Every instrument is well suited for some applications and perfectly awful for others. These next few tips discuss the more common types of flowmeters and provide an insight into how they work, when they should be used, and when they should be avoided. I will begin with a brief description of how each meter works, and then discuss the pros and cons.
NOTE: Greg McMillan provides a good discussion of Coriolis meters later in the book so that type of meter is not included here.
Concept: Vortex flowmeters can be an excellent choice for a large variety of applications. However, certain limitations associated with this type of meter can make it a poor choice in some applications.
Details: Have you ever watched a flag wave in the wind? It actually waves because the flag pole generates eddies (whirlpools, or vortices) on alternating sides of the pole, which move past the flag. The eddies are relatively high pressure areas that “push” the flag away and because they form on alternating sides of the pole, the flag weaves between them and flutters in the wind. The vortex flowmeter works the same way. It employs some kind of vertical bar or “bluff body” that generates eddies (vortices) on alternate sides as the fluid flows past. Small sensors in (or behind) the bar detect the vortices and count them. The rate of vortex creation is chiefly dependent upon the rate of flow but is also dependent upon the viscosity of the fluid. (If the fluid is too viscous or the flow too low, the meter will not shed any vortices at all.) The flowmeter converts the meter count into a fluid velocity and determines a volumetric flow rate by multiplying the fluid velocity by the cross sectional area of the meter.
Here is a quick list of the pros and cons of this type of meter:
- Works in gas, steam, and liquid applications.
- Is insensitive to fluid conductivity.
- It generally has a lower pressure drop than an orifice meter.
- It is usually much cheaper than an orifice meter for line sizes smaller than 6” because it does not require impulse lines nor any special freeze protection beyond that of the existing pipe.
- Vortex flowmeters require turbulent flow to operate and will cease to read as the fluid transitions from the turbulent flow regime to the transitional or laminar flow regime. This is called the “low flow cutoff” point of the meter, and flow rate measurement below this point is not possible. (See further information on this below.)
- Vortex flowmeters make effective (if unintended) start-up strainers. The vortex shedding bar (bluff body) across the meter is great for catching bolts, drink cans, oyster shells, and all kinds of other debris wandering down the line. When material gets caught on the body, the meter will either read inaccurately or not at all.
- Some vortex flowmeters use small ports to measure the vortices. These can plug with polymer or solids, keeping the meter from functioning. The design of the vortex measuring sensors is the chief difference between meters and the sensor design will allow some meters to work in certain applications where others will not.
- High vibration or entrained solids can be problematic for vortex meters, which may detect and count solid particles or vibrations as if they are actually flow. (Note that most meters have a “noise band” or similar adjustment that can be set to reject these vibrations, but this makes the meter less sensitive and significantly increases the low flow cutoff point.)
- Like an orifice meter, the vortex meter requires an upstream and downstream meter run to establish a good flow profile. This run should be no less than 15 diameters upstream and 5 diameters downstream, but most vendors like to see at least 25 diameters upstream and 10 diameters downstream.
- A vortex flowmeter measures volumetric flow—not mass flow. It can calculate a mass flow based on an assumed density, but if the fluid density changes, the reading will be in error.
- Beyond 6” lines the economic advantage of vortex meters falls off.
Watch-Outs: The low flow cutoff is the chief limitation of a vortex flowmeter, and it makes the meter unsuitable for any application where measuring low flows at high turndown is required. The low flow cutoff point is determined by the viscosity of the fluid and thus may vary with fluid temperature and composition. Highly viscous fluids cannot be measured with a vortex meter.
Exceptions: While most vortex flowmeters are poorly suited for measuring liquids which tend to polymerize, some meters employ a proprietary sensor design that is not easily plugged and has a constant flow of liquid around the sensors to help keep them clean. Such units can work where others tend to fail.
Insight: For high temperature applications, be sure to specify a remote mounted transmitter head. The electronics do not like being cooked, and a locally mounted transmitter will not last long. Be on the lookout for centering rings if the meter is a wafer type. The pipe fitters tend to leave these in the box and just bolt the meter between two flanges. If the meter is not centered, it will be inaccurate.
Rule of Thumb: Vortex flowmeters are a good choice for measuring the flow rate of any reasonably clean fluid where measurement at very low flows is not required. In line sizes of 6” or less, they are usually much cheaper than orifice plates, and they eliminate the need for impulse line heat tracing.