Above: New plug; Plug with short term wear; Complete failure
Between 1993 and 2010, WPC re-designed the plug stem four times in efforts to extend its lifespan. The original design was a two piece 316SST stem with ceramic tip. This had a life of one to two weeks with failure caused by the cracking of the ceramics at the threaded connection, likely due to thermal shock.
A redesign led to a two piece construction comprising a 316SST/COCRA plug with pinned 316SST stem. This extended the lifespan to three to four weeks with failure caused by plug wear and separation of the plug and stem, likely due to vibration.
A third design saw a 316SST stem with retained ceramic tip designed to alleviate plug tip wear and ensure positive connection of the stem to plug tip. However, the ceramic tip snapped off due to thermal shock and lifespan was measured in hours and days.
The fourth design was a one piece 316SST/COCRA (Alloy 6) plug stem. This lasted between six and eight weeks before erosion caused total failure of the AA control valve. The average repair cost after a plug failure was US$12,000.