Case Study: Impact of Varying Mitered Suction Elbows on Large Wastewater Pumps

Mitered elbows are commonly used for suction inlets on large vertical raw waste water pumps. While information exists regarding the appropriate number of miters or maximum angles to prevent flow separation, little information is available in regards to the impacts on the approach flow conditions into the impeller when used in pump suction piping. Pump manufacturers commonly use mitered elbows in dry-pit and occasionally in wet pit pump applications. Each manufacturer has a "typical" design with a specific number of miters that is used across a wide range of elbow sizes and turning radii. Experience has shown that in some applications, an insufficient miter design can lead to severe instability of the approach flow into the impeller. Limited design guidance is available to evaluate the impact of mitered elbow design for varying flow rates and suction sizes. The Hydraulic Institute Standard does not address mitered suction elbows mounted directly on a pump suction. This paper presents the results of a physical model study which was conducted to evaluate an existing raw wastewater pumps station housing five large (40,000 gpm) pumps. The existing station has exhibited a number of issues over the years and when a sixth pump was added, a model study was conducted to evaluate the approach flow conditions. Testing indicated that the mitered suction inlets were causing excessive flow separation and significant velocity variations at the pump inlet. Test results are presented which outline the impact of varying the miter design on pump inlet conditions.