Pressure and vacuum relief devices.

(a) Relief devices required. Each portable tank, or each independent compartment of a portable tank, must be fitted with pressure relief devices in accordance with the following:

(1) Each portable tank, or each independent compartment of a portable tank, with a capacity of more than 1893 L (500 gallons), must be provided with a primary spring-loaded pressure relief device, and, in addition, may have one or more emergency pressure relief devices that may be a spring-loaded pressure relief valve, a rupture disc or fusible element in parallel with the primary pressure relief device.

(2) Each portable tank, or each independent compartment of a portable tank, with a capacity of 1893 L (500 gallons) or less, must be fitted with a primary pressure relief device that may either be a non-reclosing device or a spring-loaded pressure relief valve.

(3) If a non-reclosing device is inserted in series with a required pressure relief valve, the space between them must have a suitable tell-tale indicator to permit detection, prior to and during shipment, of disc rupture, pinholing, or leakage which could cause a malfunction of the pressure relief system. The frangible disc must rupture at a tank pressure within the range specified in paragraph (c)(1) of this section.

(b) Location and construction of relief devices. (1) Pressure relief devices must be spring-loaded valves, rupture discs, or fusible elements. Vacuum relief devices must be capable of reclosing in any attitude. Each pressure relief device inlet must be situated in the vapor space of the tank. The discharge from any device must be unrestricted and directed to prevent impingement upon the tank shell or structural framework. Protective devices which deflect the flow of vapor are permissible provided the required vent capacity is maintained. Pressure and vacuum relief devices including their inlets must be sited on the top of the tank in a position as near as possible to the longitudinal and transversal center of the tank within the following limitation:

(i) Longitudinally on the tank within 107 cm (31/2 feet) or 1/6 the tank length, whichever is less, from the top center of the tank; and

(ii) Transversally within 12 degrees of the tank top.

(2) Except for a relief device installed in a piping system, each relief device must provide unrestricted venting under all conditions. Each pressure relief system, including any piping, must provide a venting capacity at least equal to the venting capacity specified in § 178.270-11(d) for the tank on which the system is installed.

(3) Fusible elements, when installed, must not be protected from direct communication with external heat sources.

(4) Spring-loaded pressure relief valves must be constructed in a manner to prevent unauthorized adjustment of the relief setting.

(c) Pressure settings of relief devices—(1) Primary pressure relief devices. The primary relief device required by paragraph (a) of this section must be set to function in the range of—

(i) No less than 67 percent and no greater than 83 percent of test pressure for tanks hydrostatically tested under § 178.270-13(a) of this subpart at a gauge pressure below 455 kPa (66 psig). Spring-loaded pressure relief valves must close after discharge at a pressure not less than 80 percent of start-to-discharge pressure.

(ii) No less than 67 percent and no greater than 74 percent of test pressure for tanks hydrostatically tested under § 178.270-13(a) of this subpart at a gauge pressure of 455 kPa (66 psig) or higher. Spring-loaded pressure relief valves must close after discharge at a pressure not less than 90 percent of start-to-discharge pressure.

(2) Emergency pressure relief devices. Each rupture disc, other than one used as a primary relief device in accordance with paragraph (b)(2) of this section, must be designed to burst at a pressure greater than 83 percent of and less than or equal to tank hydrostatic test pressure. Each spring-loaded pressure relief device must be set to operate at no less than 83 percent of hydrostatic test pressure and be fully open at test pressure.

(3) Fusible elements. Fusible elements must have a nominal yield temperature greater than the highest tank operating temperature and less than or equal to 121 °C (250 °F). The pressure developed in the tank at the fusible element yield temperature must be below the test pressure of the tank.

(4) Vacuum relief devices. Vacuum relief devices, when used, must be designed to provide total containment of product under normal and accident conditions and must be set to open at a nominal external overpressure of not less than 0.21 bar (3 psig) but not greater than the external pressure for which the tank is designed. Each vacuum relief device must have a minimum cross sectional flow area of 2.84 cm2 (0.44 square inches).

(d) Venting capacity of pressure relief devices—(1) Pressure relief valves (spring-loaded). Each pressure relief valve must have a minimum vent capacity of at least 170 standard cubic meters per hour (SCMH) (6,000 standard cubic feet per hour (SCFH)). The minimum total pressure relief valve vent capacity for each tank shall be 340 SCMH (12,000 SCFH) per 32.5m2 (350 square feet) of exposed tank area, but in any case at least 340 SCMH (12,000 SCFH).

(2) Total tank vent capacity. The total vent capacity of all pressure relief devices installed on each portable tank must be sufficient with all devices operating to limit the pressure in the tank to less than or equal to the test pressure. Except as provided in paragraph (d)(3) or (d)(4) of this section, the total vent capacity must be at least equal to that shown in the following table:

Table I—Minimum Total Vent Capacity [Metric units table in cubic meters of air per hour at atmospheric pressure and 15 °C] Exposed area square meters Cubic meters free air per hour Exposed area square meters Cubic meters free air per hour 2 841 37.5 9.306 3 1,172 40 9,810 4 1,485 42.5 10,308 5 1,783 45 10,806 6 2,069 47.5 11,392 7 2,348 50 11,778 8 2,621 52.5 12,258 9 2,821 55 12,732 10 3,146 57.5 13,206 12 3,655 60 13,674 14 4,146 62.5 14,142 16 4,625 65 14,604 18 5,092 67.5 15,066 20 5,556 70 15,516 22.5 6,120 75 16,422 25 6,672 80 17,316 27.5 7,212 85 18,198 30 7,746 90 19,074 32.5 8,268 95 19,938 35 8,790 100 20,790

[Nonmetric units in cubic feet of air per hour at atmospheric pressure and 59 °F] Exposed area square feet Cubic feet free air per hour Exposed area square feet Cubic feet free air per hour 20. 27,600 275 237,000 30 38,500 300 256,000 40 48,600 350 289,500 50 58,600 400 322,100 60 67,700 450 355,900 70 77,000 500 391,000 80 85,500 550 417,500 90 94,800 600 450,000 100 104,000 650 479,000 120 121,000 700 512,000 140 136,200 750 540,000 160 152,100 800 569,000 180 168,200 850 597,000 200 184,000 900 621,000 225 199,000 950 656,000 250 219,500 1,000 686,000 Note: Interpolate for intermediate sizes.

(3) Notwithstanding the minimum total vent capacity shown in table I, of paragraph (d)(2), a tank in dedicated service may have a lesser total vent capacity provided the approval certificate required by § 178.273(b)(7) specifies the hazardous materials for which the tank is suitable. The lesser total vent capacity must be determined in accordance with the following formula:

Formula for metric units

Q = 5,660,000 A0.82 (ZT)0.5 / (LC)(M0.5)

Formula for nonmetric units

Q = 37,980,000 A0.82 (ZT)0.5 / (LC)(M0.5)

where: Q = The total required venting capacity, in cubic meters of air per hour at standard conditions of 15.6 °C and 1 atm (cubic feet of air per hour at standard conditions of 60 °F and 14.7 psia); T = The absolute temperature of the vapor at the venting conditions—degrees Kelvin ( °C+273) [degrees Rankine ( °F+460)]; A = The exposed surface area of tank shell—square meters (square feet); L = The latent heat of vaporization of the lading—calories per gram (BTU/lb); Z = The compressibiliy factor for the vapor (if this factor is unknown, let Z equal 1.0); M = The molecular weight of vapor; C = A constant derived from (K), the ratio of specific heats of the vapor. If (K) is unknown, let C = 315.

C = 520[K(2/(K+1)) [(K+1)/(K−1)]]1/2

where: K = Cp / Cv Cp = The specific heat at constant pressure, -in -calories per gram degree centigrade (BTU/lb °F.); and Cv = The specific heat at constant volume, -in -calories per gram degree centigrade (BTU/lb °F.).

(4) The required total venting capacity determined by using table I or paragraph (d)(3) of this section may be reduced for insulated tanks to Qt by the following formula:

Qt = FQ1

where: Qt = The total required venting capacity of the insulated tank; Q1 = The total venting capacity required for an uninsulated tank according to table I or paragraph (d)(3) of this section; F = A coefficient with a value greater than or equal to 0.25 according to the following formula: Formula for metric units F = 8U(649-t) / 93.5 × 106

Formula for nonmetric units

F = 8U(1200-t) / 34,500

where: U = The thermal conductance of the insulation system taken at 38 °C (100 °F), in gram calories per hour square meter °C (BTU per hour square feet °F); and t = The actual temperature of the substance at loading, in °C (°F).

(5) Insulation systems, used for the purpose of reducing the venting capacity, must be approved by the approval agency. In all cases, insulation systems approved for this purpose must:

(i) Remain effective at all temperatures up to 649 °C (1200 °F); and

(ii) Be jacketed with a material having a melting point of 649 °C (1200 °F) or greater.

(6) The flow capacity rating of any pressure relief device must be certified by the manufacturer to be in accordance with the applicable provisions in Section VIII of the ASME Code (IBR, see § 171.7 of this subchapter) with the following exceptions:

(i) The ASME Code stamp is not required; and

(ii) The flow capacity certification test for spring loaded pressure relief valves may be conducted at a pressure not to exceed 120% of the set pressure provided the stamped flow capacity rating is not greater than 83% of the average capacity of the valves tested.

(e) Markings on pressure and vacuum relief devices. The following information shall be plainly displayed on each pressure relief device:

(1) The pressure or, when appropriate, the temperature at which the device is set to function;

(2) Except for vacuum relief devices, the rated flow capacity of air discharged per minute at 15 °C (59 °F) and atmospheric pressure, at:

(i) The set pressure for rupture discs;

(ii) No greater than 20% above the start-to-discharge pressure for spring-loaded relief devices; or

(iii) The fusing temperature for fusible elements.

(3) The manufacturer's name and catalog number; and

(4) The allowable tolerances at the start-to-discharge pressure and the allowable tolerances at the discharge temperature.

[Amdt. 178-65, 46 FR 9897, Jan. 29, 1981; 46 FR 24184, Apr. 30, 1981, as amended by Amdt. 178-97, 55 FR 52716, Dec. 21, 1990; Amdt. 178-99, 58 FR 51534, Oct. 1, 1993; Amdt. 178-104, 59 FR 49135, Sept. 26, 1994; 66 FR 45386, 45389, Aug. 28, 2001; 67 FR 61016, Sept. 27, 2002; 68 FR 75751, Dec. 31, 2003]