Mechanical Seal Leakage Standards and Determination: Interpretation of API 682 and GB/T Standards

Mechanical seal leakage is the most intuitive indicator of sealing performance and an important parameter for equipment acceptance and operational monitoring. However, "how much leakage counts as a leak in mechanical seals" is one of the most frequently asked technical questions on engineering sites. As a high-frequency consultation topic for the technical service team of Huahao Sealing Co., Ltd. (霍邱县华豪密封件有限公司), this article systematically interprets the leakage limit regulations in API 682 and GB/T 14295 standards, analyzes determination criteria under different conditions, and provides leak cause diagnosis and response solutions.

1. API 682 Standard Leakage Regulations

1.1 Single Mechanical Seals

API 682 standard (4th edition) specifies for single mechanical seals: under specified test conditions, the visible seal leakage shall not exceed 20 drops/minute (approximately 1 mL/min). This limit applies to conventional non-hazardous media conditions and is the minimum requirement for engineering acceptance. In actual operation, high-quality carbon graphite seal ring mechanical seals typically have leakage below 5 drops/minute.

1.2 Double Mechanical Seals

For double mechanical seals, API 682 specifies: the barrier fluid leakage from the primary seal to the media side shall not exceed 5 mL/h, and the barrier fluid leakage from the secondary seal to the atmosphere side shall not exceed 5 mL/h. Double seals using Plan 53A barrier fluid solution should have no visible leakage on the media side during normal operation.

1.3 Special Conditions Limits

For hazardous media such as volatile, toxic, or flammable substances, API 682 recommends zero-emission design, meaning no detectable media leakage during normal operation. In such cases, double seals with pressurized barrier fluid should be used to ensure no media leaks even if the primary seal fails.

2. GB/T 14295 Standard Leakage Regulations

2.1 Leakage Grade Classification

GB/T 14295 "Mechanical Seal Technical Conditions" divides mechanical seal leakage into three grades:

• Premium: ≤5 drops/minute (approximately 0.25 mL/min)
• First-grade: ≤10 drops/minute (approximately 0.5 mL/min)
• Qualified: ≤20 drops/minute (approximately 1 mL/min)

2.2 Grade Applicable Scope

Premium grade applies to precision equipment and critical conditions; First-grade applies to general industrial pumps; Qualified is the minimum acceptance standard. Carbon graphite seal ring mechanical seals from Huahao Sealing Co., Ltd. typically meet Premium grade standards.

2.3 Test Conditions

GB/T 14295 specifies test conditions as: ambient temperature clean water, pressure 1.0 MPa, speed 3000 r/min, operating time 100 hours. Leakage is measured under these conditions as the basis for grade determination.

3. Analysis of Factors Affecting Leakage

3.1 End-Face Quality

Carbon graphite seal ring end-face flatness is the key indicator for leakage control. API 682 requires end-face flatness not to exceed 0.0006 mm (approximately 3 interference bands). Measured data shows that when end-face flatness increases from 0.0006 mm to 0.001 mm, leakage can increase from 5 drops/minute to 15-20 drops/minute. End-face surface roughness should be controlled at Ra 0.1-0.2 μm.

3.2 End-Face Specific Pressure

Mechanical seal end-face specific pressure is typically 0.3-0.6 MPa. Too low specific pressure leads to insufficient end-face contact and increased leakage; too high specific pressure aggravates end-face wear and shortens seal life. For carbon graphite seal rings, due to the softer material, specific pressure is recommended at the lower-middle range of 0.3-0.5 MPa.

3.3 Media Characteristics

Media viscosity significantly affects leakage. Higher viscosity results in lower leakage. Water leakage is approximately 10 times that of hydraulic oil. Increased media temperature reduces viscosity, leading to increased leakage. Solid particles in media scratch the end face, causing dramatic leakage increase.

3.4 Installation Quality

Improper installation is a common cause of leakage. The perpendicularity error between seal chamber and shaft should be less than 0.05 mm, and the coaxiality error between seal ring and shaft should be less than 0.1 mm. Spring compression should strictly follow design requirements; excessive compression causes end-face overheating, while insufficient compression causes leakage.

4. Leakage Determination and Diagnosis

4.1 Normal vs. Abnormal Leakage

Mechanical seals have small leakage during normal operation, which is the normal phenomenon of end-face liquid film evaporation. For water media, normal operating leakage is typically 1-5 drops/minute. When leakage exceeds 20 drops/minute or becomes a continuous stream, it should be determined as abnormal leakage requiring shutdown for inspection.

4.2 Leakage Cause Diagnosis Process

1.Check leak location: seal end face, O-ring, seal chamber

2.Check media condition: temperature, pressure, solid content

3.Check seal condition: end-face wear, flatness, spring compression

4.Check installation quality: perpendicularity, coaxiality, fastening torque

4.3 Common Leak Causes and Solutions

• End-face wear: check carbon graphite seal ring wear amount; replace when exceeding 30% of original thickness
• End-face deformation: check end-face flatness; re-grind or replace if out of tolerance
• O-ring aging: check O-ring hardening, deformation, corrosion; replace if necessary
• Spring failure: check spring free height and force; replace if failed
• Media crystallization: check if media crystallizes on end face; add flush or quench solution

5. Special Requirements for Hazardous Media Conditions

5.1 Toxic Media

Mechanical seals handling toxic media (such as benzene, hydrogen sulfide) have a leakage limit of 2 drops/minute and must use double seals with Plan 53A barrier fluid solution. The barrier fluid pressure should be 0.15-0.25 MPa above seal chamber pressure.

5.2 Flammable and Explosive Media

Mechanical seals handling flammable and explosive media (such as gasoline, ethylene) should adopt zero-emission design, with no detectable leakage during normal operation. Plan 53B or Plan 54 pressurized barrier fluid solutions are recommended.

5.3 High-Temperature Media

High-temperature media (>150°C) mechanical seals have appropriately relaxed leakage standards, as media tends to vaporize at the end face. Plan 23 cooling flush solution is recommended, controlling end-face temperature below the media vaporization temperature.

6. Leakage Monitoring Methods

6.1 Visual Monitoring

Regularly check for liquid dripping at the seal, recording drops per minute. This is the simplest and most practical monitoring method, suitable for conventional conditions.

6.2 Liquid Collection Measurement

Set up collection cups below the seal, periodically measuring leaked liquid volume. Suitable for critical equipment requiring precise leakage recording.

6.3 Online Monitoring

Use VOC detectors or flow meters for online leakage monitoring, enabling real-time alarms and trend analysis. Suitable for hazardous media and critical equipment.

Conclusion

Mechanical seal leakage is a direct reflection of sealing performance, and engineers should be familiar with relevant standard limits and determination methods. For carbon graphite seal ring mechanical seals, proper end-face quality, installation quality, and operational maintenance are key to controlling leakage. Huahao Sealing Co., Ltd. can provide carbon graphite seal rings meeting API 682 and GB/T 14295 standards, along with leakage diagnosis and solution consulting services. For technical support, please contact our engineering team.