What Factors to Consider While Selecting Control Valve

Introduction

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Control valves are essential components in industrial processes that regulate the flow, pressure, and level of fluids within a system. Selecting the right control valve is crucial for ensuring optimal system performance, efficiency, and safety. This article will discuss the factors that should be considered when choosing a control valve, including process parameters, control requirements, material selection, valve performance, and maintenance and reliability.

Understanding Control Valves

Before delving into the factors influencing control valve selection, it is important to understand the basic functioning of control valves. Control valves consist of a valve body, actuator, and positioner. The valve body controls the flow rate by adjusting the position of a plug, ball, or disc to modulate the flow passage. The actuator provides the necessary force to move the valve mechanism, while the positioner ensures accurate control based on feedback signals.

Factors to Consider in Control Valve Selection

1. Process Parameters

1.1 Fluid Characteristics

The characteristics of the fluid being controlled play a significant role in determining the type of control valve required. Factors such as viscosity, corrosiveness, and erosiveness of the fluid must be considered to select the appropriate valve material and design.

1.2 Flow Rate and Pressure

The anticipated flow rate and pressure range of the system must be taken into account. Different control valves have varying capacities and pressure ratings, and selecting the right valve size is crucial to prevent excessive pressure drop or inadequate flow control.

1.3 Temperature

Temperature is another critical parameter affecting control valve selection. High-temperature applications may require valves constructed from materials capable of withstanding extreme heat, while cryogenic applications demand valves that can operate reliably in low-temperature environments.

2. Control Requirements

2.1 Valve Type

The control objective determines the appropriate valve type. Globe valves offer excellent throttling capabilities, while butterfly valves are more suitable for on-off control. Ball valves are often preferred for applications requiring quick opening or closing.

2.2 Flow Characteristic

Different control valve designs exhibit distinct flow characteristics, such as linear, equal percentage, and quick opening. Understanding the desired flow characteristic is essential to achieve accurate control and stability in the process.

2.3 Valve Size

Selecting the correct valve size is crucial for optimal performance. Undersized valves may cause excessive pressure drop, while oversized valves can lead to poor controllability and instability. Proper sizing involves considering the flow rate, pressure drop, and pipe diameter.

2.4 Actuator Type

Choosing the right actuator is important for achieving precise and responsive control. Pneumatic, hydraulic, and electric actuators offer different performance characteristics and should be selected based on the control system requirements.

3. Material Selection

3.1 Fluid Compatibility

Compatibility between the valve materials and the fluid being controlled is vital to prevent corrosion, erosion, or contamination. Materials such as stainless steel, bronze, or special alloys may be required depending on the fluid composition.

3.2 Environmental Factors

Consideration should be given to environmental conditions such as temperature, humidity, and exposure to chemicals or corrosive agents. Valves installed in harsh environments may require additional protective coatings or materials.

4. Valve Performance

4.1 Leakage

Valve leakage can have a significant impact on system efficiency and safety. Selecting valves with low leakage rates, such as those conforming to international standards like ANSI/FCI 70-2, ensures reliable operation and minimizes product loss.

4.2 Rangeability

Rangeability refers to the ability of a control valve to provide accurate control across a wide range of flow rates. High-rangeability valves offer better control resolution and can handle varying process conditions effectively.

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4.3 Response Time

The response time of a control valve affects the system&#;s ability to respond to changes in control signals. Fast-acting valves are desirable for applications requiring rapid adjustments, while slower response times may be acceptable for less dynamic processes.

5. Maintenance and Reliability

5.1 Ease of Maintenance

Consider the accessibility and ease of maintenance when selecting a control valve. Valves with removable trim, easy disassembly, and readily available spare parts can significantly reduce downtime and maintenance costs.

5.2 Availability of Spare Parts

Ensure that spare parts for the selected control valve are readily available. Valves from reputable manufacturers often have better spare parts availability, allowing for faster repairs and replacements when needed.

Conclusion

Choosing the right control valve requires careful consideration of various factors. Process parameters, control requirements, material selection, valve performance, and maintenance and reliability aspects all contribute to selecting a valve that will deliver optimal performance, accuracy, and longevity. By understanding these factors and their interplay, engineers, and operators can make informed decisions that enhance process control and efficiency.

FAQs:

1. How do I determine the appropriate valve size for my application?
   Determining the appropriate valve size involves considering the flow rate, pressure drop, and pipe diameter. Consulting valve sizing charts or utilizing valve sizing software can assist in selecting the right valve size.
2. What is the difference between a globe valve and a butterfly valve?
   Globe valves offer precise throttling capabilities and are suitable for control applications. Butterfly valves, on the other hand, are better suited for on-off control and offer lower pressure drops.
3. Can control valves be used for on-off applications?
   Yes, control valves can be used for on-off applications. However, dedicated on-off valves, such as ball valves or gate valves, are typically more appropriate and cost-effective for such applications.
4. How often should control valves be calibrated?
   Control valves should be calibrated regularly to ensure accurate performance. The frequency of calibration depends on factors such as process requirements, valve criticality, and industry regulations.
5. What are the common maintenance practices for control valves?
   Common maintenance practices for control valves include regular inspection, cleaning, lubrication, and testing of valve components. Additionally, monitoring for leaks and addressing any signs of degradation or malfunctioning is essential for optimal performance.

Control valves are called to handle all kinds of fluids at temperatures from the cryogenic range to well over 1,000F (538selecting a control valve to ensure satisfactory operation without undue initial expense.

Reputable control valve manufacturers are dedicated to helping customers select the control valve best suited for the existing service conditions. Frequently several correct choices may be available, thus it is important for customers to provide control valve manufacturers the following information.

• Type of fluid to be controlled.

• Temperature range of fluid.

• Viscosity range of fluid.

• Specific gravity range of fluid.

• Minimum and maximum flow required.

• Minimum and maximum inlet pressure at the control valve.

• Minimum and maximum outlet pressure at the control valve.

• Pressure drop across the valve expected during normal flowing conditions.

• Pressure drop across the valve at zero flow.

• Maximum permissible noise level, if pertinent, and the measurement reference point.

• Degrees of superheat or existence of flashing across the valve, if known.

• Inlet and outlet pipeline size and schedule of pipe.

When conducting an audit of existing processes, the control valve is already installed. The following guidelines can be useful in determining if the control valve installed, is suitable for the application.

Flow control processes Flow measurement signal to controller Location of control valve in relation to measuring element Wide range of flow setpoint Proportional to flow In series Linear In bypass (see note) Linear Proportional to flow squared In series Linear In bypass (see note) Equal percentage Small flow range with large changes in pressure drop across the valve In series In bypass (see note) Equal percentage characteristics should be used for applications with a small range of flow setpoint, large delta pressure at the valve, and increasing loads.
Note: When control valve closes, the flow rate as measured by the sensing element increases. Pressure control processes Liquid process Equal percentage Gas process with small volume and less than 10 ft (3 m) of pipe between control valve and load valve. Equal percentage Gas process with large volume (process has a receiver, distribution system, or transmission line exceeding 100 ft (30.5 m) of pipe). As load increases, pressure drop across the valve decreases; and the pressure drop across the valve at maximum load is >20% of the minimum load pressure difference. Linear Gas process with large volume. As load increases, pressure drop across the valve decreases; and the pressure drop across the valve at maximum load is &20% of the minimum load pressure difference. Equal percentage Level control processes Pressure drop across valve increases with load. Linear Pressure drop across valve increases greater than 2:1 with load. Quick opening Pressure drop across valve decreases with increasing load. Linear Pressure drop across valve decreases with increasing load and full load pressure drop is &20% of no-load drop. Equal percentage The above information was collected from numerous sources including, Control Engineering, Fisher Controls Control Valve Handbook, and Techmation Applications Guide.

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