Accurate, reliable, cost-effective flow measurement is more important today than it has ever been before.
The years have seen many styles and grades of flowmeter developed, because the first problem in the meter specification is to define what the measurement is for. Dispensing a volume of juice concentrate to within 2percent is a different problem to knowing and alarming when the cooling water flow drops to 50percent of the desired flow, with a system shutdown operating at 25percent. And both are different to delivering whisky or beer or petrol as a financial transaction. This article presents one view of the techniques used for liquid measurement; it does not include every type of flowmeter on the market, just some of the more familiar ones.
aCost of ownership' is often an issue for end users and suppliers but, for an OEM (original equipment manufacturer) maybe a better phrase is aconsequential costs of use'. The cost of ownership is not just the purchase price but the cost of any breakdown in the warranty period, performance loss resulting from measurement uncertainty and the power or equipment cost of running both the flowmeter and the fluid pressure drop it introduces. The cost of use for an OEM can also include any separate wiring needed, the positioning and space taken up in the equipment -- including the straight-line pipe requirements -- the weight and mounting arrangement, and any separate display or output interface.
Adding all these consequential costs to the function required can often lead to the choice of a simple mechanical vane or paddle-wheel indicator, or a mechanical totaliser -- today this might include self-contained battery-powered microelectronics; think of the modern electronic gas meter, for example. The right flowmeter for the job is the one that will perform the task actually required, satisfactorily, at an acceptable fully installed cost.
Flowmeter types
While all flowmeters can make use of the latest technology, the basic techniques are fairly well established. Flowmeters fall into six broad groups: differential pressure (DP) devices (including variable area meters and mechanical flap devices); inferential devices such as turbine meters and propeller meters; positive displacement meters (including oval gear, nutating disc and oscillating piston types); fluidic devices and vortex meters; velocity measuring devices such as electromagnetic and ultrasonic types; mass flow measurement meters (Coriolis and thermal). Each of these has its own strengths and weaknesses; every different flowmeter style within these categories was originally developed for one particular OEM or customer application.
DP flow measurement is the original method, but not necessarily the best. Generally it is used for the most mundane, and the most critical duties. The Pitot tube uses the differential pressure generated by fluid impacting the open end of a tube facing the direction of flow; a second tapping at ninety degrees to the first measures the astatic' pressure. When used on an aircraft, this measures the airspeed. Some years ago a maintenance engineer left some masking tape over the static pressure tapping on an aircraft Pitot tube; the flight engineer duly set the instruments to the ground static pressure. As the plane took off in poor weather the instruments gave nonsensical readings and the plane crashed, killing all on board. This highlighted the consequences of inadequate end-user installation and maintenance procedures.
The most common method of using this principle is with an orifice plate, a small hole within the bore of the pipe that creates an obstruction to flow: the square root of the pressure differential is proportional to the flow rate. This is where one of the problems with this type of device is encountered. If the pressure sensors have a 50:1 range the resulting range of the flowmeter is only 7:1 after the square root extraction.
It is often said that DP cells with orifice plates are the most popular form of flow measurement in the world today: but this is probably not so for OEMs -- or not unless they buy the DP cell with integral orifice complete, calibrated, pressure tested and in one unit, but then it will most likely be too costly. Without the integral unit the technique involves too many connections and parts to assemble.
There are many places where OEMs would use other DP-based devices: a classic is a V-notch weir or flume in an open channel, with a level-measuring device that can be a float, an ultrasonic level meter or similar. Flumes are available for such applications in the form of fibreglass moulds from specialist suppliers. Far more usual with OEMs are variable area flowmeters that consist of a movable afloat' in a widening aperture, but in this case the differential pressure is used to balance the afloat' against a force. This is typically gravity, as seen in a benchtop glass tube laboratory VA (variable area) meter, but some meters balance a float, flap or vane against a spring, either in a metal or glass housing, or as an insertion device; this makes them orientation-independent, but results in a higher operational pressure drop. The forces balancing the float are the mass, velocity and viscosity of the liquid. Simple visual devices are capable of ±1percent accuracy, but ±5percent is more likely. Virtually all devices today can have electronic analogue outputs added, or alarm trips, driven by a magnet in the float or flap.
There are many specific designs, such as metal-bodied units for high pressure, with magnetically driven indicators; PTFE-lined tubes for corrosive liquids; and single alarm set-point low-flow switches for domestic water heaters. All tolerate any installation pipework format, and dirt in the liquid, since the aperture just opens further if the pressure is available. Some are low-cost and easy to troubleshoot; flow range is typically between 7:1 and 10:1, and a 1percent device is possible.
Turbine and propeller meters
A turbine meter is the most common and easiest to understand of all the different inferential meters. The axial turbine meter is essentially a propeller in a pipe; with careful design the speed of the turbine is directly proportional to the flow rate; accuracies of ±0.25percent are achievable. They have many advantages: they are relatively small and are usually the same diameter as the pipe in which they are fitted, and pressure loss is quite low. Because of this tubular construction, high pressures and temperatures are readily accommodated (Fig.1). All turbines are sensitive to viscosity changes and should be manufactured and calibrated with the final application in mind. They are only as good as their bearings and the smaller the axial turbine the more important the bearing characteristics, as the energy available to overcome the bearing friction is somewhat reduced -- and the bearings, being smaller, are more sensitive to degradation.
Pelton wheel or radial-flow turbines are therefore better suited to low flows, as the bearings can be very robust; the energy available from the liquid flow around this type of enclosed awater wheel' is far greater than that available with the axial device. They are particularly suited to OEM applications such as beverage dispensing or monitoring. However, the disadvantages of this type of meter are its relatively large body compared to the line size, greater pressure drop and reduced accuracy. Advantages include low manufacturing costs, a larger dynamic range and the ability to meter very low flows.
Electronic outputs are provided either by a magnetic or optical pick-off to count the rotations of the wheel or turbine. Pick-offs are moulded into the body housing; even the electronics for a totaliser or flowrate display can be housed within the meter body (Fig.2). Typically axial or Pelton wheel devices are produced in moulded modern engineering plastics, which are corrosion resistant and can be fitted with push-on fluid connectors, hose barbs or screw threads (Fig.3). Custom engineering of these connections allows them to incorporate panel or bulkhead mounting arrangements to suit the application (Fig.4), and the electrical wiring loom and terminations can be similarly customised.
Positive displacement meters
There are many different types of positive displacement meter: gear, oval gear, sliding vane, nutating disc, oscillating piston, helical screw and many more. All have the same basic mode of operation in that they take a discrete volume of liquid and pass it from the inlet to the outlet without loss or slippage.
The better types are capable of ±0.1percent linearity over a wide flow range. By nature, these devices perform better at higher viscosities, as the increasing fluid thickness reduces the leakage rate even further and extends the useful operating range to lower flow rates. Globally the most common form of this device is likely to be the domestic water meter or the meters found in petrol dispensing equipment. Because of the way these meters operate, they often have a high-pressure drop, particularly with more viscous fluids, but some types -- including the oval gear design -- operate with a very low differential pressure, only millimetres of head in some cases.
They are highly suitable for oils, although some models in a range may be manufactured specifically for corrosive media; for example there is a version of an oval gear meter made from totally non-metallic plastic or ceramic components. In all cases the fluid should not contain any solid particles or stringy materials, as these would possibly jam the meshing gear or other mechanisms.
Electromagnetic meters have good rangeability, are available in a wide range of pipe sizes, have low pressure loss, and power requirements are constantly being reduced as better magnetic and electronic techniques are being found. They will handle sewage, slurries and paper pulp. If the materials are selected carefully the aelectrodes' are capable of handling very aggressive materials. They require the flowing liquid to be electrically conductive, but the lower limits on this are constantly decreasing. While prices are falling, they still tend to be expensive, and would only be used on slurries or other difficult liquids.
The main type of ultrasonic flowmeter currently available uses two or more transducers that fire a pulse of ultra-sound at an angle both with and against the flow; the difference in time between the signals is effectively the flow rate. Multi-path beams are used for very large pipes and in most sizes they are available as clamp-on devices. Special designs are used in small lines to increase the path length by several reflections, or the flow path is made to run along the pipe axis, as in the domestic gas meter.
The original ultrasonic meters, introduced around 1978, used one clamp-on transducer, and sensed reflected signals returned by particles or suspended solids in the flow, which shifted the transmitted frequency according to the Doppler effect. In a vibration-free pipe, with a firmly clamped or bonded transducer, these units could achieve a reasonable flow indication or ano-flow' alarm, particularly for monitoring reasonable velocity slurry or sewage flows. Unfortunately the first clamp-on reasonably priced meter systems were sometimes overused on unsuitable applications, which led to this type of meter gaining a poor reputation; there are still some suitable applications today, alongside meters that measure flow noise or particle impact sounds.
These meters are very expensive, but prices are falling. Some versions are very sensitive to errors induced by two-phase flow conditions, such as occurs if vapour is included with the liquid. This is true for all types of flowmeter, but the errors can have serious implications for mass flow meters that are attempting to achieve the ultimate in accuracy.
Thermal mass flow systems commonly used for controlling low mass gas flows are now being introduced for liquids. The technique usually uses a measure of the power required to maintain a temperature increase in the flowing liquid down a bypass capillary around an orifice.
Trevor Forster is with Titan Enterprises of Sherborne, UK. " target="_blank">www.flowmeters.co.uk
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