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ali2u (Chemical)
6 Mar 04 10:03
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.
Helpful Member!  rmw (Mechanical)
6 Mar 04 12:35
The pressure is set exactly as you supposed.  The injection steam has multiple functions.  First is to heat the BFW to the highest temperature possible for the corresponding saturation pressure that vessel design limitations dictate.  So, some of the steam is condensed as it passes its heat to the BFW.  Therefore, theorhetically speaking, you could control your steam tightly, and set the pressure by putting just enough steam in to heat the water to the desired saturation temperature, and no more.  Not a good idea.

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
rmw (Mechanical)
6 Mar 04 12:44
Oh, and ali2u,

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
ali2u (Chemical)
6 Mar 04 15:08
Thanks for the reply.

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
Fzob (Mechanical)
6 Mar 04 16:12
I'm a bit confused by your reply. The deaerator's vents main function is to remove O2 and non-condensible gases. I have never seen it used to control the feed water pressure/temp. Also if the PIC controller is functioning correctly why would you experience a high flow at the vent?

Perhaps I am missing something.
jproj (Chemical)
17 Mar 04 12:02
ali2u:

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.
DeltaCascade (Chemical)
9 Apr 04 18:38
Make-up water and condensate returns entering deaerator to become boiler feed water should be at least about 10-15 F below saturation pressure for the deaerator to work effectively.  Too hot and deaeration performance suffers.  So there will be some LP steam consumption (condensation) in heating up the condensate returns and make-up water, to the deaerator's saturation temperature.  

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).

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