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254 6-230 Safety devices operated electrically shall not depend upon the closing of a circui t to shut off the main gas supply. This requirement shall not beE'. con- strued as prohibiting the use of elctrical regulating devices, provided the re~ quired safety devices are also instllled. Controls shall be so connected that maximum inherent safety provided by such controls will be obtained. 6-231 Safety shut-off valves shall be tested to assure gas tightness of the seat when in the closed position, the valve assemble shall be gas tight in all positions. Packing glands shall be designed so that the valve will not be made inoperative by excessive tightening of the pacldng nut. Either the valve shall incorporate means for requiring a manual operation for reopening of the valve after it has been closed or the electncal circuit shall be so arranged as to require a manual operation to re-open the valve after it has been closed. In no case shall valves be able to be opened manu- ally until safety pilots are lighted and circuit completed or low-water cutoff circuit has been completed. 6-232 Where aBWitch is installed in the low-wate cutoff circuit to keep circuit to safety shut-off valve closed when blowing down water column, switch must be of the push button type so that when operator releases button the control circuit to safety shut-off valve will be normal. 6-233 On large boilers where a competent attendant is constantly on duty, it will be permissible to install a bypass around the safety valve with the valve in the bypass locked or sealed. In large boiler plants using steJllll for gen- eration of power or light and having competent operators on duty at all times, the use of limiting devices as covered in 6-228 will not be mandatory. 6-234 For bumers or a combination of bumers not exceeding a total input of 500,000 BTU per hour, a combustion control of the on-off type with quick opening control valves may be used. 6-235 For a burner or a combination of burners not exceeding 2,500,000 BTU per hour ~ input, an of-off type of control may be used provided the automatic gas valve is of the slow opening type. Valve must be so designed that an adjustable opening time of from ten (10) to thirty (30) seconds may be obtained and with a closing time not to exceed five (5) seconds. . 6-236 For a bumer or a combination of burners with a total input exceeding 2,500,000 BTU per hour, a combustion control of the modulating type must be used. Where the uptake or outlet damper is securely locked in a fixei position for maximum load condition, the control may be of the type that operates the inlet air louvre of the burner in conjunction with the gas valve by means of mechanical linkage. The control may also be of the type that operates the uptake or outlet damper in conjW1ction with the gas control valve or valves, either by mechanical linkage or be change in furnace pressure. The operating medium may be -steam, electricity J air or hydraulic. If the gas control goes to open poai tion upon I failure of operating medium, then a means for shutting off the ga.s supply upon the failure of the operating medium must be provided. 6-237 Where quick opening or slow opening gas control valve is connected to an inlet shutter on ias burner, the air inlet shutter or louvre only must be lio designed or counter-balanced as to drop to full open position in the event of breakage or failure of connecting mechanism. When the control isOthe modulating type that operates the air inlet shutter- or louvre only, shutter or louvre must -51-