The idea of using centrifugal force to transport water first appeared in a sketch by Leonardo da Vinci. In 1689, French physicist Papin invented the volute centrifugal pump with a four-blade impeller. But closer to the modern centrifugal pump is the so-called Massachusetts pump with radial straight blades, semi-open double suction impeller, and volute that appeared in the United States in 1818. From 1851 to 1875, multi-stage centrifugal pumps with guide vanes were invented one after another, making it possible to develop high-lift centrifugal pumps.
Although as early as 1754, the Swiss mathematician Euler proposed the basic equations of impeller hydraulic machinery and laid the theoretical foundation for the design of centrifugal pumps. advantages can be brought into full play. On the basis of the theoretical research and practice of many scholars such as Renault in the United Kingdom and Pfleidrell in Germany, the efficiency of the centrifugal pump has been greatly improved, its performance range and application field have also been expanded, and it has become the most widely used in modern times. The largest pump ever.
Selection and installation of centrifugal pumps
Centrifugal pumps should be selected according to the liquid to be transported, and the required performance should be checked, and the conditions of suction and discharge should be analyzed, whether it is an intermittent or continuous operation. Centrifugal pumps should normally operate at or near the pressure and flow conditions specified by the manufacturer’s design. The following review should be performed when the pump is installed:
① The size, position, and elevation of the foundation should meet the design requirements, the anchor bolts must be properly and correctly fixed in the concrete foundation, and the machine should not have missing parts, damage, or corrosion;
②According to the characteristics of the medium conveyed by the pump, the materials of main parts, shaft seals, and gaskets should be checked if necessary;
③ The leveling and alignment of the pump should comply with the provisions of the equipment technical documents, if not, it should comply with the current national standard “General Specifications for the Construction and Acceptance of Mechanical Equipment Installation Engineering”;
④ All the pipes connected to the pump body, the installation of pipe fittings, and the cleaning requirements of the lubricating oil pipes shall comply with the relevant national standards.
The use of the centrifugal pump
The test run of the pump should meet the following requirements:
①The steering of the driver should be the same as that of the pump;
② Identify the steering of the pipeline pump and the coaxial pump;
③ There should be no looseness in each fixed connection part, and the specification and quantity of lubricant added to each lubricating part should comply with the provisions of the technical documents of the equipment;
④ The parts with pre-lubrication requirements should be pre-lubricated according to regulations;
⑤ All indicating instruments and safety protection devices should be sensitive, accurate, and reliable;
⑥ The crank should be flexible and have no abnormal phenomenon;
⑦The high-temperature pump should be preheated before the trial operation, the temperature should rise evenly, and the temperature rise per hour should not be greater than 50℃; the temperature difference between the surface of the pump body and the processing pipeline with the working medium inlet should not be greater than 40℃;
⑧ Set up a connection device to eliminate the influence of temperature rise and set up a bypass connection device to provide a cooling water source.
The following points should be paid attention to when operating the centrifugal pump:
①It is forbidden to run without water, do not adjust the suction port to reduce the displacement, and it is forbidden to run under too low flow;
② Monitor the operation process, completely prevent the leakage of the stuffing box, and use new packing when replacing the stuffing box;
③ Ensure that the mechanical seal has sufficient water flow for flushing, and excessive water flow is prohibited for water-cooled bearings;
④ Do not use too much lubricant;
⑤Check according to the recommended cycle. Establish operating records, including operating hours, adjustment and replacement of packing, the addition of lubricants, and other maintenance measures and times. The suction and discharge pressure, flow rate, input power, the temperature of the washing liquid and bearing, and vibration of the centrifugal pump should be regularly measured and recorded.
⑥ The main engine of the centrifugal pump relies on atmospheric pressure to pump the water from the low place to the high place, and the atmospheric pressure can only support a water column of about 10.3m at most, so the main engine of the centrifugal pump cannot work 12 meters away from the water surface.
Maintenance of centrifugal pumps
1. Analysis of mechanical seal failure of centrifugal pump
Centrifugal pump shutdown is mainly caused by the failure of the mechanical seal. The performance of failure is mostly leakage, and the reasons for leakage are as follows:
①The main reasons for the leakage of the sealing surface of the dynamic and static rings are: the flatness and roughness of the end surface do not meet the requirements, or the surface is scratched; there is a particulate matter between the end surfaces, which causes the two ends to operate in the same way; the installation is not in place and the method is incorrect.
②The leakage of the sealing ring of the compensation ring is mainly due to: deformation of the gland and uneven preload; incorrect installation; the quality of the sealing ring does not meet the standard; the selection of the sealing ring is incorrect.
The actual use results show that the most failed parts of the sealing element are the end face of the dynamic and static rings, the sealing of the centrifugal pump, and the cracks on the end face of the static ring are common failure phenomena.
The main reasons are:
① The gap between the sealing surfaces is too large during installation, and the flushing fluid cannot take away the heat generated by the friction pair; the flushing fluid leaks from the gap between the sealing surfaces, causing the end surface to be overheated and damaged.
② The liquid medium vaporizes and expands so that the two end faces are separated by the vaporizing expansion force. When the two sealing surfaces are forced to fit together, the lubricating film is destroyed and the end surface is overheated.
③The lubricity of the liquid medium is poor, the operating pressure is overloaded, and the tracking and rotation of the two sealing surfaces are not synchronized. For example, the speed of the high-speed pump is 20445r/min, the center diameter of the sealing surface is 7cm, and the linear speed of the pump is as high as 75 m/s after running.
④The orifice plate of the sealing flushing liquid or the filter screen is blocked, resulting in insufficient water volume and invalidating the mechanical seal.
In addition, the sliding groove on the surface of the sealing surface, and the gap when the end face is attached will cause the failure of the sealing element.
The main reasons are:
①The liquid medium is not clean, and there are tiny hard particles, which slide on the sealing surface at a high speed, and scratch the surface of the end surface and fail.
②The coaxiality of the transmission parts of the pump is poor. After the pump is turned on, the end face is shaken and rubbed once every revolution, and the running track of the moving ring is not concentric, causing the end face to vaporize and overheat to wear.
③ The frequent occurrence of hydraulic characteristics of the liquid medium causes the pump unit to vibrate, resulting in the dislocation of the sealing surface and failure.
Corrosion of liquid medium to sealing elements, stress concentration, soft and hard material coordination, erosion, auxiliary seal O-ring, V-ring, the concave ring is incompatible with a liquid medium, deformation, etc. will cause damage to the mechanical seal surface, so It is necessary to comprehensively analyze the damage form, find out the root cause, and ensure the long-term operation of the mechanical seal.
2. Requirements after the centrifugal pump stops running
①After the centrifugal pump stops running, the inlet valve of the pump should be closed, and the valves of the auxiliary system should be closed in turn after the pump cools down.
② The high-temperature pump should be stopped according to the provisions of the equipment technical documents. After stopping, the crank should be turned for half a circle every 20 to 30 minutes until the temperature of the pump body drops to 50 °C.
③When the cryogenic pump is stopped, if there is no special requirement, the pump should be filled with liquid frequently; the suction valve and discharge valve should be kept normally open; the cryogenic pump with double-end mechanical seal, the liquid level controller and the sealing liquid in the pump sealing cavity The grouting pressure of the pump should be maintained.
④For pumps that transport media that are easy to crystallize, solidify, and precipitate, etc., stop the pump to prevent blockage, and flush the pump and pipeline with clean water or other media in time.
⑤ Drain the accumulated liquid in the pump to prevent rust and freeze cracking.
3. Storage of centrifugal pump
①The pump that has not been installed should be coated with a suitable anti-rust agent on the unpainted surface. The bearing lubricated with oil should be filled with appropriate oil, and the bearing lubricated with grease should be filled with only one kind of grease. Use mixed grease.
②Pump clean liquid in a short time, flush, the suction line, discharge line, pump casing, and impeller, and drain the flushing liquid in the pump casing, suction line, and discharge line side by side.
③ Drain the oil in the bearing housing, refill with clean oil, thoroughly clean the grease and refill with new grease.
④ Seal the suction port and discharge port, store the pump in a clean and dry place, protect the motor winding from moisture, and spray the inside of the pump casing with anti-rust and anti-corrosion liquid.
⑤The pump shaft rotates once a month to avoid freezing and lubricate the bearings.