More important things about Control Valves that you cannot ignore

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More important things about Control Valves that you cannot ignore

Control Valves

#1. How to specify Control Valves

About the Valve, What topic for you today… Today we recommend you should read more about the article: How to specify Control Valves.

What is important in this article?

The function of the Control valves is, it handles all kinds of fluids at temperatures ranging from the cryogenic range to well over 10000 degrees Fahrenheit (538 degree Celsius).

Anytime when select a control valve assembly requires particular consideration as: the need to provide the best available combination of valve body style, material, and trim construction design for the intended service. Besides that, the capacity requirements and system operating pressure ranges must also be considered in selecting the right control valve to ensure satisfactory operation and secure your cost, make sure without the overbudget of initial expense.

As you know, most times the control valve manufacturers and their representatives will help us 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. That’s why we write this article hopefully it will help you to understand much more about the Control Valve.

To select a given control valve for any application that you need, do not forget supply the required specification information to the manufacturer or their representative, And remember to follow the steps in the chart below:

Information Required to Specify Control Valves => Application Information => Process Information => Valve Trim Information (Where required) => Valve Actuator Information => Valve Accessories

Details:

***Application Information:

Specify the following:

  • Inlet pipe size and schedule
  • Outlet pipe size and schedule
  • Special tagging information where required
  • Valve body material
  • End connections and valve rating (screwed, class 600 RF flanged, class 1500 RT J flangers etc)

***Process Information:

Specify the following:

  • Type of fluid to be controlled
  • Temperature of fluid
  • Specific gravity of fluid
  • Flow capacity required – Maximum and Maximum
  • Inlet pressure at valve – Maximum and Maximum
  • Outlet pressure at valve – Maximum and Maximum
  • Maximum permissible noise level ad the measurement reference point
  • Degree of super heart or existence of flashing if known.

***Valve Trim Information (Where required):

Specify the Following:

  • Valve type number (optional)
  • Valve size
  • Valve body construction (angle, double-port, butterfly etc)
  • Valve plug guiding (cage style, port- guide)
  • Valve plug action (push – down –to –close or push-down-to-open)
  • Port size (full or restricted)
  • Valve trim materials required
  • Flow action (flow tends to open valve or flow tends to close valve)
  • Bonnet style (plain, extension, bellows seals etc.)

***Actuator Information:

Specify the following:

  • Actuator size
  • Instrument air supply pressure
  • Instrument control signal (3 to 15sig, 4-20mA, HART etc)
  • Fail action desired on-air failure (valve to open, close, or retain last controlled position)

***Valve Accessories Information:

Specify the following

  • Positioner type if required
  • Limit Switches if required – type, indication etc.
  • Solenoid if required –type, make, voltage etc.

#2 Rules of Thumbs for Sizing Control Valves

Sizing and the selection may be quite challenging. In the majority of applications, it is either the control valve is undersized or over-sized. Especially, in an undersized control valve, the valve is unable to deliver the required flow for each stage of the valve lift creating control problems.

Similar, poor accuracy and unstable control are often the results of over-sized control valves. The over-sized control valves will under all normal operations be confined to small openings of the valve with great risk of variable sensitivity and aggravation of any uneven movement of the valve and actuator combination.

Given a guide for the selection and sizing of control valves to help us limited the problems that could arise because of failure to size control valves accurately, following the rules below:

Rule 1: Set the design flow rate as either

Design Flow Rate = 1.3 x Normal Flow Rate

Or

Design Flow Rate = 1.1 x Maximum Required Flow Rate

Rule 2

Set the control pressure low to 50 – 60% of the frictional pressure loss of the piping system.

Rule 3

Limit the maximum flow rate through the valve: Minimum flow rate turndown should be 5:1 for linear trim valves. 10:1 for equal percentage valve trims.

Rule 4

The valve is much easier to control in the 10-80% range. So, avoid using the lower 10% and upper 20% of the valve stroke.  Consequently, the valve should be sized so as to control the required range of flow rates between 10% and 80% of valve opening. This is illustrated below:

Rule 5

Ideally, select a small control-valve body size less than the line size, i.e Select a valve that has a body size one (1) pipe size smaller than the pipe in which it is to be installed. For example, select a size 2″ valve for a 3″ pipe).But, be carefully, if this causes the valve body to be significantly less than the line size, which would reduce the valves effective Cv. then do not apply this rule of thumb.

Rule 6

Never and never select a valve larger than the pipe in which it is to be installed.

CONCLUSION

Last time we talked about The Diagnosis Performance of Control Valve article. And we mentioned about 2 important things are: 1.Selecting the Control Valve and 2. To all installation, diagnosis, and repair of the control valve must be performed by a qualified valve engineer or an experienced engineer.

Why? Because of the life of the control valve. It can be increased significantly when using PdM (Predictive Maintenance) using intelligent positioning and/or independent valve performance analyzers. And that is the reason why there has been much debate about the accuracy of the control valve test by using the original equipment manufacturer (OEM) measurements compared to the use of independent valve analyzers.

Yeah, that is very important article so, if you did not read yet, please read more here: http://www.naman-tech.vn/diagnosis-performance-control-valve/