Stephen Hall discusses the golden rules for design
CONTROL valves are used to control several important process parameters: flow rate, level, temperature, pressure, and composition. This month I am discussing rules of thumb that engineers use to select and size control valves. Some of these rules are fuzzy, and applying alternative rules may lead to conflicting answers. You can often resolve those conflicts by understanding the reasoning behind the rules and assessing how your individual problem fits.
Rules of thumb should only be used for preliminary valve selection and to reality-check submittals from vendors. Control valves are expensive high-tech products. The final selection and design should be left to experts, such as the valve manufacturer’s engineers, who can perform a complete system analysis that includes considerations for the control range, control accuracy, deadband, response time, noise, cavitation, etc. But engineers with a basic understanding can ask intelligent questions, ensure that appropriate calculations were performed by the experts, and challenge vendors to adhere to reasonable performance requirements.
The most common types of control valves are listed in Table 1.
Table 1: Common control valves and their characteristics
The first step in sizing a valve is to specify the flow rates and pressure drops at the high and low extents of the control range. Engineers often make the mistake of specifying a wider range than is necessary, thinking that this gives a conservative cushion in the sizing. When coupled with other conservative assumptions and calculations, an oversized valve is supplied, which results in suboptimal control because valves are usually most responsive when they operate closer to the fully open position (60-80% open). If you can dictate the pressure drop through the valve, use 20% of the total system pressure drop or 10 psi (0.7 bar), whichever is greater.
Refer to the Nomenclature boxout for variables definitions for the following calculations.
Calculate the required flow coefficients at the high (Cvhigh) and low (Cvlow) conditions using this simplified formula (for liquids, non-compressible):
You need to be logged in to read the rest of this article
Login Create an account
Traditionally, refineries heavily favor globe valves whereas pulp and paper plants favor Segmented ball valves and V Port ball valves. Although there are instances when one valve type is more appropriate than another, most control valve installations can be built around many types of control valves. Planning is the most important aspect including selecting materials and trim. The final selection of the type of control valve and trim should ultimately be dictated by the flow conditions required after carefully analyzing all the variables. The best way to accomplish this is by working closely with your salesperson and a Bray applications engineer. Bray Rotary Control valves have distinct advantages over linear control valves including; Higher Cv for the same size globe valve, resulting in higher control valve flow, allowing the use of smaller valve and actuation packages that reduce cost, have a smaller form factor, better packing performance and life, and Superior solids handling capabilities.
An undersized control valve will cause several problems including flow restriction (choked flow) - where the control valve is unable to deliver the required flow rate even when fully opened. Restricted flow can cause a momentary pressure drop resulting in cavitation, a phenomenon where the formation and then sudden implosion of bubbles, causes erosion. Another common control valve issue is Flashing. Flashing occurs when a liquid vaporizes due to a pressure drop across the vena contracta and is highly erosive. An oversized control valve sacrifices control resolution compared to a properly sized control valve, resulting in a loss of fidelity in the desired control range. The general rule is to size a control valve to utilize as much of the range as possible with the desired setpoint occurring at approximately 60%-80% of maximum being the ideal setpoint condition of the process.
Bray Sizing Program (BII Sizing) software is powerful control valve sizing software that performs control valve sizing calculations automatically and can help immensely with proper control valve sizing. BII sizing software produces an ISA specification sheet, based on your control valve flow requirements and can predict cavitation, noise and Cv at various openings ensuring accuracy. Although this software is provided free of charge and is generally considered to be reliable and accurate, it should not be considered a substitute for the assistance of an application engineer from the Bray team. Please consult your sales rep and or the factory for assistance with your control valve application needs.
117
0
0
All Comments (0)
Related Articles
If you are interested in sending in a Guest Blogger Submission,welcome to write for us!
Comments