Water hammer in system with two MOC? 1

[KLee777]

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

 

[BronYrAur]

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.

 

[stanier]

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

http://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

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

 

[BigInch]

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!

http://virtualpipeline.spaces.msn.com

 

[sailoday28]

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

 

[BigInch]

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!

http://virtualpipeline.spaces.msn.com

 

[KLee777]

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!

 

[BigInch]

Why do you neglect them?

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[sailoday28]

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

Regards

 

[BigInch]

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!

http://virtualpipeline.spaces.msn.com

 

[sailoday28]

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

 

[stanier]

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

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

 

[BigInch]

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!

http://virtualpipeline.spaces.msn.com

 

[BigInch]

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!

http://virtualpipeline.spaces.msn.com

 

[stanier]

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

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

 

[BigInch]

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!

http://virtualpipeline.spaces.msn.com

 

[stanier]

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

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

 

[stanier]

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

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

 

[BigInch]

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!

http://virtualpipeline.spaces.msn.com

 

[sailoday28]

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