Propeller meters are very similar to turbine meters in technology and application. The main difference between the two is in the rotating element and it's suspension in the fluid stream. A propeller is usually made of thick injection molded plastic and faces directly into the flow, suspended from a single bearing assembly. Turbines are thinner and are usually supported on both sides by two, lighter-weight bearing assemblies.
Propeller flowmeters use the mechanical energy of the fluid to rotate a “pinwheel” (rotor) in the flow stream. Blades on the rotor are angled to transform energy from the flow stream into rotational energy. The rotor shaft spins on bearings. When the fluid moves faster, the rotor spins proportionally faster.
Shaft rotation can be sensed mechanically or by detecting the movement of the blades. Blade movement is often detected magnetically, with each blade or embedded piece of metal generating a pulse. Propeller flowmeter sensors are typically located external to the flowing stream to avoid material of construction constraints that would result if wetted sensors were used. When the fluid moves faster, more pulses are generated. The transmitter processes the pulse signal to determine the flow of the fluid. Transmitters and sensing systems are available to sense flow in both the forward and reverse flow directions.
Propeller meters are accurate and relatively inexpensive and can be economical even when only needed for a short time.
Propeller flowmeters measure the velocity of liquids, gases and vapors in pipes, such as hydrocarbons, chemicals, water, cryogenic liquids, air, and industrial gases. High accuracy propeller flowmeters are available for custody transfer of hydrocarbons and natural gas. These flowmeters often incorporate the functionality of a flow computer to correct for pressure, temperature and fluid properties in order to achieve the desired accuracy for the application.
Be careful because using propeller flowmeters on fluids that are non-lubricating, because the flowmeter can become inaccurate and fail if its bearings prematurely wear. Some propeller flowmeters have grease fittings for use with non-lubricating fluids. In addition, propeller flowmeters that are designed for a specific purpose, such as for natural gas service, can often operate over a limited range of temperature (such as up to 60ºC) whereby operation at higher temperatures can damage the flowmeter.
This flowmeter can be applied to sanitary, relatively clean, and corrosive liquids in sizes up to approximately 24 inches. Smaller propeller flowmeters can be installed directly in the piping, but the size and weight of larger propeller flowmeters may require the installation of substantial concrete foundations and supports. The flow of corrosive liquids can be measured with proper attention to the materials of construction of all wetted parts, such as the body, rotor, bearings, and fittings.
Applications for propeller flowmeters are found in the water, petroleum, and chemical industries. Water applications include distribution systems within and between water districts. Petroleum applications include the custody transfer of hydrocarbons. Miscellaneous applications are found in the food and beverage, and chemical industries.
Propeller flowmeters are less accurate at low flow rates due to rotor/bearing drag that slows the rotor. Make sure to operate these flowmeters above approximately 5 percent of maximum flow. Propeller flowmeters should not be operated at high velocity because premature bearing wear and/or damage can occur. Be careful when measuring fluids that are non-lubricating because bearing wear can cause the flowmeter become inaccurate and fail. In some applications, bearing replacement may need to be performed routinely and increase maintenance costs. Application in dirty fluids should generally be avoided so as to reduce the possibility of flowmeter wear and bearing damage. In summary, propeller flowmeters have moving parts that are subject to degradation with time and use.
Abrupt transitions from gas flow to liquid flow should be avoided because they can mechanically stress the flowmeter, degrade accuracy, and/or damage the flowmeter. These conditions generally occur when filling the pipe and under slug flow conditions. Two-phase flow conditions can also cause propeller flowmeters to measure inaccurately.