Why is segmented control used for the liquid level in the turbine hot well?
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This post was last edited by hbspider on 2017-3-7 at 16:36. The turbines in our company are used to drive compressors, and the liquid level in the hot well of these turbines is regulated through step control. There are two self-regulating valves at the outlet of the condensate pump: one for sending the fluid outward and one for returning it. The degree to which the outward-flow valve opens corresponds to the degree to which the return-flow valve closes. This has always been the method used for adjustment. I suddenly asked why such adjustments are needed and what the benefits of them are. It’s possible to regulate the liquid level in the hot well by using an external self-regulating valve; isn’t this a bit wasteful? The large amount of reflux leads to an increase in the flow rate of the condensate pump. I asked other colleagues, but none of them had a satisfactory answer, and I couldn’t find it online either. So I would like to ask the experts here to come and discuss and give some advice.
For the make-up water valve, when the control signal is between 0% and 50%, the make-up water control valve remains closed; when the control signal is between 50% and 100%, the opening degree of this valve increases as the control command increases. In contrast, for the return water valve, when the control signal is between 0% and 50%, its opening degree decreases as the control command increases, while when the control signal is between 50% and 100%, the return water control valve remains closed. This ensures the continuity of the control effect of the control signal on the control valves; that is, as the control signal varies from 0 to 100%, the net flow into the condenser increases continuously due to the coordinated action of the two control valves, and it decreases continuously in the opposite direction. In automatic mode, when the condenser water level is below its set value, the control signal output by the proportional-integral regulator increases; through the action of the two control valves, the flow rate into the condenser rises, causing the water level to increase until it reaches the set value, and vice versa.
Yes, the way of adjustment here is clear, but why is it done this way?
As shown in the diagram, by removing the return valve, where does your water come from? Does it rely entirely on external chemical water supplementation?
Looking at this picture, the return valve has been removed – how can we control the amount of water entering the make-up water tank?
Our condensate process is different from yours. Here is our process; take a look. We don’t have a condensate tank.
I’ll try to explain it to you by looking at the properties of condensed water: from the relationship between saturated temperature and saturated pressure, it can be seen that condensed water is saturated water at the pressure in the condenser, and it can easily vaporize. For the condensate pump to operate properly, a certain static pressure is required at its inlet, in order to prevent the condensate at the pump inlet from vaporizing and thus preventing the pump from being able to draw in water. The way to maintain a constant static pressure is by increasing the liquid level height (the level of the hot well fluid). Therefore, at low or no load, the recirculation system (i.e., the return valve you mentioned) should be activated appropriately based on the liquid level height, in order to maintain the level of the hot well fluid and prevent the condensate water at the inlet of the pump from vaporizing, which could prevent the pump from drawing in liquid. In other words, the main function of the condensate recirculation (return valve) is to maintain the level of the hot well fluid; this valve is usually closed during normal operation, and it is only used to adjust the level of the hot well fluid under low load conditions. A variable-frequency pump can be used as a substitute for this valve.
When the outlet valve is closed, the liquid level rises; when the outlet valve is opened, the liquid level drops； Why a return valve is needed still isn’t clear
This post was last edited by jthp on 2017-4-28 08:52 for the following reason: 1. The hot well must have a certain liquid level height in order to increase the hydrostatic pressure of the liquid, thereby preventing vaporization at the inlet of the condensate pump (which is also why the condensate pump is installed in a pit beneath the hot well). Recycling is intended to return the condensate at the exit of the condensation pump back to the hot well when its level is low (essentially creating a circulation process). Without recycling valves, the level of the hot well can only be adjusted by adding water from the outside, which in turn increases production costs. 2. Condensate pumps are generally centrifugal pumps, and such pumps have a minimum flow rate requirement. If there is no recirculation and the outlet valve is closed to a certain extent, the condensate water inside the pump will vaporize, resulting in cavitation. If it is necessary to maintain an appropriate level of hot well fluid at no-load or low-load conditions, the outlet valve must be closed. However, simply closing the outlet valve can cause cavitation in the condensate pump when the flow rate falls below the minimum required level. To avoid this phenomenon, recirculation is generally used in combination with adjustments to the outlet valve, so that the flow rate of the condensate pump remains above the minimum value. If the condensate pump is variable frequency, the role of recirculation is much reduced, serving only as a backup in emergency situations. (If I still don’t understand, then there’s really nothing I can do; I hope someone more knowledgeable can answer it; P.) ）
Yes, the second point hits the mark: the water volume cannot be too low; it’s necessary to have some backflow. The key issue is cavitation in the pump, and variable-frequency pumps also don’t work well, as there may not be enough pressure in such cases