A positive displacement flow meter measures flow by ‘displacing’ a precise and known volume for each revolution of their internal parts. There are many types of internals such as two or three gear oval gear, nutating disc, rotary piston and sliding vane. Because this style of flow meter displaces a known volume, its accuracy is largely unaffected by changes in the physical properties of the liquid, such as viscosity or density. In volumetric terms, a ‘litre’ is always a ‘litre’ regardless of viscosity or density. A PD flowmeter is extremely accurate, offering linearity of better than 0.1% of reading and repeatability of better than 0.05%.
To ensure high accuracy, the clearances inside the flow meter measuring chamber must be as fine as possible without causing excessive wear. This means that the metered liquids this flow meter can measure must be clean and free from suspended solids. In most cases, it is prudent to install a suitable liquid filter on the inlet side of the meter. Positive Displacement flow meters require no upstream/downstream straight pipe lengths so, if required, a filter can be directly bolted to the inlet port.
Inevitably a very small amount of liquid will pass through the flowmeter without passing through the measuring chamber. This usually occurs between the rotor and the side plates of the meter. Although this ‘slippage’ is extremely small, it does mean that it is essential to calibrate each flow meter prior to despatch. Flowmeters are generally calibrated using a low viscosity liquid where slippage will be at its highest. As viscosity increases, ‘slippage’ will reduce as it is harder for the thicker liquid to pass by the sides of the rotor.
The flow range of PD meters is often dictated by the working viscosity. Manufacturers usually quote flow ranges at 1cP viscosity. As working viscosity increases, so does the pressure drop through the flow meter. If pressure drop reaches one bar and flow rate is below that stated for 1cP, this flow rate is regarded as the maximum flow allowed under these working conditions. On the plus side, as viscosity increases so does the minimum measureable flow (due to reduced ‘slippage’).
For many years, Positive Displacement flow meters have been extensively employed in truck delivery of fuel and chemicals. A comprehensive array of mechanical accessories can be fitted to the flowmeter to provide the user with a highly accurate measuring system. Such accessories include liquid strainer, air eliminator, batch pre-set register, large numeral reset totaliser, zero-start or accumulative ticket printer, flow non-return valve and mechanical batching valve. Where an electronic output is required the flowmeter can be fitted with a suitable pulse transmitter which can be used to input to remote instrumentation. Pulse Transmitters are available with ATEX Approval where necessary. Exd Pump Switches can be fitted inside the LNC or a Pneumatic Switch Box can be fitted to switch the air supply for pneumatic batch systems.
Positive Displacement flowmeters are ideal for applications requiring customer billing. Many flowmeters carry Weights & Measures / MID Approval and, in such cases, flowmeter registers are often lead-sealed to prevent tampering.
PD meters are also used to measure very small flows down to less than one litre per hour. They are the flowmeter of choice for many oil and pharmaceutical companies where a very high degree of accuracy is essential. Fuel oil additive metering and additive injection are typical applications where reliable mechanical metering solutions are required.
A positive displacement flow meter offers such a high degree of accuracy that many flow testing and calibration facilities install them as reference meters. These reference flowmeters are usually calibrated by a National Standards Authority so testing houses can claim accuracy of traceability to National Standards such as UKAS.
Flowquip offer positive displacement flowmeters manufactured by VAF Instruments. VAF have been manufacturing flowmeters for seventy years and are one of Europe’s leading flow specialists. Our range of VAF PD meters includes:
Milliflow (M31) piston flow meter which handles flows down to less than one litre per hour, which makes it ideal for additive injection and liquid blending processes.
LoFlow Vane meters which handle flows in the range of 1 to 50 l/min making the LoFlow ideal for continuous flow measurement and batching applications
MidFlow Vane meters which handle flows in the range of 5 to 2750 l/min making it ideal for metering hydrocarbons and chemicals. The MidFlow Vane meters are available with various register and pulse transmitter options.
HiFlow Vane meters handle flows in the range of 230 to 16000 l/min making them ideal for larger continuous flow measurement or batching applications.
All VAF flowmeters have ATEX Approval and LoFlow, MidFlow and HiFlow flowmeters can be supplied with MID (Custody Transfer) Approval.