Replacing exhaust elbows/mixers

[sd1209]

Volvo TAMD63's......replacing both 5" exhaust elbows/mixers where the outgoing saltwater mixes with the exhaust .There is a smaller elbow for the saltwater that screws into the top of the elbow......Question: what is recommended to apply to the threads on the smaller elbow where it screws into the larger mixer/elbow??

 

[READY]

Probably not helpful but... those 2 metals look very different. Should they be electrically isolated with some type of dielectric fitting?

 

[TJM]

I have used high temperature thread sealant and call it a day. The design is such that there WILL BE metal to metal contact. This topic could spiral into many paths, you are correct isolation is best, but I would not trust any non metallic fitting to the thermal cycles, vibration, and salt water. The elbow might be cast iron and the fitting bronze? Both pretty hardy in salt water. The design is a threaded joint.

 

[sd1209]

Thanks both of you for the input so far.....The picture is generic, taken off the website where I just bought the pieces (HDI Marine in Seattle).....The original fittings, which I believe are original and 23 yrs old, were cast iron and bronze smaller elbow......I am reusing the bronze fittings, they are really like new and show no wear whatsoever.....The only corrosion on the larger elbows is around the 5" opening on the downstream side...they were very badly corroded...interestingly, some corrosion was visible with it all hooked up but until I got it apart and on the bench the extent of corrosion was not visible....they may have lasted another couple of years but they needed replacing....not cheap either.....

 

[TJM]

HDI marine typically uses cast 316 alloy. What did they say to use? I replaced my wet exhaust elbow on my Northern Lights genny with theirs a few years ago.

 

[Tired Trawler]

You can use any good quality non hardening thread sealant, under no circumstance use teflon tape.

 

[Northern Spy]

You can use any good quality non hardening thread sealant, under no circumstance use teflon tape.

Why no Teflon tape?

 

[Tired Trawler]

Teflon tape is so slippery it allows you to over torque the fitting, in the past I have cracked several female fittings with the torque I would use normally on a npt fitting coated with thread sealant. Expensive lessons!

 

[Bmarler]

I will use a combination approach on threaded fittings. I’ll use one wrap of Teflon tape (leaving the end thread bare), then use a good quality liquid sealant over that. Like loctite 545 or maybe gasoila.

 

[Northern Spy]

I will use a combination approach on threaded fittings. I’ll use one wrap of Teflon tape (leaving the end thread bare), then use a good quality liquid sealant over that. Like loctite 545 or maybe gasoila.

Yup. Pretty common in the trades. Well, maybe two wraps, depending on the tape thickness.

Might not be the appropriate engineering specification, but I really haven't seen many failures with this method in 30 years of different industry.

That said, the nipple into an exhaust elbow is pretty benign in application.

Overtightening a NPT fitting into a casting certainly has the risk of cracking the cast piece if you sink the taper too deep.

 

[sd1209]

I spoke to someone at HDI and that person said all they recommend in most applications is sealant around the threads....I got a small tube of high temp thread sealant and applied that.....Reinstalled both new elbows, no leaks either exhaust or seawater.....Recommendations above all appreciated.....

Lesson learned on this one: 95% of the rust/damage to the original elbows was unable to be seen until the elbow was removed, or at least the downstream end, which had the most damage/wear, was exposed to view......Many of the openings where the seawater and exhaust mix were corroded closed....Never ran hot temps though on the gauges....