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Discuss Why would it be wrong to run a shower pump on 15mm? in the Bathrooms, Showers and Wetrooms area at UKPlumbersForums.co.uk.

  1. Ric2013

    Ric2013 Plumber Top Contributor!!

    Apart from the obvious answer which is this would not be to manufacturer's instructions and would void any warranty.

    I was having a discussion the other day with a builder who wanted to run the cold side of a 1.5Bar twin-impeller pump from a dedicated cold distributing pipe in 15mm. He claims this is what he has always done in the past and it works (I would imagine this is true, to some degree).

    I have looked at the MI, which state that the suction side of the pump should be 22mm and reduce to 15 close to the 15mm tails supplied with the pump. Common logic would be that 22 is required because the suction side is gravity, but, while I know 15mm would be wrong, I'm trying to understand WHY it would be wrong.

    My feeling is that, since 15mm on the delivery side of the pump is considered sufficient, and that the flow rate will be the same on both suction and delivery sides, 15mm is enough for flow on suction side. However, I would imagine the risk is cavitation, leading to premature failure of the pump impeller and possible the pipework or fittings on the suction side of the pump.

    Do people feel my understanding is correct?

    Thank you,


    R
     
  2. rpm

    rpm Trusted Plumber Top Contributor!!

    Many pumps insist on a larger inlet and a smaller outlet, cavitation is basically trapped air becoming very hot and popping / exploding.
     
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  3. king of pipes

    king of pipes Trusted Plumber GSR Top Contributor!!

    I have done plenty in 15mm not any dramas.
     
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  4. Harvest Fields

    Harvest Fields Trusted Plumber GSR Top Contributor!!

    Never done one myself in 15mm always have the feeds in 22mm, but I have replaced loads of existing ones that have both feeds in 15mm and they have been running for years. But best practice is definitely 22mm. I always recommend the pipes are upgraded
     
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  5. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    I've done it but only if there a short run to the pump
     
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  6. Harvest Fields

    Harvest Fields Trusted Plumber GSR Top Contributor!!

    Maybe if the CWS was straight above the cylinder. But I would still have a bad nights sleep:p:p:p
     
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  7. townfanjon

    townfanjon Trusted Plumber GSR Top Contributor!!

    I thought that was because you followed Villa lol
     
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  8. king of pipes

    king of pipes Trusted Plumber GSR Top Contributor!!

    Sleep like a baby ha ha , mind you i didnt last night have had a git of a job this week a boiler swap cylinder change and controls upgrade , electrian wired something wrong and i had air and circulation issues but got it sorted today so happy days , sacked work off tomorrow having a slack day paperwork and catching up with people
     
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  9. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    :D until someone phones in saying ive got water everywhere
     
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  10. mache

    mache Active Member

    not quite correct description. Cavitation occurs when pump impeller or sharp edge even causes a low pressure area in which a vacuum pocket forms. When that pocket collapses the violet energy change can rip material off the surface of any material it is in contact with. But I do agree that a restricted protracted inlet (e.g. full length of 15mm run) could increase the chance of cavitation occurring.
     
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  11. king of pipes

    king of pipes Trusted Plumber GSR Top Contributor!!

    Not tomorrow shauny had enough this week bud, mind you got a fancy rad to fit saturday so my weeks not over yet whats your week been like ?


    Nice one mache learn something new every day
     
  12. Harvest Fields

    Harvest Fields Trusted Plumber GSR Top Contributor!!

    Cheeky;)think we will do ok this season. Ow though. Still sacked my season ticket off though.
     
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  13. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    busy

    finished off a cloak room sink / vanity 2 cabinets new cc toilet shower head replacement

    now starting a full down stairs bathroom
     
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  14. Harvest Fields

    Harvest Fields Trusted Plumber GSR Top Contributor!!

    At least you don't have to cart the bath upstairs:)
     
  15. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    no bath and already done the upsairs one :D
     
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  16. king of pipes

    king of pipes Trusted Plumber GSR Top Contributor!!

    Glad your keeping busy mate speak soon regards kop
     
  17. rpm

    rpm Trusted Plumber Top Contributor!!

  18. mache

    mache Active Member

    Hi rpm, The link you provided is a pretty good example of caviation description. The violent energy change I mentioned is the shockwave they described. I suppose my only argument is when these papers describe 'bubbles' because that gives a mental impression of air in the fluid. Where as in fact it is a small localised pocket of nothing with high negative pressure. This is why it implodes or collapses back on itself. An air bubble cannot do that. By the way I'm no expert on it, it's just that I have investigated some examples of damaged instrumentation and learned a little about the process when reviewing what had occurred. The following wikipedia link gives some additional info...although they still annoyingly sometimes use that term bubbles as well.
    Cavitation - Wikipedia
    Best Regards
    Martin
     
  19. rpm

    rpm Trusted Plumber Top Contributor!!

    lol
    One little letter changes everything
    Violet - Violent.
    Just out the door to meet a pint with my name on it, talk later.
     
  20. mache

    mache Active Member

    Ha ha... I hadn't even spotted that.
    Enjoy that pint.
     
  21. Harvest Fields

    Harvest Fields Trusted Plumber GSR Top Contributor!!

    Ohhhh mate. I really wish right now. :(:(. But I am out tomorrow :p:p:p
     
  22. Ric2013

    Ric2013 Plumber Top Contributor!!

    Someone once told me that high flow through elbows can split the water molecule into hydrogen and oxygen, thus 'air'. I don't believe this, but I do believe the water could temporarily form water vapour, especially under low pressure. Perhaps these are what the 'bubbles' are?

    Which is what the chap in the video on the Crane Engineering link is saying: 'the other way you can boil water is by lowering its pressure'.
     
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  23. rpm

    rpm Trusted Plumber Top Contributor!!

    Last post as kind of hijacking the thread, Chesterton pumps and John Crane seals both referenced air bubbles when I worked with them and it was the Worthington- Simpson guys who spoke of air pockets when the incoming water could be pumped away faster than the incoming water which brings us back to the pipe sizing at the start of this thread.
     
  24. rpm

    rpm Trusted Plumber Top Contributor!!

    Haven`t heard that one as it is outside of the pump chamber but the difference between a sharp elbow and a swept elbow on the discharge side can reduce the head enormously.
     
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  25. Ric2013

    Ric2013 Plumber Top Contributor!!

    Thanks, rpm. I'm not bothered about hijacking. The point of my question was to expand my knowledge, which you are doing, and you are not off topic anyway!
     
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  26. Ric2013

    Ric2013 Plumber Top Contributor!!

    Elbows, valves, anywhere the restriction vs the flow is high enough. Even a badly deburred pipe: lovely picture in the Guilds textbook. But I suspect the flow has to be significant!
     
  27. rpm

    rpm Trusted Plumber Top Contributor!!

    To my thinking, the pump can only produce what it is capable of flow wise then the other stuff disturbs it.
     
  28. Ric2013

    Ric2013 Plumber Top Contributor!!

    The pump, yes. Unless pumps are designed to operate at a minimum duty? Not sure about that at all.

    I was thinking more about plumbing in general. I think some of the pitting on tap valve seats could be caused by cavitation as the water flows from behind the restriction. This being the reason the diaphragm float valve was developed: apparently the early Portsmouth type valves were eating through their brass nozzles in a matter of months in some areas (high pressure?). Which probably is what gives them that distictive noise that sounds like a Tardis parking with the handbrake left on (which is why I refuse to get rid of the Portsmouth on my feed cistern at home: I like the sound it makes ;) ).

    EDIT So if I apply that to my own question, this could potentially lead to water vapour being fed into the pump if the suction side is restrictive?
     
    Last edited: Sep 8, 2017
  29. FG28

    FG28 New Member

    it will work thats for sure
    But why do a half ar se job ?
    Do a job do it properly ,. Do a half ar se and except crap at the end ..,
     
  30. Chuck

    Chuck Top Contributor!!

    Copper pipes carrying water form a layer of copper oxide, copper carbonate, etc. This 'passivating layer' inhibits further corrosion. Flow rates high enough to cause turbulence, which tends to occur near structures like elbows, disrupt this layer so corrosion occurs typically liberating hydrogen in the process.

    You are correct, bubbles formed by cavitation contain mainly water vapor. The rate of growth and collapse affects what happens so detailed explanations are complicated and simplified explanations will seem inconsistent if you think about them carefully.

    When reading descriptions of these phenomena, one needs to keep in mind that plumbing pressures are normally 'gauge pressure', i.e. pressure above local atmostpheric pressure. Technical literature often uses 'absolute pressure'.
     
  31. Harvest Fields

    Harvest Fields Trusted Plumber GSR Top Contributor!!

    These posts are far to complex for me so I'm sorry, but I'm out.:p:p
     
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