Can someone give me a guide on the steps in writing up a datasheet for selecting control valves? I notice it is not as straight forward as other instruments.

Do all linear positioner & valve perform the same say, say opening 10% = 10% flow? If it's equal percentage they would be different?

So assuming I'm using 4-20mA I/P positioner which output instrument air, how do I match the Positioner against the Valve? Do i refer to some sort of curve graphs?

Also after installation, how does the Process Controller control the valve openings? Looking at vendor datasheets it doesn't tell much how the valves & positioner would react & their parameters. How do the engineers program the valve so correct current signal will go to positioner which output correct instrument air as end result having correct opening on the control valve?

 

Hi Papaya,

I would like to share information relating to your questions.

Some important parameter check-points for selecting a control valve are as below :

1. Valve size, Cv
The valve size and Cv are determined by doing Cv calculations. The formulae for calculating Cv is different for liquids and gas/vapors. This is addressed by ISA Standard 75.01.01. Also, as a thumb rule a valve size is selected from half the line size. This is done to avoid bending stresses resulting from widely different pipe & valve sizes and also to avoid turbulence/swage effects resulting from the difference in sizes. After the Cv is calculated an appropriate valve size to meet the required Cv is decided. For performing Cv calculation various process data like Flow rate, Inlet pressure , pressure drop, fluid density/Molecular weight, compressibility & specific heats ratio (both for compressible fluids), vapor pressure & critical pressure (for liquids), etc. are required.

2. Valve Rating
The valve's rating (rating classes are defined in ANSI Standard B16.34) depends on the material being used, the maximum design pressure encountered in the line and temperature. Usually, piping engineers define piping classes. Valve material & rating normally follow the piping class specifications.

3. End Connection
Once again the valve's end connection should match with the piping end connections. Some common end connections used are Flange (raised face flange is commonly used), Welded (Socket weld, butt weld), RTJ (Ring type joint), etc.

4. Valve Body/trim material
Body material used is normally in accordance with the piping class specifications. Trim selection is done based on the type of fluid being handled, temperature/pressure, possibility of erosion/corrosion, etc.

5. Type of valve
For control applications the type of valves preferred are as below, top being most preferred:
1. Globe type
2. Ball type
3. Butterfly type

Butterfly valves are cheap and can be used for on-off control.

There are several other details also to look for when selecting a valve, but these are the major criteria.

As for the valve characteristics - there are 2 commonly used characteristics 1.Linear 2.Equal percentage.

What you mentioned about 10% opening= 10% flow might be approx. applicable only for Linear trim. But be careful to refer Cv rather than flow, because the flow rate depends on both the valve Cv (capacity) and the pressure drop. Equal percentage is the commonly preferred characteristic.

Once you do the Cv calculation and select an appropriate valve size, the valve would have enough capacity and controllability to cater to the required flow rate in the process.

The positioner is calibrated so that the 4-20mA signal is mapped with 0-100% valve travel. For e.g., a 12mA control signal (mid-value) would produce 50% travel in a properly calibrated positioner & valve assembly.

Additionally, you can also refer to the valve's Cv characteristic curve to know how the valve Cv varies as the valve opens. Usually valve manufacturer will be able to give you the information so you can know at what % opening you need to maintain the valve in order to pass the required flow. For this you need to do a Cv calculation with the process data parameters that I mentioned.

Hope this helps you to an extent. If you have specific questions or are not clear with the explanation. Please contact me - [email protected]


Ronald Deepak.
jronalddeepak [at] gmail.com

 

Hi Ronald Deepak

iv read your explanation az well. i want to know that is that true for a pump station valve designing?

thanks for your help

 

Kindly refer the Emerson Control valve Handbook for for selecting and sizing control valve. Also refer ISA standard. You will get answer to your questions.

 

Hi dipak,

I read the information you supplied and found very useful. I want to know the operation of Equal Percentage characteristics. Consider that the valve is 25% open and flow is 50GPM, then the valve is opened 5% i.e. total valve opening is 30%. What is a flow rate at this opening and how to calculate the flow rate?

As per my knowledge the equal percentage characteristics is used for variable DP across valve, and by thumb rule the same characteristic valve is used for Pressure & Temperature control applications. Is it possible to use Linear chara's valve for pressure control application?
The linear chara's valve is used where DP across valve is constant i.e. for Level and Flow control application.

Please explain how to select the proper valve chara's.

 

Hello every body,

Please tell me what is the importance of CV in valve sizing. what i know about cv is the number of us gallons of water the valve will pass with a dp of 1psi. What is the implication of this one in valve sizing?

After computing for the cv what is the next step to what size of valve is to be used? Please give me some example.

I am a newbie in the field of instrumentation and i want to learn more about this one.

THANK YOU

 

CV is a constant relating the flow through a control valve to the pressure drop across it. The basic relationship is:

Q = CV sqrt(DP/SG) where SG is the specific gravity.

The formal definition refers to a fully-open valve, but the term CV is usually used throughout the valve stroke range.

The basic steps involve in valve selection are to:
1. Find the relevant process fluid data and design conditions.

2. Using the above equation, estimate the CV required to achieve the design flow at the associated pressure drop. Note that the flow across the control valve will not necessarily be the design pressure drop - this could occur at low flows.

3. From manufacturer's data, select a valve having the required CV at about 80% of stroke - so you have some margin.

The practical way to select a valve is to use the appropriate vendor's software as there are a LOT of other factors that need to be considered - such as the effect of pipeline reducers and expanders, any special trims required to avoid cavitation, and noise.