In March 1994, a meat processing plant in the southeast experienced an explosion in one of their freezer rooms which resulted in over $500,000.00 of property damage. Fortunately, the explosion occurred around midnight on a Sunday so there were few employees at the plant and no one was injured. An initial investigation by plant personnel determined a weld in the refrigeration system failed and leaked refrigerant. Summit Engineering was asked to assist in determining the cause of the weld failure and what caused the explosion.
The freezer room where the weld failed is commonly referred to as a "blast freezer" and is used to quickly cool the processed meat down to sub-zero temperatures. The blast freezer used an ammonia refrigeration system to provide the necessary cooling requirements. The weld that fractured is shown in Figure 1. The weld fractured approximately one-quarter of the pipe circumference along the center line of the weld.
The weld was determined to be a factory weld which was made at the refrigeration system manufacturer’s facility. The weld was sectioned and examined using optical magnification and scanning electron microscopy. In addition, the fractured weld was sectioned, polished, etched and examined. Figure 2 shows a polished sample of the fractured weld; note the lack of weld penetration and the misalignment of the pipe and elbow.
There are several standards and guidelines for ammonia refrigeration systems published by ANSI (American National Standards Institute) and IIAR (International Institute of Ammonia Refrigeration) which discuss safety, equipment, design and installation. In addition, there is an ANSI/ASME standard for Refrigeration Piping which discusses requirements for welds in detail.
The weld that failed did not meet the requirements for welds in the ANSI standard for Refrigeration Piping. There was a lack of weld penetration and a misalignment of the pipe and elbow. Both of these conditions resulted in a poor quality weld and reduced the strength of the welded joint. Internal and external loads on the refrigerant piping caused a crack to form in the weakened weld.
Once the crack propagated through the wall, the ammonia refrigerant leaked out of the piping during a defrost cycle. The ammonia began filling up the blast freezer until the flammable limits (16% to 25% by volume) were attained. At that point, an ignition source such as an electrical contact, metal fan blades rubbing against a metal fan guard, or other electrical spark in the blast freezer ignited the ammonia which resulted in an explosion.
It was concluded that a poor quality factory weld in the refrigeration piping failed, leaked ammonia which was then ignited and exploded.