Pipeline construction during the period included installation of mainline valves. A total of 175 valves are being installed on the pipeline. This is one of 80 check valves, which “checks” or prevents back flow of oil during the pipeline’s operation. When oil is flowing, a clapper inside the valve swings down into a closed position, preventing oil from flowing in the opposite direction., 1976-07, PCA0002-2a-24, Folder: PCA0002-2a. Alyeska Pipeline Service Company : Trans-Alaska Pipeline Construction Collection, 1976-1977, PCA 2. Alaska State Library - Historical Collections Finding Aids. https://alaska.libraryhost.com/repositories/2/archival_objects/17443 Accessed February 22, 2025.