Regarding the issue of closed-loop pressure
Thread Content
In a closed-loop system, for example, if the static pressure is 0.3 MPa and the head of the circulation pump is 30 meters, then when the pump starts operating, the pressure at the pump outlet becomes 0.55 MPa, and this balance is maintained thereafter. However, I encountered two instances where the system pressure kept rising, reaching 1.2 MPa in each case; I quickly stopped the pump to drain the water, and only then did the pressure drop. Can someone explain what’s going on here? How to avoid it? Also, the higher the outlet pressure of the pump is compared to the inlet pressure plus the head, then in a closed system, wouldn’t the circulation pressure become higher? How can this be explained?
This is not caused by a cyclic accumulation and pressure buildup process at all; rather, it is the pump circulation that leads to a continuous increase in the temperature of the medium, thereby causing expansion and an increase in pressure. If, as you say, the outlet pressure is 0.55 MPa and the outlet is connected to the inlet, then the inlet pressure would also be 0.55 MPa; in that case the outlet pressure would equal the inlet pressure, which is incorrect. My assumption is that the resistance in the fluid flow circuit is very low, which results in almost no back pressure at the pump outlet. As a result, the pump’s flow rate increases while its head decreases, until the pressure difference generated by the head equals the resistance losses in the circulation circuit; at this point the pump reaches equilibrium (or even experiences cavitation). The pump’s power is then primarily converted into flow losses within the circulation circuit, causing the liquid to heat up and expand, thereby increasing the system pressure until cavitation ceases
Well, our closed-loop system works like this: it is a tubular heat exchanger system used for bathing. Steam enters the heat exchanger, where it exchanges heat with water and is then converted into condensed water for discharge ； Tap water enters all the way along. A circulation pump has been added to ensure a steady supply of hot water for those taking baths at any time. The outlet of this circulation pump merges with the tap water pipeline and together they enter the tubular heat exchanger; after exiting the heat exchanger, the fluid goes to the bathhouse, and from there it returns to the inlet of the circulation pump, thus forming a closed-loop system. Under normal conditions, the pressure in this system remains relatively stable; for example, the inlet pressure of the circulation pump is 0.3 MPa and the outlet pressure is 0.55 MPa, with nothing unusual. However, I noticed that the pressure increased repeatedly on at least two occasions – the inlet pressure of the circulation pump kept rising, and as a result, the outlet pressure also increased, reaching as high as 1.2 MPa. If I don’t deal with it, the pressure will increase further. That’s the basic situation. I would also appreciate some advice from those who are more experienced.
Also, there is still pressure loss in the circulation pipeline from the pump outlet to the pump inlet, because under normal conditions the pressure remains stable, resulting in a pressure difference between the pump’s inlet and outlet.
In a closed-loop system, the pump outlet pressure is equal to the inlet pressure plus the head. Head is used to provide power to the water. Water flows inside pipes, and due to its viscosity, there is resistance when it comes into contact with the pipe walls; in addition, there are also resistances associated with valves and other components in the pipes. A low flow rate results in low pipe resistance, while a high flow rate leads to high pipe resistance. If the flow rate at normal conditions is 5 meters due to the resistance in your system, and you use a pump with a capacity of 30 meters per second, then the pressure in your system will be very high. It will reach a certain value after which it will stabilize, because as the flow rate increases, so does the resistance; equilibrium is achieved when the resistance equals the pump’s head pressure. Yours is normal.
Isn’t the friction loss along the pipeline too low?
According to LZ’s description, the main reason for the high pressure is likely due to changes in the pressure in the tap water supply system. Under normal conditions, the pressure of tap water is 0.3 bar (which is sufficient for normal use), but sometimes a sudden decrease in water consumption leads to a significant increase in pressure. It is recommended to monitor the water usage of tap water users, and at the same time install a safety valve at the pump outlet.
According to LZ’s description, the main reason for the high pressure is likely due to changes in the pressure in the tap water supply system. Under normal conditions, the pressure of tap water is 0.3 bar (which is sufficient for normal use), but sometimes a sudden decrease in water consumption leads to a significant increase in pressure. It is recommended to monitor the water usage of tap water users, and at the same time install a safety valve at the pump outlet.