Return to index New query |
(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).
Go Back to Article 6 Table of Contents