Selection
The first step in selecting the right flowmeter for the application is to compare the flowmeter specifications with the actual application. You have to look at the specific piping, pressure, and temperature conditions, and make sure that the flowmeter is going to be operating in the optimum portion of its measurement and operating range.For example, a flowmeter should not be operated much below 10% of its rated maximum flow because most flowmeters are not accurate in that region. A flowmeter should not be operated at much above 80% of its recommended temperature range, or its maximum pressure rating, since doing so may stress the product, and cause premature failure. Sometimes, selecting a flowmeter for a difficult application, such as steam, can be challenging.
Liquid flowmeters are designed to measure single-phase fluids. Introducing gases or vapors into the liquid being measured can produce poor flowmeter performance. The flowmeter should be kept full of liquid to perform properly. Similar problems occur when liquids are introduced into gas or vapor flow measurement streams. In applications such as steam flow, liquids in a gas or vapor stream can damage the flowmeter.
Installation Recommendations
The bigger problem is improper installation. You must install a flowmeter correctly. Make sure that the fluid is single-phase and that the pipe is full. Then, install the flowmeter in a location in the piping with sufficient straight run so there should be little or no turbulence present at the inlet to the flowmeter.Reducing turbulence can be achieved in a number of ways. Long straight pipes upstream of the flowmeter that contain no fittings or valves (sometimes called the "metering run" or "straight run") allow turbulence to decay naturally. Flowmeter manufacturers typically specify the length of straight pipe, in "nominal diameters" necessary for the turbulence to reach an acceptable level for accurate flowmeter operation.
Manufacturers often recommend different amounts of straight run for different piping configurations because different obstructions generate different amounts of turbulence. For example, throttled control valves can double (or more) the recommended straight run for an upstream elbow. Control valves should be located downstream of the flowmeter so the turbulence produced will not affect the flowmeter.
Sometimes "flow straighteners" or flow conditioners can be installed upstream of the meter. These devices are designed to reduce turbulence using fewer diameters than straight pipe alone. Take care when selecting a flow conditioner because some reduce swirl, others reduce velocity profile distortion, and still others reduce both. Sometimes you can use a flowmeter that is relatively immune to turbulence, as well.
In the real world, straight pipe run may be hard to find. Sometimes, the problem is how to install a flowmeter where space is limited and/or where physical obstructions exist.
Consider measuring the flow at the discharge of an existing fan. A typical flowmeter manufacturer might recommend approximately 30 diameters upstream and 4 diameters downstream straight run. For an 18 in. pipe, this amounts to approximately 50 ft of straight run piping plus 40 ft of return piping to deliver the air to its point of use.
This is impractical, because of its installed cost, the lack of existing supports, and the difficulty of penetrating the 2-ft-thick concrete wall located about 10 diameters from the start of the straight run! There's the real world component again.
Selecting the flowmeter carefully can sometimes reduce the amount of straight run you need. In this particular application, a flow conditioner was installed and produced an acceptable velocity profile within 3 diameters downstream of the flow conditioner. The final installation consisted of a flow conditioner (3 diameters) and straight run (3 diameters upstream and 1 diameter downstream) that occupied approximately 7 diameters of the 10 diameters available, leaving 3 diameters for piping to the point of use.
Accurate flow measurement is critical to the proper operation of hvac systems. It is important to select the correct flowmeter, and to install it properly. Keeping the fluid single-phase and reducing turbulence upstream of the flowmeter are critical. Sometimes it takes clever and knowledgeable engineering to achieve this goal. ES
