Does ammonia naturally separate from solution?
Does ammonia naturally separate from solution?
(OP)
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.
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.





RE: Does ammonia naturally separate from solution?
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.
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P.E. Metallurgy, Plymouth Tube
RE: Does ammonia naturally separate from solution?
RE: Does ammonia naturally separate from solution?
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:
RE: Does ammonia naturally separate from solution?
RE: Does ammonia naturally separate from solution?
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.
RE: Does ammonia naturally separate from solution?
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.
RE: Does ammonia naturally separate from solution?