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Types of Steam Flowmeter
The operation, advantages and limitations of different types of steam flowmeter, including orifice plate, variable area and vortex shedding devices.
Use the quick links below to take you to the main sections of this tutorial:
There are many types of flowmeter available, those suitable for steam applications include:
- Orifice plate flowmeters
- Turbine flowmeters (including shunt or bypass types)
- Variable area flowmeters
- Spring loaded variable area flowmeters
- Direct in-line variable area (DIVA) flowmeter
- Pitot tubes
- Vortex shedding flowmeters
Each of these flowmeter types has its own advantages and limitations. To ensure accurate and consistent performance from a steam flowmeter, it is essential to match the flowmeter to the application.
This Tutorial will review the above flowmeter types, and discuss their characteristics, their advantages and disadvantages, typical applications and typical installations.
Orifice plate flowmeters
The orifice plate is one in a group known as head loss devices or differential pressure flowmeters. In simple terms the pipeline fluid is passed through a restriction, and the pressure differential is measured across that restriction. Based on the work of Daniel Bernoulli in 1738 (see Tutorial 4.2), the relationship between the velocity of fluid passing through the orifice is proportional to the square root of the pressure loss across it. Other flowmeters in the differential pressure group include venturis and nozzles.
With an orifice plate flowmeter, the restriction is in the form of a plate which has a hole concentric with the pipeline. This is referred to as the primary element.
To measure the differential pressure when the fluid is flowing, connections are made from the upstream and downstream pressure tappings, to a secondary device known as a DP (Differential Pressure) cell.
Orifice plate flowmeter
From the DP cell, the information may be fed to a simple flow indicator, or to a flow computer along with temperature and/or pressure data, which enables the system to compensate for changes in fluid density.
In horizontal lines carrying vapours, water (or condensate) can build up against the upstream face of the orifice. To prevent this, a drain hole may be drilled in the plate at the bottom of the pipe. Clearly, the effect of this must be taken into account when the orifice plate dimensions are determined.
Correct sizing and installation of orifice plates is absolutely essential, and is well documented in the International Standard ISO 5167.
Orifice plate flowmeter installation
A few of the most important points from ISO 5167 are discussed below:
Pressure tappings - Small bore pipes (referred to as impulse lines) connect the upstream and downstream pressure tappings of the orifice plate to a Differential Pressure or DP cell.
The positioning of the pressure tappings can be varied. The most common locations are:
- From the flanges (or carrier) containing the orifice plate as shown in Figure 4.3.3. This is covenient, but care needs to be taken with tappings at the bottom of the pipe, because they may become clogged.
- One pipe diameter on the upstream side and 0.5 x pipe diameter on the downstream side.This is less convenient, but potentially more accurate as the differential pressure measured is at its greatest at the vena contracta, which occurs at this position.
Corner tappings - These are generally used on smaller orifice plates where space restrictions mean flanged tappings are difficult to manufacture. Usually on pipe diameters including or below DN50.
From the DP cell, the information may be fed to a flow indicator, or to a flow computer along with temperature and/or pressure data, to provide density compensation.
Pipework - There is a requirement for a minimum of five straight pipe diameters downstream of the orifice plate, to reduce the effects of disturbance caused by the pipework.
The amount of straight pipework required upstream of the orifice plate is, however, affected by a number of factors including:
- The ß ratio; this is the relationship between the orifice diameter and the pipe diaameter (see Equation 4.3.1), and would typically be a value of 0.7.
- The nature and geometry of the preceding obstruction. A few obstruction examples are shown in Figure 4.3.4:
Orifice plate installations
Table 4.3.1 brings the ß ratio and the pipework geometry together to recommend the number of straight diameters of pipework required for the configurations shown in Figure 4.3.4.
In particularly arduous situations, flow straighteners may be used. These are discussed in more detail in Tutorial 4.5.
Recommended straight pipe diameters upstream of an orifice plate for various ß ratios and preceding obstruction
Advantages of orifice plate steam flowmeters:
- Simple and rugged.
- Good accuracy.
- Low cost.
- No calibration or recalibration is required provided calculations, tolerances and installation comply with ISO 5167.
Disadvantages of orifice plate steam flowmeters:
- Turndown is limited to between 4:1 and 5:1 because of the square root relationship between flow and pressure drop.
- The orifice plate can buckle due to waterhammer and can block in a system that is poorly designed or installed.
- The square edge of the orifice can erode over time, particularly if the steam is wet or dirty. This will alter the characteristics of the orifice, and accuracy will be affected. Regular inspection and replacement is therefore necessary to ensure reliability and accuracy.
- The installed length of an orifice plate flowmetering system may be substantial; a minimum of 10 upstream and 5 downstream straight unobstructed pipe diameters may be needed for accuracy.
This can be difficult to achieve in compact plants. Consider a system which uses 100 mm pipework, the ß ratio is 0.7, and the layout is similar to that shown in Figure 4.3.4(b):
The upstream pipework length required would be = 36 x 0.1 m = 3.6 m
The downstream pipework length required would be = 5 x 0.1 m = 0.5 m
The total straight pipework required would be = 3.6 + 0.5 m = 4.1 m
Typical applications for orifice plate steam flowmeters:
- Anywhere the flowrate remains within the limited turndown ratio of between 4:1 and 5:1.
This can include the boiler house and applications where steam is supplied to many plants, some on-line, some off-line, but the overall flowrate is within the range.
The primary element consists of a multi-bladed rotor which is mounted at right angles to the flow and suspended in the fluid stream on a free-running bearing. The diameter of the rotor is slightly less than the inside diameter of the flowmetering chamber, and its speed of rotation is proportional to the volumetric flowrate.
The speed of rotation of the turbine may be determined using an electronic proximity switch mounted on the outside of the pipework, which counts the pulses, as shown in Figure 4.3.5.
Since a turbine flowmeter consists of a number of moving parts, there are several influencing factors that need to be considered:
- The temperature, pressure and viscosity of the fluid being measured.
- The lubricating qualities of the fluid.
- The bearing wear and friction.
- The conditional and dimensional changes of the blades.
- The inlet velocity profile and the effects of swirl.
- The pressure drop through the flowmeter.
Because of these factors, calibration of turbine flowmeters must be carried out under operational conditions.
In larger pipelines, to minimise cost, the turbine element can be installed in a pipework bypass, or even for the flowmeter body to incorporate a bypass or shunt, as shown in Figure 4.3.6.
Bypass flowmeters comprise an orifice plate, which is sized to provide sufficient restriction for a sample of the main flow to pass through a parallel circuit. Whilst the speed of rotation of the turbine may still be determined as explained previously, there are many older units still in existence which have a mechanical output as shown in Figure 4.3.6.
Clearly, friction between the turbine shaft and the gland sealing can be significant with this mechanical arrangement.
Bypass or shunt turbine flowmeter
Advantages of turbine flowmeters:
- A turndown of 10:1 is achievable in a good installation with the turbine bearings in good condition.
- Accuracy is reasonable (± 0.5% of actual value).
- Bypass flowmeters are relatively low cost.
Disadvantages of turbine flowmeters:
- Generally calibrated for a specific line pressure. Any steam pressure variations will lead to inaccuracies in readout unless a density compensation package is included.
- Flow straighteners are essential (see Tutorial 4.5).
- If the flow oscillates, the turbine will tend to over or under run, leading to inaccuracies due to lag time.
- Wet steam can damage the turbine wheel and affect accuracy.
- Low flowrates can be lost because there is insufficient energy to turn the turbine wheel.
- Viscosity sensitive: if the viscosity of the fluid increases, the response at low flowrates deteriorates giving a non-linear relationship between flow and rotational speed. Software may be available to reduce this effect.
- The fluid must be very clean (particle size not more than 100 μm) because:
- Clearances between the turbine wheel and the inside of the pipe are very small.
- Entrained debris can damage the turbine wheel and alter its performance.
- Entrained debris will accelerate bearing wear and affect accuracy, particularly at low flowrates.
Typical applications for turbine flowmeters:
- Superheated steam.
- Liquid flowmetering, particularly fluids with lubricating properties. As with all liquids, care must be taken to remove air and gases prior to them being metered.
Variable area flowmeters
The variable area flowmeter (Figure 4.3.7), often referred to as a rotameter, consists of a vertical, tapered bore tube with the small bore at the lower end, and a float that is allowed to freely move in the fluid. When fluid is passing through the tube, the float's position is in equilibrium with:
- The dynamic upward force of the fluid.
- The downward force resulting from the mass of the float.
- The position of the float, therefore, is an indication of the flowrate.
In practice, this type of flowmeter will be a mix of:
- A float selected to provide a certain weight, and chemical resistance to the fluid.
The most common float material is grade 316 stainless steel, however, other materials such as Hastalloy C, aluminium or PVC are used for specific applications.
On small flowmeters, the float is simply a ball, but on larger flowmeters special shaped floats are used to improve stability.
- A tapered tube, which will provide a measuring scale of typically between 40 mm and 250 mm over the design flow range.
Usually the tube will be made from glass or plastic. However, if failure of the tube could present a hazard, then either a protective shroud may be fitted around the glass, or a metal tube may be used.
With a transparent tube, flow readings are taken by observation of the float against a scale. For higher temperature applications where the tube material is opaque, a magnetic device is used to indicate the position of the float.
Because the annular area around the float increases with flow, the differential pressure remains almost constant.
Variable area flowmeter
Advantages of variable area flowmeters:
- Linear output.
- Turndown is approximately 10:1.
- Simple and robust.
- Pressure drop is minimal and fairly constant.
Disadvantages of variable area flowmeters:
- The tube must be mounted vertically (see Figure 4.3.8).
- Because readings are usually taken visually, and the float tends to move about, accuracy is only moderate. This is made worst by parallax error at higher flowrates, because the float is some distance away from the scale.
- Transparent taper tubes limit pressure and temperature.
Typical applications for variable area flowmeters:
- Metering of gases.
- Small bore airflow metering - In these applications, the tube is manufactured from glass, with calibrations marked on the outside. Readings are taken visually.
- Laboratory applications.
- Rotameters are sometimes used as a flow indicating device rather than a flow measuring device.
Variable area flowmeter installed in a vertical plane
Spring loaded variable area flowmeters
The spring loaded variable area flowmeter (an extension of the variable area flowmeter) uses a spring as the balancing force. This makes the meter independent of gravity, allowing it to be used in any plane, even upside-down. However, in its fundamental configuration (as shown in Figure 4.3.9), there is also a limitation: the range of movement is constrained by the linear range of the spring, and the limits of the spring deformation.
Spring loaded variable area flowmeters
However, another important feature is also revealed: if the pass area (the area between the float and the tube) increases at an appropriate rate, then the differential pressure across the spring loaded variable area flowmeter can be directly proportional to flow.
To recap a few earlier statements
- As the rate of flow increases, so does the differential pressure.
- By measuring this pressure difference it is possible to calculate the flowrate through the flowmeter.
- The pass area (for example, the size of the hole in the orifice plate) remains constant.
With orifice plates flowmeters:
With any type of variable area flowmeter
- The differential pressure remains almost constant as the flowrate varies.
- Flowrate is determine from the position of the float.
- The pass area (the area between the float and the tube) through which the flow passes increases with increasing flow.
Figure 4.3.10 compares these two principles.
Comparing the fixed area and variable area flowmeters
The spring loaded variable area principle is a hybrid between these two devices, and either:
- The displacement of the float - Option 1
- The differential pressure - Option 2
...may be used to determine the flowrate through the flowmeter.
In Option 1 (determining the displacement of the float or 'flap'). This can be developed for steam systems by:
- Using a torsion spring to give a better operating range.
- Using a system of coils to accurately determine the position of the float.
This will result in a very compact flowmeter. This may be further tailored for saturated steam applications by incorporating a temperature sensor and programming steam tables into the computer unit. See Figure 4.3.11 for an example of a flowmeter of this type.
Spring loaded variable area flowmeter monitoring the position of the float
Advantages of spring loaded variable area flowmeters:
- Turndowns of 25:1 are achievable with normal steam velocities (25 m/s), although high velocities can be tolerated on an intermittent basis, offering turndowns of up to 40:1.
- Accuracy is ±2% of actual value.
- Can be tailored for saturated steam systems with temperature and pressure sensors to provide pressure compensation.
- Relatively low cost.
- Short installation length.
Disadvantages of spring loaded variable area flowmeters:
- Size limited to DN100.
- Can be damaged over a long period by poor quality (wet and dirty) steam, at prolonged high velocity (>30 m/s).
Typical applications for spring loaded variable area flowmeters:
- Flowetering of steam to individual plants.
- Small boiler houses.
Typical installation of a spring loaded variable area flowmeter measuring steam flow
In Option 2 (Figure 4.3.10), namely, determining the differential pressure, this concept can be developed further by shaping of the float to give a linear relationship between differential pressure and flowrate. See Figure 4.3.13 for an example of a spring loaded variable area flowmeter measuring differential pressure. The float is referred to as a cone due to its shape.
Spring Loaded Variable Area flowmeter (SLVA) monitoring differential pressure
Advantages of a spring loaded variable area (SLVA) flowmeter:
- High turndown, up to 100:1.
- Good accuracy ±1% of reading for pipeline unit.
- Compact - a DN100 wafer unit requires only 60 mm between flanges.
- Suitable for many fluids.
Disadvantages of a variable area spring load flowmeter:
- Can be expensive due to the required accessories, such as the DP cell and flow computer.
Typical applications for a variable area spring load flowmeter:
- Boiler house flowmetering.
- Flowmetering of large plants.
Typical installation of a SVLA flowmeter monitoring differential pressure
Direct In-Line Variable Area (DIVA) flowmeter
The DIVA flowmeter operates on the well established spring loaded variable area (SLVA) principle, where the area of an annular orifice is continuously varied by a precision shaped moving cone. This cone is free to move axially against the resistance of a spring.
However, unlike other SLVA flowmeters, the DIVA does not rely on the measurement of differential pressure drop across the flowmeter to calculate flow, measuring instead the force caused by the deflection of the cone via a series of extremely high quality strain gauges. The higher the flow of steam the greater the force. This removes the need for expensive differential pressure transmitters, reducing installation costs and potential problems (Figure 4.3.15).
The DIVA has an internal temperature sensor, which provides full density compensation for saturated steam applications.
Flowmetering systems will:
- Check on the energy cost of any part of the plant.
- Cost energy as a raw material.
- Identify priority areas for energy savings.
- Enable efficiencies to be calculated for processes or power generation.
The DIVA steam flowmeter (Figure 4.3.16) has a system uncertainty in accordance with EN ISO/IEC 17025, of:
- ± 2% of actual flow to a confidence of 95% (2 standard deviations) over a range of 10% to 100% of maximum rated flow.
- ± 0.2% FSD to a confidence of 95% (2 standard deviations) from 2% to 10% of the maximum rated flow.
As the DIVA is a self-contained unit the uncertainty quoted is for the complete system. Many flowmeters claim a pipeline unit uncertainty but, for the whole system, the individual uncertainty values of any associated equipment, such as DP cells, need to be taken into account.
The turndown of a flowmeter is the ratio of the maximum to minimum flowrate over which it will meet its specified performance, or its operational range. The DIVA flowmeter has a high turndown ratio of up to 50:1, giving an operational range of up to 98% of its maximum flow.
The orientation of the DIVA flowmeter can have an effect on the operating performance. Installed in horizontal pipe, the DIVA has a steam pressure limit of 32 bar g, and a 50:1 turndown. As shown in Figure 4.3.17, if the DIVA is installed with a vertical flow direction then the pressure limit is reduced, and the turndown ratio will be affected if the flow is vertically upwards.
In large steam mains, the cost of providing a full bore flowmeter can become extremely high both in terms of the cost of the flowmeter itself, and the installation work required.
A Piot tube flowmeter can be an inexpensive method of metering. The flowmeter itself is cheap, it is cheap to install, and one flowmeter may be used in several applications.
Pitot tubes, as introduced in Tutorial 4.2, are a common type of insertion flowmeter. Figure 4.3.18 shows the basis for a Pitot tube, where a pressure is generated in a tube facing the flow, by the velocity of the fluid. This 'velocity' pressure is compared against the reference pressure (or static pressure) in the pipe, and the velocity can be determined by applying a simple equation.
In practice, two tubes inserted into a pipe would be cumbersome, and a simple Pitot tube will consist of one unit as shown in Figure 4.3.19. Here, the hole measuring the velocity pressure and the holes measuring the reference or static pressure are incorporated in the same device.
Because the simple Pitot tube (Figure 4.3.19) only samples a single point, and, because the flow profile of the fluid (and hence velocity profile) varies across the pipe, accurate placement of the nozzle is critical.
Note that a square root relationship exists between velocity and pressure drop (see Equation 4.2.13). This limits the accuracy to a small turndown range.
The averaging Pitot tube
The averaging Pitot tube (Figure 4.3.20) was developed with a number of upstream sensing tubes to overcome the problems associated with correctly siting the simple type of Pitot tube. These sensing tubes sense various velocity pressures across the pipe, which are then averaged within the tube assembly to give a representative flowrate of the whole cross section.
|u1 =||The fluid velocity in the pipe|
|Δp =||Dynamic pressure - Static pressure|
Advantages of the Pitot tube
- Presents little resistance to flow.
- Inexpensive to buy.
- Simple types can be used on different diameter pipes.
Disadvantages of the Pitot tube:
- Turndown is limited to approximately 4:1 by the square root relationship between pressure and velocity as discussed in Tutorial 4.2.
- If steam is wet, the bottom holes can become effectively blocked. To counter this, some models can be installed horizontally.
- Sensitive to changes in turbulence and needs careful installation and maintenance.
- The low pressure drop measured by the unit, increases uncertainty, especially on steam.
- Placement inside the pipework is critical.
Typical applications for the Pitot tube:
- Occasional use to provide an indication of flowrate
- Determining the range over which a more appropriate steam flowmeter may be used.
Vortex shedding flowmeters
These flowmeters utilise the fact that when a non-streamlined or 'bluff' body is placed in a fluid flow, regular vortices are shed from the rear of the body. These vortices can be detected, counted and displayed. Over a range of flows, the rate of vortex shedding is proportional to the flowrate, and this allows the velocity to be measured.
The bluff body causes a blockage around which the fluid has to flow. By forcing the fluid to flow around it, the body induces a change in the fluid direction and thus velocity. The fluid which is nearest to the body experiences friction from the body surface and slows down. Because of the area reduction between the bluff body and the pipe diameter, the fluid further away from the body is forced to accelerate to pass the necessary fluid through the reduced space. Once the fluid has passed the bluff body, it strives to fill the space produced behind it, which in turn causes a rotational motion in the fluid creating a spinning vortex.
The fluid velocity produced by the restriction is not constant on both sides of the bluff body. As the velocity increases on one side it decreases on the other. This also applies to the pressure. On the high velocity side the pressure is low, and on the low velocity side the pressure is high. As pressure attempts to redistribute itself, the high pressure region moving towards the low pressure region, the pressure regions change places and vortices of different strengths are produced on alternate sides of the body.
The shedding frequency and the fluid velocity have a near-linear relationship when the correct conditions are met.
The frequency of shedding is proportional to the Strouhal number (Sr), the flow velocity, and the inverse of the bluff body diameter. These factors are summarised in Equation 4.3.2.
Vortex shedding flowmeter
|f||= Shedding frequency (Hz)|
|Sr||= Strouhal number (dimensionless)|
|u||= Mean pipe flow velocity (m/s)|
|d||= Bluff body diameter (m)|
The Strouhal number is determined experimentally and generally remains constant for a wide range of Reynolds numbers;which indicates that the shedding frequency will remain unaffected by a change in fluid density, and that it is directly proportional to the velocity for any given bluff body diameter. For example:
f = k x u
k = A constant for all fluids on a given design of flowmeter.
Then the volume flowrate qv in a pipe can be calculated as shown in Equation 4.3.3:
A = Area of the flowmeter bore (m²)
Advantages of vortex shedding flowmeters:
- Reasonable turndown (providing high velocities and high pressure drops are acceptable)
- No moving parts.
- Little resistance to flow.
Disadvantages of vortex shedding flowmeters:
- At low flows, pulses are not generated and the flowmeter can read low or even zero.
- Maximum flowrates are often quoted at velocities of 80 or 100 m/s, which would give severe problems in steam systems, especially if the steam is wet and/or dirty. Lower velocities found in steam pipes will reduce the capacity of vortex flowmeters.
- Vibration can cause errors in accuracy.
- Correct installation is critical as a protruding gasket or weld beads can cause vortices to form, leading to inaccuracy.
- Long, clear lengths of upstream pipework must be provided, as for orifice plate flowmeters.
Typical applications for vortex shredding flowmeters:
- Direct steam measurements at both boiler and point of use locations.
- Natural gas measurements for boiler fuel flow.
Vortex shedding flowmeter - typical installations