Piston Rods
Piston rods are designed to resist
the loading due to gas and inertia rod loads and wear in the packing travel
area. All Ariel piston rods have rolled threads to take advantage of the
cold working and reduce the stress concentration of cut threads. The rods
are designed with an undercut section, "fail safe" section.
It is intended that a failure of the piston rod would fail in this section.
A failure at this point allows the piston rod to be supported by the rod
packing, and prevents the escape of gas from the compressor cylinder in
to the distance piece/crosshead guide, preventing further damage.
The standard piston rod material
is ETD 150 or 4150 and is Ion-nitrided
in the packing travel
area. Ion
nitrided ETD 150 and 4150 piston rods have a surface hardness that range
from 57 to 63 Rc at the surface to the base material hardness of 29 Rc
at a depth of 0.008 inches. Tungsten
carbide coating can be provided as an option.
Gases that contain significant
amounts of H2S or CO2 will require piston
rods machined from 17-4PH stainless steel material to reduce the potential
for corrosion, due to the creation of sulfuric or carbonic acids. Piston
rods of 17-4PH (UNS S17400) are
provided in the double H1150 condition with a base hardness of 33 Rc maximum
and chromium nitrided in the packing travel area. Chromium
nitride provides a surface hardness of 2500 Vickers. The
chromium nitride process has a length limitation due to manufacturing
constraints. Longer
stainless steel piston rods will be tungsten carbide coated.
Ariel piston rods have a surface
finish of 8-12 micro-inches rms.
Ariel Corporation Application Manual
14-Jul-98