LEARNING INSTRUMENTATION AND CONTROL ENGINEERING WHERE CONTROL VALVES ARE APPLIED
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In lots of applications where control valves are applied, sizing and selection can be very challenging. In nearly all applications, it's either the control valve is undersized or higher-sized. Within an undersized control valve, the valve is not able to provide the needed flow for every stage from the valve lift creating control problems.
Over-sized control valves will under all normal operations be limited to small openings from the valve with the potential risk of variable sensitivity and aggravation associated with a uneven movement of the valve and actuator combination. Poor precision and unstable control are frequently caused by over-sized control valves.
Because of the issues that could arise due to failure to size control valves precisely, the next guidelines are presented as a guide for that selection and sizing of control valves:
Rule 1
Set the look flow rate as either:
Design Flow Rate = 1.thrice Normal Flow Rate Or Design Flow Rate = 1.1 x Maximum Needed Flow Rate
Rule 2
Set the control pressure drop to 50 - 60% from the frictional pressure lack of the piping system.
Rule 3
Limit the utmost flow rate with the valve: Minimum flow rate turndown ought to be 5:1 for straight line trim valves. 10:1 for equal percentage valve trims.
Rule 4
Stay away from the low 10% and upper 20% from the valve stroke. The valve is a lot simpler to manage within the 10¡§C80% range. Consequently, The valve ought to be sized in order to control the needed selection of flow rates between 10% and 80% of valve opening. This really is highlighted below:
Rule 5
Ideally, pick a valve which has a bodily proportion one (1) pipe size smaller sized compared to pipe that will be installed e.g pick a size 2” valve for any 3” pipe). Generally, control-valve physiques are one size under the road size. However, if the causes the valve body to become considerably under the road size, which may lessen the valve's effective Cv. then doesn't apply this guideline.
Rule 6
Never pick a valve bigger compared to pipe that will be installed.