Sizing a vent nozzle of a steam flash tank

[Ding123]

Dear all:

I am coming up with a steam flash tank design. I need to size the tank and the vent nozzle. The vent is open to air.

Steam inlet: 2kg/s @490F 600psi
Water spray: 3.2kg/s @90F 30psi

How could I start with and what is the proper procedure?

Also, the required max. venting noise level is 85 dBA @ a distance of 1m.

Thanks in advance.

ding123

 

[TD2K]

You describe this as a steam flash drum but then say the 'steam' entering this vessel is 2 kg/sec. Your conditions are essentially saturation, do you mean steam condensate from upstream sources (490F @ 600F) flowing at 2 kg/sec into this drum (and flashing)?

I'm going to assume you mean 2 kg/sec of steam condensate.

1. calculate how much flash steam is produced. If you are vented to atmosphere I'd try as a first pass assuming atmospheric pressure in the tank.
2. Ignore the water (intended to condense some of the steam I assume) because this could fail. Size the tank diameter based on the Souder Brown equation for a two phase separator (since that is essentially what you are doing).
3. You need to know the pressure the tank can take and then select a desired maximum internal pressure under that pressure for the case of the water spray system failing. Size the vent so that it will pass the amount of flash steam without overpressuing the tank.

For the noise calc, I'm not sure I have a method for that. My only recommendation is to keep the velocity through the vent nozzle low. You'll start to get noise issues above 60 to 80 ft/sec. You should only have significant velocities through the vent when the water spray system isn't working, the rest of the time when it is in service the velocities will be much less (if you don't condense all the steam with the quench spray, I haven't worked out the numbers to see if that occurs or you condense only a portion).

 

[Ding123]

TD2K:

Thanks for your input.

The conditions are: the steam @490F 600psi enter the flashing tank @ a rate of 2kg/s. The quenching water is sprayed into the tank as well. The water / condensate will be drained at the bottom and the remaining steam be vented to open air.

Thanks again for your instructions.

Ding123

 

[JStephen]

Seems to me you'd generate a lot of noise from the steam entering the tank @ 600 PSI. And wouldn't the "remaining steam" be most of the steam you started with?

 

[TD2K]

One other point you need to consider. I don't know if this flash steam/condensate is entering the tank from steam traps or control valves or a combination but don't forget to allow for some of your traps failing in the open position and blowing steam into the header and thus into your tank. Similar if you have control valves dumping condensate. If they fail open, you'll blow steam through the valve once all the condensate drains out.

Don't be stingy sizing your vent. This is one case where bigger is definitely better when you start to consider various ways you 'might' get much higher flow rates of steam into your tank.