This week our colleague Steven Hocurscak speaks from experience about valve sizing:
“Still today, valve sizing & selection tools are too often based on standard equations and rules of thumb based on 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 will yield the best possible results in each specific case.
Using a sizing software similar to the one shown above can help take the guesswork out of sizing and selecting valves.
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 already prior to installation. Read on to find out some of the most important points for consideration in valve selection. These are all points that our expert valve sizing software factors into the selection process.
Achieving optimal travel is among the important factors to consider. Travel at normal flow should typically fall within a 50 to 70 percent opening angle. Travel at maximum flow should fall below 90 percent, while at minimum flow it should be above 20 to avoid erosion of the trim. It is not uncommon to have a control valve smaller than the actual pipe size. As a rule, it is recommended that to avoid valve damage caused by piping stress the valve size should not be less than half the diameter of the pipeline.
The installed gain curve is a good indicator of valve performance in specific conditions. A constant gain line at 1.0 within the control range would be ideal. For best results, 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 tied in with 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 increase surface area, helping to minimize erosion.
Aerodynamic noise is strongly related to exit velocity of media. The concern is mechanical damage caused by aerodynamic noise and subsequent 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 down 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 in a process control loop. Some of the most common problem conditions that the software can help address include noise and cavitation related issues as well as issues related to high actuator load factors. The software can also provide solutions to issues regarding media flow, such as an excessively high flow marginal and high flow velocities at both the inlet and outlet ports.
Expert sizing software is a serious control loop optimization tool that can, in the hands of knowledgeable users, harness its data and computing power to emulate control valve performance in very close simulations of the actual processing environment. The software can be of use, in varying degrees, to many different types of end-users, ranging from the customer’s plant staff to service providers, contractors and sales representatives. To find out more about how you can reap the benefits of optimized valve sizing and selection based on all the data available, please don’t hesitate to get in touch.”
Business Manager for Neles and Mapag product line