// CODE SNIPPET
63.3.9.3 Treatment Systems
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Except as provided in 63.3.9.3.1 and 63.3.9.3.2, gas cabinets, exhausted enclosures, and gas rooms containing toxic or highly toxic gases shall be provided with exhaust ventilation, with all exhaust directed to a treatment system designed to process accidental release of gas. [55:7.9.3]
Treatment systems shall not be required for toxic or highly toxic gases in storage where cylinders, containers, and tanks are provided with the controls specified in 63.3.9.3.1.1 through 63.3.9.3.1.3. [55:7.9.3.1]
Where provided, handwheel-operated valves shall be secured to prevent movement. [55:7.9.3.1.2]
Approved cylinder containment vessels or cylinder containment systems shall be provided at an approved location. [55:7.9.3.1.3]
Treatment systems shall not be required for toxic gases in use where cylinders, containers, and tanks are provided with the controls specified in 63.3.9.3.2.1 and 63.3.9.3.2.2. [55:7.9.3.2]
A gas detection system with a sensing interval not exceeding 5 minutes shall be provided. [55:7.9.3.2.1.1]
The gas detection system shall monitor the exhaust system at the point of discharge from the gas cabinet, exhausted enclosure, or gas room. [55:7.9.3.2.1.2]
The fail-safe valve shall close when gas is detected at the permissible exposure limit, short-term exposure limit (STEL), or ceiling limit by the gas detection system. [55:7.9.3.2.2.1]
For attended operations, a manual closing valve shall be permitted when in accordance with 63.3.9.3.4.3. [55:7.9.3.2.2.2]
For gases used at unattended operations for the protection of public health, such as chlorine at water or waste-water treatment sites, the automatic valve shall close if the concentration of gas detected by a gas detection system reaches one-half of the IDLH. [55:7.9.3.2.2.3]
Treatment systems shall be capable of diluting, adsorbing, absorbing, containing, neutralizing, burning, or otherwise processing stored or used toxic or highly toxic gas, or both. [55:7.9.3.3]
Treatment systems shall be capable of reducing the allowable discharge concentrations to one-half the IDLH threshold at the point of discharge. [55:7.9.3.3.2]
Treatment systems shall be sized to process the maximum worst-case release of gas based on the maximum flow rate of release from the largest vessel utilized in accordance with 63.3.9.3.4.2. [55:7.9.3.4.1]
The entire contents of the single largest compressed gas vessel shall be considered. [55:7.9.3.4.2]
Where source cylinders, containers, and tanks are used in attended process operations, with an operator present at the enclosure where the activity occurs, the volume of the release shall be limited to the estimated amount released from the process piping system within a period not to exceed 5 minutes. [55:7.9.3.4.3.1]
Such process piping systems shall comply with the requirements of 63.3.9.3.4.3.2(A) through 63.3.9.3.4.3.2(E). [55:7.9.3.4.3.2]
(A) Local Exhaust. All gas transfer operations shall be conducted within a zone of local exhaust that is connected to a treatment system. [55:7.9.3.4.3.2(A)]
(B) Gas Detection. Gas detection shall be used to provide a warning to alert the operators to emission of gas into the zone of local exhaust, and the following requirements also shall apply:
- The system shall be capable of detecting gas at the permissible exposure limit (PEL) or the ceiling limit for the gas being processed.
- Activation of the gas detection system shall provide a local alarm.
[55:7.9.3.4.3.2(B)]
(C) Process Shutdown. Operations involving the gas detected shall be shut down and leaks repaired. [55:7.9.3.4.3.2(C)]
(D) Piping System Construction. Piping systems used to convey gases shall be of all-welded construction throughout, with the exception of fittings used to connect cylinders, containers, or tanks, or any combination thereof, to the process system. [55:7.9.3.4.3.2(D)]
(E) Piping System Accessibility. Piping systems shall be designed to provide for readily accessible manual shutdown controls. [55:7.9.3.4.3.2(E)]
The time release shall be in accordance with Table 63.3.9.3.5 for the type of container indicated. [55:7.9.3.5]
Table 63.3.9.3.5 Rates of Release
| Container Type | Time Release | |
|---|---|---|
| Nonliquefied Gases | Liquefied Gases | |
| Cylinders without restrictive flow orifices | 5 minutes | 30 minutes |
| Portable tanks without restrictive flow orifices | 40 minutes | 240 minutes |
| All others | Based on peak flow from maximum valve orifice | Based on peak flow from maximum valve orifice |
[55: Table 7.9.3.5]
For portable cylinders, containers, and tanks, the maximum flow rate of release shall be calculated based on assuming the total release from the cylinder or tank within the time specified. [55:7.9.3.6.1]
When portable cylinders, containers, or tanks are equipped with reduced flow orifices, the worst-case rate of release shall be determined by the maximum achievable flow from the valve based on the following formula:
where:
| CFM | = | standard cubic feet per minute of gas of concern under flow conditions |
| A | = | area of orifice in square inches (See Table A.63.3.9.3.6 for areas of typical restricted flow orifices.) |
| P | = | supply pressure of gas at NTP in pounds per square inch absolute |
| MW | = | molecular weight [55:7.9.3.6.2] |
For mixtures, the average of the molecular weights shall be used. [55:7.9.3.6.3]
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