Because of the substantial concentrations of solids that these pumps handle, end users must understand the available sealing options.
08/27/2015
Chopper pumps are designed with an element that “chops” solids entering the pump suction, allowing them to pass through the volute. This mechanism prevents clogging of the impellers, which can cause dynamic imbalance, poor hydraulic performance, and what is commonly called “dewatering” or “ragging out.” These terms refer to the condition of flow interruption caused by the accumulation of fibrous material or solids in the volute or impeller. While they meet the unique requirements of today’s wastewater flow streams, chopper pumps are often difficult to seal because many are submerged in fluids that contain substantial concentrations of solids.
Sealing Challenges
The sealing approaches of various manufacturers differ significantly. Sealing devices in chopper pumps are highly integrated, which means the equipment manufacturer designs the seal to act as part of the equipment. Aftermarket seal designs are rare and may not consider the key elements of the equipment’s overall design. The difficult location and limited space in this equipment usually prevent the use of traditional aftermarket seal approaches. For example, barrier fluids provided by reservoir tanks or clean flush liquids are not possible in these applications. As a result, equipment manufacturers have developed alternate strategies to seal in this difficult environment.Alternative Solutions
Manufacturers typically use three main approaches to seal this difficult flow and prevent contamination of the equipment’s bearing lubricant.- Approach 1: A single-face seal with hard faces mounted below a reservoir of lubricating oil but above a chopping element.
- Approach 2: A single-face seal with hard faces mounted behind an impeller with an integral quench port. This seal is contained by a secondary seal used to remove accumulated solids inside the seal. This type is described by the American National Standards Institute (ANSI) as Piping Plan 62. It may be described as a flushless mechanical seal.
- Approach 3: A dual seal that uses the bearing lubricant as a buffer fluid. Hydrodynamic elements may be incorporated into the seal faces to discourage contamination of the bearing lubricant.