Steam heater, steam at 10 bar – how does the pressure of the condensate change under different loads?
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At the end of the flue gas purification process, a steam-flue gas heater is used to heat the flue gas, from 50 degrees to 120 degrees, using 10 bar saturated steam at 180 degrees; a drain valve is installed at the outlet. Some time ago, it was said that the drain valve wasn’t discharging water. On one hand, it was suggested that the liquid level at the inlet of the drain valve was too low (below 1 meter, which prevents the valve from functioning properly). On the other hand, it was argued that the load was too low, with a low amount of flue gas; the steam volume was around 10-20%, resulting in an excessive heating capacity of the heat exchanger and excessive heat absorption. As a result, the temperature of the water coming out of the drain valve was below 100 degrees. Some people suggested that there might be a vacuum inside the equipment. But I’m wondering whether the conditions in this place right now involve subcooled water – the temperature is low, but the pressure remains high; a pressure of 10 bar isn’t something that can simply disappear. Anyone who has experience with heat exchangers or knows something about check valves, please share some insights.
1. Based on your process diagram, the issue of too low a liquid column at the inlet of the steam trap can likely be ruled out, unless the height of the liquid column is so low that it prevents the float from moving； 2. (1) I didn’t quite understand what the other person said – that the load is too low, the amount of flue gas is small, and the steam volume is around 10-20%, which results in the heat exchanger having too much capacity and absorbing excessive heat; as a result, the temperature of the water falling to the bottom of the exchanger is below 100 degrees. You then said that the operating conditions might be those of subcooled water, but it seems like you yourself don’t understand the actual operating conditions. Suggestion: Please provide a detailed description of the operating conditions when everything was working normally in the past, as well as those under which the problem is occurring now, so that we can determine the cause of the issue.
I really want to know about point 2 of yours – there are those who insist that, when the load is low, the heat exchange capacity is too high, which can cause the equipment to produce gas; the reason given is that the condensate temperature at this time is 60 degrees, and only saturated water under vacuum conditions has a temperature below 100 degrees. But my opinion is that it’s not in a vacuum at this time; the water is at 60 degrees, so it should be supercooled water. This is where we disagree; in my opinion, it’s difficult to achieve a vacuum in the heat exchanger, and this is due to the low amount of steam entering that heat exchanger.
The steam trap has requirements regarding back pressure. You need to know what the back pressure of the steam trap is, which should be selected under differential pressure conditions. During low-load operation, although negative pressure is not generated, it is certain that the pressure decreases. With a back pressure of 10 bar selected, 9 bar might be sufficient for operation, but 8 bar will result in instability, and lower values will prevent proper water output.