×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Water hammer in system with two MOC?

Water hammer in system with two MOC?

Water hammer in system with two MOC?

(OP)
How does one analyze a piping system that involves two or more different types of pipe?  The system I'm working on involves FRP connected to HDPE...then back to FRP.  Pipe sizes are also different, but I can handle that.

Any suggestions?

Thanks,
KLee777

RE: Water hammer in system with two MOC?

This is a good one.  The time it takes for the pressure wave to travel the length of the pipe and return is equal to 2*L/a, where "L" is the length in ft, and "a" is the velocity of the pressure wave in fps.  The calculation for "a" is difficule to type out, but let me know and I will.  One of the terms of this calcualtion involves modulus of elasticity of the pipe material.  

So you can find "a" for each material and calculate the time it takes the wave to travel through each.  But, I assume you want to know the maximum water hammer pressure.  The formula is p=(0.433)*a*V/g, where "V" is the velocity of the water in fps, "g" is 32.2 fps^2, and "p" is in psig.  I suppose you could calcualte it for each material and see which one is larger, but I don't know if that is the accurate answer.  

Hope this points you in the right direction.  Sorry I can't be more helpful.

RE: Water hammer in system with two MOC?

I suggest you invest in AFT's Impulse or select a designer/consultant with this software. Go to AFT directly as they offer consulting services and will do the analysis very quickly.

Look at www.pipingdesign.com. There are two papers there on surge. Suggest you read carefully the one that addresses "risk".

Geoffrey D Stone FIMechE C.Eng;FIEust CP Eng
www.waterhammer.bigblog.com.au

RE: Water hammer in system with two MOC?

Then I presume you also know that you will get a certain amount of pressure wave reflected and transmitted onward  at each size change too.  After you read Sanier's suggested post, maybe you would also like to read my webspace "surge blog" for a discussion on surge calculations.  I won't hurt.

   Going the Big Inch! worm
http://virtualpipeline.spaces.msn.com

RE: Water hammer in system with two MOC?

KLee777 (Mechanical)


I don't fully understand the problem. But typically at the interface of two different pipe sizes one neglects the transient affects.  The assumption is that the area change at the interface (discontinuity) is more significat than the transient.  
Across the interface, a quasi steady model of conservation of mass and energy is applied.
For an incompressible fluid, use Bernoulli's with a loss coefficient and rho*Area*velocity = constant.

Upstream and downstream of the discontinuity, use the transient equations.

Regards

RE: Water hammer in system with two MOC?

Reflections depend on the area change.  If your area change isn't much, you could probably say reflections are insignificant, unless they interfere with your flow or pressure control system.  I'd say it really depends on if the magnitude of the reflections will put you over the maximum transient pressure allowable of your design code.  

   Going the Big Inch! worm
http://virtualpipeline.spaces.msn.com

RE: Water hammer in system with two MOC?

(OP)
Thanks everyone, for your input.  I received notice today that we'll soon have access to AFT's Impulse program.  I do understand that I'll get reflections at size changes, and that you neglect transient effects at size changes also.  However, this client is looking for a very detailed transient analysis also, so the software will help.

Thanks again!

RE: Water hammer in system with two MOC?

BigInch (Petroleum)Is you question, why do you neglect the transient effects at area (discontinuity) changes or something else??

Regards

RE: Water hammer in system with two MOC?

Yes.  When I make transients runs, I see reflected pressures and flow disturbences originating at area changes that are transmitted through the area change to the end of a pipeline and reflections from the area change back to the beginning of the pipeline.

Then KLee says that his client wants a very detailed analysis, and yet he apparently proposes to reflections.  Am I reading that wrong?

   Going the Big Inch! worm
http://virtualpipeline.spaces.msn.com

RE: Water hammer in system with two MOC?

Big Inch.  As I see it, KLee states "I do understand that I'll get reflections at size changes, and that you neglect transient effects at size changes also"  AND this seems to be in line with my statement of quaisy steady analysis at discontinuitys.

Regards

RE: Water hammer in system with two MOC?

In Impulse you can model a change in pipe size and the details of that change, whether it be a reducer or an abrupt change.

Geoffrey D Stone FIMechE C.Eng;FIEust CP Eng
www.waterhammer.bigblog.com.au

RE: Water hammer in system with two MOC?

Neither explains why you prefer to neglect them.

"The assumption is that the area change at the interface (discontinuity) is more significat than the transient.", doesn't mean anything to me.  Would you mind explaining how it is more significant than the transient.

   Going the Big Inch! worm
http://virtualpipeline.spaces.msn.com

RE: Water hammer in system with two MOC?

Stanier,

Do you understand why they figure they can neglect pressure wave reflections from the area change?  When starting a pump at near shutoff head with a relatively nearby "area-change" I can get high amplitude reflections well over the shutoff head of the pump.  Since many pipeline systems are designed for only pump shutoff head, its an overpressure that's often higher than the 10% transient allowable of B31.4.  

   Going the Big Inch! worm
http://virtualpipeline.spaces.msn.com

RE: Water hammer in system with two MOC?

NO I dont understand how the change can be ignored. But I come across engineers who ignore surge totally so at least this poster is on the right track and has taken advice.

Points I have made in my papers are that design codes and standards require that the design pressure include for surge. If surge is not analysed how does an engineer know what the design pressure is?

In respect of the 10% extra over in ASME B31.4 I do not think this is to accomodate surge. Although I am not familiar with that code others such as ASME B31.1 and 31.3 have similar provisions for unsustained operational loads in respect of pressure. But the "design pressure" is meant to have taken surge into account. Thus the "extra over" is not meant to cater for surge.

Geoffrey D Stone FIMechE C.Eng;FIEust CP Eng
www.waterhammer.bigblog.com.au

RE: Water hammer in system with two MOC?

I was reaching the same conclusion.  I'm originally from Texas, so I still remember what a load smells like.  If I'm wrong, I appologize in advance.

I've always taken the 10% allowance to apply to unsustained loads, which I have some memory of 4 hours as the limit.  I've copies of B31.3,4 & 8 on the way, so I reserve the right to reinterprete that, but it seems to have been drilled into my head a long time ago and I haven't hadfelt the need to reread that section for a number of years... until now.  I'll quote the text for you when I get it.

If I was their client, I'd have to bust their something or other.

   Going the Big Inch! worm
http://virtualpipeline.spaces.msn.com

RE: Water hammer in system with two MOC?

BigInch,

Here is ASME B31.3-2002  relevant para. I do not think that the versio 2004 is any different. It should be noted that the Owner's approval is required.

302.2.4 Allowances for Pressure and Temperature
Variations. Occasional variations of pressure and/or
temperature may occur in a piping system. Such variations
shall be considered in selecting design pressure
(para. 301.2) and design temperature (para. 301.3). The
most severe coincident pressure and temperature shall
determine the design conditions unless all of the following
criteria are met.
(a) The piping system shall have no pressure containing
components of cast iron or other nonductile
metal.
(b) Nominal pressure stresses shall not exceed the
yield strength at temperature (see para. 302.3 of this
Code and Sy data in BPV Code, Section II, Part D,
Table Y-1).
(c) Combined longitudinal stresses shall not exceed
the limits established in para. 302.3.6.
(d) The total number of pressure-temperature variations
above the design conditions shall not exceed 1000
during the life of the piping system.
(e) In no case shall the increased pressure exceed
the test pressure used under para. 345 for the piping
system.
(f) Occasional variations above design conditions
shall remain within one of the following limits for
pressure design.
(1) Subject to the owner’s approval, it is permissible
to exceed the pressure rating or the allowable stress
for pressure design at the temperature of the increased
condition by not more than:
(a) 33% for no more than 10 hr at any one
time and no more than 100 hr/yr; or
(b) 20% for no more than 50 hr at any one
time and no more than 500 hr/yr.
The effects of such variations shall be determined
by the designer to be safe over the service life of the
piping system by methods acceptable to the owner.
(See Appendix V.)
(2) When the variation is self-limiting (e.g., due
to a pressure relieving event), and lasts no more than
50 hr at any one time and not more than 500 hr/year,
it is permissible to exceed the pressure rating or the
allowable stress for pressure design at the temperature
of the increased condition by not more than 20%.
(g) The combined effects of the sustained and cyclic
variations on the serviceability of all components in
the system shall have been evaluated.
(h) Temperature variations below the minimum temperature
shown in Appendix A are not permitted unless
the requirements of para. 323.2.2 are met for the lowest
temperature during the variation.
(i) The application of pressures exceeding pressure temperature
ratings of valves may under certain conditions
cause loss of seat tightness or difficulty of operation.
The differential pressure on the valve closure
element should not exceed the maximum differential
pressure rating established by the valve manufacturer.
Such applications are the owner’s responsibility.

Geoffrey D Stone FIMechE C.Eng;FIEust CP Eng
www.waterhammer.bigblog.com.au

RE: Water hammer in system with two MOC?

One difficulty with analysing FRP is going to be the pseudo modulus that an engineer has to adopt to deduce  wavespeed. FRP manufacturers tend to publish wave speeds but the basis is generally not given.

Thorley quotes in Pressure Transients in Pipeline Systems wavespeed of FRP as varying as:-

1300m/s D/t 5
1070m/s D/t 20
890m/s D/t 40
760m/s D/t 60
700m/s D/t 80
650m/s D/t 100
520m/s D/t 160

These data need to carefully considered with the end fix/constraints of the piping. The amount and angle of roving affect the wavespeed. Thus  sensitivity analkysis is required using different wavespeeds.

I would be wary of a full vacuum case with FRP as it has a low strain tolerance.

The design of the FRP may be to ISO 14692, 10467, 10639 (offshore/chemical , sewage and water)and different requirements prevail.

Geoffrey D Stone FIMechE C.Eng;FIEust CP Eng
www.waterhammer.bigblog.com.au

RE: Water hammer in system with two MOC?

As long as Exxon-Mobil requires analysis including reflected and transmitted waves, EM "Pipeline Hydraulics Manual Version 1, 2003", I'm going to do it.  And if you do include them, then its presumed you have meet all the provisions of whatever code you're using.

And thanks for pointing out the FRP modulus variation.  I had noticed the wave speed figures given in some manufacturer's data and did not make the connection that it implies E varies with D/t.  Hopefully that was because I have not used plastic materials on any pipeline I've ever designed.  This is probably why it is difficult to find a value for E listed anywhere convenient.  Quite interesting, that.  So, ready for it now, if it ever comes up.

Thanks, I'll send you the paragraphs from 31.4/8 when they get here.

   Going the Big Inch! worm
http://virtualpipeline.spaces.msn.com

RE: Water hammer in system with two MOC?

Big Inch.  With regard to the discontinuity neglecting transients, I suggest you try a simple analysis of three pipes in series.  Let the upstream and downstream from the middle connecting pipe be of the same diameter and the middle connecting pipe be of different diameter, but extremely short length. I believe that you will observe, the pressure difference, flow flow difference from end to end in the short pipe to be negligible.--Meaning transient effects are negiligible,--- yet pressure difference, etc from either of the upstream/downsteam pipes to the connnecting pipe to be more significant.


Regards

RE: Water hammer in system with two MOC?

This is not something I started doing yesterday.  Been there, done that and you are wrong... if the area difference is "significant".  If the area difference is not significant you are correct.  My only point was that you cannot arbitrarily neglect reflected waves.  You must analize it (or know from experience) that reflections are insignificant before you can make your statement, and the statement is case specific.  Start up of a pump may give a significant reflection from a diameter change while a flow rate change made via an FCV may not, so the reflection is not even the same for different control actions.  Do you neglect reflection effects from pump starts?  I made a special case, just for you, that I have running now that shows a 20% over pump shutoff head maximum pressure reflected back to a pump station off a diameter change.  Assuming that MAOP of your pipe equals shutoff head, can you can show me what code you're using that would ALLOW you to neglect that?
  

   Going the Big Inch! worm
http://virtualpipeline.spaces.msn.com

RE: Water hammer in system with two MOC?

  Big Inch--The fact is that any hammer or fluid transient analysis dealing with a discontinuity such as area change treats the discontinuity as a quasi steady flow.  Now my experience is limited to one dimensional transients.  

I'd be interested to know how your program or analysis deals with a sudden area change.  What conservation equations are used?

Of course one can model reducers, etc when dealing with area change, but that is not the case of a discontinuity.  The characteristic equations for that model can also be a type of nightmare for the analyist.   

 Regards  

RE: Water hammer in system with two MOC?

Idelchik gives for an abrupt change at pages 208 & 216 respectively

Kup=(1-Aup/Adown)^2 for an expansion and
Kup= (0.5(1-Aup/Adown)^0.75)/((Aup/Adown)^2)

This is adopted by AFT's Impulse for computing the loss at the change in diameter. A separater node for such changes can be used to define the boundary between materials and diameters.

I am not about to argue with the likes of Idelchik.
Idelchik, I. E., Handbook of Hydraulic Resistance, 3rd edition, CRC Press, Boca Raton, FL, 1994.

Geoffrey D Stone FIMechE C.Eng;FIEust CP Eng
www.waterhammer.bigblog.com.au

RE: Water hammer in system with two MOC?

As described in "Exxon-Mobil Pipeline Hydraulics, 2003"

These equations are related to the velocity of sound in each segment.

Amplification Factor  for transmitted wave = ((2*A1)/a1) /(A1/a1+A2/a2).  

Amplification Factor for reflected wave = (A1/a1-A2/a2)/(A1/a1+A2/a2)

Where A = respective pipe areas up (1) and dn (2)
     a = velocity of sound in respective pipe segments


   Going the Big Inch! worm
http://virtualpipeline.spaces.msn.com

RE: Water hammer in system with two MOC?

(OP)
"I was reaching the same conclusion.  I'm originally from Texas, so I still remember what a load smells like...If I was their client, I'd have to bust their something or other."

Hold on there, BigInch.  Geez.  IN NO WAY was I saying that I am totally ingnoring pressure reflections, much less surge in general (what kind of engineer would that make me)?  Check out my other threads and you'll see I'm not one prone to neglect--I ask questions so that I'll know what I *don't* know!  (Thanks to stanier for recognizing this.)  However, I may not have been as clear as I should have been in my post and I apologize for that.  Perhaps it has something to do with these 16 hour days I've been working....

The area change in this system is *very* small (4.0" I.D. pipe going down to 3.94" I.D.).  I was originally more concerned with the change in MOC anyway.  Since I'm no expert in hydraulic calculations, I discussed this at length with several of my company's fluid transient "experts," that is, once I was finally able to get in touch with them.  All agree that any reflections due to area change alone would be virtually negligible, especially since the area change occurs ~5,600 ft from the pump, and we're pumping wastewater at <3 ft/sec.  It's not like this stuff is screaming through the pipes.

Turns out we're leaving the calculation to the more experienced guys, even though it comes with a bigger price tag for the specialists.  That's what we do when we want to get it right.  Although, I'm not relinquishing this altogether--I'm going along for the ride so that I can learn, too.  It never hurts to be more informed.

"Then KLee says that his client wants a very detailed analysis, and yet he apparently proposes to reflections.  Am I reading that wrong?"

Yes, you are wrong, the client does not oppose, WE do in this case--but if the client wants the transient analysis, we'll provide it.  And please, don't automatically assume I'm a guy...it gets annoying sometimes.  I do appreciate the rest of the discussion, but I ask everyone to please watch their tone and/or assumptions when posting so as not to offend.  The first quoted statement kind of left a bad taste in my mouth, even with advance apology.

Thanks,
KLee

RE: Water hammer in system with two MOC?

KLee,  

You're right!  I'm, not as diplomatic as I should be all the time.  I (and a few select others) have "buttons", of which apparent ignoring transient analysis is one and it obviously gets me ballistic, however I am not saying this as an excuse for poor choice of words.  What I will say though is that it wasn't ment for you.  It was aimed at sailorday's statement, "treats the discontinuity as a quasi steady flow"  (to whom I will also offer the same appology) but, only after he gives us his equations. I still believe he is only thinking steady state pressure loss in the short section in relation to steady state pressure loss in the other longer segments, which I think everybody pretty much has that down after fluid mechanics 1001 and is completely irrelevant to transient reflections and transmissions from a reducer/expander.  Actually what I understand about the hydraulics of his statement is so obvious, that I can't believe he is making a comment about transient analysis at all.  

Sorry, I didn't mean for you to be a victum of friendly fire machinegun.  I assumed you were just asking for advice when you made no attempt to debate the issue one way or another.
  

   Going the Big Inch! worm
http://virtualpipeline.spaces.msn.com

RE: Water hammer in system with two MOC?

I'm going on vacation for about a week, but will give a fuller explanation of "my???" equations later.

Regards

RE: Water hammer in system with two MOC?

(OP)
Thanks, BigInch.  I understand that everyone has their buttons, I have mine too!  Just wanted to say that I didn't respond right away because I simply did not have the time.  Sit back and give it a few hours next time to see if the poster will respond.  Otherwise it feels sort of like an attack (love the machine gun guy, by the way)!

Thanks again to all for their input.  This forum has always been a place for good information and debate, and I hope it continues to be such. peace

Regards,
KLee

RE: Water hammer in system with two MOC?

I don't mean to take things to a personal level.  Its just my way to talk about things directly and impartially as I see it.  I have grand delusions of Dennis Weaver in "McCloud".  I'll see if I can't keep them in the holster a little longer.

Well, I tried.  Didn't do too well.  I need professional help.  Just shows you it wasn't personel, just my way with everybody. I'm just a bull in a china shop.  You might want to check out my latest response over on,

http://www.eng-tips.com/viewthread.cfm?qid=161768

   Going the Big Inch! worm
http://virtualpipeline.spaces.msn.com

RE: Water hammer in system with two MOC?

PS I wouldn't even count anything less than a full diameter change as an "area change", but, as Stanier says, watch the different effects from Youngs modulus variation with the FRP's t/D.  I'm still greatful for that comment.

   Going the Big Inch! worm
http://virtualpipeline.spaces.msn.com

RE: Water hammer in system with two MOC?

u=velocity    A =area   p= pressure

   1[---------------------]23[--------------------]4-

Consider stations 1 far to the left of sudden change in are 2 to 3. Stations 1 to 2 have the same diameter and stations 3 and 4 , the same diameter but different from 1 or 2

For simplicity negelect friction and elevation changes and assume sound speed is constant through out.
A right characteristic traveling from 1 to 2 travels at dx/dt =u+c, along that characteristic du+dp/rho/c=0
(u2-u1) + (p2-P1)/(rho*c)=0.  equation 1
  Along a left traveling characteristic which travels at dx/dt=u-c from 4 to 3. Along that characteristic du-dp/rho/c=0
(u3-u4)-(p3-p4)/(rho*c)=0.    equation 2
Quasi steady conditions are taken across the discontinuity.
Between stations 3 and 4 if flow is left to right (p3-p4)/rho=(u4^2-u3^2)/2   equation' 3
and from continuity  u3*A3=u4*A3   equation 4

4 equations  and the unknowns are p2, p3,u2, u3
It is assumed that conditions at 1 and 4 are known,
For reflections
1 and 4 will be impacted by characteristics traveling back from 2 to 1 at dx/dt =u-c  
and 3 to 4 along characteristics traveling at dx/dt=u+c
Equations 1-4 are easily solved with algebra.

The above consider highly incompressible fluids.
With compressible flow the curves of x vs t anre not nearly as straight as for incompressible.

I'll stay glued to the computer for a little while as I leave tomorrow for a week.

Regards

Regards


                                                                                      

RE: Water hammer in system with two MOC?

Please correct my equation 4 to read
and from continuity  u3*A3=u4*A4   equation 4

RE: Water hammer in system with two MOC?

I thought there was something else.  I'm still not sure why you neglect reflections.  Apparently you are neglecting them simply because the sign is negative and of relatively small amplitude.  Correct?  That assumes that the only effect you are interested in examining is the maximum pressure of the pipeline.

I will now ask you to consider what the impact of that reflection might be on equipment at the beginning of the pipeline, if, as Stanier mentions, no vacuum, or that might be trying to maintain a constant discharge pressure or flow, or for a typical scenairo I see a lot of, the effect on a high speed, high pressure pump with a 1500 m static head that has just started a runup?  I don't think you can say, neglect reflections, then justify it by saying, "Well.  The pipeline must be flat and it must be incompressible flow.

Hope you had a good vacation.

   Going the Big Inch! worm
http://virtualpipeline.spaces.msn.com

RE: Water hammer in system with two MOC?

In respect of the "button pushing" I would like to enquire of Klee777 why the analysis is only done if the client wants it. Or did I get that wrong.

Standards and codes require surge to be included in the design pressure. So the designer has to do the analysis to determine the design presure and hence wall thickness of the pipe.

Surge analysis is not optional. It is not something to be guessed. Yes it is complex. There are tools available for the engineer to use to determine the surge in a system. It beggars belief that companies will spend a small fortune of software for fancy presentations, risk analysis and stock control but wont give engineers the tools to address design parameters that are mandated in standards and codes. I am more bemused by the ignorance in the piping industry of these requirements.

Geoffrey D Stone FIMechE C.Eng;FIEust CP Eng
www.waterhammer.bigblog.com.au

RE: Water hammer in system with two MOC?

A client I have did a transient analysis for several large refined product storage sites as part of an initial design contract, but the transient analysis was done only for the design flowrates.  The analysis was limited to examining the effects of closing critical valves and stopping pumps and for the design of a surge control system, but nothing else.  We have found that doing an additional comprehensive transient analysis covering the entire range of flowrates and all possible operating scenarios from start_up to maximum possible flowrate can be quite valuable.  Design deficiencies and bottlenecks can be identified.  Operating temperature ranges of pumps can be determined.  Intermediate operational scenarios can be identified and control points developed.  Control sequences can be verified, corrected and even the timings adjusted.  Pump, compressor and instrument parameters can be evaluated.  Pressure setpoints for smooth start-ups can be determined.  Optimal pump configurations and horsepower settings can be predetermined.  And when the comprehensive transient analysis is finished, the model can be attached to a training simulator for pipeline operators, so they can be trained in what to expect when they turn the key or a particular pump doesn't start on time and the pipeline pressure upstream goes high.  Common possible operational mistakes can be identified and recovery methods examined.  Leaks can be simulated and spill volumes at various points can be determined to assist preparation of emergency procedures.  Proposed upgrade modifications to the pipeline can be checked in advance.  The original transient analysis can be the starting point to a highly leveraged operational tool useful for the life of the pipeline.



   Going the Big Inch! worm
http://virtualpipeline.spaces.msn.com

RE: Water hammer in system with two MOC?

BigInch,

You certainly know how to push buttons. Fancy wanting to do more engineering to optimise the design, construction and operation of a facility. In today's modern world dominated by "project managers" this is surely heresy and you will burnt at the stake.

$1 spent at concept design stage saves
$10 spent in design saves
$100 spent at detail design saves
$1000 spent in procurement saves
$10000 spent in construction saves
$100,000 spent in operation saves
$1,000,000 when the lawyers are called in

Geoffrey D Stone FIMechE C.Eng;FIEust CP Eng
www.waterhammer.bigblog.com.au

RE: Water hammer in system with two MOC?

All those numbers are correct, except for the last, where you must multiply by (1+ RAND(20)/100MM project_cost).  For offshore work you can probably 2X to 4X those.

Today its easier to justify a project by using the "avoided legal cost" method.

"Way back when" in the early '80s, my boss was picking up some complaints from the project management group about my offshore pipeline projects taking too much engineering and design time.  During those years we were making numerous short pipeline tie-ins between existing lines and new platforms, so each engineer did 4 to 6 of those a year, so I knew quite a lot of data was available.  I went to the Accounting dept. and got the financial records for all projects done in the previous 5 years.  My projects were taking 33% more time than other engineers, but fortunately were still being finished within overall schedule.  Thinking I was in deep... water, I had to come up with a way to justify my engineering costs.  It seemed to me that the construction managers never gave me as much trouble as the other engineers, (one they cruicified on a daily basis) so I started checking my project's construction costs.  I found my pipelines had little if any extra field work charges that were not weather related and their costs, when including construction, were 20% less per diameter-mile with the average cost being around $1xE6 less.  I had to assume that the savings were due to the extra engineering hours spent doing detailed dimensional checks on the platforms, double checking all drawings, making accurate material take-offs, requisitioning proper quantities and tripple checking all mark numbers against drawings and requisitions.  2000 hours extra was returning 1 million dollars, when the average engineering hour was around 25.00  Spending $1.00 on engineering netted a payback of around $20.  I presented that case and got a salary increase.  Don't let anybody tell you its not true.  

   Going the Big Inch! worm
http://virtualpipeline.spaces.msn.com

RE: Water hammer in system with two MOC?

After reading through this lengthy thread, I wonder if the original inquirer yet knows exactly how to determine the design pressure when different pipe materials of construction are all thrown together?  I guess while it might be judged over-conservative by some, in the absence of more definitive field research I kind of like the first thoughts of BronYrAur, that if I understood the post correctly may have been to determine a conservative surge as if the pipeline were at least a little more simplistic system of one stiffer material and go with the higher number (at least on paper this might be harder to at some point in someone else's forensic work judge unsafe?)  More detailed analysis might well require the services of a "sure 'nuff" specialist or expert in pipeline transients, who additionally is willing to stand by their detailed surge analysis of composite pipelines involving multiple materials.  I noticed similar or related  subjects were brought up in a couple other threads a few years ago, including  http://www.eng-tips.com/viewthread.cfm?qid=125930&amp;page=10 and   http://www.haestad.com/hmicom/listserv/archive/default.asp~action/thread/messid/10829/v/n/searchid/76066.  I suspect different materials of construction, and I will note also variations in stiffness of encasements and penetrations of buildings/structures etc. are probably more a rule than an exception in many contemporary systems.       

RE: Water hammer in system with two MOC?

rconner,

The scenario can be readily modelled in Impulse (www.aft.com) with all changes of diameter, materials, abrupt diameters etc.

The original poster has taken the advice and is going to get it analysed using this software by people that know what they are doing. The original post has been answered and the rest is just cats whaling on the fence post at night.

Geoffrey D Stone FIMechE C.Eng;FIEust CP Eng
www.waterhammer.bigblog.com.au

RE: Water hammer in system with two MOC?


u=velocity    A =area   p= pressure

   1[---------------------]23[--------------------]4-

Reflections must be included.  I only gave a simple portion of an analysis.

Refering to my previous post, the pressures velocity etc are solved at stations 2 and 3 and say time t1.

The reflections from 2 to 1 along characteristic dx/dt=u-c
and u1-u2-P(p1-p2)/rho/c=0
The reflection from 2 to 3 along dx/dt =u+c and hammer formulation u4-u3+(p4-p3)/rho/c=0.
The above equations are solved for pressure and temperatures and the new time(s) that they occur.

The equations stated above and on the previous post were simply given to show how the discontinuity of area is accounted for.

Typical MOC will approximate the characteristics dx/dt as +or - c.  This is because the velocities, u are generally much less than the sound speed, c.

Big inch "I will now ask you to consider what the impact of that reflection might be on equipment at the beginning of the pipeline,"


u=velocity    A =area   p= pressure

   1[---------------------]23[--------------------]4-


Consider known condtions of velocity and pressure and station2 and 3 at time t. This would have been obtained from the previously solved equations

Let us assume that at station 1, similar to a H-Q curve we know the Pressure1, velocity1  relation.
Along the reflected characteristic dx/dt=-c
U1-U2-(P1-P2)/rho/c=0    P2 and U2 are known and with this equation AND a relation between P1 and U1, the pressure and velocity at station 1 are determined along with the new time.  
Let's say the pressure at4 is know.  The the equations along the dx/dt=+c characteristic are solved.


I hope this is answering your question(s).
I have tried to give a simplifed explanation to the use of the discontinuity.  I have assumed that use of characteristics dx/dt and du+/=dp/rho/c were already known.

Believe me, using constant sound speed and low velocities and relatively incompressible fluids makes analysis simple compared to compressible flow.
Regards

RE: Water hammer in system with two MOC?

We've passed the equation issue some time ago.  

I do believe, which is exactly why all of my remaining questions and statements were attempting to discover why you would think about making such an oversimplification?  

   Going the Big Inch! worm
http://virtualpipeline.spaces.msn.com

RE: Water hammer in system with two MOC?

Biginch-The equation issue, dx/dt plus/minus c, etc is in response to your question of reflections.  Further, I have tried to answer questions related to boundary conditions, such as inlet and outlet of the simple system.
MOC is basically solving equations.  One could make the simplification more complex, by considering friction, heat addition, etc.  I don't see what that would accomplish.
Could you be specific with your question?

Regards

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources