Failures of mechanical seals

It is best to check the shaft play after stopping the pump; the maximum value of this shaft play should not exceed 0.05 mm. For the inspection of shaft runout, burrs and sharp edges on the shaft and shaft sleeve at the sealing locations must be removed. For the perpendicularity check, the maximum value of perpendicularity shall not exceed 0.005 mm for every 10 mm of the sealing chamber diameter. It would be best to specify the important parameters of your hydrogen peroxide pump, as this will facilitate analysis.

Failures in mechanical seals are generally detected only after phenomena such as abnormal leakage, abnormal wear, and abnormal torque occur. The reasons for the failure are roughly as follows: 1. Incorrect design and selection of the mechanical seal ； 2. Poor quality of mechanical seal ； 3. The precision of the machine itself, which uses or is equipped with a mechanical seal, does not meet the required standards ； 4. Errors in machine operation. V. Causes and Analysis of Seal Failure (I) Seal failure is mainly due to the following three reasons: 1. Opening of the seal surface. When repairing mechanical seals, 85% of cases of seal failure are not caused by wear; leakage occurs before wear even sets in. Once the sealing surface is opened, solid particles in the medium enter the sealing surface under the influence of liquid pressure; once the sealing surface is closed again, these solid particles become embedded in the surface of the soft ring (usually the right O-ring). This essentially acts as an \"abrasive wheel\" that can damage the surface of the hard ring. Since the moving ring or rubber ring is fixed to the shaft (shaft sleeve), when the shaft moves, the moving ring cannot fit in place in time, which causes the sealing surface to open. Moreover, the delayed closure of this sealing surface allows solid particles to enter it. There are also solid particles between the coaxial shaft (shaft sleeve) and the sliding components, which affect the sliding of the rubber ring or moving ring (a common fault relative to the dynamic sealing point). Furthermore, the medium also forms crystals at the friction points between the rubber ring and the shaft (shaft sleeve), and solid substances accumulate at the springs; all of these can cause the sealing surface to open. 2 Overheating: Since heat is generated on the sealing surface, the operating temperature of the rubber ring should be below the specified limits. Fluororubber and polytetrafluoroethylene can be used at temperatures of 216°C, while nitrile rubber can be used at 162°C. Although all of them can withstand high temperatures, the heat generated at the sealing surface is high, posing a risk of further vulcanization of the rubber ring, which ultimately leads to a loss of elasticity and leakage. (Cold zones require consideration for cold brittleness.) Between the sealing surfaces, heat can also cause the medium to crystallize, such as the formation of carbon deposits, which leads to the sticking of sliding components and the clogging of the sealing surfaces. Moreover, some polymers coking due to overheating, and some fluids lose their lubricating properties due to overheating, or even catch fire. Overheating not only changes the condition of the medium but also accelerates its corrosion rate. To prevent deformation of metal parts, cracking of alloy surfaces, and cracks in certain coatings, a balanced mechanical seal should be selected in the design in order to reduce the specific pressure and avoid overheating. 3 Out of tolerance: Proper assembly tolerances are essential for installing mechanical seals. The shaft (shaft sleeve) must have an appropriate surface roughness and correct dimensions, but manufacturers rarely provide these tolerance values, which are crucial for installation. (Relying on experience and common sense) The dimensional accuracy and form and position tolerances of mechanical seals must meet the requirements specified in the drawings; deviations from these requirements can lead to premature failure of the seal.

Thank you to the fellow sailors upstairs for their answers. All our other pumps are working fine; the problem only occurs with those used for hydrogen peroxide. The mechanical seal is made of cemented carbide – should we consider using a different material instead? I’ve been wondering if it would work to replace it with silicon carbide.

The design and selection of the mechanical seal are incorrect； The quality of the mechanical seal is poor ； Machines that use or are equipped with mechanical seals do not meet the required precision standards ； Error in machine operation.

Cemented carbide is primarily composed of micron-sized powders of carbides of highly hard refractory metals (WC, TiC). In an oxidizing atmosphere, all these carbides are prone to oxidation at high temperatures, which can be considered a major weakness of carbides.

Reply to 7# Waiter: Senior, then what type of mechanical seal should be chosen?

Reply to 8# huangzx6688: We haven’t used it before; could we consider using ceramic or graphite instead?

Reply to 9# Waiter, thank you! I’ll consider it

It is better to use a canned pump; otherwise, the seal material should be ceramic or glass-filled PTFE. Hydrogen peroxide is a strong oxidizing agent, and it is said that at room temperature or with slight heating, it releases oxygen which can combine with the carbon in the seal surface or with the carbon black in the secondary seal, leading to chemical erosion and thus causing the seal to fail easily.