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A cartridge seal design should be used whenever possible to ensure proper seal assembly and optimum mechanical seal MTBFs. The use of cartridge mechanical seals significantly minimizes installation errors. While cartridge seals are more expensive than component seals, the additional cost can be justified in many cases based on the material costs and revenue losses of component seals.
Mechanical seals are typically categorized into two major types; pusher and non-pusher. A pusher-type seal consists of a primary sealing ring assembled with an ‘O’ ring and springs (can be one or multiple). The purpose of this is to force the sealing liquid across the face and keep it from leaking to the ID (atmospheric) side of the seal.
The dynamic ‘O’ ring is designed to move axially (be pushed) along the shaft or sleeve (in a cartridge seal). The surface underneath the dynamic ‘O’ ring must therefore be very smooth (<32 RMS) to allow for axial movement. If solids are abundant in the sealing fluid, they can build up on the ‘O’ ring and prevent this axial movement (hang up).
A non-pusher type seal consists of a bellows assembly. The bellows is a component that acts as both the load element (like a spring in a pusher type) and a secondary sealing element (like an ‘O’ ring in a pusher type). Because the bellows prevents any leakage to the atmospheric side of the seal, and has a large clearance between itself and the shaft or sleeve, it can move freely in the axial direction (no dynamic ‘O’ ring), reducing the potential for hang up.
Pusher type seals are used more commonly in low S.G. (<0.7) services. In a bellows seal, the secondary sealing element (bellows) generally has a larger diameter than a pusher seal, therefore the closing area is less. Since the closing area is larger and the width of the primary ring face is limited (cannot be too large or it won’t fit in the bellows assembly), the balance ratio cannot be varied as much as in a pusher seal.
With light S.G. fluids, it is important to be able to have a range of balance ratios to control where the fluid will vaporize across the faces. It is for this reason that a pusher type seal is desirable in light S.G. services. Some applications have a S.G. of less than 0.7 and contain solids. In these applications, it is still recommended to use a pusher-type seal, however provisions need to be made to ensure the seal will not hang up in operation.
An advantage for using the bellows seal, apart from being less likely to hang up, is that they typically utilize ‘grafoil’ packing rings as their secondary seals. Grafoil packing rings can withstand temperatures of approx. 425 degrees Celsius (800 degrees Fahrenheit), allowing metal bellows seal to be used in refinery bottoms applications with great success.
Modifying existing component mechanical seals for cartridge seal assemblies, based on a life cycle cost analysis, has resulted in significantly higher seal MTBFs. Many clients have standardized on cartridge seals for all pumps that can accommodate them, justified by a life cycle cost analysis which compares the material, maintenance and lost revenue costs of component seals to the additional costs of cartridge seals.
This best practice has been used since 2000 for new projects, and recommends modifications to cartridge seals for plant ‘bad actor’ seals. This best practice has resulted in seal MTBFs of greater than 48 months, compared with previous MTBFs below 12 months.