Seal of approval
Gas-lubricated mechanical seals perform well in high-temperature hydrogen service

At Leuna in eastern Germany, Domo Caproleuna manufactures hydroxyl ammonium sulphate (HAS) by burning ammonia. The ammonia combustion reactors are preheated by burning hydrogen fed from a rotating distributor that requires a shaft seal. The original packed-gland seal caused many problems, so in 1998 they were replaced by Burgmann type AGS 451 gas-lubricated mechanical seals. These have performed excellently ever since.

Domo Caproleuna, a company in the Chemicals & Polymers division of the Domo group, has two production sites at the Leuna chemical park in eastern Germany. Among other products, these sites manufacture caprolactam, a raw material for polyamide fibers such as Nylon. Caprolactam itself is made from hydroxyl ammonium sulfate (HAS), which Domo Caproleuna also produces.

HAS in turn is made from nitrogen monoxide (NO) obtained from the catalytic combustion of ammonia (NH3). The ammonia combustion reactors used by Domo Caproleuna were manufactured by energy and environmental engineering specialist Steinmüller Engineering, Gummersbach, Germany. To start up an ammonia combustion reactor, the catalyst bed must first be heated to 220 °C. To do this, 135 Nm3/h of hydrogen is fed down a hollow shaft to a distributor that rotates above the catalyst bed, where it is ignited. When the catalyst reaches its operating temperature, the distributor drive motor is switched off, the hydrogen supply is isolated, and ammonia is introduced to the reactor. Once the ammonia combustion is under way, the temperature of the catalyst bed rises rapidly to 900 °C.
The original method of sealing the rotating distributor shaft was with a stuffing box. The hydrogen was fed into a lantern ring and thence through holes in the wall of the hollow shaft. This arrangement caused some problems: periodic tightening of the packing caused the lantern ring to move, partially blocking the holes in the hollow shaft, and restricting the hydrogen supply. The only way to correct this was to shut down the reactor and dismantle the whole top assembly—a time-consuming operation.
The difficulty of sealing hydrogen, and the fact that the sealing system operates at a temperature of around 180 °C, meant that there was also a serious risk of setting the packing on fire.
To make the ammonia combustion reactors more reliable, the stuffing box packings and lantern rings were replaced with Burgmann AGS gas-lubricated mechanical seals. The hydrogen is used as a buffer gas and is simultaneously fed into the center of the hollow shaft through a hole between the atmosphere and product sides of the mechanical seal. To make this work satisfactorily, the hydrogen supply pressure was increased from 2 to 3 bar a. A hole in the shaft sleeve limits the hydrogen flowrate to a maximum of 135 Nm3/h.
Since the retrofit in 1998, the seals have operated with no failures and to the complete satisfaction of the customer. The Burgmann solution significantly increased system availability and substantially improved safety. The resulting decrease in servicing and maintenance costs made the plant significantly more cost-effective.