May 2008

Flow

Behind the scenes of steam flow measurement

By Jesse Yoder

Steam, or vaporized water, is an important part of our daily lives. It is easy enough to experience steam; simply boiling water will produce it. But steam has equally important uses as a source of power for ships, in paper production, and for cooling and heating buildings. Probably the most important use of steam is as a source of power in electricity production.

Steam is difficult to measure accurately, mainly because of its sensitivity to changes in temperature and pressure. Steam is often classified into three types, which reflect the different pressure and temperature conditions that steam is subject to.

Wet steam contains steam and condensed water. The amount of water in wet steam is designated by the term quality. The quality of steam refers to the percent of steam flow that is steam and water by weight. Steam that is 90% steam and 10% water is known as 90% quality steam. Wet steam is the most difficult type of steam to measure, and it is also known as quality steam.

Saturated steam exists at one pressure and corresponding temperature, and it does not contain any water. Saturated steam is saturated with heat. If heat is added to saturated steam, the temperature of the steam is increased above the boiling point. If heat is removed from saturated steam, condensation occurs, and the fluid becomes quality steam.

Superheated steam exists when the temperature of the steam is above the boiling point. The term "degrees of superheat" refers to how far above the boiling point the steam temperature is.

Two important applications for steam are in power plants and district heating. In both applications, steam is generated to serve as a source of energy.

Powerplants

In a power plant, steam is generated in a boiler, flows from the boiler to a turbine, and then passes through the turbine and flows to a condenser. Here the steam is converted back to water so it can be reused as boiler feedwater.

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District heating

In district heating, a centralized heating system provides heat for a building, a group of homes, or even a whole town. District cooling works in a similar way, except the group of buildings or houses is cooled instead of heated. In district heating, either water or steam serves as the medium for distributing heat.

Other steam flow applications

Measuring steam at a boiler outlet and district heating are important applications for steam flow measurement. Heating process fluids could occur in a process plant, and steam injection takes place in oil production. Other applications mentioned include heating, ventilating, and air conditioning and custody transfer. This survey reinforces the importance of steam flow measurement in power plants and district heating, but also shows it is important in other contexts as well.

Steam flow measurement

Most steam flow measurement happens either with differential pressure (DP) flowmeters or with vortex meters. DP flowmeters consist of a DP flow transmitter together with a primary element. The primary element creates a constriction in the flowstream, resulting in a reduction in pressure downstream from the primary element. The DP flow transmitter senses the difference in pressure upstream and downstream from the primary element, and uses this value to compute flow rate. Bernoulli's Principle is used to perform this calculation.

The most common type of primary element used in flow measurement is the orifice plate. Orifice plates are also widely used for measuring steam flow. However, other types of primary elements are also used. Flow nozzles work well with steam flow measurement because they can readily handle the high velocities and pressures associated with steam flow. Other types of primary elements used to measure steam flow include Venturis and averaging Pitot tubes.

Vortex flowmeters also see use to measure steam flow. Vortex meters work well because they cause minimal pressure loss and can tolerate high temperatures and pressures common in steam flow. In district heating, a volumetric vortex meter is often combined with a pressure transmitter and temperature sensor. A flow computer uses these values to compute mass flow. Multivariable vortex meters measure volumetric flow, pressure, and temperature, and use these values to calculate mass flow. Multivariable vortex meters are a fast-growing product segment within vortex flowmeters.

ABOUT THE AUTHOR

Jesse Yoder, Ph.D., is president of Flow Research, Inc. (www.flowresearch.com), in Wakefield, Mass. Contact him at jesse@flowresearch.com.