PVC Exhaust

A friend is fitting 2 High RPM engines with big turbos kicking out 250HP each.

It is a power boat so it is likely the engines will be running at least 80% all the time.

He is making his exhaust from 90mm PVC from the hardware shop, elbows and joins as you do.

Very light weight in comparison to a stainless steel job and needs minimal support, no dump box or mufflers straight down a foot then up and out.

 

Good luck with that. PVC has a very low melting point, it will not last very long if we are talking about typical hardware store low pressure drain pipe.

 

Your mate might make abetter job of it by molding the tubes in vinylester resin & glass.

 

Yes PVC pipe from the hardware shop but thickest at about 3mm wall.

Hot water from the cooling system is injected into the exhaust!!!

 

PVC !!!!!!!!!!!!!!!!!!!!!!!!


It gets soft very fast with heat. The classic way a building contractor bends a pvc pipe in the field is to take the tube and jam it over the exhaust pipe of his pickup truck. Fire up the truck engine, let idle for a few minutes and force hot exhaust thru the pvc pipe. presto...pipe is flexible like rubber.

Good trick to remember if you need to bend a pvc pipe and your heat gun is kaput

 

Schedule 30 PVC is rated for 140°F. It melts at 176°F and bends lower than that.

 

So if the water thats injected keeps the PVC lower than that it will be ok

I do believe water can not exceed that temp therefore 100 is the hottest it can get.

 

Heat distortion in PVC occurs at 158 degrees and at 230 for CPVC. If he uses CPVC, he has a chance it might stay together, but it needs to be well supported or it'll sag badly. PVC will not work with most engines as the thermostat will open at 150 to 160 degrees. PVC will start to sag at about 130 degrees. He will also need to use CPVC cleaner and cement, or the pipe will hold, but the joints will soften and leak. I'll bet the CPVC system will still fail, because when you shut of the engine, the water flow stops, but the head will still be hotter than an ugly girl on prom night after 5 beers.

 

This is 3mm wall blue PVC 90mm from the hardware shop meant for toilet soil piping.

It is PVC cemented together with 90 elbows.

There is 6 inch of 85mm stainless pipe from the turbo mixer inserted into the PVC.

The engine will be running at 3800 constant. There is no water injection relief ---all injected water goes out the wet exhaust

I dont know what CPVC is.

 

CPVC was invented to address the temperature issues found in hot water supply lines. Most residential hot water systems are about 125 degrees, but they can be turned up to about 140, which is over the practical limit for PVC. There was a time before CPVC and it broke at the joints and deforemed pipes regularly enough for CPVC to be developed.

The stuff your buddy is using is rated at 140 degrees, but will likely fail before this temperature, with sufficient exposure. This pipe (the blue stuff) is AWWA C905 compliant and intended for potable water and waste lines, in under ground applications.

I don't think it will tolerate a steel head pipe for long (minutes) and I would strongly recommend a regular exhaust pipe used as an isolation (fairly short length of reinforced rubber exhaust hose) piece before the plastic pipe. Naturally, CPVC is the only commonly available pipe that will comply, though polybutylene (PB) or crosslinked polyethylene (PEX) will both take sufficient temperatures to hold for a while.

 

As a practical matter I don't see how your friend can justify making his exhaust system out of plastic pipe. I congratulate him for thinking out of the box, but he's failed to recognize a few basic problems inherent in his design.

1) With the engines at idle his cooling system will not be providing enough water flow to keep the upper portions of the plastic pipe immersed. The hot exhaust gases at the top of the pipe will simply melt his exhaust system.

2)90* elbows commonly used for pvc pipe have too short a radius. This creates a high pressure area where the exhaust gases and cooling water are forced to change direction. Joint failure would be a major concern in at least four points in this design.

3) Elbows are also notorious hot spots caused by friction and the disruption in water flow. So not only do you have the potential for the joint to fail on the upstream side, but the elbow itself can fail just aft of the bend.

4) In the event your friends cooling system fails, for any reason, he will have an instant and catastrophic failure of a second major component onboard his boat.

Not only is he courting disaster by failing to build his exhaust system using suitable material, he's giving himself less then a razors edge of a safety margin should ANYTHING malfunction. A loose or worn water pump belt, an obstruction covering the water intake, a worn water pump impeller, almost anything including normal operations could lead to serious problems.

Your friend would be better off building his exhaust system out of black iron pipe and exhaust hose, at least then he might be able to make it out of the harbor.

MM

 

Wise words Midnitmike, as you say it's not just about the water, those exhaust gases are hot. Failed exhaust systems can be fatal.

 

For comparision standard marine flexible exhaust hose is designed to operate between -30° and + 100° C, with brief peak temperatures of 115° C


http://www.pyacht.com/vetus-exhaust-hose.htm

 

Exactly--so rubber hose is less likely to be successful than PVC even though it is sold for the job.

Could it be possible that is some strange way it could be that you are all wrong.

Surely you engineers know that water can not get hotter than 100.

 

Frosty,
I may not be an engineer but I do know the exhaust gases in that system get a heck of a lot hotter then a 100 Celsius, and that is what your friend should worry about. Sure you can build this exhaust system out of plastic pipe, but you're courting disaster if you do. I've bulit plenty of wet exhaust using heat resistant glass tubing, and it works fine. So too do those built using copper pipe, and even iron pipe when joined together with exhaust hose.

In your case the entire system is a weak link with almost no ability to absorb excess heat except via sustained water flow from water injection. In the event the cooling system experiences any malfunction of almost any kind the entire exhaust system is in jeopardy of a complete meltdown.

Personally I would NEVER let a friend actually build something like this..not if I wanted to have him around for a while. You and your friend can do what you want, but when it falls apart I'd rather not have to say I told you so.

MM