A guide to selecting a Author: Martin Cuthbert hydraulic flow meter This guide looks at what flow is, how to decide what flow meter to use, and at some of the more common flow measurement methods that are currently available for use on hydraulic systems. What is flow? Flow is the measurement of the volume of a liquid that passes a fixed point in a unit of time. For most hydraulic applications, flow is measured in litres per minute (lpm), U.S. gallons per minute (US gpm), or, occasionally, U.K. gallons per minutes (UK gpm). Multiply By To obtain US gpm 3.785 lpm UK gpm 4.546 lpm 1
Why measure hydraulic flow? Flow is to the hydraulic engineer what current is to the electrician, while pressure is the hydraulic equivalent of voltage. Measuring one without the other can lead to a completely wrong diagnosis of why a system isn t performing. For example, if a hydraulic cylinder isn t moving, you could measure the pressure at the inlet and see that it is 200 bar (3000 psi), as expected. And if you looked no further, you might conclude that the hydraulic power pack is fine and that the fault must lie with the cylinder. However, if you measure the flow into the cylinder, you might find that there is none. This would immediately make you question whether the pump is working and have you focus on the components that can affect flow, such as the pump or an incorrectly set relief valve. Flow measurement If you had no flow meter and needed to get an idea of the flow rate in a hydraulic system, a crude way to measure the flow would be to time how long it takes to fill a bucket with oil. From the volume of the bucket and the time it takes to fill it, you could calculate the volume per time, or flow. The accuracy would depend on many things, including the volume of the bucket and the type of stopwatch that is used. However, apart from being quite dangerous, the solution is not very practical; once the oil is inside of the bucket, it is out of circulation, exposed to contamination and no longer inside of the hydraulic system. What should you consider when selecting a flow meter? When searching for a flow meter for use in a particular hydraulic application, these five questions can really help: 1. What are the fluid properties? 2. What are the hydraulic system operating conditions? 3. Why are you measuring flow; how accurately do you need to measure flow? 4. What effect might the flow meter have on the fluid and vice-versa? 5. How important is it to measure flow; what is your budget? If you can answer all of these questions, it becomes a lot easier to specify the right flow meter for your application at the right price to ensure that you get the right results. What are the fluid properties? First of all, is the flow meter going to be used on the same fluid all of the time? This is typical when a flow meter is permanently installed. However, if it s portable and taken to different machines in the field, it might be used with a range of different fluids. It is important to know about the fluid(s) you are measuring, as the characteristics of the fluid can greatly influence your choice 2
of flow meter. Of particular interest are the fluid properties: Is it corrosive or a natural lubricant, and what is its material compatibility and viscosity characteristics? This information can be found on the fluid s datasheet under such headings as physical properties, lubricity and anti-corrosion. What are the hydraulic system operating conditions? Consider these two requirements: You need to measure high flows that don t vary much, such as between 1000-1100 lpm (264-291 US gpm), and you need to measure a large range of flows, such as between 1 and 400 lpm (0.26 and 106 US gpm). The first example, with such a high flow, might look more extreme. However, it s actually much more straightforward to measure than the second example. Knowing the minimum and maximum flows you are looking to measure will directly influence the type of flow meter you need to buy and the price of that meter. The ratio of the highest flow to the lowest flow is called the turndown ratio. In the first example, this was 1.1:1, while in the last example, this was 400:1. The higher this ratio is, the harder it will be to find one flow meter that will cover the whole range with consistent accuracy. This could mean that you will need two or more meters to cover that range, or you might choose to reconsider the minimum operating range and decide that 10-400 lpm (2.6-106 gpm) is acceptable, which reduces the ratio to 40:1. The other information you need to know is the typical operating conditions of the hydraulic system. This will include knowing the system s typical cleanliness level, especially if the system isn t very clean, as some flow meters are more sensitive to contamination and can easily jam and stop working if cleanliness isn t kept below certain limits. Oil cleanliness There are lots of guides to defining the right cleanliness level for your hydraulic system. I ve found the articles and books by Brendan Casey to be very informative; search online for Brendan Casey - hydraulic fluid cleanliness. The maximum operating pressure will influence the flow meter material type and, thus, the price. For example, monitoring the tank line pressure at 10 bar can be much easier and cheaper than monitoring the pressure line at 350 bar. In the first case, the meter could be made from a diecast aluminium housing, while in the second, it might be machined from high-grade aluminium or stainless steel. The ambient and system temperature range also matter. Once you know the fluid properties and the system s operating temperature range, you can quickly look up or calculate the kinematic viscosity for the fluid over that temperature range. For example, in a typical hydraulic system with an ISO32 fluid used between 40ºC and 60ºC, the kinematic viscosity will vary between about 34 and 15 cst. Fluid viscosity As the temperature of a hydraulic oil increases, the kinematic viscosity goes down. The rate of change of the fluid s viscosity is described by the viscosity index. The higher the viscosity index, the lower the rate of change of viscosity for every degree of change in the temperature. The effect of the change in oil viscosity will affect some flow meter technologies more than others. So, if the oil viscosity of the hydraulic system is expected to be quite stable and in the range of 10-100 cst, then all flow meter types listed below could be used. However, if the oil is used for high-pressure lubrication and its viscosity is above 100 cst all of the time, or if the oil is actually a low-viscosity biodegradable hydraulic fluid, the meter has to be suitable for high or low viscosities respectively. This will limit the types of meters that can be used. Possibly the most difficult situation is when the fluid is subject to wide temperature swings, resulting in large viscosity changes, as this can directly affect flow accuracy. In this case, the chosen meter will need to be 3
relatively insensitive to the effect of viscosity, have built-in viscosity compensation or will need to be calibrated for an average viscosity, with allowances being made when the temperature/viscosity is out of range. Viscosity For further information on viscosity of hydraulic oils, go to www.webtec. com and search for viscosity in the top right-hand search box. Why are you measuring flow; how accurately do you need to measure flow? For some applications, flow measurement is required to monitor trends, such as answering the question of whether the flow is more or less than last week. At other times, flow measurement is required to compare performance with other systems or against a manufacturer s specification. In the first example, repeatability is more important, while in the second example, you will need repeatability and much greater accuracy. Output signals Depending on the application you may also need to take a signal from the flow meter to send to a PLC or data logger, so that you can record the flow going through the system at a particular time. Some basic flow meters, often used for monitoring trends, only have an analogue dial and these can rarely be upgraded. However, many flow meters offer an output signal as standard and there are a wide range of different signal options available. They can be broadly split into analogue and digital and linear and non-linear. Many flow meters will have onboard electronics that convert the raw measurement, for example a non-linear frequency, to ensure the output signal is linear. For optimal accuracy a linear digital signal such as CAN will give you the least error and most reliable results, regardless of cable length. Many customers prefer to use traditional linear analogue signals like 0-5V, 0-10V or 4-20mA, often as they have existing readouts and they are easy to wire-up and faultfind. However, if accuracy is critical you should remember devices with an analogue output can introduce digital-analogue conversion errors. Finally, care must be taken when using longer cables (over 5m / 16 ft ) with voltage devices as you can get a significant voltage drop across the cable which will in turn increase the error in the displayed values. Flow meter accuracy Accuracy is normally quoted by the flow meter manufacturer as a percentage value to indicate the acceptable error band. This should be traceable and be based on when the flow meter was last calibrated and carried out under the conditions stated by the manufacturer. Typically, accuracies are quoted as a percentage of either side of the maximum or full-scale value 4
or as a percentage of either side of the measured or indicated reading. Typically, it costs more to have higher accuracy for the same flow range; thus, it costs more to have a device that gives traceable readings that are accurate as a percentage of the measured value than a device that only quotes readings as a percentage of the full-scale value. For example: Accuracy of +/- 1% of the full scale value (1% FS) means that a flow meter rated to 400 lpm (106 US gpm) should measure flow to within plus or minus 4 lpm (1.06 US gpm) (0.01 x 400 = 4 lpm). If the same meter is being used to measure 40 lpm, the possible error is still +/- 4 lpm (1.06 US gpm), though this is equivalent to an error of 10% of the measured value. Hence, a meter that will measure to within 1% of the measured or indicated reading (1% IR) will generally cost more, as it s much harder to achieve that 1% FS. Flow measurement For further information on the wide range of flow meters available, not just for hydraulics, including the theory behind how different flow meters work, see Introductory Guide to Flow Measurement by Roger C Baker, ISBN: 0791801985. What effect might the flow meter have on the fluid and vice-versa? This might seem like a strange question, as the effect on the fluid will depend on whether the flow meter is intrusive or not and also on the flow meter technology. This effect can be measured by the energy loss due to the presence of the meter, better known as the pressure drop ( P) across the device. This has two effects: Increasing the upstream pressure and generating heat. For example, in a tank line of a piston pump, the flow may be quite low, under 10 lpm (2.6 US gpm), and the pressure may not be allowed to exceed 10 bar (145 psi) without risking damage to the seals. In this case, the pressure needs to be kept as low as possible by using the right type of flow meter and by ensuring the ports are large enough to minimise the pressure drop, especially in the event of a sudden surge in flow. In a high-flow, high-pressure system, if the flow meter has a large pressure drop, the heat generated by the pressure drop might be more important, especially because much of that heat will go into the fluid. This wasted energy might have been avoided by specifying a different type or size of meter. Regarding the effect of the fluid on the meter, this isn t normally a problem if you know about the fluid properties before selecting the flow meter. The two most common problems we see are excessive contamination and incompatible fluids. Both can severely reduce the life of the flow meter and cause it to malfunction. In the case of the wrong fluid being used this can result in corrosion and the flow meter sticking or damage to the seals resulting in leaking. This is often due to a misunderstanding, such as when a flow meter fitted with Viton seals is used on a phosphate ester fluid (such as Skydrol ). Had the flow meter been fitted with EPDM or equivalent seals, the problem could have been prevented. Viton is a registered trademark of DuPont Performance Elastomers L.L.C. Skydrol is a registered trademark of the Eastman Chemical Company Maintenance & recalibration Lastly, it s worth considering the longer-term effects of the fluid on the flow meter, especially if the fluid is known to be carrying high levels of contaminant or if the fluid has very low lubricity. In any case, it s important to not only identify the correct flow meter in the first place, but also to consider how frequently it should be removed for maintenance and recalibration. The quoted accuracy can only be guaranteed to be true on the day it was last calibrated. If the accuracy is important, then the quality department will add the flow meter to the list of instruments that need recalibration. The exact recalibration frequency will depend on the meter type, the duty cycle and manufacturer s recommendations. The typical period for recalibration is every 12 months. 5
Turbine type flow meter Turbine type flow meter schematic Variable orifice type flow meter Variable orifice type flow meters 6 How important is it to measure flow; what is your budget? A better question might be, What happens if you don t measure flow? If the answer is Nothing; it s just one of many indicators we look at, then you already know that you re working with a very small budget. However, if the answer is, the PLC will think that the machine lubrication has failed and stopped working, then you can already visualise how much the Operations guy will be sweating, how important it is to measure the flow and how much you might budget to stop that from happening. Just because you have a budget doesn t mean you have to spend it. And answering these five questions will enable you to determine what s required of a flow meter so that you can specify the right model and explain to the Ops guy why certain features are important. In addition, you can often save money as well because you will be much less likely to over-specify a flow meter just to be safe. How much will the flow meter cost? All of the factors mentioned above will influence the price of the meter, so there is no hard and fast rule, but the overview of the different technologies below will hopefully be a useful guide. Flow measurement is a big subject. All of the flow meters referred to here are volumetric, which means that they are measuring the volume of fluid passing through a pipe, not the mass. The models we are looking at here are the types most frequently used for hydraulic flow measurement. To aid in the comparison of these various models, there is a simple table of key factors next to each flow meter type, rated at 1 5, with 1 the lowest and 5 the highest. The icons used are as follows: Comment Accuracy Pressure drop ( P) Viscosity sensitivity Price Other comments: Where 1 = low; 5 = high Score Variable orifice flow meters The idea of flow displacing an object, usually a piston or a ring, forms the basis of simple variable orifice flow meters. The momentum of the fluid exerts a force on a piston that is held in place by a spring. As the flow increases, the piston moves, and the orifice size increases, along with the spring force on the piston. The piston is linked to the analogue readout via a magnet. The flow indicator is purely mechanical and is ideal for looking at trends rather than the exact measurement of flow. Such meters typically have an accuracy of between 2-5% of full scale. The flow indicator comes in a range of sizes; any one size will typically cover a 15:1 range. The flow indicator offers a low-cost????
solution, with a typical pressure drop of about 1.5 to 2 bar (22-29 psi) at 400 lpm (106 US gpm). Comment Accuracy Pressure drop ( P) Viscosity sensitivity Price Other comments: normally uni-directional Score Gear type flow meters These are positive displacement flow meters. On the inside, they look similar to a gear-type motor. Fluid passes around the outside of a pair of intermeshed gears, rotating the gears on their shafts. A transducer mounted above one of the gears generates a pulse each time a gear tooth passes under it. The rotation of the gears is proportional to the flow rate. Sometimes, two transducers are used to measure direction and improve resolution. 1 2 3 1 These flow meters are relatively insensitive to changes in fluid viscosity and work best with higher-viscosity fluids that are greater than 10 cst to as high as 1000s cst. With low viscosity fluids, less than 10 cst, you can get leakage across the top of the gears, which will reduce the flow meter accuracy. Depending on the flow meter size and sensor-linearisation technology, accuracies of +/-0.3% to 0.5% of the indicated reading are achievable. However, gear type flow meters will usually have a very high pressure drop, such as 9 bar (130 psi) at 10 lpm (2.6 US gpm), and can be quite noisy. Comment Accuracy Pressure drop ( P) Viscosity sensitivity Price Score Other comments: can be very noisy 5 5 1 5 without linearisation. The same 1 turbine flow meter when used with a look-up linearisation table will operate over a wider range, with an accuracy of 1% of the indicated reading. A turndown ratio of 30:1 is common. The use of a turbine instead of gears means that the meter requires less energy to operate and has a very low pressure drop, such as 3 bar (44 psi) at 400 lpm or lower, depending on the bore size. The disadvantage with this type of meter is that it is quite susceptible to changes in viscosity. Thus, it is usually used for fluids under 100 cst. Comment Accuracy Pressure drop ( P) Viscosity sensitivity Price Score Other comments: shouldn t be mounted near a bend in the pipe 4 2 4 3 The relationship between the frequency measured by the transducer and the flow rate is shown using the meter or K factor (K factor = frequency / flow). Given a constant K factor, the flow can be easily calculated from the frequency. A gear type flow meter will give a very precise measure of flow. Typically, a gear flow meter will have a large turndown ratio of at least 30:1, and it can sometimes go as high as 200:1. It will measure the flow to an accuracy of better than 1% of full scale without additional linearisation. Turbine-type flow meters In a turbine-type flow meter, a turbine rotor is mounted on a shaft between two sets of flow straighteners. The fluid passes through the flow meter and rotates the turbine blade. As for a gear-type flow meter, a transducer is mounted above the turbine and generates a pulse each time a blade passes under it. The frequency from the transducer is proportional to the flow over a limited range. For example, a 1 turbine flow meter might typically have an accuracy of +/-1% of full scale Gear type flow meter 7
Other meter types Oval gear meters are similar to conventional gear meters, but use two elliptical gears that rotate together at 90º to one another inside a housing. The fluid is swept around the chamber by the gears, and the frequency of rotation is directly related to the volume of fluid through the meter. This type of meter generally works best with higher viscosity fluids, but has a lower pressure drop than an equivalent conventional gear-type meter. The teeth on the gears tend to be very fine, resulting in the flow meter being more susceptible to fluid contamination than other meter types. About the author Martin Cuthbert studied Mechanical Engineering at Sheffield, has worked in the field of hydraulic test equipment for over 20 years and is Managing Director of Webtec. Webtec designs and manufactures a wide range of hydraulic components and test equipment. For further details and to discuss your flow measurement application with a qualified hydraulic sales engineer, please go to www.webtec.com In theory, non-intrusive meters, such as ultrasonic meters, are very attractive in that they don t require breaking into the system and have no pressure drop. However, they aren t widely used in hydraulics due to three main reasons. First, hydraulic pipe sizes are often quite small, even under 25 mm (1 inch), thus making transit times very short and harder to measure. Second, hydraulic pipes are usually flexible and the material within them is often not known; this can make the installation and calibration of the meter very difficult. Third, correctly installing the transmitter and receiver parts of the flow meter can be very time-consuming, especially if the meter is for portable use. Milwaukee, WI 53235, USA Tel: +1 (414) 769-6400 sales-us@webtec.com St. Ives, Cambs. PE27 3LZ, UK Tel: +44 (0) 1480 397 400 sales-uk@webtec.com www.webtec.com Hydraulic measurement and control Designed and produced by Webtec - HYDFLOW-ED-ENG-3331.pdf - 02/15 8