ARE YOU INSPECTING ALL VULNERABLE COMPONENTS OF PIPING AS PER STANDARDS?

At 11:15 pm, high-pressure steam pipe elbow at 120 bar and 520°C connected to catalytic reforming equipment burst. The elbow and pipe , a 40-kg block of steel, was projected longitudinally. Inside the shop area, a walkway was ripped off its supports damaging an access ladder. It then flew over an ammonium nitrate conveyor belt only to land 230 m away in a parking zone for tanker cars full of ammonia awaiting shipment, which on that day happened to be empty. The steam tore apart the asbestos cement cladding on the wall located 20 m from the original rupture and escaped into the atmosphere with a load noise. 

The two employees present in the unit at the time shut down production operations and cooled the steam reformer with nitrogen. 

A metallurgical assessment of the elbow indicated that slow creep had initiated on the outer skin, which combined with flow into the material layer was the cause of pipe rupture. This hypothesis relied on the detection of oxidised, yet non-deformed, pipe openings in the presence of intergranular microcracks on both the pipe and its elbow. The origin of this creep was explained by the metallurgical composition of the elbow, i.e. ordinary carbon steel containing no alloys and not adapted to temperatures above 425°C. The pipeline was made of a slightly-alloyed P22 type steel, which was more resistant to creep and compliant with the original specifications defined 32 years prior for both material elements. 

Inspections carried out at the time on the elbow did not detect any noncompliance of the steel, given that non-destructive technology had not yet come of age. This creep might have been accelerated as a result of heat treatment performed at 700°C during equipment installation, once the assembly had been welded. Periodic inspections dedicated to pressurised equipment on the damaged pipe were only recorded into the log 25 years after service start-up, at the time of applying for facility recertification. The initial recertification was rendered official without any underlying structural documentation (misplaced), and subsequent inspections never focused on the section of pipe that would burst.

Source: Aria database