【Weekly Topic】Production Technology 2011.11.06~11.13 What are the precautions for using centrifugal pumps?

Reply to 1# sun-rock: Precautions when using centrifugal pumps:
I. Pre-start preparations
1. Manually rotate the motor fan blades; the impeller should rotate freely without any jamming or friction. 　　2. Open the inlet valve and the exhaust valve to allow the liquid to fill the entire pump chamber, then close the exhaust valve. 　　3. Rotate the pump by hand to allow the lubricant to reach the mechanical seal faces. 　　4. Operate the motor in jog mode to confirm whether the direction is correct. 　　II. Starting and Operation 　　1. Open the inlet valve fully and close the outlet line valve. 　　2. Connect the power supply. After the pump reaches its normal speed, gradually open the valve on the discharge pipeline and adjust it to the desired operating conditions. 　　3. Pay attention to observing the instrument readings and checking for any leaks in the shaft seal. Under normal conditions, the mechanical seal should leak less than 3 drops per minute. Also, verify that the temperature rise at the motor and bearings does not exceed 70°C. In case of any abnormalities, they must be addressed promptly. 　　III. Parking 　　1. Gradually close the valve on the discharge pipeline and cut off the power supply. 　　2. Close the inlet valve. 　　3. If the ambient temperature is below 0°C, the liquid inside the pump should be drained to prevent it from freezing and cracking. 　　4. If it is to be out of use for an extended period, the pump should be disassembled, cleaned, and stored in packaging.

I. Installation instructions for centrifugal pumps: 1. At the joints of the piping connected to the centrifugal pump, especially at all pipe connections, a reliable and leak-tight connection must be ensured. 2. When a centrifugal pump is installed above the liquid level to be pumped, a foot valve must be installed at the inlet of the suction pipe. For self-priming pumps, however, no foot valve is required at the suction pipe inlet as long as the pump operates within its self-priming range. 3. If the liquid being transported contains hard objects such as bamboo, wood, stones, or metal that are too large or too long in size, a filter must be installed at the water inlet of the suction pipe, to prevent these objects from being drawn into the pump and causing the impeller to get stuck, thereby damaging the motor or the components of the water pump. 4. Centrifugal pump units should generally be equipped with overload and short-circuit protection devices, such as fuses and overcurrent relays, and the setting values of these protection devices should be adjusted according to the current specified on the motor’s nameplate. II. Preparations before starting 　　1. Turn the motor blades by hand; the impeller should rotate smoothly without any jamming. 　　2. Open the inlet valve and the exhaust valve to allow the liquid to fill the entire pump chamber, then close the exhaust valve. 　　3. Rotate the pump by hand to allow the lubricant to reach the mechanical seal faces. 　　4. Operate the motor in jog mode to confirm whether the direction is correct. 　　III. Startup and Operation 　　1. Open the inlet valve fully and close the outlet line valve. 　　2. Connect the power supply. After the pump reaches its normal speed, gradually open the valve on the discharge pipeline and adjust it to the desired operating conditions. 　　3. Pay attention to observing the instrument readings and checking for any leaks in the shaft seal. Under normal conditions, the mechanical seal should leak less than 3 drops per minute. Also, verify that the temperature rise at the motor and bearings does not exceed 70°C. In case of any abnormalities, they must be addressed promptly. 　　IV. Shutdown
1. Gradually close the valves on the discharge piping and cut off the power supply. 　　2. Close the inlet valve. 　　3. If the ambient temperature is below 0°C, the liquid inside the pump should be drained to prevent it from freezing and cracking. 　　4. If it is to be out of use for an extended period, the pump should be disassembled, cleaned, and stored in packaging.

Reply to 1# sun-rock: I. Inspection and preparation work before starting (testing) the pump 1. Remove dirt and debris from the pump, motor, and their surrounding area. Prepare items such as an “F” wrench, stethoscope, rag, and a notebook for recording. 2. Check whether the pump inlet and outlet flanges, valves, and oil sealing lines are intact and free from leaks; ensure that the cooling water flow is unobstructed, and that the oil sealing lines of the hot oil pump are complete and functional ； Check whether the process flow of the towers, vessels, and pipelines connected to the pump is correct ； Whether the relevant process parameters meet the conditions for pump startup. 3. Check whether the manual valve of the pressure gauge is open, whether the pressure gauge is in good condition, and whether its measurement range is appropriate. If the pressure gauge does not meet the requirements, it should be replaced promptly. 4. Check (replace) the lubricating oil in the bearing housing. If the lubricating oil in the original bearing housing contains a large amount of impurities (solid particles, sludge, water) or is emulsified, the bearing housing should first be cleaned with a light-grade oil. After all the impurities have been removed from the housing, the lubricating oil should be replaced 1–2 times, filling the housing to about 1/2 to 2/3 of the level indicated by the sight glass. For pumps that use grease as a lubricant, during inspection the oil cup cover should be opened to check the quality and quantity of the grease inside the cup; if the amount of grease is insufficient, it should be topped up, and if there are impurities in the grease, it should be replaced. 5. Turn the pump 3–4 times in its operating direction; apply even force while doing so. Check whether the rotating parts move smoothly, and whether there is any tightness or jamming. Also check for any abnormal noises inside the pump, and install protective covers on the drive wheels. (Note: If the motor wires are removed for maintenance, the motor must be tested after maintenance to check whether its rotation direction matches that of the pump. 6. Open the inlet and outlet valves of the cooling water valve, check whether the cooling water pipeline is unobstructed, and open the cooling water valve on the machine seal gland as appropriate. 7. With the outlet valve closed, slowly open the inlet valve. While doing so, check whether there is any leakage at the pump’s sealing areas and flange connections (if the leakage is severe, close the inlet valve promptly to address it). Fully open the outlet valve, then slowly open the drain valve (vent valve); once all air inside the pump has been removed, close the drain valve (vent valve) tightly. 8. The hot oil pump should be inspected first: close the drain valve, open the inlet valve, and use the preheating valve (or slightly open the outlet valve by 1–2 turns) to preheat the pump body so that its temperature is at least 40°C higher than the temperature of the incoming oil. The preheating rate should be 50°C per hour; during preheating, turn the pump shaft 180 degrees every half hour and check for any leaks. If a leak is detected, contact the relevant personnel to handle it. 9. Fill out the power supply work order as required, and inform the electrician to supply power. 10. During the preheating process of the hot oil pump, seal oil should be added as appropriate (the seal oil must be thoroughly dehydrated). The cooling water for the mechanical seal gland must not be interrupted. II. Starting and Operation 1. After checking that everything is normal prior to starting the pump, start the motor. Pay attention to whether the rotation direction and sound of the pump are normal after it starts; check the readings on the pressure gauge and ammeter (at this point, the pressure gauge should show the maximum head at the pump’s outlet, with a stable pointer reading, while the current indicated by the ammeter should be at the minimum allowed level). Check for any abnormal noises; feel the bearing housing and motor areas to check for vibrations and overheating of the pump. If any abnormalities are detected, shut down the pump for inspection. 2. Once the pump outlet pressure and motor current return to normal, in coordination with the operator at the pump station, slowly open the outlet valve until they are within the specified control limits. 3. Check for seal leakage; it shall meet the following requirements: (1) When a mechanical seal is used, the leakage rate for light oils shall not exceed 10 drops per minute, and the leakage rate for heavy oils shall not exceed 5 drops per minute ； (2) Use a packing seal: the leakage rate for light oil should be no more than 20 drops per minute, and for heavy oil, it should be no more than 10 drops per minute. (A small amount of leakage from the face seal is allowed when the pump starts up, but it should decrease over time until no leakage occurs; if the leakage increases, the pump must be stopped immediately.) ) 4. Check whether the oil sealing, cooling water, and lubricating oil systems are operating properly. After the hot oil pump is operating properly, adjust the seal oil manual valve so that the seal oil pressure is about 0.5–1.0 kg/cm2 higher than the pressure in the sealing chamber. 5. It is strictly prohibited to run the pump in reverse; after startup, it is not allowed to operate continuously for more than 5 minutes with the outlet valve closed. It is strictly prohibited to use the inlet valve to regulate flow rate. 6. The motor must not be started continuously; 2–3 starts are allowed when it is cold, while only 1 start is permitted when it is hot.

1. Before starting the pump, it is necessary to fill the pump with liquid until liquid emerges from the vent nozzle at the top of the pump casing. 2. Close the outlet valve before starting the pump. When started with zero flow, the pump requires minimal power, thereby preventing the motor from burning out due to overload during startup. After the pump is started, the outlet valve can be gradually adjusted to achieve the desired flow rate. 3. During pump operation, it is necessary to regularly check for leaks and overheating in the packing layer, as well as the tightness of the packing. It is appropriate if the shaft can be turned by hand when the pump is stopped, and if liquid seeps out drop by drop. 4. When shutting down the pump, first close the outlet valves to prevent backflow of liquid, which could cause the impeller nuts to loosen. If the pump will be idle for an extended period, drain all liquid from the pump and piping to prevent corrosion and freezing in winter. 5. When it is necessary to change the flow rate, this can be achieved by adjusting the opening degree of the pump’s outlet valve. Although this requires more power due to increased fluid resistance, it enables rapid adjustment of the flow rate as well as continuous changes, which is suitable for the characteristics of chemical production. 6. Note that it is absolutely forbidden to control the pump by adjusting the opening degree of the inlet valve. Doing so can lead to “cavitation”. When cavitation occurs, the pump body vibrates and makes noise due to the impact; the flow rate and head decrease, and in severe cases, the pump fails to operate properly.

Reply 1# sun-rock: The turntable should be flexible with no sticking； 2 The temperature of the preheating pump should be close to that of the medium, with a temperature difference of no more than 50℃ ； 3 Install the pressure gauge and open its valve. 4 Check the tightness of all screws ； 5 Check the lubrication oil and cooling conditions ； 6 Check for leaks at all sealing points ； 7 Before starting the pump, the inlet valve should be opened to its maximum position, while the outlet valve should remain closed. Open the drain valve to prime the pump.

Here, we use centrifugal pumps in systems at -29°C. A series of problems have occurred in the past; let’s try to list them out. 1. A priming pump is a necessary step; otherwise, air entrapment is likely to occur. 2. After the pump starts, the outlet valve must open slowly; otherwise, large fluctuations in flow rate can lead to unstable process parameters and may even trigger an ESD. 3. I’ve encountered situations multiple times where problems occurred with the shaft seals and liquid seals of centrifugal pumps, resulting in a significant increase in noise levels. Regular inspections are necessary; one must pay close attention to the condition of the shaft seals and liquid seals. 4. When switching pumps, the standby pump must be turned on first, and then the pump that is currently in operation should be turned off. 5. If the pump is to be stopped, the outlet valve must be closed first; this should be clear to everyone.

The trial operation of the pump shall meet the following requirements: ① The rotation direction of the driver shall be opposite to that of the pump; ② Determine the rotation directions of pipeline pumps and coaxial pumps; ③ All fixed connections shall be secure, and the type and quantity of lubricant applied to all lubricated parts shall comply with the provisions in the equipment technical documentation; ④ For parts requiring pre-lubrication, such pre-lubrication shall be carried out as specified; ⑤ All indicating instruments and safety protection devices shall be sensitive, accurate, and reliable; ⑥ Cranking shall be smooth, with no abnormal phenomena; ⑦ Before the trial operation of cryogenic pumps, the pump body must be pre-warmed; the temperature shall rise uniformly, with a maximum hourly temperature increase not exceeding 500°C; the temperature difference between the pump body surface and the process piping at the outlet where the medium exits shall not exceed 4090; ⑧ Install connection devices to mitigate the effects of temperature rise, and install bypass connection devices to provide a cooling water source. 　　When operating centrifugal pumps, the following points should be noted: ① Running the pump without water is prohibited; do not adjust the suction inlet to reduce the discharge volume. Operating the pump at excessively low flow rates is also prohibited. ② Monitor the operation process carefully; completely prevent any leakage from the stuffing box. When replacing the stuffing box, use new packing. ③ Ensure that there is sufficient water flow for flushing the mechanical seal; for water-cooled bearings, excessive water flow must be avoided. ④ Do not use too much lubricant. ⑤ Perform inspections at the recommended intervals. Maintain operation records, including operating hours, adjustments and replacements of fillers, addition of lubricants, as well as other maintenance actions and the time taken for them. For rotary pumps, the suction and discharge pressures, flow rate, input power, temperatures of the wash liquid and bearings, as well as the vibration status should be measured and recorded periodically.

Preparations before starting 1. Turn the motor blades by hand; the impeller should rotate smoothly without any jamming. 　　 　　2. Open the inlet valve and the exhaust valve to fill the entire pump chamber with liquid, then close the exhaust valve. 　　 　　3. Manually turn the pump to allow the lubricant to enter the mechanical seal faces. 　　 　　4. Turn on the motor to verify whether the direction is correct. 　　 Starting and Operation　　　1. Fully open the inlet valve and close the outlet valve. 　　 　　2. Start the pump; once it reaches its normal operating speed, gradually open the outlet valve and adjust it to the desired operating conditions. 　　 　　3. Pay attention to the gauge readings and check for shaft seal leaks; if any abnormalities are detected, they should be addressed promptly. 　　 Shutdown　　　1. Gradually close the outlet valve and stop the pump. 　　 　　2. If the ambient temperature is below 0°C, the inlet valve should be closed to drain the liquid inside the pump, thereby preventing it from freezing and cracking. If it is not used for an extended period, the pump should be disassembled, cleaned, and stored in packaging.

1. High-head pumps are used for low-head pumping; many operators believe that the lower the pumping head, the less load is placed on the motor. Under the influence of this mistaken perception, when selecting a water pump, people often choose a pump with a very high head pressure. In fact, for centrifugal water pumps, once the pump model is determined, the power consumption is proportional to the actual flow rate of the pump. The flow rate of the water pump decreases as the head increases; therefore, the higher the head, the lower the flow rate, and consequently the lower the power consumption. Conversely, the lower the head, the greater the flow rate, and thus the more power is consumed. Therefore, to prevent the motor from overloading, it is generally required that the actual pumping head of the water pump not be lower than 60% of the rated head. Therefore, when high head is used for pumping at too low a head, the motor is prone to overload and overheat, and in severe cases the motor can be damaged. In case of emergency use, a gate valve must be installed on the water outlet pipe to regulate the flow rate (or the small outlet can be blocked with wood or similar materials) in order to reduce the flow and prevent the motor from overloading. Pay attention to the motor’s temperature rise; if the motor overheats, reduce the flow rate at the water outlet or shut it down promptly. This can also lead to misunderstandings; some operators believe that blocking the outlet and forcing a reduction in flow rate will increase the load on the motor. In fact, it’s quite the opposite. On the discharge pipes of standard high-power centrifugal pump irrigation units, gate valves are installed. To reduce the motor load during startup, the gate valve should be closed first; only after the motor has started can it be gradually opened. This is the rationale behind this procedure. 2. Using a large-diameter water pump with small pipes for pumping water: Many operators believe that this approach with centrifugal pumps can increase the actual head pressure, but in fact, the actual head pressure of a water pump = total head pressure – loss head pressure. Once the pump model is determined, the total head remains constant ； The head loss is primarily caused by pipeline resistance; the smaller the pipe diameter, the greater the resistance, and thus the higher the head loss. As a result, when the pipe diameter is reduced, the actual head output of the pump does not increase – instead it decreases, leading to a drop in the pump’s efficiency. Similarly, when a small-diameter water pump is used to pump water through a large-diameter pipe, its actual head does not decrease. On the contrary, the head loss is reduced due to the decreased resistance in the pipeline, thereby increasing the actual head. Some operators believe that when using large pipes to draw water with pumps of small diameter, it will inevitably **increase the load on the motor. They argue that as the pipe diameter increases, the pressure of the water in the outlet pipe on the pump’s impeller rises, which in turn **increases the load on the motor. What is not known is that the magnitude of liquid pressure depends only on the height of the lift, and not on the cross-sectional area of the pipe. As long as the head remains constant and the impeller size of the pump does not change, the pressure acting on the impeller is constant regardless of the pipe diameter. Simply put, when the diameter increases, the flow resistance decreases, which leads to an increase in flow rate; at the same time, power consumption also increases slightly. As long as it remains within the rated head range, the pump can operate properly regardless of the increase in pipe diameter; moreover, this reduces pipeline losses and improves the pump’s efficiency. 3. When installing the water inlet pipeline, ensure that the horizontal section is level or slightly tilted upward; otherwise, air will accumulate inside the inlet pipe, reducing the vacuum level in the pipe and the centrifugal pump, which in turn lowers the water pumping height and reduces the flow rate of water. The correct approach is: its horizontal section should be slightly inclined in the direction of the water source; it should not be horizontal, nor should it tilt upward. 4. Too many elbows are used in the inlet pipeline; if there are many elbows in the inlet pipeline, it will increase local water flow resistance. Moreover, the elbow should turn vertically; turning horizontally is not allowed to prevent air from accumulating. 5. The water inlet of the pump is connected directly to the elbow, which causes uneven distribution of the water flow as it passes through the elbow and enters the impeller. When the diameter of the water inlet pipe is larger than that of the centrifugal pump’s inlet, an eccentric reducer should be installed. The planar part of the eccentric reducer should be installed on top, while the inclined part should be installed at the bottom. Otherwise, air will accumulate, the water output will decrease or no water can be drawn up, and there will be knocking sounds and similar issues. If the diameter of the water inlet pipe is equal to that of the water pump’s inlet, a straight pipe should be added between the pump’s inlet and the elbow, with the length of this straight pipe being no less than 2 to 3 times the diameter of the pipe. 6. The lowest section of the water inlet pipe equipped with a bottom valve is not vertical; when installed in this way, the valve cannot close on its own, resulting in leaks. The correct installation method is: for the inlet pipe equipped with a bottom valve, the lowest section should preferably be vertical. If vertical installation is not possible due to topographical constraints, the angle between the water pipe axis and the horizontal plane should be 60° or more. 7. The inlet position of the water inlet pipe is incorrect. (1) The distance between the inlet of the centrifugal pump’s water inlet pipe and the bottom as well as the walls of the water tank is less than the diameter of the inlet. If there is sediment or other debris at the bottom of the tank, and the distance between the water inlet and the bottom of the tank is less than 1.5 times the diameter, it will result in poor water intake during pumping, or sediment and other particles being drawn in and blocking the water inlet. (2) When the water entry depth at the inlet of the water intake pipe is insufficient, it causes vortices to form on the water surface surrounding the pipe, which affects water intake and reduces the flow rate. The correct installation method is: for small and medium-sized water pumps, the depth of submersion should be no less than 300–600 mm, while for large water pumps, it should be no less than 600–1000 mm. 8. If the outlet is above the normal water level of the discharge tank, although it increases the head of the centrifugal pump, it reduces the flow rate. If, due to topographical constraints, the outlet must be at a higher level than the water level in the outlet tank, elbows and short pipes should be installed at the pipe outlet to create a siphon effect, thereby reducing the height of the outlet

1. Pay attention to the pressure at the pump’s inlet and outlet to prevent blockage of the inlet filter or failure of the pump to operate. 2. Ensure the liquid level in the pump inlet tank is stable to prevent vacuum conditions that could damage the pump. 3. Regularly check the level of lubricating oil and the temperature of the pump bearings, etc., to ensure proper daily maintenance.