Selection of control valves. In industrial automation systems, devices designed to change the flow rate of the working medium in compliance with the required adjustment characteristics are of great importance.
The main elements of these devices are control valves that serve to change the flow of the working medium and drives designed to control control valves.
By type of actuator, control valves are available in the following versions:
- with pneumatic diaphragm spring drives,
- with electric and electro-hydraulic drives,
- with pneumatic piston drives,
- with manual drives
- without drives.
The selection of control valves for specific operating conditions should analyze the following issues:
- valve design type
- material execution,
- nominal pressure,
- throughput,
- expense characteristic,
- the design of the internal parts of the valve (trim),
- gland and stem seal type,
- connection type,
- hardening of the internal parts of the valve,
- drive selection,
- harmful effects during valve operation.
Many types of valves are available, each with its own advantages and limitations. The main requirements and selection depend on their ability to perform certain functions, such as:
- Possibility to throttle or control the flow rate.
- No turbulence or flow resistance in the fully open position as turbulence reduces head.
- Quick opening and closing mechanism - quick response is often required in emergency or safety situations.
- Hermetic sealing prevents leaks due to high pressure drop.
- Allowing flow in only one direction prevents backflow.
- Opening at a given pressure - control procedure to prevent equipment damage
- Capable of handling abrasive media - hardened material prevents rapid wear.
There are two main types of control valve design, depending on the action of the closing element: straight-stroke or rotary. Straight-stroke valves use linear motion to move the closure element in and out of the seat. Rotary valves use a rotary motion to rotate the closure element in and out of the seating surface.
Control valve selection requires special consideration to ensure the best available combination of valve body type, trim material, and design. Because there are often several possible correct valve designs for the same application, it is important to get all the information you need. What are questionnaires used for?
The valve selection process can be broken down into 6 parts:
- Determine initial conditions. P1, ∆P, Q, T1, process fluid, acceptable noise, etc. Select the appropriate valve body and trim pressure class.
- Calculate preliminary required Kv. Check noise and cavitation levels.
- Determine trim design. If no noise or cavitation occurs, use standard trim. If the noise level exceeds the allowable limits, use a noise reducing trim (eg perforated plug). If cavitation is present, use an anti-cavitation trim (such as a multi-stage plug).
- Select valve body and trim type. Select valve body type and trim size according to required Kv. Pay attention to stem travel, trim configuration and cut-off parameters (pressure drop in closed position, tightness class)
- Select trim materials. Select trim materials according to operating conditions. Verify that the selected trim is available for the selected valve size.
- Consider additional options. Cover design, stem seal materials, etc.
