Mechanical seals consist of a rotating element and a stationary element, each with a lapped, precision-smooth mating face (see Figure 1). Seal performance is determined primarily by the condition of the faces and the pressure applied to them. Other key factors are vibration, heat and pumpage characteristics. Depending on the application and user's needs, different seal types may be appropriate.
Figure 1. Common mechanical seal (Images and graphics courtesy of EASA)
For many larger centrifugal pumps, users have the option of installing either component or cartridge mechanical seals. Understanding the advantages and limitations of each can help determine the best solution for a particular application.
Component Mechanical Seals
Standard mechanical seals are typically component seals. When users order a replacement, they typically receive a box containing seal faces, holding brackets, O-rings, boots and other parts that require the skills of an experienced pump technician to install and adjust properly (see Image 1).
Image 1. Component (left) and cartridge (right) mechanical seals
Incorrect installation and adjustment are common causes of component seal failure. For example, if the seal faces are not properly seated on the shaft or in the seal housing, they will be misaligned. Sliding O-rings and elastomers over shaft shoulders, keyways or sharp edges of the seal housing can also cause damage to these parts and result in incorrect seal tension.
The seal housing often provides limited access, so successful adjustments require precision and accuracy. While an experienced pump technician can successfully install and adjust any component seal, this process provides opportunity for error.
Cartridge Mechanical Seals
Cartridge mechanical seals and component seals use similar components, but the stationary components of cartridge seals are preassembled in a housing, and the rotating components are preassembled on a shaft-mounted sleeve that is sealed with an O-ring. The cartridge seal housing typically replaces the gland cover plate and seals to the pump housing with a gasket, an O-ring or other elastomer. Since cartridge mechanical seal components are preassembled onto the sleeve and into the cartridge housing, errors in parts installation are unlikely.
The amount of spring tension applied to the seal faces is an important factor that affects successful seal installation. On component seals, technicians can set this tension manually by adjusting the length of the installed seal spring. With cartridge mechanical seals, the spring tension is preset. To ensure the proper tension, a retaining device holds the rotating and stationary elements in alignment until after the seal is mounted.
While the details of whether a cartridge mechanical seal can be fitted to an application are complex, one criterion is whether the seal installs from the wet side or dry side of the seal chamber. Pumps with a seal that installs from the wet side, behind the impeller, are generally not candidates for cartridge mechanical seals.
In addition, submersible pumps, which are usually fitted with dual component seals, cannot be converted to cartridge mechanical seals because the seals install from the wet side of the pump.
Component vs. Cartridge
For end users deciding between a component and cartridge seal, the primary considerations are cost and ease of installation. If a competent pump technician services the pump during overhaul under good working conditions, ease of installation may seem like a minor issue. However, the concern will be for subsequent seal replacement during an emergency outage.
Cartridge mechanical seals may cost two to three times component seals, so unless otherwise stated, competitive repair bids are typically for component seals. Despite the higher initial investment, however, a cartridge seal can be a more cost-effective, long-term solution, given the expectation that pump maintenance may require in-service seal replacement. Potential savings accrue from lower labor costs and less production downtime when subsequent seal replacement is needed. Projected savings also include the elimination of seal failures resulting from improper installation of component seals.
Dual seals are effective solutions for many pumping environments and applications that are tough on seals, including high temperatures, high pressures and foul pumpage laden with abrasives. Dual seals have a chamber between the seals into which barrier fluid can be pumped to provide cooling, lubrication and protection from abrasives in the pumpage. While redesigning a single-seal pump to accept dual component seals would be challenging, the precision components of a cartridge mechanical seal can be designed as a dual seal that can easily fit in the same space as a single component seal (see Image 2).
Image 2. A cartridge dual mechanical seal
Some pumps, particularly those with semi-open impellers, require periodic adjustment of the impeller face clearance. Users can make this adjustment by moving the pump shaft axially, which can change the tension on the seal. On a component seal, resetting the seal tension requires significant disassembly of the pump. Most cartridge mechanical seals have retaining devices that can be reinstalled to align the stationary and rotating elements. This makes it easy to reset the seal tension after the impeller face clearance has been adjusted.
Split Cartridge Mechanical Seals
Replacing a mechanical pump seal, component or cartridge usually requires pump disassembly. One way to avoid this is to use a split seal. The faces and other circumferential components are split in half so they can be installed without disassembling the pump. Since each circumferential component must be properly fitted and joined together, installation of split component seals can be problematic and requires a high degree of technical ability. If any mistakes are made, the seal won't work.
Recent developments in seal technology have led to the production of split cartridge mechanical seals, which greatly simplify the installation of split seals. Whether a split cartridge mechanical seal is the best option depends on ease of installation versus cost. The additional cost may be justified if, historically, the application has required in-service seal replacement and if pump disassembly is difficult.
Making the Right Choice
If users are aiming to find the most cost-effective, long-term solution to pump maintenance and they anticipate in-service seal replacement, a cartridge mechanical seal will likely be a good choice. Incorporating a cartridge mechanical seal also allows the conversion from a packing seal to a mechanical seal. When low initial cost is important, component seals—and a well-trained pump technician—are the best option.