Reverse flow measurement – an expert view

Most flowmeters are setup for forward flow measurement. That is, they have a preferred direction for operation and that is adopted by the manufacturer/designer and advised to the customer.

Some flowmeters are symmetrical in the forward and reverse direction and will work in the reverse direction, too. Some are symmetrical and may not function or function well, due to meter design or principle, in reverse.

Finally, some meters may detect that the flow is in reverse and, better still, measure in either direction to the same accuracy.

We use the term flowmeter generically. When discussing flow direction and measurement we should consider the instrumentation as well as the flow sensor itself.

Distinctions:

Type A – not designed for Reverse flow, may cause damage:
Type B – can accept reverse flow, no detection
Type C – can accept reverse flow, detects direction of flow
Type D – Can accept reverse flow, measures accurately but doesn’t output direction
Type E – Accepts reverse flow and provides flow velocity and direction

B, C, D, E may need special instrumentation to extract the desired information.  The type designation just helps us understand the designs – it isn’t used outside of this article.

Flowmeter element Type
Orifice plate B, C, D or E
Wedge B, C, D or E
Venturi tube B, C, D or E
Flow Nozzle B or C
Pitot tube B
Elbow meter B, C, D or E
Target meter B or C
VA A or E (E with a special meter)
Positive Displacement A, B, C, D or E depends on design and sensing
Turbine A, B, C, D or E depends on design and sensing
Pelton A, B, C, D or E depends on design and sensing
Vortex A or B
Electromagnetic B, C, D or E – most units would be E
Ultrasonic(Doppler) B, C, D or E – most units would be E
Ultrasonic(Time-of-travel) B, C, D or E – most units would be E
Mass Coriolis B, C, D or E – most units would be E
Thermal Mass Insertion B
Thermal Mass Capillary B
Weir A
Flume B

 

Litre Meter flowmeters

Pelton Wheel

The LM range have a jet (apart from the LM220 and LM330). With a jet concentrating the flow onto the rotor, a reverse flow is very inefficient and the rate of rotation is much reduced for the same flow. The 220 and 330 are broadly symmetrical in some models and have different pipe layouts in most versions. These have never been fitted with technology to provide direction.

The same can be said of the larger, orifice plate based, MM meter. The inlet and outlet holes to the pelton wheel chamber are much different in size, so function poorly in reverse. A sole MM was manufactured with two pelton wheels and two sensors facing in different directions. By comparing the magnitude of the signals it was simple to tell which direction was active.

VFF

All VFF (rotary piston positive displacement) meters are perfectly symmetrical in design with rotors able to rotate clockwise or anticlockwise. There are small differences in machining so there are small differences in meter performance forward and reverse. There is a prescribed forward direction (purely for consistency) but the client can select the other direction as forward.

With a standard sensor the output is the same whether the flow is forward or reverse – just a series of pulses. When two sensors are fitted then the direction and magnitude of flow can be determined if the right instrumentation is used.

There are two suitable instruments used by Litre Meter and at least one system used by clients with varying degrees of sophistication. Sometimes two sensors are fitted for redundancy purposes. The instrument monitors one sensor. After a certain time period, if there isn’t a pulse attention is switched to the other sensor. The period is set to be a few seconds longer than the frequency for lowest flow achievable or the clients lowest expected flow.

Litre Meter use two sensor setups both based on reed switch sensing of the magnet in the VFF rotor. Litre Meter are developing a 3-D magnetometer based field sensor that can determine the position of the rotor in the chamber for finer pulse output.

If reed A switches before reed B then direction is forward. If B before A then direction is reverse.

The two reed switch setups are similar in concept but packaged differently. The original reed switch is situated in a sensor hole. When two reeds are used there are two parallel holes generally situated along the radius of gyration of the magnet. They are spaced to produce a clear lead or lag depending on rotor direction. The Fluidwell F115 is designed to interpret the lead/lag to produce a display with directionality.

With the introduction in 2015 of the CIFM versions of the VFF and the Litre Meter FlowPod the sensor was repackaged, with two reeds as standard, in an M8 stainless sensor body. Only one sensor hole is required as the reeds are side by side. The F115 and FlowPod can both interpret the CIFM sensor output.

F115

Fluidwell manufacture a wide range of instrumentation which Litre Meter have used since 2002. The F115 version is specifically designed for directionality. It does not have linearisation.

Description

The flow rate / totalizer model F115-P is a microprocessor driven instrument designed to show the flow rate, the total and the accumulated total. This model is able to detect the flow direction and to show a positive or negative flow rate, the totals for both directions and the cumulative totals.
This product has been designed with a focus on:

  1. ultra-low power consumption to allow long-life battery powered applications (type PB/PC),
  2. intrinsic safety for use in hazardous applications (type XI);
  3. several mounting possibilities with aluminum or GRP enclosures for harsh industrial surroundings;
  4. ability to process all types of flowmeter signals;
  5. transmitting possibilities with analog / pulse and communication outputs.
Flowmeter input

Two sensors with a phase difference of 90 or 270 degrees, can be connected to the F115-P.

Standard outputs
  1. Pulse output to transmit a pulse that represents a totalized quantity as programmed.
  2. Negative / positive pulse output indication – i.e. a flag.
  3. Linear 4-20mA analog output to represent the actual flow rate as programmed. The 4-20mA signal limits can be tuned.

FlowPod

Description

The FlowPod was designed from the outset with the CIFM sensor which is fitted with two closely positioned reed switches in one M8 stainless steel package. They are positioned in the flowmeter to provide a two separate pulses along the path of the spinning rotor. They are fairly close together such that there is a distinct time difference between reed A and B depending on direction.

In most installations the second reed is for redundancy purposes. The FlowPod monitors reed A. After a certain time period, if there isn’t a pulse, attention is switched to reed B. The period is set to be a few seconds longer than the frequency for lowest flow achievable or the lowest expected flow.

For directionality the FlowPod monitors reed A and B and interprets lead and lag to determine flow direction. Within the software forward direction can be set as ‘A before B’ or vice versa. Redundancy is still offered, although, of course, without direction.

Linearisation is standard on the FlowPod. The curve of flow rate versus frequency is similar in forward and reverse.