Chemical Process Plant Upset: Flow-induced Erosion/Corrosion of Piping

By Stephen P. Andrew, T. Kim Parnell, Robert Caligiuri, Lawrence Eiselstein

A process upset at a chlorine production facility resulted in a release that forced the partial evacuation of a nearby town. Investigations revealed that the events commenced with the failure of a shell and tube heat exchanger used to condense chlorine gas.

A process upset at a chlorine production facility resulted in a release that forced the partial evacuation of a nearby town. Investigations revealed that the events commenced with the failure of a shell and tube heat exchanger used to condense chlorine gas. Post-incident inspections revealed a cloth at the liquefier coolant inlet that accelerated the flow in that region, causing certain tubes to be breached. As a result, the water-based brine liquefier coolant was entrained in the chlorine process stream, forming a highly acidic oxidizing mixture. This corrosive mixture then flowed to the chlorine storage tanks destroying an elbow in the tank inlet piping and rendering the tank shut-off tank valve ineffective, thus allowing chlorine to vent into the atmosphere.

This paper discusses the multi-disciplinary approach used to investigate this incident. In particular, this paper discusses the detailed physical inspections performed, the finite element analyses used to determine the flow conditions, and the corrosion testing conducted to evaluate the failure.

Chlorine Production Process

The subject facility produced chlorine by passing a calcium chloride solution through electrolytic cells. The resulting gas was subsequently dried, cooled, and compressed. For storage and shipping, the gas was condensed into a liquid since liquid chlorine occupies approximately 1/480 the volume of an equivalent mass of gas. The gas was condensed via two vertical shell and tube heat exchangers (liquefiers) that were connected in series. Chlorine gas flowed downward through the tubes of each liquefier, and was condensed by a chilled (-10oF), counter-flowing calcium chloride brine solution circulating around the outside surface of the tubes. The first liquefier (primary) was designed to provide initial condensing of the gas. The remaining uncondensed gas was sent to the secondary liquefier where additional condensing occurred. Liquid chlorine was collected at the bottom of the liquefiers and transported by gravity to a manifold. The manifold diverted the liquid chlorine to one of the eight storage 150-ton capacity storage tanks in the ?tank farm? for storage and later shipment to customers.

Incident Chronology

Immediately prior to this incident, chlorine was being produced and directed to Storage Tank Number 4. Approximately 60 tons of chlorine were contained in this storage tank when the incident began. Chlorine was also being pumped from this storage tank to a rail car. In the early morning hours, chlorine alarms sounded in the storage tank area. Upon investigating the alarm, plant personnel observed a 1-inch to 1-1/2 inch hole in a 2-inch schedule 80-pipe elbow connected to Storage Tank No. 4. Chlorine was reportedly spraying approximately 1 foot to 1-1/2 feet in each direction from this hole. In an attempt to stop the leak, production was diverted from Tank No. 4 to tank no. 5. However, this did not stop the flow of chlorine through the hole.“a corrosive medium had been formed in the chlorine”

The storage tank head contained a plug-type Monel (copper-nickel) angle valve that accepted flow from the horizontal production line piping and transported it downward through a dip leg that extended to the bottom of the tank. By design, there was no siphon break on this dip leg; consequently it was suspected that chlorine was siphoning from the storage tank through the pipe elbow.

Consequently, this valve was closed in an attempt to stop the chlorine flow. While this slowed the flow, chlorine was again noted to be rapidly leaking from the hole a short time later. Plant personnel made several additional unsuccessful efforts to stop the leak. The chlorine flow finally ceased when steel and Teflon blinds were installed in a flange on the line several hours after the leak began, at which time, much of the liquid chlorine had escaped.

Evidence Examination

Investigation of this incident began with the inspection of the process equipment and piping at the facility. These investigations began at the tank farm and systematically proceeded upstream.

Tank Farm

Among the damage near Storage Tank No. 4 was a approximate 1-inch by 2-inch oval hole in an elbow in the piping leading to the tank as well as damage to the production line angle valve. Examination of the elbow revealed damage initiating at the inside of the pipe and progressing toward the outside. Damage to the angle valve connecting the production line to the storage tank revealed degradation in both the valve seat and plug. This damage was such that the plug would not seat and the valve could not provide a seal that would adequately stop the flow of chlorine.

From these observations it was apparent that a corrosive medium had been formed in the chlorine. Immediately upstream from the tank farm were the two liquefiers that used a chilled calcium chloride brine coolant. Since chlorine dissolves and readily reacts with water to form highly corrosive hydrochloric and hypochlorous acids, the introduction of water into the process stream from these liquefiers was a likely source for the formation of the corrosive medium.

Liquefier inspection

Both liquefiers were single pass, shell and tube heat exchangers that were operated in series. Chlorine flowed through the inside of the tubes and calcium chloride brine circulated around the tubes and within the outer shell of the liquefier. Consequently, if a hole was present in the tubes, calcium chloride could potentially become entrained into the chlorine process stream and create a highly acidic mixture.

Both liquefiers were pressure-tested in-situ to evaluate tube integrity. In this procedure, air pressure at 100 psig was applied to the tube side of each liquefier. The primary liquefier was observed to hold this pressure. However, the secondary liquefier was observed to leak air into the brine storage tank, indicating that hole(s) were present in the tubes.

To identify the number and location of the leaking tubes, the secondary liquefier was removed to a warehouse where it could be tested more thoroughly. This evaluation was accomplished by removing the heads on both ends of the liquefier to expose the tube sheet. The brine outlet on the shell was sealed with a blind flange and air pressure (5 to 10 psig) was applied at the brine inlet flange. Each tube was then successively plugged at one end, and airflow was monitored at the other to determine which tubes were leaking. Using this technique, three tubes were identified as containing holes.

To determine the exact location of the holes in the tubes, the top half of the shell was cut and removed from the rest of the liquefier. In addition to revealing the locations of the leaking tubes, a cloth was found wrapped around the brine impingement plate area at the brine inlet flange. Coincidentally, all of the tubes with holes were also found near the cloth.

Other equipment

Other equipment in the process stream was examined for damage and potential contribution to the incident. No such damage was found.

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