Traditional Technology Flowmeters
These flowmeter technologies were introduced prior to 1950. There is less product development among these meters than among new-technology meters. And traditional meters typically perform at a lower level than new-technology meters in categories such as accuracy and repeatability. Traditional meters include differential pressure (DP), positive displacement (PD), turbine, open channel, thermal, and variable area.
* A Difference in Pressure: Differential pressure transmitters have been used for more than a century to measure flow. In conjunction with a primary element, such as an orifice plate or a Venturi tube, they measure the pressure drop across an obstruction in the pipe. This is a well-understood technology and is especially popular in the U.S.
One of the most important developments in DP flowmeters in the past five years has been the packaging of DP transmitters together with primary elements to create a true DP flowmeter. Instead of having to buy and install an orifice plate or averaging pitot tube separately, this new style of DP flowmeter comes with the primary element already integrated with the transmitter. Some models also come with pressure and temperature sensors, and the ability to measure mass flow. The ProBar and the Mass ProBar from Emerson Rosemount are examples.
Important advances have been made in multivariable flowmeters, which are a type of new-technology meter. These flowmeters compute flow by measuring two or more process variables. They typically use a DP transmitter and the values from a pressure and temperature sensor to compute mass flow. Foxboro and ABB are among the companies that recently introduced multivariable transmitters.
* Positive Disposition: The history of positive displacement meters goes back to 1815, with the invention of a PD meter for measuring gas flow. Nutating disc meters, used to measure water flow, were invented in 1830. The popular oval gear flowmeter, used today for industrial applications, was patented in Germany by Bopp & Reuther in 1932. Bopp & Reuther is still selling this meter today, and it is also manufactured in Brazil by Metroval.
Positive displacement meters operate by capturing the fluid in a small volume whose dimensions are precisely known. They then count how many times this operation occurs. This need for counting is what gives rise to the registers that adorn the top of many positive displacement meters. The counting registers give the meters a personality with their face-like appearance (Figure 2).
Figure 2: Say "Nutate"
Counting registers that commonly adorn the tops of many positive displacement flowmeters appear to some observers to give them a distinctive face-like appearance. (Source:Tuthill Transfer)
PD meter suppliers have been using advanced manufacturing techniques and chronometry to more accurately measure the dimensions of PD meters and produce more precise flowmeters. Due to their precision, PD meters are still widely used for custody transfer measurement of hydrocarbons and water. They are also used for custody transfer of gas.
* A Spinning Thing: Reinhard Woltman invented the first turbine meter in 1790. Since that time, at least six different types of turbine meters have been invented. However, the modern use of turbine meters to measure fuel flow is more recent. It stems from attempts made during World War II to develop a reliable flowmeter for measuring fuel consumption on military airplanes.
There are at least 140 suppliers of turbine flowmeters worldwide, more than for any other type of flowmeter. There are also more turbine flowmeters sold annually than any other type of meter in terms of units. In fact, more turbine meters were sold in 2002 than the sum total of all new-technology flowmeters worldwide. The same equation does not hold true for dollars, however, since the average selling price of turbine meters is considerably less than the average selling price of new-technology meters.
Turbine suppliers are improving their meters by using more durable materials to increase their reliability. For example, Hoffer Flow Controls uses hybrid ceramic ball bearings to increase longevity. Hoffer also has introduced clamp-on insertion turbine meters to compete with plastic paddlewheel meters.
* Open to Many Channels:Open channel flowmeters are used to measure flows of rivers and streams and also to measure water flow in partially filled large pipes. There are a variety of methods used, including weirs and flumes, area velocity, and dilution methods. When weirs or flumes are used, a variety of methods are used to measure level.
Some advances in open channel flow measurement revolve around electronic enhancements and improvements in communication. There is also room for improvements in sensor technologies. Some improvements of this type have already occurred with Doppler technology.
* Taking the Temperature of the Flow: Thermal flowmeters were developed in the 1960s and 1970s by several independent companies in California. The technology evolved out of hot-wire anemometry, the origins of which go back to the early 1900s. Hot-wire anemometers were used in velocity profile and turbulence research. Because they are fragile and are susceptible to error caused by dirt, they are not suited to industrial environments.
Fluid Components Intl. (FCI) first developed thermal flow switches to give a flow or no-flow reading for oil being pumped out of wells in California. The process of creating these flow switches begin in 1964; it wasn't until 1981 that FCI began producing thermal flowmeters. These meters were designed to measure gas flow. Sierra Instruments independently developed thermal flowmeters during the same period.