Steam Escaping from Drip Pan Elbow Gap

[BronYrAur]

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.

 

[racookpe1978]

Hmmmn.

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.

 

[Compositepro]

Your pipe will fill with refluxing condensate before any steam gets out the end. Water can't drain out the bottom of the line as steam is going the other way.

 

[georgeverghese]

Problem description is poor
How does steam set off a smoke alarm?

 

[BronYrAur]

I don't know the inner workings of smoke detectors, but the presence of steam is making them go off. I am not always on site to make any detailed sketched, but here is a rough diagram to attempt to illustrate.

 

[pmover]

without analyzing the relief valve data . . .

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?

 

[georgeverghese]

Ran a quick check on this steam flow, assuming that the flash steam is at 80-100degC or so; even at full rate with 24000lb/hr through 200ft of pipe, backpressure on PSV exit is approx 15kpa, which arguably isnt much for this narrow internal sleeve to external pipe 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.

 

[StoneCold]

What about covering the gap with a rubber joint so the steam is forced out but the pipe can still grow as it is heated.
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

 

[Duwe6]

Problem is pretty obvious -- you are blasting steam & flashing condensate into 200-ft of pipe at slightly over 25 psi. Why are you surprised when the back-pressure of such a long run of exhaust pipe causes some steam to escape the pan gap? A drip pan is supposed to develop a venturi, and sweep out the steam/condensate. To use one, your pipe needs to be a LOT shorter or a LOT bigger.

 

[macmet]

I agree with Duwe6. For proper function of the vent pipe, there needs to be a venturi effect that can help suck the steam up and outside. Because of your lengths, that may be a problem for you, but I would not recommend wrapping around the gap.
"

 

[davefitz]

Prevention of blowback in the drip pan is the basis of the design rules in ASME B31.1 appendix II (non-mandatory) . Basically, the occurrence of blowback implies the outlet stack diameter ( from drip pan to atmosphere) is too small for the flow resistance that occurs( elbows, length ).

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