Have you ever wondered what a “No. 54 drill-size Opening” is? This is a phrase used in OSHA’s 1910.111, Storage and handling of anhydrous ammonia. In simple terms, OSHA has established that if the potential leak is through an opening of No. 54 drill-size or smaller then, the opening need not be protected by an excess-flow valve. Here’s what I am talking about…
Here is a nice graphic showing us a “No. 54 drill-size Opening” (see highlighted box below). As we can see this is an extremely small opening, and thus OSHA exempts these openings from being protected by an “excess flow valve.”
Here are the exemptions from 1910.111, Storage and handling of anhydrous ammonia:
1910.111(b)(6)(ii) All connections to containers except safety relief devices, gaging devices, or those fitted with No. 54 drill-size orifice shall have shutoff valves located as close to the container as practicable.
1910.111(b)(6)(iv) Liquid-level gaging devices that require bleeding of the product to the atmosphere and which are so constructed that outward flow will not exceed that passed by a No. 54 drill-size opening need not be equipped with excess flow valves.
1910.111(b)(6)(v) Openings from the container or through fittings attached directly on the container to which pressure gage connections are made need not be equipped with excess flow valves if such openings are not larger than No. 54 drill size.
1910.111(b)(14)(iii) Gaging devices that require bleeding of the product to the atmosphere such as the rotary tube, fixed tube, and slip tube devices shall be designed so that the maximum opening of the bleed valve is not larger than No. 54 drill size unless provided with an excess flow valve. (This requirement does not apply to farm vehicles used for the application of ammonia as covered in paragraph (h) of this section.)
1910.111(c)(2)(ii) All liquid and vapor connections to containers except filling pipes, safety relief connections, and liquid-level gaging and pressure gage connections provided with orifices not larger than No. 54 drill size as required in paragraphs (b)(6) (iv) and (v) of this section shall be equipped with excess-flow valves.
As an additional note, the standard also refers to “No. 60 drill-size Opening” as well:
1910.111(b)(6)(vii) Excess flow valves shall be designed with a bypass, not to exceed a No. 60 drill-size opening to allow equalization of pressures.