Control valve - CV at 100% open 3

[sheiko]

Hello people,

How can i obtain the the CV value of a control valve at 100% open, knowing the CV value at 80% open for example?

Thanks

"We don't believe things because they are true, things are true because we believe them."
"Small people talk about others, average people talk about things, smart people talk about ideas and legends never talk."

 

[gerhardl]

I believe you can't. As the cv is the dimensioned measurement of flow (kv = m3/h of water at 20 deg C with a pressure drop of 1 bar, cv = kv x 1,2 (approximate definition)) there is no direct automatic mathematical connection between the flow at 100% and any lesser opening degrere.

The flow (cv) at any opening will be depending on the internal geometrical lay-out of the valve at the different opening degrees. Measured flow will give you the flow-curve (cv at different openings).

The flow curve may approximate a straight line, or a curved (normally convex) line.

The manufacturers of valves do not usually test each single valve for the flow curve, but select certain sizes and certain opening degrees on the curve, and give the rest as approximations based on the known measured data.

The best you can hope for with your valve to guess the flow is the same. If you cant obtain the real flow curve for your valve you may compare and compute according to values from a similar (very similar!) known valve.

 

[JLSeagull]

Standard curves exist for control valves. You need to determine which trim exists. Some manufacturers provide tables showing the percentage travel and the valve coefficient Cv at each 10%, or each 10 degrees for rotary valves.

A common characteristic selection for throttling valves is described as equal percentage. With an equal percentage characteristic curve, equal change in the travel provide equal percentage changes in the flow coefficient. Cage style globe valves can be machined for several common characteristic curves. Equal percentage is likely the most common, followed by linear and fewest control valves being quick opening. A similar curve for old style plug and seat valves was known as modified parabolic.

This site includes common control valve characteristic curves:

http://www.engineeringtoolbox.com/control-valves-flow-characteristics-d_485.html

Even if you cannot find such data for a specific manufacturer and model number, if you know which characteristic curve applies, you could calculate or ratio against tables provided by Emerson, Dresser, Flowserve, etc.

 

[JimCasey]

The 100% Cv should be stamped on the Nameplate.
I am curious how you would know the 80% (travel) Cv and not know the 100% Cv. That seems odd.

 

[gerhardl]

JLSeagull - star for reminding me of the engineering toolbox (and your answer), the toolbox is included in my 'favourites' now.

 

[sheiko]

JimCasey,

Using the masoneilan software, you input pressure drops at min, norm and max flow and the software outputs give the Cv a certain valve opening which the max is not 100% but close to 80%...

"We don't believe things because they are true, things are true because we believe them."
"Small people talk about others, average people talk about things, smart people talk about ideas and legends never talk."

 

[JLSeagull]

Normally the output from sizing software pertains to the valve coefficient at that specific operationg condition. Often the process data includes minimum, normal and maximum cases. The Cv calculated is the requirement for the specified condition - not the rated valve Cv. If a specific valve was selected, the software may identify that specific valve opening that corresponds to that calculated Cv.

 

[insult2injury]

I have not used the masoneilan software, but most vendor programs have the ability to output the full open cv, cv curve, or tabulated %open/cvs. I would expect that the masoneilan software is no different.

I2I

 

[bcd]

For what it's worth at this point as it is not germain to the initial discussion. Just adding some information in response to JimCasey's posting.

Cv at 80% open is common for butterfly valves due to dynamic torque effects. Going beyond 80% is when the leading edge of the disc typically passes by the shaft centerline on double and triple offset valves. Before 80% open, torque from flow tends to shut the valve. Note, this is with flow with the shaft downstream. Beyond 80% open, there is a large dynamic torque reversal trying to open the valve. Some suppliers set the open stops to limit travel on these types of valve to 80%.

While it was only an example in the original posting, there are cases where Cv at only 80% open is used.

bcd

 

[sheiko]

During the study of relief valves scenarios (control valve failure), API states to use the CV at 100%. This was my intent.

"We don't believe things because they are true, things are true because we believe them."
"Small people talk about others, average people talk about things, smart people talk about ideas and legends never talk."

 

[roydm]

Some of the Fisher tables don't show 100% either, we always try to size a valve so it's operating ~ 75% of full flow so the 100% value is irrelevant.
BTW Cv is how many USG will flow at 1 psid
Roy

 

[sheiko]

Looking at tthe link provided by JLSeagull, it seems that for linear type control valve, one could approximate the CV at 100% opening from the Cv at 80% opening using:
CV(100%) = 100*CV(X%)/X

"We don't believe things because they are true, things are true because we believe them."
"Small people talk about others, average people talk about things, smart people talk about ideas and legends never talk."

 

[insult2injury]

The linear valve characteristic may not intercept at 0 due to the dwell at the seat (i.e. at low open% the relationship may not be linear).

I2I

 

[roydm]

"Using the masoneilan software, you input pressure drops at min, norm and max flow and the software outputs give the Cv a certain valve opening which the max is not 100% but close to 80%.."
The reason for that is you never size a valve to operate at 100% open. Always leave a margin to cover possible errors in process data etc.
If you give us some more information someone will tell you what the wide open Cv is.
Roy

 

[sheiko]

You are right insult2injury.

Also i would like to remind the objective of this post: calculate flow through control valve during inadvertent valve opening relief scenario when sizing relief valves. API 521 states to use the CV coefficient at 100% opening. Then i assume this coefficient does exist and i would like to know where or how to find this it?

"We don't believe things because they are true, things are true because we believe them."
"Small people talk about others, average people talk about things, smart people talk about ideas and legends never talk."

 

[psafety]

Can you get the serial number off the valve? I run into this quite often and call the supplier, he always had a record of the valve supplied to us.
I too use 100% for maximum flow.

 

[JimCasey]

This thread seems to have gone all over the place.
You asked how to get the 100% Cv of a valve, knowing the Cv at 80%.

If that's all the information you have, you can only estimate.


MOST control valves have equal percentage characteristic. These vary, but 80% of TRAVEL with =% is about 45% of CAPACITY. So if you have (for example) a Cv of 20.7 at 80% of stroke, the full 100% travel Cv is 46. If it's linear characteristic, you can estimate pretty closely by multiplying the 80% Cv by 1.25. The characteristic also should be on the serial plate. Cv46P means that the Cv is 46, percentage.

You mentioned the Masoneilan software. I haven't used it recently but I did have a relationship with Masoneilan for 18 years, and their software, (and their sliderule), were all keyed to the 100% Cv which has always been posted in their literature. And if it's a Masoneilan valve, the serial number plate was stamped with the rated Cv, and the Characteristic. BTW: The numbers in the previous paragraph were from the Masoneilan 21000 series valve bulletin.

Psafety gives you an effective "Plan B" by calling the Manufacturer with the serial number and asking for a data sheet.

Many valve manufacturers engrave the part number of the plug in the flats at the top of the stem. If the serial number plate is gone, you might be able to read the plug part number, then check the parts supplement or the manufacturer's customer service rep, and get the Cv of the plug with that part number.

 

[sheiko]

Thank you,

In pages 126-128 from Fischer handbook (

http://www.documentation.emersonprocess.com/groups/public/documents/book/cvh99.pdf),

i have found some CV values.

1/ Could you please tell if the CV in the very first table (page 126) are at 100% opening (as there is no mention of degree of valve opening)?

2/ More generally, what is the differences between the 2 tables (single-ported versus rotary shaft)?

"We don't believe things because they are true, things are true because we believe them."
"Small people talk about others, average people talk about things, smart people talk about ideas and legends never talk."

 

[insult2injury]

1/ The Cv values listed in that table are general guidelines for Fisher globe and ball valves. They may be vastly different from the specific valve/trim you have selected.

2/ The two tables are for different valve geometries (globe valves and ball/butterfly)

I2I

 

[sheiko]

1/ thanks but at which opening?

"We don't believe things because they are true, things are true because we believe them."
"Small people talk about others, average people talk about things, smart people talk about ideas and legends never talk."