The secondary piston rings of the 4M125 compressor are broken in large numbers; please analyze what could be the causes
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In the hydrocracking units of our company, extensive fractures were found in the secondary piston rings of the 4M125 compressors in both units that were taken apart for inspection. The piston rings had been in operation for 6,000 hours since the units started running; the diameter of the secondary cylinder is 390 mm, and the thickness of the piston rings is 19.2 mm, with a wear thickness of 18–18.5 mm – so the degree of wear isn’t very significant. The side clearance during installation is around 25 microns, and the opening of the piston rings is approximately 100 mm. Medium: hydrogen; secondary inlet pressure 3.5–4 MPa, outlet pressure 7.5–8.2 MPa. I hope everyone can help analyze the cause of the fracture.
Wrong, the piston ring opening is about 10 mm
Bend the new piston ring to see if it will break; if it does, it means the polytetrafluoroethylene has not been fully sintered.
It’s difficult to analyze just from these few pictures; it’s not clear whether there is wear on the cylinder liner or on the piston ring grooves. It seems that the carbon content in these rings is too high； So, regarding brittleness, apart from material issues, one can check whether the specific pressure of the piston rings is sufficient. The PTFE-filled piston ring is an elastic open-ring; in its free state, it has an open gap. When inserted into the cylinder, it is forced to close up into a circular shape, with a thermal expansion gap at the cut area. Piston rings possess elasticity; in their free state they fit tightly against the inner wall of the cylinder, and the restraining force acting on them is known as the initial elastic compressive stress. For piston rings to achieve sealing, an initial elastic compressive stress is required in order to create a pressure difference, and this initial elastic compressive stress of the piston rings is closely related to the free opening clearance of the rings. When the compressor is operating normally, the sealing specific pressure increases as the pressure difference between the inside and outside of the cylinder increases; therefore, the piston rings provide a self-sealing effect. From the perspective of sealing alone, the higher the sealing specific pressure, the better. However, if this pressure is too high, it can cause severe wear on the piston rings or cylinder liners, thereby affecting the proper operation of the compressor. For the piston rings of the original design model, the initial spring pressure is given by: PK = AE/(7.07D(D/t–1)3) = 0.035 MPa. In this formula, A represents the free opening size; D is the inner diameter of the cylinder, which is 35 cm; t is the radial thickness of the ring ； E is the elastic modulus of the ring material, 1.5 × 105 MPa; P is the initial elastic pressure of the ring on the cylinder wall, in MPa, with the empirical value for PLICF rings ranging from 0.03 to 0.05 MPa. Calculate the specific pressure of your piston ring by yourself to see if it’s a design issue
Is the fracture occurring in the piston ring that is near the piston face? ? ? ? ?
Plastic piston rings? ? ? What is the operating temperature? ? I think the opening gap is a bit small! ! 1. Give it a try – the opening gap should be larger! ! It is possible that the clearance at the opening is small, and the piston ring is stuck, resulting in an excessive sealing pressure difference for a single piston ring; this in turn causes damage in the areas of the piston ring where its structure is looser ! ! Just my personal opinion! ! 2. Change the formula of the piston rings! !
I think there’s still an issue with the material of the piston rings :)
Tetrafluoroethylene impregnated graphite; too much graphite, resulting in high brittleness after sintering