On April 16, 2016, a slurry pump in a Jamaican alumina refinery catastrophically failed severely injuring a nearby technician. The fluid being pumped was a hot, caustic fluid. A large portion of the pump casing was blown off. The event happened during a daily routine pump start-up procedure. Shortly after the pump was started up, a manual valve in the pump discharge line broke and its disc was slammed by the pump pressure into a closed position. This sent a waterhammer pressure wave back towards the pump discharge. The pump casing material is known to be susceptible to brittle fracture and the slurry pumps at the facility had a history of casing damage and leaks. Photos of the system, damaged pump and broken valve are shown. A waterhammer model indicated that pressures at the pump could have been as high as ~100 bar (1400 psi) at the pump discharge flange.

Authors:
Trey Walters, PE, Applied Flow Technology, USA; Sam Chamberlain, PE, Carib Engineering, Jamaica; Eval Robotham, Jamalco, Claredon, Jamaica
Presented at the 14th International Conference on Pressure Surges  |  April 12-14, 2023, Eindhoven, Netherlands

A catastrophic pump failure resulted from a combination of pump casing material properties, the caustic slurry high-temperature process, pump operation and maintenance philosophy, and a failed valve which caused a waterhammer event. 

A waterhammer model shows the pressure at the pump discharge could have been as high as 100 bar (1400 psi). The pump was rated to a maximum pressure of 6.9 bar-g (100 psig).

In all of the cases that were analyzed, the highest pressure at the pump discharge occurred during the first waterhammer pressure wave event. Moderate to substantial transient cavitation occurred in the discharge line after the first reflection at the valve and a period of low pressures occurred in the entire line. Since the highest pressure occurred before any cavitation occurred a higher confidence is placed on this first pressure wave. Since the valve closure is unknown, and no high frequency data exists, then the only view into what happened in the discharge line was obtained by varying the valve transient and looking at results.

1.  Introduction

Jamalco is a mining and alumina refining entity in Jamaica. The Jamalco predecessor entity started as a bauxite mining venture by Alcoa in 1959 and exported its first shipment of bauxite in 1963. The venture grew to a raw bauxite processing enterprise in 1972 where alumina was produced and shipped from Clarendon, Jamaica (Fig. 1).

The use of white irons in centrifugal pump casing material selection is standard in slurry applications because of the improved wear life in solids hydro (slurry) transport applications. There have been concerns expressed because of the microstructure of the metals especially 28% chrome iron, ASTM 532 Class III, Type A, which is widely used in slurry applications.  The concern is that in some situations these castings fail by brittle fracture which can result in catastrophic failure of pump-wetted components. (1)

On Saturday, April 16, 2016, the alumina refinery experienced a serious waterhammer event. Like many engineering failures, the event resulted from several independent factors which fatefully came together at one time and led to a catastrophic failure – in this case, a centrifugal pump. The pump casing exploded which resulted in a hot, caustic fluid being blown out of the pump and onto nearby facility personnel and equipment. One technician, unfortunately, suffered serious and permanent injuries.

The event occurred during a routine morning procedure involving a manual switchover from a single operating pump to a different parallel pump. The fluid being pumped was a caustic slurry operating at about 110 C (230 F) at near atmospheric pressure at the pump suction. In this case the operating pump was referred to as the #4 Digester Blow-Off (DBO) Pump. The pump being switched to (and which failed) was the #3 DBO Pump. During this routine procedure, a 300 mm (12-Inch) angle valve located on the #3 DBO Pump line was manually opened. The valve was located at the pump discharge roughly 7 m (23 ft) downstream. When the valve was opened the pump was off according to the standard procedure. The #3 DBO Pump was then started and shortly thereafter the discharge valve threaded spindle completely severed. The disc on the discharge valve could then move freely. The pressure and flow from the pump caused the valve disc to slam into the closed position. This sent a high-pressure waterhammer wave back towards the pump which resulted in the pump casing failing catastrophically. It is estimated that this pressure could have been as high as ~140 bar (2000 psi) at the valve and ~100 bar (1400 psi) at the pump discharge flange. The pressure rating for the pump was 6.9 bar-g (100 psig).

2.  Aluminum Production and the Bayer Process

The production of aluminum (or aluminium in some English-speaking countries, periodic table element “Al”, atomic number 13) is done by smelting a chemical compound called alumina (Al₂O₃) which is also referred to as aluminum oxide. Alumina is obtained from a raw mineral called bauxite. An alumina refinery takes in raw bauxite and through a complicated, multi-step process, creates the hydrated oxide (Al₂O₃.XH₂O) which is then calcined to create alumina – the final product for the refinery. The alumina is then shipped to other facilities which smelt the alumina into the aluminum metal with which we are all familiar. (2)

The most common process for processing bauxite into alumina is the Bayer Process (Fig. 2). The first major step in the process is Digestion where the hydrated oxide is dissolved from bauxite in a sodium hydroxide solution at elevated temperature and pressure. The resulting slurry is flashed cooled and pressure reduced in a series of vessels and discharged as DBO to Clarification where the pregnant liquor is separated from undigested solids. The failed pump was pumping DBO slurry to the Sand Removal Unit in the Clarification Process (Fig. 2, bottom). The supply tank for the pump was a “blow-off” tank at the end of the flash cooling and is maintained at near atmospheric pressure. At this point the fluid in the supply tank was a slurry mixture of undigested bauxite, sand and the sodium aluminate liquor. As mentioned previously, the slurry temperature was at roughly 110 C (230 F).

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