Brief Summary: The flowmeter is a volumetric measuring turbine type. The flowing fluid engages the vaned rotor causing it to rotate at an angular velocity proportional to the fluid flow rate. The angular velocity of the rotor results in the generation of an electrical signal (AC sine wave type) in the pickup. The summation of the pulsing electrical signal is related directly to total flow. The frequency of the signal relates directly to flow rate. The vaned rotor is the only moving part of the flowmeter.
Turbine Flow Meter
The Turbine flow meter (axial turbine) was invented by Reinhard Woltman and is an accurate and reliable flow meter for liquids and gases. It consists of a flow tube with end connections and a magnetic multi bladed free spinning rotor (impeller) mounted inside; in line with the flow. The rotor is supported by a shaft that rests on internally mounted supports.
The Supports in Process Automatics Turbine Flow Meters are designed to also act as flow straightners, stabilizing the flow and minimizing negative effects of turbulence. The Supports also house the unique open bearings; allowing for the measured media to lubricate the bushes – prolonging the flow meters life span. The Supports are fastened by locking rings (circlips) on each end.
The rotor sits on a shaft ,which in turn is suspended in the flow by the two supports. As the media flows, a force is applied on the rotor wings. The angle and shape of the wings transform the horizontal force to a perpendicular force, creating rotation. Therefore, the rotation of the rotor is proportional to the applied force of the flow. Because of this, the rotor will immediately rotate as soon as the media induces a forward force. As the rotor cannot turn thru the media on its own, it will stop as soon as the media stops. This ensures an extremely fast response time, making the Turbine Flow Meter ideal for batching applications.
A pick-up sensor is mounted above the rotor. When the magnetic blades pass by the pickup sensor, a signal is generated for each passing blade. This provides a pulsed signal proportional to the speed of the rotor and represents pulses per volumetric unit.; and as such the flow rate too.
Also Read: Turbine Flow Meters Troubleshooting Tips
Advantages & Disadvantages
The cost is moderate. Very good at clean, low viscosity fluids of moderate velocity and a steady rate. Turndown is very good as it can read very low compared to the maximum flow. They are reliable if put in a clean fluid especially if it has some lubricity. AGA and API approved for custody transfers. They do cause some pressure drop where that may be a factor such as gravity flows. Not reliable for steam. Bearings wear out.
In order of magnitude from largest to smallest, these are used in oil and gas, water and wastewater, gas utility, chemical, power, food and beverage, aerospace, pharmaceutical, metals and mining, and pulp and paper.
Cautions for Turbine Flowmeters
Turbine 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. Turbine 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, turbine 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 turbine flowmeters to measure inaccurately.
Turbine Meter Advantages
• Wide flow rangeability including low flow rates
• Turndown ratio is up to 35:1
• Good level of accuracy at an economic price
• Simple, durable construction
• Easy to install and maintain
• Flexible connection to flow instruments for flow control
• Wide variety of process connections
• Turbine meters can operate over a wide range of temperatures and pressures
• Low pressure drop across the turbine
• Provides a convenient signal output
Turbine Meter Limitations
• Requires constant backpressure to prevent cavitation
• Accuracy adversely affected by bubbles in liquids
• Turbine meters can be used with clean liquids and gases only (may need to install a strainer upstream to prevent damage from particulates)
• Not applicable for measuring corrosive fluids
• Requires a turbulent flow profile (consistent fluid velocity across the pipe diameter) for accuracy
• Sensitive to changes in fluid viscosity
• Require a straight run of pipe before and after the turbine meter to allow swirl patterns in the flow stream to dissipate
• May not function properly with high viscosity fluids where the flow profile is laminar
Also Read: Flow Switch Working Animation