Ralph Merullo has been in the rotating equipment industry for more than 25 years—designing, manufacturing and specifying mechanical sealing devices. He is manager of applications engineering and sales support at A.W. Chesterton Company in Groveland, Massachusetts.
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.
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.
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.
Designers select seal face materials based on their inherent lubricity, abrasion resistance, corrosion resistance, machinability and cost. Seal designers may select a softer material for the narrow face and a harder material for the wide face to allow the narrow face to “lap in” as it runs. Lapping in is defined as wear of the softer material as it rubs against the harder material to obtain a flatness that is better than when the equipment was originally manufactured.
As the mating surfaces rub, they become more flat in clean fluids, which increases their ability to seal and minimizes visible leakage. In dirty fluids, however, solids can become embedded in a soft seal face material, which can grind a groove in the wide seal face and allow visible leakage.
To avoid this phenomenon, seal designers will select hard faces. Common seal face materials include tungsten carbide or silicon carbide, which are technically ceramic in nature. Once lapped by the manufacturer, they will maintain their flatness. Lapping in of a hard-face material against a mating hard seal face takes many days of service. Lapping in of a softer material, on the other hand, takes only hours.
A quench fluid is a low-pressure liquid introduced between the rotating shaft or sleeve and the stationary seal face (also known as the seat) inside diameter. This fluid will flow in and out of this cavity to remove an accumulation of solids that may occur as the sealed liquid lubricates the narrow space between the seal faces. Once the sealed fluid evaporates, solids remain and could potentially clog the springs or O-ring grooves. A spring loaded lip seal could prevent this fluid from leaking from the back of the mechanical seal.
A dual seal uses two sets of seal faces with a clean fluid sealed between them. The fluid can be pressurized above the barrier fluid or below the actual sealed fluid known as the buffer fluid. Chopper pump dual mechanical seals typically use the bearing lubricant as a buffer fluid to protect and lubricate the outboard seal. The fluid acts to buffer and prevent leakage in order to avoid immediate seal failure.
One especially innovative approach to sealing chopper pumps is the incorporation of hydrodynamic grooves etched into a seal face. These grooves use the energy of shaft rotation to expel solids and sealed fluid from between the seal faces. They are described by manufacturers as “micro-pumping grooves” because the depth of the groove may be on the scale of ten-thousands of an inch. The shaft rotates the grooves, which then channel leakage back in the direction of the sealed fluid.
The Right Selection
All approaches will deliver different levels of performance depending on the concentration and hardness of the solids in the flow stream. If the pumped fluid often causes dewatering, then the dual approach may be more successful because some lubrication is provided by the buffer fluid. If the solids in the flow stream will crystallize inside the seal, then the flushless or dual seal approach may be successful. If the flow stream includes solids that are easily broken down and do not crystalize or cause dry running, then a conventional seal with hard faces mounted below a reservoir of oil should be successful.
The key is to understand the properties of the fluid and how it will behave after the chopper blades break it down in the volute. This knowledge will lead to a fact-based selection of the ideal sealing solution.