Here’s a tip from a Siemens flow specialist that may help you with the accuracy of your flow meter.
Recently, we had a case where a customer complained about the accuracy of his magnetic flow meter. After some investigation, it turned out that the meters installed were oversized for the flow rates and the sensors were reading in the very bottom of the operating scale.
When discussing the sizing of magnetic flow meters to the application, there are two rules to follow:
Rule number one: Never size the meter to the pipe. Always size it to the flow rates.
Rule number two: Refer back to rule number one.
This is where most of the problems come into play. Example: A customer said that his flow meter was not reading what the drawdown test showed. After surveying the application onsite, we found a 24-inch Mag 5100W meter installed on a 24-inch pipe. When we asked what the flow rate was during the test, the customer indicated it was 500 gpm (gallons per minute).
When asked what the maximum flow would ever be, the customer said it would never go over 700 gpm. We then ran the flow instruments sizing program, and the sizing program indicated that the maximum flow capability of the 24-inch Mag 5100W meter was 44,817 gpm. The minimum flow rate through a 24-inch meter that is needed to maintain accurate flow measurement was 2,154 gpm at a velocity of 1.58 ft/s. The customer’s flow rate of 500 gpm was about 0.38 ft/s well below the suggested flow velocity.
This, of course, is an extreme example but it shows what can happen. But how can you prevent this from happening? The first step is to understand the right way to size a meter to prevent the over sizing of the meter as indicated in the example. A good rule of thumb is to size the meter so the average flow rate is around 15 to 25% of the maximum flow capability of the meter.
For example, on a meter with a maximum flow rate of 4000 gpm, the average flow should not be less than 500 to 1000 gpm. This flow rate will maintain sufficient velocity through the meter and still give the customer room for expansion. This, of course, is not always possible. Many installations are designed for expansion in the future, so large size pipes are installed. In this case, you must look at the minimum flow expected, and ensure that the average flow should never fall below 2 ft/s, or in this case, 300 gpm.
If you cannot reduce the overall size of the pipe to accommodate the proper size meter, you should install a reducer in the line located about 3 diameters upstream of the flow meter. You can then install an expander downstream of the meter and return to the original pipe size. This will ensure proper flow measurement now, and still allow you to remove the smaller meter in the future and replace it with a larger meter. This may sound extreme, but in the long run, you will be happier with an accurate meter.
How do you select the proper size magnetic flow meter for your application?