Mechanical Seal Design Protects Submersible Pumps from Abrasive Wastewater

Pump manufacturing companies have made strides in improving design, efficiency and energy usage in recent years through developments in impeller profile and the use of optimized motors and variable frequency drives. All of these changes enhance end users' bottom lines by reducing energy costs. But operators have more options to improve efficiency and reliability while reducing costs of fluid management. They can, for example, extend the period between necessary maintenance activities and minimize the cost of spare parts required during maintenance. One particular pump application that consistently accrues a high maintenance bill is wastewater movement, especially on a site that lacks pump operation experience. The most convenient and commonly adopted design for this demanding application is a waterproof pump—complete with an upper and lower set of mechanical seals designed to protect the bearings and motor—with a vertical shaft that is submerged in the pumped liquid. Such submersible pumps can be lowered into sumps, storage tanks, storm drains, cesspits, bilge and ballast tanks, rivers, flooded foundations or workings of all kinds to draw the fluid out and move it to an area where it can be processed or disposed of as needed. Two contributory factors have a negative influence on the life of mechanical seals inside submersible pumps.

• These pumps handle a typically contaminated aqueous fluid that has a high solids content with a wide range of particle sizes, from microns to large pieces of masonry, all of which are potentially damaging to mechanical seals.
• The pumps are often neglected, either because they are underwater and out of sight or are being operated by contractors with little knowledge about or training on the requirements of correct pump operation. The pumps frequently run dry once the fluid level has been reduced to the minimum and the impeller is no longer submerged.

Submersible pumps are often regularly repaired. In the case of rental companies, the pumps are repaired after each user and are often recalled for repair during service because of seal failure. Since these pumps have several O-rings that ensure adequate water proofing as well as upper and lower sets of mechanical seals, the inventory of spares required each time one of these pumps is repaired has a relatively high value. The typical design of a submersible pump requires a shorter than standard mechanical seal. This is because this design of submersible pump has two sets of seals mounted in close proximity on the vertical shaft. Because no common design standard exists, each manufacturer designs the sealing chambers according to its own policies. As a result, each pump design has a unique seal chamber that requires a custom mechanical seal designed specifically to fit the space available. This design methodology means that industry standard mechanical seal designs cannot be fitted to the vast majority of submersible pumps. This situation has caused frustration for many pump users in the past. Without the option of a standard, off-the-shelf seal from a third-party manufacturer, repairers had to use the standard original equipment manufacturer (OEM) part instead of specifying an alternative replacement part. A mechanical seal company can provide repair shops with the seals that are needed, rather than producing a range of industry standards and leaving the repair shop to find what is required on its own. Many seal companies offer parts that have a close visual similarity to the original parts, as well as necessary O-rings and any fitting tools. For example, an OEM's seals may come in two basic types with upper and lower seal faces in one seal that is easier to install. However, some of these wearing parts may be composed of low-cost materials and will need frequent replacement. A seal company, on the other hand, could create several seal ranges that fit into the seal chambers without modification to the equipment and with parts made from higher-end materials chosen for a specific application. These customized seals' characteristics could help prevent premature seal failures, especially in an arduous application requiring submersible pump service.