Yoshitake AF-7, Instruction Manual, Page: 5 / 8

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Operation
Discharge
When the pressure at the inlet side of the safety valve rises and approaches the set pressure, the
fluid pressure which pushes the valve
⑦
approaches the pressure of the spring
⑮
which is
holding down the valve
⑦
. Pre-leaking which is as low as 3% of the discharge pressure occurs at
this point. (The pressure at this point is called the commence to discharge pressure. In case of
safety valve for gases, the set pressure is generally the commence to discharge pressure.)
The fluid which has started to pre-leak gradually accumulates in the valve holder
⑩
, and when it
reaches the prescribed discharge pressure, the valve pops. (The pressure at this point is called the
popping pressure. In case of safety valves for steam, the set pressure is generally the popping
pressure.)
Closing
When the valve
⑦
pops, and the fluid is discharged into the air, the pressure on the safety valve
inlet decreases, lowering the pressure of the fluid to raise the valve. Thus, the resilience of the
spring
⑮
becomes stronger, and the valve closes. When the safety valve is discharging, the
pressure from the fluid coming in from the back of the valve holder
⑩
(back pressure) adds to the
pressure for the valve to close.
6. Size selection tables
6.1 For steam (Saturation temperature)
[Pressure vessel structure specifications
・
Boiler structure specifications ] (kg/h)
Set pressure (MPa)
size
(mm)
Bore of
throat
(mm)
Blowout
area
(mm 2 )
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
25 16 200.9
180 250 335 421 504 586 668 748 829 911
40 25 490.6 440 612 818 1028 1230 1432 1631 1828 2026 2225
50 32 803.8 721 1003 1341 1685 2016 2347 2673 2996 3320 3645
65 40 1256.0 1127 1567 2095 2633 3151 3667 4177 4682 5188 5697
80 50 1962.5
1761 2446 3274 4115 4923 5730 6526 7315 8106 8901
100 65 3316.6
2976 4139 5533 6954 8320 9684 11029 12363 13700 15043
6.2 For air (20
℃
) [Pressure vessel structure specifications] (kg/h)
Set pressure (MPa)
size
(mm)
Bore of
throat
(mm)
Blowout
area
(mm 2 )
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
25 16 200.9 278 424 570 716 862 1008 1154 1300 1446 1592
40 25 490.6 680 1036 1393 1749 2106 2462 2819 3175 3532 3888
50 32 803.8 1114 1698 2282 2866 3451 4035 4619 5203 5787 6371
65 40 1256.0 1742 2654 3567 4479 5392 6305 7217 8130 9042 9955
80 50 1962.5 2722 4148 5573 6999 8425 9851 11277 12703 14129 15555
100 65 3316.6 4600 7010 9419 11829 14239 16649 19058 21468 23878 26288