Safety Valves - How They Work

Safety valve

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

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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 that the disc moves from the closed position to the position required for discharge.
  • Backpressure: The pressure built upon the outlet of the safety valve 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 1400 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. This allows the safety valve 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. 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 leads to the media working against a larger surface area when the valve opens, increasing the force acting on the disc and opening the disc rapidly.
Safety valve with spring mechanism: expansion chamber (A), spring (B), disc (C), nozzle ring (D), and nozzle (E).

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

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 that the media interacts with determine the input force. According to Pascal’s Law, force is equal to 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 open fully in a short period 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 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).

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).

Safety valves of this type have a significant disadvantage: they are very susceptible to back pressure. Backpressure can negatively affect the valve's safety.

Balanced bellows

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

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 at which pressure the pilot poppet opens.
  • Pilot valve: The pilot valve (Figure 5 labeled B) opens at a set pressure, leading to a pressure differential that allows the main valve to open.
  • Main spring: The main spring (Figure 5 labeled C) holds the main valve shut until the pilot valve opens.
  • Main valve: The main valve (Figure 5 labeled D) opens to allow flow from inlet to 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.

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).

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 for the valve to fall back into its seat. This type of valve is only suitable for stationary applications.

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

Safety valves are mainly used in industrial applications to protect against overpressure, which can cause dangerous situations such as fire or explosions. 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, 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

Varying safety valve symbols

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 and verifies that it meets the minimum requirements under the Pressure Equipment Directive (PED) 2014/68/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 4126

The ISO 4126 standard is a general specification for pressure relief valves, regardless of the application’s media.

FAQs

What does a safety valve do?

A safety valve rapidly reduces a system’s pressure in the case of said pressure rising to unsafe levels. The safety valve continues to operate until system pressure returns to well within 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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