Steam Condensate Tank

[murphymok]

Hi,

I have a question about steam condensate tank for a building. 200 psig steam is supplying to the building and it will be reduced to 15 psig after the pressure reducing valve. There is only one single condensate receiver tank for the building's low pressure condensate returns and the 200 psig condensate return from the trap drip leg off the main steam line. The existing tank is experiencing water hammer or flashing becuase of the two differenct pressure condensates. What can I do to resolve this problem? Do I need two separate tanks for each pressure condensate? I have seen a lot of buildings that use only one tank?

Thanks.

 

[sailoday28]

Please clarify:
How have you determined that the tank is experiencing water hammer problems?
OR flashing problems.
How is the pressure controlled in this tank? ie. by venting elsewhere, etc?

 

[murphymok]

There is a banging sound in the tank and I can see steam going out the venting pipe to outside the building. The tank itself does not have any pressure control. Basically it acts as a condensate receiver tank for both pressure condensates.

The banging noise appears when the steam system is shut off and the only condensate going into it is the high pressure condensate from the drip leg. I am not sure what will happen when the steam is turned back on for the winter because this project is given to me during summer time.

Thanks.

 

[sailoday28]

It seems that the receiver is an atmosheric flash tank.
What are the sources for the tank when "the steam system is shut off"?
Is it from the 200psig drain line? AND no other source?

 

[murphymok]

When the steam system is shut off for the summer, the only condensate going into the flash tank is from the steam trap drainage off the 200 psig line.

 

[sailoday28]

Based on the weight rate of inflow of drains from the 200psig line at a liquid enthalpy of Hin(214.7psia sat liq)]
The flashed (vapor) portion of the 14.7 psia should be
flashed rate =Hin/Hfg(14.7psia)
and liquid rate- inflow-flashed rate.

Based on the above, the vent/ drains should be appropiately sized to handle the corresponding flows.
The other steam system flows should then be addressed.

 

[TBP]

The drip trap is probably failed open, blowing live steam into the tank. Shut the trap off for a little while, and see if your problem disappears. If it does (and I'll bet it will), just change the trap.

 

[murphymok]

I think my ultimate question is that can I have both high pressure and low pressure condensate going into the same flash tank?

 

[sailoday28]

""I think my ultimate question is that can I have both high pressure and low pressure condensate going into the same flash tank?""
First the hammer should be resolved. Was there a failed mechanical trap in the drip leg feeding the tank?
The operating pressure of the flash tank will be determined by the vent line (I have assumed it vented to the atmosphere). If the vent and drain lines are sized appropriately, there should not be a problem.
Previously, I assumed the tankdrain was via a loop seal, also to the atmosphere. If so could it have blown out if the vent/drain lines are not sized properly.

 

[friartuck]

Seems like you need to reduce the amount of flash created by the HP steam/condensate.

What about fitting sub-cooling legs and thermostatic traps which allow the condense to cool.
Alternatively, run a coil of HP condensate in your tank, which will cool off before entering the tank.

Thirdly, could you install a flash recovery vessel to utilise the high temp condensate. The condensate can be safely flashed off to feed another l;ower pressure process...or even flashed off to re-feed the lower pressure circuit.



Friar Tuck of Sherwood

 

[Joesteam]

We have this application a lot here in New York City with high pressure Con Ed steam. We use what we call a dilution tank, which is a centrifugal device used to separate the flash from the condensate. Some of these had a manufacturing problem where the condensate inlets were below the water level in the tank, causing water hammer in all of the installations. (these tanks were vented to atmosphere and also used to cool the condensate before dumping into the sewer).
For another type of tank, make sure that the inlet is in the 'air space' in the tank and not below the water level. And also make sure that there are segregated pipes for high and low pressure entering the tank. But if the vent is sized right and not obstructed, there should be no problem with both high and low pressure condensate in one properly sized atmospheric flash tank.

 

[hacksaw]

what sort of steam trap are you using on the 200 # line?

 

[murphymok]

The 200 psig line is using a thermodynamic trap.

To Joesteam's comment: I should be able to have one single tank servicing both high and low pressure condensate, as long as the flash tank is sized properly (or big enough?).

 

[Joesteam]

Yes, one tank should be OK if sized and configured properly

 

[friartuck]

Joesteam

I was interested to note that you discharge the condensate above the water level. I guess that this would eliminate the banging but waste a lot of steam since you would not be pre-heating the water in the feed tank as efficiently. The vapours would simply blow out the vent pipe. Isn't this the same (well nearly the same) as simply blowing the condensate to atmosphere. The only difference is that the remnants of the condensate dribble into the tank but a large proportion disappears to atmosphere. Seems a wee bit wasteful?


Friar Tuck of Sherwood

 

[Joesteam]

You are correct, sir Friar, for every place but here in NYC, where Con Ed does not accept returned condensate and the building must dump the return to the sewer. Whats even more wasteful is that they use fresh cold city water to cool this hot condensate.
Sometimes we install a heat exchanger (water to water) to preheat the city water used for domestic hot water using this hot condensate.

But also, discharging the condensate below the water level will give you water hammer without a sparging tube, which I don't really trust to last in condensate applications.

 

[hacksaw]

thermostatic traps are notorious for hammer, can you use a modulating type?

 

[TBP]

I agree with hacksaw - thermostatic traps subcool condendensate, and this can often cause a back-up, into the main.

I've had no problems at all with thermodynamic traps on high pressure drip service. BUT, you have to take the time to size them properly. It's often amazing just how small they need to be. On long piping runs, we've installed a pair of half capacity traps in parallel to handle the start-up air venting and condensate surge. Install them in the vertical plane on the drip legs, and leave them both valved-in, all the time. On a start up, the lower trap won't handle the condensate load, so the drip leg backs up condensate until the upper trap starts to work as well. Once the main is hot, the condensate to be handled will drop off, and the lower trap alone will handle the job. The upper trap only sees steam, so it doesn't open. We've used this method on drip services on up to 400' of 12" pipe operating at 125 PSIG. A pair of 3/4" TDs, one over the other, works fine. Start ups are fast, smooth and quiet, and the trap that's in regular service is the right size. Many have lasted well over 5 years in 24 hr a day year-round service. And the flash steam is minimal.