The paper presents a graphical method for estimating the excess pressure in penstocks under partial hydraulic hammer. Based on the proposed method, the maximum design load under hydraulic hammer in a highpressure penstock of the Zaramagskaya HPP-1 was determined. Strength of metal shell and crescent rib of the penstock fork was evaluated under a specific load combination typical for hydraulic hammer. Variants of the fork structure reinforcement were considered. Comparison of strength reliability was performed for the variants of steel and composite steelconcrete construction of penstock fork.