How does a surface safety valve work?

Learn About Steam | Safety Valves | Spirax Sarco

Introduction to Safety Valves

As soon as mankind was able to boil water to create steam, the necessity of the safety device became evident. As long as 2000 years ago, the Chinese were using cauldrons with hinged lids to allow (relatively) safer production of steam. At the beginning of the 14th century, chemists used conical plugs and later, compressed springs to act as safety devices on pressurized vessels. Safety valves have come a long way since these early innovations.

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Early in the 19th century, boiler explosions on ships and locomotives frequently resulted from faulty safety devices, which led to the development of the first safety relief valves. Around 1848, Charles Retchie invented the accumulation chamber, which increased the valve’s efficiency by enlarging the compression surface and enabling rapid opening within a narrow overpressure margin.

Nowadays, strict health and safety regulations mandate that steam users install safety devices to prevent dangerous conditions. The primary function of a safety valve is to protect life and property by releasing excess pressure from the system.

Function of Safety Valves

Basic Operation

Safety valves are designed to release a volume of fluid from a system when a predetermined maximum pressure is reached, reducing excess pressure in a safe manner. Given that safety valves may be the last defense against catastrophic failures, it's essential that they function reliably under all conditions. They are typically installed in systems where the maximum allowable working pressure (MAWP) might be exceeded.

Common Applications

In steam systems, safety valves protect boilers from overpressure but are also applied downstream of pressure reducing controls and for other process operations to prevent product damage from excess pressure. Some scenarios that can generate excess pressure include:

• Imbalance in fluid flow due to isolation valves being inadvertently opened or closed.
• Cooling system failures allowing fluid or vapor expansion.
• Failures in compressed air or electrical power affecting control instrumentation.
• Pressure surges or exposure to plant fires.
• Heat exchanger tube failures.
• Uncontrollable exothermic reactions in chemical processes.
• Changes in ambient temperature.

Types of Safety Valves

The terms ‘safety valve’ and ‘safety relief valve’ are used to describe various pressure relief devices. Different valves are available to meet diverse applications and performance requirements, and they need to comply with multiple national standards. For example, in the USA, the term ‘safety valve’ typically refers to what is called a ‘safety relief valve’ or ‘pressure relief valve’ in Europe. Detailed standards and definitions, such as those given by the ASME/ANSI PTC25.3, outline precise terminology and application criteria.

Common terms defined by these standards include:

• Pressure relief valve: A spring-loaded device that opens to relieve excess pressure and closes to prevent further fluid flow after normal conditions return. It is characterized by rapid-opening ‘pop’ action or proportional opening according to the pressure increase.
• Safety valve: A pressure relief valve actuated by inlet static pressure and characterized by rapid opening.
• Relief valve: A device that opens gradually in proportion to the pressure increase over the opening pressure, commonly used in liquid systems for lower capacities and thermal expansion duties.
• Safety relief valve: Combining features of safety and relief valves, it can open rapidly or proportionally, suitable for both liquid and compressible fluid systems.

For further detailed information, check resources like the Post-Tensioning Institute > Home, Collets, and 304 Polished Stainless Steel Sheet Supplier for more insights.

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Oil Well Subsurface Safety Valves - Completions

Ensuring Reliability with Advanced Technology

Surface-controlled subsurface safety valves (SCSSVs) are essential for well completions, preventing uncontrolled flow in case of severe wellhead damage. These valves must close failsafe to ensure safety but remain open for well production. Schlumberger’s surface-controlled subsurface safety valves exceed ISO 10432 and API Spec 14A requirements for pressure integrity and leak prevention.

Schlumberger's safety valve systems, built on decades of innovation and expertise, feature robust flapper systems and multizone dynamic seal technology for hydraulic actuation. This advancement enhances reliability when compared to traditional seal systems. Their validation included rigorous performance and longevity tests using a unique sapphire crystal bore to simulate real-time wellbore conditions.

The multizone seal technology is available in the GeoGuard high-performance deepwater safety valves, validated to API Spec 14A V1 and V1-H. These safety valves represent the best in technology and reliability.

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