Principles for Selecting Control Valves in Chemical Plants

Overview of Control Valve

Control valves are crucial in regulating fluid flow, pressure, and temperature in chemical plants. Proper selection enhances system efficiency, minimizes operational costs, and prevents unnecessary energy losses. This article provides a detailed overview of the principles for selecting control valves, including performance requirements and calculating CV values.

Basic Principles for Selecting Control Valves

Meeting Process Requirements

Control valves should be selected based on specific process parameters such as the characteristics of the medium, operating pressure, temperature range, and flow requirements to ensure the valve meets process demands.

Selecting the Appropriate Flow Characteristics

Control valves come with various flow characteristics (e.g., linear, equal percentage, quick-opening):

• Linear: Suitable for processes with small load variations.
• Equal Percentage: Ideal for applications with large load variations and high accuracy requirements.
• Quick-Opening: Commonly used for on-off control.
• Economy and Durability
  The valve material and structure should be appropriate to avoid over-engineering and ensure long-term stability in environments with high temperatures or corrosive media.
• Compatibility with Actuators
  The control valve must be compatible with the actuator’s response speed and adjustment accuracy to achieve stable automatic control.
• Ease of Operation and Maintenance
  Select valve designs that are easy to install and maintain, reducing operational workload.

Calculation and Selection of CV Value (Flow Coefficient)

Definition of CV Value

The CV value indicates the flow rate (in gallons per minute) of water passing through the valve when fully open under a pressure drop of 1 psi. It is a critical parameter for selecting control valves to ensure they meet actual flow requirements.

CV Value Calculation Formulas

The formula for calculating CV value varies based on the type of medium:

• For Liquid Media:

• Where:
  • Q: Flow rate (m³/h or gpm)
  • ΔP: Pressure drop across the valve (bar or psi)
  • G: Relative density of the medium (based on water, which has a density of 1)
• For Gaseous Media:

• Where:
  • Q: Flow rate (standard cubic meters per hour)
  • ΔP: Pressure drop across the valve (bar or psi)
  • T: Absolute temperature of the medium (℃)
  • Z: Compressibility factor (approximately 1 for ideal gases)
  • 1: Absolute inlet pressure (bar or psi)
• For Steam Media:

• Where:
  • Q: Steam mass flow rate (kg/h)
  • V: Specific volume of steam (m³/kg)

CV Value Selection Principles

• Design Margin: Typically, a 10%-20% margin is added to the calculated CV value to accommodate fluctuations.
• Working Range: Control valves operate best within 30%-70% of their fully open CV value, ensuring precise and sensitive regulation.

Selection of Valve Materials and Structures

Material Selection

The material should be chosen based on the medium’s characteristics and process conditions:

• Carbon Steel: Suitable for normal temperature and non-corrosive media.
• Stainless Steel: Appropriate for mildly acidic, alkaline, or corrosive media.
• Alloy Materials: Designed for high-temperature, high-pressure, and strongly corrosive environments.
• PTFE/Rubber: Used for low-temperature or weakly corrosive media.

Structural Selection

• Single-Seat Control Valves: FFor forlow-pressuredrop and small flow applications, offering minimal leakage.
• Double-Seat Control Valves: For high-pressure drop and large flow, though with slightly higher leakage.
• Cage-Guided Control Valves: Combining the benefits of single and double-seat valves, suitable for high-pressure drop, and large flow.
• Ball Valves: Ideal for high-pressure or particle-containing media.
• Butterfly Valves: Suitable for large diameters and low-pressure applications.

Installation and Commissioning Considerations

Position and Orientation

• Control valves should be installed on horizontal pipe sections, with the valve body orientation matching the flow direction indicated.
• Avoid installation in areas with high vibration to prevent reduced valve lifespan.

Bypass Pipeline Design

• For critical control loops, a bypass pipeline should be installed to allow system operation during valve maintenance or replacement.

Commissioning and Calibration

• During commissioning, adjust the actuator’s response speed and control range to ensure the valve responds promptly and accurately to control signals.

Practical Example

In a chemical plant, a liquid medium is transported at a flow rate of 30 m³/h. The upstream pressure is 5 bar, and the downstream pressure is 3 bar. The medium is water (relative density = 1).

Calculate the required CV value: