Selection of a good flowmeter is basically dependent upon the necessities of the specific application. Hence, a considerable amount of time must be spent in the nature analysis of the process fluid and its overall installation. Specifications stating the application requirements should be developed systematically and step-by-step.
Following are the steps which need to be followed in the process of the flow sensor selection:
- First of all, determine if the flowrate information should be continuous or totalized. Also decide, whether this information is required locally or remotely. In case, remotely needed then establish the type of transmission preferred. Transmission can be analog, digital and shared. If the transmission is shared type then ascertain the minimum needed data-update frequency.
- After finding answers to all the above questions, two main evaluations are done. First evaluation is related to properties and flow characteristics of the process fluid, and another one is of the piping which will accommodate the flowmeter.
- Now, the next step is to decide the essential meter range by identifying minimum and maximum flows (mass or volumetric) to be measured.
- Once this is done, the requisite accuracy of flow measurement is determined.
In general accuracy is specified in
- percentage of actual reading (AR)
- in percentage of calibrated span (CS)
- in percentage of full scale (FS) units
The accuracy requirements should be discretely stated at minimum, normal, and maximum flowrates. Without judging these requirements, our meter’s performance can not reach the acceptable level over its full range.
Fluid and Flow Characteristics
Major fluid and flow characteristics which need to be specified for the selection procedure are mentioned below:
- Name of the fluid
- Allowable pressure drop
- Density (or Specific gravity),
- Vapor pressure at maximum operating temperature
It should also be indicated that how these properties may vary or interact. Additional information which should be provided along with above data includes:
- All safety or toxicity information
- Detailed data on the fluid’s composition,
- Presence of bubbles, solids such as abrasive or soft, size of particles, fibers
- Tendency to coat
- Light transmission qualities like opaque, translucent or transparent
- Expected minimum and maximum pressure and temperature values in addition to the normal operating values
Following questions need to be answered while selecting right flow sensor
- Whether flow can reverse
- Whether it does not always fill the pipe
- Whether slug flow can develop (air-solids-liquid)
- Whether aeration or pulsation is likely to happen or not
- Whether sudden temperature changes can take place
- Whether special care is to be taken during cleaning and maintenance
Following information need to be specified pertaining to the piping and the area where the flowmeter is to be located:
- Piping direction (Downward flow in liquid applications should be avoided)
- Flange-pressure rating
- Up or Downstream turns
- Available straight-pipe run lengths
In relation with the area, some other issues need to be addressed by the engineer if:
- vibration or magnetic fields are present or possible
- electric or pneumatic power is available
- the area is classified for explosion hazards
- there are other special requirements such as compliance with sanitary or clean-in-place (CIP) regulations