flash tank vent blowing water 1

[mfi2000]

Hello all, first time post here. We have a flash tank next to our main steam lines in the subcellar. The tank is being fed from 2 condensate line off the steam legs. The tank has a vent going up to the 3rd floor into a plenum. The tank is supposed to drain into a trap, past a check valve, and into a condensate pump setup. The vent on the 3rd floor blows a ton of water out, filling up the plenum. We changed the check valve, and had the trap tested. I still believe that the trap may be bad. What could cause this condition. The vent and the drain line are connected. My thinking is if the trap does not open, the pressure is causing the condensate to blow up the vent.

 

[katmar]

mfi2000,

What are the relative heights of the flash tank, the trap and check valve, and the condensate pump? As you are working in a cellar it sounds as though there may simply not be enough head to drive the condensate through the trap and check valve. In order to build up the required driving force the level in the flash tank has to rise, and if it rises into the vent pipe the flash steam will carry the condensate over like an air pump.

The most important question to ask in this type of problem is "was this problem always there?". If the problem was not always there, and only started occurring some time after the plant started up, it tells you that the original design and equipment selection was good, and obviously something has stopped working the way it was used to work. Also ask "was anything changed at the time the problem was first observed".

regards
Harvey

 

[jherbert]

A diagram would be nice, however a few observations:

All the steam trapping should be carried before the flash vessel, primarily to prevent the flash vessel becoming a steam space which in turn prevents condensation cooling (necessary to avoid pump damage)

When was the trap added?

do this happen with/without the condensate pump operating?

water exiting the vent is a big problem in my view, it would indicate that the vessel is over-pressure, and that would contravenes H&S regs in most countries.

I'd consider getting a good steam engineer to take a look ASAP.

cheers,
John

 

[TBP]

Troubleshooting situations like this can be a bit of an Easter egg hunt. The actual source of the problem, and spot where the problem shows-up, are often some distance apart.

Has the condensate tank become pressurized for some reason, so the trap has no pressure differential across it? How about the traps on the drip legs that feed the flash tank - do you have one or both of them blowing through, sending far more steam than normal to the load off the flash tank vent? From your description, the traps feeding this rig are just a couple of drip traps - they shouldn't be sending much steam to the flash system. If they are, why? Other drip traps failed somewhere else, or big chunk of insulation missing, so there's a load of condensate showing up at these traps that wasn't there before?

Assuming the traps have valves & unions in the right places, valve out the traps one at a time, and break the downstream union. Carefully open the upstream valve, and see what happens. This is the absolutely dead-sure way to verify failed traps - whether failed open, or closed. If you know that the trap works when discharging to atmosphere, but doesn't seem to get rid of the condensate when it's fully valved-in, that suggests back-pressure in the condensate system that wasn't there before. Why?

 

[mfi2000]

Hello again, back at work. I'm going to have to open the main trap up sometime to see if it is working. The plant is going full blast, too hot to do it now. I asked the senior engineer about this and he said it has been happening for some time. I will also put a gauge on the tank. I'm wondering if the vent is filling up with so much water that it is causing the tank to pressurize, enough pressure builds up then blows the water out. It comes out with pretty good force. The check valve was just replaced, seems good. I need to get the trap report which was just done. I just shut the drains and will try to catch it when it goes off. Too bad the vent is 4 floors above the system.
As to what katmar wrote. The pumps do not feed a trapped and checked line, only a line that goes next door to the sumps. This problem has to be caused by something before the check. Something is pressurizing the tank, need a gauge to see how much. More to follow.

 

[katmar]

Hi mfi2000,

Thanks for the extra information and feedback. I did not think that the pumps fed into a trapped and checked line. In your first post you wrote "The tank is supposed to drain into a trap, past a check valve, and into a condensate pump setup". From this, I took it that the condensate flows from the flash tank through a trap and a check valve to the suction of the pump. From your latest post I see that the pumps deliver the condensate to a sump on the same level. Is this correct?

If my understanding of your setup is correct, it is a rather unusual and potentially problematic arrangement. Are these centrifugal pumps? You should generally not have traps and check valves on the suction line into a centrifugal pump. The biggest problem with using centrifugal pumps for condensate is getting enough NPSH. Additional fittings like traps and check valves will only make this problem worse. Do you get cavitation in these pumps?

How do you control the pumps? A common method in this type of installation is to have a level detector on the tank which controls a valve on the DISCHARGE of the pump. You have not mentioned control at all, and reading between the lines it looks like the designer was trying to use the trap in place of a proper control system.

The advice that TBP gave about checking the traps which feed into the condensate flash tank was right on the money. The flash tank would have been designed to allow the flash steam to disengage from the condensate at a given flowrate, and if you have excess steam (and therefore higher velocities than expected) it will carry the condensate with it up the tank and into the vent.

regards
Harvey

 

[UtilityLouie]

Maybe I don't understand the problem - but is your vent undersized?

I've had problems with high velocities in the vent blowing water that is condensing on the sides of the pipe out the top. You may not even be able to insulate this problem away because the steam has a quality less than one and you may just be collecting moisture.....

I've had this happen on lines with drains at the bottoms. The condensing water doesn't make it down to the drains.

Just an idea....

 

[mfi2000]

Well it turned out to be a simple problem. A bad trap. When I asked the boss if the traps had tested good he said yes. I guess he didn't read the report. One of the traps feeding the tank tested bad. Even if didn't I would have replaced them, there are only 3. I spent the night listening and watching the system. Once I heard the water hammer in the vent and watched it blow it was clear what the prob was. Once I E-mailed him and the chief about it, all of a sudden he starts reading the report. Amazing how they promote guys who have no clue. Thanks for the input, this is a great site that I'm sure I will learn a lot from.

 

[katmar]

mfi2000, Thanks for the feedback - I'm glad your problem got nailed. Well done to TBP for sussing it out straight away.