Oil Industry Flowmeters
Flowmeter Selection and Principles
Meter Selection
The operating principles of the VFF and Helical Screw flowmeters are of positive displacement type where the flow is divided into discrete 'pockets' There is little slippage past the rotating elements and the fluid is therefore 'positively' metered with high accuracy.
Alternatively 'Inferential ' turbine flowmeters, infer the flow by the rotor being 'spun' as fluid passes through the meter and provide less inherent accuracy. For chemical injection applications involving low flow rates and hydraulic lines where the fluid can be static and cold, the fluid viscosity can be high and a positive displacement meter would be the preferred principle. The Pelton Wheel Flowmeter would be selected for such applications as wash water and other low viscosity fluids such as methanol.
VFF Series Meters
The basic design of our VFF meters is very well established and appears in the British Standard BS7405: 1991 section 3.3 which reviews recognised meter types. The flow causes a rotor to move within a measuring chamber. This movement is sensed, giving an output representing an increment of volume flow. Typically repeatability is better than 0.25% and a system accuracy of 1% can be obtained.
The rotor is basically a disc shape with an annular groove on its underside capable of holding and transporting flow from the chamber inlet to the outlet. Some fluid is also transported in a cavity formed between the rotor outside wall and the chamber wall. A centre 'peg' under the rotor is constrained to run in a circular groove in the body. A web in the body is engaged with a slot in the rotor and this modifies the rotation to that of an oscillation as flow passes. It is this oscillation that produces the compartmentation of the fluid into positively displaced 'pockets'. The rotor top has a magnet encapsulated in 316 stainless steel directly above the 'peg' on the underside and so this also has a circular path which allows it to engage and disengage a reed-switch sensor located in the top cap above. An Intrinsically safe volt-free contact closure output signal is given for each oscillation representing a volume increment. The fluid is transported in a 'positive' manner at all times.
Helical Screw Flowmeters
The Helical Screw Flowmeter has rotor scrolls which accept and release continuous flow. While the flow is being transported through the centre portion of the meter, it is trapped by the 'flights' of the scrolls, these form positive displacement 'pockets'. The meter inherently provides a high resolution pulse output from a many toothed polewheel on the end of one of the scrolls. The polewheel is sensed by the Litre Meter high pressure sensor and Intrinsically Safe transmitter electronics which provides a strong 3mA constant current square wave sinking pulse. This pulse can be transmitted long distances and has good noise immunity. The meter materials of construction are compatible with MEG and mineral or synthetic hydrocarbon hydraulic line fluids. The meter is ideal for fuel oils and has been widely used in the marine industry.
Pelton Wheel Flowmeters
The Litre Meter Pelton Wheel flowmeters employ a single size straight bladed rotor manufactured in chemically resistant PFA and sapphire bearing balls. The rotor bearing balls run in hemispherical sapphire bearing cups forming two 'floating' low friction precision bearings. Ferrites sealed in the PFA rotor are sensed by a pick-up coil in the Litre Meter high pressure sensor and Intrinsically Safe transmitter system. The flow path for the LM series meters is shown where all of the flow travels with the Pelton Wheel and exits from the flowmeter. The meter provides a high resolution NPN pulse output. PNP and TTl are optional outputs.
LM Series meters are specifically designed for low flows (for example the LM24 size flowrange is 0.03 to 4.3 l/min) and have a wide turndown ratio of up to 280:1. For the MM Series an identical Pelton Wheel ring assembly is located in a by-pass chamber. The rotor movement is directly proportional to total flow through the flowmeter. This gives a wide rangeability, typically 50:1 for the MM orifice/by-pass meter.
Our Products
VFF Rotary Piston
LMX Pelton Wheel
Sierra Mass Gas Flowmeter
Helical Screw