Hydraulics Analysis versus Piping Stress Analysis

[yogiemaradona]

Dear Experts,

Currently I've got problem with fluid hammering. Piping has displaced around 300mm from it's normal position. I'm working to find out the rout case why it happened.

In parallel third party company has conducted hydraulic analysis to find the root case, unfortunately the result is different with what I've understood with commonly occurred when we're analysed by piping stress analysis, such as hammering to occur if valve closure is shorter than back forth wave travelling 2*L/ c (L=length of pipe to be analysed ; c= speed wave) but this hydraulic was reported hammering was happened with valve closure larger than wave travelling.

In my case valve closure should be smaller than 0.45 sec then fluid hammering will occur, but hydraulic analysis reported 1 sec.

Please advice can I perform response spectrum or time history following hydraulic analysis result or not ? and please advice me why this discrepancies was occur?

Thanks,
Yogie. M

 

[BigInch]

Some hydraulic programs consider the pipe as non-expandable in the radial direction, some programs consider that the pipe is fixed all along the axial direction. Expandable pipe would slow wave speed, while axially fixed pipe would tend to increase wave speed to your value. Some liquids have sensitivity to speed vs pressure & temperature. Maybe you are considering an average wave speed and the program is considering many segments with different wave speeds in each segment.

What fluid, pipe material, lengths, fixity are you talking about and which program was used for the hydraulic analysis?

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[yogiemaradona]

Hi..BigInch,

Thank for response, This line for LNG services, Temperature -162 degC, pipe material A 358 24" - Sch.10s.

Please find attachment for detail dimension and my question.


Regards,
Yogie

 

[BigInch]

Nice iso, but not much help to me without having any buttons to click on.
I think your pipe stress program may be simplifying a little too much. That seems to be considering response time as length of pipe/speed of sound.

"People will work for you with blood and sweat and tears if they work for what they believe in......" - Simon Sinek

 

[stanier]

AFT's Impulse allows a force file to be generated that can be used in Caesar II.

That said when I do a waterhammer analysis I am able to plot the time history at nodes. This data can then be input into a pipe stress program. This is not ideal but gives some idea of the forces involved.

It is essential that that the hydraulics model is verified against actual field data if this is so sensitive. Firstly does the hydraulic model match the field in steady state?

“The beautiful thing about learning is that no one can take it away from you.”
---B.B. King

http://waterhammer.hopout.com.au/

 

[SNORGY]

Was this a one time event or is it a chronic problem every time a valve closes?

 

[yogiemaradona]

BigInch,

I've considered the response time as length and wave speed as well as attached on my sketch, but I can't do pipe stress dynamic simulation since the valve closure is of the range for fluid hammer travelling load to occur on each straight pipe segment.


Strainer,

The hydraulic calculation was calculated by other then I don't have any native file to be converted, and to be informed this phenomenon has been suspected was occurred because of two event condition, Firstly closing the check valve at the top Tank-2 following by the closing of control valve at node 78 (please see my sketch attached). However can AFT simulate hammering with 2 event condition's as described?


Snorgy,

This was a one time event which suddenly displace all the pipe shoes fallen down from its sleeper, approximate 300mm pipe movement was measured at the elbow (node 50).


Thanks all for the response, and waiting further opinion to solve my problem.


Best Regards,
Yogie. M

 

[stanier]

Hi Yogiemaradonna,

AFT Impulse can similulate multiple transients occurring in a system. There are ways to include Boolean logic to simulate a real situation. This isnt documented but the guys at AFT know how its done. Hopefully in AFT Impulse 5 there will be Boolean algebra included to get logical steps to emulate a PLC system of interlocks and timers.

“The beautiful thing about learning is that no one can take it away from you.”
---B.B. King

http://waterhammer.hopout.com.au/

 

[waterpipe]

Hi,
Just an idea about your different results.
Equally important as the valve's closure time, is its closure behavior or closure curve (relationship between the area of the opening and the position of the valve’s spindle). This is normally non-linear for most of the valves’ types. Many circular gate valves are as much as 85% close before there is a 15% reduction in flow. So a system could experience two different pressures under a same closure time with two different closure curves.

Therefore, 0,45 second closure time that has 85% of full closure at 0.1 sec and the reaming 15% in 0,35 sec "might" result lower surge pressures comparing to 1 sec closure time with a linear closure curve, which closes the last 15% of full closure in 0,15 sec. This can be modeled with a surge program.

 

[SNORGY]

yogiemaradona

Thanks for that. Is it possible that it wasn't a transient per se, but a slug?

 

[Ktracid]

Yogie,

I have also used AFT Impulse and found it very useful for cases like yours (i.e. rapid valve closure).

Regards,
Pirooz

 

[stanier]

I use Impulse to model complex networks. A typical scenario may be one pump trip, valve closes in 3s after a 5sec telemetry delay follwoed by a valve opening in 10s to relieve surge pressure to a reservoir, other valves in the network can be programmed to open if flow, pressure, time etc in a nominated pipe exceed or is below a certain value. Download the demo from

http://www.aft.com

and play with it.

“The beautiful thing about learning is that no one can take it away from you.”
---B.B. King

http://waterhammer.hopout.com.au/

 

[MortenA]

Does your software consider column separation? Im not quite sure which valve closes, but if its USVE2020 this could be an issue. Try to get a pressure transient and see how low the pressure gets at node 78. If its ever below vapour pressure then you may have collumn separation: Column separation may cause waterhammer that is (much) higher than what Youkowsky predicts.