Flow meters are important to various industries because they help determine the amount of gas, liquid, or vapor present in a particular area. If you plan to get one for whatever purpose, you’ll have to make sure you choose the right type of digital flow meter and that the device you get is of top quality. Never make the mistake of choosing based on price alone and then trying to justify your choice with the amount of money you saved with the purchase.
In choosing a flow meter, you’ll need to take a number of factors into consideration, including the ease of calibration and maintenance, durability, accuracy of reading, the credibility/track record of the manufacturer, and the availability of spare parts. The cost of the device should only be considered once all of the other important considerations have been taken into account.
Choosing the Right Digital Flow Meter
How do you make sure that the flow meter you get is the right one? The first thing you need to do is determine whether you need the flowrate information to be totalized or continuous. You also need to decide if you will only be getting the information locally or if you need to get it remotely as well. If the information needs to be gathered remotely, you’ll have to determine whether it will be best transmitted via analog, digital, or shared means. If you go for shared transmission, you’ll have to determine the minimum required data-update frequency.
When you have all of these data on hand, can you already consider yourself ready to start shopping for a flow meter? Not quite. There is still a second phase you need to go through in order to be absolutely sure you won’t be wasting your money on the wrong device. This second phase involves evaluating the flow characteristics and properties of the process fluid as well as the piping to be used for your flow meter.
Where the process fluid is concerned, there are things like temperature, pressure, allowable pressure drop, specific gravity, viscosity, conductivity, and vapor pressure at the maximum operating temperature to consider. The manner in which all of these properties are likely to interact and/or vary also has to be determined and indicated, along with safety and toxicity information as well as detailed information on the composition of the fluid. Data on the possible presence of solids and bubbles in the process fluid, a possible tendency to coat, and its light transmission qualities should also be provided.
In the case of the piping and the area where your flow meter will be located, the following factors will have to be considered: the direction of the piping (a downward flow will have to be avoided for liquid applications), the size of the pipes, the piping material, the flange pressure rating, the area’s accessibility, upstream and downstream turns, regulators, valves, and straight-pipe run lengths.
Of course, you don’t have to make all these determinations yourself, unless you really are familiar with the process. You can hire the services of a professional engineer who can assist you not only in the planning and preparation, but even in choosing the right flow meter and in the installation process. The engineer will also be responsible for determining whether there are vibrations or magnetic fields in the area, if the area has been classified for explosion hazards, if pneumatic or electric power is available in the area, and if there are any special requirements you need to fulfill prior to installation.
The third phase in ensuring that you get the right flow meter involves determining the necessary meter range. This is done by identifying what the minimum and maximum flows to be measured are (mass or volumetric?). You then need to determine what the necessary flow measurement is. Make sure your accuracy requirements are stated at the normal, minimum, and maximum flowrates. All of these requirements are important because they help you ensure that the performance of your flow meter is acceptable through its full range.
As a recap of what you need to do in order to make sure the flow meter you get is the right one for your needs, here are the key questions you need to ask before you go out to shop for a flow meter:
- What kind of fluid are you planning to measure?
- Will you be doing rate measurement or totalization measurement?
- If you plan to measure liquid other than water, what is the viscosity of the liquid you’ll be measuring?
- Will a local measurement display on the flow meter be enough for your purposes or will you require an electronic signal output?
- What are the minimum and the maximum flow rate?
- What are the minimum and the maximum process pressure?
- What are the minimum and the maximum process temperature?
- Is the process fluid chemically compatible with the wetted parts of the flow meter?
- If what you’re planning is a process application, how big is the pipe?
The answers to these questions will make it a lot easier for you to determine which particular flow meter you should get.
It’s not enough for you to make sure you buy the right flow meter; you should also make sure it is installed properly so that it functions exactly as it should. For one thing, you have to make sure that a flow meter used for liquid service remains full of liquid while it is installed because any amount of gas or vapor in it can change its geometry and negatively affect the meter’s accuracy. For flow meters to be used in gas/vapor service, the opposite is true: it has to remain full of gas/vapor because the presence of moisture or liquid can alter its geometry.
Note as well that any disturbance such as control valves and pipe elbows can also affect reading accuracy, so it is best to place them downstream, so the disturbance in the flow will not be reflected by the flow meter measurement.