Control Valve Selection

 

A previous blog-post discussed the importance of control valve sizing and energy optimization opportunities. This blog-post will focus more on the topic of control valve selection although, both topics shouldn’t be considered to be mutually exclusive. 

When selecting a control valve for process plant, there are many things to be considered. These can include the valve flow characteristic, size, valve body and trim materials, noise, potential for damage from cavitation or flashing, actuator type and size, dynamic response to changes in control signal etc. This summarises the typical considerations when making a control valve selection. For more at van cầu nối bích

 

Selecting an improperly sized control valve can have serious consequences on safety, operation and productivity. The following list outlines some of the things to consider when making a control valve selection:  

1. Give careful consideration to selecting the correct materials of construction. Take into consideration the components of the valve that come in to contact with the process fluid such as the valve body, the valve seat or any other valve components exposed to the process fluid. 
2. Consider the operating temperature and pressure the control valve will be exposed to. Consider the local ambient atmosphere and any corrosives that can occur which may affect the exterior of the valve. 
3. Consider the degree of control you require and ensure the selected valve is mechanically capable of achieving the desired operating conditions. 
4. Consider the inherent flow characteristic of the control valve you are selecting. Different valve types have different flow characteristics. The flow characteristic can be generally thought of as the change in rate of flow in relationship to a change in valve position. This item is discussed in a little more detail later. 
5. Aim for optimal valve travel. When a valve is sized correctly, the range of operation will correspond well to the control range of the valve. Some industry literature recommends travel at normal flow should fall within 50 to 70 percent opening angle. Travel at maximum flow should fall below 90 percent whereas travel for minimum flows should be above 20 percent open to avoid erosion of the trim. When modelling a control valve in FluidFlow, the software enunciates a warning message if the valve position falls outside of an optimal operating range. Users can also adjust the desired settings for minimum and maximum valve positions. This helps prompt the designer into considering valve position for the given design operating conditions. Ultimately, it also helps the designer make a better and more appropriate valve selection for the application in hand. 
6. Avoid oversizing a control valve. If the control valve is too large for the required application, only a small percentage of travel is required. This is due to only a small change in valve position having a large effect on flow which in turn makes the valve hunt. This can cause excessive wear. Some published literature sources recommend sizing a control valve at about 70% to 90% of travel. 

This list represents just some of the criteria to be considered when selecting a control valve. It is generally recommended that the final valve selection is discussed with an appropriate and experienced supplier or manufacturer before making your final selection. For more at van cầu hơi nóng

 

Control Valve Flow Characteristic

The flow characteristic of a valve represents the inherent relationship between the valve opening and flow rate. As a valve gradually opens, the flow characteristic allows a certain amount of flow though the valve at a particular opening percentage. This permits predictable flow regulation through the valve. The most common flow characteristics are linear, quick opening and equal percentage. 

Linear Flow Characteristic

This flow characteristic exhibits a linear relationship between valve position and flow rate. The flow through the valve varies directly with valve stem position. 

Quick Opening Flow Characteristic

The flow characteristic of a quick opening valve is such that for a relatively small initial change in valve stem travel, a large increase in flow occurs. The noticeable characteristic of this valve type is that maximum flow is achieved at a relatively low percentage of the valve stem range. 

Benefits to plant operators 

Demanding business or manufacturing environments require the most accurate and reliable control of production processes possible. The failure to meet and achieve specific operating standards can produce an inherently inefficient plant, can lead to serious consequences for quality and safety and can significantly affect the financial margins for the final product. Optimum control valve performance is therefore vital in preventing such scenarios. 

Industrial organisations can benefit greatly from working closely with their manufacturer representatives or instrumentation suppliers to initially specify appropriate measurement and control devices. This collaboration can achieve important performance criteria including: For more at y lọc inox.

 

• Precise flow and pressure control. This produces stable and consistent production results. 
• Efficient energy usage.
• Reduced operating costs.
• Fewer unplanned and undesirable plant shutdowns.
• Increased plant availability.
• Lower maintenance and repair costs resulting in longer valve trim life. 

Control valves are required to withstand the erosive effects of the flowing fluid while maintaining an accurate position to control the process. In order to successfully perform these tasks, control valves need to be sized accurately and correctly for the application as well as being designed, built and selected such that it is appropriate for the process operating conditions.