What is the primary cause of valve internal leakage?

The primary cause is the poor sealing performance between the valve disc and seat due to inadequate sealing pressure or wear.

How does sealing surface material affect valve performance?

Non-metallic materials offer better elasticity and sealing but are limited by temperature and pressure conditions, while metallic materials require hardening treatments for durability.

Why is machining precision important for valve sealing surfaces?

Higher precision reduces microscopic gaps, improving sealing effectiveness but also increasing production costs.

What is the significance of guide rail precision in valve operation?

Precise and durable guide rails ensure even wear of the valve disc, maintaining long-term sealing performance.

Which standards are commonly used for valve leakage control?

API 598, API 6D, ASME B16.34, MSS SP61, ASME B16.104, and ISO 5208 are commonly used standards for valve leakage control.

Analysis of Valve Internal Leakage

Pipeline valve

Overview

Valves are critical for controlling the flow in pipelines, and their performance is judged by how well they can shut off flow. This shutoff capability depends on the sealing between the valve disc and seat. When the seal is poor, leakage occurs, which can hinder maintenance and pose safety risks, especially with hazardous materials like flammable, explosive, or toxic substances.

Types of Valve Shutoff Seals

Valve shutoff seals can be grouped into three main types:

Forced Sealing
- Mechanism: Sealing pressure increases by turning the handwheel.
- Examples: Wedge gate valves, globe valves, orbit ball valves.
- Advantages: Reliable seal, compensates for wear, maintains performance over time.
Semi-Forced Sealing
- Mechanism: Sealing pressure is preset during assembly and cannot be increased manually.
- Examples: Fixed ball valves, non-mechanical balanced plug valves.
- Characteristics: Sealing pressure is set by design features like springs or cam mechanisms.
Passive Sealing
- Mechanism: Sealing pressure relies entirely on the medium's pressure.
- Examples: Floating ball valves, check valves.
- Limitations: Less effective at low pressures, not suitable for large valves with low-pressure media.

Factors Influencing Internal Leakage

Apart from sealing pressure, other factors affecting internal leakage include:

Sealing Surface Material: Non-metallic sealing materials, or metallic materials embedded with non-metallic seals, offer better sealing due to their compressibility and elasticity. However, they are limited by temperature, pressure, and fire safety requirements.
Wear and Cavitation Resistance: Hardening treatments on metallic sealing surfaces enhance wear and cavitation resistance, crucial for maintaining long-term sealing performance. The choice of materials and treatments should consider the operating conditions and medium characteristics.
Machining Precision and Surface Finish: Higher precision and better surface finish reduce microscopic gaps on metallic sealing surfaces, improving sealing effectiveness but also increasing production costs.
Guide Rail Precision and Durability: Valves with long travel, such as gate valves and globe valves, require precise and durable guide rails to ensure even wear and maintain sealing performance. High-precision guide rails with hard-facing alloys can shift wear away from the sealing surface, enhancing valve reliability.

Valve Internal Leakage Classification and Control

Standards such as API 598, API 6D, ASME B16.34, MSS SP61, ASME B16.104 (FCI 70-2), and ISO 5208 provide leakage control criteria. Among these, API 598 is the most widely used, though it does not classify leakage levels. For hazardous media, stricter control based on ISO 5208, which classifies leakage into levels A, B, C, and D, is recommended.

A Level: Suitable for non-metallic sealed valves or harsh media environments, referred to as "tight sealing".
B and C Levels: Suitable for metallic sealed valves in harsh media environments, referred to as "close sealing".
D Level: Similar to API 598 standards, for general media applications, referred to as "standard sealing".

For further details on API 598 and API 6D leakage allowances, please contact us.

Conclusion

Valve internal leakage is a critical factor in valve performance and safety. Understanding the types of sealing mechanisms and factors influencing internal leakage can guide the selection and maintenance of valves for optimal performance in various industrial applications.

FAQs

What is the primary cause of valve internal leakage? The primary cause is the poor sealing performance between the valve disc and seat due to inadequate sealing pressure or wear.
How does sealing surface material affect valve performance? Non-metallic materials offer better elasticity and sealing but are limited by temperature and pressure conditions, while metallic materials require hardening treatments for durability.
Why is machining precision important for valve sealing surfaces? Higher precision reduces microscopic gaps, improving sealing effectiveness but also increasing production costs.
What is the significance of guide rail precision in valve operation? Precise and durable guide rails ensure even wear of the valve disc, maintaining long-term sealing performance.
Which standards are commonly used for valve leakage control? API 598, API 6D, ASME B16.34, MSS SP61, ASME B16.104, and ISO 5208 are commonly used standards for valve leakage control.