Flow meter offers more positive displacement

Dean Palmer reports on a new, positive displacement flow meter for paint where aggressive fillers do little harm and very low flow rates can be measured Most of the positive displacement flow meters Eureka is aware of have some kind of rotating parts inside. However, a company based in Chesterfield has developed a new flow meter based on sliding piston technology. The company, Needham Specialised Machines, claims the wear on its new device when used with filled liquids, is less than with gear (or other rotating part) flow meters - less than a third in most cases. This is because the volume of liquid in a piston and chamber is large when compared to the volume trapped by a gear tooth and the periphery of the gear chamber. Whereas a gear meter counts the number of teeth and amplifies the signal, Needham's patent-pending dispensing/metering method is to recover as much information as is available and emulate the output during the slightly longer delays between output signals. This emulation and the flow meter's shape and size mean that the company's '1-Flo' meter can, in most cases, directly replace any type of gear flow meter. Needham has also spent the last two years developing a two-pack mechanical mix ratio controlling device similar to the meter mix unit featured in Eureka's August 2003 issue, developed by Phil Thompson of Kettering-based company Failsafe Metering. This system used electronic feedback to check the output whereas Needham's unit is mechanically self-timed using the previous spool movement. David Needham, managing director of Needham Specialised Machines, explained the origins of 1-Flo to Eureka: "The flow meter project started because an existing client had a paint with silica filler which was very abrasive. This aggressive paint was damaging the customer's gear type flow meters. We had been working with piston flow control devices and knew we could design and build a meter that would closely replicate a gear meter." He added that other problems clients had been experiencing with flow meters was if de-watered fillers collected at a point of turbulence prior to the meter and then broke away, falling into the gear teeth, which stopped the gears rotating. "Due to the high pressure, some liquid is then forced past the meshing part of the gear and quickly damages the side plates, requiring the meter to be at best calibrated, or even a complete overhaul of the damaged unit," said Needham. According to Needham, another customer found a two pack filled paint which was supplied with what appeared to be "grated cheese" in one component. As Needham explained: "Some two pack paints have a quantity of the base or activator ready mixed into each other but not in sufficient quantities to change its liquid state." In the case of Needham's client, the cheese-like material was very quickly filling filter systems and without filters was causing problems at the gear meters. This turned out to be poorly-blended component during the paint manufacture. "1-Flo meters have resolved both problems as any soft, stringy particles and dry fillers are cut up and pass through the mechanics of our meter," stated Needham. The 1-Flo meter is tolerant of larger particles and so the filtration mesh can be increased, improving the service intervals on machinery. And, from field service data collected over the last 18 months, Needham suggested that 1-Flo will have "three times the life of a gear meter when used on abrasive filled liquids". Needham continued: "Our first clients for 1-Flo meters were epoxy resin pipeliners who typically need flow meters from one to eight litres per minute, so this was our original range. When we tried to market this meter to automotive paint finishing users, they needed meters with one litre per minute or less, and they seemed content with their gear meters for this." But Needham discovered two areas where gear meters were causing problems for automotive companies - low flow rates (5 to 50cc/min) and densely filled liquids. He explained: "Gear meters give a very unstable output until the flow rate reaches the equivalent of 600 teeth movements per minute and below 200 teeth per minute they can stall. We made a smaller version of 1-Flo and tested it on hydraulic oil of only 5.5 centistokes viscosity. We kept turning the flow rate down and found that we could consistently read a flow rate of 0.1 cc per minute. We are not claiming that this meter gave a great deal of useful data at this very low flow, but crucially it did keep moving." 1-Flo has a built-in electronics package which works on a DC supply from 12 to 30 volts and is operator programmable to give outputs in the range 1 to 4,800 Hz in the form of a two-channel offset square wave. The output can be in the format 0 to 10 Volt or 4-20mA if the user prefers. Calibration can be performed at 100 separate points throughout its flow range, increasing accuracy and making the flow meter suitable for low flow gases. Working pressure is up to 400 Bar and Needham suggested that the device could also take on some applications in the food industry normally performed by Coriolis meters, especially where vibration is present. Pointers * Needham's 1-Flo meter has no internal gears or bearings, needs no maintenance, achieves outputs from 1 to 4,800 pulses per second and is accurate throughout a range of viscosities * The meter can be manufactured with a stainless steel body and plastic pistons making them useful for the food industry * The gentle action of filling and emptying of chambers compared to the crushing, grinding action of gears is the main reason for reduced wear on 1-Flo's internal components