Sea Chest Sizing

Discussion in 'Boat Design' started by SAMUEL TAN, Apr 8, 2021.

  1. SAMUEL TAN
    Joined: Apr 8, 2021
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    Location: Singapore

    SAMUEL TAN New Member

    Dear All,

    I have facing an issue on the sea chest sizing, currently my company having 2 55m Landing Craft, both vessel are identical. The first vessel we using 2x24" seachest(attach with 2"vent pipe) connected to sea main with multiple consumers and we having difficulties for main engine to suck sea water when engine running at high RPM. For the second vessel, we using 2 x 16" sea chest(attach with 2"vent pipe) connected to sea main with multiple consumers and doesnt facing any issue.
    Can anyone experience in this explain to us what is the cause, thank you.
     
  2. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    I have come across serious ventilation/cavitation problems in cases where inlet for ballast pumps and scrubber pumps are located too close to each other, and the chest design created a "shortcut" from the hull side to the suction pipes. In addition to the shortcut problem, the pumps are "always" too big, meaning that they are operated at part flow, which creates strong internal cross-flows and pre-rotation in the pump inlet. This will cause an accumulation of gas in the center of the inlet eye.

    Could you please attach some sketches/pictures/drawings of your installations? And info on required flow capacity might help as well.
     
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  3. SAMUEL TAN
    Joined: Apr 8, 2021
    Posts: 3
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    Location: Singapore

    SAMUEL TAN New Member

    Thank you for the reply, kindly find the following snapshot for our seachest and seamain arrangement, it shown that the sea main connected to multiple consumers including 2 sw cooling pump(75m3/h) and 2 Fire/GS Pump(30m3/h). Both sea chest have same size(24") but different height. Pleas kindly advice if you have an explanation on the sw suction issue, thank you.
    upload_2021-4-9_9-53-31.png
     

    Attached Files:

  4. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    The main issue I think is the chest arranged in the flat bottom area; there is a high risk of a stream of bubbles following the skin and ending up in the top of the seachest. Providing the strainers are not full of debris, and all valves that should be open are actually so, my first measure would be to increase the fluid residence time in the chest, and to force the aerated water to the top, where it can release the air, see sketch attached.

    Also, air bubbles tend to follow solid surfaces, so the suction opening should have a collar inside the chest wall; to reduce losses it should be conical, ~45-60 degrees. In really bad situations, there may even be an accumulation of air in the common manifold, you should consider a manually operaded vent on top of the manifold pipe.

    The suction pipe to one of the CW pumps (nearest to the camera in the photo) is really bad practice, with a 90 degree bend immediately in front of the inlet; there should be at least two pipe diameters length of straight pipe between the bend and the inlet!!!!

    If you have any pressure readings from the pump inlet (and delivery as well), they could show if there is something else going on.
    Seachest 001.jpg
     
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  5. SAMUEL TAN
    Joined: Apr 8, 2021
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    Location: Singapore

    SAMUEL TAN New Member

    Thank you very much for your helpful and detail solution Sir.
     
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  6. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    Sir, indeed.

    I am not a fluid dynamics expert. But, personally, I think the fast 90 is a major problem. I am glad someone helped you with real wisdom. I only saw the 90 before @baeckmo commented and was afraid to comment for lack of comprehensive knowledge of these systems. I was curious about the 2 pipe diameter standard mentioned and did some googling (apologies, but not for curiousity).

    I found an interesting research paper that may edify a few readers. I don't like to be that google guy, but here it is anyway.

    Changing the angle might be difficult, but sometimes careful plumbing of 45s beats bad 90s..if you can make the space..I don't know how using 45s would change the 2 diameter length advise either, or whether it would help.

    Effect of 90° elbows on pump inlet flow conditions https://link.springer.com/article/10.1007/s13201-020-01255-7
     

  7. baeckmo
    Joined: Jun 2009
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    Location: Sweden

    baeckmo Hydrodynamics

    The "close bend-syndrome" is particularly common among marine installations, unfortunately. All pump manufacturers specify a minimum undisturbed length, which is consequently neglected by the piping designers..... And, yes, with some extra thought a lot of 90 degree bends could be avoided; but pipe designers and builders have 90-degree coordinates stamped into their brains. In fact, most of the pipe losses onboard a ship comes from armature and bends and very little from pipe friction. So, saving bend losses pays directly in fuel savings.

    Now, in this case the pumps seem to be a self-priming type (Desmi, probably), which has an internal straight length between flange and impeller, thus my recommendation for two diameters. But I wonder what components there are after the gate valve at the manifold. There is something hidden behind the pillar (a fine-mesh strainer?) and after that an additional something, is that a non-return valve? If so, that is a big no-no in the suction pipe!!! It should be placed after the pump in order to protect the pump from transient pressure waves ("water hammer") at power-out cases.
     
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