Surge forces on elbows

I am having to do a force time history surge analysis and am having problems working out the forces to be applied to the bends.
I have been provided with force time files from a CFD package.

I had assumed that there would be 2 forces acting on a bend due to pressure which would give me a resultant force direction of 1/2 the bend angle & resultant magnitude for a 90 bend of 1.41 (2 sin Angle/2).
The above assumption would seem to be confirmed by the paper "Dynamic pipe stresses during water hammer III Complex stress relationships" by R. Leishear.

While looking to see how I input the force time history files into CII I noticed that water hammer examples showed the force acting in an axial direction only.

So which way is right or am I over complicating things for a simple beam calc. program?

Paul,

Maybe both are correct...

Your resultant force has two components that relate to the pipe entering the bend and the pipe exiting the bend. Your surge is a pressure wave coming from the pipe entering the bend and going to the pipe exiting the bend. If you focus on the load, you will have your constant loads on all elbows except the the one we're talking about and all these loads will be pointing away from the bend in the direction you state.

But if, instead, you focus on the imbalance, the imbalance exists in each run, one after another. The surge is ending the imbalance on the entering pipe and initiating the imbalance on the exiting pipe. The dynamic load is the imbalance and that can be easily entered as a load along the axis of each pipe.

Does that make sense?

It sort of makes sense Dave
If ive got this right it really depends upon the data provided in the force time history files.
1. If the force time history files have the imbalanced forces on elbow elbow pairs then you need only apply the loads in an axial direction.

2. If the forces in the history files are based on actual pressures at each elbow at a given time then you are going to need to input a resultant force.

We have been provided with the 2nd of the above maybe we should have asked for different data, what do most companies doing surge ask for?

Just had another thought on this.
Do the forces in the time history file from a CFD program have to ramp up from zero or can CII cope with starting from the steady?

The force/time profiles that you add in a Time History analysis are the unbalanced forces. While Caesar II can handle a non-zero starting point, doing so in my opinion implies an unbalanced force to start with. I guess you can experiment with it to see if the results make sense.

Paul,

My understanding of the problem is that the Force should be applied in two directions.

The unbalanced load is due to higher pressure ( p+dp) in one elbow and p in the other.Now the unbalance starts when pressure pulse reaches first weld line of the entering( first elbow). The unbalance reaches its peak when the pulse covers the length of the elbow , it remains till the time the pulse does not reach the first weld line of the exiting or second elbow and then slowly dies down as it travels across the second elbow and goes to the second weldline.

In the above phenomenon the pulse clearly acts in both the directions and hence I would prefer to go by the option of giving the force input in both directions.

Typically we get the input from our Pipeline group in the form of pressure vs time data at different elbvow points and from this data we get the maximum differentials.However the maximum differential does not mean that the DLF is maximum at that time.Hence we have to talk to our CFD analyst to get fine tuned data at specific points of interest.

Regards

Loren,

The data was requested from the CFD people in its current form prior to me starting at this company & it was only when I was working with the history files that I saw a problem.
I can see the point of using unbalanced forces rather than the raw data.
From tests I have done with another pipe stress program it would suggest that when using raw force data a start up ramp is required to get to the forces created by the steady state pressure, if you don't provide a ramp the results should be conservative but upto 2x the equivalent results for the ramped case.


Anindya,

What you are saying about the loads is exactly what I felt would be happening, my mistake was to skim read the CII manual.
If I had read the manual properly I would have realised that Coade were talking about a different set of force data.
If people had listened to the CFD people at the start I wouldn't need to be asking questions & changing things now.

More speed less haste needs to be my new motto.

Haste makes waste... Benjamin Frasnklin

It certainly has done in this case