You can get flowmeters in insertion versions that are paddlewheel, propeller, turbine, magnetic, vortex and differential pressure sensors. Insertion flowmeters are popular in many industries, because they appear to be easy to install, inexpensive and come in technology variations that mimic full-pipe meters.
But with no exceptions, insertion flowmeters are not the same as their full-pipe counterparts. In fact, there is some evidence for David W. Spitzer's claim in his book Industrial Flow Measurement that insertion flowmeters are a type all of their own.
How Does This Work?
In Figure 1, you see turbulent flow and laminar flow. These are based on the concept of the Reynolds number, which is a dimensionless number relating to the ratio of viscous to inertial forces in the pipe. Laminar flow where the flow profile is straight and smooth occurs at Reynolds numbers of less than about 2500. Turbulent flow, where there are eddies, vortices and swirls in the pipe, occurs above 4500 Reynolds numbers. Transitional flow, which is neither fully laminar nor fully turbulent, occurs between about 2500 and 4500 Reynolds numbers. Laminar flow profiles are usually visualized as being bullet-nosed, while turbulent flow profiles are seen as plug flows.
Without getting too far into the math, flow studies have shown that in a pipe with a fully developed flow regime, either fully turbulent or fully laminar, the average velocity in the line can be found at a point somewhere between 1/8 and 1/10 of the way in from the side wall, depending on the flow study you read.
Insertion Paddlewheels, Propellers and Turbines
There are three very similar types of insertion flowmeter that use a rotor that spins with the velocity of the fluid.
The first is a paddlewheel, because the rotor is parallel to the centerline of the pipe, just like a paddlewheel steamer. Paddlewheels range from very inexpensive to inexpensive, and are designed to be disposable. The least expensive ones use polymer bearings, which go out of round and cause the rotor to wobble before the rotor shaft cuts through a bearing and goes downstream. The best use jeweled bearings and ceramic shafts so that they have much more longevity and less drag.
The spinning of the rotor is sensed by either a magnetic pickup that generates a sine wave the frequency of which is proportional to velocity, or a Hall effect sensor that generates a proportional square wave.
Read Also: Going with the Flow
The advantage of the magnetic pickup is that it generates the sine wave without additional power. The advantage of the Hall effect sensor is that it does not cause "stiction" (the momentary friction stop when the rotor sees the magnetic pickup's magnet). Hall effect sensors generally are able to read lower flow rates and are more accurate at lower flow rates as well.
Paddlewheel flow sensors are designed to be easily inserted into a small hole cut into the pipe using a custom fitting. Some paddlewheel sensors can be inserted into the pipe using a hot tap assembly, which allows the sensor to be inserted and retracted without shutting down the flow or relieving the pressure in the pipe.
Propeller meters use a prop shaped very much like an outboard motor's propeller and are generally connected to a mechanical or electromechanical totalizer with a cable very much like a speedometer cable. In fact, there is a legend that one very large manufacturer of propeller meters made a very inexpensive lifetime buy of Ford Pinto speedometer cables and used them for years. Some more modern propeller meters use embedded magnets and either magnetic pickups or Hall effect sensors. Like paddlewheels, propeller meters have a pulse output that is proportional to the average velocity in the pipe. Because propeller meter rotors are large and located at the centerline of the pipe, they are likely to be quite accurate, and even insertion propeller meters have been certified for billing purposes for decades.
Propeller meters, because their prop is significantly larger than a paddlewheel, are inserted using a flange that mounts into the upright member of a tee fitting.
Turbine meters come in both electronic and electromechanical styles, but the only insertion turbine flowmeters are electronic. They, like paddlewheels, use either a mag pickup or a Hall effect sensor to produce an output pulse that is proportional to the velocity of the fluid. Like paddlewheels, they must be inserted to the "average velocity point" that exists somewhere between 1/8 and 1/10 of the inside diameter away from the pipe wall.