An in-depth analysis of centrifugal pumps

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Centrifugal pumps are widely used in chemical industries such as petrochemical and coal chemical industries to transport liquids of different properties, and to provide the pressure and flow rate required for chemical reactions. There are many types of centrifugal pumps, which can be classified into acid pumps, alkali pumps, clean water pumps, slurry pumps, etc., depending on the properties of the medium to be transported. Since the operating temperature and pressure of the medium being transported vary, effectively extending the service life of centrifugal pumps and reducing the need for maintenance plays a significant role in improving the economic efficiency of factories. 1. Selection and installation of centrifugal pumps: Centrifugal pumps should be selected based on the liquid to be pumped, and their required performance characteristics must be evaluated. Factors such as the conditions for suction and discharge, as well as whether the pump will operate intermittently or continuously, also need to be taken into account. Centrifugal pumps should generally operate at or near the pressure and flow conditions specified by the manufacturer. The following checks should be carried out when installing the pump: ① The dimensions, position, and elevation of the foundation must meet the design requirements; the foot bolts must be properly and correctly fixed in the concrete foundation, and the machine must not be missing any parts, be damaged, or corroded ； ②Depending on the properties of the medium pumped by the pump, it is necessary to verify the materials of the main components, shaft seals, and gaskets as required ； ③The leveling and alignment of the pump shall comply with the provisions in the equipment’s technical documents; in the absence of such provisions, it shall conform to the requirements of the current **standard \"General Specifications for the Construction and Acceptance of Mechanical Equipment Installation Projects\" ； ④All pipes connected to the pump body, the installation of pipe fittings, and the cleaning of lubrication oil pipelines must comply with the provisions of relevant **standards. 2. Use of centrifugal pumps: The trial operation of the pump shall meet the following requirements: ① The rotation direction of the drive mechanism shall be the same as that of the pump ； ②Determine the rotation direction of the pipeline pump and the co-axial pump ； ③All fixed connections should be secure, and the type and quantity of lubricant applied to all lubrication points must comply with the specifications stated in the equipment’s technical documents ； ④Areas that require pre-lubrication should be pre-lubricated as specified ； ⑤All indicating instruments and safety protection devices must be sensitive, accurate, and reliable ； ⑥The turning gear should operate smoothly, without any abnormal phenomena ； ⑦Before trial operation, the pump body of a high-temperature pump should be preheated, with the temperature rising evenly; the temperature increase per hour should not exceed 500℃ ； The temperature difference between the surface of the pump body and the process pipeline with the working medium inlet should not exceed 4090 ； ⑧Install connection devices to eliminate the impact of temperature rise, and set up bypass connection devices to provide a cooling water source. The following points should be noted when operating a centrifugal pump: ① Operation without water is prohibited; do not adjust the suction inlet to reduce the flow rate, and operation at excessively low flow rates is also prohibited ； ②Monitor the operation process to completely prevent leaks in the packing box; use new packing when replacing it ； ③Ensure that the mechanical seal has an adequate flow of water for flushing; excessive water flow is not allowed for water-cooled bearings ； ④Do not use too much lubricant ； ⑤Conduct inspections at the recommended intervals. Maintain operation records, including hours of operation, adjustments and replacements of fillers, addition of lubricants, as well as other maintenance actions and the times at which they were carried out. The suction and discharge pressures, flow rate, input power, as well as the temperatures of the fluid and bearings of the centrifugal pump, along with its vibration levels, should all be measured and recorded regularly. 3. Maintenance of centrifugal pumps 3.1 Analysis of mechanical seal failure in centrifugal pumps. The shutdown of centrifugal pumps is mainly caused by the failure of their mechanical seals. The most common symptom of failure is leakage, and the reasons for leakage include the following: ① Leakage at the sealing surfaces of the stationary and rotating rings; the main causes are that the flatness and roughness of these surfaces do not meet the required standards, or there are scratches on the surfaces ； There are particulate materials between the end faces, preventing the two end faces from operating in the same manner ； It was not installed properly; the method used was incorrect. ②The leakage of the compensation ring seal is mainly caused by: deformation of the gland and uneven pre-tightening force ； Incorrect installation ； The quality of the sealing ring does not meet the standards ； The wrong sealing ring was selected. Actual usage experience shows that the areas where sealing elements fail most frequently are the end faces of the rotating and stationary rings. Cracking on the end faces of these rings in centrifugal pump seals is a common failure phenomenon. The main reasons for this include: ① An excessive gap between the sealing surfaces during installation, which prevents the flushing fluid from removing the heat generated by the friction between the components ； The flushing fluid leaks through the gaps in the sealing surfaces, causing overheating of the end faces and resulting in damage. ②The vaporization and expansion of the liquid medium cause the two end surfaces to be separated due to the forces resulting from this expansion. When the two sealing surfaces are pressed together tightly, the lubrication film is disrupted, leading to overheating of the surface of those end surfaces. ③The liquid medium has poor lubricity, and coupled with excessive operating pressure, the two sealing surfaces do not rotate in sync. For example, in a high-speed pump with a rotational speed of 20,445 r/min and a center diameter of the sealing surface of 7 cm, the linear velocity of this surface can reach as high as 75 m/s once the pump is in operation. If one of the sealing surfaces lags behind and fails to keep up with the rotation, the resulting instantaneous high temperature can cause damage to that sealing surface. ④The orifice plate or filter screen of the sealing flush fluid is clogged, resulting in insufficient water flow and thus causing the mechanical seal to fail. Furthermore, surface grooves on the sealing surface, as well as gaps that occur when the end faces come into contact, can lead to the failure of the sealing element. The main reasons for this are: ① The liquid medium is not clean and contains small, hard particles that move at high speeds across the sealing surface, scratching its surface and causing failure. ②The coaxiality of the pump’s driving components is poor; once the pump starts operating, the end face comes into contact and rubs against something once for each rotation. The path taken by the moving ring is not concentric, which leads to vaporization of the end face and excessive wear. ③Frequent changes in the hydraulic properties of the liquid medium cause vibration in the pump set, leading to misalignment of the sealing surfaces and their failure. Corrosion of the sealing elements by liquid media, stress concentration, the combination of soft and hard materials, erosion, auxiliary sealing O-rings, V-rings, and concave rings that are incompatible with the liquid medium, as well as deformation, can all cause damage and failure of the mechanical seal surface. Therefore, it is necessary to conduct a comprehensive analysis of the forms of damage in order to identify the root causes and ensure the long-term proper operation of the mechanical seal. 　　3.2 Requirements after the centrifugal pump stops operating: ① After the centrifugal pump stops, the inlet valve of the pump should be closed; once the pump has cooled down, the valves of the auxiliary systems should be closed one by one. ② The shutdown of high-temperature pumps must be carried out in accordance with the specifications provided in the equipment’s technical documents. After shutdown, the pump shaft should be turned half a turn every 20–30 minutes until the temperature of the pump body drops to 50°C. ③When a cryopump is shut down, unless there are special requirements, the pump should always be filled with liquid ； The intake valve and exhaust valve should remain in an open state ； In cryopumps equipped with double-end face mechanical seals, the liquid level controller and the sealing fluid in the pump’s seal chamber must maintain the pump’s filling pressure. ④Pumps used to transport media that are prone to crystallization, solidification, or precipitation should be prevented from becoming clogged after shutdown, and the pump and pipelines should be promptly rinsed with clean water or other appropriate media. ⑤Drain the liquid accumulated in the pump to prevent rust and freezing damage. 3.3 Storage of centrifugal pumps ① For pumps that have not yet been installed, a suitable rust inhibitor should be applied to their un painted surfaces. Bearings that are lubricated with oil should be filled with the appropriate oil, while bearings lubricated with grease should be filled with only one type of grease; mixed greases should not be used. ②Pump in clean liquid for a short period to flush the suction line, discharge line, pump casing, and impeller, then drain the flushing liquid from the pump casing, suction line, and discharge line. ③Drain the oil from the bearing housing, refill it with clean oil, thoroughly clean the grease, and then fill it with new grease. ④Seal the intake and exhaust ports, store the pump in a clean, dry place to protect the motor windings from moisture, and spray the inside of the pump casing with rust preventive and anti-corrosion fluids. ⑤The pump shaft is rotated once a month to prevent freezing, and the bearings are lubricated