How Do Safety Valves Work?

In the new, well new in , EU Standard brings together the Safety, Pressure Relief, Safety Relief Valves into a single Standard – Safety Valves. Interestingly, the American Standard ASME / ANSI still divide these three functions. Due to the potential risk to life caused by failing systems, most nations around the world have a National Standard covering Safety Valves.

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SAFETY VALVE OPERATION

Safety Valves used within the Building Services Industry follow a design that uses an applied force (a spring) to directly oppose the system force.

This valve design utilises three states:

SAFETY VALVE SIZING

The sizing of Safety Valves is particularly important. 

• Valves that are undersized will not be able to discharge the maximum capacity at the specified overpressure and as a consequence the system pressure will not drop but continue to rise. 
• Valves that are oversized will not achieve the fully open state.  The rising system pressure could only slightly lift the disc off the seat, releasing the excess pressure, allowing the valve to close.  The valve starts a cycle of opening / closing as the design discharge was not achieved, this is often referred to as ‘chattering’.

NABIC have a sophisticated programme to ensure correct selection.

SAFETY VALVE INSTALLED POSITION

Safety valves should only be installed in a vertical position with the spring chamber, ie the bonnet, in an upwards position. An isolation valve between the safety valve and the system is strictly prohibited.

The installation should avoid:

• Long runs of pipework between safety valve and system
• Fittings and bends between safety valve and system
• Inverted safety valve outlet

SAFETY VALVE MAINTENANCE

• Testing
  • A site risk assessment should establish a maintenance regime for testing the function of safety valves, we would specify that this should be at least every 3 months
• Inspection
  • The NABIC design of safety valves allows for the internal examination and cleaning without altering the set pressure
• Re-calibration
  • A site risk assessment should establish a period for re-calibration.  In resent years, insurance companies and boiler inspectors have insisted that safety valves are re-calibrated on a yearly basis

TYPICAL SYSTEM FAILURES

Within Building Services, there are four typical system failure modes that bring the safety into action:

• Blocked outlets
• External heat
  • Uninsulated external pipe runs heated by external source, ie the sun
• Thermal expansion
  • Failure of boiler temperature controls
• Pipework component failure
  • Control valves failure

TYPICAL TERMS

Set Pressure

BS EN ISO state ‘The pressure at which the valve is about to open, ie  the equilibrium state. In practice, it’s not possible to find ‘when the valve is about to open’, only when the valve is closed or just opened. ASME state ‘The pressure at which the valve starts to open’.  In practice this position is easier to establish than the BS EN equilibrium position, ie ‘about to open’.

Overpressure

A pressure increase over the set pressure, usually expressed as a percentage.

Accumulation

The pressure increase over the maximum allowable working pressure of the system during discharge through the safety valve, expressed as a percentage of pressure.

Blowdown

The difference between the actual set pressure and the actual reseating pressure, expressed as a percentage of pressure.

Backpressure

The static pressure existing in the discharge system ‘from’ the safety valve.

Introduction to Safet​y Valves

As soon as mankind was able to boil water to create steam, the necessity of the safety device became evident. As long as 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 pressurised vessels.

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.

In , Charles Retchie invented the accumulation chamber, which increases the compression surface within the safety valve allowing it to open rapidly within a narrow overpressure margin.

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Today, most steam users are compelled by local health and safety regulations to ensure that their plant and processes incorporate safety devices and precautions, which ensure that dangerous conditions are prevented.

The primary function of a safety valve is therefore to protect life and property.

The principle type of device used to prevent overpressure in plant is the safety or safety relief valve. The safety valve operates by releasing a volume of fluid from within the plant when a predetermined maximum pressure is reached, thereby reducing the excess pressure in a safe manner. As the safety valve may be the only remaining device to prevent catastrophic failure under overpressure conditions, it is important that any such device is capable of operating at all times and under all possible conditions.

Safety valves should be installed wherever the maximum allowable working pressure (MAWP) of a system or pressure-containing vessel is likely to be exceeded. In steam systems, safety valves are typically used for boiler overpressure protection and other applications such as downstream of pressure reducing controls. Although their primary role is for safety, safety valves are also used in process operations to prevent product damage due to excess pressure. Pressure excess can be generated in a number of different situations, including:

• An imbalance of fluid flowrate caused by inadvertently closed or opened isolation valves on a process vessel.
• Failure of a cooling system, which allows vapour or fluid to expand.
• Compressed air or electrical power failure to control instrumentation.
• Transient pressure surges.
• Exposure to plant fires.
• Heat exchanger tube failure.
• Uncontrollable exothermic reactions in chemical plants.
• Ambient temperature changes.

The terms ‘safety valve’ and ‘safety relief valve’ are generic terms to describe many varieties of pressure relief devices that are designed to prevent excessive internal fluid pressure build-up. A wide range of different valves is available for many different applications and performance criteria.

Furthermore, different designs are required to meet the numerous national standards that govern the use of safety valves.

A listing of the relevant national standards can be found at the end of this module.

In most national standards, specific definitions are given for the terms associated with safety and safety relief valves. There are several notable differences between the terminology used in the USA and Europe. One of the most important differences is that a valve referred to as a ‘safety valve’ in Europe is referred to as a ‘safety relief valve’ or ‘pressure relief valve’ in the USA. In addition, the term ‘safety valve’ in the USA generally refers specifically to the full-lift type of safety valve used in Europe.

The ASME/ANSI PTC25.3 standards applicable to the USA define the following generic terms:

• Pressure relief valve - A spring-loaded pressure relief valve which is designed to open to relieve excess pressure and to reclose and prevent the further flow of fluid after normal conditions have been restored. It is characterised by a rapid-opening ‘pop’ action or by opening in a manner generally proportional to the increase in pressure over the opening pressure. It may be used for either compressible or incompressible fluids, depending on design, adjustment, or application.

This is a general term, which includes safety valves, relief valves and safety relief valves.

• Safety valve - A pressure relief valve actuated by inlet static pressure and characterised by rapid opening or pop action.

Safety valves are primarily used with compressible gases and in particular for steam and air services. However, they can also be used for process type applications where they may be needed to protect the plant or to prevent spoilage of the product being processed.

• Relief valve - A pressure relief device actuated by inlet static pressure having a gradual lift generally proportional to the increase in pressure over opening pressure.

Relief valves are commonly used in liquid systems, especially for lower capacities and thermal expansion duty. They can also be used on pumped systems as pressure overspill devices.

• Safety relief valve - A pressure relief valve characterised by rapid opening or pop action, or by opening in proportion to the increase in pressure over the opening pressure, depending on the application, and which may be used either for liquid or compressible fluid.

In general, the safety relief valve will perform as a safety valve when used in a compressible gas system, but it will open in proportion to the overpressure when used in liquid systems, as would a relief valve.

The European standard EN ISO -1 provides the following definition:

• Safety valve - A valve which automatically, without the assistance of any energy other than that of the fluid concerned, discharges a quantity of the fluid so as to prevent a predetermined safe pressure being exceeded, and which is designed to re-close and prevent further flow of fluid after normal pressure conditions of service have been restored.

Typical examples of safety valves used on steam systems are shown in Figure 9.1.1.

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