Pumps & Systems, August 2007
How many gallons per minute are REALLY flowing out of that pump?
Perhaps the OEM pump manufacturer tested this pump at the factory - or perhaps not. You now have a pump, or perhaps it has been there for some time, and you suspect that it is no longer pushing the flow as it once did or is supposed to. The problem is there is no inline metering device to monitor the actual number of gallons. What can you do?
Installing a magnetic flowmeter or a similar device is the obvious solution to pursue when the line can be scheduled out of service. But in the meantime, installing a flowmeter can be intrusive because cutting into the pipe is expensive and could bring the entire operation to a screeching halt. A permanent flowmeter (a mag meter, for instance) would work well (and they are usually very accurate), but it is expensive and often impractical to install a flowmeter with every pump.
Instead, to quickly troubleshoot a flow problem an externally mounted flowmeter can do a good job.
Externally mounted flowmeters come in different shapes and sizes and utilize varying technology. Clean liquids require different types of flowmeters compared to dirty liquids.
For example, transit time meters are used mainly for clean liquid flow surveys. The newest of these versatile flowmeters now feature advanced measuring systems that are highly accurate. Their non-invasive transducers can be installed quickly onto piping systems and can be configured and operational within minutes.
In contrast, ultrasonic Doppler flowmeters are used for liquid flows with suspended solids and are ideal for sewage, sludge, slurries, and other fluids with suspended solids or high aeration. These devices feature some of the most advanced non-invasive flow measurement technology available. They are capable of measuring full-pipe, bi-directional, liquid flows to 40-fps in pipe sizes from 1-in to 100-in.
Because a permanent attachment takes time and troubleshooting flow problems often allows no such luxury, there are certain tricks to consider. First of all, positioning your meter can be tricky: finding a 10-ft to 15-ft straight run of pipe is a good thing (if available!). Sometimes an elbow actually helps (believe it or not), because it creates additional vortices of which the ultrasonic meter can bounce off the signal to get a better reading.
Given the realities of plant piping configurations, locating a good spot to mount a flowmeter for troubleshooting can be a real challenge, especially since pipes are covered with insulation and can be hard to reach. They can also be hot, steamy, nasty, rusty and dirty.
Due to these reasons (and because there are deadlines to meet), an ultrasonic flowmeter does not really need to be attached to the pipe, although it would help if you plan to do repeated measurements.
For the quickest troubleshooting, you can simply hold the sensors in your hands. Make sure, however, the sensors are well-lubricated with the appropriate lubricant between the base of the sensors and the pipe, and the pipe is clean of rust.
As for a battery, you do not really want to drag an electric cord along with you. A good meter will have a large powerful battery that lasts a long time, typically 100-hrs of busy measuring work.
Accuracy? This depends on how accurate your readings really need to be and how permanent you want to make the installation. If you have the time and can find a good location for a meter, the readings can be as accurate as any other type of device.
Because troubleshooting work often forces you to take the best spot available, accuracy can suffer somewhat. If you are doing flow certification, this may be not good enough, meaning you might need to find a more permanent spot to take readings, which then can be used for repeated trending measurements as well.
To help the Flow-Measurement-Community, Pumping Machinery, LLC is partnering with Instruments Direct, Inc., to form the Flow Measurement Forum to exchange ideas, compare notes, learn new tricks, and better understand what works well, what is practical, and what is not. We will be posting your emails at the "Flow Poster Hints" Board section of our website: www.pumpingmachinery.com/consulting/flow_measurements/flow_measurements.htm