Steam Escaping from Drip Pan Elbow Gap
Steam Escaping from Drip Pan Elbow Gap
(OP)
I know this question is difficult to quantify, but how much steam should escape from the air gap on an drip pan elbow? I have a 6" valve rated for 23,468 lbs/hr that lifts at 25 psig and fills a small 25'x30' room with steam when it lifts. It is enough to set off the smoke alarm. It only serves a flash tank that harvests low pressure steam and sends it back into a 15 psi system. The problem occurs when there is a power outage or other issue on the load side of that low pressure system. The steam has no where to go, but the high pressure condensate entering the flash tank is coming from multiple sources. It keeps coming and there is no where for the low pressure steam to go except through the relief valve.
What is reasonable? The vent line is I believe a 10" that goes up probably 200 equivalent feet, including 7 stories up and out the roof. Could that be my problem? Too much back pressure?
More fundamentally, why do I need the gap? There is a drain on the bottom of the elbow to allow any condensate or rain water to drain out. If I didn't have the gap, there would never be any water in the pan itself. The connection through the roof is sealed, so no water is coming back down on the outside of the pipe.
Thanks for your help.
What is reasonable? The vent line is I believe a 10" that goes up probably 200 equivalent feet, including 7 stories up and out the roof. Could that be my problem? Too much back pressure?
More fundamentally, why do I need the gap? There is a drain on the bottom of the elbow to allow any condensate or rain water to drain out. If I didn't have the gap, there would never be any water in the pan itself. The connection through the roof is sealed, so no water is coming back down on the outside of the pipe.
Thanks for your help.





RE: Steam Escaping from Drip Pan Elbow Gap
Please give us a sketch - as close to scale as possible - of the side view of the inlet, gap, drip pan, drip pan drain, and the vent. Something is going on I can't visual.
RE: Steam Escaping from Drip Pan Elbow Gap
RE: Steam Escaping from Drip Pan Elbow Gap
How does steam set off a smoke alarm?
RE: Steam Escaping from Drip Pan Elbow Gap
RE: Steam Escaping from Drip Pan Elbow Gap
i have seen residual steam come out from that gap before, but not to the degree of causing other problems.
my first thought is possible restriction(s) in exhaust pipe. steam will take the path of least resistance.
something to think about, are there any restriction in exhaust pipe (i.e. wasp mud dobbers, dropped beer cans, other solid matter, etc.). can the 90° elbow 130' downstream with a drain have a boroscope inserted (no boroscoping w/ potential of relief valve open!) for inspection?
RE: Steam Escaping from Drip Pan Elbow Gap
The piping folks may have set this up with this internal pipe sleeve for piping flexibility reasons, and may be to minimise mechanical loads on the flash tank RV nozzle, due to expansion?
a) Is there a bird or bug screen on the vent exhaust?
b) Run a pressure / purge test on the vent and see if you develop any backpressure
c) How is steam prevented from escaping through this small low point drain line on the elbow? - where does this drain line terminate - in this room ? Is this line normally open?
Even after you find and fix this, would say this is not a safe design - in time, corrosion products from the vent pipe could block up the PSV exit.
RE: Steam Escaping from Drip Pan Elbow Gap
I think I would have blown down to a blowdown tank and then let that tank free vent, more than likely out the side of the building and not up 7 stories, but maybe they had their reasons.
Regards
StoneCold
RE: Steam Escaping from Drip Pan Elbow Gap
RE: Steam Escaping from Drip Pan Elbow Gap
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RE: Steam Escaping from Drip Pan Elbow Gap
"Nobody expects the Spanish Inquisition!"