Vent size requirement to avoid vacuum in tank. 1

[lctommyboy]

How do I determine an appropriate vent size to alleviate the vacuum effects for the following senario:

60 inch diameter, 144 inch long horizontal 10 Ga 304 SST tank with flanged and dished heads at both ends which is empty but at a temperature of 160 degrees F with 50 degree water being sprayed at a flow rate of 40 gal/minute.

Currently, we're leaving an 18 inch diameter manway open on one of the ends to provide this protection, but would like to close it and vent out the top of the tank.

Any help would be greatly appreciated.

 

[Montemayor]

Tommyboy:

I’m getting old. So for my sake please explain where/how you expect to create a vacuum in this horizontal stainless tank. All you’ve said is that the tank is at a metal temperature of 160 ^oF and you are spraying 40 gpm of cold water (at 50 ^oF) into it. (I have to presume you are spraying the water into or within the tank – probably to rinse it out).

If you don’t have any condensables inside the tank – such as steam vapors – you are not going to create a vacuum; rather, you’re going to create a pressure with the cold water fill unless you vent out the air equivalent of the 40 gpm. Here, I have to assume that you are not spraying the exterior surface of the tank. Why would you? If you were, then you’d create a slight in-breathing effect as the hot interior air started to cool down and occupy less space. This effect is no big deal and an a priori estimate on the vent size such be sufficient – 2” should be more than sufficient.

I have, in the past, had to design for the vacuum scenario resulting from a failed “steam-out” procedure on a storage tank. This is the scenario where you are spraying in live steam inside the tank to dissolve and extract oily waxes or other soluble contaminants as preparation for a change-of-service or for subsequent human entry and inspection or repairs. The hazard is introduced when a sudden, cool rain shower appears and the hot tank (filled with steam vapor) is rapidly cooled down by the rain with a resulting internal condensation and vacuum created. Storage tanks have been “sucked-in” via this scenario – it’s not hard to do. But you don’t mention any steam or other condensables, so I can’t assume that’s what you have.

What is it that is creating your vacuum?

 

[JStephen]

How a vacuum forms: From cooling the air in the tank. That's all it takes. With that much of a temperature drop, it won't suck that tank down like a beer can or anything, but could probably crinkle a side okay.

There are several approximate ways to figure the cooling. I'd suggest this: Assume ALL the water you spray in is heated to the 160 degrees. That gives you a rate of heat transfer to the water, and you must have that same rate of heat transfer out of the air. That gives you a maximum rate of cooling in the air, and from that you can figure rate of volume change and thus inflow. That initial assumption could be very conservative. But it's quick and easy. Check with it and see what size vent you come up with. If it seems too large, then try to get into more detail.

Be sure and cneck allowable vacuum for the tank, which may be fairly low with a 10ga tank.

 

[lctommyboy]

Montemayer,

The pre-chilled water introduction senario is a residual humid air environment after 160 degree liquid contents has been dumped. Not actually steam-like.

If I understand you correctly, the venting requirements established for the volume displacement of the chilled water will be greater than any in-breathing requirements do to further cooling in the tank atmosphere after the chilled water flow has stopped. That makes sense, and to be perfectly honest, our vacuum concerns were supposed and not actually experienced.

 

[Montemayor]

Tommyboy:

Anytime you take the time and effort to make a sound and professional analysis of what could be a hazardous scenario, you are doing the right thing – professionally and morally.

I estimate your tank weighs 1,390 lbs and, with a heat capacity of 0.116 for stainless steel, the tank should be giving up 9,676 Btu when cooled down to 100 ^oF. This is not a lot of duty for 40 gpm of 50 ^oF water. Besides the water will be displacing 5.35 ft³/min (which is the approximate amount of air being pushed out. On a practical level, you will find that the heat transfer from the cold water to the hot tank metal will be much more rapid than the heat transfer from the cold water to the air. This is because the air has a horrendously bad heat transfer coefficient. Of course I’m assuming you’ll be dumping the water against the hot steel metal and not spraying it in fine droplets into the internal air pocket. Regardless, you still have the incoming 5.35 ft³/min of water volume that is trying to occupy the tank space. That is in your favor and I believe it will be more than sufficient to compensate for any tacit air cooling (and “shrinkage”) occurring. Again, a 2” nominal vent nozzle should suffice, giving approximately a max air velocity out of 220 ft/sec.

I would document all your calculations and basis for the record and to make sure others are aware this has been analyzed and provided-for.

I hope this helps.

 

[lilliput1]

Rule of the thumb is make the vent size at least equal to the size of the inlet. Make it at least the same size as the pipe size feeding the 40 gpm.

Also vent discharge height should be such that if the tank & vent pipe is filled, the pressure excerted by the static head of the fluid should be less than the design pressure of the tank.

 

[lctommyboy]

One more clarification which may effect the results are that the chilled water is being sprayed into the tank and then draining out a 2.5 inch port. It isn't actually building volume in the tank as it is sprayed in.

Would this be considered the same senario as if a heated tank suddenly experienced a cool rain?