Selection and Application of Gear Pump Seals

Why is it necessary to seal a gear pump?

The sealing of a gear pump primarily occurs in two critical areas:

1. Shaft seal (main shaft seal): This is the most critical seal, located at the point where the pump shaft extends from the pump body. Its function is to prevent the medium (liquid) inside the pump cavity from leaking along the pump shaft to the external environment, while simultaneously

Prevent external air from entering the pump.

2. Static Seals: Such as O-rings and gaskets between the pump body and front/rear end covers. These are seals for fixed connections, typically installed once with high reliability.

The term "sealing selection" commonly refers to the selection of shaft seals.

 Primary Seal Types and Their Characteristics

The commonly used shaft seals for gear pumps are mainly divided into the following three types, each suitable for specific operating conditions.

```Packing seal```

This is a traditional and economical sealing method.

Structure: Several layers of soft woven packing materials (such as graphite, PTFE, or aramid fibers) are inserted into the sealing cavity and compressed by the gland bolts, generating radial pressure between the shaft and the housing to achieve sealing.

Advantages:

  Low cost: Both the initial cost and maintenance cost are relatively low.

   High resistance to particulate wear: Well-suited for applications involving fine solid particles in the medium.

   Easy to maintain: Relatively simple to replace.

 Disadvantages:

  Permit leakage: A small amount of leakage (typically dripping, such as 1-60 drops per minute) is required to lubricate and cool the packing, but it is not suitable for applications where leakage is not permitted.

   Wear on the shaft: Prolonged operation can cause wear on the pump shaft.

  High power consumption: Due to significant friction with the shaft, the energy consumption is relatively high.

Application: Clear water, non-hazardous, non-valuable media, and scenarios allowing minor leaks, such as low-pressure irrigation, wastewater treatment, and certain basic industrial processes.

2. Mechanical Seal

This is currently the most mainstream and widely used sealing form in industrial gear pumps.

Structure: Composed of a pair of smooth and flat sealing rings (stationary ring and moving ring) perpendicular to the axis, they are tightly fitted under the action of fluid pressure and spring force, forming a seal through relative rotation.

```Advantages:```

   Extremely low leakage: Under normal conditions, it is almost zero leakage, meeting environmental protection and safe production requirements.

   Low friction power consumption: Long service life.

   Wide Applicability: By selecting different friction pair materials and structures, it can be used in high-temperature, high-pressure, corrosive, and toxic/hazardous environments.

Drawbacks:

   High cost: The manufacturing precision requirements are high, and the price is more expensive than that of packing seals.

   Sensitive to media cleanliness: Solid particles can scratch the sealing surface, leading to failure.

   High installation requirements: Professional installation is required; otherwise, it may be easily damaged.

Common types:

   ```In-line vs. external: In-line is more commonly used, where the medium pressure acts on the seal, aiding its performance; external type is employed for highly corrosive or easily crystallizing media.```.

  Single-sided vs. Double-sided:

     Single mechanical seal: Features only a pair of friction components. Simple structure and low cost. Suitable for general non-hazardous media.

     Double-ended mechanical seal: There are two pairs of back-to-back friction pairs, forming a "sealing cavity" between the two pairs of seals. An external isolation fluid (barrier fluid) needs to be introduced, with its pressure slightly higher than

Pump chamber pressure.

    Application: For media that are toxic, hazardous, flammable, explosive, highly corrosive, high vacuum, or prone to crystallization. The isolation fluid serves both as a sealing agent and as a lubricant and coolant, playing a crucial role in ensuring safety

3. Magnetic drive seal (sealless pump)

This is a technology that completely solves the leakage problem.

 Structure: The direct connection between the pump shaft and the motor shaft was eliminated. An isolation cover fully separates the pump impeller (inner magnetic rotor) from the motor drive end (outer magnetic rotor), transmitting torque through magnetic field coupling.

Disadvantages:

The highest cost.

Low efficiency: There is eddy current loss.

Power limited: Not suitable for high-power applications.

Prohibit dry running and overload: otherwise it will quickly demagnetize and fail.

Application: Highly toxic, corrosive, radioactive, and extremely expensive media, such as certain chemical and pharmaceutical processes.

Key considerations for sealing selection:

When selecting a seal, the following operating parameters must be comprehensively considered:

1. Properties of the conveying medium:

Corrosivity: determines the selection of sealing metal components (springs, seat rings) and friction pair materials (such as Hastelloy, 316 stainless steel).

Lubrication: For media with poor lubrication (such as liquefied gas and hot water), sealing surface materials with good self-lubricating properties (such as silicon carbide to silicon carbide) should be selected.

Viscosity: High viscosity media can affect spring action and heat dissipation, and may require enhanced cooling.

Whether it contains solid particles: For media containing particles, priority should be given to packing seals or mechanical seals with hard to hard friction pairs (such as silicon carbide to silicon carbide), and flushing schemes may be required.

Danger: Toxic, flammable, and explosive media must be sealed with double end mechanical seals or magnetic pumps.

2. Operating conditions:

Temperature: High temperature will accelerate the aging of seals and affect the performance of springs. High temperature resistant materials (such as fluororubber, perfluoroether rubber, polytetrafluoroethylene PTFE, graphite, etc.) and cooling structures should be selected.

Pressure: High pressure conditions require the selection of balanced mechanical seals to reduce the specific pressure on the sealing end face.

·Rotational speed: High rotational speed requires higher dynamic balance and cooling of the seal.

Continuous or intermittent operation: Intermittent operation is prone to dry friction and requires higher sealing materials.

Environmental and safety regulations: Whether regulations allow leakage is the primary determining factor in choosing between packing seals or mechanical seals.

Application Selection Guide:

Recommended preferred sealing type for working conditions. Remarks

The most economical choice for sealing with clean water, low cost, and drip packing, with simple maintenance

The most widely used single end mechanical seal for general industrial oils (hydraulic oil, lubricating oil) is reliable and has minimal leakage

Single end mechanical seal for light corrosive chemicals (material upgrade). Upgrade the sealing surface (such as ceramic/silicon carbide) and O-ring material (such as fluororubber, EPDM) according to the medium

Double end mechanical seals or magnetic pumps for flammable, explosive, toxic, and highly corrosive media prioritize safety, and require isolation liquid systems for double end seals

Mechanical seals with hard friction pairs containing a small amount of solid particles (such as silicon carbide to silicon carbide) or packing seals may consider using an external flushing solution to flush the sealing surface

Single end mechanical seal for high viscosity media (such as resin, asphalt), with strengthened cooling. Pay attention to the insulation or cooling of the medium to prevent coking at the sealing point

Extremely dangerous, no leakage allowed, magnetic drive pump (unsealed) ultimate solution, high cost

Common causes and prevention of seal failure:

Improper installation: such as uneven tightening of the gland bolts and the entry of impurities during installation. Prevention: Strictly follow the installation instructions and maintain cleanliness.

Dry operation: When the pump operates without a medium, the sealing surface is instantly burned due to high temperature. Prevention: Ensure that the pump is fully filled before starting.

Gas or vaporization in the medium: leading to poor lubrication of the sealing surface. Prevention: Ensure sufficient import pressure to prevent cavitation.

Cooling/flushing system failure: Especially for double end seals and high-temperature conditions, interruption of isolation fluid or cooling water can cause rapid seal failure. Prevention: Monitor the pressure/flow rate of the cooling system.

Choosing a gear pump seal is a systematic decision-making process, there is no "best" seal, only the "most suitable" seal. The core logic is:

1. Safety and compliance priority: Is the medium dangerous? Does the regulation allow leakage?

2. The characteristics of the medium determine the material: the sealing material and structure are selected based on the corrosiveness, lubricity, and particle content of the medium.

3. Configuration is determined by operating conditions: whether cooling, flushing, or balancing the structure is required is determined based on temperature, pressure, and other factors.

In practical applications, it is strongly recommended to have in-depth communication with gear pump manufacturers or professional sealing suppliers, provide detailed operating parameters, and have them recommend the most optimized and reliable sealing solutions for you