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Replacement Exhaust Elbow


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#11 Island Time

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Posted 20 August 2019 - 05:00 PM

A minute of research shows diesel combustion temps around 650 deg C. And this;

 

 

9849885c7eda9a8d54764fe5028521360e57ad57

 

Construction Of Exhaust Risers

The risers are connected at the middle or aft of the manifolds at one end and to the exhaust hose at the other end. Basically marine engine risers are in shape of conventional pipes; however, in order to fight the heat of the engine exhaust and also to cool it, the risers have two layers one inside the other, for the passing of cooling water. The engine exhaust passes through the inner layer, and outer layer surrounding it acts as water-filled compartment or water jacket. The water jacket lasts only uptil the edge of the rise and does not continue into the exhaust hose. The water compartment of the riser ensures that the engine exhaust cools down completely before it reaches the exhaust hose so that the later doesn’t get damaged due to high temperature of the exhaust.

Marine Engine Exhaust Riser & Exhaust Manifolds: What it is and its Failure
Introduction

Marine engine exhaust riser is an important part of any marine engine exhaust system, whether big or small. Attached to the exhaust manifolds of the engine cylinders, exhaust risers act as the connection between the cylinder manifold and exhaust hose. The risers, sometimes also known as elbows mainly because of their horseshoe shape, are located near the top and along the side of the engine. Both the manifold and riser are usually made from cast iron and are painted on the outside to prevent corrosion.

The number of risers and manifolds in a system depends on the type of marine engine. Generally, the number of risers and manifold are equal in all types of marine engines. Moreover, all the risers are water cooled to prevent the effect of high temperature exhaust gases and also to cool it. Thus, the exhaust risers performs two main functions in the engine’s exhaust system – It provides the cooling water for the engine exhaust cooling and also prevents cooling water from entering the engine through the exhaust manifold.

 

9849885c7eda9a8d54764fe5028521360e57ad57

 

Construction Of Exhaust Risers

The risers are connected at the middle or aft of the manifolds at one end and to the exhaust hose at the other end. Basically marine engine risers are in shape of conventional pipes; however, in order to fight the heat of the engine exhaust and also to cool it, the risers have two layers one inside the other, for the passing of cooling water. The engine exhaust passes through the inner layer, and outer layer surrounding it acts as water-filled compartment or water jacket. The water jacket lasts only uptil the edge of the rise and does not continue into the exhaust hose. The water compartment of the riser ensures that the engine exhaust cools down completely before it reaches the exhaust hose so that the later doesn’t get damaged due to high temperature of the exhaust.


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#12 Island Time

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Posted 20 August 2019 - 05:02 PM

So the water injection normally hits an internal pipe, cooling it and the gases before they actually mix....


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#13 Cazzate

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Posted 20 August 2019 - 05:28 PM

Beautifully welded up frank but I agree with IT . Your original elbow had the water injection cooling the first bend before entering the exhaust . You could slice in half a slightly larger diameter of pipe than that injection pipe and weld it on at the first bend to roughly copy the wee jacket on your old elbow
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#14 BOIGuy

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Posted 20 August 2019 - 06:02 PM

I have made a few of these in the past, some pretty complex, some just a pipe into an elbow. You definitely want to inject the water further back, it needs to be well mixed (the arrangement is referred to as a mixer in some documents) before it hits the hose.

Inserting the tube into your first elbow after the flange and pointing down the pipe a couple of inches would be good, pays to have something to encourage the mixing, maybe even a solid bar the water hits just after injection.

If you have room you could add a length of pipe to what you have made, cut your tube off back to the elbow and put a deflector of some sort in front of it, the pipe will get very hot but will handle it, you may want some insulation on the metal bits to stop heat affecting nearby parts.

Sorry I didn't say anything sooner, I saw your picture of the parts before welding and assumed you were going to insert the tube vertically down through the first elbow with the tube finishing along the first nipple, was pretty impressed with the simplicity at that stage.


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#15 Frank

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Posted 20 August 2019 - 07:59 PM

Dang ! but all good comments  thanks ! and I cant lay claim to the welding it was done by a mate and he is a pro !.

 

If you look carefully (it is not obvious) at the sectioned part the water is injected at the first elbow but then travels down an internal gallery and mixes at the last elbow. So while I guess it provides some overall cooling its probably not very effective.

 

It makes sense however that the water is  deflected downwards by the elbow geometry thus spreading/forcing it into the gas flow.

 

I will talk it over with my welder mate but I think if I trim the existing pipe flush with the internal wall and add a horizontal deflector bar say 3 x 10 mm then I would probably get away with it, particularly if I extend with another socket and nipple as BOI mentioned. Other wise I will mod it  and fabricate something to enter at the first elbow perhaps extending into the first nipple as mentioned. If I do that I might machine a two piece bolted standoff which  mimics the OEM setup. More updates to follow !


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#16 Fish

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Posted 20 August 2019 - 08:17 PM

If you are shifting the injection point back up, remember to ensure there is no risk of the cooling water going back the wrong way toward the engine...

Fixing a hydra-lock up will be more expensive than buying a genuine elbow.


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#17 Frank

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Posted 21 August 2019 - 04:39 AM

Yes that was in the back of my mind as well which is why I might go with a mod to the as-welded set up, it will be closest to the original design. 


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#18 Frank

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Posted 21 August 2019 - 08:01 PM

Attached File  20190821_185023.jpg   1.32MB   0 downloadsAttached File  20190821_184948.jpg   1023.91KB   0 downloadsOK , I have trimmed the pipe right back and added an extension piece and deflector plate, I reckon it should work fine after final welding.


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#19 Jason128

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Posted 21 August 2019 - 08:17 PM

Sorry to be blunt, but I would not want this near my boat.

If you look at the original, you have a long length that runs in the casting parallel. This cools the casting, before the water is eventually injected into the exhaust.

I doubt you will keep the end of that pipe at below 100 degrees where your hose is connected on.
If I’m wrong, and you do, you will have a say 300 degree pipe cooling to under 100 over the length of inch. This will lead to fatigue cracking in stainless.

I would be nervous this will fail, and of course at the worst possible time
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#20 Frank

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Posted 21 August 2019 - 09:44 PM

There is some irony in your comments Jason because all three OEM elbows lasted about three seasons and then failed by cracking ! they also clogged up internally and rusted from the inside out.

When they cracked they leaked hot seawater on to the back of the engine causing yet more corrosion ! 

They were low quality sand castings so the metal was porous-  not helpful. True the top part of the elbow was cooled but the lower  part  still got very hot.

 

The elbows are investment cast SS my money says they will outlast the cast iron by a factor of two but I accept they will eventually fail nothing lasts forever. The metal is ductile enough that I don't expect a failure to be catastrophic, we'll see.

 

I have a laser thermometer so I will report back on the temperature gradient,  ;-)


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