high pressure restraints

[vodeni]

I am moving approximately 5000 gpm of water (inside the plant) at 130 psi. Do we need additional restrain on piping (rather than regular flanges. We have surge relief system that activates on appearance of transient pressure.

Thanks for your input

 

[pennpiper]

130 psi is not high pressure and 5000gpm is not a high flow rate. Depending on the other factors you have not shared with us the answer may be either yes or no.

What is the line size?
What is the material?
What is the operating Temperature?
What is the source of the water?
What kind of pump are you using?
What is the destination of the water?
What is the configuration of the suction and discharge lines?

 

[vodeni]

the pipe sizes are 18" parallel force main, I also have a relatively long 8" surge relief piping. It's all ductile iron and the liquid is effluent water in a sewage treatment plant.

 

[BigInch]

You need to consider what changes in fluid momentum during the surge event and other causes of velocity and fluid accelerations in the system, how and where they will occur and whether they will exert undue forces on any of the pipe supports.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[eadwine]

Ever heard of "water hammer"? Normally this is a problem at startups and shutdowns. It also happens during valve closure with short closure times. These transients can make pipelines "shake, rattle and roll" significantly which can play havoc with supports and pipe. These kinds of problems become apparent with use and experience with the system in question and need to be corrected when discovered.

 

[c2sco]

vodeni,
Someone has obviously considered water hammer (also known as surge or transient flow) there since you refer to a "surge relief system".
As eadwine states, such conditions occur whenever there are flow rate changes. Normally these are slow enough not to give much problem with pipe anchors, but it is possible to operate valves quickly, have pressure relief valves operate, have pump startups and trips, or flow in partially full lines that can give severe flow rate changes that can generate forces high enough to break supports and even collapse pipe bridges.
The forces produced can be frighteningly high - tens or hundreds of tons on large pipes! Fortunately most transients are brief, and the forces only apply for brief periods such that the pipe hardly moves despite the "bang".
Analysis of pressure surge to ensure your system is protected against overpressure is a specialised topic using modern software. Software like Flowmaster, AFT Hammer, HiTrans and others is good, but complex to use.
If you go to my web site page on this -

http://www.cedcs.com/pages/waterHammer.php

you can read about it, and if you go to the downloads page

http://www.cedcs.com/pages/downloads.php

you can get a Powerpoint presentation and Excel workbook that hopefully will enlighten you!
Regards,
Stuart

 

[c2sco]

PS I should also have said that these programs only work out pressure-time history, they don't calculate forces on pipes, but I can help with that if needed.
Stuart

 

[BigInch]

Forces on restraints can be roughly estimated as the change in pressure between restraints x cross sectional area of pipe. That is very rough and may be considerably affected by the stiffness of the piping configuration, so its just to help give you an idea of where they might turn into problems.

**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch

http://virtualpipeline.spaces.live.com/

 

[c2sco]

Yes, that's correct in some situations. However you need to be aware of the amplifying effect of harmonic motion in the pipe and to represent this there is a so-called "Dynamic load factor", which can be up to 2. If unknown, the safe assumption is to multiply the answer from BigInch's method by 2, and that's a fair starting point. In reality as I said most often the duration of sustained pressure differences that give a force on the pipe are quite short.

 

[BigInch]

If you use the dynamic pressure differences, it "works" as an estimate for dynamic loads too.

Designing for dynamic loads often requires an impact factor, so I wouldn't call that a "harmonic" factor, as harmonic implies a load due to cyclic or vibration loads. Surge pressures might hopefully be estimated by taking the normal load X 2, if you didn't know the surge pressures themselves.

Short times or not, the acceleration and fluid masses can be high and its those that make the dynamic force, F = m*a

**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch

http://virtualpipeline.spaces.live.com/

 

[c2sco]

Dynamic load factor does come from cyclic loading as the load is taken up and released. Surge loads can ONLY be estimated when you know the surge pressures, and that comes from transient flow modelling of the system. I can send you a paper on the subject if you like.

 

[BigInch]

I treat cyclic loading as a fatigue problem that reduces the (long term) allowable stress. Usually the B31.3 method is sufficient.

Transient surge loads seldom exceed 50% normal operating pressure, if they are that high. Most of the time they generally fall into 20% to 30% of OP or so, as a rule of thumb, so 2X usually gives a very conservative answer. If you're happy with that, you could use it. If its a short pipeline, maybe you can afford to do that. Most of the time my clients can't afford it because the pipelines are very long with no room for such extras, so I use Stoner Pipeline Simulator for steady and transient surge and steady and transient thermal analysis whenever I need more detailed knowledge.


**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch

http://virtualpipeline.spaces.live.com/

 

[BigInch]

At least that's true for normal pipe design where velocities, operation pressures, pressure drops have the customary and typical correspondence. It won't be true for extreme velocities, but then you're out of the normal framework of things. The secret is to know when you can use the rules of thumb and when you can't.

**********************
"The problem isn't working out the equation,
its finding the answer to the real question." BigInch

http://virtualpipeline.spaces.live.com/