Failure Analysis and Solutions for the Mechanical Seal of the Catalytic Slurry Pump

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1. Introduction The P207 slurry pump is a bottom tank pump used in the catalytic cracking FCC units of Shandong Shida Technology Group to transport fluid containing catalyst particles. The pump model is 100PYS-130, a single-stage cantilever centrifugal pump with an axis power of 75 kW, a flow rate of 45.5 m3/h, and a head of 130 m. The conveying medium is oil slurry, with a medium temperature of 375°C and a pump suction pressure of 0.2 MPa. Although this pump has always used welded bellows mechanical seals, due to various factors such as the properties of the medium in use, the mechanical seals often leak during actual operation. According to the statistics from 2007, the shortest service life of mechanical seals was 20 days, while the average service life was around 3 months, which severely affected the normal operation of the processing equipment. To completely eliminate potential risks, we conducted a feasibility analysis and modification of the mechanical seal system of this pump, thereby extending its operational life and ensuring stable operation of the equipment. 2. Fault cause analysis 2.1 Sealing structure before improvement and fault statistics The mechanical seal used in the catalytic slurry pump (hereinafter referred to as P207) is a DBM-90 welded bellows mechanical seal produced by Dandong Klon Group. The characteristics of this sealing structure are as follows: 1. The moving ring friction pair is made of a low-expansion alloy, while the stationary ring friction pair is made of antimony-doped graphite ； 2. The rotating ring is connected to the shaft sleeve flange via screws ； 3. The impeller compresses the shaft sleeve to achieve sealing of the shaft sleeve gasket ； 4. Stationary metal bellows structure. According to the annual failure statistics for 2007, P207 experienced 4 cases of seal failure, including 1 case of leakage in the shaft sleeve gasket. The following explores the causes of mechanical seal leakage based on actual on-site maintenance practices. The working medium of the slurry pump is catalytic slurry; it has a high temperature and contains solid particles such as catalysts, resulting in relatively harsh operating conditions. Through several maintenance tasks and an analytical examination of the original mechanical seal, it was found that the main reasons for the failure and leakage of the mechanical seal were as follows: 2.2.1 Wear of the friction pair along with thermal cracks. The material used for the friction pair of the static ring in the original DBM-90 welded bellows mechanical seal was antimony-doped graphite; upon disassembling the seal, it was observed that there were gaps on the inner and outer edges of the static ring’s friction pair, and end-face cracks were present in most cases. The content of solid catalyst particles in the slurry is ≤6 g/L; it rises to around 12 g/L when production is not operating properly. Slight fluctuations in the process conditions during operation can cause the catalyst particles to enter the sealing surfaces, scratching those surfaces and disrupting the continuity of the liquid film, which in turn leads to leaks. Under normal sealing conditions, the sealing surfaces are in a state of boundary lubrication or semi-liquid lubrication, with the two surfaces separated by a boundary film. When catalyst particles are present between the sealing surfaces or when there is wear on the rotating ring’s sealing surface, the thickness of the liquid film between these surfaces increases significantly, resulting in substantial leakage of the oil slurry. 2.2.2 The high temperature and high density of carbon deposits and coking at the troughs on the inner side of the corrugated tube are the main causes of slurry coking and carbon deposition. When P207 is used at 375°C, the slurry will gradually settle or solidify in the gaps of the bellows, forming carbon deposits. Over time, coking and carbon deposition within the bellows gaps prevent the bellows from undergoing axial stretching or compression, causing it to lose its elasticity. As a result, the bellows is unable to provide an axial force that changes with the pressure of the medium, failing to perform its compensatory function; this leads to a decrease in the pressure of the liquid film at the end faces, causing a reduction in the back-pressure coefficient of the liquid film, and ultimately resulting in a decrease in the end-face specific pressure, which in turn causes the seal to fail. 2.2.2 Scaling forms between the stationary ring and the shaft sleeve, preventing the stationary ring from moving freely. This seal utilizes forced internal cooling; the cooling water is recycled water from the power plant’s water system, and its quality is poor. Cooling water will form scale between the static ring bellows and the shaft sleeve, preventing the static ring from moving freely. The specific analysis results are shown in Table 1: Table 1 Analysis of cooling water quality. Parameter, Control limit, Test result: (COD) milligrams/liter <10, 227.8; (Iron ions) milligrams/liter <25.55; (Magnesium ions) milligrams/liter <137, 44.05; (Calcium ions) milligrams/liter 102.1; (Suspended solids) milligrams/liter <30, 149; pH 7–9, 7.83. It can be seen from Table 1 that various parameters such as those related to suspended solids in the cooling water **exceed the specified control limits. The temperature at which scale generally forms is around 80°C, but the temperature of the cooling water supplied to the mechanical seal of high-temperature pumps is higher than this; as a result, calcium and magnesium salts crystallize and deposit between the bellows and the baffle, forming scale. Due to the small gap between the bellows and the baffle sleeve, the formation of scale prevents the bellows from undergoing axial stretching or compression, causing it to lose its elasticity. 2.2.4 The shaft sleeve gasket is prone to leakage. The DBM90 seal uses a key-driven transmission system, and the sealing of the shaft sleeve is achieved by the impeller pressing against the gasket located inside the shaft sleeve. The main reasons for leakage of the shaft sleeve gasket are as follows: 1. Reverse rotation of the impeller or too rapid preheating of the pump body causes the impeller locking nut to loosen, resulting in the gasket not being pressed tightly and thus leakage occurring ； 2. During installation, impurities enter the shaft sleeve gasket, causing it to lose its sealing properties and resulting in leakage. 3. Solutions and Measures: In response to the aforementioned faults, we conducted thorough analysis and evaluation, and implemented effective solutions for the DBM90 type mechanical seal. The specific measures are as follows: 3.1 Using a metal bellows as the rotating moving ring – Since the mechanical seal originally used had a stationary structure, oil slurry could easily accumulate, coking, and form carbon deposits in the gaps of the bellows. To prevent coking and carbon deposition in the gaps of the mechanical seal bellows, we designed the metal bellows as a rotary structure. The rotary bellows seal can clean itself under the action of rotational centrifugal force, reducing deposits on the outside and coking on the inside of the bellows, and it also prevents deformation of the bellows due to sudden cooling. 3.2 Slotted beveled compression sleeve sealing structure: To address leaks in the sealing sleeve gaskets, we adopted a slotted beveled compression sleeve sealing structure. This type of positioning drive is reliable, easy to install and remove, and does not damage the shaft. A limit plate is also provided to facilitate adjusting the compression amount of the seal outside the pump. A 45° bevel is provided at one point on the inner diameter of the bellows to distribute stress and extend its lifespan. The auxiliary seal uses flexible graphite instead of other sealing materials, enabling it to withstand high temperatures of up to 425°C. The design of this structure effectively prevents leakage from the sealed shaft sleeve gasket. 3.3 Use of wear-resistant friction pair materials: Since the medium in the slurry pump contains solid particles, for the selection of materials for the friction pair, we chose chromium oxide (Cr2O3) for the surface coating of the rotating ring, while YG6 was used for the stationary ring; this combination represents a \"ceramic-to-ceramic\" type of friction pair. Due to their different hardnesses, it prevents simultaneous damage to the sealing surfaces of the stationary and rotating rings, while also avoiding the occurrence of thermal cracking. To address the issue of seal ring embedding structures coming loose in high-temperature environments, we use a monolithic structure seal ring. The sealing compression amount is set at 3.5 mm, effectively reducing the excessive frictional heat at the sealing surface. 3.4 Improving the cooling and flushing effects of the seal: In high-temperature slurry pumps, due to the high temperature of the fluid, along with short-term mechanical or thermal loads, the stable liquid film between the seal surfaces turns into vapor; the temperature difference during this transformation causes radial cracks to form. To improve the cooling effect of the sealed friction pair, I increased the diameter of the inlet and outlet cooling water holes in the original sealing gland by 2 mm, which doubled the amount of cooling water and **improved the cooling effect**. To prevent the catalyst particles from sticking and blocking the flushing tube channels, we increased the diameter of the flushing holes from Φ5mm to Φ8mm, and kept the flushing pressure at 3kg. Through improvements in self-flushing, the temperature rise of the sealed end face was effectively controlled, the liquid phase area was increased, and the friction condition was improved. 4. Conclusion Through the analysis of the failure causes of the mechanical seal in the slurry pump, we made effective improvements in aspects such as its material and structure. At present, the mechanical seals for the slurry pumps that we have modified have been applied in industrial production. Before the renovation, the effective operating time of this pump was on average around 3 months; after the renovation in April 2008, it was able to operate continuously for up to 8,000 hours in total, without any leakage problems despite periods of high-load operation and frequent switching operations. Through the modification and application of the mechanical seal for the slurry pump, not only were the problems affecting the stable operation of the unit resolved, but significant economic benefits were also achieved. It can be seen that the modification of the mechanical seal for the slurry pump in the catalytic unit was quite successful.