Check Valves Provide Diffusion Value on Sea Floor

Introduction

The Water Authority of Istanbul, which owns a biological wastewater treatment plant, serves a population of about 15 million people in the city of Istanbul, Turkey. To meet stricter environmental regulations, the Water Authority approached Marine Consultants Group (MCG) in the spring of 2017 to find a suitable product to provide adequate diffusion for their marine wastewater outfall.

MCG worked with Proco to design and engineer duckbill diffusers to accommodate outfall sewage from the treatment plant through an 87 in. diameter steel pipe. The pipe extends 1.35 miles (2230 meters) along the bottom of the ocean and discharges at about 154 ft below sea level.

There were several challenges Proco needed to keep in mind for this project. One concern was that diffusers and duckbill check valves might be damaged by passing marine vessels. To address this concern, Proco needed to determine the appropriate diffuser length, which in this project was about 741.5 ft at the furthest end of the 1.5-mile runout fall pipe.

Previously, engineers and contractors were often required to blind flange some of the ports so that they would be functional with less flow rates and opened to allow for future maximum flow rates. Maximum flow was calculated to handle requirements with a 20-year future outfall rate. However, the present flow rate will not be functioning at maximum output, as a result, Proco and MCG needed to design appropriately sized check valves to alleviate this concern.

The Process

MCG recommended that the Water Authority use the Proco ProFlex Style 710 duckbill check valves for this project, and each port discharge was equipped with two 10-in. diameter (250 mm) duckbill check valves. 

Duckbill valves are used to prevent back flow in deep marine outfall/diffuser projects. The Proco ProFlex 710 duckbill check valves prevented the need to blind cap some of the ports during the first years of operations. In addition, the use of Proco ProFlex 710 duckbill check valves eliminates the need to blind flange ports since the lines can operate with full efficiency from the initial startup, which decrease maintenance and reduces diver time to add additional valves in the future.

Using duckbill check valves for this project was to prevent saline intrusion into the line when there is no inlet pressure. Different head pressures between seawater and discharge water cause seawater to enter the line when there is low head pressure or when there is no inlet flow. If this back flow is not prevented, the ports may be blocked or plugged, and marine life can enter the lines. As a result, the calculated and engineered dilution ratios will then not be achieved. Duckbill check valves increase the jet velocity and provide a dilution value different than the traditional flap gates, even with low flow rates.

In addition, the Proco ProFlex Style 710 duckbill check valves’ engineered elastomer maintains its memory after a long period of staying open or staying in a closed position, and the valves are 100% barnacle and algae resistant, which is an important feature for a salt water project. Proco’s engineering team also provided full assistance with supplying the head loss calculations.