Problem with exhaust cooling

Discussion in 'Inboards' started by seastar, May 20, 2013.

  1. seastar
    Joined: May 2013
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    seastar Junior Member

    Hi, I put two Mitsubishi 4D55T diesels (from mid 80s Ford pickups) in a 1947 Monk, formerly powered with straight 8 Chrysler gas pots. To see the beast check out my FB albums:
    https://www.facebook.com/media/set/?set=a.1029970270787.2005698.1271273247&type=1&l=4676b0af2b

    I used ZF 25 gear so I still have counter rotation at the props. I use the original keel coolers. I put a crank pulley on each motor with five A section slots. I have a 3/4" Jabsco to push the coolant around and it all works fine. I run a little 1/2" Jabsco off the collant pump to push raw water into the exhaust. The exhaust is dry and wrapped up to and over a gooseneck. Below the gooseneck is the water injection point as the pipe goes straight down. At the bottom of that drop, there's a metal 45 and a coupler into 3" stainless tubing leading 20 odd feet back to the rubber mufflers and then out of the boat in stainless.

    Problem: For some reason my exhaust raw water coolant pumps lose prime. I'm wondering if the exhaust pressure is too high for the pump's output. GlenL has a formula that says I only need a gallon and a half a minute, so that's how I built it, but then I tried upping the pump speed on one side, which worked well at the dock, but lost prime about an hour out under way. It's got unobstructed flow for sure, antisiphon as it comes into the boat. Any ideas:?:
     
  2. tom kane
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    tom kane Senior Member

    You can get pumps that have presure control on them by just turning a knob. No ideas about the lose of prime it just should not happen IF water flow is always available.
     
  3. CDK
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    CDK retired engineer

    The Jabsco pump can easily handle many times the exhaust pressure and even if it couldn't, that doesn't cause loss of prime.
    Therefore the cause must be on the intake side of the pump, like a small leak where air can enter or a thru hull that isn't always submerged at speed. Leakage along the shaft can also cause this if there is excessive belt tension.
     
  4. michael pierzga
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    michael pierzga Senior Member

    Those jabsco impellor pumps work best with positive water pressure. How far above the waterline is the pump located.

    At what speed are you loosing water...perhaps your thru hull is cavitating or pulling a vacume.

    How many devices are pulling water off your thru hull water supply...is one water user starving your jabsco
     
  5. seastar
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    seastar Junior Member

    Thanks to everyone for your replies. I have no other connections on the raw water through hull, one for each motor, total lift above water line to pumps is about one foot. In fact the same pumps (these are all rebuilt, new shafts and impellers) were used on the old straight eight Chryslers, so I just assumed the basics of the plan were good.
    This is a slow cruiser so the pickups are always in the water BUT, the suggestion I'm getting air from below is interesting because my depth sounder is often useless at speed (8 knots cruise) and I figure that's due to air passing by. It would be a real pain to relocate the pumps below water line but I know that's a consideration.
    I appreciate the mention of air leaking on the shafts under high belt tension. I checked them the other day and thought that I've got them tighter than is really needed. And with anti reverse flow valves on the through hulls, the only way the output can go dry is if the exhaust pressure was to push the water back to the pump and out on the shaft, but I have not such leakage. Air intrusion would seem most likely. Thanks, at least I've got some good ideas to follow up!
     
  6. michael pierzga
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    michael pierzga Senior Member

    I use essentially the same pump to feed sea water to a Cummins 4 cylinder diesel.

    No problems in 20 years of operation...but the pump is located below the waterline and always has positive pressure to it.
     
  7. tom kane
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    tom kane Senior Member

    If you have a pump with rubber impeller it may need replacing they may look OK but they can be deceving and I think should be replaced often. The biggest threat to a marine motor is loss of cooling fluid and can ruin a motor in seconds or start a fire from a water injected exhaust not being cooled.
    I prefer a radiator or heat exchanger cooled motor with raw water only used to suplement cooling when tempratures get high. that way not much chance of cooking a motor or damaging a motor from water regressing bach to motor.
     
  8. seastar
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    seastar Junior Member

    Tom,
    As I mentioned, the problem is pumping raw water to cool the exhaust piping from each riser in the engine room and aft through the boat and the pumps are rebuilt. The pumps are the same spec (operating with less lift) as was used with the original motors for 60 years.
    I agree with suggestions that I relocate the pump down at or below the waterline.
    Thanks
    Brian
     
  9. tom kane
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    tom kane Senior Member

    If I had your problem I could not find the cause unless I could do some tests. It may seem strange but after hundreds of years of using pumps it is not understood by many how pumps and siphons really work.
    Vacuums do not suck.. it is the atmospheric weight of the air that pushes the water into a pump or siphon. A pump or siphon displaces the air and the atmospheric pressure pushes the water into the pump. Your pump may not be able to pull in water because air is being entrained in the water. A scoop for the water may keep water flowing into the pump at all times.These are often fitted when the inlet to the pump is under the hull.
     
  10. seastar
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    seastar Junior Member

    Thanks, you're right about vacuum. I've done vacuum infusion fibreglass work so I understand the physics. I tried a priming loop and changed belts and pulleys to speed up the pumps and still nothing. The big deal is they lose prime while under way. But these pumps have been used and have been reliable in boats for years.
    Like I say, the setup is virtually identical to the original that fed the old straight 8s. During the refit I pulled out the through hulls, cleaned them, reblocked and bedded them, new ball valves, anti reverse flow valves and hoses. And when they prime, they pump. But underway, suddenly they're not pumping. There's no water leaking out so it could be some air getting in from under the boat, but the more I think about it, this layout worked before, so I'm going to try and figure a way to test the pumps. I've got a spare to start with.
     
  11. sdowney717
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    sdowney717 Senior Member

    if those pumps loose prime and stop pumping water the impellers burn up quickly.

    How are the end plates, worn out? I filled up one of my worn out end cover plates with a layer of lead solder.

    What do you have for intakes in the water? Scoops facing forward, or just thru hulls? I have the forward facing scoops.

    On my setup, I have rubber underwater exhaust extensions. I can not see the output water, so I ran a 1/4 inch polyethylene tube diverting a tiny part of the output from pump where it does the final dump into the oil and trans cooler. Now I can see water flow at the transom.
    My pumps are about 6 inches above the waterline, and no problems.
     
  12. seastar
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    seastar Junior Member

    Well like I said, this set-up is almost identical to the original Chrysler straight 8s, with the same scoops under water as before, less lift by about 10" and the only thing I think is different is that the injection is into 1.5" exhaust after the turbo, 6" below the gooseneck and at the bottom of the drop the exhaust splices into 3" stainless then turns to do the run to the transom (and mufllers just ahead of the transom). The original exhaust was 3" all the way. The pumps are original but rebuilt with new shafts and impellers. And I have a spare that I've swapped in and rotated around and nothing seems to consistently work or not work. They worked fine in the Chrysler set-up, even with intermittent use in the 10 years up to when I bought her. So it's not like the basic design is particularly different that original except as mentioned about the turbo and small diameter exhaust at injection but then again, these are tiny 2.3 litre motors. I haven't had time to get back to her but I will soon. I want to test the exhaust pressure at the injection point and the pump pressure.
     
  13. sdowney717
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    sdowney717 Senior Member

    The exhaust pressure should not be affecting the pump output at all.
    Diesels also don't run well with backpressure in the exhaust.

    If the pumps actually stop pumping they do destroy the rubber impellers if they start sucking air, they will run dry.
    I had a pump fail in less than a minute, rubber impeller massive destruction, engine overheated and I think it was cause a jelly fish got sucked in.

    Is your system splitting the pump output of cooling water many different ways?

    So that perhaps more water is flowing out one way and much less another being diverted or blocked by as your saying exhaust pressure?

    Does your exhaust lines from engines run downhill all the way to the exit at the transom, or does it run uphill with some low spots? Or is it mostly flat which might cause the back pressure to rise, tubes filling up with water.
     
  14. seastar
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    seastar Junior Member

    I appreciate anyone's help but please read the thread first before asking questions that have been clearly answered earlier. I have clearly detailed the construction of the system already.

    Each motor system is independent of the other including exhaust cooling. The raw water pump on each side simply has one inlet and one outlet. The exhaust is well constructed of Sch 40 steel, from turbo to the riser or gooseneck, then a vertical fall past the injection point, then into 3" stainless, then a bend to the main run that is a smooth consistent fall to the marine mufflers and then heavy wall stainless out the transom with anti splash flaps on the tips.

    There IS some pressure in an exhaust system especially from a turbo diesel. Motors do pump a lot of air, just put your hand close your car's tailpipe and have someone hit the gas to half throttle, then tell me there's no pressure. Water injection into the exhaust requires water pressure greater than the exhaust pressure or the pump will cavitate.

    The Jabsco spec explains how to adjust the shaft nut to allow a bit of dripping, therefore when the pumps are off, the system can drain half of the pump and anything uphill - it needs to prime. If you know hydraulics, you know the fundamental rule that pumps pump volume not pressure. Pressure is a form of friction. Pumps are only capable of pumping volume against some resistance measurable as pressure. When the pressure is too great, the pump can still be primed and just cavitate, or it could lose prime altogether if the fluid can be pushed backwards in the system.

    I have had the injection line disconnected when the pumps are pumping, and there appears to be substantial water pressure but I haven't measured it. Often the pumps work fine at the dock and then one or other (but often not both) appear to lose prime later. I've then disconnected the injection line for the side that's getting hot, restarted the motor and confirmed at that point that the pump isn't pushing water at all. I cannot prime the things properly while underway, but if I prime them at the dock, they run fine. And I can come back days or weeks later and at the dock, they run fine. Then I go out and one or other, once in a while both, with stop pumping.

    I have pulled one of the pumps apart and it looked fine inside and when I prime them, they do pump but not consistently underway. There was no indication of wear. The comment about these pumps burning up impellers is contrary to everything everyone has ever told me. They would not work fine sometimes and sometimes not if they were toast. If I prime them pressure test them and the exhaust, then I'll know more.

    Perhaps the pressure of the exhaust is enough to cause the pumps to stall, especially when the turbos really spool up. In heavy weather with the boat pitching for and aft, I don't have this problem. Probably the boat pitching down causes some push of water into the through hulls. I've swapped in a spare rebuilt pump with no particular change in the intermittent performance.

    The main run of the exhaust on either side (after injection and the bend at the bottom of the drop from the riser or gooseneck) runs at about 100 degrees F most of the time at half to 3/4 throttle. But at times with the motors only running half throttle the temps run up to about 140 at times while the ambient air and water temps are not elevated.
    I originally was running the pulley drive ratio to pump 2 gallons per minute (according to GlenL marine conversion of engines guide to exhaust cool ratio) and it worked fine at times. I recently changed one pump to run more than double that and initially it pumped very well at the dock and the pipe was obviously cooler, running about 75 F but an hour out it suddenly increased to match the other side. Of course as I said, I cannot see if it is pumping while underway but the temps would indicate that it quit.

    It is ultimately frustrating that this system is basically the same as the original 60+ year old system, only every hose connection is new and the pumps are rebuilt. The lift is about 10", which is probably 6 or 8" less than the original design and the output is about 10" more than before. The exhaust injection point on the old system was into a pipe substantially larger in diameter (about 3" for a very slow rev'ing 320 cu in motor), so perhaps the pressure in the smaller pipe (1.5") with a 140 cu in turbo diesel running about the same rpm could be a factor.

    One writer suggested wisely that there may be air under the boat in sufficient quantity to upset flow. I know my sounder does not perform well at even 5 knots, probably due to air bubbles flowing across the transducer. Again though I have to question, why was this not a problem before?

    I'll put in a section of clear tube and a nylon ribbon telltale in the injection line, so I can confirm flow. I've tried a priming loop on one side but it didn't appear to help but like I say, I'm interpreting temps as a direct indication of water flow which may be misleading.

    I'll try to also install a priming tank with on/off valves feeding to the pumps. When either pump starts, it should then refill the tank before I shut the tank off. This would at least give me assurance that the pumps are primed and pushing water at the outset.

    It should not be necessary to do as suggested earlier and put the pumps down closer to or below the waterline, though it seems like a good idea. The original system worked fine and it had greater lift and was very poorly maintained. A pump installed below the water line would only be safe if I shut of the raw water through hull valves whenever the boat was unattended. As it is now, the valves can be left on because they are tight piped to the pumps above the water line.

    I'll plug the injection point with a pressure gauge to get some idea of what typical exhaust pressures are (not just at idle) and then do likewise on the pump lines to find out the pressure capability of the pumps. At the same time I can check the pumps for any sign of leaks at the shaft.
     

  15. sdowney717
    Joined: Nov 2010
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    sdowney717 Senior Member

    Huh?

    If the pumps run out of inlet water like some people are saying loosing prime, then the rubber impellers fail. You say the impellers are fine, so they must not be running out of water.

    Rubber impeller pump has vanes. They are positive displacement pumps.
    If the pressure is too great, the vanes will flex and they wont pump any water out. Essentially just spinning in water. So then the impellers will look ok.

    Even impellers that have taken a set can compromise water pumping with bent vanes. However on my boat that does not stop then from functioning.

    Internal friction of vane against housing when the water is not there will overheat the rubber and it disintegrates quickly, that is my experience. I have read about 10 seconds then damage if the water runs out, not available to pump.

    http://www.westmarine.com/webapp/wc...1&storeId=11151&page=Impeller-Replacement-101

    http://seagrant.uaf.edu/bookstore/boatkeeper/water-pumps.pdf
     
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