An accidental release of 12 tons of anhydrous ammonia at a storage/dispensing facility resulted in the hospitalization of five people and the evacuation of 550 residents. The unexpected release of anhydrous ammonia is believed to be due to a design error that may have caused a 3/4" (1.91 cm) schedule 80 vent stack on the liquid ammonia line to fail at the point where it was threaded into the liquid ammonia line.
ANSI K-61.1-1981, Safety Requirements for the Storage and Handling of Anhydrous Ammonia, Section 2.9.10, specifies that "A hydrostatic relief valve or equivalent shall be installed in each section of piping between shut-off valves to relieve pressure that could be developed by trapped liquid ammonia." In addition, OSHA Standard 29 CFR 1910.111(c)(6)(i) requires that all valves and other appurtenances be protected against tampering and physical damage.
Where more than one storage tank is in use, it is a common practice to plumb both tanks together on one relief manifold system. In this accident, two 30,000 gallon (113,200 liter) anhydrous ammonia storage tanks had their relief vent header plumbed together on a common manifold system. A vent stack (3/4" (1.91 cm) schedule 80 pipe) with a relief valve was installed on a liquid ammonia line between the shut-off valves on the storage tanks (See attachment). During a replenishing operation of anhydrous ammonia from a tanker truck to one of the two storage tanks, the vent stack with the relief valve catastrophically failed resulting in the release of approximately 12 tons (10,886 kg) of anhydrous ammonia into the atmosphere. Investigation has determined that the 3/4" (1.91 cm) pipe was not properly secured, putting undue strain at the point where it was threaded into the liquid ammonia line. The strain resulted in a stress fracture causing the vent pipe to separate from the liquid ammonia line.
The system was equipped with an excess flow valve located in the storage tank(s). The valve was designed to detect a sudden drop in pressure due to the release of ammonia through an opening equivalent to the diameter of the liquid ammonia line and to stop its flow. The valve in this accident did not stop the flow of liquid ammonia because the liquid ammonia line was 3" (7.62 cm) in diameter and the vent pipe opening was only 1-1/4" (3.18 cm), meaning the excess flow valve did not see “excess flow” and remained OPEN. The ammonia was discharged through the 1-1/4" vent pipe opening.
It appears that the type of installation described above is common. Since Farmland Industries of Kansas City, Missouri, installed the system involved in the accident, the Bismarck Area OSHA Office contacted Farmland to determine the effective means for correcting the apparent design error. Farmland Industries recommended that the 3/4" (1.91 cm) schedule 80 vent stack and the relief valve be replaced with a 1/4" (.64 cm) hydrostatic relief valve installed directly into the main shut-off valve. It should be noted that the selection and installation of relief valves should be consistent with the valve manufacturer's specifications and recommendations.
To prevent similar accidents, it is recommended that anhydrous ammonia facilities be informed of the potential failure of vent pipes as described above. They should be made aware that the vent pipe on the liquid ammonia line should be evaluated for mechanical integrity and adequacy of design. For systems manufactured by Farmland Industries, the manufacturer recommends that the vent pipe be removed from the liquid ammonia line, the opening plugged and as stated in the previous paragraph, a 1/4" (.64 cm) hydrostatic relief valve be installed directly into the main shut-off valve on the liquid ammonia line. If, however, this cannot be done, Farmland recommends ensuring that the vent pipe be properly secured and supported, and that a rain cap be attached to the top of the pressure relief valve.
Furthermore, it is recommended that additional measures for safety be taken. These measures include inspecting all other piping and appurtenances for damage and corrosion to prevent the unexpected release of anhydrous ammonia, and establishing an emergency plan as required in the 29 CFR 1910.38 standard, Employee emergency plans and fire prevention plans."