FAILURE ANALYSIS AND MODIFICATION OF MECHANICAL SEALS OF CATALYTIC RESIDUAL OIL PUMP

A petrochemical company has a 800 kU/a catalytic device, it introduced two screw pumps in 2006, and the conveying medium was residual oil. The medium temperature of the two screw pumps is 200 C, the inlet pressure is normal pressure, and the outlet pressure is 0.47 MPa.

Since the installation of these two pumps, there have been frequent failures of mechanical seal leakage. The service life of the mechanical seal is only about 2 months on average. Frequent maintenance makes the accessories and maintenance costs very high. The nature of the medium make the maintenance very troublesome for the workshop and the maintenance unit. Therefore, it is urgent to solve the problem of frequent leakage of the two pump seals.

1. Causes Analysis of Frequent Leakage of Mechanical Seals

1) The structure of mechanical seals

The pump has a twin-screw structure. The 4 sets of mechanical seals used were originally molded mechanical seals selected by pump manufacturers, which are elastomer bellow seals. Between the mechanical seal static ring and the static ring gland, a PTFE sealing ring with a rectangular cross section is used for sealing. During assembly, the static ring is clamped by the squeezing deformation force of the sealing ring to position the static ring axially and radially. The bellows moving ring is fixed on the shaft with 6 jacking screws.

2) Causes analysis of mechanical seal failure

From the analysis of seal structure, material and pump operation, it is believed that the frequent failure of the mechanical seal is due to the following reasons.

A. There is evacuation at the seal.

B. Problems with the sealed static ring.

C. Process operation problem.

2. Modification of Mechanical Seals

After the above analysis, it is decided to modify the mechanical seal of the screw pump. Since the static metal bellows seal is the main choice for high temperature applications, so the static ring of the mechanical seal is changed to a bellows structure. After the static ring is installed in the gland, it is bolted. It is sealed with a compact graphite gasket in the middle. The coaxial connection structure of the moving ring is similar to the previous one, except that the dynamic and static rings are reversed. The sealing of the static ring and the gland adopts the graphite seals (which is a common gland packings) commonly used in most high-temperature pumps.

Through the test, the formula of passivating agent is achieved: potassium dichromate 22 g / L + Ammonium molybdate 3.8 g / L + potassium chloride 5.5 g / L + A5 g / L + sodium fluoride 2 g / L. The passivation process is: pH = 4 and t = 10 min at room temperature.

3. Causes Analysis of Plating off and Its Process Solutions

The main reason for plating off is the small bonding force between the coating and the substrate. The main factors affecting the bonding strength of Ni-P plating are insufficient pre-plating treatment, incomplete degreasing and activation of the surface, and low temperature of the workpiece plating bath. Therefore, we increase the preheating process of the workpiece before plating, the preheating temperature is 85 C. The hydrochloric acid originally used to remove the oxide film and rust on the metal surface is changed to a compound acidic activator with better activation effect, and the activation time is controlled.

Test the adhesion of the coating. after improving the pre-plating process, place the plated parts in an oven at a temperature of (250 + 10)℃, keep it warm for 2 h, then quickly put it into cold water after taking it out, and quench it after 5 min. There is no blistering and no peeling on the plating. The inspection by the scratch method and the bending method shows that the binding force is significantly enhanced than before.

Through the above analysis, we can understand the failure and modification of mechanical seals of the catalytic residual oil pump well, which is very helpful for the installation and maintenance of catalytic residual oil pumps.