Venting, deaerator

Knox has provided the following graphs for estimating the required vent steam from boiler feedwater deaerators. Vent steam rate depends upon the type of deaerator (spray or tray type) and the percentage of makeup water (in contrast to returning condensate). Low makeup water rates require relatively lower steam vent rates, but there is a minimum rate required to remove CO2 from the returning condensate. 

In addition, the solids may cause fouling problems such as reduced deaerator venting, sticking check valves, and blocked feed pump. 

NOTE Some recirculation of ammonia and amine takes place within the overall boiler plant system, although at higher pH much of the ammonia is lost at the deaerator vent. In practice, this recirculation coupled with low er-than-theoretical C02 liberation (as a result of the incomplete breakdown of sodium bicarbonate when present in the boiler) typically results in a reduced amine-demand for any particular boiler pressure. This reduced demand, compared with the apparent demand, results in real cost savings. 

Scale and corrosion debris transport results in FW line/feed pump blockages Impingement and erosion Poor deaerator venting Sticking check valves Fe, Cu, Ni, Zn, Cr oxides Crystalline magnetite... 

Deaerator spray heads and trays are subject to oxygen corrosion and erosion damage. The FW storage area is also subject to risks of oxygen corrosion. The deaerator venting system may block with corrosion debris. 

The available energy flou through five major sections of sulphuric acid plant is given in figure 2. The major inputs to this system are sulphur and pouer, with demineralised (DM) water uet air, process water and cooling water from environment. The useful outputs from the system are sulphuric acid and steam. Losses to environment include heat losses from various equipments blowdown water steam from deaerator vent warm water and stack gas. 

As methanol emissions continue to be monitored more closely, there are some methods of reducing the methanol in the deaerator vent. The vent stream could be condensed and sent to the reformer or the steam system. Catalytic combustion could be used to reduce the methanol. A scrubber system could be added to remove the methanol. In some instances a condensate stripper is added instead of the deaerator to remove the ammonia, methanol, and other contaminants from the condensate. This system recycles the vent stream to the reformer as process steam and the bottoms are mixed with the incoming boiler feedwater makeup. However, this system adds considerable capital cost to a project. 

On the low pressure side 90/10 cupronickel was used to tube Nos 1 and 2 LP heater, and the deaerator vent condenser, as at low temperatures of operation no serious corrosion was expected on the steam side even In the presence of oxygen. On the water side most of the released corrosion products will be removed by the full flow Ion exchange treatment given by Powdex resin Interposed beyond the second LP heater. Typical analysis for water which has been given this treatment are 5 ppb maximum for metals normally encountered In feed-water circuits. Unfortunately It Is clear from power station experience that a significant pick-up of corrosion products on the water side takes place In the high pressure feed heaters downstream of the clean-up plant. 

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