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
Any suggestions?
Thanks,
KLee777





RE: Water hammer in system with two MOC?
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?
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?
Going the Big Inch!![[worm] worm](https://www.tipmaster.com/images/worm.gif)
http://virtualpipeline.spaces.msn.com
RE: Water hammer in system with two MOC?
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?
Going the Big Inch!![[worm] worm](https://www.tipmaster.com/images/worm.gif)
http://virtualpipeline.spaces.msn.com
RE: Water hammer in system with two MOC?
Thanks again!
RE: Water hammer in system with two MOC?
Going the Big Inch!![[worm] worm](https://www.tipmaster.com/images/worm.gif)
http://virtualpipeline.spaces.msn.com
RE: Water hammer in system with two MOC?
Regards
RE: Water hammer in system with two MOC?
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] worm](https://www.tipmaster.com/images/worm.gif)
http://virtualpipeline.spaces.msn.com
RE: Water hammer in system with two MOC?
Regards
RE: Water hammer in system with two MOC?
Geoffrey D Stone FIMechE C.Eng;FIEust CP Eng
www.waterhammer.bigblog.com.au
RE: Water hammer in system with two MOC?
"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] worm](https://www.tipmaster.com/images/worm.gif)
http://virtualpipeline.spaces.msn.com
RE: Water hammer in system with two MOC?
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] worm](https://www.tipmaster.com/images/worm.gif)
http://virtualpipeline.spaces.msn.com
RE: Water hammer in system with two MOC?
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'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] worm](https://www.tipmaster.com/images/worm.gif)
http://virtualpipeline.spaces.msn.com
RE: Water hammer in system with two MOC?
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?
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?
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] worm](https://www.tipmaster.com/images/worm.gif)
http://virtualpipeline.spaces.msn.com
RE: Water hammer in system with two MOC?
Regards
RE: Water hammer in system with two MOC?
Going the Big Inch!![[worm] worm](https://www.tipmaster.com/images/worm.gif)
http://virtualpipeline.spaces.msn.com
RE: Water hammer in system with two MOC?
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?
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?
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] worm](https://www.tipmaster.com/images/worm.gif)
http://virtualpipeline.spaces.msn.com
RE: Water hammer in system with two MOC?
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?
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
Going the Big Inch!![[worm] worm](https://www.tipmaster.com/images/worm.gif)
http://virtualpipeline.spaces.msn.com
RE: Water hammer in system with two MOC?
Regards
RE: Water hammer in system with two MOC?
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.
Regards,
KLee
RE: Water hammer in system with two MOC?
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
http://www.eng-tips.com/viewthread.cfm?qid=161768
Going the Big Inch!![[worm] worm](https://www.tipmaster.com/images/worm.gif)
http://virtualpipeline.spaces.msn.com
RE: Water hammer in system with two MOC?
Going the Big Inch!![[worm] worm](https://www.tipmaster.com/images/worm.gif)
http://virtualpipeline.spaces.msn.com
RE: Water hammer in system with two MOC?
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?
and from continuity u3*A3=u4*A4 equation 4
RE: Water hammer in system with two MOC?
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] worm](https://www.tipmaster.com/images/worm.gif)
http://virtualpipeline.spaces.msn.com
RE: Water hammer in system with two MOC?
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?
Going the Big Inch!![[worm] worm](https://www.tipmaster.com/images/worm.gif)
http://virtualpipeline.spaces.msn.com
RE: Water hammer in system with two MOC?
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?
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] worm](https://www.tipmaster.com/images/worm.gif)
http://virtualpipeline.spaces.msn.com
RE: Water hammer in system with two MOC?
RE: Water hammer in system with two MOC?
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?
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] worm](https://www.tipmaster.com/images/worm.gif)
http://virtualpipeline.spaces.msn.com
RE: Water hammer in system with two MOC?
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
RE: Water hammer in system with two MOC?
Going the Big Inch!![[worm] worm](https://www.tipmaster.com/images/worm.gif)
http://virtualpipeline.spaces.msn.com
RE: Water hammer in system with two MOC?
Amen
Geoffrey D Stone FIMechE C.Eng;FIEust CP Eng
www.waterhammer.bigblog.com.au