How to Specify Control Valves

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Control valves handle all kinds o fluids at temperatures ranging from the cryogenic range to well over 10000 degree Fahrenheit (538 degree Celsius). Selection of a control valve assembly requires particular consideration to provide the best available combination of valve body style, material, and trim construction design for the intended service. Capacity requirements and system operating pressure ranges must also be considered in selecting the right control valve to ensure satisfactory operation without undue initial expense.

Most times the control valve manufacturers and their representatives will help in selecting the most appropriate control valve for a given service conditions. However they require the right set of information to be able to do this as there are several possible correct choices for any given application.

To select a given control valve for any application, follow the steps in the chart below

Control Valve Body Materials

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The medium or substance flowing through a valve will often determine the type of valve body material. Some valve body materials are best suited for high temperature applications while others are best suited for their corrosion resistance to fluid substance still others are selected based on a low cost factor. Most times, manufacturers of valves will aid you in selecting the right valve for a given application after supplying the needed information.

Smart Valve Positioners

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Smart valve positioners are digital valve controllers, microprocessor-based, current to pneumatic instruments with internal logic capability. They are designed to convert a current signal to a pressure signal to operate a valve. The ability to embed software commands into the memory of these smart digital controllers represents the real difference between digital and analog I/P devices. The software capability allows automatic configuration and setup of the valve when equipped with a digital controller. Most importantly, it allows two-way communication for process, valve, and instrument diagnostics.

Reasons for the Use of Smart Positioners
In most respects smart positioners perform the same basic function as the basic analogue positioners combined with current to pneumatic converters. However, the key advantages for using smart positioners are:
  • Automatic calibration and configuration. Considerable time savings are realized over traditional zero and spanning.
  • Valve diagnostics. Through the Distributed Control System (DCS), PC software tools, or handheld communicators, users can diagnose the health of a control valve while it is in the line.
  • Reduced cost of loop commissioning, including installation and calibration.
  • Use of diagnostics to maintain loop performance levels
  • Improved process control accuracy that reduces process variability
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