A used O ring will show some flattening where it was held in the groove. The question is whether it can still press against the sealing surfaces when pressure and temperature change. If it has lost too much recovery, leakage may show on startup, after cooling, or at the next pressure cycle.
Record the full duty, not just the product temperature. On a food or pharmaceutical line that includes the cleaning fluid, its concentration, contact time, rinse, steam temperature and how often the cycle runs. A material that survives the product may still have trouble with the cleaning cycle.
Keep an unused ring from the same batch if possible. Compare its cross section with the failed ring. Photograph any flattening, cracks, blisters or swelling and measure the groove rather than sizing a replacement from the used ring. Check that the groove gives the right squeeze at assembly and across the working temperature range.
A flatter ring does not by itself prove the compound was wrong. High temperature, excessive squeeze, time in service and contact with the fluid can all affect recovery. Confirm the exact compound and its approval requirements before changing material. A simple like for like replacement can bring the line back today and cause another stoppage later.
Related: EPDM or FFKM for SIP and Steam Duty, FFKM O-Rings and Large Diameter O-Rings.