Technology of end seals of centrifugal pumps: comparison of single, double and cartridge systems

1. Introduction

The efficiency and reliability of operation of centrifugal pumps at industrial enterprises of Ukraine critically depend on the condition of the shaft sealing units. Fluid leaks through stuffing boxes or outdated designs lead to emergency shutdowns, environmental fines and significant productivity losses. According to our engineering practice at UNITEC-D GmbH, more than 40% of unscheduled repairs of pumping equipment in the chemical, metallurgical and energy industries are related precisely to the failure of end seals. The correct choice of design - single, dual or cartridge - determines the MTBF (mean time between failure), which for modern systems should exceed 25,000 hours of continuous operation.

2. Fundamental principles

The operation of the end seal is based on the creation of a hermetic joint between two flat surfaces that rotate relative to each other. One friction pair is fixed on the shaft (rotating ring element), and the second - in the pump body (fixed ring, stationary). Hermeticity is achieved thanks to the axial force created by the spring block and the hydraulic pressure of the environment.

The key physical process is the formation of a microfilm of liquid between the friction surfaces (face gap). The thickness of this film is usually from 0.5 to 3 micrometers. If the film is too thin, dry friction, thermal overload and destruction of silicon carbide or carbon graphite occur. If it is too thick, the volumetric leakage of the medium increases. The calculation of friction parameters is based on the hydrodynamic load coefficient ($K$), which is determined by the formula:

K = (P_3 - P_b) / (P_s - P_b)

where $P_3$ is the pressure in the sealing chamber, $P_b$ is the pressure on the friction surfaces (balancing), $P_s$ is the ambient pressure.

3. Technical specifications and standards

The design and manufacture of end seals is regulated by international and national standards that ensure interchangeability and safety:

• ISO 2858 — End dimensions and nominal characteristics for centrifugal pumps with axial suction (class PN 16).
• ISO 21049 / API 682 — Specifications for shaft sealing systems of centrifugal and rotary pumps. Defines the classification of structures (Category 1, 2, 3) and types of flushing plans (Plan 11, 32, 53A, etc.).
• DIN 24960 — German standard for dimensions and classification of stationary and rotating circles.
• DSTU ISO 5199 — Technical requirements for centrifugal pumps. Class II.
• DSTU EN 809 — Pumps and pumping units for liquids. General safety requirements.

Certification according to the CE and UkrSEPRO standards is mandatory for the supply of equipment to the Ukrainian market, which guarantees compliance with the industrial safety standards of NPAOP 0.00-1.59-18.

4. Selection and selection: Single, double and cartridge systems

The engineering selection of the seal is based on the physico-chemical properties of the pumped medium (temperature, viscosity, toxicity, presence of abrasive), as well as on the working pressure and shaft rotation speed.

5. Best Practices for Installation and Commissioning

Errors during installation are the cause of 50% of premature failures of seals. Compliance with the regulations guarantees the project resource:

1. Check the radial runout of the shaft in the area of the oil seal. It should not exceed 0.05 mm for speeds up to 3000 rpm.
2. Make an axial displacement of the shaft (axial backlash) - the permissible value is no more than 0.13 mm.
3. Clean the seats. Use neutral lubricants to facilitate installation of rubber O-rings (do not use mineral lubricants with ethylene-propylene EPDM elastomers).
4. When installing the cartridge seal, tighten the flange bolts evenly (criss-cross) to the specified torque.
5. After fixing the seal on the shaft, be sure to remove the setting clips, otherwise mechanical destruction of the surfaces during startup is inevitable.
6. Pre-press and fill the sealing chamber with liquid. Running "dry" even for 3 seconds leads to thermal shock and destruction of carbon graphite circles.

6. Failure modes and root cause analysis (RCA)

A visual inspection of the friction surfaces after disassembly allows you to accurately determine the cause of the accident:

• Thermal stress (Thermal Stress): Radial cracks on solid rings (silicon carbide SiC, tungsten carbide WC). Reason: operation without coolant or shift of the flushing flow.
• Abrasive wear (Abrasive Wear): Uniform wear of the ends with the formation of deep grooves. Reason: high content of solid particles in the medium, lack of cyclone separator (Plan 31) or filter.
• Chemical corrosion (Chemical Attack): Swelling, softening or destruction of secondary seals (elastomers). Reason: wrong choice of material (for example, using Viton in an environment with ammonia or ketones).
• Cavitation Erosion: Surface pitting on metal and carbon parts. Reason: pump operation outside the operating characteristic (Q-H curve), low cavitation margin ($NPSH_a < NPSH_r$).

7. Predictive maintenance and condition monitoring

Modern diagnostic methods are used to prevent accidents at critical pumping units of Ukrainian enterprises:

• Vibration analysis: Measurement of high-frequency vibrations (spectral analysis by the demodulation/HFE method) allows you to detect imbalance, cavitation and the initial stage of self-oscillations of the seal.
• Thermometry: Continuous monitoring of the temperature in the sealing chamber (or barrier liquid). An increase in temperature above +85°C for water environments indicates deterioration of lubrication or clogging of flushing lines.
• Leak monitoring: For double seals — monitoring the level and pressure of the buffer fluid in the supply system tank (Plan 53A). A drop in pressure indicates damage to the inner or outer seal.

8. Comparative matrix of configurations

9. Conclusion

The choice of end seal technology for centrifugal pumps is an engineering decision that directly affects the safety and economy of production. The transition from oil seals to modern balanced and cartridge systems reduces frictional energy consumption and completely eliminates uncontrolled leaks. UNITEC-D GmbH engineers are ready to provide expert support in the selection of optimal components, the calculation of API 682 support systems and the supply of original spare parts for your enterprise.

Check out the catalog of reliable industrial components on our website: https://www.unitecd.com/e-catalog/.

10. Links

• ISO 21049:2004 — Pumps — Shaft sealing systems for centrifugal and rotary pumps.
• API Standard 682—Shaft Sealing Systems for Centrifugal and Rotary Pumps, 4th Edition.
• DIN EN 12756 — Mechanical seals — Principal dimensions, designation and material codes.
• DSTU ISO 5199-2001 — Centrifugal pumps. Class II. Technical requirements.
• Technical Whitepapers & Engineering Guidelines, UNITEC-D GmbH Laboratory Archives, Augsburg, 2023.