knocking sound in the water line

[donald1]

Originally Posted by Local16 Omaha

Dishwashers have a very fast closing valve that will send a "shockwave" up the supply line and it will "hammer" against the nearest bend or fitting upstream.

yeah u tell him absolutely correct solution.

[CHRISM]

"Hi Phishfood" - Thanks for explaining that the `Hammer Arrestors` in the U.S. are NOT adjustable - here in the U.K. they are available with a `schrader` valve - the same as on a car tyre - to which we attatch a tyre pump - with a gauge - to set the vessal pressure.
The vessals are pressurised - mini versions of those used in `sealed system` Heating Systems and Hot Water Systems - with a neoprene membrane inside - and an internal pressure set at [for U.K.] 50 psi. - which in the U.K. is approximately 3.4 Bar - Bar pressure being approx. 14.7 psi.[ 1 Atmosphere] - the vessals are approx. 4" - 5" in diameter.
The Water Pressure in the U.K. varies enormously from region to region - even in London where I am based - an `average` is approx. 30 psi. -because of old supply pipework [not able to take higher pressures] - BUT I know areas where pressure reducing valves have to be used because the `Mains` pressure is 80 psi and above [mainly at night].
Our Water Pressure will be seen by the U.S. Plumber to be VERY low - BUT our `Mains` Water supply pipework in some areas is over 100 Years old - gradually being replaced - where new pipework has been installed - higher pressures exist, although MOST of our Water Heaters/ Combi Boilers etc are designed for `Low Pressure`[compared to what I read for U.S.]
When I stated that adjustment would sometimes be needed to the vessal pressure - not knowing that you do not have the same type in the U.S. - I was explaining that for example - the incoming `Mains` pressure was 30 psi and the purchased vessal pressure was pre-set at 50 psi - just installing the vessal and hoping that it would stop the `water hammer` - would not work because the 50 psi vessal pressure would NOT allow any `give`/deflection in the neoprene membrane - when the pressure surge `hit` the vessal - it would be the same as a `dead end` - as if no vessal was fitted, because the 50 psi behind the membrane would not allow any deflection.
To acheive the `deflection` of the membrane - the vessal pressure would have to be `nearer` the water pressure.
I am assuming from your description of the Water Hammer Arrestor - sometimes being installed in places which will become `inaccessable` - that in the U.S. the `Arresstors` are likely to be just a tubular form - with no membrane - which just `traps` an amount of air when the pipework is `commissioned`- and that the air `acts as a `buffer` to stop the water hammer ?? - perhaps like one could make using a larger bore of copper pipe and an end cap etc. ?? - although `sizing`this would be `trial and error`- for some situations. - We do have that type available in the U.K. - BUT I don`t think they would be used IF the purchaser KNEW about the `Pressure Vessal` type.
It was good of you to reply to my post - thanks for the information, there WILL BE plenty of situations where I will NOT be completely correct in regard to the U.S. Regulations/Codes - and I will never try to answer any question where Codes are involved - except perhaps to state what the U.K. Regulations/Codes are and make `Technical` comments.
In fact on the same day that I registered with Plumbing Forum - when I realised that it was U.S. based - I Emailed Keith to ask whether I should NOT participate - because of that.
I have `posted` on a couple of other subjects - regarding : Gas Leaks and Bathroom Vent [leaking water] - IF you can find them - I would really appreciate YOUR and other U.S. Plumbing Professionals input regarding the `relevence` of my `Posts` to the U.S. industry.
There is NOTHING `Technical` in those posts - I have not found any thing of a `VERY Technical nature to reply to - as yet. "Regards" CHRISM.

[phishfood]

A further note on hammer arrestors in use here. The type I am reffering to operate on a piston principle, with a actual physical barrier between the water and the air/gas. If there is not a physical seperation, eventually all of the air in the chamber will be dissolved into the water, and the chamber will become inoperative. We have installed "jobmade" air chambers in the past by, as you mentioned, using a section of pipe with a cap on it. These have fallen out of favor for the reason I mentioned above.

I think that you have presented your thoughts on the matter clearly and with sufficient warnings that your comments are that of a plumber from the UK. In the end, we are dealing with the same issues/problems, and your input is just as valuable as mine.

[CHRISM]

I would have been `surprised` if the U.S. did not have a design of `arrestor` that was more advanced than a `capped pipe` section.
When you said that the U.S. do not have the `Pressure Vessel` type that I was describing - and not having seen what you subsequently described as the `Piston type` - I had seen the `capped pipe section` used on a Canadian [?] T.V. program about correcting `Building Disasters`- including Plumbing faults - called `Holmes on Homes` - and although as you stated it is obvious that the Air which is `trapped` in the capped pipe would `dissolve`
its gasses into the water - I thought that adding those sections of pipe MIGHT have been `common practice` - I am glad to hear that it is not.

"Regards" CHRISM.