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Safety Relief Valve

Safety relief valves are designed to protect pneumatic and hydraulic systems from overpressure caused by faulty equipment, incorrect operation, or external forces. On crossing the designed safety thresholds, safety valves open and discharge the system’s media quickly until the pressure drops to within the safe limit. Continue reading to learn more about their construction, working, and the applications that use safety valves.

Applications of safety relief valves

Safety valves are used in various industries to prevent excessive pressure buildup. They are typically used in the following applications:

  • Petroleum: Subsurface safety valves shut down a system quickly if the components malfunction or break. This rapid action prevents further media flow preventing dangerous explosions.
  •  HVAC: Safety valves help prevent blocked media discharge and thermal expansion that can damage HVAC systems.
  • Power: Safety valves protect gas and compressed air in power plants against over-pressurization.
  •  Food, beverage, and pharmaceutical: Systems that demand paramount protection against contamination safety valves made of stainless steel.  

Types of safety relief valves

The primary characteristic feature of a safety valve is its ability to open quickly because of its design. The closing mechanism of the valve (the top of the valve’s disc) has a lip separated from the media until the disc opens. As the disc opens, the media quickly interacts with a higher surface area. This, in turn, increases the force acting on the disc, pushing it to open to nearly  60% of its completely open state. There are several types of safety relief valves used in various industries, including spring-supported single-action, pilot-operated, and balanced bellows.

Spring mechanism

A Spring-loaded or direct-acting safety valve is the most common type of safety valve in use. They are used in various applications with a working pressure that ranges approximately from 1 – 1400 bar (14.5 – 20305 psi). The valve has a spring that keeps the disc shut and controls the relief valve’s set pressure. When the media pressure exceeds the spring force, the disc cracks and opens rapidly. The spring can be adjustable or non-adjustable and is factory set. Spring-supported safety relief valves have the following parts:

  • Nozzle: The media flows from the inlet into the nozzle.
  • Nozzle ring: The nozzle ring controls the relief valve’s reseating pressure. If the pressure value is set too low, the disc will fluctuate, and if it is set too high, it will reseat too late.
  • Closing mechanism: A disc or a poppet acts as the closing mechanism.
  • Expansion chamber: The expansion chamber accommodates the disc’s wider top, which ensures the pressure drops within the operating limits. 
  • Spring: The spring mechanism controls the disc’s cracking pressure.

Pilot-operated safety valve

A pilot-operated safety valve is composed of two separate valves: the main valve and a pilot valve. The pilot valve controls the safety valve’s cracking pressure through a spring mechanism which can either be adjustable or non-adjustable. 

The main valve’s spring is relatively soft and non-adjustable. The pilot hole enables the free flow of the media above and below the main valve. This allows pressure to be equalized on both sides of the main valve, which ensures that the closing mechanism remains closed.

Media also flows to the pilot valve. When the pressure rises enough to open the pilot valve, the media above the main valve flows through the pilot valve and out of the relief valve’s outlet. This creates a difference in pressure above the main valve, which allows the free flow of the media through the safety valve at standard flow rates.

Balanced bellows safety valve

A balanced bellows safety valve can either be single-action or pilot-operated. The difference between the two lies in the position of the bellows with respect to the main disc. The bellows are used in applications that have a high degree of back pressure. Uncontrolled backpressure can cause damage to the top portion of the disc. The bellows come into action here by distributing the pressure evenly across the top of the safety relief valves. This helps in ensuring that no single zone experiences excessive pressure.

How to select the right safety valve

There are several parameters to consider while selecting a safety relief valve.

  • Set pressure: In applications where the system has a constant maximum allowable working pressure, a non-adjustable safety valve can be used. In applications where the maximum allowable pressure is expected to fluctuate within a range, an adjustable safety valve can be used.
  • Backpressure: Choose a balanced bellows safety valve if the pressure is likely to rise downstream of the safety valve’s discharge.
  • Discharge capacity: A safety valve must be able to discharge the media rapidly. Therefore, the safety valve’s discharge capacity must be able to handle the system’s normal flow rates.
  • Operating temperatures: When selecting a valve, the maximum permissible temperature of the material of the valve should also be taken into consideration. If the temperature of the medium is outside the operating temperature of the valve, the probability of failure increases.
  • Sealing material: Choose a sealing material that is suitable for the system’s media to reduce any failure or malfunction.

Certifications

Manufacturing products within set standards and specifications help in creating products that are reliable and safe while reducing the chances of malfunctioning. The top three certification bodies that publish safety certifications for safety valves are TÜV, ASME, and ISO 4126.

  • TÜV: TÜV ensure that products are built according to the requirements registered in Pressure Equipment Directive (PED) 2014/68/EU. The PED sets the grades for designing and manufacturing steam boilers, safety pressure valves, pipelines, and pressure vessels.
  • ASME: ASME certifies the accreditation and specification of pressure relief valves, boilers, and pressure vessels.
  • ISO 4126: ISO 4126 is a general specification for all pressure vessels, regardless of the media involved.

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