Mechanical seals are key components used to prevent fluid leakage in rotating machinery (such as pumps and compressors). They can be divided into single mechanical seals and double mechanical seals according to their structure. The design principles, application scenarios and performance characteristics of the two are significantly different. The following is a detailed comparison:

1. Structure and working principle

(1) Single Mechanical Seal:

Structure: It consists of a set of seal faces (dynamic ring + stationary ring), elastic elements (springs), auxiliary seal rings (O-rings, etc.), installed between the rotating shaft and the stationary housing.

Principle:

• The dynamic ring rotates with the shaft, and the stationary ring is fixed on the seal chamber. The two fit together to form a seal face, which is kept in close contact by spring force.
• An extremely thin liquid film (or air film) is formed between the seal faces to prevent the leakage of the internal medium. At the same time, the liquid film plays a role in lubrication and cooling.

Features:  simple structure, easy installation, but only rely on a single sealing surface to achieve sealing.

(2) Double Mechanical Seal

Structure: It consists of two sets of seal faces (inner dynamic ring + stationary ring, outer dynamic ring + stationary ring). The two sets of sealing faces are arranged in parallel to form an isolation cavity in the middle.

Principle:

• The inner sealing surface (primary seal) prevents the leakage of process media (such as liquid, gas);
• The outer sealing surface (secondary seal) prevents the leakage of isolation fluid (or buffer), and the isolation chamber needs to introduce external fluid (such as clean water, lubricating oil).
• The isolation fluid pressure is usually higher than the process medium pressure, forming a "barrier fluid" to further block the leakage path of the medium.

Features: Double sealing surface + isolation chamber design, higher sealing reliability, but complex structure and high cost.

2. Differences and contrasts

3. Analysis of key application scenarios

(1) Application scenarios of single mechanical seals

Medium properties: clean and free of impurities (such as water, oil, alcohol); non-high-risk (non-toxic, non-flammable, non-corrosive).

Working conditions: pressure ≤ 1.5 MPa, temperature ≤ 100℃; medium rotation speed (such as ordinary centrifugal pump).

Examples:

Household tap water pipeline pump;

Juice delivery pump in the food and beverage industry;

Refrigerated water pump in air conditioning system.

(2) Typical scenarios for double mechanical seals

Handling of high-risk media: flammable liquids such as liquefied natural gas (LNG) and gasoline; highly corrosive media such as sulfuric acid and caustic soda; toxic gases such as chlorine and ammonia.

Severe working conditions: high pressure (such as oil field water injection pumps, pressure > 10 MPa); high temperature (such as refinery vacuum tower bottom pumps, temperature > 300°C); containing solid particles (such as slurry pumps, sewage treatment pumps).

Examples: nuclear power plant reactor coolant pumps (to prevent leakage of radioactive media); petrochemical industry hydrocracking unit pumps (high pressure, high temperature, hydrogen media); coal slurry delivery pumps in coal chemical industry (containing particles, requiring isolation fluid to flush particles).

4. Maintenance and selection suggestions

(1) Maintenance points

Single Mechanical Seal:

• Regularly check the seal face for wear (such as cracks and scratches);
• Avoid dry operation (the seal face is easily burned when the medium is insufficient).

Double Mechanical Seal:

• Focus on monitoring the isolation fluid system (pressure, flow, temperature);
• Regularly replace the isolation fluid (to prevent contamination or deterioration);
• Check whether there are signs of medium leakage in the isolation chamber (such as abnormal color or odor).

(2) Selection principles

Single Mechanical Seal is preferred: if the medium and working conditions meet the clean and low-risk conditions to reduce costs and maintenance complexity.

Double Mechanical Seal must be selected: when the medium involves safety, environmental protection or process stability (such as leakage may cause explosions and pollute the environment), or the working conditions exceed the limits of single mechanical seal (high pressure, high temperature, particles).

Intermediate solution: If the medium contains a small amount of particles but does not reach a high-risk level, a single mechanical seal with flushing can be used (clean the sealing surface through external flushing fluid to extend the service life).

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