How to replace the mechanical seal of a centrifugal pump?

Replacing the mechanical seal of a centrifugal pump is crucial for ensuring optimal pump performance and maximum service life. Several warning signs indicate that it may be time to replace a failed mechanical seal. Fortunately, the mechanical seal replacement process is relatively simple.

• When to replace the mechanical seal

Mechanical seals are among the most critical components of a centrifugal pump – they prevent leakage of pumped liquid or air. In a standard centrifugal pump assembly, the mechanical seal sits between the pump housing and the impeller, allowing the impeller to rotate without allowing the pumped liquid to leak. Like any mechanical component, seals wear out and eventually need replacement. When a centrifugal pump struggles to maintain the correct pressure, seal leakage is often part of the problem. These failures may not immediately manifest as seal-related issues. For example, the pump might be consuming more liquid than normal – an early sign that liquid is leaking somewhere, such as at the seal.

• Centrifugal pump seal replacement process

Mechanical seal replacement typically follows a standard procedure, although each pump may have slightly different requirements. The following are the standard steps to follow:

1. Pump Preparation:

Turn off and disconnect the pump power (to prevent accidental pump start-up) to begin preparing to disassemble the pump. Additionally, disconnect the pump from the liquid – ensure the pump inlet and outlet valves are securely closed, then drain the liquid from the pump casing.

2. Pump Disassembly:

This stage varies depending on the pump. Many modern centrifugal pumps use a pull-back design, in which the mechanical seal can be accessed simply by removing the bearing assembly without removing the pump casing from the inlet and outlet lines – remove the seal gland and the fasteners (gaskets and bolts) connecting the seal chamber to the seal to remove the mechanical seal from the pump. However, some centrifugal pumps do not use this easily accessible design. For these pumps, the pump and its casing must be completely disconnected from the inlet and outlet lines and/or the core must be removed before removing the bearing assembly to reach the seal. In some pumps (such as certain vertical pumps), the motor may also need to be removed. Regardless of whether the pump casing needs to be disconnected, for some pumps, the impeller needs to be removed to replace the mechanical seal.

3. Mechanical Seal Replacement

At this stage, you may see one set of seals on the pump shaft and housing (e.g., cantilever and vertical suspension pumps), or two sets of seals (e.g., pumps with end supports). Nowadays, many centrifugal pumps use cartridge mechanical seals, which are very easy to replace. Remove the seal gland fasteners, gently push the seal gland away from the sealing cavity using the set screws, and then carefully remove the old seal. Once the old seal is removed, the new seal can be installed. Remember that a new O-ring or gasket is required first. During installation, pay close attention to the specific conditions of the new seal, such as cleanliness and surface quality. Caution: Touching the sealing surface with your hands may introduce grease or debris onto the sensitive sealing surface, which could damage the new seal. Also, always carefully check the instructions for the specific pump used for the rotary seal. These pumps often have different installation requirements.

4. Pump Reassembly

For pumps that require impeller removal to replace the mechanical seal, first reinstall the impeller onto the pump shaft. Similar to new seals, new O-rings or washers should be used when reinstalling the impeller. Once the impeller is in place, the pump cartridge can be reinstalled into the pump housing. For pumps removed from the piping, the pump inlet and outlet flanges can be reconnected to the inlet and outlet pipes, and then the coupling can be installed. During pump reinstallation, patiently align the pump and motor – this step may take some time. Once everything is correctly in place, tighten the pump housing with bolts and reconnect any other bolts or screws as needed.

5. Restart the pump