Selection, use and maintenance of centrifugal pump

1. Selection and installation of centrifugal pump
Centrifugal pump should be selected according to the liquid delivered, and check the required performance, analyze the suction and discharge conditions, whether intermittent or continuous operation, etc. The centrifugal pump shall normally operate at or close to the pressure and flow conditions specified by the manufacturer. The following rechecks shall be carried out during pump installation:
(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. The machine should not be missing, damaged or rusted;
(2) according to the characteristics of the medium delivered by the pump, the material of main parts, shaft seals and gaskets should be checked if necessary;
(3) the leveling and alignment of the pump shall comply with the provisions of the technical documents of the equipment. If there is no regulation, it shall comply with the provisions of the current national standard "general code for construction and acceptance of mechanical equipment installation engineering";
(4) the installation of all pipes and fittings connected with the pump body and the cleaning requirements of lubricating oil pipes shall comply with relevant national standards.
2. Use of centrifugal pump
The test run of centrifugal pump shall meet the following requirements:
The steering direction of the driving machine should be the same as that of the pump;
(2) find out the steering direction of pipeline pump and coaxial pump;
(3) each fixed connection part shall be free from looseness, and the specification and quantity of lubricant applied to each lubricating part shall comply with the provisions of technical documents of the equipment;
(4) parts with pre lubrication requirements shall be pre lubricated according to regulations;
(5) all indicating instruments and safety protection devices shall be sensitive, accurate and reliable;
(6) the turning gear should be flexible without abnormal phenomenon;
(7) the pump body shall be preheated before the test run, and the temperature shall rise evenly, and the temperature rise shall not be greater than 50 ℃ per hour; the temperature difference between the pump body surface and the process pipeline with working medium inlet shall not be greater than 40 ℃;
(8) set up connection device to eliminate the influence of temperature rise, and set bypass connection device to provide cooling water source.
Centrifugal pump operation should pay attention to the following points:
(1) it is forbidden to operate without water, do not adjust the suction population to reduce the displacement, and it is forbidden to operate under low flow rate;
(2) monitor the operation process to prevent the leakage of the stuffing box, and use new packing when replacing the stuffing box;
(3) ensure that the mechanical seal has sufficient flushing water flow, and excessive water flow is prohibited for water-cooled bearings;
Do not use too much lubricant;
(5) check according to the recommended period. Establish operation records, including operating hours, packing adjustment and replacement, lubricant addition and other maintenance measures and time. The suction and discharge pressure, flow rate, input power, temperature of washing liquid and bearing and vibration of centrifugal pump should be measured and recorded regularly.
(6) the main engine of the centrifugal pump pumps the water from the low place to the high place by relying on atmospheric pressure, which can only support about 10.3M water column at most, so the main engine of the centrifugal pump cannot work when it is 12 meters away from the water surface.
3. Maintenance of centrifugal pump
3.1 failure analysis of centrifugal pump mechanical seal
The shutdown of centrifugal pump is mainly caused by the failure of mechanical seal. The performance of failure is mostly leakage, and the leakage causes are as follows:
The main reasons for the leakage of the sealing surface of the dynamic and static rings are as follows: the flatness and roughness of the end face do not meet the requirements, or the surface is scratched; there is particulate matter between the end faces, which causes the two ends of the seal surface to not run the same way; the installation is not in place and the way is incorrect.
The main reasons for the leakage of the compensation ring seal ring are: the gland is deformed, the preload is not uniform, the installation is not correct, the quality of the sealing ring does not meet the standard, and the type selection of the sealing ring is not correct.
The practical application results show that the most failure parts of the sealing elements are the end faces of the dynamic and static rings. The cracking of the end faces of the static rings is a common failure phenomenon when the centrifugal pumps are sealed
(1) during installation, the clearance between the sealing faces is too large, and the flushing fluid has no time to take away the heat generated by the friction pair; the flushing fluid leaks out from the seal face gap, causing the end face to be overheated and damaged.
(2) the liquid medium vaporizes and expands, so that the two end faces are separated by the vaporization expansion force. When the two sealing surfaces are forced to fit together, the lubricating film will be destroyed and the end surface will be overheated.
(3) the lubrication of liquid medium is poor, and the operating pressure is overloaded, and the tracking rotation of the two sealing faces is not synchronous. For example, the rotating speed of high-speed pump is 20445r / min, and the center diameter of sealing surface is 7cm. After the pump is running, its linear speed is as high as 75 m / s. when one sealing surface lags behind and cannot track the rotation, the instantaneous high temperature will damage the sealing surface.
(4) the orifice plate or filter screen of sealing flushing fluid is blocked, resulting in insufficient water and failure of mechanical seal.
In addition, the sliding groove on the surface of the sealing surface and the gap during the end face fitting lead to the failure of the sealing element
(1) the liquid medium is not clean, and there are tiny hard particles, which slide onto the sealing surface at a high speed, which will scratch the surface of the end face and become invalid.
(2) the coaxiality of the transmission parts of the pump is poor. After the pump is started, the end face is shaken and rubbed once every revolution. The moving ring running track is not concentric, resulting in the vaporization of the end face, overheating and wear.
(3) the frequent occurrence of hydraulic characteristics of liquid medium causes the vibration of pump set, resulting in dislocation of sealing surface and failure.
The corrosion of liquid medium to sealing elements, stress concentration, matching of soft and hard materials, erosion, auxiliary seal 0-ring, V-ring, concave ring and liquid medium incompatibility, deformation, etc. will cause mechanical seal surface damage and failure, so the damage form should be comprehensively analyzed to find out the root cause to ensure the long-term operation of mechanical seal.
3.2 requirements after the centrifugal pump stops running
(1) after the centrifugal pump stops running, close the population valve of the pump, and then close the valves of the auxiliary system successively after the pump cools down.
(2) the shutdown of high temperature pump should be in accordance with the technical documents of the equipment