What are the common mechanical seal flushing schemes for multistage centrifugal pumps?

       As a general mechanical equipment, multistage centrifugal pumps, which are almost ubiquitous in industrial and agricultural production, are used very frequently in daily operations. Therefore, the stable operation of multistage centrifugal pumps is crucial to system safety, and the reliability of mechanical seals directly affects the service life and efficiency of the equipment. As a key maintenance method for mechanical seals, the flushing scheme needs to comprehensively consider factors such as medium characteristics, operating parameters, and working conditions. This article from Zoomlian Pump Industry will help you understand the adaptation strategies under different working conditions, providing practical references for optimizing seal performance and reducing leakage risks.

     Ⅰ、Core Purposes of Mechanical Seal Flushing

The design of the mechanical seal flushing scheme needs to meet the following core requirements:

1. Heat Dissipation and Temperature Control: High temperature is generated by the friction between the rotating dynamic ring and the static ring. The flushing fluid is required to take away the heat to avoid vaporization of the end face liquid film or failure of lubrication. When the temperature of the seal chamber exceeds 80°C, the lubrication performance decreases significantly, and forced cooling (such as PLAN21 or PLAN23) is required.
2. Cleaning and Anti-blocking: Prevent solid particles, crystals, or impurities from accumulating on the seal end face to avoid dry friction or wear of the seal. For media containing particles, external flushing (such as PLAN32) or additional hydrocyclones should be used.
3. Lubrication and Pressure Maintenance: Maintain the pressure of the seal chamber higher than the vaporization pressure of the medium to prevent cavitation or flashing. For high-temperature hydrocarbon media, cooling is required to increase the critical range of vaporization.
4. Environmental Safety: For toxic and flammable media, double mechanical seals (such as PLAN52/53) and an isolation fluid system must be used to prevent leakage.

     Ⅱ、Skills for Selecting Flushing Schemes Under Key Working Conditions

1. High-temperature Working Condition (>80°C):

Preferred scheme: Adopt self-flushing with a cooler (PLAN21) or internal circulation cooling (PLAN23). PLAN23 circulates the cooling fluid through the liquid delivery ring in the seal chamber, which only needs to cool the local liquid and has higher efficiency.

Notes: When the medium temperature is >300°C and there is no external flushing condition, a high-temperature bellows seal + PLAN21/23 can be used instead of PLAN32 to save costs.

2. High-pressure/High-viscosity Media:

Preferred scheme: Self-flushing (PLAN11) needs to be equipped with an orifice plate to control the flow rate to avoid high-pressure impact on the seal end face. When the viscosity is >300cP, PLAN21 is recommended to ensure smooth circulation.

3. Media Containing Solid Particles:

Preferred scheme: External flushing (PLAN32) with a hydrocyclone, or self-flushing + filtering device. Forced external flushing is required when the particle concentration is >5%.

Extreme conditions: For pulp pumps or slurry pumps, PLAN31 (cyclone + self-flushing) can be used.

4. Easily Crystallizable/Polymerizable Media:

Preferred scheme: Double mechanical seal (PLAN53A/B) with an isolation fluid pressurization system. The pressure of the isolation fluid must be 0.05~0.1MPa higher than that of the seal chamber.

5. Light Hydrocarbon Media Prone to Vaporization:

Preferred scheme: PLAN52 uses a siphon tank to circulate the isolation fluid to avoid vaporization and leakage of the medium. It is necessary to monitor the liquid level of the isolation fluid and set a low liquid level alarm.

Ⅲ、Strategies for Balancing Economy and Reliability

1. Simplify the System: When the medium is clean and the temperature is moderate, priority should be given to self-flushing (PLAN11/13) to reduce the complexity of the equipment.
2. Modular Design: For high-temperature or particle-containing working conditions, standardized flushing modules (such as API PLAN21/23) are adopted to facilitate maintenance and spare parts management.
3. Redundant Design: For key pump units (such as reactor feed pumps), double mechanical seals + PLAN53 are used. Although the cost increases, it can avoid unplanned shutdowns.

The selection of the mechanical seal flushing scheme for multistage centrifugal pumps needs to comprehensively consider medium characteristics, working condition parameters, and industry standards. The core is to extend the seal life through heat dissipation, cleaning, and pressure control. In practical applications, it is recommended to select mature schemes in conjunction with the API682 standard to balance reliability and economy.