ISO 4126-4:2013 pdf download.Safety devices for protection against excessive p ressure-Part 4:Pilot operated safety valves.
This part of ISO 4126 specifies general requirements for pilot operated safety valves, irrespective of the fluid for which they are designed. In all cases, the operation is carried out by the fluid in the system to be protected.
It is applicable to pilot operated safety valves having a valve flow diameter of 4 mm and above which are for use at set pressures of 0,1 bar gauge and above. No limitation is placed on temperature.
This is a product standard and it is not applicable to applications of pilot operated safety valves.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 4126-7:2013, Safety devices for protection against excessive pressure — Part 7: Common data
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
pilot operated safety valve
self-actuated device comprising a main valve and an attached pilot
Note 1 to entry: The pilot responds to the pressure of the fluid without any other actuating energy than the fluid itself and controls the operation of the main valve. The main valve opens when the fluid pressure that keeps it closed is removed or reduced. The main valve re-closes when the pressure is re-applied.
Note 2 to entry: See Figure 1 for a list of main components.
3.2
main valve
parts of a pilot operated safety valve, through which the discharge capacity is achieved
3.3
flowing pilot
pilot which discharges the fluid throughout the relieving cycle of the pilot operated safety valve
3.6
modulating
action characterized by a gradual opening and closing of the disc of the main valve which is a function of the pressure, proportional but not necessarily linear
Note 1 to entry: This is an action of the pilot operated safety valve.
3.7
set pressure
predetermined pressure at which the main valve of a pilot operated safety valve under operating conditions commences to open
Note 1 to entry: It is the gauge pressure measured at the main valve inlet at which the pressure forces tending to lift the main valve disc for the specific service conditions are in equilibrium with the forces retaining the main valve disc on its seat.
3.8
maximum allowable pressure
PS
maximum pressure for which the protected equipment is designed
3.9
opening sensing pressure
pressure at which the pilot commences to open in order to achieve the set pressure
3.10
overpressure
pressure increase over the set pressure, usually expressed as a percentage of the set pressure
3.11
reseating pressure
value of the inlet static pressure at which the main valve disc re-establishes contact with the seat or at which the lift becomes zero
3.12
cold differential test pressure
inlet static pressure at which a pilot operated safety valve is set to commence to open on the test bench
Note 1 to entry: This test pressure includes corrections for service conditions, e.g. back pressure and/or temperature.
ISO 4126-4 pdf download.