Preliminary Sea-Chest Sizing from Grating Intake Velocity and Cooling Requirement

Sea-chest sizing is a process that is often based on rules of thumb rather than a set scientific method. This paper aims to create such a method by incorporating real world data and sound engineering principles. Specifically, the use of example species to set a flow velocity that will avoid impingement on fish, comparing engine indicated power to obtain an early estimate for flow rate, and ratio of open area to grating area to size the sea-chest. The main use case of this method is for future Marine Mechanical Design students to size sea-chests on their third-year project vessels. A large part of the Marine Mechanical Design program at the Marine Institute of Memorial University of Newfoundland is creating the third-year design project. This involves taking a previous year’s Naval Architecture third-year project vessel and designing the auxiliary systems for that ship. The sea main and sea chest of the project vessel used in this paper can be seen in figure 1. The students are given structural drawings and minimal information and tasked with making decisions that will turn the bare hull into a working ship. This paper aims to create a preliminary method to size sea-chests for the use of future Marine Mechanical Design students in the creation of their third-year projects. To do this, three key pieces of information are required. The flow velocity through the grating, the flowrate, and the grating open area. A review of current regulations found very little in the way of hard and fast rules about the flow velocity of sea chests. The United States Code of Federal Regulations states: "…each cooling water intake structure at your facility to a maximum through-screen design intake velocity of 0.5 ft/s;". (United States Code of Federal Regulations, 2013, Title 40 Chapter I Subchapter D Part 125.84), however, this regulation is intended for application to cooling water intake structures of civil installations in freshwater bodies rather than for a vessel. Given the limited availability of guidance of velocity through the grating, a suitable metric needed to be determined.