes are not recommended because it is difficult to find the insertion depth corresponding to the average velocity and because the pitot element produces such a small pressure differential. The use of a pitot venturi does improve on this situation by increasing the pressure differential, but cannot help the problem caused by the elongated velocity profile.
Averaging Pitot Tubes Averaging pitot tubes been introduced to overcome the problem of finding the average velocity point. An averaging pitot tube is provided with multiple impact and static pressure ports and is designed to extend across the entire diameter of the pipe. The pressures detected by all the impact (and separately by all the static) pressure ports are combined and the square root of their difference is measured as an indication of the average flow in the pipe (Figure 2-12). The port closer to the outlet of the combined signal has a slightly greater influence, than the port that is farthest away, but, for secondary applications where pitot tubes are commonly used, this error is acceptable. The number of impact ports, the distance between ports, and the diameter of the averaging pitot tube all can be modified to match the needs of a particular application. Sensing ports in averaging pitot tubes are often too large to allow the tube to behave as a true averaging chamber. This is because the oversized
Averaging pitot tubes offer the same advantages and disadvantages as do single-port tubes. They are slightly more expensive and a little more accurate, especially if the flow is not fully formed. Some averaging pitot sensors can be inserted through the same opening (or hot tap) which accommodates a single-port tube.
Area Averaging Area-averaging pitot stations are used to measure the large flows of low pressure air in boilers, dryers, or HVAC systems. These units are available for the various standard sizes of circular or rectangular ducts (Figure 2-13) and for pipes. They are so designed that each segment of the cross-section is provided with both an impact and a static pressure port. Each set of ports is connected to its own manifold, which combines the average static and average impact pressure signals. If plugging is likely, the manifolds can be purged to keep the ports clean. Because area-averaging pitot stations generate very small pressure differentials, it may be necessary to use low differential d/p cells with spans as low as 0-0.01 in water column. To improve accuracy, a hexagonal cell-type flow straightener and a flow nozzle can be installed upstream of the area-averaging pitot flow sensor. The flow straightener removes local turbulence, while the nozzle amplifies the differential pressure produced by the sensor.
Installation Pitot tubes can be used as permanently installed flow sensors or as portable monitoring devices providing periodic data. Permanently installed carbon steel or stainless steel units can operate at up to 1400 PSIG pressures and are inserted into the pipe through flanged or screw connections. Their installation usually occurs prior to plant start-up, but they can be hot-tapped into an operating process. In a hot-tap installation (Figure 2-14), one first welds a fitting to the pipe. Then a drill-through valve is attached to the fitting and a hole is drilled through the pipe. Then, after partially withdrawing the drill, the valve is closed, the drill is removed and the pitot tube is inserted. Finally, the valve is opened and the pitot tube is fully inserted. The velocity profile of the flowing stream inside the pipe is affected by the Reynolds number of the flowing fluid, pipe surface roughness, and by upstream disturbances, such as valves, elbows, and other fittings. Pitot tubes should be used only if the minimum Reynolds number exceeds 20,000 and if either a straight run of about 25 diameters can be provided upstream to the pitot tube or if straightening vanes can be installed.
Vibration Damage Natural frequency resonant vibrations can cause pitot tube failure.
VL = 5253(M x Pr x D)/L
Where M = mounting factor (3.52 for single mount); Pr = probe factor (0.185 for 3/8-in diameter probes; 0.269 for 1/2-in; 0.372 for 3/4-in; and 0.552 for 1-in); D = probe diameter (inches); L = unsupported probe length in inches, which is calculated as the sum of the pipe I.D. plus the pipe wall thickness plus: 1.25 in for 3/8-in diameter probes; 1.5 in for 1/2-in; 1.56 in for 3/4-in; and 1.94 in for 1-in diameter probes. Once the velocity limits have been calculated, make sure that they do not fall within the range of operating velocities. If they do, change the probe diameter, or its mounting, or do both, until there is no overlap.
Variable area flowmeters (Figure 2-15) are simple and versatile devices that operate at a relatively constant pressure drop and measure the flow of liquids, gases, and steam. The position of their float, piston or vane is changed as the increasing flow rate opens a larger flow area to pass the flowing fluid. The position of the float, piston or vane provides a direct visual indication of flow rate. Design variations include the rotameter (a float in a tapered tube), orifice/rotameter combination (bypass rotameter), open-channel variable gate, tapered plug, and vane or piston designs. Either the force of gravity or a spring is used to return the flow element to its resting position when the flow lessens. Gravity-operated meters (rotameters) must be installed in a vertical position, whereas spring operated ones can be mounted in any position. All variable area flowmeters are available with local indicators. Most can also be provided with position sensors and transmitters (pneumatic, electronic, digital, or fiberoptic) for connecting to remote displays or controls.
Purge‹Flow Regulators If a needle valve is placed at the inlet or outlet of a rotameter, and a d/p regulator controls the pressure difference across this combination, the result is a purge-flow regulator. Such instrumentation packages are used as self-contained purge flowmeters (Figure 2-16). These are among the least expensive and most widely used flowmeters. Their main application is to control small gas or liquid purge streams. They are used to protect instruments from contacting hot and corrosive fluids, to protect pressure taps from plugging, to protect the cleanliness of optical devices, and to protect electrical devices from igniting upon contact with combustibles. Purge meters are quite useful in adding nitrogen gas to the vapor
As shown in Figure 2-16, purge meters can operate in the constant flow mode, where P2 - P0 is held constant at about 60 to 80 in H2O differential. In bubbler and purge applications, the inlet pressure (P1) is held constant and the outlet pressure (P0) is variable. Figure 2-16 describes a configuration where the outlet pressure (P0) is held constant and the inlet pressure (P1) is variable. They can handle extremely small flow rates from 0.01 cc/min for liquids and from 0.5 cc/min for gases. The most common size is a glass tube rotameter with -in (6 mm) connections, a range of 0.05-0.5 gpm (0.2-2.0 lpm) on water or 0.2-2.0 scfm (0.3-3.0 cmph) in air service. Typical accuracy is ±5% FS over a 10:1 range, and the most common pressure rating is 150 psig (1 MPa).
Rotameters The rotameter is the most widely used variable area flowmeter because of its low cost, simplicity, low pressure drop, relatively wide rangeability, and linear output. Its operation is simple: in order to pass through the tapered tube, the fluid flow raises the float. The greater the flow, the higher the float is lifted. In liquid service, the float rises due to a combination of the buoyancy of the liquid and the velocity head of the fluid. With gases, buoyancy is negligible, and the float responds mostly to the velocity head. In a rotameter (Figure 2-15), the metering tube is mounted vertically, with the small end at the bottom. The fluid to be measured enters at the bottom of the tube, passes upward around the float, and exits the top. When no flow exists, the float rests at the bottom. When fluid enters, the metering float begins to rise. The float moves up and down in proportion to the fluid flow rate and the annular area between the float and the tube wall. As the float rises, the size of the annular opening increases. As this area increases, the differential pressure across the float decreases. The float reaches a stable position when the upward force exerted by the flowing fluid equals the weight of the float. Every float position corresponds to a particular flowrate for a particular fluid's density and viscosity. For this reason, it is necessary to size the rotameter for each application. When sized correctly, the flow rate can be determined by matching the float position to a calibrated scale on the outside of the rotameter. Many rotameters come with a built-in valve for adjusting flow manually. Several shapes of float are available for various applications. One early design had slots, which caused the float to spin for stabilizing and centering purposes. Because this float rotated, the term rotameter was coined. Rotameters are typically provided with calibration data and a direct reading scale for air or water (or both). To size a rotameter for other service, one must first convert the actual flow to a standard flow. For liquids, this standard flow is the water equivalent in gpm; for gases, the standard flow is the air flow equivalent in standard cubic feet per minute (scfm). Tables listing standard water equivalent gpm and/or air scfm values are provided by rotameter manufacturers. Manufacturers also often provide slide rules, nomographs, or computer software for rotameter sizing.
Design Variations A wide choice of materials is available for floats, packing, O-rings, and end fittings. Rotameter tubes for such safe applications as air or water can be made of glass, whereas if breakage would create an unsafe condition, they are provided with metal tubes. Glass tubes are most common, being precision formed of safety shielded borosilicate glass.
Floats typically are machined from glass, plastic, metal, or stainless steel for corrosion resistance. Other float materials include carboloy, sapphire, and tantalum. End fittings are available in metal or plastic. Some fluids attack the glass metering tube, such as wet steam or high-pH water over 194°F (which can soften glass); caustic soda (which dissolves glass); and hydrofluoric acid (which etches glass). Floats have a sharp edge at the point where the reading should be observed on the tube-mounted scale. For improved reading accuracy,
A correlation rotameter has a scale from which a reading is taken (Figure 2-15). This reading is then compared to a correlation table for a given gas or liquid to get the actual flow in engineering units. Correlation charts are readily available for nitrogen, oxygen, hydrogen, helium, argon, and carbon dioxide. While not nearly as convenient as a direct reading device, a correlation meter is more accurate. This is because a direct-reading device is accurate for only one specific gas or liquid at a particular temperature and pressure. A correlation flowmeter can be used with a wide variety of fluids and gases under various conditions. In the same tube, different flow rates can be handled by using different floats.
Small glass tube rotameters are suitable for working with pressures up to 500 psig, but the maximum operating pressure of a large (2-in diameter) tube may be as low as 100 psig. The practical temperature limit is about 400°F, but such high-temperature operation substantially reduces the operating pressure of the tube. In general, there is a linear relationship between operating temperature and pressure.
Glass-tube rotameters are often used in applications where several streams of gases or liquids are being metered at the same time or mixed in a manifold, or where a single fluid is being exhausted through several channels (Figure 2-17). Multiple tube flowmeters allow up to six rotameters to be mounted in the same frame.
It also is possible to operate a rotameter in a vacuum. If the rotameter has a valve, it must be placed at the outlet at the top of the meter. For applications requiring a wide measurement range, a dual-ball rotameter can be used. This instrument has two ball floats: a light ball (typically black) for indicating low flows and a heavy ball (usually white) for indicating high flows. The black ball is read until it goes off scale, and then the white ball is read. One such instrument has a black measuring range from 235-2,350 ml/min and a white to 5,000 ml/min.
For higher pressures and temperatures beyond the practical range of glass, metal tube rotameters can be used. These tubes are usually made of stainless steel, and the position of the float is detected by magnetic followers with readouts outside the metering tube.
Metal-tube rotameters can be
Plastic-tube rotameters are relatively low cost rotameters that are ideal for applications involving corrosive fluids or deionized water. The tube itself can be made from PFA, polysulfone, or polyamide. The wetted parts can be made from stainless steel, PVDF, or PFA, PTFE, PCTFE, with FKM or Kalrez® O-rings.
Accuracy
Laboratory rotameters can be calibrated to an accuracy of 0.50% AR over a 4:1 range, while the inaccuracy of industrial rotameters is typically 1-2% FS over a 10:1 range. Purge and bypass rotameter errors are in the 5% range.
Rotameters can be used to manually set flow rates by adjusting the valve opening while observing the scale to establish the required process flow rate. If operating conditions remain unaltered, rotameters can be repeatable to within 0.25% of the actual flow rate.
Most rotameters are relatively insensitive to viscosity variations. The most sensitive are very small rotameters with ball floats, while larger rotameters are less sensitive to viscosity effects. The limitations of each design are published by the manufacturer (Figure 2-18). The float shape does affect the viscosity limit. If the viscosity limit is exceeded, the indicated flow must be corrected for viscosity.
Because the float is sensitive to changes in fluid density, a rotameter can be furnished with two floats (one sensitive to density, the other to velocity) and used to approximate the mass flow rate. The more closely the float density matches the fluid density, the greater the effect of a fluid density change will be on the float position. Mass-flow rotameters work best with low viscosity fluids such as raw sugar juice, gasoline, jet fuel, and light hydrocarbons.
Rotameter accuracy is not affected by the upstream piping configuration. The meter also can be installed directly after a pipe elbow without adverse effect on metering accuracy. Rotameters are inherently self cleaning because, as the fluid flows between the tube wall and the float, it produces a scouring action that tends to prevent the buildup of foreign matter. Nevertheless, rotameters should be used only on clean fluids which do not coat the float or the tube. Liquids with fibrous materials, abrasives, and large particles should also be avoided.
Other Variable-Area Flowmeters
Major disadvantages of the rotameter are its relatively high cost in larger sizes and the requirement that it be installed vertically (there may not be enough head room). The cost of a large rotameter installation can be reduced by using an orifice bypass or a pitot tube in combination with a smaller rotameter. The same-size bypass rotameter can be used to measure a variety of flows, with the only difference between applications being the orifice plate and the differential it produces.
Advantages of a bypass rotameter include low cost; its major disadvantage is inaccuracy and sensitivity to material build-up. Bypass rotameters are often provided with isolation valves so that they can be removed for maintenance without shutting down the process line.
Tapered plug flowmeters are variable-area flowmeters with a stationary core and a piston that moves as the flow varies. In one design, the piston movement mechanically moves a pointer, while in another it magnetically moves an external flow rate indicator. The second design has a metallic meter body for applications up to 1,000 psig.
One gate-type variable-area flow-meter resembles a butterfly valve. Flow through the meter forces a spring-loaded vane to rotate, and a mechanical connection provides local flow rate indication. The inaccuracy of such meters is 2-5% FS. The meter can be used on oil, water and air, and is available in sizes up to 4 inches. It also is used as an indicating flow switch in safety interlock systems.
References & Further Reading
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