Gene Vogel
EASA Pump & Vibration Specialist
For many larger centrifugal pumps, there are options for installing “component” or “cartridge” mechanical seals. Understanding the advantages and limitations of each will allow you to recommend the best solution for customer applications.
There are many suppliers for both types of seals, and a wealth of detailed information is available from any competent supplier for a specific application. Here we will try to present the basics.
Mechanical seals have two elements: a rotating element and a stationary element. Each element has a precision smooth lapped face which mates against the other element face. See Figure 1. The condition of the faces and the pressure applied to them are the primary factors in how well the seal performs. Other key factors are vibration, heat and the pumpage characteristics.
Component mechanical seal
Standard mechanical seals are generally component seals. When you open the box for a replacement component seal, you have ‘components’ – seal faces, holding brackets, O-rings, boots, etc. See Figure 2. The pump technician needs the knowledge and skills to properly install and adjust the seal components. Incorrect installation practices are a common cause of mechanical pump seal failure. The seal faces may not be properly seated on the shaft or in the seal housing, resulting in misaligned faces. The seal tension may not be set properly, or O-rings and elastomers may be damaged by sliding over shaft shoulders, keyways or sharp edges of the seal housing. Often the seal housing provides limited access, so small hands and precision adjustments are needed. While a good experienced pump technician can do a fine job of installing any component seal, the opportunities are there for mistakes.
Cartridge mechanical seals
Cartridge mechanical seals (see Figure 3) use components similar to those in component seals, but the shaft-mounted components are pre-assembled onto a mounting sleeve and the stationary components are pre-assembled into a housing. The sleeve mounts over the shaft and seals to the shaft with an O‑ring. Typically the cartridge seal housing replaces the gland cover plate and seals to the pump housing with a gasket, O‑ring or other elastomer.
Since the components of the cartridge mechanical seal are pre-assembled onto the sleeve and into the cartridge housing, there is no opportunity for mis-installation. An important factor in a successful seal installation is the amount of spring tension applied to the seal faces. With a component seal, this may be set manually by adjusting the installed seal spring length. With cartridge mechanical seals, the spring tension is preset and the rotating and stationary elements are held in alignment by a retaining device. The retaining device is removed after the seal is mounted in place so that the proper tension is assured.
Applications
Cartridge mechanical seals are not a solution for many pump designs. The details of whether a cartridge mechanical seal can be fitted to an application are complex. But a basic concern is whether the seal installs from the wet side or dry side of the seal chamber. Pumps where the seal installs from the wet side, behind the impeller, are generally not candidates for cartridge mechanical seals. In addition, submersible pumps, which are most often fitted with dual component seals, cannot be converted to cartridge mechanical seals, because the seals install from the wet side of the pump.
Decisions
There are two main concerns when deciding between a component and cartridge seal: ease of installation and cost. In the service center, ease of installation may have little value, since presumably the work is done by a competent pump technician under good working conditions. So the concern is for subsequent seal replacement at the customer’s facility, likely by their technicians. If the repair is being quoted in a price competitive situation, a component seal would be the obvious choice. Cartridge mechanical seals may cost 2 to 3 times more than a component seal. But if the customer wants the most cost-effective, long-term solution, and expects that maintaining the pump may include in-service seal replacement, the cartridge seal can be shown to be worth the higher initial investment.
The savings come from lower labor costs and less production downtime when subsequent seal replacement is needed. Further savings can be projected from eliminating seal failures that result from improper installation of component seals by technicians with limited knowledge and skill in seal installation, and less than optimum installation conditions.
Other decision factors
There are many pumping environments and applications that are tough on seals: high temperature, high pressure, foul pumpage laden with abrasives and lots of others. For many of the tough applications, a double seal provides a very effective solution. Double 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. But it would be very challenging to redesign a single seal pump to accept dual component seals. Cartridge mechanical seals can be a solution. The precision components of a cartridge mechanical seal can be designed so that dual seals can easily fit in the space where a single component seal had been. See Figure 4.
Impeller clearance adjustment
Some pumps, particularly those with semi-open impellers, require periodic adjustment of the impeller face clearance. The adjustment is often achieved by moving the pump shaft axially. This affects the tension on the seal as the shaft is moved. Resetting the tension on a component seal would require significant disassembly of the pump. However, most cartridge mechanical seals are provided with retaining devices, which can be reinstalled, allowing easy resetting of the seal after the impeller face clearance has been adjusted.
Split cartridge mechanical seals
Replacing a mechanical pump seal, component or cartridge, usually requires disassembly of the pump. One way around having to disassemble the pump is to use a “split” seal. The split seal faces and other circumferential components are split in two halves so they can be assembled on the pump without disassembling the pump. Since each circumferential component has to be properly fitted and sealed back into a single piece, installation of split seals can be problematic and requires a high degree of technical ability. Any mistakes and the seal – won’t. Recent developments in seal technology have made available split cartridge mechanical seals, which greatly simplify the installation of split seals. The decision on choosing a split cartridge mechanical seal vs. other options is again ease of installation vs. cost. If the application has historically required in-service seal replacement, and pump disassembly is difficult, the additional cost may well be justified.
Summary
If a service center is working with a customer to find the most cost effective, long-term solution to pump maintenance, and in-service seal replacement is expected, a cartridge mechanical seal will likely be a good choice. Also, when converting a pump from a packing type to mechanical seal, a cartridge mechanical seal offers an excellent means of engineering the change. For quotes where low initial cost is an important concern, component seals – and a well trained pump technician – provide the best option.
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