Analysis and Improvement of Mechanical Seal Failure in Alkali Liquid Circulation Pumps

　　The concentrated caustic solution circulation pump in the compression system of the ethylene plant is the bottom-of-tower circulation pump for the caustic scrubbing tower. It pumps the caustic solution—after it has been treated by caustic scrubbing of the cracked gas—up to the top of the tower, enabling the caustic solution to be recycled. In actual operation, this pump frequently experiences leaks. The mechanical seal starts to leak slightly about a week after installation and operation, and within one to two months, the leakage increases significantly, rendering the pump unusable. This seriously affects the plant’s ability to meet environmental standards and maintain long-term stable operation.

　　 1. Basic Information:

　　This pump is a single-suction, cantilevered, horizontal centrifugal pump. The main technical parameters are shown in the table below:

　　Medium composition: 20% NaOH liquid containing colloidal and particulate impurities.

　　Medium temperature: 45℃

　　Medium viscosity: 1070 CP

　　Flow rate: 30 m³/h

　　Head 18m

　　Inlet pressure: 1.078 MPa

　　Discharge pressure: 1.272 MPa

　　Shaft power: 2.7 kW

　　Motor speed: 2950 r/m

　　 Disassemble the Nester mechanical seal after it has failed; the damage condition is as follows:

　　(1) The gap between the moving ring, the moving ring seat, and the shaft sleeve is blocked.

　　(2) The moving ring and the stationary ring both show ring-shaped wear marks.

　　(3) The sealed cavity contains impurities such as charred polymers, particles, and crystalline substances.

　　(4) The spring is severely corroded, jammed by impurities and crystalline deposits, and lacks sufficient elasticity.

　　 2 Failure Analysis:

　　The main causes of seal failure generally include failure of the liquid film on the end faces, incompatibility between the sealing material and the medium, as well as manufacturing and installation issues. End-face specific pressure is an important performance parameter for mechanical seals; it must remain within the designed range (typically 0.2 MPa to 0.4 MPa for internal mechanical seals) to ensure proper mating of the mechanical seal's end faces and to enable the formation of an effective liquid film between the sealing surfaces.

　　Fb (face force) = Ft (spring force) + Fp (medium acting pressure) - Fm (liquid film acting pressure). The calculation formula for the face specific pressure is:

　　Pb = Pt + Pp - Pm

　　Pb = Pt + (K - λ)P

　　Among them: d2 = 50 mm, d1 = 32 mm, d2 = 38 mm

　　Spring specific pressure Pt = Ft (spring force) / [π/4(d2² - d0²)] = 0.3 MPa (manufacturer’s design value)

　　Load factor K = (d2² - d0²) / (d2² - d1²) = 0.715

　　Membrane pressure coefficient λ = 0.742

　　Medium pressure P = 0.45 MPa

　　The calculated specific pressure Pb on the end face of the original 8B1-P mechanical seal is 0.28 MPa, which complies with the design specifications and can meet production requirements under normal operating conditions. This finding is consistent with the actual situation: in the initial stage of use, the mechanical seal does not leak. So what then could be the cause of its failure? To answer this question, we must start by examining the working environment of the mechanical seal and conduct a comprehensive analysis.

　　Since the pump is located at the bottom of the caustic washing tower, rust from pipelines and equipment, as well as impurities and high-molecular-weight polymers present in the cracked gas, all accumulate at the bottom of the tower. Meanwhile, within the tower, NaOH reacts with H2S and CO2 components, forming Na2S and Na2CO3, which in turn give rise to crystallization. The pump’s inlet filter was originally rated at 40 mesh; however, in actual production, it frequently became clogged, causing the pump to run dry. We subsequently switched to a 20-mesh filter, which largely resolved the inlet blockage issue. Unfortunately, this change also resulted in the alkaline solution entering the pump body containing large amounts of impurities and crystals. Given that the pump employs a self-flushing mechanism, these impurities and crystals inevitably accumulate in the sealing chamber, obstructing the clearance between the moving ring, the moving-ring seat, and the shaft sleeve. As a result, the moving ring’s axial movement becomes inflexible or even completely jammed, while the spring suffers corrosion, becomes stiff, and loses its elasticity, failing to maintain the original designed end-face specific pressure of 0.28 MPa. Consequently, the sealing surfaces can no longer make proper contact, disrupting the liquid film equilibrium. At this point, the seal begins to exhibit minor leakage. As crystals and impurity particles continue to enter the friction pair’s sealing surfaces, these surfaces gradually wear down. Moreover, the moving ring’s insufficient axial compensation further exacerbates the situation, leading to an increase in the seal’s leakage rate and ultimately resulting in complete seal failure.

　　 3 Improvement Measures

　　Based on the causes of sealing failure, we carried out targeted technical improvements in the following areas:

　　(1) Modify the sealing structure design

　　Since the medium is an alkaline solution containing impurities, from the perspective of the sealing structure, static-ring compensation is more suitable than the originally designed dynamic-ring compensation. Based on the specific operating conditions of this pump, we have selected the EIGER 3055 cartridge-type single-face mechanical seal (packaged type), whose main features are as follows:

　　Precise centering is achieved through an automatic locking ring.

　　The adjustable gland can be directly used with common bolts, and its modular design makes installation convenient.

　　The clearance between the stationary ring and the shaft sleeve is located on the non-media side, ensuring the flexibility of axial compensation for the stationary ring.

　　The spring is located on the outside of the auxiliary sealing ring and is completely isolated from the medium, thereby preventing corrosion and seizing of the spring.

　　(2) Change the materials of the friction pair and eliminate seal flushing.

　　Both the moving and stationary sealing rings in the cartridge-type seal are made of tungsten carbide. Tungsten carbide boasts extremely high strength and hardness, as well as excellent wear resistance and corrosion resistance. This makes it highly resistant to damage from impurities and crystalline deposits. Moreover, the tungsten carbide friction pair exhibits outstanding thermal conductivity and a very low linear thermal expansion coefficient, enabling it to maintain reliable sealing performance even at elevated temperatures. For the auxiliary sealing ring, we have selected fluororubber FPM2461, which offers superior corrosion resistance and excellent elasticity, with a maximum operating temperature of 250°C.

　　Due to the presence of impurities and crystalline substances in the alkaline solution, the original mechanical seal’s self-backflushing mechanism has difficulty achieving its intended purpose. Moreover, there is no other suitable medium available at the production site that could serve as a flushing fluid. Given these conditions, coupled with the relatively low operating temperature of the medium (45℃), the Model 3055 cartridge mechanical seal can operate normally even without the self-backflushing feature. This not only reduces energy and material consumption caused by flushing but also simplifies the daily maintenance of the mechanical seal.

　　By rationally selecting the mechanical seal structure for the alkali pump and choosing appropriate friction pairs and auxiliary O-ring materials, we have improved the performance of the mechanical seal, enabling it to operate stably even without sealing flush. This has reduced equipment maintenance and repair costs, ensuring continuous and stable operation of the plant.