Fluid Menu

High & Low Viscosity Flow Measurement

 

VFF Rotary Meters

 

Design and Principles

 

 

Accurate Positive Displacement Metering

 

Design

 

The basic meter design is very well established and appears in the British Standards BS7405: 1991 Section 3.3 which reviews recognised meter types. Litre Meter has applied the design in producing a range of specialised high performance meters particularly capable of measuring low flows. The flow causes a rotor to move within a measuring chamber. The VFF Series is manufactured in 316 stainless steel with a chemically resistant carbon graphite rotor or with a rotor in metal selected for the fluid service. The top cap O-ring seal is of FPM, PTFE covered silicon or other materials. High pressure meters usually have silver plated Inconel seals. The meter is optionally fully submersible rated IP68 to 20m with the enclosure for the signal output having an M20 gland entry. The output is volt-free reed-switch contact for computers or instruments.

 

Installation & Maintenance

 

The meter is installed in a horizontal pipework with the unit vertical. The pipe size can differ from the meter entry size and pipe bends etc are allowed prior to the meter with no accuracy degradation. Filtration to 100 microns is desirable. Single or multiple, long-life reed-switch sensors are used. At the current levels used in our insturmentation life of these sensors is in excess of 30 years operation at maximum flow. They can be replaced with the meter in service. As a stand-alone item only two-wire field cabling is required. The flowmeter is exceptionally robust and the construction is extremely simple. The unit is easy to dis-assemble from above with the meter body remaining in the line.The rotor sits freely in the meter body and can be easily removed for cleaning etc. No special refitting techniques or tools are needed.

 

Performance

 

The precision manufacture of Litre Meter VFF flowmeters ensures that the maximum accuracy is obtained over a wide flow range and that low flowrate performance is assured. Repeatability of measurement is better than 0.2% with a metering accuracy of 1% of actual for most applications. The meter provides an excellent performance and even measures bi-directional flows. Accuracy can often be further improved by the use of instrumentation with linearisation facilities

 

Meters Of Quality

 

The VFF Series is manufactured in 316 stainless steel as standard with a chemically resistant carbon graphite rotor or with a rotor in metal selected for the fluid service. The top cap O-ring seal is of FPM, PTFE covered silicon or other materials. High pressure meters have silver plated Inconel seals. The meter is fully submersible rated IP68 to 20m with the enclosure for the signal output having an M20 gland entry. Other varaitions are available for sub-sea duty. The output is volt-free reed-switch contact for computers and other flow instruments.

 

Hazardous Area Use

 

The VFF meter output is considered 'simple apparatus' and is covered under European standards by clause 1.3 of EN50 014 and BS 5501 Part 1 as suitable for up to EEx ia IIc T4 class installations. The output is isolated for hazardous area use by either suitable zener barriers or a galvanic isolator device located in a safe area. The safe area can also be provided by an EEx d enclosure. Battery-powered, Intrinsically Safe rate and total readout units can be fitted to the VFF meter. They are certified EEx ia IIB T4 for hazardous area use.

 

Principle Of Operation

 

 

 

The flow causes a rotor to move within a measuring chamber. This movement is sensed, giving an output representing an increment of volume flow. 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 top of the rotor is equipped with a powerful magnet directly above the 'peg' that is 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. A volt-free contact closure output signal is given for each oscillation which represents a volume increment. The fluid is transported in a 'positive' manner at all times. The typical metering repeatability is better than 0.2% and a meter accuracy of 1% actual reading is usually obtained over a substantial flow range. For lowest flows the meter will under-read the actual flow in a consistent manner. This allows an improved wide-range system accuracy to be gained by the use of a linearising electronics instrument such as the Litre Meter Micro.