Surge Loads in Pipework
Surge Loads in Pipework
(OP)
Hi There,
I have a general question regarding Surge Loads. If you were presented with the results from a hydraulic surge analysis and wanted to perform a Time History Analysis of the pipework system to ensure stress levels are acceptable and also to determine the support loads would it be relevant to apply a Dynamic Load Factor to the calculated loads. I ask because there are two schools of thought within my Company. I beleive a DLF is applicable whereas others seem to beleive the DLF is already taken care of within the Hydraulic analysis.
Regards
DSB123
I have a general question regarding Surge Loads. If you were presented with the results from a hydraulic surge analysis and wanted to perform a Time History Analysis of the pipework system to ensure stress levels are acceptable and also to determine the support loads would it be relevant to apply a Dynamic Load Factor to the calculated loads. I ask because there are two schools of thought within my Company. I beleive a DLF is applicable whereas others seem to beleive the DLF is already taken care of within the Hydraulic analysis.
Regards
DSB123





RE: Surge Loads in Pipework
Have a look at this free calculator maybe you can compare your results and get an answer.
http
desertfox
RE: Surge Loads in Pipework
I have already had a look at this but it does not answer my question.
RE: Surge Loads in Pipework
I looked at the theory at the bottom of of tab2 in the spreadsheet and it says a DLB should be applied if the force you are looking at from the hydraulic surge analysis
is the product of the pressure wave size * pipe area, then a DLB of 2 should be applied to err on the safe side.
I looked at the surge calculator on tab6 and the DLB is included.
Surely whoever did the surge analysis can tell you whether he just used peak pressure * pipe area or whether he put a factor of 2 in.
regards
desertfox
RE: Surge Loads in Pipework
please read DLF where I wrote DLB
desertfox
RE: Surge Loads in Pipework
http://virtualpipeline.spaces.msn.com
RE: Surge Loads in Pipework
What I have done in the past is taken the variation of pressure versus time from Impulse results, converted this to a force and then applied the force vs time to Pipepak at the node in question. I have not applied a DLF as I have simulated a dynamic loading.
I figure the designer of the structure taking the loads is going to apply their design factor and having factor on factor is overkill.
The study of fluid structure interaction is very complex. When you consider that when a dynamic event occurs the structure will deflect and the stresses will be relieved. The main concern is from buckling. The structural engineer has to be concerned as to how lean the structural design is before the hydraulic or piping designer starts adding factors.
The factors that are applied depend upon the risk profile of the design. A risk profile of a nuclear reactor or refinery will be somewhat different to water pump station in a remote location.
Geoffrey D Stone FIMechE C.Eng;FIEAust CP Eng
www.waterhammer.bigblog.com.au
RE: Surge Loads in Pipework
Stanier,
You say you have not applied a DLF and just converted the pressure to a force vs time loading and applied this to the pipework via Pipepak. I am not convinced this is correct as the results from the Hydraulic analysis is just a pressure vs time. The actual loading on the nodes within the piping system is an effective force vs time and without the DLF does not "model" the dynamic effect of the pressure wave travelling through the pipework.
BigInch,
Thanks for the reply but can you advise whether you think that a DLF should be included in the actual loadings (from the pressure X area) vs time used in a pipe stress analysis or not.
Regards
DSB123
RE: Surge Loads in Pipework
The dynamic results are change in pressure (force) with time. The rate at which the change occurs is the significant point. If the force were to increase in 0.1s it would have a different impact to a change in 5 seconds.
The use of 2 times DLF is a sledgehammer of a way of looking at things. The profile generated is not being used but some number that has little engineering basis but to feel good about something.
Geoffrey D Stone FIMechE C.Eng;FIEAust CP Eng
www.waterhammer.bigblog.com.au
RE: Surge Loads in Pipework
http://virtualpipeline.spaces.msn.com
RE: Surge Loads in Pipework
I know a DLF of 2 is OTT generally but do you consider that some value of DLF is applicable or not?
RE: Surge Loads in Pipework
http://virtualpipeline.spaces.msn.com
RE: Surge Loads in Pipework
The short answer is that a DLF should not be applied to the restraint loads reported by a time history analysis.
The tricky bit is ensuring that you've got the time profile of the surge pressures correct and of course that your system is realistically modelled. You can easily see for yourself the effect of dynamic amplification on restraint loads by varying the load-time profile in a simple model. Better yet read a book on Structural Dynamics.
If on the other hand you are conducting a pseudo-static analysis you should make your own estimate of DLF and apply that to the static loads to be input to the stress program.
Also I would not expect the surge analysis to have any DLF included. Remember, the force from the surge does not somehow get increased, it is simply that the restraints must resist the load as well as decelerate the pipe.
regards
MB
RE: Surge Loads in Pipework
I don't follow what you mean by this?
"the force from the surge does not somehow get increased, it is simply that the restraints must resist the load as well as decelerate the pipe".
I would say that F = M x Acceleration where acceleration could also mean deceleration, so of course there is an increase in total load to the support, surge load = F, total load to the support = initial load + F
Load factors are not for "somehow increasing" any calculated surge load. Load factors are used to compensate for,
1.) That a calculated load itself may contain errors (ex. maybe the mass of all the moving liquid was not properly included) and,
2.) The support's margin of failure range, given material type and assumed failure mode (concrete explosive fracture),
3.) Impact, which considers the time history of the applied loads and the materials ability "toughness" to resist local high energy levels for a sufficiently long period to distribute them without reaching failure conditions.
all of which can be combined to some equivalent,a multiple of, the calculated load, such that the total effect could reasonably be assumed to have an equivalent load less than or equal to the upper limit of the calculated load's value multiplied by the chosen load factor.
Time history is not important to the design of the support, if the material is sufficiently tough to resist the impact, as the design load is always the maximum load F, which does not change if the load is applied slowly or quickly. The rest of the time history effect is only the resulting vibration frequencies and vibratory displacements.
http://virtualpipeline.spaces.msn.com
RE: Surge Loads in Pipework
Steel is cheap so the structural engineer doubling the hydraulic load is not going to cost a lot more and allows the engineer to sleep at night.
4) variation in material properties
5) errors in fabrication
6) damage during installation
7) lack of certainty in design of weldments
8) the life of the structure for it may corrode
You will find in researching the history of standards that the factors have little basis in fact. Take stress intensification factors for piping. All based on testing done in the 1950's for ferrous materials. Yet they are applied to alloys not available at the time. Why have they not bee updated? The cost of the research is prohibitive in the western world. perhaps it is time for the developing nations with lower labour costs to step up and do the research instead of relying on work done by others.
Geoffrey D Stone FIMechE C.Eng;FIEAust CP Eng
www.waterhammer.bigblog.com.au
RE: Surge Loads in Pipework
http://virtualpipeline.spaces.msn.com
RE: Surge Loads in Pipework
I agree with your point below, either I didn't make myself clear or you have read something unintended into my statement. The point I was making is that while the total load on a support may be increased, it is not due to the driving force (surge pressure) increasing, but is due to the combination of driving force plus inertial effects.
RE: Surge Loads in Pipework
Certainly in Pipepak I can plot a force vs time output from such an event. Perhaps Ceasar II handles this differently. Now in Impulse 4 the force vs time can be output. there is even a capability to generate a Ceasar II input file.
Then the DLF is applied to design the support to resist this predicted load.
I think we may be in violent agreement here just expressing it differently.
Geoffrey D Stone FIMechE C.Eng;FIEAust CP Eng
www.waterhammer.bigblog.com.au
RE: Surge Loads in Pipework
COADE: Piping Stress Analysis application Ceasar II with the additional; Paulin Research Group www.paulin.com BOS Fluids:
BOS Fluids is an engineering software package that analyzes fluid transients in pipe systems and relates this information back to the mechanical piping system transferring the fluid.
For years, piping engineers have labored with simplifying hand methods, cumbersome analogue computers, or user-unfriendly software products when needing basic steady state and transient fluid analysis capability. BOS Fluids is written specifically to address the needs of the piping engineer for fluid reaction forces, and to provide a system whereby the fluid simulation results can be easily integrated back into the piping system design and analysis.
BOS Fluids is an interactive computer simulation package that models steady state and transient flow in liquid or gas carrying piping systems. The procedure is easy to use and interfaces with most pipe stress programs. The package contains the elements required to model most common unsteady flow conditions. The elements included in the simulation package are pipes, valves, pressure relief valves, vacuum breaker, air valves, pumps, equipment, surge vessels, inlets, outlets, and orifices. BOS Fluids makes fluid simulation simple and easily accessible and yet gives the analyst pressure transients and dynamic force results with an engineering accuracy.
The present friction model used in BOSFluids is Colebrook-White. The Darcy-Weisbach flow model is used for steady state pressure drop calculations and the basic theory applied in BOS Fluids can be found in Wylie & Streeter's "Fluid Transients" published by FEB Press. BOS Fluids is capable of simulating both the steady and transient behavior of liquid carrying closed conduit systems of pipes, valves, pumps and surge relief devices.
Typical analyses using BOS Fluids include:
Water Transmission and Distribution Systems
Main Cooling Water Systems for Chemical Plants
Sewage Water Systems
Combined Power and Drinking Water Cycle Power Stations
Oil Product Transport Lines
Tanker Loading and Unloading Systems
Dynamic Behavior of Chemical Liquid Transport Lines
Acoustic Analyses for Compressors and Pumps
Regards
Leonard Stephen Thill
L S THILL
RE: Surge Loads in Pipework
Thanks for the information. I am aware of BOS Fluids and Paulin.
There is nothing in your message that Impulse does not do except for the acoustics analysis of compressors & Pumps. I have been using Impulse for ten years now and find that it meets all my needs. In fact in Australia I do work for many of the local and overseas consultants. They have other software packages but still get me to the analysis and design.
Geoffrey D Stone FIMechE C.Eng;FIEAust CP Eng
www.waterhammer.bigblog.com.au
RE: Surge Loads in Pipework
In the AFT product range what is the difference between Impulse and Fathom?
Best regards
Morten
RE: Surge Loads in Pipework
Thanks for the responses from everyone. There seems to be a lot of differing opinions on this - not a concise yes or no to the application of a DLF or not. Also when you look at the Caesar II applications guide then there seems to be two inputs that are required. The first is the forcing function (time history information from a hydraulics analysis) and a DLF spectrum input which applies Force Multipliers if I am not mistaken dependant on frequency. To me this suggests that the Caesar II software does apply a DLF to the basic Hydraulic time history input.
Any comments of what Caesar II actually does? Am I interpreting the application guide correctly?
Regards
DSB123
RE: Surge Loads in Pipework
http://virtualpipeline.spaces.msn.com
RE: Surge Loads in Pipework
Thanks for the quick response. So basically I am right when I say that you need to apply a DLF to the time history information (i.e. loads) when doing a pipe stress analysis.
RE: Surge Loads in Pipework
http://virtualpipeline.spaces.msn.com
RE: Surge Loads in Pipework
These descriptions are off the top of my head. The website has more comprehensive descriptions. I run Fathom and Impulse but have used Arrow as well.
Fathom is a steady state analysis program with the capability of heat transfer, extended period simulation, goal seeking and costing. It handles Newtonian and non Newtonian incompressible fluids
Impulse is an unsteady state program for Newtonian and non Newtonian fluids. It does have the Fathom steady state engine as part of the software however does not have heat transfer, goal seeking and extended period simulation
Arrow is a steady state program for compressible fluids with the same functionality as Fathom.
Mercury is another even more sophisticated modelling program for a complete facility that is designed to critically model Capex and Opex costs with the ability to determine optimum line sizes, pumps operation and far more.
Geoffrey D Stone FIMechE C.Eng;FIEAust CP Eng
www.waterhammer.bigblog.com.au
RE: Surge Loads in Pipework
http://virtualpipeline.spaces.msn.com
RE: Surge Loads in Pipework
So, the answer to your question about if you have to apply a DLF in your mechanical analysis is: only is you are solving the problem statically. If you are solving an actual dynamic problem, i.e. you are applying a dynamic load (time history load vs. time at each loaded point of the system) and you are solving the problem dynamically (dynamic equilibrium: M d2u/dt + C du/dt + K u = P(t) ) you don“t have to apply a DLF, because the time history response of the problem ( u(t) ; F(t) = K u(t) ) will provide you the actual response of the structure, and obviously, its maximum values.
Regards.
RE: Surge Loads in Pipework
http://virtualpipeline.spaces.msn.com
RE: Surge Loads in Pipework
Yes, Anybody know what Caesar II is doing? Ask WWW.COADE.COM
L S THILL
RE: Surge Loads in Pipework
So the capability for dynamic analysis is apparently there. Getting back to the original question, "I beleive a DLF is applicable whereas others seem to beleive the DLF is already taken care of within the Hydraulic analysis." I'd still say NO. Its not taken care of in the hydraulic analysis.
http://virtualpipeline.spaces.msn.com