Subject: PRINCIPLES OF PRECISION LIOUID FLOW MEASUREMENT DEVICES, Gary E. Collins

Transcription

1 TECHNICAL PAPER TP Subject: PRINCIPLES OF PRECISION LIOUID FLOW MEASUREMENT DEVICES, Gary E. Collins INTRODUCTION: Flow measurement devices surround us in our daily lives. From the water meter in ones houses, to the large meters found in manufacturing processes, these devices and their associated control elements have proven themselves invaluable in the monitoring of our process lines, resources and even our environment. I would like to focus my discussion of the process aspect of the liquid flow measurement as it relates to the sanitary industry. The first subject I would like to address is defining a flow meter. A FLOW METER OR TRANSMITTER IS A DEVICE WHICH MEASURES THE FLOW OR QUANTITY OF A LIQUID OR GAS. FUNCTIONS OF A FLOW METER: Flow measurement devices are placed in plants for a number of functions. I have broken down these functions into four categories. A. TO MEASURE AND RECORD FLOW Meters are used throughout a plant to measure and record rates and volumes of product flow. Through the use of recording devices such as chart recorders and flow indicators, we are able to maintain consistency of the end product. B. TO CONTROL FLOW By measuring the rate of flow, we are also able to use the information from the flow-measuring device to control the rate of flow. This is normally done through such devices as flow controllers which receive the information from the meter and in turn modulate the valves or control pump speeds to maintain a specific flow. By controlling flow rate, we are then able to blend on-line, two or more products thus making our process more efficient. 18 Regan Road, Units 28 & 29, Brampton, ON CA L7A 1C2 Tel: Fax: conflow@conflow.ca Website:

2 C. TO ESTABLISH CUSTODY OF A PRODUCT Most of us own a motor vehicle. These vehicles require fuel 1n order to run. When we go to a gas station, we measure the fuel through a flow meter into our gas tank. At the end of the measuring process, we are then obligated to pay for the fuel because we have taken custody of the fuel. These meters are usually very precise and must be approved by a local Weights and Measures authority for use in trade. On a larger scale, many plants have bulk reception systems that are also approved for the reception of raw materials. D. TO CONTROL INVENTORY OF A PRODUCT In today s ever-competitive market, it is important for a plant to control its inventory. By controlling inventory, it is possible to also identify exactly where in-plant losses occur and take steps to minimize these losses. In-plant losses account for millions of dollars per year as well as countless hours in trying to identify exactly where the loss occurred. By installing a flow measurement device at strategic points in the process line, it is possible to accurately control ones inventory and increase efficiency. Accuracy and Repeatability: These are the two key words when it comes to flow measurement. I have noticed in my travels, that there appears to be a great deal of confusion concerning this subject. It's been said, that it doesn't matter if a meter is accurate as long as it repeats, the figures can be adjusted. This raises the question, What does one adjust the figures to?" This problem is being noticed more and more by plant personnel in trying to resolve their inventory control problems. For example, most modern beverage plants now have measuring devices but the question keeps raising itself, Which one is right?" For a measuring device to be precise, it must be both accurate and repeatable. ACCURACY: A MEASURING DEVICE IS CONSIDERED TO BE ACCURATE IF IT HAS BEEN CALIBRATED TO A KNOWN STANDARD AND IS DESIGNED TO MAINTAIN A CLOSE TOLERANCE CALIBRATION UNDER VARYING CONDITIONS. REPEATABILITY: A MEASURING DEVICE IS CONSIDERED TO BE REPEATABLE, IF IT CAN CONSISTENTLY REPEAT ITS ERROR ALONG THE SAME CALIBRATION CURVE UNDER VARYING CONDITIONS. I realize that this sounds complicated, but it is really quite simple if one thinks about it. It is possible to have a meter that because of poor design or misapplication is neither accurate nor repeatable. In other words, it doesn't work. On the other hand, one can also have a device that is repeatable but not accurate because it has not been correctly calibrated. The curve illustrated is an example of an electro-magnetic flow transmitter which was tested in an independent laboratory using a certified volumetric prover. It illustrates both accuracy and repeatability along its entire flow range. Page 2 of 9

3 Types of Sanitary Flow Meters: The Positive Displacement Meter: This type of meter has been in existence for many years. It works on the principle of having a precision-machined chamber of a known volume and an oscillating piston of a known displacement. Product is pumped into the chamber which in turn causes the piston to oscillate. Because the volume of the chamber is known and the displacement of the piston is also known, it is possible to accurately measure a precise volume of product for each revolution of the piston. The number of revolutions of the piston is transferred through a magnetic drive mechanism to a registration unit which is calibrated in engineering units. These meters are usually very precise and to date, are the only type of meter that can be legally used for custody transfer in many countries. Page 3 of 9

4 ADVANTAGES Normally highly accurate. Good repeatability. Can be used for custody transfer (some models). Can be used for oils and deionized products. Long service life. DISADVANTAGES Normally very expensive. High maintenance cost (older models). Periodic recalibration required. Readily measures air. Cannot measure products containing particulates. Limited viscosity range (most models). In the last category under Disadvantages, I have included limited viscosity (most models). Recently, a manufacturer has designed a new positive displacement meter for viscous products. It is comprised of a stainless chamber containing two specially constructed FDA approved plastic impellers. As product enters the meter inlet, it proceeds toward the inner wall, where it is captured in the pockets formed between the impeller lobes and casing walls. The impellers are forced to rotate with the number of rotations corresponding to a known volume. Through magnets imbedded in the impeller lobes and an electronic sensor on the outside of the meter casing to sense the magnets passing it, a pulsed signal is transmitted for further processing. It is currently being used very successfully in the soft drink industry in the United States, for liquid sugar measurement along with various syrup concentrates. Page 4 of 9

5 THE TURBINE METER: The turbine meter was very popular a number of years ago in the food and beverage industries. There are still a few models being sold but because of poor accuracy and high maintenance costs, many plants are discovering that they are more trouble than they are worth. The turbine meter consists of a multi-bladed rotor which is permeable and free spinning. The rotor is housed in a stainless steel body. As the product passes through the meter, the rotor spins. As each blade passes a magnetic transducer, a pulse is generates which in turn is divided into engineering units electronically. ADVANTAGES Cost Can be steam cleaned. Excellent for Deionized and WFI water. DISADVANTAGES Not suitable for viscous products. Moving parts - maintenance issue. Rotors can be attacked by acid sanitizers resulting in calibration drift over time. Not suitable for liquids with particulate. Readily measures pure air. Page 5 of 9

6 THE MASS METER: The mass meter was developed to measure in units of mass rather than volume. Based on the Coriolis force, the most popular mass meter is the vibrating U-tube type. As the flow passes through the tube, it creates a deflection vertically in the tube, which is proportional to the velocity and, mass of the product. This twisting of the tube is measured by the sensors and then converted electronically into a mass flow rate hence a volume or weight. It is relatively new to the market and appears to work fairly well. It is difficult to calibrate because there are few standards readily available to which it can be calibrated. It is also subject to line shock which can limit its applications. ADVANTAGES Relatively good accuracy. Good repeatability. Low maintenance. Not affected by viscosity of product. Will not measure pure air. DISADVANTAGES Expensive. Subject to zero drift. Influenced by hydraulic line shock. Tubes can crack under prolonged stress. THE ELECTRO-MAGNETIC FLOW TRANSMITTER/METER The electro-magnetic flow meter has been manufactured for many years. The principle of this device is based on Faraday s Law of Magnetic Induction. Simply stated, when a stable magnetic field is inducted and a conductor is passed through the field at right angles to it, a voltage proportional to the velocity of the conductor will be generated. Until recently, this type of meter was very large, bulky and had poor accuracy or repeatability. A very few manufacturers have been successful in producing a mag meter with tight specifications. The ones that have are using a microprocessor based electronic system. Through the use of this type of electronics, they have been successful in producing compact, extremely precise devices. In fact, one manufacturer has Page 6 of 9

7 been successful in gaining Weights and Measures approval in Europe and Canada for his standard meter, for use in the custody transfer of product. Also, much of the guesswork has been taken out of using these meters, because they have their own artificial intelligence and can actually tell you when you are using them improperly or if a malfunction occurs in any part of the device. These microprocessor-based devices represent the latest in technology and so far have proven to surpass most other designs of meters in both performance and repeatability. ADVANTAGES DISADVANTAGES * High accuracy. + Moderately expensive. * Good repeatability. Cannot measure oils or nonconductive products. * Can measure products with Cheaper models have poor accuracy particulates. and repeatability specifications. * Microprocessor based. * Can communicate directly with an in-house computer. * Will not measure pure air. ** Not affected by product viscosity. Low maintenance. * -Few models only + -High accuracy models **-DC excited field models only Page 7 of 9

8 THE DIPSTICK: I felt the dipstick is worth mentioning not because I consider it a form of accurate measurement, but because many other people do. I have often heard people comparing a meter to a dipstick rather than the other way around. Even the worst specification meter is more accurate than a dipstick. The errors created by measuring with a dipstick are extremely large. Such conditions as deformation of tanks, as well as the broad measuring surface in a horizontal tank, render this form of measurement inaccurate. I was personally involved in a programme where we tried to compare a dipstick to a proving device and a meter. We were able to get the prover and the meter to agree within tolerances, but when we tried to get some form of agreement between the prover and the dipstick and the meter and the dipstick, we found it to be impossible to obtain accuracy or repeatability. In one case, the error was as high as ten percent. For this reason, Weights and Measures authorities do not consider a dipstick as a form of measurement. Controlling Your Process: By installing properly applied high quality flow measurement systems, it is possible to control your plant process from the time your liquid products arrive in the raw stage, to the time the finished product is packaged. With the advent of computerized central control operations, it is even more important that the computer receive the correct information. If the primary measuring device is inaccurate or unrepeatable, then the information it is sending is of little value. High quality, properly applied flow measurement equipment can save a great deal of money and time in determining where product is lost or unaccounted for. By using a properly installed system, it is possible to perform on-line blending. For example, some beverage plants are using flow meters with associated controllers to accurately blend concentrates, sugar and water on the fly without interrupting the process. This allows for greater efficiency in the process as well as improved product consistency. Batch control is another application for flow measurement equipment. From large vats, to small "bag in a box fillers." Batch control ensures product consistency as well as accurate inventory control figures. Independent tests on a bag in a boxfilling machine where a precision electro-magnetic flow meter was used have yielded accuracies of + or % on fill volumes as low as six litres. Internal process control is also important. From the time product arrives in the plant to the time it leaves the plant, there are losses. It is important to identify where these losses occur and through this take steps to minimize them, thus yielding greater profits. Page 8 of 9

9 Summary: When choosing a flow-measuring device, it is important to explore your application with a qualified person in the flow measurement field. Too many devices have been misapplied and too many plant managers have become disillusioned with any type of flow meter when in many cases there is nothing wrong with the flow meter at all; just how it is applied. The beverage industry is now striving for a better form of control of its product. It is important to remember that: IF YOU CAN'T MEASURE IT, YOU CAN'T CONTROL IT. Gary E. Collins President Conflow Technologies Inc. Page 9 of 9