1. Packing seal (packing seal)
Structure and working principle
Core Components:
·  Packing box: An annular cavity wrapped around the pump shaft to hold the packing.
·  Soft packing: Multi-layer wound fiber material.
·  Gland: Bolts down the packing to form an axial compression seal.
·  Packing ring (optional): Built-in flushing water channel to cool and lubricate the packing.

Sealing mechanism:
The packing is pressed against the shaft surface to form a dynamic seal by friction and material elasticity. External flushing water (e.g. fresh water) is required to reduce frictional heat and wash away particles, preventing packing burnout or shaft wear.

Advantages and disadvantages
Advantages:
· Simple construction, low installation and replacement costs.
· Can accommodate some degree of shaft eccentricity or vibration.

Disadvantages:
· Leakage is unavoidable: a small amount of leakage (about 1~5 drops/minute) is allowed during normal operation for lubrication and heat dissipation.
· Frequent maintenance: the gland bolts need to be adjusted regularly (usually checked every 1 to 3 months) and the packing needs to be replaced when it wears out.
· Higher energy consumption: the friction between the shaft and the packing leads to additional power loss (approx. 3%~5% efficiency drop).

Applicable scenes
· Media characteristics: low concentration of solid particles (≤30%), no strong corrosive (pH 5~9).
· Working conditions: low pressure (≤1.6MPa), room temperature (≤80℃), low speed (≤1500rpm).
· Typical industries: mine tailings transportation, coal washing plant circulating water pumps, construction mud pumps, etc.

2. Mechanical seals
Structure and working principle
Core components:
· Dynamic ring: hard material (e.g. silicon carbide, tungsten carbide) that rotates with the shaft.
· Static ring: Soft material (e.g. graphite, PTFE) fixed on the pump casing.
· Spring/Bellows: Provides axial pressure to ensure that the movable and static rings fit together.
· Flushing system (e.g. PLAN 11/23): external cleaning fluid cools the sealing surfaces and flushes out particles.

Sealing mechanism:
The dynamic ring and the static ring form a very thin liquid film (approx. 1~3μm) at high rotational speeds, which realizes dynamic sealing through contact or non-contact design. The double face mechanical seal is further isolated from the medium by a separating fluid (e.g. glycerin or synthetic oil).

Advantages and disadvantages
Advantages:
· Very low leakage: single end face leakage ≤ 0.1mL / h, double end face can realize zero leakage.
· Long life: the life of high-quality mechanical seals up to 1 ~ 3 years (packing seals usually 3 ~ 6 months).
· High adaptability: can withstand high pressure (≤ 10MPa), high temperature (≤ 250 ℃) and strong corrosive media.
Disadvantages:
· High cost: the price of a single set is about 5 to 10 times that of the packing seal (thousands to tens of thousands of dollars).
· Complicated installation: requires high-precision centering (axial deviation ≤ 0.05mm), otherwise it is easy to lead to bias wear failure.
· Dependent on auxiliary system: need to be equipped with flushing, cooling or isolation fluid device, increasing the complexity of the system.

Applicable Scenarios
· Media characteristics: highly abrasive particles (such as quartz sand), strong acid / alkali (pH 1 ~ 14), high temperature slurry.
· Working conditions: high pressure (≥2.5MPa), high speed (≥3000rpm), continuous operation.

Typical industries: chemical process pumps, metallurgical slurry pumps, power plant desulfurization pumps, etc.

3. Secondary impeller seal
Structure and working principle
Core components:
· Secondary impeller: a closed impeller installed at the back of the main impeller, rotating synchronously with the pump shaft.
· Pressure reducing cover: fixes the secondary impeller and forms a pressure balancing chamber.
· Parking seal: rubber lip seal or packing seal, used to seal when the pump is stationary.

Sealing mechanism:
· Dynamic sealing: During operation, the centrifugal force generated by the secondary impeller counteracts the pressure inside the pump, forming a “negative pressure barrier”.
· Static sealing: When stopping, the parking seal (e.g. lip seal) automatically presses against the shaft surface to prevent backflow.

Advantages and disadvantages
Advantages
· No leakage: no leakage at all during operation, excellent environmental friendliness.
· Low shaft wear: no packing friction, extended shaft life (suitable for high hardness shaft materials such as stainless steel).

Disadvantages:
· Dependent on the operating state: only effective after the pump starts, start and stop frequently when the parking seal is easy to wear.
· Power loss: the secondary impeller consumes extra energy (efficiency reduced by about 2%~4%).
· Limitations: Need to cooperate with other sealing methods (e.g. parking seal), the complexity of the system increases.

Applicable scenes
· Media characteristics: containing coarse particles (particle size ≤ 10mm), high concentration (solids content ≥ 40%).
· Working conditions: high head (≥ 50m), continuous operation (start and stop frequency ≤ 5 times / day).

Typical industries: long-distance slurry pipeline pumps, dredging pumps for dredging vessels, deep-sea mining pumps, etc.