F&T Trap sizing on steam main

Gateacre · December 16

Hello everyone. The photo attached is of the end of a 4" steam main (roughly 70ft long) in a 130 year old commercial property in eastern Canada. The trap is a 3/4" Dunham 30-2A that's been there for many decades and been subject to catastrophic water hammer caused by terrible piping on our previous boiler and I suspect the internals have had it.

This was going to be my first attempt at rebuilding an F&T trap (there are more to do) but I'm now wondering if it's actually undersized. I know Dan cautions against "line sizing" these traps but dropping from 4" to 3/4" seems extreme.

Thoughts?

delcrossv · December 16

Probably not, it's passing condensate.

You'll want to get rid of the arch in your flex hose

Jamie Hall · December 17

Probably not undersize — or at worst, one size too small. As @delcrossv says, you are only passing condensate, and people wildly overestimate the volume of condensate in a steam system. For example. Suppose that that steam main is handling 200,000 BTUh, and all of the condensate resulting needs to go through that one trap (which is very unlikely — most of it is likely dripped elsewhere). Even if all of it goes through that trap, that's only half a gallon per minute.

EBEBRATT-Ed · December 17

The weigh to properly size an end of the main drip is something most people do not know how to do and they usually install traps far larger than needed.

You measure the length of the main and accounting for any reductions in pipe size you add up the WEIGHT of all the pipe, valves and fittings. Then you take the specific weigh of the steel pipe and knowing the weight and specific heat you calculate how much condensate will be formed bringing the steel pipe from say 50 degrees to the steam temp (212) or whatever, Then when you get the condensate volume from that you apply a 1.5 safety factor to that.

In most cases a 3/4 F&T is more than large enough.

Alot of peopel put on a steam trap as large as the drip leg which makes it 4x the size it needs to be and wire drawing will shorten the trap life.

Gateacre · December 17

https://forum.heatinghelp.com/discussion/comment/1827295#Comment_1827295

That makes alot of sense. Thankfully, it seems I can replace like with like and avoid the calculations, but I appreciate the information.

Cheers

Gateacre · December 17

In fact nearly all the branch lines to radiators are at the end of this long main. It's like a palm tree. It's an odd old cluster of buildings. There's also an inverted bucket trap about 2/3rds of the way along the line where the pipe steps up about a foot in elevation, so between the two, I think we'll be ok.

Thanks

dopey27177 · December 17

The 3/4" trap is ok. if you size up to 1" it has the same capacity as the 3/4 trap.

The best way to increase the capacity of the trap is to drop the trap to 12 inches below the steam main.

the steam pressure from the steam main will push the condensate into the return line!

Typically the element in the vent side of the trap blows out first.

check the condensate pump or boiler feed pump and see if is pumping out enough condensate water to keep the return line dry.

Jake

(dopey27177@aol.com)

F&T Trap sizing on steam main

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