One Question per Day on Chemical Equipment 2011.04.15

Reply 1# —Lantian— 1: The minimum flow line for the pump is necessary only in cases where it is not possible to control the pump’s minimum flow rate using valves； 2. Compared with ordinary centrifugal pumps, the disadvantages of screw pumps are: high requirements for manufacturing and processing, and significant effects of viscosity changes on the pump’s performance.

Learn*: 1. The minimum flow line is primarily designed for high-pressure centrifugal pumps, with a designed minimum flow rate of 30%. The reasons for this are as follows: 1. The outlet valve of the centrifugal pump must be closed before starting the pump; otherwise, the starting current will be too high, which can easily damage the motor. 2. However, when the outlet valve of a high-pressure centrifugal pump is closed, the pressure generated is too high, and this can cause damage to the pump body and its inlet flange. For these reasons, a minimum flow line is designed. The minimum flow line of a centrifugal pump serves the following purposes: 1. When the centrifugal pump operates at low flow rates, fluid separation can occur inside the pump, which can lead to vibration and noise. By setting a minimum flow rate for the pipeline, it is ensured that a certain amount of fluid flows through the pump at all times after it starts up, thereby preventing cavitation ； 2. When a centrifugal pump with high power is operated with the outlet valve closed, the fluid inside the pump becomes increasingly hot; in severe cases, this can damage the bearings. By installing a minimum flow line, it is possible to ensure that a certain amount of fluid flows through the pump at all times while it is running, thereby preventing such issues from occurring ； 3. When operating at low flow rates, the pump shaft is subject to additional radial forces. By installing a minimum flow line, it is ensured that a certain amount of fluid flows through the pump at all times during operation, thereby preventing the pump shaft from being subjected to excessive radial forces. National standards require a minimum return line for systems of 200KW and above. As for the types, there are roughly three. 1. The most economical option is a flow control orifice plate, but for larger pumps, the operating costs increase. 2. Install a control valve on the return line, using the pump’s outlet flow rate for control; this approach requires higher investment and is generally used for pumps with relatively high power – a reference value of over 300 KW applies, and the specific type can be chosen accordingly. 3. Now, pump protection valves have been introduced; a single valve is sufficient to address the issue of minimum backflow, and it can replace the shut-off valve at the pump outlet, the control valve assembly on the minimum flow line, as well as the flow meter at the pump outlet. The investment is higher than that of a flow-limiting orifice plate, but much cheaper than installing a control valve. But it seems that only imports are possible now. The specific choice should be made based on the particular circumstances of the project. II. The disadvantage of screw pumps is that the processing and assembly of the screws require high precision ； The performance of the pump is relatively sensitive to changes in the viscosity of the liquid. Screw pumps have the following advantages: 1. A wide range of pressure and flow rates. The pressure ranges from about 3.4 to 340 kilogram-force/cm2, and the flow rate can reach 18,600 cm3 per minute ； 2. A wide range of liquid types and viscosity levels are available for transportation ； 3. Since the inertial force of the rotating components inside the pump is low, very high rotational speeds can be used ； 4. Good suction performance, with self-priming capability ； 5. Uniform and continuous flow, minimal vibration, low noise ； 6. Compared with other rotary pumps, it is less sensitive to the gases and contaminants that enter ； 7. Robust structure, easy to install and maintain.

1. Under what circumstances is it necessary to establish a minimum flow line for the pump? What is the flow rate of the pump? What type of pump? A minimum flow line should be installed when the minimum flow rate required for operation is close to the pump’s minimum continuous flow rate; if the flow rate is too low, a return orifice plate or return valve must be installed. 2. What are the disadvantages of screw pumps compared to ordinary centrifugal pumps? High manufacturing and processing requirements are needed, and the working properties are sensitive to changes in viscosity

National standards require a minimum return line for systems of 200KW and above. As for the types, there are roughly three. 1. The most economical option is a flow-limiting orifice plate, but for larger pumps, the operating costs increase. 2. A control valve is installed on the return line, with control being achieved through the flow rate at the pump outlet; this approach requires higher investment and is generally used for pumps with relatively high power – a reference value of over 300 KW applies, and the specific type of valve can be selected accordingly. 3. Now, pump protection valves have been introduced; a single valve is sufficient to address the issue of minimum backflow, and it can replace the shut-off valve at the pump outlet, the control valve assembly on the minimum flow line, as well as the flow meter at the pump outlet. The investment is higher than that of a flow-limiting orifice plate, but much cheaper than installing a control valve. But it seems that only imports are possible now. The specific choice should be made based on the particular circumstances of the project. The disadvantage of screw pumps compared to centrifugal pumps is that they require higher precision in screw processing and assembly ； The performance of gear oil pumps is quite sensitive to changes in the viscosity of the liquid.

Most pumps designed abroad are equipped with a minimum flow line as well as flow-limiting orifice plates; a slight increase in energy consumption during long-term operation is of greater importance. It’s rarely used in domestic applications! The minimum flow valve is there to ensure that there is enough flow to remove the heat from the pump, thereby preventing overheating; hence, the higher the power of the pump, the greater this flow rate needs to be. Pumps with low power requirements don’t need this; dissipating heat into the environment is sufficient.

1. If the pump has a high flow rate and its power exceeds a certain level, it is necessary to include a minimum flow line. According to national standards, a minimum flow line must be added when the power exceeds 200 KW. 2. Compared with centrifugal pumps, screw pumps require higher standards in terms of processing and assembly； The performance of the pump is sensitive to changes in the viscosity of the liquid

Come in and learn*. My answer: The minimum flow line of a centrifugal pump serves the following purposes: (1) When the centrifugal pump operates at low flow rates, fluid separation can occur inside the pump, which can lead to vibration and noise. By setting a minimum flow rate for the pipeline, it is ensured that a certain amount of fluid flows through the pump at all times after it starts up, thereby preventing cavitation ； (2) When a centrifugal pump with high power is operated with the outlet valve closed, the fluid inside the pump becomes increasingly hot; in severe cases, this can damage the bearings. By installing a minimum flow line, it is possible to ensure that a certain amount of fluid flows through the pump at all times while it is running, thereby preventing such issues from occurring ； (3) When operating at low flow rates, the pump shaft is subjected to additional radial forces. By installing a minimum flow line, it is ensured that a certain amount of fluid flows through the pump at all times during operation, thereby preventing the pump shaft from being exposed to excessive radial forces ； The disadvantage of screw pumps is that the processing and assembly of the screws require high precision ； The performance of the pump is relatively sensitive to changes in the viscosity of the liquid.

This post was last edited and replied to by netcraft on 2011-4-15 08:45. Reply 1# —Lantian— 1. It is usually set, but not necessarily used. When the pump’s operating point is close to the minimum continuous flow rate point, the minimum flow line needs to be kept active. The selection point for the pump should be the rated flow rate plus the minimum continuous flow rate. This line can also be used to start up the pump. Centrifugal pumps have this issue. 2. Disadvantages: 1. High cost 2. Inadequate for high flow rates

According to relevant regulations: for powers above 200KW, a minimum return line must be installed. Choose specifically based on the particular circumstances of the project

1. National standards require a minimum return line for systems of 200KW and above. As for the types, there are roughly three. a. The most economical option is a flow control orifice plate, but for larger pumps, the operating costs increase. b. A control valve is installed on the return line, with control being achieved through the flow rate at the pump outlet; this approach requires higher investment and is generally used for pumps with relatively high power – a reference value of over 300 KW applies, and the specific type of valve can be selected accordingly. c. Now, pump protection valves exist; a single valve is sufficient to address the issue of minimum backflow, and it can replace the shut-off valve at the pump outlet, the control valve assembly on the minimum flow line, as well as the flow meter at the pump outlet. The investment is higher than that of a flow-limiting orifice plate, but much cheaper than installing a control valve. But it seems that only imports are possible now. 2. The procurement cycle for spare parts is long, the costs for on-site technical support from manufacturers are high, and it’s difficult to get contacts to arrive at the site. Things get even more complicated when the spare parts need to be replaced; the procurement cycle can last half a year or a whole year, which is extremely frustrating. Additionally, the initial investment is high, making it a difficult choice for projects with fixed budgets.