# Water Hammer Reduction w/ Swing Joints



## michael34667 (Jun 2, 2009)

I was recently talking to an engineer about the hydrodynamics of "water hammer". If I have a basic single handle moen old style (non-positemp) shower valve at the end of a several hundred foot run of 3/4" PEX, CPVC, Copper, Etc. shutting off the valve rapidly, causes water hammer. 

Will installing one or more swing joints in the run help eliminate water hammer??? And If so, why does the change in direction via a swing joint help eliminate this problem??? 

Obviously I could use a prefabed arrestor, air chambers, and other methods and/or devices to elimate this type of problem. I am just not sure how this swing joint concept works, if it does.

I am a 4th generation plumbing contractor in Central Florida w/ about 20+ yrs exp. and have always used other methods mentioned above to eliminate this type problem.

Wondering if someone could enlighten me on this swing joint concept, if it works.

Thanks,

Michael. CFC, CGC


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## Protech (Sep 22, 2008)

I say it doesn't work at all. But I was wrong one time.......

BTW. Welcome to the zone big bro:thumbup:


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## Protech (Sep 22, 2008)

huh, this 3rd poster seems to agree with the concept: http://www.plbg.com/forum/read.php?1,305889


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## michael34667 (Jun 2, 2009)

Thanks little bro, this site kicks a$$. I am sure their is some hydrologist out there that knows.


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## Herk (Jun 12, 2008)

If it's threaded pipe, I'd guess that the water hammer would mess up the threads after a while. Better to have slow-closing valves.


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## Tankless (Jun 12, 2008)

That poster said the duel TK's were the cause of the hammer.....I'll bet that was his best educated guess. Tankless units do not aid or help remove hammer. At lease in all the ones I have installed and seen. Long runs with proper pressure will still hammer when the valve is slammed shut. Velocity plays a big part in it.


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## Tankless (Jun 12, 2008)

michael34667 said:


> I was recently talking to an engineer about the hydrodynamics of "water hammer". If I have a basic single handle moen old style (non-positemp) shower valve at the end of a several hundred foot run of 3/4" PEX, CPVC, Copper, Etc. shutting off the valve rapidly, causes water hammer.
> 
> Will installing one or more swing joints in the run help eliminate water hammer??? And If so, why does the change in direction via a swing joint help eliminate this problem???
> 
> ...


One of my first bosses used to tell me that at each turn the hammer's energy would dissipate. I would assume based on that theory that a swing joint would equate to enough turns at its operating pressure to dissipate the energy. I never installed one so...take it with a grain.

There was one time I couldn't get rid of hammer, I added a 3/4" line from the service riser about 150'. about 15' was above ground and braced with strut. The rest was about 20" below ground, that was a pex run. I brought it up against a footer and udes a drop ear'd 90 with a few tap cons....point is, there was no pipe wobbling around and when I hit the 1/4 turn hose bib to stop it would ping about 4 or 5 times all within maybe 2 or 3 seconds. Pressure was 70. It was just a hose bib line so I didn't spend a great deal of time on it and it wasn't worth it to arrest.

I think the engineers comments are correct in theory...but in real world...I dunno, I never used one.


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## rogeru (May 9, 2009)

For water hammer I wouild install factory made shock arrestors as close to the offending faucet as possible. Putting in the swing joints/offset would not hurt either but not sure if it wouild solve the problem. Size the arrestors according to manufacterers recomendations.
Roger


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## stillaround (Mar 11, 2009)

At my own risk Ill weigh in. Had to call an engineer who said he usually gets 1500-1800 for this stuff so with his angered english accent he walked me thru the basic causes of hammer. Large runs uninterupted have a weight of water that at quick close , that mass, creates a shock that travels very fast. The more the line can be shortened with arrestors the less mass for shock. The swing joints (imo) while slowing velocity a touch wouldnt do it.


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## stillaround (Mar 11, 2009)

***


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## SPH (Nov 4, 2008)

Little formula for you to figure out how much pressure you have at the end of your run, you can change the variables to suit your application
_dp = 0.070 v l / t (1)_
_where_
_dp = increase in pressure (psi)_
_v = flow velocity (ft/s)_
_t = valve closing time (s)_
_l = upstream pipe length (ft)_​EXAMPLE

_dp = 0.070 (5 ft/s) (300 ft) / (0.4 s) _
_ = 262.5 (psi)_


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## Protech (Sep 22, 2008)

good to know:thumbsup: thanks sph


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## stillaround (Mar 11, 2009)

I like that formula better. Relying on high school physics to grab a concept can be trumped by a good formula. 
http://en.wikipedia.org/wiki/Water_hammer after reading some of this I lost some interest in the topic


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## para1 (Jun 17, 2008)

*"who wants icecream?":whistling2:*


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## leak1 (Mar 25, 2009)

hi girls-come to daddy!


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## ROCKSTARPLUMBER (Dec 14, 2008)

Para!! Did you hack into my computer again??????????


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## user4 (Jun 12, 2008)

The process with the swing joints you are refering to is a good way to ease thermal expansion on long pipe runs, it will do nothing to prevent water hammer.


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## Protech (Sep 22, 2008)

I agree.


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## user4 (Jun 12, 2008)

Protech said:


> I agree.


The hell you say.


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## Protech (Sep 22, 2008)

I agree with what you are saying. I know, it's hard to believe.


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## michael34667 (Jun 2, 2009)

stillaround said:


> I like that formula better. Relying on high school physics to grab a concept can be trumped by a good formula.
> http://en.wikipedia.org/wiki/Water_hammer after reading some of this I lost some interest in the topic


 
I checked out the link above and here is a direct quote. Not sure how reliable the source is though. Looks pretty creditable to me. The physics are a little beyond my understanding right now. (College Physics is still 1 semester away for me & Hydrodynamic Engineering is about 5 semesters away right now).

"Shorter lengths of straight pipe, i.e. add elbows, expansion loops. Water hammer is related to the speed of sound in the fluid, and elbows reduce the influences of pressure waves."
====================================









In this expression[5]:

overpressurization δP is expressed in Pa;
Q is the volumetric flow in m3/s;
Zh is the hydraulic impedance, expressed in kg/m4/s.
The hydraulic impedance Zh of the pipeline determines the magnitude of the water hammer pulse. It is itself defined by:







with:

ρ the density of the liquid, expressed in kg/m3;
A area of the pipe, m2;
Beff effective modulus of compressibility of the liquid in the pipe, expressed in Pa.

*This seems to verify the original swing joint theory.*

Thanks for posting the link.


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## Protech (Sep 22, 2008)

I didn't catch that.......

All I know is that the hammer is due to the "train wreck" of water into the valve. As previously stated, the variables that determine how bad that train wreck is are the mass and the speed that the mass is moving and the rate of deceleration. I don't see where making the water go around a turn changes any of those variables.


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## Protech (Sep 22, 2008)

Plus, I don't see that listed as an option here:

Mitigating measures
Water hammer has caused accidents and fatalities, but is usually less threatening. In many cases damage is limited to breakage of pipes or appendages. An engineer should always assess (at least qualitatively) risk of a pipeline burst. Pipelines with hazardous goods should always receive special attention and should be thoroughly investigated.
The following characteristics may reduce or eliminate water hammer:

Low fluid velocities. To keep water hammer low, pipe-sizing charts for some applications recommend flow velocity at or below 5 ft/s (1.5 m/s).
Slowly closing valves. Toilet flush valves are available in a quiet flush type that closes quietly.
High pipeline pressure rating (expensive).
Good pipeline control (start-up and shut-down procedures).
Water towers (used in many drinking water systems) help maintain steady flow rates and trap large pressure fluctuations.
Air vessels work in much the same way as water towers, but are pressurized. They typically have an air cushion above the fluid level in the vessel, which may be regulated or separated by a bladder. Sizes of air vessels may be up to hundreds of cubic meters on large pipelines. They come in many shapes, sizes and configurations. Such vessels often are called accumulators or expansion tanks.
A hydropneumatic device similar in principle to a shock absorber called a 'Water Hammer Arrestor' can be installed between the water pipe and the machine which will absorb the shock and stop the banging.
Air valves are often used to remediate low pressures at high points in the pipeline. Though effective, sometimes large numbers of air valves need be installed. These valves also allow air into the system, which is often unwanted.
Shorter branch pipe lengths.
Shorter lengths of straight pipe, i.e. add elbows, expansion loops. Water hammer is related to the speed of sound in the fluid, and elbows reduce the influences of pressure waves.
Arranging the larger piping in loops that supply shorter smaller run-out pipe branches. With looped piping, lower velocity flows from both sides of a loop can serve a branch.
UPS (uninterruptible power supply) is sometimes installed to dampen the initial pressure wave by keeping the system running for some time after a power trip.[_citation needed_]
Flywheel on pump.
Pumping station bypass.
Hydroelectric power plants must be carefully designed and maintained because the water hammer can cause water pipes to fail catastrophically.


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## michael34667 (Jun 2, 2009)

Okay Kenny, Obviously we disagree on this subject. We will have this conversation again in a few years after we both take Hydrodynamic Engineering. Better yet I will ask one of the Engineering Instructors at University of Florida. I will take the word of an Hydrodynamic Engineer with a PhD over any plumbing contractor. 

My quote is from the 10th bullet down on your last post.

I still think this theory could work, with the logic of the shockwave being somewhat dissapated (could be negligible), when having to navigate multiple 90 degree turns.
I know I have seen several 90 degree turns at energy plants & I don't believe it is for the sole purpose of thermal expansion. 

Logically I think any shockwave will travel easier in a strait line, rather than having to navigate multiple turns (refracting). I think that when the shockwave hits a turn it does dissapate some of the energy. You could be right though & I could be wrong. I still got nothing but love for you brother.


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## Protech (Sep 22, 2008)

There may be something to your "shock wave" trap concept.

I would equate it to trying to dampen the sound of a bullet to save the life of the person it's going to hit. The bullet hits first and does the damage, the "after shock" of reverberations come after the damage has been done.

Talk to your engineer buddy. I love talking to people smarter than me and I’m curious what he has to say about it.:thumbsup:


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## SPH (Nov 4, 2008)

I think you may have a point with the 'swing joints' disapating the shockwave. But why not isolate the water hammer at the point of use, rather than restrict flow on the whole system? I guess each application may differ.


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## michael34667 (Jun 2, 2009)

SPH said:


> I think you may have a point with the 'swing joints' disapating the shockwave. But why not isolate the water hammer at the point of use, rather than restrict flow on the whole system? I guess each application may differ.


 
We are just talking theory here. "Point of Use" is always the way I have intalled stuff. Either prefabbed arrestors and/or air chambers, usually does the trick. This is just a concept that my litte bros and I (both Plumbing Contrctors) along with the other 8 Plumbing Contractors spread over the Southeast U.S. in my family have had a discussion about at one time or another. Some say yes it works, some say no. I think it could work, but I dunno. I'll find out soon.

:jester:


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## Double-A (Aug 17, 2008)

Consider that all of the equations for figuring the pressure wave use the area of the cross section of the pipe and the length of the run. Not the developed length.

The hammer is a sound pressure wave. It will respond to changes in direction by losing energy in the turn. This is the same basic concept as a muffler. Make the thing change directions enough times, and the energy is reduced to "safe" or negligible levels.

Think of it this way. If you shoot a marble through a pipe, it will have a higher velocity if it travels through a long straight run of pipe than if it has to travel through the same developed length of pipe, but with two 45 degree offsets. 

The only difference is, in this example the thing doing the speed of sound is a marble, in the real water hammer situation, its a sound pressure wave moving through the water.


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## Plumbus (Aug 4, 2008)

Another way to avoid hammer is to keep velocities below 5 ft/sec., as the wikipedia entry mentiones (first bullet). Recently, I had to do calculations to assure flows below 4'/sec on a job, as called for by the acoustical engineer [a very careful man]. Here are my calculations (using a formula given to my by an on line engineer budy) using the Hunter Curve (shown at the bottom of the page) to convert GPM's to WSFU's.

Velocity = 
gpm x 0.3208
A
where A = area of the pipe, in square inches, 
for: 
1/2" A = 0.1963 Max WSFU-2 
3/4" A = 0.4418 Max WSFU-5
1" A = 0.7854 Max WSFU-10
1 1/4" A = 1.2272 Max WSFU-20
1 1/2" A = 1.7671 Max WSFU-35
2" A = 3.1416 Max WSFU-80
2 1/2" A = 4.9087
3" A = 7.0686

To convert GPM to WSFU's see CPC 2007 
Appendix A, Chart A-3 [curve 2](Hunter Curve)


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## leak1 (Mar 25, 2009)

yep! thats what i thought!


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## TDB (Jun 25, 2008)

Protech said:


> Talk to your engineer buddy. I love talking to people smarter than me and I’m curious what he has to say about it.:thumbsup:


zinger!


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