I. Fundamental Selection Principles
The chemical industry involves a wide variety of media and complex operating conditions. Flowmeter selection must comprehensively consider five major aspects: instrument performance, fluid characteristics, installation conditions, environmental conditions, and economic factors. Selection should be based on process parameters, including fluid velocity, state, temperature, density, viscosity, and pressure under both maximum and design operating conditions, while also accounting for non-normal operating conditions such as commissioning, start-up, and emergency scenarios.
1. Media Compatibility as the Primary Prerequisite
Clearly identify the physical and chemical properties of the measured medium, including type, corrosivity, viscosity, cleanliness (presence of particulates), and volatility. Different media have significantly different compatibility requirements regarding flowmeter materials and structure.
2. Material Grade of Wetted Parts
The material grade of instrument components that come into contact with the process medium shall be equal to or higher than the grade required by the process. When measuring corrosive fluids, both the measuring tube and the sensor housing must possess corresponding corrosion resistance, and safety protection measures should be in place to prevent hazards in the event of measuring tube rupture.
3. Reasonable Flow Range Coverage
The measurement range shall cover the actual flow variation range with an appropriate margin. Never select the flowmeter size based solely on the pipeline diameter; instead, determine the meter size based on the actual flow rate. An oversized meter will result in excessively low flow velocity, leading to inaccurate measurement or no reading at all. An undersized meter will cause excessive pressure loss and potential sensor damage.
4. Balance Between Cost-Effectiveness and Accuracy
Determine the required accuracy class based on the measurement purpose. High accuracy (e.g., ±0.1% class) is required for custody transfer (trade settlement), while ±0.5% class is sufficient for internal process control. While meeting measurement requirements, consider cost factors and avoid pursuing unnecessarily high accuracy grades.
5. Pressure Loss Requirements
The pressure loss induced by the flowmeter itself shall be less than the maximum allowable pressure loss in the process pipeline. For flammable and explosive media, ensure the flow velocity does not exceed safe velocity limits to prevent static electricity generated by friction from causing detonation.
