TYPICAL FAILURE ANALYSIS OF MECHANICAL SEALS OF THE MAIN OIL PUMP

West Gansu Oil and Gas Branch Company of CNPC Pipeline United Co., Ltd. uses multi-stage centrifugal crude oil pumps at the oil delivery station. The shaft ends of the drive and non-drive ends use a set of mechanical seals. Since it was put into production, multiple pumps on the drive and non-drive ends have experienced various degrees of oil rejection and leakage. During the operation process, the faulty mechanical seal was disassembled on the spot combined with the phenomenon of mechanical seal failure, the causes were found, maintenance measures were taken and field tests were conducted, and good results have been achieved.

The mechanical seal is a shaft seal device that keeps a pair or pairs of end face that are perpendicular to the shaft and slide relative to the shaft, under the action of the fluid pressure and the elastic force of the compensation mechanism, they maintain attached and matched with the auxiliary seal to achieve leakage resistance. Commonly used mechanical seal is composed of static ring, rotating ring (moving ring), elastic component, spring seat, set screw, auxiliary sealing ring of rotating ring, and static ring, etc. The anti-rotation pin is fixed on the cover to prevent the static ring from rotating.

Reasons and Solutions of the Oil Leakage of Mechanical Seals

1. Leakage under Static Conditions

At the initial stage of pump operation, the mechanical seal leakage occurred under the non-operating conditions of stamping. According to the factory design requirements of the pump, the population pressure of the pump is 0.4 to 1.6 MPa, and the outlet pressure is 11.2 to 14.7 MPa. Because this type of mechanical seals adopts the PLAN31 plane, and the population pressure is increased according to the valve closing head and the impeller head, that is: seal chamber pressure = (0.4 ~ 1.6) + 1455x 9.81 x 0.8437 / 1000 = (0.4 ~ 1.6) + 1.72 = 2.12 ~ 3.32 MPa. The dynamic pressure of this model mechanical seal is 51.7har, and the static pressure is 72bar, fully meeting the pressure level of the on-site seal cavity, so the mechanical seal selection is appropriate. Then we carried out the mechanical seal disassembly inspection, after opening the mechanical seal, it was found that yellow scale adhered to the surface of the dynamic and static ring and the spring seat. After cleaning the internal of the mechanical seal and replacing the dynamic and static rings, re-installing the stamping test, the phenomenon of static pressure leakage disappears.

2) When the pump is running dynamically, there is intermittent oil leakage of the mechanical seal.

When the pump unit is in operation, the amount of oil thrown by the mechanical seal in summer is significantly less than the amount of oil thrown in winter. By taking the average of the physical property parameters of the oil products transported in winter and summer, it was found that there is a certain relationship between the change of oil properties and the oil leakage of the mechanical seal on site. In summer and winter, the two mechanical seals that have oil leakage were disassembled, it was found that there are more deposits in the static ring seat of the mechanical seal of the running pump in winter, and the surface of the compensation spring is adhered by thick condensate. Due to the high viscosity of the crude oil and poor fluidity, coupled with the low outdoor temperature in winter, the temperature of the sewage pipeline is not enough, and the crude oil leaked in the sewage pipeline cannot be completely flowed into the Plan65 system in time, or the Plan65 system is blocked due to the condensation of crude oil, resulting in some crude oil being thrown out of the mechanical seal lock ring.

Measures: first add an annular groove between the static ring and the throttle ring, which is conducive to the storage of crude oil leaking from the end face and quickly lead to the discharge port and enter the PLAN65 collection pipeline, maximize the leakage of crude oil into the drain and flowing into the collection system to avoid throwing along the axis. Secondly, rearrange the heating system of the mechanical seal leakage collection sewage pipeline to ensure that the internal temperature of the sewage pipeline is enough to allow the leaked crude oil to flow downstream in time.

3) Reasons and solutions for excessive leakage of mechanical seals

Since the large leakage of this pump seal had caused the phenomenon of stopping the pump during operation, we conducted on-site inspection of the defective mechanical seal, it was found that wax and colloids phenomena occurred inside the seal. The situation of accumulation of condensate on both sides of the dynamic O-ring and ring surface was determined. Wax formation, colloids, etc, are the key reasons that cause the seal to not be well compensated for floating and thus the amount of leakage is excessive large.

Measures: 

A. When the pump unit is stopped, the temperature of the pump body is low, which will precipitate wax or gums. You can warm the pump before starting the pump.

B. When colloids and waxes appear during the normal operation of the pump, it is necessary to test whether the precipitation is caused by the excessive low temperature of the crude oil. If it is not due to physical reasons, it belongs to the design selection defect of the special working condition of mechanical seal.

Later, by adopting the modification of the mechanical seal, the spring seal was changed to the elastomer bellow seal. First, the pressure resistance is enhanced, and the linear relationship of the elastic force is adjusted. The internal diameter is designed at a 45°, which will prevent the viscous particles of spring seals from being thrown out and realize the automatic cleaning function. Secondly, the modified bellows mechanical seal installs the compensation part in the static ring seat. Through the modification, it not only greatly reduces the incomplete bonding of the dynamic and static rings caused by the spring being glued, but also avoids the 0-ring in the moving ring seat from being condensed by the glue. The leakage is significantly less and the effect is good after modification.