In brief
A safety valve is a self-acting valve that opens when a set pressure is exceeded and protects vessels or pipelines against overpressure.
Contents
What is Safety valve?
Safety valves are the last line of protection against impermissible overpressure in steam boilers, pressure vessels, compressed air and process plants. When the pressure reaches the set pressure, the valve opens without auxiliary energy and discharges medium until the pressure falls again and it closes. It should be distinguished from the hydraulic pressure relief valve, which limits system pressure during normal operation.
Most safety valves are spring-loaded; there are also pilot-operated and weight-loaded designs. Basic requirements are described in the EN ISO 4126 series of standards (DIN EN ISO 4126). Key parameters are set pressure, blowdown, discharge capacity or discharge coefficient, minimum flow area, inlet and outlet connection sizes, materials and the medium. Within the EU, safety valves are classed as safety accessories within the meaning of the Pressure Equipment Directive.
Causes of failure include deposits and corrosion that prevent the valve from lifting, damaged sealing surfaces after frequent discharge and unsuitable settings. Selection, setting and periodic inspection are governed by the plant documentation and the applicable regulations; in individual cases, the assessment by competent persons or the responsible inspection body is binding.
In practice: maintenance and procurement
A safety valve is only replaced with a valve that has the same set pressure, sufficient discharge capacity, suitable connections and the required marking. An enquiry requires the nameplate data, medium, operating and set pressure, temperature and the required capacity; the seal and inspection records should be documented when replacing the valve.
Synonyms and abbreviations: pressure safety valve (PSV) · safety relief valve · overpressure valve · SV
Frequently asked questions
What is the difference between a safety valve and a pressure relief valve?
A safety valve is a protective device that only responds in the event of a fault and protects a vessel or plant from dangerous overpressure. A pressure relief valve, by contrast, is part of normal operation, especially in hydraulics, and limits the system pressure during operation. The requirements for design, marking and inspection differ accordingly.
What does set pressure mean for a safety valve?
The set pressure is the pressure at which the valve begins to open under operating conditions. It is set and secured at the factory or by an authorised body, usually with a seal. The set pressure must not exceed the permissible pressure of the protected plant and is stated on the nameplate.
What information is needed to replace a safety valve?
Required are the set pressure, medium and its physical state, operating temperature, required discharge capacity, nominal sizes of inlet and outlet, connection type, materials and the data from the nameplate. It is also important to know which marking and documentation are required for the plant. Replacing a safety valve on the basis of thread size alone is not permissible.
How often does a safety valve need to be inspected?
Inspection intervals depend on the type of plant, the medium and the applicable national law and are specified for each plant. They often include a function test by lifting the valve and an inspection as part of periodic plant inspections. The operating manual and the requirements of the responsible inspection body are decisive; this entry does not replace a legal assessment.