All too often, valve sizing and selection tools are based on standard equations and rules of thumb shaped by performance data collected under laboratory conditions. While laboratory data may be considered extremely accurate, they are no substitute for real-world data collected under actual operating conditions over an extended period of time.
This data, collected over a span of decades, in combination with the manufacturer’s own laboratory data can be utilized to model line-specific conditions to aid in valve sizing and selection that yields the best possible results in each specific case.
To make this process as efficient and fast as possible, many manufacturers have created expert sizing and selection software. The use of such software can frequently deliver a huge payback in terms of improved process safety and productivity. Successful control valve sizing and selection always depends on knowing the actual process conditions in the system in question. The software can help accurately predict performance prior to installation of the valve assembly. Read on to learn a few important points for consideration in valve selection. These are all points that are factored into the selection process by our expert valve sizing software.
Important points in valve selection
Achieving optimal travel is among the important factors to consider. Travel at normal flow should ideally fall within a 50 to 60 percent opening angle. Travel at maximum flow should fall below 90 percent for rotary valves and 70 percent for linear valves. Minimum flows should always be above 20 percent to avoid erosion of the trim, especially on linear valves. It is not uncommon to have a control valve smaller than the actual pipe size. As a rule, the recommended valve size should not be less than half the diameter of the pipeline to prevent valve damage caused by piping stress.
The installed gain curve is a very good indicator of valve performance in specific conditions. A constant gain line at 1.0 within the control range is ideal. For the best results, the gain should be between 0.5 and 3.0, and the ratio of maximum to minimum gain should be ≤ 2.0.
Erosion damage is closely associated with the velocity of the media. Maintaining a proper velocity, especially when media is erosive or flashing, is important for extending your valve life. In such cases, moving to a larger valve may be considered to decrease velocities and thus potential damage to the valve.
Aerodynamic noise is strongly related to exit velocity of media. The concern here is mechanical damage caused by intense vibrations. To prevent mechanical damage to your valve assembly, noise levels should be kept below 110 dBA.
Hydrodynamic noise is used to predict cavitation intensity. Generally, if you want to eliminate damage you must lower noise to an acceptable level, which in turn means lowering the intensity of the cavitation. For specific noise recommendations, review your specific valve sizing software for noise levels applicable to your specific size valve.
Well-developed expert sizing and selection software solutions also offer a broad set of tools that can be used to detect a wide range of problems that can occur. Some of the most common problems that the software can help address include noise and cavitationrelated issues as well as issues related to high actuator load factors. The software can also provide solutions for media flow issues, such as an excessively high flow marginal and high flow velocities at both the inlet and outlet ports.
Successful control valve sizing and selection depends on knowing the actual process conditions in which the valve is to be installed. This information is rarely available firsthand. The more assumptions one has to make on flow conditions, the less accurate the control valve sizing is going to be.
Process uptime, safety and productivity are topics that always count when speaking about industrial processes and process plants. Correctly selected process valves are essential to achieve the set uptime, safety and productivity targets and also help minimize the maintenance costs and waste of energy and raw materials.
Our guest writer Jon F. Monsen shows how an analysis using our control valve sizing and selection software’s unique ability to graph the installed gain of a control valve can be helpful in selecting the optimum pump for a balance of good controllability and minimum energy use.