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Säkerhetsventil/Safety Relief

Destination of safety valves and basic qualifications
An assignment of safety valves is protecting of pressure devices (boilers, tanks, pipelines etc.) before excessive height pressure above the limiting value.
Mechanism of his work is following:
- after reaching the opening pressure of valve must he begin to open;
- at further height of pressure he should take over the stream of factor and stably take off
- after pressure drop in the system he should close tight again
In direct action safety valves which are object of the present elaboration, the process of opening and closing takes place only under power exerted by working factor and opposed power of valvate mechanism which is spring or weight.
The process of variability of pressure in device which is protected with direct action safety valve in function of time describes following graph.

Under notion of working parameters of safety valve should be understand:
- po – pressure at which follows the beginning of opening of safety valve (opening pressure)
- p1 – pressure at which the disc of valve get maximum lift (drop pressure) - pz – pressure at which follows hermetic close of valve (closing pressure)
- b1 – proportional increase of opening pressure before safety device, necessary to get full lift and consequently maximum of capacity
- b2 – proportional drop of opening pressure before safety device, necessary to tight close - pr – working pressure of safety device (pressure of correct work of device)
The area of work of safety valve is situated among the pressure admitted for safety device (pmax), and maximum pressure, which can appear in device after opening of safety valve (1,1 pmax).
Working pressure of device (pr), lower than admitted pressure (pmax), permit free regulation of pressure in device in range pr —> pmax, without causing of opening of safety valve.

Correctly selected safety device should comply with two main conditions:
- should effectively protect pressure device against pressure increase above values exceeding admissible pressure of device up to 10 %
- should not interrupt during its operation the correct work of protected pressure device
The fulfilling of these tasks demands from the designer the proper assignment of working pressures of safety device to the zones of specific pressures for the protected pressure device.
General information by definition of the value of safety valve opening pressure.
In case of small diameters of valves seats (do < 20 mm), sealing surfaces are so small that the reached execution tolerances have crucial influence on the opening pressure and the tightness of closure.
Therefore it is recommended to increase difference between the working pressure of protected pressure device and the opening pressure of safety valve (working difference of pressures).
Similarly by low opening pressures the working difference of pressures is basically bigger than in case of the higher opening pressures.

The mentioned below diagram shows the recommendations in this matter according to German resources:
Furthermore, external influences like mechanical hit of medium from inflow side or stream pulsation (e.g. in piston compressors), demand also the bigger working difference of pressures.

Connection cords of safety devices
Below mentioned information are given on basis of WUDT-UC-WO/A regulations, as well as on basis of composition of Central Laboratory of Technical Supervision „Devices protecting against increase of pressure”.

Under notion „connection cords” you should understand both inlet and out connections, joined directly to the connection terminal of safety device.
The connection cords should be as short as possible, and their geometrical shape possibly simple. The inlet connection cords should be joined basically directly to the terminal connections of protected pressure areas, and their construction, used materials and resistance calculations or technical norms should be strictly adjusted to the working parameters of protected pressure device. The connection cords should be lead with the possibly smallest changes of flow directions. Generally should fulfill the following demands:

  • inside diameter of inlet/outlet connection should be not smaller than the biggest outside diameter of terminal inlet/outlet connection of safety valve
  • axle of inlet connection should be rising line on the whole length
  • respective sections of outlet connection should not be siphon bent
  • changes of flow directions should be executed by the presence of angles not smaller than 90o
  • radius of connection bents should be not smaller than tripled diameter of connections

Furthermore, the connection cords should be designed taking into consideration thermal extensions’ compensation, and fastening of safety valve body, as well as connection cords, should comply with the static and dynamic interaction of working medium.
Discharge of working medium from valve should not create danger for the environment. The connections discharging flammable, caustic, poisonous and explosive mediums – should be executed in the way which assures the safety.

The usage of cut-off fittings on the connection cords of safety valve is not admissible. For the containers with flammable, caustic, poisonous or explosive medium, the respective supervisory authority can permit to use cut-off fittings which fulfill the supervisory regulations. On the inlet and outlet connections of safety valves can be applied switching valves with construction which excludes simultaneous cut off of all valves, but the not cut off safety valves should have sufficient capacity. The area of free passage of switching valves should be not smaller than the biggest area of connection cord intersection.
The outflow connection i.e. joining the safety device with atmosphere or discharge area, should be lead on the whole length with the appropriate decrease designed with the consideration of possibilities of effective discharge of condensate, and protection against freezing.
Applying the expanders or sound silencers on the outlet connections you should also consider hints given in the supervisory or specific regulations, but generally they should not interrupt the work of valve, and additional flow resistances should be considered by the calculations of inlet connection and capacity of safety valve.

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