1. The analysis of losing efficacy of mechanical seal to centrifugal pump
The closing down of centrifugal pump is mainly caused by losing efficacy of mechanical seal. The performances of losing efficacy are leakage. There are some reasons contributing to the leakage:
①The main reasons for the leakage of dynamic and static ring sealing surface are: the end surface roughness and roughness did not meet the requirements, or surface scratches; there are particle materials between the end faces, resulting in both same ends can not run; installation is not in place, and the way is incorrect.
②The main reasons for the leakage of the sealing ring are: gland deforms and pre tightening force is uneven; installation is not correct; the quality of the sealing ring does not meet the standard; the sealing ring selection is wrong.
The practical applications show that the most parts of the sealing element failure are the end face of the moving and stationary ring; the failure of most part of the sealing element is due to the end face of the moving and stationary ring. The centrifugal pump is sealed and does not move. The cracking of the end of the static ring is a phenomenon of common failure, and it is mainly due to:
①When installing, the clearance of the sealing surface is too large, and the flushing liquid can not take away the heat produced by the friction pair; the flushing liquid is leaked from the clearance of the sealing surface, and the end face is overheated and damaged.
②The liquid medium evaporates and then expands, making the ends of the two surfaces separated by vaporization and expansion force. And when the two sealing faces are pressed together, the lubricating film is damaged, thereby causing the surface of the end face to overheat.
③The lubricating property of liquid medium is poor, coupled with operating pressure overload, two close cover tracking rotation is not synchronized. For example, the high speed pump is 20445r/min, the central diameter of the sealing surface is 7cm, the pump runs, and its line speed is as high as 75m/s. When there is a sealing surface lag, it can not track the rotation, making the instantaneous high temperature damage the sealing surface.
④The sealing irrigation plate or filter screen block, resulting in insufficient water. So the machine seal fails.
In addition, the surface of the sealing surface slips into the ditch. When the end of face joints, it will cause the sealed elements lose efficacy. The main reasons are:
①The liquid medium is not clean, which has subtle hard particles. The particles will slide into the sealed face at a very high speed, leading to the scoring of the end of faces and to lose efficiency.
②When the pump is opened, the end face of the pump is shaken and rubbed once, and the running track of the moving ring is not concentric, which causes the end surface to vaporize and overheat wear.
③The frequent occurrence of hydraulic characteristics of liquid medium causes the vibration of pump group and causes the sealing surface to be dislocated.
The corrosion of components of the sealing liquid medium has stressful concentration. With the cooperation of soft and hard materials , erosion, 0 seal ring, V ring, concave ring which is not compatible with liquid medium and deformation, the surface of mechanical seal will damage. Thus, we have to have a comprehensive analysis to its damaging form to find out the essential reasons in order to make sure the mechanical seal can run for a long time.
2.The requirements of closing down of centrifugal pump
①When the centrifugal pump stops running , the population valve of the pump shall be closed, and the valves of the attached system shall be closed in turn after the pump cools.
②High temperature pump should be in accordance with the provisions of parking equipment technical documents. After parking, for every 20 to30mins, there should be a partial turning half circle until the pump body temperature goes down to 50 degrees centigrade.
③When low-temperature pump stops and there is without no special requirements, the pump should always be filled with liquid; the suction valve and discharge valve should be kept open; to adopt mechanical seal with double ends of low temperature pump, and the liquid level controller and the sealing liquid in the pump seal cavity should keep the grouting pressure of the pump.
④To transport easy crystallization, easy solidification, easy precipitation and other media pump. When the pump stops, it should be prevented clogging, and it should be timely used water or other media to flush the pump and pipe.
⑤To discharge the liquid within the pump in case that the pump will rust and frost crack.
3. The maintenance of centrifugal pump
①When the pump is not installed well, a suitable rust inhibitor shall be applied on the uncoated surface, and oil lubricated bearings shall be filled with proper grease. Grease lubricated bearings shall be filled with only one kind of grease, but do not use mixed greases.
②To put clean water into the pump , flush, take out and suck up the pipe line and then settle down the pipe line. For the case of the pump and impeller, to clean the case of the pump, and then take out as well as suck up the pipe line. Then, to discharge the flushing fluid of the pipe line.
③To drain the oil in the bearing box, then add clean oil, thoroughly clean the grease, and then fill the new grease.
④To rotate the pump shaft once a month so as not to freeze and lubricate the bearings.
⑤To seal the suction outlet and discharge outlet; to clean the pump and store it in a clean and dry place; to protect the motor windings free from moisture, and then spray the antirust liquid and anti corrosion liquid into the inside parts of the case of the pump.