Reliability Study: Use of Chrome Plating on Special Purpose Steam Turbines

An 18,000 HP special purpose steam turbine is driving a propane compressor in a Gas Treatment facility in one of Saudi Aramco gas plants. This turbine is designed with two 10-inch steam inlets, and each inlet has an independent, hydraulically actuated Emergency Shutdown Valve (ESV). The core function of these valves is to rapidly isolate the high pressure (600 psi) steam supply to the steam turbine whenever an Emergency Shutdown Device (ESD) signal is activated in a reaction time less than 200 milliseconds. An inspection was carried out for the turbine (ESV's) by inspecting the hydraulic section drain lines through port B when actuated to open positions. The inspection revealed that oil flow through the drain line is continues at a rate of 30 ml/s. While oil flow through port B should stop when the hydraulic section is at its ultimate open position. Initial review for the hydraulic section internal components revealed that the section was designed in a way that does not require any soft sealing components to provide oil sealing between the cylinder and piston during oil pressurization when valves are opened. It was only when valves were dismantled that it was found that the oil sealing between cylinder and piston is achieved by a very tight clearance between the two sliding components. The required Original Equipment Manufacturer (OEM) clearance was 0.9 to 1.9 thou, and the max "as found" clearance was close to 4 thou in both actuators. A plan was set to refurbish either the cylinder or the piston in an effort to reduce the internal clearance between the sliding components. A "weld-buildup" which is one the most common and traditional repair methodology was proposed on the internal side of the cylinder. However, due to the sheer difficulties to produce uniform manual weld on the inside surface of the cylinder, in addition to the risk of cracks formation and propagation caused by welding on cast iron were two main reasons to discard such repair proposal. Since the objective was to decrease the clearance between the cylinder and piston it was evaluated that using a chrome plating repair methodology on the outside diameter of the piston in attempt to increase its overall outside diameter while maintaining the inside diameter of the piston. It was concluded that the use of chrome plating will produce far superior outcomes than that of the use of traditional weld-build up repair alternative.