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.

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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.

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.

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.

Safety Valve Types and Working Principle

Figure 1: Safety valve

A safety valve protects a system against overpressure. Overpressure occurs when the system's pressure exceeds the Maximum Allowable Working Pressure (MWAP) or the pressure for which the system is designed. Safety valves can open very quickly compared to relief valves. A safety valve opens from a set pressure; the valve first opens a little, then opens fully so that the unwanted pressure is removed from the system as quickly as possible.

Safety valves prevent pressure increases that lead to malfunctions, fire hazards, or explosions. The system's media fully actuates a safety valve, keeping it working in a power failure. Safety valves only have mechanical parts, which operate when electronic or pneumatic safety devices fail.

Table of contents

Important terminology

• Overpressure: Excess pressure over the set pressure of the safety valve.
• Operating pressure: The pressure at which the system works under normal operating conditions.
• Set pressure: The pressure at which the safety valve's disc begins to lift and open.
• Lift: The distance the disc moves from the closed position to the position required for discharge.
• Backpressure: The pressure built upon the safety valve outlet during flow. Backpressure = Built-up backpressure + Superimposed backpressure.
• Built-up backpressure: The pressure at the outlet when the safety valve opens.
• Superimposed backpressure: The pressure at the outlet of a closed safety valve.
• Maximum allowable working pressure (MAWP): The maximum allowable pressure at a designated temperature under normal operating conditions. MAWP is the maximum pressure that the system's weakest component can handle.
• Blowdown: The difference between the pressure at which the disc lifts and the pressure at which the valve closes. Blowdown is generally expressed as a percentage.
• Blow-off capacity: The rate at which the safety valve can release excess pressure.

Safety valve types

There are different safety valve types: valves with a spring-loaded mechanism, valves with balanced bellows, and pilot-operated safety valves. Each type has an advantage in a specific situation.

Spring mechanism

The most common safety valve is a spring-loaded or direct-acting safety valve. An advantage of this type is that it is available for pressure ranges from approximately 1 to bar. The mechanism consists of the following components:

• Expansion chamber: The expansion chamber (Figure 2 labeled A) increases the surface area that the system's media pushes against to open the safety valve, allowing it to open rapidly.
• Spring: The spring&#;s (Figure 2 labeled B) stiffness determines at which pressure the system&#;s media can begin to open the valve.
• Disc: The disc (Figure 2 labeled C) sits on the nozzle and moves up and down to allow or prevent flow through the safety valve.
• Nozzle ring: The nozzle ring (Figure 2 labeled D) affects the pressure at which the disc reseats. A high setting can cause the disc to reseat too late, while a low setting can lead to the disc randomly opening and closing when it should not.
• Nozzle: The nozzle (Figure 2 labeled E) controls the disc surface area that the media interacts with before the valve opens. This causes the media to work against a larger surface area when the valve opens, increasing the force acting on the disc and opening the disc rapidly.

Figure 2: Safety valve with spring mechanism: expansion chamber (A), spring (B), disc (C), nozzle ring (D), and nozzle (E).

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The balance between a safety valve's spring force and the input force controls the valve's opening and closing. Inlet pressure and the disc's surface area with which the media interacts determine the input force. According to Pascal's Law, force equals the product of pressure and area. Therefore, as the area of the disc that the media interacts with increases, so does the force.

The most important characteristic of safety valves is that they quickly open entirely to reach maximum blow-off capacity in minimal time. This is possible because the valve's disc has a larger diameter than the nozzle. As soon as the inlet pressure is high enough, the disc lifts. At this moment, the disc surface at which the medium can reach becomes larger. This results in an input force much greater than the spring force, and the valve completely opens.

Special safety valve versions exist for incompressible and compressible media and gasses/vapors. Safety valves for gasses and vapors often open before the set pressure is reached and open to at least 50% lift at the response pressure (see Figure 3). Safety valves of this type have a significant disadvantage: they are very susceptible to back pressure, which can negatively affect the valve's safety.

Figure 3: Safety valve mechanism for gasses and vapors (left): nozzle ring (A) and flow pattern (B). Blow-off characteristic of a safety valve for gasses and vapors (right): set pressure (1) and lift (2).

Balanced bellows

Figure 4: Safety valve with balanced bellows: guide (A), metal bellows (B), disc holder (C).

Balanced bellows safety valves are not susceptible to the negative impacts of backpressure. Bellows (Figure 4 labeled B) above the disc ensure that backpressure is evenly distributed above and below the disc. Furthermore, the spring does not encounter the media, preventing undesirable influence on the spring from the media. The downside of balanced bellows safety valves is that their MAWP is lower than direct-acting safety valves. They operate up to a maximum of 15.9 bar.

Pilot-operated safety valve

In a pilot-operated safety valve, the pressure required to open the disc is much closer to the system&#;s working pressure. This eliminates unnecessary pressure increases beyond the working pressure. The following components work together to make this possible:

• Pilot spring: The pilot spring (Figure 5 labeled A) controls the pressure at which the pilot poppet opens.
• Pilot valve: The pilot valve (Figure 5 labeled B) opens at a set pressure, leading to a differential that allows the main valve to open.
• Main spring: The main spring (Figure 5 labeled C) shuts the main valve until the pilot valve opens.
• Main valve: The main valve (Figure 5 labeled D) opens to allow flow from the inlet to the outlet.
• Adjusting knob: The adjusting knob on the pilot valve (Figure 5 labeled E) allows for adjustment of the set pressure.

As long as the inlet pressure is lower than the set pressure, the valve remains closed (Figure 5 left). As soon as the inlet pressure rises above the response pressure, the pilot valve moves toward the open position, allowing flow through the pilot hole and out of the valve (Figure 5 middle). This causes a pressure difference over the main valve, causing it to move upwards, allowing the remaining media to flow freely to the outlet (Figure 5 right). The valve closes when the inlet pressure drops below the response pressure again.

Figure 5: Pressure relief valve with guide control (left): pilot spring (A), pilot valve (B), main spring (C), main valve (D), and adjustment knob (E). The pilot valve opens in response to high enough inlet pressure, allowing flow through the pilot hole and out of the valve (middle). The main valve in the open position (right).

Dead weight safety valve

A deadweight safety valve is the simplest type of safety valve. It consists of a gunmetal valve on top of a boiler's vertical steam pipe. When pressure within the boiler rises enough, steam lifts the valve until it reduces enough to fall back into its seat. This type of valve is only suitable for stationary applications.

Safety valve actuation types

Safety valve actuation types are crucial for how a valve responds and fits into different systems. Here's a simplified overview of the three main types:

• Electric safety valves: These use electrical signals for control, making them great for systems that need precise adjustments but not necessarily fast responses. They're often used in automated setups where the valve can be programmed to react to specific conditions like pressure changes.
• Pneumatic safety valves: These valves use compressed air to work, offering quick responses. They're ideal for places where electricity isn't an option or is too dangerous. You'll find them in industries like oil and gas or chemical processing, where they help quickly relieve pressure to prevent accidents.
• Hydraulic safety valves: Operated by fluid pressure (usually oil), these valves are for heavy-duty use where you need a lot of force to open or close the valve. They provide smooth operation and are perfect for high-pressure situations where a sudden pressure drop could cause problems.

Selection criteria

To protect your system against overpressure, it is essential to understand the five selection criteria below. Please read our technical article on selecting safety valves to better understand these criteria.

• Set pressure
• Backpressure
• Discharge capacity
• Operating temperatures
• Valve and sealing material

Applications

A safety valve's purpose is primarily for industrial applications to protect against overpressure, which can cause dangerous situations such as fire or explosions. Industrial safety valves are often found in:

• Oil, gas, and petroleum industry: For example, subsurface safety valves, or downhole safety valves, are common on offshore oil wells. In the case of equipment malfunction, a safety valve can shut off rapidly to prevent oil and gas from flowing up the well in unsafe conditions.
• Energy: Safety valves in power plants are common for compressible gasses such as steam and air.
• Sanitary: Stainless steel safety valves are ideal for industries that require sanitary conditions. For example, in the food, beverage, and pharmaceutical industries.
• HVAC: Safety valves relieve pressure in the case of blocked discharge, thermal expansion, or external heat that can damage the components.

Safety valve symbol

Figure 6: Varying safety valve symbols

Safety valve certifications

Safety valves must comply with various national and international standards for safety and quality. To ensure that the product complies, please consult local standards.

TÜV

The TÜV certification assesses a product's safety. It verifies that it meets the minimum requirements under the Pressure Equipment Directive (PED) /68/EU. The PED outlines the standards for designing and manufacturing pressure equipment such as pressure relief devices, steam boilers, pipelines, and pressure vessels operating at a maximum allowable pressure greater than 0.5 bar.

ASME

The ASME (American Society of Mechanical Engineers) ensures the specification and accreditation of pressure vessels, boilers, and pressure relief devices.

ISO

The ISO standard is a general specification for pressure relief valves, regardless of the application&#;s media.

Pressure safety valve vs relief valve

Pressure safety valves and relief valves have important similarities and differences.

• Similarities: Both valves types are safety devices designed to automatically open at a set pressure level to prevent overpressure in a system.
• Differences: Pressure safety valves are designed to open rapidly and quickly relieve pressure. Pressure relief valves open more gradually to control the system's pressure decrease.

Learn more in our comprehensive pressure safety and relief valve comparison article.

FAQs

What does a safety valve do?

A safety valve rapidly reduces a system's pressure when it rises to unsafe levels. The safety valve continues operating until system pressure returns to safe levels.

What is the difference between a relief valve and a safety valve?

A relief valve will not immediately halt the operation of downstream components, whereas a safety valve will.

What are the types of safety valves?

Common safety valve types are direct-acting, pilot-operated, and balanced bellows.

What is an ASME safety valve?

An ASME safety valve meets the requirements of the ASME pressure vessel code&#;s Section I. These valves must have a large constant flow rate at no more than 10% overpressure.

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