Aq. ammonia storage tank vent 1

[handee]

I'm spec'ing out a 5000-gal storage tank for a solution that is essentially 9wt% aqueous ammonia. Perry's says that for 9.5% NH3 on a hot day (at 90F) total vapor pressure can reach almost 4psia.

Sizing the conservation vent is not a problem, I'll go with Groth or Protectoseal, but my plans for using an atmospheric HDPE tank might be going out the window. Any one have experience with this? From my understanding, plastic tanks aren't rated for any kind of pressure at all.

Thx.

 

[TD2K]

I don't see the problem from the pressure point of view.

4 psia means that nearly 75% of the vapor space is filled with air. The tank won't be pressurized, if that is what you are thinking about, other than due to atmospheric heating and expansion of the vapor space or filling, both of which will be handled by your conservation valve.

Where are the vapors going to be vented to?

 

[TD2K]

I don't know where you are located.

Are there any regulations on the maximum vapor pressure you can put in that tank? While the 4 psia might not be a problem from the physical ability of the tank to withstand the solution, it might not be acceptable from a regulatory point of view.

 

[handee]

Thanks TD2K

So you are saying that the 4psia does not mean 4psig...ok, I'll have to think about that. But the folks in the plant tell me that their 300-gal totes build up a good 5psi on hot days. The conservations vents periodically pop-off to let out ammonia vapors.

No 5000 gal polyethylene tank that I know of is rated for 5psi by the manufacturer. Strictly atmospheric, and I don't think we'll want to take that chance.

 

[handee]

I should add that although the tank will be vented to the atmosphere, we need to keep the ammonia vapors contained as much as possible for odor control and to prevent the pH and chemical composition from changing.

 

[TD2K]

http://www.rmtech.net/technical_information.htm

Interesting source of data on ammonia. I haven't worked with ammonia so keep that in mind. If I'm reading the tables correctly, I agree with your estimate of about 4 psia vapor pressure for a 9.5 wt% ammonia solution at 90F. I don't see how you could get 5 psig pressure in a tank unless you are basically filling it with the vent closed or you are heating it somehow.

 

[JStephen]

My understanding is that the vapor pressure as used there means the partial pressure of that component of the vapors in the vapor space. The actual total pressure in the vapor space could be atmospheric, or higher, or lower.

You mention the increase in pressure due to heating. That would require some form of venting or a pressurized tank. Of course, a larger tank might heat up much more slowly than a smaller one. You'll also have vapor moving as you fill or empty the tank. But the pressure that is built up is not identical to the vapor pressure, but would depend on the temperature rise, phase changes in the tank contents, etc.

 

[sshep]

handee:
If the tank starts cold and warms up, there will be outbreathing ofair and ammonia vapor until the vapor composition gets to the equilibrium partial pressure of ammonia. At night there will be inbreathing to keep from pulling vacuum. Your issue will be the amount of ammonia lost to breathing, and the consequential cost (due to excessive ammonia losses), odor and exposure effects in the surrounding areas. Maintenance on the top of the tank could even be a fresh air job at these levels of ammonia.

If you wish to use an atmospheric tank, I think your best bet is to fit an internal floating roof.

best wishes, sshep

 

[BenThayer]

i worked in a complete ammonia facicility for nearly 20 years and we made 29.6% NH3 in water or Aqua Ammonia. It sometimes went off spec on the low side during production but never on the high side. it was in an atmospheric tank. look up the vapor pressure and you'll see why it never went off spec on the high side.

the 4 psia that you will have will result in a mixture that is ~27% NH3 in the vapor space. that would be 27,000 ppm right? NH3 IDLH (Immediate Danger to Life & Health) was ~400 ppm. if not used to the odor, you can detect it at 3-5 ppm and your eyes start watering at 10-25 ppm. your lips will start to burn at 50 ppm. the moisture/sweat in your groin will start to cause irritation above 100 ppm. you'll be in canister mask at this point.

you will want more measures than a simple CBV. you should consider some type of scrubber, nitrogen pad/depad, floating roof, etc. depending on how the rest of the system is designed.

some 29.6% systems are designed with a vapor return to the trucks so that as the truck is pumped into the tank, the vapors are returned to the truck. if the truck is pressured off, where will that pressure with ammonia fumes be vented prior to leaving the site?

the comments regarding environmental issues is very valid. do the calculations on the breathing of the tank based on fill rates and usages. and don't forget scenarios like adding water to flush and clean the tank.

 

[handee]

Thanks for those references.

Apparently this material simply off-gases and builds up head space pressure, which now I realize is unrelated to the vapor pressure numbers. We were hoping to use a poly tank, but that looks unlikely.

Thanks for your input, folks.

 

[jdarco]

Check with your ammonia supplier as well. We had a situation where they were reluctant to deliver to a site that had a poly tank because the tanker pneumatically off loaded and they were concerned about tank failure AND vapors in the building. It would be shame to do the project twice!

 

[mullet]

We are a suipplier of nitorgen blanketing valves/ pressure vacuum vents and emergency vents. THE L & J Technologies Company- Contact us and we can send you sizing software that will help with tank venting etc.
Brent Mullen
L & J Sales Associate
brent#fluidequipment.com

Thanks

 

[bimr]

Wow, lots of opinions. This is a relatively common application and there is no need to speculate on what tank designs will work.

HDPE is not suitable. The Harrington corrosion chart shows a temperature rating of 100 Deg. F. HDPE with aqueous ammonia. Bear in mind that the strength of the plastic lessens with increasing temperature. The loss in strength doesn't occur at just 100 Deg F., but gradually as the temperature increases.

The recommended (as well as the most common) tank design is carbon steel construction with a .125" corrosion allowance. The tank should be designed to API-650 (not weak top design) with a design pressure of 3 psig. Operating pressure should be atmospheric pressure. Vent the tank to a hot well.

You can also use a carbon steel centrifugal pump to transfer the material.

 

[mbeychok]

Ben Thayer:

Your 27,000 ppm is incorrect. 27 percent by volume is 270,000 ppm by volume.

Milt Beychok
(Visit me at www.air-dispersion.com)

 

[BenThayer]

thanks for the correction!

 

[handee]

Again thanks for your input everyone. It seems I was somewhat ill-informed. Because of other salt complexes in this mixture, it actually contains less than 1% free ammonia in solution. And the history of pressure building up in storage totes was another miscommunication. There has never been a buildup of pressure in totes of this particular solution. Geez.

But this was a very useful discussion for me. Cheers.

 

[mullet]

Typically if you put a conservation vent on a tank it will see some pressure and vacuum. Minimum settings are ususally at .5 oz. I would not recommend using a plastic tank- These are normally reated for atmopheric conditions only. Look at Shand & Jurs /

http://www.ljtechnologies.com

-They have a special sealing called expanda seal -

Brent Mullen

http://www.fluidequipment.com