Regulating valve

Q1: What factors need to be considered for control valve selection?

Factors include: process parameters (flow, pressure, temperature, medium), differential pressure, flow characteristic, leakage class, rangeability, noise requirements, cavitation/flashing risk, actuator type, etc. It is recommended to provide a complete process data sheet for professional selection.

Q2: What are cavitation and flashing? How to avoid?

Cavitation is when local liquid pressure drops below vapor pressure, forming bubbles that collapse in high pressure zones, causing metal surface damage. Flashing is when liquid pressure drops below vapor pressure and vaporizes. Avoidance methods: use anti-cavitation trim (cage type), reduce differential pressure, increase downstream pressure, use hardened materials.

Q3: What does rangeability mean?

Rangeability (R) is the ratio of maximum controllable flow to minimum controllable flow. Typical control valve rangeability is 30:1 to 50:1. Higher rangeability means wider control range.

Q4: How to choose between pneumatic and electric actuators?

Pneumatic actuator: fast response, explosion-proof, fail-safe (with spring return), but requires air supply. Electric actuator: power convenient, precise positioning, no air supply needed, but slower response. Prefer pneumatic for explosion-proof requirements.

Q5: What is a positioner? What is its function?

A positioner is an accessory device for control valves. It receives control signal (4-20mA) and compares with stem position, adjusting actuator air pressure to make valve opening precisely correspond to control signal. Positioners improve control accuracy, dynamic response, and enable split-range control.