How to use a pressure relief valve
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The company has a multi-stage centrifugal pump with a capacity of 150 m3 and a head of 300 meters, which is used to supply water to the boiler’s water wall; after passing through the heat exchanger, the water returns to the tank. To maintain the pressure inside the water-cooling wall tubes, a pressure relief valve, also known as a pressure maintaining valve, is installed in the return pipeline between the outlet of the heat exchanger and the water tank. This valve helps to keep the pressure before it around 3 Mpa, while the pressure after the valve can be reduced to 2–3 kilograms per square centimeter. However, there is a problem: although the pressure relief valve helps to maintain the pressure before it, the flow rate decreases significantly due to the need to hold back that pressure. When the variable-frequency pump operates at 45 Hz, the pump heats up and cannot operate at full capacity. By reducing the pressure before the pressure relief valve to around 2 Mpa, the flow rate increases again, and the pump no longer overheats. I would like to ask: if we want to maintain a pressure of 3 Mpa before the valve, how should the flow rate be controlled to ensure that it remains at 150 cubic meters per hour? There are no bypasses in the entire system.
Use a flow control valve to regulate the pressure before and after the control valve
When determining the pump’s head and flow rate, it was necessary to take into account the pressure drop caused by the valve at the pump outlet, the height difference in liquid levels, as well as the conditions related to pressure buildup. This pump has a flow rate of 150 cubic meters per second and a head of 300 meters; if we calculate based on an efficiency of 55%, the shaft power required will be 230 kW, so a motor with a capacity of at least 300 kW is needed. 1) The flow rate and head of a pump depend not only on the pump’s own characteristics but also on the resistance properties of the pipes. According to your description, the pressure before the valve must be reduced to 2 MPaG for the flow rate to meet the requirements; if the valve opening is further reduced, the flow rate will naturally decrease. In such cases, the only way to address this issue is by altering the pump’s characteristics, such as increasing its speed through frequency control or enlarging the diameter of the impeller ； 2) Generally, multi-stage centrifugal pumps with a motor power of ≥160 kWd require a minimum flow protection mechanism ； When a pump with high power operates with the outlet valve opened to a smaller degree, the fluid inside the pump becomes increasingly hot; in severe cases this can damage the bearings. Minimum flow protection ensures that fluid continuously flows through the pump during operation, thereby preventing overheating
According to your estimate, would a head of around 400 meters be more appropriate for this pump? It’s not possible to replace the pump at the moment; between increasing the rotational speed and enlarging the impeller diameter, which option is easier to implement?
Increasing the speed by upscaling the frequency and setting backflow protection are relatively easy options to use