Ammonia Piping Purging 2

[JeffHofacre]

When performing maintenance on ammonia system piping I assume you would close valves as needed and then start your work. Is it necessary to purge the piping before breaking into the piping? Do certain industries require this? Where do you purge to? Do you purge with air or nitrogen?

In addition, is monitoring for ammonia leaks required? Per MSDS ammonia is fatal at 5000 ppm.

 

[dimitris]

Hi,
Normally it is necessary to purge ammonmia piping systems prior to access them. It all depends on the pressure of the system of the overall ammonia hopld up and of the extent of maintenance you will perform.
The best way is to isolate the area you want to do work at, properly blind it, and then purge only this area.
DO NOT purge with air for two reasons.
- Risk of forming explosive mixtures.
- Corrosion risks.
You can purge with Nitrogen and the best way to do so is by pressurizing - depressurizing your pipes (watch out though when you pressurize!!). Another very effective way to access ammonia piping is by inserting water in the pipes as Ammonia is very soluble and then drain the system, but once again you must be very cautious when doing it because of the heat of mixing effect of Ammonia - Water mixtures.
Regards,
Dimitris..

 

[JeffHofacre]

The ammonia injection system I have has about 20 feet of 1" piping which when in service can vary in pressure from 50 psig to 275 psig. As soon as ammonia injection stops the pressure will drop to 0 psig.

The reason I asked about purging is that the manufacturer of the skid has on the P&ID drawing a connection labeled N2 purge. I think they are just being overcautious.

After consulting CGA G-2 Anhydrous Ammonia - Compressed Gas Association, INC. It says in 9.4.2.2 that if ammonia vapor is released, the irritation effect of the vapor will force personnel to leave the area long before they have been exposed to dangerous concentrations.

Since this is an outdoor application, I don't believe I need to purge with nitrogen before maintenance. There is a bleed connection which can be opened to verify that the line is not under pressure. Thanks for your comments.

 

[Bala0610]

Hi

Just a word of caution...

While usage of water would be a nice way to clear Ammonia, besides the heating mentioned by dimitris, also be aware that the high solubility can cause vacuums. Your mention about the N2 purge makes me wonder if the same is located on the dosign tank. If so, unless your dosing tank is designed for vacuum (or can stand it) or you have protection against vacuum occuring, dont introduce water in the tank.

Regards

bala