BFW system: Deaerator
BFW system: Deaerator
(OP)
Hi All,
I am working in a project on a boiler feed system for a hydrogen plant.
We are using LP steam as injection instead of a coil inside the deaerator. I want to know whether the amount of LP steam injected to the deaerator determines the pressure inside the deaerator and thereby the deaerated BFW outlet temperature. How is the pressure in the deaerator set?
I am looking forward for a good discussion, if any.
Thanx.
I am working in a project on a boiler feed system for a hydrogen plant.
We are using LP steam as injection instead of a coil inside the deaerator. I want to know whether the amount of LP steam injected to the deaerator determines the pressure inside the deaerator and thereby the deaerated BFW outlet temperature. How is the pressure in the deaerator set?
I am looking forward for a good discussion, if any.
Thanx.





RE: BFW system: Deaerator
The purpose of heating the water, aside from boiler heat transfer reasons, is to raise the water temperature to a point where the oxygen soluability is as low as possible. Generally speaking, 15 psig, which is 250F reaches a point where you have gotten a lot of the benefit that you are going to get, but 50 Psig deaerators are not uncommon.
But, you will notice that deaerators have devices, like spray nozzles, trays, packing, etc., that distribute the water for good contact with the steam, and hence, maximize good heat transfer.
Excess steam, over and above what is requred to just do the heating and pressure regulation, acts as a 'scrubbing' medium to get the oxygen, which now has low soluability, "liberated" from it's bond with the water, and swept away with the excess steam to the vent, where it is returned to the atmosphere. That is why every deaerator you see has a steam plume. (some do not have enough, however. Be careful) The trays and sprays, and/or packing all distribute the water in such a way as the scrubbing steam can have maximum contact with the water to get the scrubbing done.
How much turndown you require will dictate whether you go with sprays, trays, or packing. Whether or not your BFW has a lot of dissolved oxygen or not, also drives your design.
I personally speaking, do not like the idea of the coil, unless for some reason your BFW is already very pure condensate, coming from a hotwell that already has an ultra low oxygen concentration, as you don't get the type of scrubbing with the boiling off of the coil surface that you get with the methods mentioned above.
I hope this is enough to start the conversation.
rmw
RE: BFW system: Deaerator
As an old power boiler guy, I pretty much limited my explaination to oxygen, which make up water is typically loaded with, and, as such, carry the soluability numbers in my head, but I am not a chemical process guy, so they will have to advise you of what other types of non condensibles that you are going to encounter in a H2 plant, and have to remove with your DA.
rmw
RE: BFW system: Deaerator
The LP steam that is is injected into the deaerator is as I impose and you have explained controled by a PIC-controller. However, if the amount of LP steam is too high, the high vent flow will increase. The orifice for the steam vent is normally designed for max flow 2000 kg/h. That means that you actually can control the pressure in the deaerator by means of vent flow.
If the flow vent flow icreased max design, you can imagine what will happen.
If anyone has some other good points, I am looking forward to hear.
ali
RE: BFW system: Deaerator
Perhaps I am missing something.
RE: BFW system: Deaerator
The DA pressure should be controlled be the steam PRV, not the vent valve. Normally, steam PRV's are pilot operated valves that sense the downstream pressure and throttle the flow accordingly (too much pressure, valve closes. not enough pressure, valve opens).
Yes, as pressure increases, the flow across the vent orifice will increase, but the vent valve should not be used to control the pressure. In practice, the vent flow should only be adjusted to minimize the steam loss while minimizing the outlet oxygen concentration.
If the vent flow is exceeding the design flow, then your pressure is too high and the problem is with your steam valve, not your vent valve.
RE: BFW system: Deaerator
In addition, a typically well operated deaerator vent will vent at a rate equal to approximately 0.25% of boiler feed water flow (if memory serves correct).