O-rings have been used for many years to energize u-cup style seals with great success. Ever-increasing application demands in terms of pressure and temperature, however, have made o-ring energizers less practical than other (metal) type energizers. The primary problem with using o-rings in these HP-HT applications is the fact that the o-ring, over time, will take a compression set and lose its ability to energize the seal effectively. The illustrations to the right demonstrate this phenomenon. Seal 1 shows the o-ring as installed while Seal 2 shows the o-ring after it has been in service. The top and bottom surfaces of the o-ring are flattened out and the o-ring can no longer energize the seal. This will result in seal leakage.