Technical Committee on
LP-Gas at Utility Gas Plants
Addendum to the Agenda
Sheraton Denver Downtown
1550 Court Place
Denver, CO 80202
August 7-8, 2013
The following items relate to item 5.B of the Agenda:
Attachment A: Suggested change to pressure relief valve sizing
Attachment B: Suggested change in legislative text
Attachment C: Recommended substantiation for revision
Attachment A:
Suggested Change to Pressure
Relief Valve Sizing
10.2.3 For ASME containers holding propane, with a maximum pressure of 300 psi or less, the minimum rate of discharge of
installed pressure relief valves shall be in accordance with Table 10.2.3 or shall be calculated using the following formula:
F 53.632A0.82
where:
F = flow rate (SCFM air)
A = total outside surface area of container (ft2)
10.2.4 For ASME containers not covered by 10.2.3, the minimum rate of discharge of installed pressure relief valves shall be
sufficient to relieve vapor produced by vaporization that occurs when a heat input of Q=34,500A0.82 is applied to the vessel,
where:
Q = heat input (Btu/hr)
A = total outside surface area of container (ft2)
Table 10.2.3 Pressure Relief Valve Flow Capacity as a Function of Container Surface Area
Surface Area (ft2) Flow Rate (SCFM)
Surface Area (ft2) Flow Rate (SCFM)
Surface Area (ft2)
626
170
3620
600
20
25
751
175
3700
650
30
872
180
3790
700
35
990
185
3880
750
40
1100
190
3960
800
45
1220
195
4050
850
50
1330
200
4130
900
55
1430
210
4300
950
60
1540
4470
1000
65
1640
230
4630
1050
70
1750
240
4800
1100
75
1850
250
4960
1150
80
1950
260
5130
1200
85
2050
270
5290
1250
90
2150
280
5450
1300
95
2240
290
5610
1350
100
2340
300
5760
1400
105
2440
310
5920
1450
110
2530
320
6080
1500
115
2630
330
6230
1550
120
2720
340
6390
1600
125
2810
350
6540
1650
130
2900
360
6690
1700
135
2990
370
6840
1750
140
3080
380
7000
1800
145
3170
390
7150
1850
150
3260
400
7300
1900
155
3350
450
8040
1950
160
3440
500
8760
2000
165
3530
550
9470
—
This table is applicable to containers holding propane with a maximum pressure of 300 psi or less.
For SI units, 1 SCFM = 0.0283 m3/min.
Note: Flow rate in SCFM air.
Flow Rate (SCFM)
10,170
10,860
11,550
12,220
12,880
13,540
14,190
14,830
15,470
16,100
16,720
17,350
17,960
18,570
19,180
19,780
20,380
20,980
21,570
22,160
22,740
23,320
23,900
24,470
25,050
25,620
26,180
26,750
27,310
—
Annex D Relief Device Sizing
This annex is not a part of the requirements of this NFPAdocument but is included for informational purposes only.
D.1 Nonrefrigerated Containers.
D.1.1 Containers Holding Propane, with a Maximum Pressure of 300 psi or Less.Table D.1.1 should be used to size pressure
relief valves.
Surface area equals the total outside surface area of the container in square feet. Where the surface area is not stamped on
the nameplate or where the marking is not legible, the area can be calculated by using one of the following formulas:
(1) Cylindrical container with hemispherical heads:
Area = Overall length × outside diameter × 3.1416
(2) Cylindrical container with other than hemispherical heads:
Area = (Overall length + 0.3 outside diameter) × outside diameter × 3.1416
(3) Spherical container:
Area = Outside diameter squared × 3.1416
Flow rate of air (ft3/min) = required flow capacity in cubic feet per minute of air at standard conditions, 60°F (15.6°C), and
atmospheric pressure [14.7 psia (101 kPa)].
The rate of discharge can be interpolated for intermediate values of surface area. For containers with a total outside surface
area that is greater than 2000 ft2 (186 m2), the required flow rate can be calculated using the following formula:
Flow rate of air ft3 /min53.632A0.82
where A = total outside surface area of the container (ft2) (0.4719 L/s)
D.1.2 All Other Nonrefrigerated Containers. Pressure relief valves should be sized by calculating the heat input to the
container due to fire exposure, calculating the resulting rate of vaporization, and converting the rate of vaporization to a flow rate
of air.
Heat input can be calculated using the following formula:
Q = 34,500A0.82
where:
Q = heat input (Btu/hr)
A = total outside surface area of the container (ft^2)
The rate of vaporization can be calculated using the following formula:
Wvap = Q / L
where:
Wvap = rate of vaporization (lb/hr)
Q = heat input (Btu/hr)
L = latent heat of vaporization of the fluid (Btu/lb)
The minimum rate of discharge of the pressure relief valves can be calculated using the following formula:
√
(
)
√
√
where:
F = relieving capacity in SCFM of air
and:
ka = 1.4, ratio of specific heats of air
Ma = 28.97 lb/lbmol, molecular weight of air
Ta = 520°R, absolute temperature of air at standard conditions
and the following quantities are evaluated at relieving conditions:
Wvap = relieving capacity in lb/hr of product vapor
kv = ratio of specific heats of product vapor
Mv = molecular weight of product vapor
Tv = absolute temperature of product vapor
Zv = compressibility factor of product vapor
(
)
Table D.1.1 Minimum Required Rate of Discharge in Cubic Feet per Minute of Air at 120 Percent of the Maximum
Permitted Start-to-Discharge Pressure for Safety Relief Devices to Be Used on Nonrefrigerated Containers Other than
Those Constructed in Accordance with U.S. Department of Transportation Specifications
Surface Area (ft2) Flow Rate (SCFM)
Surface Area (ft2) Flow Rate (SCFM)
Surface Area (ft2) Flow Rate (SCFM)
626
170
3620
600
10,170
20
25
751
175
3700
650
10,860
30
872
180
3790
700
11,550
35
990
185
3880
750
12,220
40
1100
190
3960
800
12,880
45
1220
195
4050
850
13,540
50
1330
200
4130
900
14,190
55
1430
210
4300
950
14,830
60
1540
4470
1000
15,470
65
1640
230
4630
1050
16,100
70
1750
240
4800
1100
16,720
75
1850
250
4960
1150
17,350
80
1950
260
5130
1200
17,960
85
2050
270
5290
1250
18,570
90
2150
280
5450
1300
19,180
95
2240
290
5610
1350
19,780
100
2340
300
5760
1400
20,380
105
2440
310
5920
1450
20,980
110
2530
320
6080
1500
21,570
115
2630
330
6230
1550
22,160
120
2720
340
6390
1600
22,740
125
2810
350
6540
1650
23,320
130
2900
360
6690
1700
23,900
135
2990
370
6840
1750
24,470
140
3080
380
7000
1800
25,050
145
3170
390
7150
1850
25,620
150
3260
400
7300
1900
26,180
155
3350
450
8040
1950
26,750
160
3440
500
8760
2000
27,310
165
3530
550
9470
—
—
This table is applicable to containers holding propane with a maximum pressure of 300 psi or less.
Note: ft2 = 0.09 m2; ft3/min = 0.4719 L/s.
Attachment B:
Suggested Change in Legislative Text
10.2.3 For ASME containers holding propane, with a maximum pressure of 300 psi or less, The the minimum rate of discharge of
installed pressure relief valves installed in ASME containers shall be in accordance with Table 10.2.3 or shall be calculated using
the following formula:
F 53.632A0.82
where:
F = flow rate (SCFM air)
A = total outside surface area of container (ft2)
10.2.4 For ASME containers not covered by 10.2.3, the minimum rate of discharge of installed pressure relief valves shall be
sufficient to relieve vapor produced by vaporization that occurs when a heat input of Q=34,500A0.82 is applied to the vessel,
where:
Q = heat input (Btu/hr)
A = total outside surface area of container (ft2)
Formatted: Font: Not Bold
Formatted: Font: Not Bold
Formatted: Superscript
Formatted: Font: Italic
Formatted: Font: Italic
Table 10.2.3 Pressure Relief Valve Flow Capacity as a Function of Container Surface Area
Surface Area (ft2) Flow Rate (SCFM)
Surface Area (ft2) Flow Rate (SCFM)
Surface Area (ft2)
626
170
3620
600
20
25
751
175
3700
650
30
872
180
3790
700
35
990
185
3880
750
40
1100
190
3960
800
45
1220
195
4050
850
50
1330
200
4130
900
55
1430
210
4300
950
60
1540
4470
1000
65
1640
230
4630
1050
70
1750
240
4800
1100
75
1850
250
4960
1150
80
1950
260
5130
1200
85
2050
270
5290
1250
90
2150
280
5450
1300
95
2240
290
5610
1350
100
2340
300
5760
1400
105
2440
310
5920
1450
110
2530
320
6080
1500
115
2630
330
6230
1550
120
2720
340
6390
1600
125
2810
350
6540
1650
130
2900
360
6690
1700
135
2990
370
6840
1750
140
3080
380
7000
1800
145
3170
390
7150
1850
150
3260
400
7300
1900
155
3350
450
8040
1950
160
3440
500
8760
2000
165
3530
550
9470
—
This table is applicable to containers holding propane with a maximum pressure of 300 psi or less.
For SI units, 1 SCFM = 0.0283 m3/min.
Note: Flow rate in SCFM air.
Flow Rate (SCFM)
10,170
10,860
11,550
12,220
12,880
13,540
14,190
14,830
15,470
16,100
16,720
17,350
17,960
18,570
19,180
19,780
20,380
20,980
21,570
22,160
22,740
23,320
23,900
24,470
25,050
25,620
26,180
26,750
27,310
—
Annex D Relief Device Sizing
This annex is not a part of the requirements of this NFPAdocument but is included for informational purposes only.
D.1 Nonrefrigerated Containers.
D.1.1 Containers Holding Propane, with a Maximum Pressure of 300 psi or Less. Table D.1.1 should be used to size
pressure relief valves.
Surface area equals the total outside surface area of the container in square feet. Where the surface area is not stamped on
the nameplate or where the marking is not legible, the area can be calculated by using one of the following formulas:
(1) Cylindrical container with hemispherical heads:
Area = Overall length × outside diameter × 3.1416
(2) Cylindrical container with other than hemispherical heads:
Area = (Overall length + 0.3 outside diameter) × outside diameter × 3.1416
(3) Spherical container:
Area = Outside diameter squared × 3.1416
Flow rate of air (ft3/min) = required flow capacity in cubic feet per minute of air at standard conditions, 60°F (15.6°C), and
atmospheric pressure [14.7 psia (101 kPa)].
The rate of discharge can be interpolated for intermediate values of surface area. For containers with a total outside surface
area that is greater than 2000 ft2 (186 m2), the required flow rate can be calculated using the following formula:
Flow rate of air ft3 /min53.632A0.82
where A = total outside surface area of the container (ft2) (0.4719 L/s)
D.1.2 All Other Nonrefrigerated Containers. Pressure relief valves should be sized by calculating the heat input to the
container due to fire exposure, calculating the resulting rate of vaporization, and converting the rate of vaporization to a flow rate
of air.
Heat input can be calculated using the following formula:
Q = 34,500A0.82
where:
Q = heat input (Btu/hr)
A = total outside surface area of the container (ft^2)
Formatted: Indent: First line: 0"
Formatted: Centered, Indent: First line: 0"
Formatted: Superscript
Formatted: Indent: First line: 0"
The rate of vaporization can be calculated using the following formula:
Wvap = Q / L
where:
Wvap = rate of vaporization (lb/hr)
Q = heat input (Btu/hr)
L = latent heat of vaporization of the fluid (Btu/lb)
Formatted: Centered, Indent: First line: 0"
Formatted: Indent: First line: 0"
The minimum rate of discharge of the pressure relief valves can be calculated using the following formula:
√
Formatted: Centered, Indent: First line: 0"
(
)
√
√
where:
F = relieving capacity in SCFM of air
(
)
Formatted: Indent: First line: 0"
and:
ka = 1.4, ratio of specific heats of air
Ma = 28.97 lb/lbmol, molecular weight of air
Ta = 520°R, absolute temperature of air at standard conditions
Formatted: Indent: First line: 0"
and the following quantities are evaluated at relieving conditions:
Wvap = relieving capacity in lb/hr of product vapor
kv = ratio of specific heats of product vapor
Mv = molecular weight of product vapor
Tv = absolute temperature of product vapor
Zv = compressibility factor of product vapor
Formatted: Indent: First line: 0"
Table D.1.1 Minimum Required Rate of Discharge in Cubic Feet per Minute of Air at 120 Percent of the Maximum
Permitted Start-to-Discharge Pressure for Safety Relief Devices to Be Used on Nonrefrigerated Containers Other than
Those Constructed in Accordance with U.S. Department of Transportation Specifications
Surface Area (ft2) Flow Rate (SCFM)
Surface Area (ft2) Flow Rate (SCFM)
Surface Area (ft2) Flow Rate (SCFM)
626
170
3620
600
10,170
20
25
751
175
3700
650
10,860
30
872
180
3790
700
11,550
35
990
185
3880
750
12,220
40
1100
190
3960
800
12,880
45
1220
195
4050
850
13,540
50
1330
200
4130
900
14,190
55
1430
210
4300
950
14,830
60
1540
4470
1000
15,470
65
1640
230
4630
1050
16,100
70
1750
240
4800
1100
16,720
75
1850
250
4960
1150
17,350
80
1950
260
5130
1200
17,960
85
2050
270
5290
1250
18,570
90
2150
280
5450
1300
19,180
95
2240
290
5610
1350
19,780
100
2340
300
5760
1400
20,380
105
2440
310
5920
1450
20,980
110
2530
320
6080
1500
21,570
115
2630
330
6230
1550
22,160
120
2720
340
6390
1600
22,740
125
2810
350
6540
1650
23,320
130
2900
360
6690
1700
23,900
135
2990
370
6840
1750
24,470
140
3080
380
7000
1800
25,050
145
3170
390
7150
1850
25,620
150
3260
400
7300
1900
26,180
155
3350
450
8040
1950
26,750
160
3440
500
8760
2000
27,310
165
3530
550
9470
—
—
This table is applicable to containers holding propane with a maximum pressure of 300 psi or less.
Note: ft2 = 0.09 m2; ft3/min = 0.4719 L/s.
Attachment C:
Recommended Substantiation
for Revision
Substantiation:
This proposed change to NFPA 59 aims to correct a deficiency in the sizing method for nonrefrigerated
container relief devices. Jim Stannard raised the concern in Committee Inputs 57, 58, and 59 that the
current relief valve sizing method, which was added in the 1953 edition, uses a fixed value for the latent
heat of vaporization of the product. The committee statement associated with CIs 57, 58, and 59 says,
“The 1953 caclulation, which is still in effect in the most recent editions of both NFPA 58 … and
NFPA 59 … is based upon a total heat input Q: Q=34,500A0.82 Btu/hr. The heat flux value of
34,500 Btu/ft2-hr is also adopted in several NFPA codes and standards with some modification
for larger vessels. The calculations that led to the adoption of ‘CFM air=53.632A0.82’ in the 1953
edition were apparently based upon a single value for the ‘latent heat of vaporization’ (∆h)
regardless of either pressure or composition. (i.e. butane or propane)”
The latent heat of vaporization used in the equation from the 1953 edition was L=110 Btu/lb, which is
the latent heat of vaporization for saturated propane at 300 psig. Using this same value for lower
relieving pressures is conservative, because the latent heat of vaporization increases with decreasing
pressure, which in turn lowers the relieving flow rate needed. However, if the relief valve has a relieving
pressure greater than 300 psig, using the same value for L would result in undersized relief valves.
Similarly, the latent heat of vaporization of butane is always less than that of propane at the same
conditions, so using the same value of L=110 Btu/lb would result in undersized relief valves.
The proposed change to section 10.2.3 addresses this issue by limiting the applicability of the equation
F=53.632∙A0.82 to propane only, with a maximum pressure of 300 psig. Most applications will fall under
this section, and the same equation and Table 10.2.3 may continue to be used. In other cases, the new
section 10.2.4 must be used. This section requires that the rate of discharge of the relief valve be
enough to handle the same heat input of Q=34,500A0.82 Btu/hr, but doesn’t proscribe a formula or
specific values for the amount of vapor produced or the equivalent air flow rate. The intention is that
the system designer will have to perform their own calculations to justify that the relief valve can handle
the same heat input on the tank.
A suggested, nonbinding method to calculate relief valve size is added to the annex in section D.1.2. This
method follows the same method used in the derivation of the 1953 equation, but stops short of
substituting in the material properties of the product.
Derivation of D.1.2:
ASME BPVC Section VIII Division 1 Appendix 11, “Capacity Conversions for Safety Valves,” provides the
following equation for relief valve capacity.
√
Here, W is the capacity in pounds per hour, K is the coefficient of discharge, A is the discharge area of
the safety valve, in square inches, P is the absolute relieving pressure, M is the molecular weight of the
gas or vapor, Z is the compressibility factor, and T is the absolute temperature at the safety valve inlet. C
is in turn defined by the following equation in terms of k, the ratio of specific heats.
√ (
)
We apply this in turn to the product vapor, designated with the subscript v, and test air, designated with
the subscript a. We will assume that the compressibility of air is 1, so it will drop out. Note that K and A
are properties of the safety valve, so they will not change. The inlet pressure will be the same when
relieving product as when testing the valve on air, so P will be constant as well.
√
√
We can also substitute in the formula for C.
√
(
√
(
)
√
)
√
Dividing these equations, and cancelling like terms, we get the following.
√
(
)
√
√
(
)
Finally, we relate the mass flow rate of air to the standard volume flow rate of air using its standard
density. Here, F is in SCFM and Wa is in lb/hr.
Substituting and rearranging yields the final equation.
√
(
)
√
√
(
)