Does ammonia naturally separate from solution? 1

[gcmaus]

We have an outdoor system at a power plant that was experiencing a freezing issue last winter in Maryland. The system pumps a 19% aqueous ammonia solution to an injection unit that helps treat emissions. The solution freezing point is -30F, but somehow we were still experiencing freezing.

Is it possible that the ammonia was separating out of solution while it sitting stationary in the pipes (during times when the system is not circulating the fluid)? I suppose this could leave a lower % solution with properties closer to water (higher freeze point) and enabling the freezing at ambient temps?

The system is essentially sealed, though the storage tank has a vacuum breaker and there might be other components that allow a bit of vapor leakage to atmosphere. I suppose even if there isn't an escape of the ammonia vapor, it could still be separating and leaving certain parts more vulnerable to freezing. Thanks for any assistance you all can offer in understanding this issue.

 

[EdStainless]

Since 19.7% is -31F and 15.8% is -15F you wouldn't have to loose much to be in serious trouble.
And yes, some ammonia is naturally lost from solution.
In general the vapor pressure of the ammonia is roughly 1 atm.
The entire system has to be very well sealed, and kept at a slight positive pressure to keep the solution intact.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[georgeverghese]

Leaks commonly occur through pump shaft seals, valve stem gland packing and instrumentation compression tubing fittings. Try to devise some easy way of detecting these leaks - one way may be to waft a wick dipped in dilute HCl for instance at a suspect leak point - if you get white vapors, this would be ammonium chloride.

 

[gcmaus]

Thanks for the responses. We have a few leak detectors mounted in the surrounding areas, but those are more for personnel safety and generally not paid much mind if not in alarm state (20-25 ppm). The system is mostly stainless pipe w/ socket welds and flanges, but there are some small tubing lines for instrumentation. Pumps are mag-drive gear pumps and isolation valves are ball valves.

We generally operate at about 82 psig on the discharge line but it bleeds down to 4-7 psig when the pumps are shut off. I'm assuming the pressure is lost back into the supply tank, which has an air volume in it. The vacuum breaker on the tank is set at -3.5" W.C., but I assume is fully-seated otherwise.

I know some of the valves on the downstream mixing system are not 100% shutoff classification, so there might be a way to leak that direction.

Pic of system that was experiencing freezing:

 

[georgeverghese]

Presume you've checked the PSV or outbreathing safety valves on the tank also. If you're recyling a much more dilute ammonia solution from a vent scrubber ( generated during the fill operation ) back to tank, the resultant NH3 solution woud drop - how is this checked? You'd also need good mixing of this weak soln in the tank to prevent concentration layers building up.

 

[gcmaus]

Not sure if the tank PSV's (30 psig setpoint) have been checked since they were installed. The supply tank is 10,000 gallons and periodically filled by whichever vendor sells them the ammonia solution. I'm not familiar with vent scrubbers, but I don't think we have one on this system. When the vendor's trucks come to fill the tank, they connect to both a supply hose and a vapor return hose, so that any vapor they displace in the filling process ends up in the tanker truck. The plant operator did send out a sample of the system fluid to get tested, but found it was 18%, which was within the tolerances of what the vendor agreed to provide. I don't think they make a habit of these tests though, and there is no instrument in the system that tests solution quality.

To give you some ideas on the fluid usage/turnover,
When generating power:
Flow leaving tank: 1.5-2.5 gpm
Flow recirculating to tank: 0.5 gpm or less (included in above flow)

I'm estimating the plant generates power for an average of 4 hours per day during non-peak periods (including winter), so those numbers work out to:
Volume leaving tank per month: 10,800-18,000 gallons
Volume recirculating to tank per month: 3600 gallons at max (included in above volumes)

Generation hours vary alot, so some days they might not run at all, but others 8-12 hrs. I'd also add the the re-circulation part of the system is not necessarily designed for mixing. It's just to protect the piping/pumps from over-pressure.

 

[georgeverghese]

Okay, so you dont have a vent scrubber. Instead, the fill truck pumps this ammonia in to this tank and also receives displacement vapors.

As you say this may be another reason :

"Is it possible that the ammonia was separating out of solution while it sitting stationary in the pipes (during times when the system is not circulating the fluid)? I suppose this could leave a lower % solution with properties closer to water (higher freeze point) and enabling the freezing at ambient temps?"

Yes, can see how this could happen on days when there is a significant swing in temperature. If this is the root cause, some means of keeping the pump discharge piping pressurised to prevent ammonia from flashing out after the pump has stopped may be the solution - a suitably pressurised gas charged pulsation dampener ? This dampener would be located downstream of the pump discharge check valve and downstream of any pressure or flow recycle valve.

 

[georgeverghese]

On second thought, the sat vap pressure of a 19% wt ammonia solution is about 14.5psia at 120degF, so the boiling point at a residual pressure of 20psia ( when the pump is not running) would have to be even higher to result in some flashing of this solution. Given that you wouldnt have such temperatures in Maryland, I'd think it is highly unlikely that this is the cause of the dilution.