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5．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 ]
Bore of
Blowout
size
throat
(mm)
(mm)
(mm
25
16 200.9
40
25 490.6
50
32 803.8
65
40 1256.0
80
50 1962.5
100
65 3316.6
6.2 For air (20℃)
Bore of
Blowout
size
throat
area
(mm)
(mm)
(mm
25
16
200.9
40
25
490.6
50
32
803.8
65
40 1256.0
80
50 1962.5
100
65 3316.6
area
0.1
0.2
2
)
180
250
440
612
721
1003
1127
1567
1761
2446
2976
4139
[Pressure vessel structure specifications]
0.1
0.2
2
)
278
424
680
1036
1114
1698
1742
2654
2722
4148
4600
7010
Set pressure (MPa)
0.3
0.4
0.5
335
421
504
818
1028
1230
1341
1685
2016
2095
2633
3151
3274
4115
4923
5533
6954
8320
Set pressure (MPa)
0.3
0.4
0.5
570
716
862
1393
1749
2106
2282
2866
3451
3567
4479
5392
5573
6999
8425
9419 11829 14239 16649 19058 21468 23878 26288
- 4 -
0.6
0.7
0.8
586
668
748
1432
1631
1828
2347
2673
2996
3667
4177
4682
5730
6526
7315
9684 11029 12363 13700 15043
0.6
0.7
0.8
1008
1154
1300
2462
2819
3175
4035
4619
5203
6305
7217
8130
9851 11277 12703 14129 15555
■EＰＤＴ－１６８a■
(kg/h)
0.9
1.0
829
911
2026
2225
3320
3645
5188
5697
8106
8901
(kg/h)
0.9
1.0
1446
1592
3532
3888
5787
6371
9042
9955
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