Nearly all process inputs are flows. The measurement of flow is important for process analysis, metrics, and modeling, reducing variability of process inputs, enabling feedforward and ratio control, and isolating valve nonlinearities by secondary flow loops. Inline flow meters offer the best accuracy and rangeability. Here is a list to help select and install an inline flow meter.
The straight run requirement for inline meters has not been detailed to the extent of orifice meters, flow tubes, and venturi differential head flow meters. The supplier is typically the source of most of the expertise. In an attempt to provide some guidance for the user I have roughly correlated straight run requirements for vortex, turbine, and magnetic flow meters to the ASME guideline for flow tubes. More details need to be consolidated that quantify piping system requirements and effects on inline meter accuracy, noise, and rangeability.
The following checklist is not intended to cover all the specification requirements but some of the major application details to be addressed for inline meters (Coriolis, magnetic, turbine, and vortex meters). The following list assumes the materials of construction have been properly specified, the meter will work safely and reliably with acceptable accuracy for the maximum possible temperature, and electrical connections and enclosure meet electrical area classification and codes in plant. For more information on flow measurement see the March 2012 Control Talk column "Going with the Flow." For a detailed understanding see Chapter 4 in the ISA book Essentials of Modern Measurements and Final Elements in the Process Industries. Reliability, precision (noise, repeatability, resolution, and threshold sensitivity), and turndown (rangeability) are most important.
- Do the meters’ threshold sensitivity, repeatability, and drift meet application requirements?
- Does the meters’ rangeability and permanent pressure loss meet application requirements? (Maximum possible rangeability: 15:1 vortex, 50:1 turbine, 100:1 magmeter, 200:1 Coriolis)
- Do O-rings and gaskets meet worst case corrosive and temperature operating conditions?
- Are gaskets not projecting into flow stream?
- Is meter centerline concentric with piping centerline?
- Do the upstream and downstream straight run lengths for vortex meters meet the ASME guideline for 0.8 beta ratio flow tubes (e.g. 20 pipe diameters upstream for long bends)?
- Do the upstream and downstream straight run lengths for turbine meters meet the ASME guideline for 0.6 beta ratio flow tubes (e.g. 10 pipe diameters upstream for long bends)?
- Do the upstream and downstream straight run lengths for magnetic flow meters meet the ASME guideline for 0.4 beta ratio flow tubes (e.g. 5 pipe diameters upstream for long bends)?
- Have asymmetric profiles and swirling been minimized by piping design and straightening vanes by special conditioners for profile distortion for turbine and vortex meters?
- Is the maximum kinematic viscosity less than the maximum for vortex meters?
- Is the minimum and maximum velocity within limits for magnetic, turbine, and vortex meters?
- Is the minimum Reynolds number greater than the minimum for vortex meters?
- Are flow meters in vertical lines installed with flow up?
- Is maximum vacuum (e.g. after steam cleaning) less than maximum for lined magnetic meters?
- Is the minimum fluid conductivity (e.g. low water) greater than minimum for magnetic meters?
- Are there no bubbles in magnetic, turbine, and vortex meters?
- Is maximum % bubbles and solids less than maximum permitted by Coriolis meter software?
- Is particle abrasion negligible for U-tube Coriolis, magnetic, turbine, and vortex meters?
- Is particle concentration high enough to require a straight tube Coriolis meter?
- Is the minimum fluid lubricating effect better than minimum for turbine meter bearings?
- Is the fluid always a liquid (e.g. no flashing) for magnetic meter?
- Are Coriolis and magnetic flow meters completely full at zero flow?
- Is signal grounded to zero when no flow to prevent sloshing errors?
- Is maximum piping vibration less than the maximum permitted by Coriolis and vortex meters (e.g. is there a vibration damper for isolation)?
- Are bubbles and solids not trapped in U-tube Coriolis, magnetic, turbine, and vortex meters?
- Are magnetic meters properly grounded to earth and for lined pipe are there ground straps between pipe flanges and meter?
For more information on the extensive opportunities afforded by flow measurements see the Jan-Feb 2010 InTech article "Advances in flow and level measurement enhance process knowledge" and the March-April 2011 InTech article "Feedforward enables flexible, sustainable manufacturing."
