Pressure acting on pipe wall

[feng09]

Hi all,

I am working on high pressure valve design and need help from piping experts..

when we pass a clean water at 390 bar (5656 psi) through pipe line, then what amount of pressure will act on wall of the pipe ?

as a beginner, i feel 390 bar pressure will not directly act on pipe as this pressure is converted into velocity to generate flow....also; i think this reduced pressure will act on internal parts of the valve....

I may be wrong but please comment on this..

thank you very much in advance

 

[LittleInch]

You're wrong.

Valves tend to go in pressure rating steps and you need to assume that all the pressure acts on the inside of the valve.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[zdas04]

A gentleman named Archimedes described this process very well a while back (about 225 BCE). Pressure acts in every direction all the time. The flow is driven by a differential pressure, internal pipe stresses (primarily we are concerned with hoop stress, but longitudinal stress is non-zero) are a function of the pressure in the line and the geometry of the line.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

 

[1gibson]

How much does a pound of feathers weigh?

 

[MJCronin]

390 bar ???........ you are a beginner ?????...... pressure not acting directly on the pipe ?????

Welcome to valve engineering in the third world and ......please, please stand far back !!!!

 

[feng09]

thank you all, for amazing replies...i think question was quit simple as you all experienced guys got misunderstand ....it was about conservation of energy - "pressure converted in to velocity" to generate flow then what would be the reduced pressure. i presume in valve engineering it is called "pressure drop" ,let say a pipe of 2" dia for 30 meters , at one end pressure will be 390 bars and at other end i imagine it wont be 390 bars ..then what would be that reduction in pressure
pressure acts on body uniformly is a universal law i don't want to challenge on that.

hope i made my self clear..

Thank again for your replies...

 

[BigInch]

Yes, correct. There is a static pressure and velocity pressure potential (read from a pitot tube). When velocity is reduced, static pressure increases to the maximum.... LIKE WHENEVER THE VALVE IS CLOSED. If you can close your valve, you'd better consider the maximum static pressure.

Independent events are seldomly independent.

 

[racookpe1978]

Yes, when a theoretical valve is open in a theoretical pipe under ideal conditions in an ideal world under ideal conditions, there will be a little bit less pressure on the pipe wall because of the flow energy in the ideal liquid moving past the ideal pipe wall.

And whenever your theoretical valve is closed rapidly in real a pipeline in the real world, the valve will see multiple pressure surges significantly ABOVE those simple static pressures just mentioned above of the theoretical no-flow design condition.

Recommendation: Design for the real world.

 

[feng09]

yes, when the valve is closed hydro-static pressure should be considered ..here in this case full 390 bar..
but when valve is open ...hydro-dynamic pressure will in to the picture i think that will act on internal of valve body ..it is a base rule in valve engineering ..that valve should be design at maximum pressure (actually 1.5 times the maximum pressure) but it will be more precise if we can find hydro-dynamic pressure...

come on guys its a real engineering world ...there is fact written somewhere ...There should not be any kind of force which is out of control while designing a component..

 

[LittleInch]

You're "working on high pressure valve design". The valve needs to be designed for the highest pressure it will ever see in operation, even if only once during its life. If it can withstand that presusre (and be tested for bidy stregnth at 1.5 times that pressure), then it can withstand any lower pressure. I therefore can't understand what your question now is.

Pressure is not "converted" into velocity. Will the pressure in the pipe be lower further along - Maybe, maybe not. All depends on pipe size, flow rate, viscoisity, elevation, density, roughness of pipe etc etc. If the flow is low and the pipe vertical down, the pressure in the pipe might be higher if the static head increase is greater than the friction losses.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[feng09]

Yes your right, if the valve is withstand at test pressure then it will safe for any other lower pressure,
but generally valve testing is done at static condition hence no dynamic forces are consider. however in real world due do tremendous velocity of fluid dynamic forces may cause damage to graphite seals, soft (PTFE) seats or abrasion in metal seats hence i am looking for hydro-dynamic pressure acting on valve components.

 

[BigInch]

Why not test (take a real beating on) your theories over at the Valve Forum.

Independent events are seldomly independent.

 

[LittleInch]

Come on big bro' don't encourage him to double post... He'll just get red flagged.

Now what you're talking about is something completely different. In an ideal world, flow is controlled / governed / stopped and started using valves deisgned for the purpose, i.e. control valves. Isolation valves isolate static flow and open against low differential pressure.

However in the big bad world we actually live in valves are regualrly destroyed by being opened and closed under larege differential pressure or flowing conditions. Soft seats in these instances last for only a few operations and metal seats can be destroyed by hard particles as the velocity reaches huge numbers in the milli second before closure / as the valve opens. As far as i know this is virtually impossible to predict / calculate, but unless you are trying to make a valve that can do everything, then you need to rely on the data sheet which says that your valve is for isolation only....

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[BigInch]

He can red flag this post as the duplicate.

Independent events are seldomly independent.

 

[LittleInch]

Yeh, but until he works out what his question really is, then there's no hope...

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[KLee777]

MJCronin, my thoughts exactly!

I'm so confused as to what he's really driving at in this post. Apparently so is everyone else. Let's hope you have a more experienced engineer reviewing your work, feng09!

 

[zdas04]

He is trying to find support for the idea that if dynamic pressure is driving flow then it is not available to push on the pipe walls. People really want to anthropomorphise fluid flow. The concepts of dynamic pressure and static pressure are among the hardest for people to get straight in their heads. Once you start down a wrong road, everything on the right road looks and sounds wrong.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"

 

[77JQX]

Strictly speaking, he's right. By Bernoulli, the pressure in a moving fluid is less than than in a static fluid (at the same elevation, etc.). What starts as potential energy is partly converted to kinetic energy. Not that the change amounts to anything at 390 bar and speeds you'd want in a pipe.

 

[zdas04]

Yep, it would be less. Less by several milibars. Measurable. Insignificant.

David Simpson, PE
MuleShoe Engineering

"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The plural of anecdote is not "data"