What is the introduction of safety valve?

Device for releasing excess pressure in a system

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A safety valve is a device designed to act as a fail-safe mechanism. A common example is a pressure relief valve (PRV), which automatically releases a substance from a boiler, pressure vessel, or other systems when the pressure or temperature exceeds preset limits. Another specialized type is the pilot-operated relief valve. Additionally, a lower-cost, leak-tight option for emergency use is a rupture disk.

Safety valves were first created for steam boilers during the Industrial Revolution. Early boilers that lacked safety valves were highly susceptible to explosions unless operated with extreme caution.

Vacuum safety valves (or combined pressure/vacuum safety valves) prevent a tank from collapsing while emptying or during cold rinse water use after hot cleaning or sterilization procedures. The calculation method for sizing these valves is not defined by any standard, especially in the hot cleaning/cold water scenario, though some manufacturers have developed simulations for sizing.

The term safety valve can also be used metaphorically.

Function and Design

A cross-section of a proportional safety valve illustrates its functionality. The earliest safety valves were used in steam digesters, utilizing weight to retain steam pressure, a design still found in pressure cookers. However, early models were easily tampered with or could accidentally release. On the Stockton and Darlington Railway, the safety valve would activate whenever the engine hit a bump. To address this, a less sensitive model was developed using a spring to contain steam pressure, but these could still be adjusted to exceed design limits, a dangerous practice sometimes employed to marginally increase steam engine performance.

In 1856, John Ramsbottom invented a tamper-proof spring safety valve that became universal on railways. His design featured two plug-type valves connected by a spring-loaded pivoting arm. Adjustments to one valve would cause the other to lift off its seat, preventing unauthorized pressure adjustments. Only by disassembling the entire valve could it be adjusted, which eliminated the risky practice of tying down the valve for increased power. To facilitate operation, the pivoting arm extended into the locomotive cab, allowing crews to confirm proper operation.

Safety valves have evolved to protect various equipment, including pressure vessels and heat exchangers, with the term safety valve primarily applied to compressible fluid applications (gas, vapor, or steam).

In industrial contexts, two main types of protection exist: thermal protection and flow protection.

For liquid-filled vessels, thermal relief valves are characterized by their relatively small size, sufficient to protect against excess pressure due to thermal expansion. Most liquids are nearly incompressible, so a small amount discharged can significantly reduce pressure.

Flow protection involves larger safety valves designed to handle significant quantities of gas or liquids quickly to maintain vessel or pipeline integrity. An alternative method for protection is the installation of a high integrity pressure protection system (HIPPS).

Technical Terms

In various industries, including petroleum refining, chemical manufacturing, and pharmaceuticals, the term safety valve is often linked with pressure relief valve (PRV), pressure safety valve (PSV), and relief valve. The generic term is Pressure Relief Valve (PRV) or Pressure Safety Valve (PSV). Despite common misconception, PRVs and PSVs serve different functions; PSVs feature a manual lever for emergency operation.

• Relief Valve (RV): An automatic valve that acts based on static pressure in a liquid-filled vessel and opens proportionally with increased pressure.
• Safety Valve (SV): An automatic valve that relieves static pressure in a gas and usually opens completely with a distinct popping sound.
• Safety Relief Valve (SRV): An automatic valve that relieves static pressure on both gas and liquid.
• Pilot Operated Safety Relief Valve (POSRV): Operated remotely by a pilot with the static pressure connected from the equipment to protect.
• Low Pressure Safety Valve (LPSV): Relieves static pressure on gas when the vessel pressure and ambient pressure differ slightly.
• Vacuum Pressure Safety Valve (VPSV): Similar function as LPSV but for negative pressures close to atmospheric levels.
• Low and Vacuum Pressure Safety Valve (LVPSV): Handles static pressure on gas with small variations, both negative and positive.

RV, SV, and SRV are spring-operated, while LPSV and VPSV can be either spring-operated or weight-loaded.

Legal and Code Requirements in Industry

Many countries mandate the use of relief valves to protect pressure vessels and related equipment. Compliance with equipment design codes from organizations such as ASME, API, and ISO is typically required. These codes prescribe design standards for relief valves, including schedules for periodic inspection and testing post-removal by company engineers.

Today, hygienic safety valves are essential in industries like food, cosmetics, and pharmaceuticals, where they must be fully drainable and cleanable in place. Most are constructed from stainless steel, with the chief hygienic standards being 3A in the USA and EHEDG in Europe.

Development of the Safety Valve

Deadweight Lever Valves

The first safety valve, invented by Denis Papin, was designed for steam digesters. A weight acting through a lever held a valve in place. By using a "steelyard" lever, a smaller weight could effectively regulate pressure. Early safety valves were viewed as vital controls that required constant supervision. A historical explosion highlighted the dangers associated with neglecting safety valves.

By the turn of the century, Trevithick had begun implementing pairs of safety valves, one for adjustments and the other sealed with a fixed weight, ensuring it released at a higher, predetermined pressure for added safety.

When fitted on locomotives, these valves were susceptible to rattling and leakage, releasing steam continuously.

Direct-Acting Deadweight Valves

Despite their convenience, lever safety valves were too sensitive to locomotive motion. Consequently, a simpler configuration using stacked weights directly on the valve was adopted, although this sometimes led to inadequate pressure relief and potential explosions.

Direct Spring Valves

To address the challenges of weighted valves, Timothy Hackworth developed lightweight spring valves. Although the metallurgy of that era limited their designs, these valves allowed for easier manipulation and tampering was minimized by enclosing them in robust brass casings.

Salter Spring Balance Valves

The Salter coil spring balance made significant advances in valve design employing spring steel for enhanced strength. Lever mechanisms allowed these valves to act effectively under significant boiler pressures, often doubling as pressure gauges.

Lockable Valves

Despite advancements, the risk of tampering persisted through easily adjustable components. To combat this, many Salter valves were used in pairs, with one adjustable and another sealed within a lockable cover.

Paired Spring Balance Valves

Paired safety valves often operated at slightly different pressures, with one as a control measure and another securely set at a higher threshold.

Ramsbottom Safety Valves

In 1856, John Ramsbottom introduced a safety valve designed with improvisation and security in mind. His valve system involved pairs of plug valves and a spring-loaded lever designed to prevent tampering and ensure safe operation.

These Ramsbottom valves improved reliability and were designated for unique fittings within a boiler, showcasing both versatility and potential maintenance challenges due to their complexity.

Naylor Valve

Introduced around 1880, Naylor valves featured a bellcrank design that managed the strain on springs more effectively, marking a significant improvement in valve engineering.

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