| Steam plays a very important role in our lives today. It is used in the production of paper, in food preparation, for cooling and heating buildings, and as a source of power for ships. Perhaps its most important use is a source of power in electricity production.
It is safe to say that steam is the most difficult fluid to measure, when compared to liquids, gases, and air. Steam is very easily influenced by changes in temperature and pressure, and to that extent can be considered an unstable fluid. DP and vortex flowmeters work well to measure steam flow because they can tolerate the high temperature and pressures associated with steam flow measurement. Despite the existence of other technologies, vortex and DP flowmeters still do the lion’s share of steam flow measurement.
While steam flow may be measured less often than liquid and gas flow, it is increasingly important at a time when the world is becoming more energy conscious. Steam flow measurement plays a major role in energy production, and the rising price of crude oil and natural gas has resulted in a major push for more efficiency in energy production.
Steam power plants obtain heat from a variety of sources, including fossil fuels such as coal, oil, or natural gas. Some plants get their power from nuclear fuels. Others get fuel from waste fuels, municipal sold waste, solar energy, and other sources. With each of these power sources, heat energy is released. With fossil fuels, this is done through combustion, while nuclear plants use fission. Heat is typically transferred to water in these plants. The heated water generates steam. Steam in a power plant is generated in a system called a boiler. The steam flows to a turbine, where drives a generator for the purpose of producing electricity. After it passes through the turbine, the steam flows to a condenser. Here the steam is converted back to water so it can be reused as boiler feedwater.
Steam is also used for well injection, for heating process fluids, and for boiler outlet measurements. Many companies are taking a closer look at their steam flow measurement to see if they can increase efficiency, gain higher accuracy, or improve their measurement in some other way. There has never been a better time to look at the steam flow measurement market.
The World Market for Steam Flow Measurement includes:
- Worldwide market size and market shares for flowmeters used to measure steam flow in 2007
- Forecasts of market growth for flowmeters used for steam flow measurement through 2012
- Industries where flowmeters are used for steam flow measurement, including growth areas
- Applications for flowmeters used for steam flow measurement
- Product analysis for the main companies selling into the steam flow measurement market
- Strategies to manufacturers for selling into the steam flow measurement market
- Company profiles of the main suppliers of flowmeters used to measure steam flow
In conjunction with the study, we also completed a User Perspective Survey on Steam Flow Measurement. The purpose of the survey was to find out what types of flowmeters are being used for steam flow, and what changes to expect in the next several years. The survey also reveals why end-users are making the decisions they are making about steam flow measurement.
The survey provides another perspective from which to evaluate steam flow measurement, and serves as a cross-check of the supplier data for the steam flow study. It is also helpful in the growth forecasts. The following are some of the subjects included in the steam flow end-user survey:
- Line sizes of steam flowmeters
- Steam flowmeter applications
- Types of steam being measured (wet, saturated, superheated)
- Volumetric vs. mass flow measurement of steam
- Accuracy requirements for steam flowmeters
- Drivers of accuracy requirements
- Types of primary elements used with DP flowmeters measuring steam flow
- Prospective changes in primary elements
- Prospective changes to other flowmeter types
- Repair and maintenance issues for steam flowmeters
Best flowmeters for steam
Steam flow is measured mainly by vortex and differential pressure (DP) flowmeters. However, some new technologies are now being used to measure steam flow, namely Coriolis and ultrasonic flowmeters. The following flowmeter types, including primary elements, are used for steam and included in this study:
- Differential pressure (DP)
- Primary elements
- Vortex
- Coriolis
- Ultrasonic
- Turbine
- Variable area
- Target
Vortex flowmeters
Vortex flowmeters have a small obstruction in the flowstream called a bluff body, and it is the bluff body, not the transmitter, that is exposed to the heat of the steam. The presence of the bluff body in the flowstream generates alternating vortices, which are circular appearing ripples. The velocity of the flow is proportional to the frequency of the vortices. The flowmeter determines vortex frequency by one of several methods, including ultrasonic, thermal, and pressure. Flowrate is calculated, based on vortex frequency and the area of the pipe.
Vortex flowmeters have a number of advantages. While they are not as accurate as Coriolis flowmeters, they still have good accuracy. Vortex flowmeters also are quite reliable. While they are more intrusive than magnetic and ultrasonic flowmeters, they are less intrusive than DP flowmeters, especially those using orifice plates. Vortex flowmeters offer good accuracy and reliability for a reasonable price.
Steam flow. Vortex flowmeters are well suited for measuring steam flow, and they are widely used for this purpose. Steam is the most difficult fluid to measure. This is due to the high pressure and high temperature of steam, and because the measurement parameters vary with the type of steam, including pressure.
Steam is often measured in process plants, and for power generation. Magnetic flowmeters cannot measure steam flow. In addition to their ability to tolerate high process temperatures and pressures, vortex meters have wide rangeability. This allows them to measure steam flow at varying velocities. In process and power plants, steam is often measured coming from a boiler.
District heating is a growing application for vortex flowmeters. In district heating, a centralized heating system is created that can provide heat for a building, a group of houses, or even an entire town. A similar concept applies to district cooling. In district heating, a volumetric flowmeter is often combined with a temperature sensor and pressure transmitter to measure mass flow. A flow computer is used to calculate mass flow. Both vortex and ultrasonic flowmeters are used to measure water flow, and steam flow is also measured.
District heating is a growing application in Europe, and it is also beginning to be more widely used in Asia. It has never caught on in the United States, perhaps because of the individualistic mindset of U.S. citizens. However, it is likely that district heating will start being used more widely in the United States, as the costs of energy increase. District heating is already used in Canada.
Multivariable vortex. Sierra Instruments introduced the first multivariable vortex flowmeter in 1997. This meter includes an RTD temperature sensor and a pressure transducer with a vortex shedding flowmeter. By using the information from these sensors, the flowmeter can determine volumetric flow, temperature, pressure, fluid density, and mass flow. This makes it unnecessary to use a flow computer to make the flow calculation.
This multivariable flowmeter is one of a growing number of multivariable new-technology flowmeters, including multivariable magnetic flowmeters and multivariable DP flowmeters. Multivariable ultrasonic flowmeters are used in district heating applications. Since Sierra brought out the multivariable vortex, several other companies have introduced their own multivariable vortex flowmeters, including Yokogawa and J-Tec.
DP flowmeters
DP flowmeters — which consist of a DP transmitter and a primary element — are also used to measure steam flow. The effectiveness of DP flowmeters varies with the type of primary element used. The DP transmitter itself is not exposed to the heat from the steam, and the primary element is the only component of the flowmeter that is exposed to the heat. The primary element places a constriction in the flowstream. The constriction creates a pressure drop in the line. DP flowmeters determine flowrate based on the difference in pressure upstream of and downstream from the primary element.
There are many different types of primary elements. The main types are orifice plates, Venture tubes, flow nozzles, primary flow elements, and averaging pitot tubes. Other types include wedge elements and V-Cones. There are also a number of different types of orifice plates. Orifice plates are the most widely used type of primary element.
Primary elements create a pressure drop that makes the flow calculation possible. This pressure drop is considered a disadvantage of this method of measuring flow, due to pumping costs. The amount of pressure drop depends on the type of primary element used. Orifice plates typically create the highest amount of pressure loss.
Suppliers of DP transmitters have made important advances to make their products more appealing, and to keep up with advances in flow technology. Just as suppliers of vortex flowmeters have introduced multivariable products, so suppliers have introduced multivariable DP transmitters. These products typically include pressure and temperature transmitters, and use these values to compute mass flow. This makes it unnecessary to use a flow computer to perform the mass flow calculation.
Another advance by DP transmitter suppliers is to include an integrated primary element with the multivariable DP transmitter. This makes it possible to calibrate the primary element together with the transmitter prior to shipping. Multivariable DP transmitters, with and without integrated primary elements, are widely used to measure gas and steam flow. The temperature and pressure values are used to compute density, which in turn is used to calculate mass flow.
Suppliers of pressure transmitters have made other important advances, including the introduction of high tier products. High tier pressure transmitters set new benchmarks in terms of transmitter accuracy and stability. These advances apply to differential pressure, as well as gage and absolute pressure transmitters.
Emerging technologies for steam
Both ultrasonic and Coriolis flowmeters are beginning to be used to measure steam flow. However, this is a difficult measurement for both technologies. Panametrics introduced a clamp-on ultrasonic flowmeter designed to measure steam flow. Expect to see additional steam flow products from both ultrasonic and Coriolis suppliers .
Another type of meter used to measure steam flow is the target meter. However, there are relatively few manufacturers of this type of meter. Target flowmeters are sometimes used for large pipe applications. Besides steam, they are also used to measure the flow of liquids and gases. |
