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Subchapter 1. Unfired Pressure Vessel Safety Orders
Article 6. Anhydrous Ammonia

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§511. Safety Relief Valves.

(a) Every vessel used in anhydrous ammonia service shall be fitted with 1 or more safety relief valves in direct communication with the vapor space. These safety relief valves shall be of the spring-loaded type suitable for anhydrous ammonia service. The discharge from safety relief valves shall be full size and be directed away from the vessel, and shall discharge upward and unobstructed to the open air.

All relief-valve discharge openings shall have suitable loose-fitting rain caps that will allow free discharge of the vapor and prevent entrance of water. Provision shall be made for draining condensate which may accumulate in the discharge pipe.

The safety relief valves for anhydrous ammonia service shall be set to start to discharge with relation to the allowable working pressure of the vessel as follows:

Pressure vessels constructed in accordance with: Minimum Maximum

Code paragraphs U-68 and U-69

110% 125%
Code paragraphs U-200, through 1974 edition 100% 110%
API-ASME Code 100% 110%
DOT Cylinders As required by DOT Regulations

(b) Except for code paragraphs U-68 and U-69 tanks, the discharge capacity of safety relief valves for anhydrous ammonia tanks shall be sufficient to prevent pressure in the tank from exceeding 120 percent of the allowable working pressure of the tank. ASME code paragraphs U-68 and U-69 tanks shall have safety valve capacity sufficient to prevent pressure in the tank from exceeding 135 percent of the allowable working pressure of the tank. All safety relief valves required by this Order shall be ASME rated and stamped. The minimum required rate of discharge of safety relief valves for anhydrous ammonia tanks shall be in accordance with the following table:

Surface area  Flow rate	Surface area  Flow rate	Surface area    Flow rate 
  sq. ft       CFM air    sq. ft       CFM air     sq. ft        CFM air  
20 or less .....258   	185 ...........1,600      900 .........  5,850
25 .............310   	190 ...........1,640      950 .........  6,120
30 .............360   	195 ...........1,670    1,000 .........  6,380
35 .............408   	200 ...........1,710    1,050 .........  6,640
40 .............455   	210 ...........1,780    1,100 .........  6,900
45 .............501   	220 ...........1,850    1,150 .........  7,160
50 .............547   	230 ...........1,920    1,200 .........  7,410
55 .............591   	240 ...........1,980    1,250 .........  7,660
60 .............635   	250 ...........2,050    1,300 .........  7,910
65 .............678   	260 ...........2,120    1,350 .........  8,160
70 .............720   	270 ...........2,180    1,400 .........  8,410
75 .............762   	280 ...........2,250    1,450 .........  8,650
80 .............804   	290 ...........2,320    1,500 .........  8,900
85 .............845   	300 ...........2,380    1,550 .........  9,140
90 .............885   	310 ...........2,450    1,600 .........  9,380
95 .............925   	320 ...........2,510    1,650 .........  9,620
100 ............965   	330 ...........2,570    1,700 .........  9,860
105 ............1,010   340 ...........2,640    1,750 ......... 10,090
110 ............1,050   350 ...........2,700    1,800 ......... 10,330
115 ............1,090   360 ...........2,760    1,850 ......... 10,560
120 ............1,120   370 ...........2,830    1,900 ......... 10,800
125 ............1,160   380 ...........2,890    1,950 ......... 11,030
130 ............1,200   390 ...........2,950    2,000 ......... 11,260
135 ............1,240   400 ...........3,010    2,050 ......... 11,490
140 ............1,280   450 ...........3,320    2,100 ......... 11,720
145 ............1,310   500 ...........3,620    2,150 ......... 11,950
150 ............1,350   550 ...........3,910    2,200 ......... 12,180
155 ............1,390   600 ...........4,200    2,250 ......... 12,400
160 ............1,420   650 ...........4,480    2,300 ......... 12,630
165 ............1,460   700 ...........4,760    2,350 ......... 12,850
170 ............1,500   750 ...........5,040    2,400 ......... 13,080
175 ............1,530   800 ...........5,300    2,450 ......... 13,300
180 ............1,570   850 ...........5,590    2,500 ......... 13,520

Surface area = total outside surface area of container in square feet. When the surface area is not stamped on the nameplate or when 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 in feet times outside diameter

in feet times 3.1416

(2) Cylindrical containers with other than hemispherical

heads.

Area = (overall length in feet plus 0.3 outside

diameter in feet) time outside diameter in feet times

3.1416

(3) Spherical container.

Area = outside diameter in feet squared times 3.1416

Flow rate -- CFM Air = cubic feet per minute of air required at standard conditions, 60 degrees Fahrenheit and atmospheric pressure (14.7 psia).

The rate of discharge may be interpolated for intermediate values of surface area. For containers with total outside surface area greater than 2,500 square feet, the required flow rate can be calculated using the formula: Flow Rate CFM Air = 22.11A[0.82], where A = outside surface area of the container in square feet.

The minimum required rate of discharge of safety relief valves for anhydrous ammonia DOT cylinders shall be in accordance with the regulations of the Bureau of Explosives.

(c) Safety relief valves shall be so designed and installed that the possibility of tampering will be minimized. If the pressure setting is external, the relief valves shall be provided with acceptable means for sealing the adjustment.

(d) Shutoff valves shall not be installed between the safety relief valve and tank, except that a shutoff valve may be used where the arrangement of this valve is such as always to provide full required capacity flow through sufficient relief valves to properly protect the tank. [This exception is intended to permit 3-way valves, mechanically interconnected valves, etc., to be installed between the tank and safety valve where the installation and arrangement will always permit the required number of valves to be in communication with the tank to provide the relief capacity required by (b) above.]

The flow capacity of the relief valve shall not be restricted by any connection to it on either the upstream or downstream side.

(e) Each safety relief valve used on anhydrous ammonia tanks shall be plainly marked with the following information:

(1) With the letters "AA".

(2) The pressure in pounds per square inch gage (psig) at which the valve is set to start to discharge.

(3) The rate of discharge of the valve at its full open position in cubic feet per minute (cfm) of air.

(4) The manufacturer's name and catalog number.

(5) The symbol of the ASME Code.

HISTORY:

1. Amendment filed 12-8-72 as procedural and organizational; effective upon filing (Register 72, No. 50).

2. Amendment of subsections (a) and (d) filed 3-28-75; effective thirtieth day thereafter (Register 75, No. 13).

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