Safety valves - How do they work

Figure 1: Safety valve

Figure 1: Safety valve

A safety valve is used to protect the system against overpressure. Overpressure occurs when the 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, after which it opens fully so that the unwanted pressure is removed from the system as quickly as possible.

Safety valves are used to prevent pressure increases, leading to malfunctions, fire hazards, or explosions. Safety valves only have mechanical parts, so they are used when electronic or pneumatic safety devices fail. A safety valve is fully actuated by the system's media, keeping it working in a power failure.

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Table of Contents

Important terminology

  • Overpressure: The 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 disc begins to lift and the safety valve opens.
  • Lift: The distance that the disc has moved from closed position to the position required for discharge.
  • Backpressure: The pressure built upon the outlet of the safety valve during the flow. Backpressure = Built-up backpressure + Superimposed backpressure.
  • Built-up backpressure: The pressure at the outlet once the safety valve has opened.
  • Superimposed backpressure: The pressure at the outlet of the 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 weakest component of the pressure device can handle.
  • Blowdown: The difference between the actual pressure at which the disc lifts and the actual pressure at which the valve closes. It is generally expressed in 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

This safety valve is the most commonly used version, called direct-acting safety valves because it is only held closed by a spring. An advantage of this type is that they are available for pressure ranges from approximately 1 to 1400 bar. The mechanism consists of a nozzle, a spring, and a poppet valve (see Figure 2).

Figure 2: 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 opening and closing of a safety valve are determined by the balance between the spring force and the input force. In other words, the force of the medium on the poppet valve. The input force is determined by the inlet pressure, and the poppet valve's available surface area: F = P*A.

An essential characteristic of safety valves is that they open fully within a short period to reach maximum blow-off capacity in minimal time. This is possible because the poppet valve has a larger diameter than the nozzle. As soon as the inlet pressure is high enough, the poppet valve is lifted (smothered). At this moment, the poppet valve surface at which the medium can reach becomes larger. This results in an input force much greater than the spring force, and the valve will open completely.

Standard versions have a flat poppet valve, but there are versions for specific types of gases and liquids. The main difference is in the poppet valve"s shape; in these versions, the poppet valve is equipped with an expansion chamber (see Figure 3). This chamber provides more lift (lifting of the poppet valve) and will be open for a shorter time. These versions are mainly used in applications where the allowable overpressure is low.

There are special versions for incompressible and compressible media and gases/vapors. Safety valves for gases and vapors often open before the set pressure is reached and open to at least 50% lift at the response pressure (see Figure 3).

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

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

Safety valves of this type do have a significant disadvantage: they are very susceptible to backpressure. This backpressure can affect the safety of the valve. The next type of safety valve discussed is much more resistant to backpressure.

Balanced bellows

Figure 4: 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)

Safety valves with a spring mechanism are susceptible to backpressure. For this purpose, special versions have been made in which metal bellows have been incorporated. The bellows ensure that the back pressure is exerted on an equally large surface at the poppet valve's top and bottom. As a result, the backpressure does not influence the operation of the valve.

Furthermore, in this version, the spring mechanism is always isolated from the media, so there is no undesirable influence on the spring force itself. The space in which the spring is located is also vented to the atmosphere. See Figure 4 for an example of a safety valve with balanced bellows. Please note that this version only works up to certain counter pressure. Typical safety valves with balanced bellows operate up to maximum backpressure of 15.9 bar (pressure gauge).

Pilot operated safety valve

The third type of safety valve is the pilot-operated safety valve type. With this type, the poppet valve is held on the nozzle by the inlet pressure itself. The higher the working pressure, the tighter the main poppet valve is pressed onto the nozzle.

An advantage of the guide control is that the response pressure can be closer to the working pressure without leaks or unwanted openings. The most important components are shown in Figure 5 are:

  • Guide spring (A)
  • Guide plate valve (B)
  • Mainspring (C)
  • Main plate valve (D)
  • Adjusting knob (E)

As long as the inlet pressure is lower than the set pressure, the valve remains closed. As soon as the inlet pressure rises above the response pressure, the guide moves to the open position allowing the media to flow to the outlet via the pilot tube. This causes a pressure difference over the main valve, causing it to move upwards, allowing the media to flow freely to the outlet. When the inlet pressure drops below the response pressure again, the valve closes again.

Figure 5: Pressure relief valve with guide control (left): spring control (A), guide plate valve (B), mainspring (C), main plate valve (D), adjustment knob (E). The valve in the open position (right)

Figure 5: Pressure relief valve with guide control (left): spring control (A), guide plate valve (B), mainspring (C), main plate valve (D), adjustment knob (E). The valve in the open position (right)

Selection criteria

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

  1. Set pressure
  2. Backpressure
  3. Discharge capacity
  4. Operating temperatures
  5. Valve and sealing material

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 the 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, pressure vessels, etc., 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 media used for the application.

Applications

Safety valves are mainly used in industrial applications to protect against overpressure. This overpressure can lead to dangerous situations, fire, or explosions. Safety valves are often found in:

  • The oil, gas, and petroleum industry. Equipment failure can lead to fire or explosions, in which case a safety valve must discharge the hazardous substance.
  • The energy sector. They are often used to discharge overpressure in steam, air, gas or liquid pipelines, or vessels/boilers.

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