6 replies to this topic

[Mark-TR]

Dear all,

 

I’m studying a surge analysis for liquid filled systems in a project at its preliminary stages.

 

The scenario I’m studying is the failure close of an on-off.

 

I´m in the preliminary stages and then I do not have yet vendor information.

 

Under my preliminary result I have had the next observations:

 

-In general terms, the full open CV of an on-off valve is very high to modify the flowrate through line. It only starts to affect the flowrate once a minimum value is achieved.

 

As an example: if the CV of an On-off valve 2500 , it will only start to change the hydraulics of the system if a CV lower than 500 is obtained due to the closure of the valve.

 

-Due to this fact the travel vs Cv can highly affect the results (see attachement).  example.pdf   94.51KB   54 downloads

 

If we consider a linear closure from 100% open to completely close in 10 seconds, a linear valve will change the flow from 100% flow to zero  in 2 seconds, whereas an equal percentage will change the flow from 100% flow to zero in 4 seconds.

 

A linear valve will provide a higher pressure for surge calculations.

 

Then I would like to know if it is typical that on-off valves are designed to be equal percentage or if they are designed to be linear. Additionally, I’d like to know if the degrees of travel vs time is typically linear or equal-percentage.

 

At the end my objective is to know how the CV changes with time.

 

I have tried to contact vendors in different projects, may be the service in spain is not good, but they don’t usally answer this kind of questions. They usually have information about control valves, nevertheless they do not have it for On-Off valves.

 

Then I’d like to know if somebody In the forum could advise me regarding how the of On-off valve reacts with time during its closure

 

Kind regards

Edited by Mark-TR, 30 January 2014 - 07:08 AM.

[Steve Hall]

I believe that Cv vs. % open charts are readily available for all types of valves, but you have to state which type of "on-off" valve you are considering. And what size?

[Ajay S. Satpute]

Hi,

 

Please check attached file.

 

Consider using hysys for valve sizing.

 

Regards.

 

Ajay S. Satpute

Attached Files

•  Flowserve Cv tables.pdf   744.29KB   134 downloads

[Mark-TR]

Dear Steve and Ajay,

 

Thanks for your promptly response. the sizes I'm considering goes to 6" to 24" inches.

 

According to your answers it is my understanding that on-off valve can be linear or equal-pecentage. Correct me If I'm wrong. 

 

What I'd like to also know is how the %open varies with time.

 

On services where surge conditions the typical closure time is 20 seconds.

 

Therefore, it will not be the same if the %open vs time is linear or equal percentage.

 

For example, if the % open is linear with time. I can directly use the table provided by Ajay. I just will need to transform the curve %Cv vs %open by changing only the time variable for %open proportionally.

 

ex

%open=100, time=0 seconds

%open=50,   time=10 seconds

% open=0.    time=20 seconds

 

If the time vs % open is not linear this analysis will be different: 

ex

%open=100, time=0 seconds

%open=50,   time=5 seconds

% open=0.    time=20 seconds

 

Then I would appreciate your advise on how it is the curve %open vs time for on-off valves.

 

kind regards

[Ajay S. Satpute]

Mark,

 

Please go through below link for learning more about the valve flow characteristics;

http://www.engineeri...tics-d_485.html

 

Below link states "Fast opening valves tend to be electrically or pneumatically actuated and used for 'on / off' control".
http://www.spiraxsar...acteristics.asp

 

Shape of the trim should help you identify the valve flow characteristics. Refer second link for more details.

 

The Control valve type is used to calculate an assumed design Cv at 100 % Open. The valve opening characteristics is approximated as shown below:

Linear: Cv% = Valve opening %
Equal percentage: Cv% = (Valve opening %)^3
Quick opening: Cv% = (Valve opening %)^0.5

 

The flow shall follow above equations. If you set valve opening as a function of time, then it you shall get flow at given time. I believe this is what you stated in your earlier post.

 

Your first post mentions an attachment. I do not see any attachment.

 

Are you doing the surge analysis in PIPENET-TRANSIENT?

 

Regards.

 

Ajay S. Satpute

[Mark-TR]

Thanks Ajay,

 

I didn´t notice I forgot the attachment. I have updated the query.

 

Your answer will be very usefull, I'm currently modeling my surge analysis with AFT Impulse. it is less complicated and not so detailed than Pipenet but effective.

 

The article on spirax sarco clarifies very well how the inherent curve is related with type of valve used.

 

I finally have obtained an answer from our Instrument department, Metso ( a valve constructor) has confirmed us that the valve opening % is linear with time for an on-off valve, unless a ramp is provided.

 

The closure time of an On-off can be specified on its datasheet, a typical value is 1 second per inches of diameter

 

With that information and the help that I have received on this forum, I can proceed with my analysis. 

 

Thanks a lot

[farid.k]

Have you considered to relocate the valve to upstream? The lesser pipe length the chances for the surge is smaller then valve closing time can be faster.