This is what the inside of a GE/Alstom HRSG looks like. The Chrome Moly pipe in this photo holds water & steam on the inside and sees temperatures up to 1200F on the outside. The pipes here grow up to 9″ in length from cold to hot. When they’re hot they protrude further out the bottom of the HRSG. Every pipe penetration has an expansion joint on it to account for this growth and movement.