Flowmeters Are Not All Alike
- Monday, 01 December 2008
Pipe Size, Configuration
When evaluating a flowmeter installation, users should know the flow direction (avoid downward flow in liquid applications), pipe size, pipe material, flange-pressure rating, accessibility, up or downstream turns, valves, regulators, and available straight-pipe run lengths in their facility.
The amount of available installation space may dictate your choice of flowmeter technology. Where there is limited room, larger instrument designs like Coriolis meters may not be feasible. Vortex, turbine and PD meters, as well as a handful of other technologies, are a wiser choice under these circumstances.
Nearly all flow instruments must be installed with a significant run of straight pipe before and after the location of the meter. Elbows, reducers, chemical injection ports, filters, screens and valves can cause radial, tangential and axial swirling effects within the pipe. In combination, these changes can rapidly distort the velocity profile, degrading the flowmeter’s accuracy and repeatability.
Many flow measurement instruments require straightening vanes or straight upstream piping to eliminate distorted patterns and swirls. This involves the installation of additional diameters of straight pipe run before the flow straightener, and between the straightener and the flowmeter itself.
In the pharmaceutical, food & beverage, and biotech industries, contamination- free processing is critical. The integrity of the sanitary manufacturing application is essential for full compliance to the validation process.
The potential for contamination increases with the introduction of peripheral components, such as flow, temperature and pressure measuring instrumentation required to ensure process parameters remain within acceptable limits. As a result, these inline devices must themselves meet standards set by governing agencies to ensure there are no weak links in the sanitary chain.
Sanitary standards for flowmeters and other measurement devices address issues such as process connections, materials of construction, surface finishes, electronic instrument housings, autoclave sterilization, and diaphragm seals.
As a rule of thumb, flowmeters with few or no moving parts require less attention than more complex instruments. Meters incorporating multiple moving parts can malfunction due to dirt, grit and grime present in the process fluid. Meters with impulse lines can also plug or corrode, and units with flow dividers and pipe bends can suffer from abrasive media wear and blockages.
Moving parts are a potential source of problems, not only for the obvious reasons of wear, lubrication and sensitivity to coating, but also because they require clearance spaces which sometimes introduce “slippage” into the flow being measured. Even with well-maintained and calibrated meters, this unmeasured flow varies with changes in fluid viscosity and temperature. Temperature changes also change the meters internal dimensions and require compensation.