How to Perform Stress Analysis for Water Hammer Loads

Read about START-PROF pipe stress analysis software

In piping systems sudden changes in flow cause pressure unbalanced forces between elbows, valves, tees, reducers, and other in-line components. These forces damage the piping system when it is not properly designed to accommodate them.

The main advantage of using PASS/START-PROF and PASS/HYDROSYSTEM is that 3D piping model can be easily converted between these software packages both ways.

3D piping model from PASS/START-PROF can be converted into 3D piping model in PASS/HYDROSYSTEM, then generate 3D water hammer loads for all nodes at certain moments of time (not only just in one node!) and pass it directly into stress analysis software PASS/START-PROF automatically.

The water hammer loads are calculated using PASS/Hydrosystem software, then exported to PASS/START-Prof to perform the pipe stress analysis. The algorithm is following:

1. The piping model is created in PASS/START-PROF and saved into the file (.ctp)
2. The file .ctp imported in the PASS/Hydrosystem software
3. Then transient process analysis (waterhammer calculation) is performed in the PASS/Hydrosystem software and loads are exported into the .ctpf file
4. The loads are imported from .ctpf file into the PASS/START-PROF software using the operation mode editor (Add > Force Loads from File)

The loads that exported to PASS/START-PROF are calculated in PASS/Hydrosystem the following way:

• during water hammer program is calculating the pressure at the nodes of the pipeline and how it changes in time
• then it calculates the difference between these pressure values and value of “normal” pressure (at the steady-state conditions)
• then this difference is divided to the cross-sectional area of pipeline in all direction where the forces are directed and the vector sum of the forces in node is calculated
• then the program determines the times when the peaks of these resulted forces occur (using a special algorithm)
• and then peak values are multiplied on the DLF (dynamic load factor) that takes into account that when dynamic loads are applied to the system, deformations can turn out to be 1.0–2.0 times greater than those from static loads. Since the calculation in the "START-PROF" from the water hammer loads is performed by the static method, to take into account the dynamic nature of the load, we recommend to use by default the worst value of the dynamic coefficient DLF = 2.0 (but you can change it manually on “Water hammer” tab for the pipeline in Hydrosystem)

You can find more detailed description of this algorithm in PASS/Hydrosystem user guide.

PASS/START-PROF and PASS/HYDROSYSTEM allows us to model it very easy, just in several clicks, and help to find a way to reduce water hammer damage of the piping system. Let’s study how to perform stress analysis of the piping system from water hammer loads.

The simple piping system created in PASS/START-PROF software is shown on below screenshot:

After saving the PASS/START-PROF file, we can open it using PASS/HYDROSYSTEM software.

Set pressure 0.15 MPa at node 1, and 0.16 MPa at node 11

Choose water steam library

Define the period of surge analysis 2.5 sec, data output stem 0.01 sec, and dynamic load factor DLF=2.0.

Select Valve in node 12 and in the “Waterhammer” tab set that valve is immediately closed.

Run Isothermal Flow analysis

Run water hammer analysis

After that export unbalanced pressure forces into (.ctpf) file. Also HYSROSYSTEM allows to export data into (.frc) file that can be opened by CAESAR II.

(.ctpf) file contains forces multiplied by DLF factor for all nodes in piping models at several most dangerous moments of time. The most dangerous moments of time is found automatically by HYROSYSTEM software.

This is the (.ctpf) file structure

This means, that PASS/HYDROSYSTEM generates unbalanced pressure loads for all nodes in the piping system at a certain moment of time.

Now open PASS/START-PROF, open Operation Mode Editor and import waterhammer forces into first operating mode.

Eight additional force-based load cases are imported from the file

You can check the force values at any node properties

Now run piping stress analysis and see results

Deformed Shape at any moment of time

Stress table at any moment or time

And maximum stresses from all load cases (static and all moments of time during waterhammer)

The same table can be checked for displacements, support loads, flange leakage, expansion joint deformations, etc.