Water and wastewater treatment is not a glamorous or popular topic. However, it is a critical part of our everyday lives that is often taken for granted. For example, the last thing on anyone’s mind during a major sporting event, such as the Super Bowl, is water and wastewater. During halftime, though, thousands of treatment facilities handle the demand when millions of people use their facilities at the same time.
Municipal treatment facilities are sophisticated operations that use chemical and biological processes to contain and convert dangerous waste into an environmentally safe and often reusable material. Since most of these facilities are operated and funded by a municipal governing body, cost control, efficiency and reliability are top priorities. The review of common applications, rotating equipment and sealing options found in a typical treatment facility is important.
Like most process facilities, wastewater treatment plants have a variety of rotating equipment to handle the volume and different fluids being processed. These include centrifugal pumps (vertical and horizontal), submersible pumps, progressive cavity pumps and large axial flow pumps. Sealing options that provide reliable and economical solutions to contain the fluids being pumped are available for each type. These include designs for the wide range of pump sizes used in treatment facilities.
Sealing Options—Compression Packing or Mechanical Seal
The most common sealing option is compression packing. In addition to providing adequate leakage control, it is economical and provides additional bearing support to the shaft. However, it experiences wear and requires regular adjustments and a clean water flush. Further, it can wear the shaft sleeve that it seals upon.
Image 1. Typical components of a bellows seal.
Mechanical seals are also widely used and provide reliable, safe and economical containment of pumped liquids. Different seal types are used depending on the service conditions and fluid character. Component seals—such as the elastomer bellows seal (see Image 1)—are used extensively and have demonstrated excellent reliability in many applications within a treatment plant.
Cartridge, pusher type seals with stationary springs that are installed out of the pumped fluid are effective for handling the abrasive solids and are simple to install and maintain. The seal type or design almost becomes an owner/operator preference.
A solution growing in popularity and preferred during the past decade is the split seal. Some of the equipment found in a treatment plant is very large, creating unique challenges to installing and maintaining the seal. The development of split seal technology has made installation much easier and faster, especially with between bearing horizontal pumps and large pumps. The maintenance of large pumps typically requires special lifting equipment, and replacing a typical seal can take many hours.
Split seals significantly reduce the time needed for installation or replacement and can eliminate the need for special tools or lifting equipment. The bottom line may be a significant reduction in life-cycle cost of the equipment. With split seal technology, seal installation is just like the installation of compression packing, except the leakage is decreased, and the friction on and the wear of the shaft sleeve is eliminated.
Feedwater—Handling Solids and Abrasives
Wastewater treatment facilities process solids-containing liquid from a wide range of sources. Typically, ambient temperatures prevail and large volumes of liquid and every kind of solid imaginable are processed. The amount of solids at any given time will be a guide to the optimum mechanical seal arrangement. However, the design will almost always include hard faces (silicon carbide) to handle the abrasives that are present.
Since water is the base fluid, stainless steel is the most common and acceptable metallic component of the seal for most applications. However, some applications with excessively high or low pH or those in which other corrosives are present might require a higher base metallurgy. The abrasives are the most common and challenging part of wastewater applications.
The amount of solids present, typically defined as a weight percentage of the fluid being handled, will drive the decision to use a single or dual seal arrangement. The typical single seal with two hard faces can handle as much as 10 percent solids when installed in a big bore seal chamber without flushing or in conjunction with API Plan 02. In a pump that is equipped with a standard bore seal chamber, the typical single seal is limited to a few percent solids unless water is injected into the seal chamber per API Plan 32. A throat bushing in the bottom of the seal chamber should be used with API Plan 32 to minimize water consumption while providing a clean environment for the seal. Treated water is commonly used as a flush medium because it is readily available and does not require removal later.
For solid concentrations above 10 percent, single seals typically can be used, but an external flush will be imperative. If an external water flush is not desirable or available, dual pressurized seals should be considered for high solid concentrations. The most common flush plan for dual seals is API Plan 54. API Plans 53 A and B are rarely used. Single mechanical seals are usually preferred over dual seals because of the lower initial cost and simplicity of operation and maintenance.