Non-condensable Venting

There is always a small amoimt of CO in steam. It originates from residual carbonates in the BFW. Compared to steam, the CO is a noncondensable with a limited solubility in H O. As the steam condenses, the few ppm of CO will be trapped on the steam side of the tube bundle inside the channel head. However, none of the CO will be trapped above the pass partition baffle shown in Fig. 13.1. And all of the COj will be trapped below the pass partition baffle. Therefore, the problem is that, venting from valve C in Fig. 13.1 will vent off steam, but no COj. To purge the accumulated CO from the tube bundle, vent valve B must be used. 

If this non-condensable is not removed from the channel head, then my clients have observed (over a period of days, not hours) a progressive loss in heat transfer capacity. The proof that this is a buildup of non-condensables is that briefly blowing the condensate seal restores heat transfer capacity. The CO increases the tube-side heat transfer film resistance, rather similar to a temporary fouling deposit. I call this vapor binding. 

In 1983,1 was told by a Shell engineer that their standard is to continuously vent 0.5 percent of the steam through a restriction orifice to purge COy I still follow this practice, even though I suspect that just a small fraction of this rate of venting is required, if the steam is produced from demineralized water. 

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One problem with flooded condenser pressure control is related to the need to occasionally vent non-condensables. This vent valve must not leak when closed. Since the drum is normally full of liquid, a leaking vent valve will pass liquid. Many pounds of product can be lost in this way. A butterfly control valve with a soft, rubberized seat is a good choice for a remotely controlled, non-condensable vent valve. 

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