Surge Loads in Pipework 2

[DSB123]

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

 

[desertfox]

Hi DSB123

Have a look at this free calculator maybe you can compare your results and get an answer.

http://www.cedcs.com/Simple peak pressure force free.xls

desertfox

 

[DSB123]

thanks desertfox,
I have already had a look at this but it does not answer my question.

 

[desertfox]

Hi DSB123

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

 

[desertfox]

Hi

please read DLF where I wrote DLB

desertfox

 

[BigInch]

Don't count on that! It is extremely doubtful that a hydraulic analysis included any consideration of the pipe stresses at all. The general design process considers hydraulics for hydraulic design and pipe stress for pipe stress and there is seldom any interaction between the two disciplines. A hydraulic analysis typically makes sure only that pressures are within the design pressure and that the design flowrates are possible, given the rotating equipment engineer buys the right pumps. A transient hydraulic analysis will include the additional consideration that any transient pressures are within the design pressure plus allowable transient pressure. Most hydraulic engineers will give little thought to the pipe support manner at all and will know nothing about any "pipe load factors". Any assumption made about the rigidity of the pipe supports (usually assumed totally rigid) should be stated in the analysis, but it is probably not as the hydr engr just left the default condition checked (and might not even know he did that), and whatever was said, if anything, should be verified by the stress engineer anytime he might think it was important to do so.

http://virtualpipeline.spaces.msn.com

 

[stanier]

Impulse 4 has just been released by AFT and has the ability to provide a load file for Caesar II. I am still investigating what is behind this and will revert. The difficulty I have is that i do not use Caesar II but use Algor's pipepak.


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

http://www.waterhammer.bigblog.com.au

 

[DSB123]

Thanks for the replies,

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

 

[stanier]

DSB123

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

http://www.waterhammer.bigblog.com.au

 

[BigInch]

DSB its the same thing, Pressure x Area = Force x Time = impulsem and the effects of the pressure wave traveling through the pipe at more or less 3000 ft/sec on a short pipe means it happens pretty much all at the same time, so Stainer's is correct (for short pipes). With a pipe 1000 meters long, I doubt that applying the load to one end and then opposite end 1 second later would be any different as far as the pipe supports are concerned than applying the loads at the same time.

http://virtualpipeline.spaces.msn.com

 

[DSB123]

stanier,
I know a DLF of 2 is OTT generally but do you consider that some value of DLF is applicable or not?

 

[BigInch]

IMO, a DLF should be used for support analysis, the magnitude of the factor applied is set either by the structural (or other?) code, or lacking a suitable code or standard, by the engineer's opinion of the support's failure mechanism and the overall consequences of a failure of the support. A typical no impact type (static) live load design factor for concrete member design is 1.7, so 2 could be entirely reasonable for an impact loaded steel compression member failing in a column or lateral buckling mode, so similar situation.

http://virtualpipeline.spaces.msn.com

 

[MBlackman]

DSB123,

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

 

[BigInch]

MBlackman,
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

 

[stanier]

The situation is even more complex in that the properties of steel are time dependent. We have all learnt the static properties of steel. When you go deeper into material research you will find that when a load is applied rapidly the material property increases. None of this benefit is taken into account in codes and standards.

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

http://www.waterhammer.bigblog.com.au

 

[BigInch]

Stanier, I'd say its because there's no money to be made or saved in reducing the stress intensification factors any more than they are now, or have been since the 50's as you say. I think a different approach has taken root, probaby via competition between material manufacturers, and we have seen an increase the yield strengths and other engineering properties, which have made many of the materials so thin we have to worry more now about things such as local buckling and excessive thinning when bending, etc. in addition to the realization that stress redistribution and plastic design is possible with these materials as well. Heck a lot of them are plastics anyway. I think there are even some acrobatic airplanes made now without any wood or metal in any load carying member, nor a rivit or bolt either. With d/t ratios so low as they are now, there is not much economic necessity to reduce the stress intensification factors even more, unless you just happen to stumble onto a most unusual situation.

http://virtualpipeline.spaces.msn.com

 

[MBlackman]

BigInch

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.

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

 

[stanier]

Surely that is what you analyse for in the stress package. By inputing a time history from the hydraulics software, the stress software develops the forces that result in the inertia of the piping being overcome? These forces are then resisted by the support.

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

http://www.waterhammer.bigblog.com.au

 

[LSThill]

stanier (Mechanical)

COADE: Piping Stress Analysis application Ceasar II with the additional; Paulin Research Group

http://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

 

[stanier]

Hi IstHill

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

http://www.waterhammer.bigblog.com.au