Differential Pressure Flow Meters (DP Flow Meters) …… Still a Force to be Reckoned With, by Trevor Forster
A recent report [Global Industry Analysis Inc.] stated that globally one in six new systems purchased in 2014 use differential pressure flow meters & techniques and I have heard it stated that over 40% of existing flow measurement installations are differential pressure flow meter devices (DP flow meters). This surprised me. I was aware of the historic usage but assumed that engineers would today prefer to install more hi-tech equipment. Although difference pressure flow meter methods are losing ground it is evident that demand for this traditional flow metering technology is still very strong. The only technology that exceeds DP devices for units sold was electromagnetic flowmeter systems which accounted for just under 18% of all units sold in 2014. For this reason I felt I should review differential pressure flow meters.
Having been around for quite some time – there are today hundreds of types of differential pressure flow meters. To keep this feature article concise and readable we focus upon the common DP methods and simply mention some of the others.
Advantages of Differential Pressure Flow Meters
The advantages of differential pressure flow meters are numerous and include that they are usually inexpensive ; based on an easily understood principle their construction is robust and simple ; they can be used to meter a wide variety of fluids and gases ; there is a long established and constantly rationalized database of published information relating to the technique and modern secondary instrumentation related to DP flowmeters is relatively inexpensive.
Orifice plates are usually based on concentric discs introduced into a pipe. These create a restriction which generates a pressure drop. Pressure tappings are then positioned in accordance with the chosen standard of which there are numerous. Typically these are one diameter upstream and half a pipe diameter downstream, this is to read the line pressure and the pressure at the lowest point (highest velocity) in the vena-contracta. Corner tappings are also used which effectively measure the impact pressure and something approaching the vena-contracta pressure. The pressure drop that is generated is proportional to the square of the flow thus the resulting delta P must have a square root extraction to give flow rate, double the flow quadruples the pressure drop. This has the disadvantage that a 100:1 range pressure sensor will only give a 10:1 operating flow range although a typical range for this type of device is normally more like 4 or 6:1.
Whole books have been written on this subject and the system may require regular checking to ensure long term accuracy. The flow must be “turbulent” as laminar flow has an entirely different characteristic. The upstream edge of the plate must retain a sharp edge to maintain accuracy so these cannot be used on abrasive or dirty fluids.
Laminar flow element: Unlike the square law relationship with an orifice plate a laminar flow element uses low velocity fluid not in the turbulent region. This has a great advantage in that the pressure drop across the element is directly proportional to flow. This makes these devices very useful for small flows usually of gasses.
The element itself is usually a type of honeycomb offering a large surface area and low fluid velocities. Due to the operating principle these devices are usually only used for low flows as large meters of this type are very expensive to manufacture.
Pitot tubes most commonly encountered by the general public on the outside of aircraft where they are traditionally used to measure the airspeed. In engineering they are used in a wide spectrum of applications. They can be impact, cantilever or static and each type generally has a square law characteristic the same as the orifice plate
The advantage of the Pitot is there is very little pressure loss, the disadvantage is it is a point measurement and a full tube traverse or a multi-hole type should be used in an attempt to integrate the velocities across the whole conduit. They are ideal for checking things like air flow in heating/cooling ducts but can be used on quite large pipes with liquids as well.
Venturi nozzles: In the illustration for the orifice plate the illustrated flow lines form a contraction and expansion of the fluid. If you make your pipe form following this shape no secondary vortices are formed und there is no extra turbulence in the system. This controlled flow does not impose a high pressure loss on the system and is very energy efficient as the flow is contracted and expanded slowly conserving virtually all the energy.
In technical terms the discharge coefficient is near unity and almost constant in all flow velocities in the turbulent region, close to the ideal. Square root extraction is still required however. Different manufacturers have developed their own versions of these meters making them more compact with little compromise of performance. They are expensive to produce and install.
Nozzle devices are an attempt to produce a device that maintains some of the characteristics of the Venturi with the simplicity of an orifice plate. They have several advantages, they are compact, inexpensive (comparatively) to produce can handle dirty fluids as they do not have a sharp edge as mentioned in the orifice plate section. Corner tappings are usually used and the square root extraction is required.
For certain applications in gasses a version of this type of meter can be used called a “sonic nozzle”. These use the characteristics of gasses to give a constant flow and are very useful as a calibration reference as the throughput is fixed provided a certain upstream pressure is exceeded and the velocity of the gas in the throat is “sonic”. A simple and effective reference device.
Variable aperture devices is a big area as well and I am not going to attempt to cover all the meters in this category. Many are probably not considered as differential pressure flow meters as their operation appears different to the meters described above but they do use a differential pressure to make them work so in my mind belong in the same category.
Even my pond filter has a meter from this category. It is a simple drag/target meter with a moulded, hinged flap which is driven open by the mass of the fluid hitting it and the pressure drop behind causing the flap to open. Anything less than 45° open and the filter need’s a clean.
Commercial, well-engineered versions of this type of flow meter are available. Rotameters also fall into the same category as the height of the ball or specially shaped float is driven by differential pressure operating against gravity or in more industrial versions a spring.
Although this technology is slowly receding there does appear to be some very good reasons to continue with this low-tech, highly reliable method of measurement. Titan is concentrating its future development program on the Atrato ultrasonic flow meter as this technology appears to be the fastest expanding method of flow measurement.