ISO 4126-5:2013 過度の圧力から保護するための安全装置—パート5：制御された安全圧力解放システム（CSPRS） | ページ 5

3.1

controlled safety pressure relief system

CSPRS

system consisting of a main valve in combination with a control unit

Note 1 to entry: See Figure 1 for the components of a CSPRS.

Note 2 to entry: On reaching the set pressure, the operating forces on the main valve are by means of the control unit automatically applied, released or so reduced that a main valve discharges a specified quantity of the fluid so as to prevent the predetermined pressure being exceeded. The system is so designed that the main valve re-closes and prevents a further flow of fluid after normal pressure conditions of service have been restored.

Note 3 to entry: Specific types of CSPRS are installed to protect the downstream system by preventing further fluid input (safety shut-off valve). In this case the closing function shall meet the same requirements as the opening function of the relief valve (see 5.1.5).

3.2

main valve

valve consisting of the parts of a CSPRS through which the discharge capacity is achieved, and the actuator

3.3

relieving principle

principle in which a main valve opens when the operating force is released or reduced, and in which the main valve closes when the operating force is re-applied

Note 1 to entry: See Figure 2, Type 1.

3.4

loading principle

principle in which a main valve opens when the operating force is applied, and in which the main valve closes when the operating force is removed

Note 1 to entry: See Figure 2, Type 2.

3.5

control unit

unit which establishes the opening and closing of the main valve

Note 1 to entry: The arrangement shall consist of redundant individual control paths in operation (see 5.1.13 and 5.1.15). The individual control path may consist of pressure tapping line, pressure sensor, sensing line, control module and control line [see Figures 1 a), 1 b) and 1 c), principle for two control paths].

3.6

pressure tapping line

line to the pressure sensor

3.7

sensing line

line between the pressure sensor and control module

3.8

control line

line between the control module and the main valve

3.9

pressure sensor

comparator in which a predetermined adjustable value of pressure is compared with the actual system pressure

Note 1 to entry: On reaching the predetermined pressure, a signal is transmitted to the control unit. The signal to the control unit is removed when the system pressure has been lowered to a predetermined pressure.

3.10

control module

module which transforms the signal from the pressure sensor into a force to operate the actuator of the main valve

3.11

closed circuit principle

principle characterized by the fact that on failure of the external control energy, the control unit effects the loading or relief of the main valve

3.12

open circuit principle

principle characterized by the fact that on failure of the external control energy the control unit does not change the loading or relief of the main valve

3.13

operating force

force which causes the main valve to operate

3.14

set pressure of a CSPRS

predetermined pressure at which a main valve disc 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 open the main valve for specified service conditions are in equilibrium with the forces retaining the main valve disc on its seat.

3.15

maximum allowable pressure

ps

maximum pressure for which the protected equipment is designed

3.16

overpressure

pressure increase over the set pressure, usually expressed as a percentage of the set pressure

3.17

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

3.18

cold differential test pressure

inlet static pressure at which the main 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.

3.19

relieving pressure

pressure used for the sizing of a CSPRS which is greater than or equal to the set pressure plus overpressure

3.20

blowdown

difference between set and reseating pressures, normally stated as a percentage of set pressure except for pressures of less than 3 bar when the blowdown is expressed in bar

3.21

opening sensing pressure

3.22

closing sensing pressure

predetermined pressure which deactivates the pressure sensor

3.23

back pressure

pressure that exists at the outlet of a safety valve as a result of the pressure in the discharge system

Note 1 to entry: The back pressure is the sum of the superimposed and built-up back pressures.

3.24

built-up back pressure

pressure existing at the outlet of the main valve caused by flow through the main valve and the discharge system

3.25

superimposed back pressure

pressure existing at the outlet of the main valve at the time when the device is required to operate

Note 1 to entry: It is the result of pressure in the discharge system from other sources.

3.26

balanced bellows

bellows device which minimizes the effect of superimposed back pressure on the set pressure of a safety valve

3.27

lift

actual travel of the main valve disc away from the closed position

3.28

flow area

minimum cross-sectional flow area (but not the smallest area between seat and disc) between inlet and seat which is used to calculate the theoretical flow capacity, with no deduction for any obstruction

3.29

flow diameter

diameter corresponding to the flow area

3.30

theoretical discharge capacity

calculated capacity of a theoretically perfect nozzle having a cross-sectional flow area equal to the flow area of the main valve

Note 1 to entry: Expressed in mass or volumetric units.

3.31

coefficient of discharge

value of actual flowing capacity (from tests) divided by the theoretical flowing capacity (from calculation)

3.32

certified (discharge) capacity

that portion of the measured capacity permitted to be used as a basis for the application of the CSPRS

Note 1 to entry: It may, for example, equal: a) the measured capacity times the de-rating factor; or b) the theoretical capacity times the coefficient of discharge times the de-rating factor; or c) the theoretical capacity times the certified de-rated coefficient of discharge.

3.33

DN (nominal size)

alphanumeric designation of size that is common for components used in a piping system, used for reference purposes, comprising the letters DN followed by a dimensionless number having an indirect correspondence to the physical size of the bore or outside diameter of the component end connection

Note 1 to entry: The dimensionless number does not represent a measurable value and is not used for calculation purposes.

Note 2 to entry: Prefix DN usage is applicable to components bearing PN designations according to ISO 7268.

Note 3 to entry: Adapted from ISO 6708:1995, definition 2.1.

3.34

opening time

time interval for the main valve disc to move from the closed to the fully open position

3.35

reseating time

time interval for the main valve disc to move from the fully open to the closed position

3.36

opening dead time

time interval from the detection of the opening sensing pressure and the commencement of the opening of the main valve

3.37

reseating dead time

time interval from the detection of the closing sensing pressure and the commencement of the closing of the main valve