Sec. 599.18. Venting, and pressure/vacuum/thermal monitoring

(a) General venting requirements.

(1) All tanks must be protected from over-pressurization and excessive vacuums such as may be caused by operator error, filling, emptying, atmospheric temperature changes, pumping, refrigeration, heating and fire exposure. Protection must be provided by one or a combination of the following means: vents, rupture discs, pressure/vacuum relief devices, controllers, fail-safe vessel designs or other means determined by a qualified engineer.

(2) If a pilot-operated relief valve is used, it must be designed so that the main valve will open automatically and will protect the tank in the event of failure of the pilot valve or another essential functioning device.

(3) Open vents must be provided with a flame-arresting device, if used on a tank containing a flammable hazardous substance or if used on tanks containing a hazardous substance that is heated above its flash point.

(4) All vent discharge openings must be designed and constructed to prevent interference of operation due to precipitation.

(5) Discharge from vents must not terminate in or underneath any building if the discharge could pose a fire, health or safety problem.

(6) All vents must have provisions for draining any condensate which may accumulate.

(7) Vents must be so arranged that the possibility of tampering will be minimized.

(8) Vents must have direct contact with the vapor space of the tank.

(9) The capacity of the vent must not be restricted below design.

(10) Tanks fitted with relief valves must not be equipped with an isolation valve below it unless two or more relief valves are provided and isolation valves are interlocked.

(11) All cooled tanks with sealed double-wall construction must have a pressure relief valve on the outer wall in addition to a pressure relief valve or safety disk on the inner tank.

(b) Normal vents.

(1) All atmospheric tanks and all low-pressure tanks must be equipped with normal vents designed to accommodate:

(i) inbreathing resulting from maximum outflow of hazardous substances from the tank;

(ii) inbreathing resulting from contraction of vapors caused by maximum decrease in atmospheric temperature;

(iii) outbreathing resulting from maximum inflow of hazardous substances into the tank and maximum evaporation caused by such inflow; and

(iv) outbreathing resulting from expansion and evaporation that result from maximum increase in atmospheric temperature (thermal breathing).

(2) Normal vents may consist of a pilot-operated relief valve, a pressure relief valve, a pressure-vacuum valve, a conservation vent, an open vent or an equivalent device or combination of devices.

(c) Emergency vents.

All atmospheric, low-pressure and high-pressure aboveground tanks must have emergency vents to insure that the safe pressure for the tank is not exceeded. Emergency vents must be designed by a qualified engineer in accordance with generally accepted engineering practices. Emergency vent designs may include: larger or additional open vents, pressure-vacuum valves, pressure relief valves, a gauge hatch that permits the cover to lift under abnormal internal pressure, a manhole cover that lifts when exposed to abnormal internal pressure; or other practice for pressure and vacuum relief.

(d) Labeling of safety/pressure/vacuum relief valves.

Where safety, pressure relief or vacuum relief valves are used, each must be permanently labeled with the information listed below. The labeling may be provided on the valve itself, or on a plate or plates securely fastened to the valve. Labels may be stamped, etched, impressed or cast in the valve or nameplate. The label must include the following information:

(1) the name or identifying trademark of the manufacturer;

(2) the manufacturer's design or type number;

(3) the pipe size of the inlet;

(4) the set pressure or vacuum, in PSIG;

(5) the full open pressure or vacuum, in PSIG; and

(6) the capacity at the indicated pressure or full open vacuum in either cubic feet of gas per minute or cubic feet of gas per hour, and be so designated.

(e) Pressure, vacuum and thermal monitoring.

(1) All tanks subject to failure due to pressure or vacuum, must be provided with pressure/vacuum gauges and pressure/vacuum controllers.

(2) Thermal monitors, pressure/vacuum indicators, and their corresponding alarms must be provided for all tanks where a reaction may cause damage to the tank system or endanger human health, safety or the environment.

(3) All heated or cooled tanks must be equipped with a temperature and pressure gauge and appropriate thermal controls.

(4) Special precaution against overheating or overcooling must be provided for heated or cooled tanks in accordance with generally accepted engineering practices. Protection must be provided by one or a combination of the following means: temperature controllers, insulation, alarms, fail-safe cooling systems, material selection, or other means determined by a qualified engineer.