Frank 157 Posted August 14, 2019 Share Posted August 14, 2019 Hi All, any guidance appreciated ! So the cast iron exhaust elbow on my Volvo MD7B has rusted out, no surprises there as they only last about four seasons. I have been quoted $800 for a replacement so I am looking at fabricating something in Stainless Steel. I would appreciate any input from someone who has done this, for instance what grade of Stainless would you recommend ? I have sliced it in half to show the inner geometry, my thoughts were to use to fabricate the unit using cast butt weld elbows from the Anzor catalogue with some suitable straight pipe in the middle. From there I would weld a smaller dia pipe in top to inject the cooling water. Thoughts ? chrs Frank Quote Link to post Share on other sites
Island Time 1,211 Posted August 14, 2019 Share Posted August 14, 2019 Like this? https://www.boatsandoutboards.co.uk/Engine-Spares-for-sale/volvo-penta-md6-stainless-steel-exhaust-elbow-md7-stainless-/1563 Stainless often has similar life to cast iron. It does not like hot salt water.... Or this? https://www.frenchmarine.com/product/Volvo-Penta-833998-Exhaust-Elbow-921-172 Quote Link to post Share on other sites
wheels 543 Posted August 14, 2019 Share Posted August 14, 2019 316 is best as it has the greatest resistance to Chloride (Salt) and yes, the cast is best as 316 is prone to cracking from vibration and heating/cooling. So a decent wall thickness is needed, good welds and decent thickness for flanges. I have used SST for the same use and it has lasted several years. Quote Link to post Share on other sites
Frank 157 Posted August 14, 2019 Author Share Posted August 14, 2019 Thanks Guys and good spotting Matt ! The UK SS version seems like a good fit, I will make enquiries. Quote Link to post Share on other sites
Frank 157 Posted August 15, 2019 Author Share Posted August 15, 2019 Game on Quote Link to post Share on other sites
Frank 157 Posted August 18, 2019 Author Share Posted August 18, 2019 It will be welded by Mon, the bits cost $80 at ANZOR, I will post another Pic. Quote Link to post Share on other sites
Ex Machina 365 Posted August 18, 2019 Share Posted August 18, 2019 Nice Scottish Jandals mate ! Quote Link to post Share on other sites
Frank 157 Posted August 20, 2019 Author Share Posted August 20, 2019 The Jandals are a special type for welding, here it is all welded up, Im experimenting with injecting the hot seawater down stream from the exhaust elbow. The specs of the exhaust hose say it is good up to 100 deg c so we shall see. Quote Link to post Share on other sites
Island Time 1,211 Posted August 20, 2019 Share Posted August 20, 2019 IMO, that wont work. The exhaust will be well over 100c until AFTER the water injection, which is why the injection is normally above the rubber pipe connection. However, I've never measured it on a diesel, so I dont know what it ought to be... 1 Quote Link to post Share on other sites
Island Time 1,211 Posted August 20, 2019 Share Posted August 20, 2019 A minute of research shows diesel combustion temps around 650 deg C. And this; Construction Of Exhaust RisersThe 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 Marine Machinery, Engines & Controls / By Raunekk / Marine Engineering 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. 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. Quote Link to post Share on other sites
Island Time 1,211 Posted August 20, 2019 Share Posted August 20, 2019 So the water injection normally hits an internal pipe, cooling it and the gases before they actually mix.... Quote Link to post Share on other sites
Ex Machina 365 Posted August 20, 2019 Share Posted August 20, 2019 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 Quote Link to post Share on other sites
Cantab 341 Posted August 20, 2019 Share Posted August 20, 2019 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. Quote Link to post Share on other sites
Frank 157 Posted August 20, 2019 Author Share Posted August 20, 2019 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 ! Quote Link to post Share on other sites
Fish 0 Posted August 20, 2019 Share Posted August 20, 2019 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. Quote Link to post Share on other sites
Frank 157 Posted August 20, 2019 Author Share Posted August 20, 2019 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. Quote Link to post Share on other sites
Frank 157 Posted August 21, 2019 Author Share Posted August 21, 2019 OK , I have trimmed the pipe right back and added an extension piece and deflector plate, I reckon it should work fine after final welding. Quote Link to post Share on other sites
Jason128 6 Posted August 21, 2019 Share Posted August 21, 2019 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 Quote Link to post Share on other sites
Frank 157 Posted August 21, 2019 Author Share Posted August 21, 2019 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, 1 Quote Link to post Share on other sites
Steve Pope 243 Posted August 21, 2019 Share Posted August 21, 2019 Schedule 80 black pipe fittings will cope with the hot / cold much better that the shiny SS and will generally last considerably longer, Hi temp paint (2 stroke expansion chamber paint) will look after the appearance. Quote Link to post Share on other sites
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