Hi guys!

I need some help here.

I am analyzing a 40" LNG line, with a surge pressure of 9.14 barg and a volumetric flowrate of 14,000 m^3/hr. I need to solve for the force develop by this surge pressure on my piping system, so that I can anticipate precisely the loads acting on my directional anchors and guides. I just don't know the right formula for it, or how to calculate it precisely. Anyone have any ideas on how to calculate surge forces? Please help me out here folks.. I just need it badly..

Thank you very much!

Calculating surge loads is not a straight forward exercise unfortunately. Most times I have experienced this the approach has been very, very conservative. ie: P x A x DLF, which is basically assuming an instant valve closure and assuming the same pressure rise for the entire piping system regardless of the length or configuration.

It is worth speaking to an experienced process engineer who has practical piping and site knowledge. You need to know the surge rise, the valve closure time, the valve type (to determine effective closure time), acoustic velocity and some other things. A process engineer can tell you all those things, and they should be able to help work out realistic loads developed from elbow to elbow.

If you work with any sub-sea or pipelines engineers they should also be able to help you, as they would deal with substantial surge loads due to long straight lengths of pipe.

Hello Kuramadenz,
Please confirm whether the pressure you have stated is the surge pressure or the line pressure.
Usually for a main LNG lines a surge analysis will be studied during shut down operation in a specialised software(eg, Pipenet). This will give the rise in pressure in the line during shut down operations. Since the valves are not stoped a sudden the surge pressure will not be that high. Since the line sizes are very high in your case it will be too conservative to consider the absolute pressure difference as the surge pressure.

Hello There,

For valve closure and surge analysis, PIPENET Transient module generates Force File (FRC file) containing uni-axial applied forces on piping systems. CAESAR II can import FRC files and generates Dynamic input (Water Hammer) automatically.
You may do the same analysis by using BOSFluids and CAESAR II.
For water hammer calculation, refer to CAESAR II application guide.

Hello Fai Maz,

The surge pressure is really that 9.14 barg. Actually I have already the surge analysis report from the process group. They have 7 cases of ESD of control valves. I chose the case that would give me a conservative result, and that case registered a peak surge pressure of 9.14 barg.

Statically, the force that would be expected at each nodes, is around 600kn for my case, assuming of course that the pressure rise time is zero. But the process had given me a rise time of 15 seconds after ESD. So I run a dynamic analysis, and get a low loads on my supports. I'm just not sure if this is a right procedure.

Hello

If you have the surge analysis report from process group which contains the time-Pressure data at each nodes. First change the pressure into rised pressure by using P = P0 + Delta P(Rised pressure) and then change the rised pressure into force by using F = P x A.
Finally, run Time history analysis in CaesarII with Time-force data at
each nodes.
This is my just idea

Does anybody have any idea about above?

If applying the calcualted surge forces in CEASAR II by "static equivalent", it might require to multiply the loads by DLF factor = 2 ?

Du Wei,

DLF for an instantaneously applied constant load (not a harmonic or impact load) is always exactly 2.

In my opinion,

F = Delta_P x Ai x DLF.

What the surge report gives you is maximum surge pressure (or peak pressure) P in chart result. And P0 is steady state pressure (or operating pressure) which is available from line list or can be read from chart at Time O seconds

Delta_P (maximum unbalanced surge pressure) can be taken from chart result (by using formula Delta_P = P - P0) or detail discussion with process.

If using P = P0 + Delta_P for calculating F will give you a very big value which leads to unnecessary over design of pipe support or pipe over stress

Looking for other opinions

Van Ha,

Have a look in a fluid dynamics book for Joukowsky, this deals with surge pressure due to rapid valve closure.

Farhad,

The value of DLF does not EXCEED 2.0, however it may be far lower and is certainly NOT always exactly 2. It is a function of load duration and system natural frequency.

MoverZ,

Can you clearly specify your comment? I'm happy to hear correction from you.

What I'm saying is for static analysis (not dynamic analysis)

Van Ha,

I was not referring to Caesar II dynamics. I meant look at a Fluid Mechanics book that deals with fluid flow type dynamics.

MoverZ,

Thanks for your comment. What I stated regarding DLF=2, was "static equivalent" load factor. In dynamic analysis, for each natural frequency, there is a specific DLF (max. 2).