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The A dimension

kali
kali Member Posts: 1
Why does water hammer not occur in the A dimension? Isn’t it where steam and water meet? Doesn’t that alone cause water hammer?

Comments

  • Jamie Hall
    Jamie Hall Member Posts: 19,147
    Steam and water meet all over the place in a steam system. That's not what causes water hammer. The proximate cause of water hammer is a wave or slug of water -- could easily be as little as half a cup in a smaller pipe! -- being pushed along at speed by the steam behind and sweeping over it -- and then hitting an obstacle, such as the elbow or a T. Whang.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    ethicalpaul
  • ethicalpaul
    ethicalpaul Member Posts: 3,751
    Which is exacerbated by hot steam (there's no other kind really haha) flowing over a pool of cool collected water. This is why often hammer occurs most after a longer period of cooldown time.

    The steam will rapidly collapse, which causes a massive, rapid change in pressure that can "suck up" the sitting water and sling it.

    At the wet return you won't have this because the steam isn't flowing over the cool water...there is a cushion of air above the water that prevents the steam from even touching it.
    1 pipe Peerless 63-03L in Cedar Grove, NJ, coal > oil > NG
  • dopey27177
    dopey27177 Member Posts: 873
    The A dimension is a factor in returning water back to the boiler.
    There are two a dimensions.
    1. for boilers less than 100,000 BTUs
    2. for boilers greater than 100,000 BTUs

    Dimension "A" is the factor needed to put water back into the boiler.
    A failure to have sufficient height in dimension "A" will not only cause banging in the steam system it will cause erratic heat in the system as the boilers water levels will vary in short periods of time, where if the boiler has a LWCF the burner will shut down frequently and sufficient steam supply needed for heating will not happen.

    See Attachment

    This example was taken from my book Steam the Perfect Fluid for Heating and Some of the Problems.

    Jake

    Sale of book $35.00 and $5.00 for shipping

    Jake
    Alan (California Radiant) ForbesEdTheHeaterMan
  • STEAM DOCTOR
    STEAM DOCTOR Member Posts: 1,500
    Just to add a bit. The steam will only travel as far as the last air vent. 
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 3,332
    edited April 12

    Just to add a bit. The steam will only travel as far as the last air vent. 

    That sounds about right. (unless there is another way for the air to exit.)

    I always though of the Banging in a steam system was a drop if water in a cooler pipe. As the water expands from the steam flowing over it, the water level in the pipe rises and there is a drop of water that spills over some fitting and drips on to a really hot steam pipe and instantaneously explodes into steam. about 1700 times its volume of water, in a small space makes a small bang. Larger volumes of water (Like a whole teaspoon) make larger bangs. and a Cup of water... well that will rock the building.


    So proper pitch and a proper path for the condensate to find its way back to the boiler without that water coming into contact with a superheated piece of pipe, is what the dead Men designed. (superheated in it's technical sense meaning at least one degree above the saturation temperature of the water/steam). As long as the condensate has a path there will be no hammer.

    PS. I remember some factoid about a Dynamite explosion is an expansion of gases by a factor of 900. Water to steam is a factor of 1700. Just think about that when you hear the steam banging in some pipe somewhere.
    Edward Young
    Retired HVAC Contractor from So. Jersey.
    Services first oil burner at age 16
    P/T trainer for EH-CC.org
  • Jamie Hall
    Jamie Hall Member Posts: 19,147
    That's one cause. More often, however, I think is that water lying in a pipe -- even if it is flowing somewhat -- gets made into waves by the high velocity steam moving over it (watch a breeze -- even a mild one -- making waves on a lake). If the wave gets big enough, it blocks or nearly blocks that high velocity steam and, instead of flowing sweetly along, gets picked up and propelled by the steam at close to the steam velocity. When that slug of fast moving water comes to a change of direction, it can't make the change -- and slams into the fitting making the bang (it can be remarkably rhythmic on occasion). In extreme cases there is enough momentum in that slug of water to actually physically move the pipe. In very extreme cases there is enough momentum that the pipe can't take the stress and breaks... but that would be pretty extreme in a residential system (it's not in high pressure steam, where the velocities and pressures can be much higher -- a slug of water and it can be pretty much game over).
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • ethicalpaul
    ethicalpaul Member Posts: 3,751
    Steam doesn't move very fast in residential pipes, certainly not in mine. Even when the pipes are already warm, there's no way it's fast enough to pick up water.

    What I have seen is the steam will come across a cool pool of water (due to poor pipe pitch), and the steam will instantly condense when it hits that water. That will cause a pressure drop which is what picks up and slams the water around.

    This is why banging is much more common at the first steam production after a long delay, such as a call for heat in the morning after a setback. After all, the water is always sitting there in the valley, so why else would hammering be greater after a delay?

    So in my opinion, it is basically the opposite of Ed's description above (nothing personal Ed!)...it is the collapse of steam that causes it, not some instantaneous flashing into steam.
    1 pipe Peerless 63-03L in Cedar Grove, NJ, coal > oil > NG
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 12,143
    Its the steam condensing (reducing in volume 1700 times) when the steam condenses it causes a vacuum and something races in to fill the void, water air or steam and it moves so fact it hammers.
    ethicalpaul