Gas Compression Magazine

October 2016

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A client sent photos of a damaged compressor. The piston rod had failed several threads inside the crosshead. The compressor had experienced a prior failure of a variable vol- ume clearance pocket (VVCP). Causal analysis on the first fail- ure had focused on the new VVCP design. Since both failures were on the first stage, liquid slugging was now suspected. The process engineers insisted, as they had after the first failure, that it couldn't be liquids. The process control data showed no upsets in the upstream process prior to the rod failure. When the rod broke inside the crosshead, the piston and rod were pushed to the head end of the cylinder where they remained. Figure 1 shows the piston and rod at the end of the outward stroke and after removal of loose debris. The crosshead jamb nut was not a traditional hex nut. Rather, it used eight small compression bolts pressing on a thick bear- ing washer resting against the face of the crosshead. When the piston and rod came to rest against the outer head and the crosshead retracted, the bearing washer was freed. It dropped into the crosshead guide but remained vertical. The washer was large enough that it covered the broken end of the piston rod. When the crosshead extended, the washer was between the cavity in the crosshead and the end of the piston rod that needed to re-occupy the cavity. The resulting impacts sheared the washer before the compressor stopped on high vibration (Figure 2). The damage to the compressor was extensive. The force of the impacts damaged the piston and VVCP and com- promised the connecting rod and associated bearings. The location of the failure was curious. Piston rods are not supposed to break in the threads. Rod threads are rolled, rather than cut, to forge the grain structure and reduce stress risers. This has been codified in ISO 13707, which is based on API 618, and ISO 13631, based on API RP 11P. 1-4 The former includes the requirement for a polished thread relief area. This relief area is an undercut at the end of the threads and is the smallest cross section on the rod. The small diameter area should act like a fuse. This is where we normally expect to find gross failures and it is where we want them to occur. Figure 3 shows the piston and rod pushed inward against the cross- head. The undercut is visible where the rod enters the jamb nut. Note also the gap between the jamb nut and the cross- head indicating the thickness of the washer that was sheared. A failure at this fuse location should generate very little, if any, collateral damage. The piston and rod should be pushed to gascompressionmagazine.com | OCTOBER 2016 6 CASE STUDIES By STeVen B. TOdARO Why did iT BREak in ThE WROng plaCE? Figure 1: The piston and rod are shown here at the end of the outward stroke, after removal of loose debris. Figure 2: The damage to the compressor was extensive. The force of the impacts damaged the piston and VVCP and compromised the connecting rod and associated bearings. the outer head (the failure occurring in tension) where they remain. The crosshead will continue to reciprocate until the imbalance causes a vibration shutdown. The only interference

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