Brief analysis of important ways to extend the application cycle of centrifugal pumps
Centrifugal pumps are widely used in petrochemical, coal chemical and other chemical industries to transport liquids of different properties and provide the pressure and flow required for chemical reactions. There are many types of centrifugal pumps, which can be divided into acid pumps, alkali pumps, clean water pumps, mud pumps, etc. according to the nature of the conveying medium. The working temperature and working pressure of the conveying medium are different. Therefore, it is effective to extend the service life of the centrifugal pump and reduce the maintenance amount, which has a great effect on improving the economic efficiency of the factory. 1. Selection and installation of centrifugal pump
The centrifugal pump should be selected according to the liquid to be transported, and the required performance should be checked, and the suction and discharge conditions should be analyzed, whether it is intermittent operation or continuous operation, etc. Centrifugal pumps should usually be operated at or close to the pressure and flow conditions specified by the manufacturer. The following rechecks should be carried out 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 be missing, damaged or rusted;
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;
③The leveling and alignment of the pump should meet the requirements of the equipment technical documents, if there is no requirement, it should meet the requirements of the current national standard “General Specification for Construction and Acceptance of Mechanical Equipment Installation Engineering”;
④All the pipes connected to the pump body, the installation of pipe fittings and the cleaning requirements of the lubricating oil pipes should meet the requirements of relevant national standards.
2, the use of centrifugal pump
The test run of the pump should meet the following requirements:
① The steering of the driver should be the same as that of the pump;
②Find out 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 lubrication part should meet the requirements of the equipment technical document;
④ The parts with pre-lubrication requirements should be pre-lubricated according to regulations;
⑤All indicating instruments and safety protection devices should be sensitive, accurate and reliable;
⑥ The cranking should be flexible and free of abnormal phenomena;
⑦The high temperature pump should be preheated before trial operation, the temperature should rise uniformly, and the hourly temperature rise should not be greater than 500℃; the temperature difference between the surface of the pump body and the process pipeline with the working medium inlet should not be greater than 4090;
⑧ Set up a connection device to eliminate the effect of temperature rise, and set up a bypass connection device to provide a source of cooling water.
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 to reduce the displacement, and it is forbidden to run under too low flow;
②Monitor the operation process, completely prevent the stuffing box from leaking, and use new stuffing when replacing the stuffing box;
③Ensure that the mechanical seal has sufficient flushing water flow, and excessive water flow is prohibited for water-cooled bearings;
④Do not use too much lubricant;
⑤Inspect according to the recommended cycle. Establish operating records, including operating hours, adjustment and replacement of fillers, adding lubricants and other maintenance measures and time. The centrifugal pump suction and discharge pressure, flow rate, input power, temperature of washing liquid and bearing, and vibration should be measured and recorded regularly.
3. Maintenance of centrifugal pump
3.1. Analysis of the failure of the mechanical seal of the centrifugal pump
The downtime of the centrifugal pump is mainly caused by the failure of the mechanical seal. Most of the failure manifestations are leakage. The reasons for leakage are as follows:
① The main reasons for the leakage of the dynamic and static ring sealing surfaces are: the flatness of the end surface, the roughness does not meet the requirements, or the surface is scratched; there is particulate matter between the end surfaces, which causes the two ends to not operate at the same time; the installation is not in place and the method is incorrect.
②The main reasons for the leakage of the sealing ring of the compensation ring are: the gland is deformed and the pre-tightening force is uneven; the installation is not correct; the quality of the sealing ring does not meet the standard; the type of the sealing ring is incorrect.
The actual use effect shows that the most failure of the sealing element is the end face of the dynamic and static ring. The centrifugal pump seals the dynamic and the end face of the static ring is a common failure phenomenon. The main reasons are:
①When the sealing surface gap is too large during installation, the flushing fluid is too late to take away the heat generated by the friction pair; the flushing fluid leaks from the sealing surface gap, causing the end surface to overheat and damage.
② The liquid medium vaporizes and expands, so that the two end faces are separated by the vaporization and expansion force. When the two sealing faces are firmly attached, the lubricating film is destroyed and the end face surface is overheated.
③The liquid medium has poor lubricity, and the operating pressure is overloaded, and the tracking and rotation of the two sealing surfaces are not synchronized. For example, the high-speed pump speed 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 the pump is running. When a sealing surface lags behind and cannot track the rotation, the instantaneous high temperature causes the sealing surface to be damaged.
④ The orifice plate or filter screen of the sealing flushing fluid is blocked, resulting in insufficient water volume and failure of the mechanical seal.
In addition, the surface of the sealing surface is slippery, and gaps appear when the end faces are attached, which leads to the failure of the sealing element. The main reasons are:
①The liquid medium is not clean and has tiny hard particles. It slides on the sealing surface at a very high speed, and the end surface is scratched and invalidated.
②The coaxiality of the transmission parts of the pump is poor. After the pump is turned on, the end face is swayed and rubbed once per revolution, and the moving ring runs out of concentricity, causing the end face to vaporize, overheating and abrasion.
③ The frequent occurrence of hydraulic characteristics of the liquid medium causes the pump unit to vibrate, causing the sealing surface to be misaligned and fail.
The corrosion of the sealing element by the liquid medium, stress concentration, soft and hard material coordination, erosion, auxiliary sealing 0-ring, V-ring, concave ring and liquid medium incompatibility, deformation, etc. will cause damage and failure of the mechanical seal surface, so The damage form must be analyzed comprehensively to find the root cause to ensure the long-term operation of the mechanical seal.
3.2, the requirements after the centrifugal pump stops running
① After 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 has cooled down.
②The shutdown of the high-temperature pump should be carried out in accordance with the equipment technical documents. After the shutdown, it should be turned for a half circle every 20 to 30 minutes until the temperature of the pump body drops to 50°C.
③When the cryopump is stopped, when there is no special requirement, the pump should always be filled with liquid; the suction valve and the discharge valve should be kept normally open; the cryopump with double mechanical seals, the liquid level controller and the sealing liquid in the pump seal cavity The grout pressure of the pump should be maintained.
④ Pumps that transport media that are easy to crystallize, solidify, precipitate, etc., should be prevented from clogging after stopping the pump, and flush the pump and pipeline with clean water or other media in time. ⑤Drain the liquid accumulated in the pump to prevent rust and frost cracking.
3.3. Storage of centrifugal pump
①The un-installed pump should be coated with a suitable anti-rust agent on the unpainted surface. Bearings lubricated with oil should be filled with proper oil, bearings lubricated with grease should be filled with only one kind of grease, do not Use mixed grease.
②Pump clean liquid in a short time, flush, suction pipeline, discharge pipeline, pump casing and impeller, and drain the flushing liquid in pump casing, suction pipeline and discharge pipeline side by side.
③Drain the oil in the bearing box, add clean oil, thoroughly clean the grease and then fill with new grease.
④ Seal the suction port and discharge port, store the pump in a clean and dry place, protect the motor windings from moisture, and spray the inside of the pump housing with anti-rust and anti-corrosion liquid.
⑤The pump shaft rotates once a month to avoid freezing and lubricate the bearings
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