Technical Committee on LP-Gas at Utility Gas Plants

Transcription

1 Technical Committee on LP-Gas at Utility Gas Plants Addendum to the Agenda Sheraton Denver Downtown 1550 Court Place Denver, CO 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

2 Attachment A: Suggested Change to Pressure Relief Valve Sizing

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 or shall be calculated using the following formula: F A 0.82 F = flow rate (SCFM air) A = total outside surface area of container (ft 2 ) For ASME containers not covered by , 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=34,500A 0.82 is applied to the vessel, Q = heat input (Btu/hr) A = total outside surface area of container (ft 2 )

4 Table Pressure Relief Valve Flow Capacity as a Function of Container Surface Area Surface Area (ft 2 ) Flow Rate (SCFM) Surface Area (ft 2 ) Flow Rate (SCFM) Surface Area (ft 2 ) Flow Rate (SCFM) , , , , , , , , , , , , , , , , , , , , , , , , , , , , , This table is applicable to containers holding propane with a maximum pressure of 300 psi or less. For SI units, 1 SCFM = m3/min. Note: Flow rate in SCFM air.

5 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 (2) Cylindrical container with other than hemispherical heads: Area = (Overall length outside diameter) outside diameter (3) Spherical container: Area = Outside diameter squared 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 A 0.82 where A = total outside surface area of the container (ft2) ( 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 = 34,500A 0.82 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: W vap = Q / L W vap = 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: F = relieving capacity in SCFM of air and: k a = 1.4, ratio of specific heats of air M a = lb/lbmol, molecular weight of air T a = 520 R, absolute temperature of air at standard conditions and the following quantities are evaluated at relieving conditions: W vap = relieving capacity in lb/hr of product vapor k v = ratio of specific heats of product vapor M v = molecular weight of product vapor T v = absolute temperature of product vapor Z v = compressibility factor of product vapor

6 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 (ft 2 ) Flow Rate (SCFM) Surface Area (ft 2 ) Flow Rate (SCFM) Surface Area (ft 2 ) Flow Rate (SCFM) , , , , , , , , , , , , , , , , , , , , , , , , , , , , , This table is applicable to containers holding propane with a maximum pressure of 300 psi or less. Note: ft2 = 0.09 m2; ft3/min = L/s.

7 Attachment B: Suggested Change in Legislative Text

8 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 or shall be calculated using the following formula: F A 0.82 F = flow rate (SCFM air) A = total outside surface area of container (ft 2 ) For ASME containers not covered by , 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=34,500A 0.82 is applied to the vessel, Q = heat input (Btu/hr) A = total outside surface area of container (ft 2 ) Formatted: Font: Not Bold Formatted: Font: Not Bold Formatted: Superscript Formatted: Font: Italic Formatted: Font: Italic

9 Table Pressure Relief Valve Flow Capacity as a Function of Container Surface Area Surface Area (ft 2 ) Flow Rate (SCFM) Surface Area (ft 2 ) Flow Rate (SCFM) Surface Area (ft 2 ) Flow Rate (SCFM) , , , , , , , , , , , , , , , , , , , , , , , , , , , , , This table is applicable to containers holding propane with a maximum pressure of 300 psi or less. For SI units, 1 SCFM = m3/min. Note: Flow rate in SCFM air.

10 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 (2) Cylindrical container with other than hemispherical heads: Area = (Overall length outside diameter) outside diameter (3) Spherical container: Area = Outside diameter squared 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 A 0.82 where A = total outside surface area of the container (ft2) ( 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 = 34,500A 0.82 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: W vap = Q / L W vap = 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: Formatted: Indent: First line: 0" Formatted: Centered, Indent: First line: 0" Formatted: Superscript Formatted: Indent: First line: 0" Formatted: Centered, Indent: First line: 0" Formatted: Indent: First line: 0" Formatted: Centered, Indent: First line: 0" F = relieving capacity in SCFM of air and: k a = 1.4, ratio of specific heats of air M a = lb/lbmol, molecular weight of air T a = 520 R, absolute temperature of air at standard conditions and the following quantities are evaluated at relieving conditions: W vap = relieving capacity in lb/hr of product vapor k v = ratio of specific heats of product vapor M v = molecular weight of product vapor T v = absolute temperature of product vapor Z v = compressibility factor of product vapor Formatted: Indent: First line: 0" Formatted: Indent: First line: 0" Formatted: Indent: First line: 0"

11 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 (ft 2 ) Flow Rate (SCFM) Surface Area (ft 2 ) Flow Rate (SCFM) Surface Area (ft 2 ) Flow Rate (SCFM) , , , , , , , , , , , , , , , , , , , , , , , , , , , , , This table is applicable to containers holding propane with a maximum pressure of 300 psi or less. Note: ft2 = 0.09 m2; ft3/min = L/s.

12 Attachment C: Recommended Substantiation for Revision

13 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=34,500A 0.82 Btu/hr. The heat flux value of 34,500 Btu/ft 2 -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=53.632a 0.82 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 addresses this issue by limiting the applicability of the equation F= A 0.82 to propane only, with a maximum pressure of 300 psig. Most applications will fall under this section, and the same equation and Table may continue to be used. In other cases, the new section 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=34,500A 0.82 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.

14 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 W a is in lb/hr. Substituting and rearranging yields the final equation.

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