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Valve Sizing and Selection

Dec 05 2011 03:20 PM | Chris Haslego in Fluid Flow ****- Share this topic:
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Selecting a Valve Type


When speaking of valves, it's easy to get lost in the terminology. Valve types are used to describe the mechanical characteristics and geometry (Ex/ gate, ball, globe valves). We'll use valve control to refer to how the valve travel or stroke (openness) relates to the flow:
1. Equal Percentage: equal increments of valve travel produce an equal percentage in flow change
2. Linear: valve travel is directly proportional to the valve stoke
3. Quick opening: large increase in flow with a small change in valve stroke


So how do you decide which valve control to use? Here are some rules of thumb for each one:
1. Equal Percentage (most commonly used valve control)
a. Used in processes where large changes in pressure drop are expected
b. Used in processes where a small percentage of the total pressure drop is permitted by the valve
c. Used in temperature and pressure control loops

2. Linear
a. Used in liquid level or flow loops
b. Used in systems where the pressure drop across the valve is expected to remain fairly constant (ie. steady state systems)

3. Quick Opening
a. Used for frequent on-off service
b. Used for processes where "instantly" large flow is needed (ie. safety systems or cooling water systems)

Now that we've covered the various types of valve control, we'll take a look at the most common valve types.

Gate Valves


Attached Image: valve5.gif
Best Suited Control: Quick Opening

Recommended Uses:
1. Fully open/closed, non-throttling
2. Infrequent operation
3. Minimal fluid trapping in line

Applications: Oil, gas, air, slurries, heavy liquids, steam, noncondensing gases, and corrosive liquids

Advantages:
1. High capacity
2. Tight shutoff
3. Low cost
4. Little resistance to flow

Disadvantages:
1. Poor control
2. Cavitate at low pressure drops
3. Cannot be used for throttling



Globe Valves


Attached Image: valve6.gif
Best Suited Control: Linear and Equal percentage

Recommended Uses:
1. Throttling service/flow regulation
2. Frequent operation

Applications: Liquids, vapors, gases, corrosive substances, slurries

Advantages:
1. Efficient throttling
2. Accurate flow control
3. Available in multiple ports

Disadvantages:
1. High pressure drop
2. More expensive than other valves



Ball Valves


Attached Image: valve7.gif
Best Suited Control: Quick opening, linear

Recommended Uses:
1. Fully open/closed, limited-throttling
2. Higher temperature fluids

Applications: Most liquids, high temperatures, slurries

Advantages:
1. Low cost
2. High capacity
3. Low leakage and maint.
4. Tight sealing with low torque

Disadvantages:
1. Poor throttling characteristics
2. Prone to cavitation



Butterfly Valves


Attached Image: valve8.gif
Best Suited Control: Linear, Equal percentage

Recommended Uses:
1. Fully open/closed or throttling services
2. Frequent operation
3. Minimal fluid trapping in line

Applications: Liquids, gases, slurries, liquids with suspended solids

Advantages:
1. Low cost and maint.
2. High capacity
3. Good flow control
4. Low pressure drop

Disadvantages:
1. High torque required for control
2. Prone to cavitation at lower flows



Other Valves


Another type of valve commonly used in conjunction with other valves is called a check valve. Check valves are designed to restrict the flow to one direction. If the flow reverses direction, the check valve closes. Relief valves are used to regulate the operating pressure of incompressible flow. Safety valves are used to release excess pressure in gases or compressible fluids.

References


Rosaler, Robert C., Standard Handbook of Plant Engineering, McGraw-Hill, New York, 1995, pages 10-110 through 10-122
Purcell, Michael K., "Easily Select and Size Control Valves", Chemical Engineering Progress, March 1999, pages 45-50



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16 Comments

Great Post! Very helpful.
Thanks
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MAGICAL_RUPI
Jun 09 2012 07:06 AM
Magnificent post !!!
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ecovarrubias
Aug 14 2012 10:37 AM
Congratulations... Great Post!!!! Thanks
Thanks for the info
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Emad Elgebesy
Aug 28 2012 05:56 AM

Hi, Chris
I'm Emad Elgebesy. Senior process engineer in Worley Parsons. First of all, thanks for your clear efforts on the community, And im willing to share my experiance in this regard.
In Worley Parsons, we did the valve calculation for, single phase & two phase. The above mentioned calculation did not demonstrate which phase is, But i think this liquid phase valve calculation only.

After calculating the Cv, Supplier should verify the strock range (10% to 90%) is commnly used, noise level& body size as well.
Finally, there is many free control valve software of FirstVue do regirous calculation

Regards
Emad

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Chris Haslego
Aug 28 2012 11:57 AM
Thanks for your feedback. You are correct, this article only addresses valve sizing for liquid phase design.
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khaerullah ibnu umar
Dec 28 2012 03:50 AM
so when will you post about valve sizing for vapor phase??
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ashish dubey
Feb 02 2013 01:03 PM

thanks for the info Chris

Dear Chris,

 

If the vendor did not supply the valve chart, is there any we can get the control valve opening by using calculated Cv and rated Cv? 

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Chris Haslego
Feb 17 2014 12:39 PM

Dear Chris,

 

If the vendor did not supply the valve chart, is there any we can get the control valve opening by using calculated Cv and rated Cv? 

 

Cv charts are generally made for experimental data.  If your valve supplier doesn't have a Cv chart for a valve that they sold to you...find a new valve supplier...that is not acceptable.

Chris,

Nice article!

I'd like to make a suggestion for a follow-up article. There are many times that we size liquid control valves in systems where the source pressure comes from a gas in the system instead of a pump (I am thinking of scrubber dump valves). In these situations, after sizing the liquid valve, we often need to determine the "gas blowby" rate, which is how much gas would flow through the valve if the liquid level control loop failed and left the valve wide open. This is often required when sizing pressure safety valves further downstream in the system. It would be great to add an additional step to this article to calculate the maximum gas flow through the selected liquid control valve.

Regards,

Paul

Great job

Thanks Chris
Really Helpful post for someone like me who is recent graduate, explained in very easy way.

I have few questions.

1. is this procedure and equations are useful for compressible fluids/ Gases

2. Do you have anything for selecting control valve for compressible fluids/gases ? (Explaining Nice and easy with thumb rules for selection criteria, like gain should never be less than 0.5 etc.)

I found a handbook from Emerson on control valve. That procedure seems to be too complicated or theoretical.( Too many contents no thumb rules ) Page 118 of Fisher control valve handbook, Emerson (its free on google - couldn't post the link)

 

Beautiful thanks Chris

Dear all,

In maximum case of flowrate, why the pressure dropcontrol valve should have the minimum limitation?

For example, when calculating hydraulic balance, at max case ( 120% normal flowrate), the pressure drop of control valve for liquid service is from 0.5 to 1 kg/cm2?

In my opinion, if the pressure drop of CV is too small, the Cv of CV will be very big. Therefore, even though the CV is full open at max. case, CV can not control the flowrate is the case, right?