[Code of Federal Regulations]
[Title 49, Volume 2]
[Revised as of October 1, 2007]
From the U.S. Government Printing Office via GPO Access
[CITE: 49CFR179.15]
[Page 1066-1068]
TITLE 49--TRANSPORTATION
CHAPTER I--PIPELINE AND HAZARDOUS MATERIALS SAFETY ADMINISTRATION,
DEPARTMENT OF TRANSPORTATION
PART 179_SPECIFICATIONS FOR TANK CARS--Table of Contents
Subpart B_General Design Requirements
Sec. 179.15 Pressure relief devices.
Except for DOT Class 106, 107, 110, and 113 tank cars, tanks must
have a pressure relief device, made of material compatible with the
lading, that conforms to the following requirements:
(a) Performance standard. Each tank must have a pressure relief
device, made of materials compatible with the lading, having sufficient
flow capacity to prevent pressure build-up in the tank to no more than
the flow rating pressure of the pressure relief device in fire
conditions as defined in appendix A of the AAR Specifications for Tank
Cars (IBR, see Sec. 171.7 of this subchapter).
(b) Settings for reclosing pressure relief devices. (1) Except as
provided in paragraph (b)(2) of this section, a reclosing pressure
relief valve must have a minimum start-to-discharge pressure equal to
the sum of the static head and gas padding pressure and the lading vapor
pressure at the following reference temperatures:
(i) 46 [deg]C (115 [deg]F) for noninsulated tanks;
(ii) 43 [deg]C (110 [deg]F) for tanks having a thermal protection
system incorporating a metal jacket that provides an overall thermal
conductance at 15.5 [deg]C (60 [deg]F) of no more than 10.22 kilojoules
per hour per square meter per degree Celsius (0.5 Btu per hour/per
square foot/per degree F) temperature differential; and
(iii) 41 [deg]C (105 [deg]F) for insulated tanks.
(2)(i) The start-to-discharge pressure of a pressure relief device
may not be lower than 5.17 Bar (75 psig) or exceed 33 percent of the
minimum tank burst pressure.
(ii) Tanks built prior to October 1, 1997 having a minimum tank
burst pressure of 34.47 Bar (500 psig) or less
[[Page 1067]]
may be equipped with a reclosing pressure relief valve having a start-
to-discharge pressure of not less than 14.5 percent of the minimum tank
burst pressure but no more than 33 percent of the minimum tank burst
pressure.
(3) The vapor tight pressure of a reclosing pressure relief valve
must be at least 80 percent of the start-to-discharge pressure.
(4) The flow rating pressure must be 110 percent of the start-to-
discharge pressure for tanks having a minimum tank burst pressure
greater than 34.47 Bar (500 psig) and from 110 percent to 130 percent
for tanks having a minimum tank burst pressure less than or equal to
34.47 Bar (500 psig).
(5) The tolerance for a reclosing pressure relief valve is 3 psi for valves with a start-to-discharge pressure of
6.89 Bar (100 psig) or less and 3 percent for
valves with a start-to-discharge pressure greater than 6.89 Bar (100
psig).
(c) Flow capacity of pressure relief devices. The total flow
capacity of each reclosing and nonreclosing pressure relief device must
conform to appendix A of the AAR Specifications for Tank Cars.
(d) Flow capacity tests. The manufacturer of any reclosing or
nonreclosing pressure relief device must design and test the device in
accordance with appendix A of the AAR Specifications for Tank Cars.
(e) Combination pressure relief systems. A non-reclosing pressure
relief device may be used in series with a reclosing pressure relief
valve. The pressure relief valve must be located outboard of the non-
reclosing pressure relief device.
(1) When a breaking pin device is used in combination with a
reclosing pressure relief valve, the breaking pin must be designed to
fail at the start-to-discharge pressure specified in paragraph (b) of
this section, and the reclosing pressure relief valve must be designed
to discharge at not greater than 95 percent of the start-to-discharge
pressure.
(2) When a rupture disc is used in combination with a reclosing
pressure relief valve, the rupture disc must be designed to burst at the
pressure specified in paragraph (b) of this section, and the reclosing
pressure relief valve must be designed to discharge at not greater than
95 percent of the pressure. A device must be installed to detect any
accumulation of pressure between the rupture disc and the reclosing
pressure relief valve. The detection device must be a needle valve,
trycock, or tell-tale indicator. The detection device must be closed
during transportation.
(3) The vapor tight pressure and the start-to-discharge tolerance is
based on the discharge setting of the reclosing pressure relief device.
(f) Nonreclosing pressure relief device. In addition to paragraphs
(a), (b)(4), (c), and (d) of this section, a nonreclosing pressure
relief device must conform to the following requirements:
(1) A non-reclosing pressure relief device must incorporate a
rupture disc designed to burst at a pressure equal to the greater of
100% of the tank test pressure, or 33% of the tank burst pressure.
(2) The approach channel and the discharge channel may not reduce
the required minimum flow capacity of the pressure relief device.
(3) The non-reclosing pressure relief device must be designed to
prevent interchange with other fittings installed on the tank car, must
have a structure that encloses and clamps the rupture disc in position
(preventing any distortion or damage to the rupture disc when properly
applied), and must have a cover, with suitable means of preventing
misplacement, designed to direct any discharge of the lading downward.
(4) The non-reclosing pressure relief device must be closed with a
rupture disc that is compatible with the lading and manufactured in
accordance with Appendix A of the AAR Specifications for Tank Cars. The
tolerance for a rupture disc is +0 to -15 percent of the burst pressure
marked on the disc.
(g) Location of relief devices. Each pressure relief device must
communicate with the vapor space above the lading as near as practicable
on the longitudinal center line and center of the tank.
(h) Marking of pressure relief devices. Each pressure relief device
and rupture disc must be permanently marked in
[[Page 1068]]
accordance with the appendix A of the AAR Specifications for Tank Cars.
[Amdt. 179-52, 61 FR 28678, June 5, 1996, as amended by Amdt. 179-52, 61
FR 50255, Sept. 25, 1996; 62 FR 51561, Oct. 1, 1997; 64 FR 51919, Sept.
27, 1999; 66 FR 45390, Aug. 28, 2001; 68 FR 75759, Dec. 31, 2003]