Water lock pressure loss calculation

Discussion in 'Boat Design' started by Aurel, May 30, 2012.

  1. Aurel
    Joined: May 2012
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    Aurel New Member

    Hi,
    I'm trying to calculate the back pressure of my exhaust line and I was wondering how can I calculate the pressure loss in the water lock or where can I found informations about it?
    Thanks in advance.
     
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  2. Petros
    Joined: Oct 2007
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    Petros Senior Member

    A little more explanation please. What exhaust? Engine? If so, what size, type, displacement, etc.. What size is the exhaust pipe, length, etc?

    What are you calling a "water lock", not sure I have heard that experession before. Only time I have seen the term used is when an engine ingests water into the intake and stops the engine, it is often called water lock. It can do damage to the engine, but I do not think that is what you mean here.

    Generally, when talking about engines, the more resistance to pushing out the exhaust, the less available power. And the more fuel it will consume going the same speed.
     
  3. Aurel
    Joined: May 2012
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    Aurel New Member

    The water lock is a water separator, on a wet exhaust.
    I only need an estimation or a way to find the pressure loss (or backpressure) of this kind of device.
    That's for a generator with an exhaust pipe of 100mm.
     
  4. daiquiri
    Joined: May 2004
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    daiquiri Engineering and Design

    Petros,
    waterlock is a pretty common term which indicates a component of a wet-exhaust system which collects water and serves both as a silencer and as a lock against salty air from flowing back into engine when it is stopped.
    An example: http://viewer.zmags.com/publication/06fa8bec#/06fa8bec/68
    or: http://www.yanmarhelp.com/i_exhaust.htm

    Aurel,
    the formula you are looking for probably doesn't exist, for a very obvious reason - the exhaust tube on the exit side of a waterlock does not contain just gas or just water, but a mixture of the two.
    Take a look at this pic, taken from the Vetus catalogue:
    Waterlock.jpg

    The backpressure due to the waterlock would be the sum of:
    1) the friction losses due to the roughness of the hose;
    2) pressure losses of the waterlock "labyrinth";
    3) hydrostatic pressure of the vertical part of the exit tube (which height I've indicated with H).​

    The friction losses (n.1) can be reduced by choosing a bigger diameter of the exhaust hose. A pressure loss in the waterlock barrel (n.2) is a minor player, because exhaust gas slows down considerably inside the waterlock (due to a big cross-section area), and hence have a very small loss.

    Remains the problem of evaluating the hydrostatic pressire due to the column of water/gas mixture inside the vertical exhaust hose (n.3). If it was a single-phase flow, this term would be given by the well-known hydrostatic equation:
    Δp = ρ g H

    But the average density ρ is unknown, becuase it depends on the gas/water mixing ratio which, in turn depends on the engine RPM and load. At high RPMs and high loads the temperature of the exhaust gas before mixing with cooling water is very high, which implies that water will evaporate at a bigger rate, thus diminishing the liquid phase in the gas/water mixture - and that means lower mixture density. At low RPMs and low loads the exhaust temperature is low, so less water will evaporate in contact with the gas coming out of cylinders and the mixture density will increase.
    So you can comprehend that the worst case happens at idling RPMs, where a high hydrostatic backpressure pushes against the low engine-driven exhaust gas pressure. The exact numerical value of gas/water mixture density will depend on the engines RPM, size, waterlock capacity. The exhaust-gas temperature considerations can also change if the engine is turbo-charged.

    Sorry for such an inconclusive reply, but at least you have a picture of why it is so inconclusive...

    Cheers
     
  5. FAST FRED
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    FAST FRED Senior Member

    If you are considering a water lift muffler a larger diameter will lower the back pressure the most.

    These are great , as they can make a really quiet exhaust , but become really large with BIG engines.

    FF
     
  6. bertho
    Joined: Aug 2006
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    bertho bertho

    back pressure on the exhaust is generally check in small yard with a simple small clear plastic hose connected with a temporary fitting after the mixing elbow, you nail this hose (diameter 0.6mm) fill up with water, on a small plank, install vertically, with a loop under the point 0 and do measurement how high the water is going up when the engine is running at different speed, most of the time 0.3 to 0.5 meter is the maximum admitted by engines constructor. (equivalent at 0.5 atmosphere)
    engine with turbo are much more sensible than atmospheric engines.
    but how to calculate other than by experience..?, i don't know !! :) !
    follow engine supplier advices for length and diameter..each elbow is a big penalty..
    it's one important control to do at new boat/engine commissioning..
    best regards
    bertho
    www.fusionschooner.blogspot.com
     
  7. daiquiri
    Joined: May 2004
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    daiquiri Engineering and Design

    Agreed up to 0.5 (even 0.6) meters is ok, which equals 0.05 (not 0.5 ;) ) to 0.06 atm, or 5 to 6 kPa.

    Good point about turbocharged engines, as an excessive backpressure would noticeably decrease the efficiency of the turbocharger.
     
  8. bertho
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    bertho bertho

    daiquiri,
    sorry, absolutely right, wrong typing, one meter is 0.1 atmosphere... so 50 cm is 0.05
    but still to much for a turbo !!
    .. apologize..
    cheer's
    bertho
     
  9. Petros
    Joined: Oct 2007
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    Petros Senior Member

    Thanks,

    I have seen those devices but did not know what they were called. I have owned almost 20 boats, but none had motors (sail, paddle or oar powered). I know a lot about engines, I have even worked as an automotive engineer for several very large companies, and worked for a professional auto racing team. I have never seen a calculation for back pressure vs. power loss. So unless you find something that a manufacturer puts out based on testing, not sure there is a mathematical way to express it accurately. Perhaps a simple power consumption estimate will give you an indication.

    The whole "back pressure" thing is usually misunderstood anyway, one of my pet peeves. Properly designed exhaust systems will have exhaust pressure below atmospheric pressures because of the system dynamics, presumably the manufacturer of the water lock knows how to take advantage of this and can design for minimal loss.
     
  10. Luc Vernet
    Joined: May 2004
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    Luc Vernet Senior N.A.

    A very easy way to control the backpressure of a waterlift muffler (or ANY water injected exhaust) is to fit a three-way valve on the water injection pipe, one oultlet sending the water to the exhaust, the other to outside of the boat via separate fitting, OR right on the exhaust outlet which has the advantage of cooling this outlet. The more water goes into the muffler, the more silent but the higher the pressure. The less water: opposite effect.
     

  11. Frosty

    Frosty Previous Member

    Thats why turbo power boats dont use locks or dump buckets. Exhausts are kept free and open hence they tend to be a bit raspy.
     
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