There are many types of flowmeters available that are highly suited to steam applications, examples include:

• Orifice plate flowmeters
• Turbine flowmeters (including shunt or bypass types)
• Variable area flowmeters
• Spring loaded variable area flowmeters
• 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 series of Tutorial articles will review the above flowmeter types, and discuss their advantages and disadvantages and their typical applications.

Read other tutorials on Flowmeters, here and here.

Tutorial 1: Orifice Plate, Turbine and Variable area flowmeters

Orifice Plate flowmeters for steam applications

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• Simple and rugged.
• Good accuracy.
• Low cost.
• No calibration or recalibration is required provided calculations, tolerances and installation comply with ISO 5167.

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• 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.

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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.

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Turbine flowmeters for steam applications

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• 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.

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• 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

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• 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.

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Variable Area flowmeters for steam applications

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• Linear output.
• Turndown is approximately 10:1.
• Simple and robust.
• Pressure drop is minimal and fairly constant.

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• The tube must be mounted vertically
• 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.

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• 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.

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The next tutorial will cover:

• Spring loaded variable area flowmeters
• Direct in-line variable area (DIVA) flowmeter
• Pitot tubes
• Vortex shedding flowmeters

If you have any questions in the meantime, contact us today!