The rotating shaft of a centrifugal pump enters the pump casing at the rear end through a stuffing box or seal chamber. The reason to use a mechanical seal in a pump is to stop liquid from escaping to the atmosphere along the rotating shaft. Therefore, selecting the correct mechanical seal for each pump application is critical to safety and reliability.
Mechanical seal failure is one of the most common causes of pump failure. This blog will discuss installing a mechanical seal in your centrifugal pump without causing downtime.
The Function Of Mechanical Seal In A Pump
A mechanical seal requires contact between two particularly smooth and apartment (lapped) seal faces, and one rotates with the shaft and the other fixed to the housing. Secondary seals secure these seal encounters to their mounts (O-rings or gaskets).
The sliding surfaces are mechanically loaded and flexible to contact and move to compensate for static and dynamic misalignment and wear. The seal faces require fluid lubrication and degrade this fluid when it attempts to leak through the seal faces, resulting in low vapor leakage.
Things To Consider When Replacing Mechanical Seals In Centrifugal Pumps
mechanical seal can be installed efficiently and quickly if you follow these ten simple steps.
Turn The Power Off
The first step is to stop the centrifugal motor. Next, turn off the primary power source and ensure the machine cannot be restarted.
Remove The Liquid From The Pump
Close the inlet and outlet shut-off valves, then remove the housing drain plug to drain the pump housing.
Disconnect The Centrifugal Pump
Remove the spacer in the pump coupling if your pump is a “back-pull-out” design. Then loosen the housing screws and slide the pump out of the housing. The mechanical seal is now accessible without removing the casing from the inlet and outlet pipes. If the pump is not “back-pull-out,” you must remove the entire pump after releasing the connection between the pump and the motor shaft.
If the pump is close-coupled (the pump uses the motor shaft as the shaft, and the motor is attached directly to the back of the pump), the complete pump/motor must be removed. Next, remove the housing screws and the housing itself.
Remove The Impeller
On the pump shaft, the mechanical seal is located behind the impeller. Impellers are either screwed or bolted onto the shaft. To remove a bolt-on impeller from the shaft, hold it with a wrench and turn the impeller clockwise until it is completely unscrewed. To remove a bolt-on impeller, hold the shaft and remove the bolt.
Detach The Seal
The spinning and stationary sealing pieces can now be accessed directly. Set screws are commonly used to secure the spinning parts to the shaft. First, the set screw should be removed, and pulling off the rotating parts of the seal should be done. Next, remove the stationary component of the seal from the housing or seal chamber bore.
Replace The Seal
Now it’s time to install a new mechanical seal on the shaft. First, carefully slide the components of the replacement seal onto the shaft. Then, press the stationary component into the housing or seal chamber bore using a new O-ring or seal material. Finally, follow the instructions to reattach the rotating element to the shaft properly. This is an important step.
Important Note
Mechanical seals should permanently be installed in a clean working environment. Do not touch the front of the seal faces; they are sensitive to body oils and may not function properly if connected. Keep the seal in its original packaging until you can use it.
Install The Impeller
Hold the shaft with the wrench while screwing the impeller onto the pump shaft with a new impeller O-ring or seal. Instead, use the impeller bolt and a new O-ring or seal to secure the impeller to the end of the shaft.
Use The Original Casing Bolt To Reconnect Casing
Slide the rear extension component against the installed housing and bolt it in place for back extension designs. After step 9, check the alignment of the pump. Next, reassemble the casing using the casing bolts for close-coupled or non-rear extension systems. Finally, tighten the casing bolts according to the pump’s Installation, Operation, and Maintenance (IOM) manual.
Reconnect The Pump
For rear extension designs, only reattach the spacer to the existing coupling hubs along the pump and motor shafts and reattach the back extension design mounting feet to the pump base plate. Adjust the pump and motor. Reinstall the pump and, for close-coupled pumps, reconnect the inlet and outlet piping. Replace the pump on the base plate, reconnect the inlet and outlet pipes, screw the pump back onto the base plate, reconnect the coupling, and then realign the pump and motor for non-reverse-extension pump and motor designs.
Restart The Machine
Check that the pump casing is refilled by opening the shut-off valves at the inlet and outlet. Specific pump designs require venting; therefore, refer to the IOM of your pump. Before operating the pump, it is advisable to check the motor rotation to ensure it is correct. This is done before attaching the pump and motor couplings. Check the rotation of the motor by bumping it. The pump is now ready to start when you reconnect it.
Take Proper Safety Precautions
Always read the safety precautions in the pump operating manual. Always use the pump instruction manual when working on the pump. Install the mechanical seals according to the manufacturer’s instructions. Make sure the pump and motor are correctly aligned. Mechanical seal failure can be caused by misalignment.
Conclusion
Mechanical seals play a crucial role in the proper functioning of centrifugal pumps. They are designed to prevent leakage of the pumped liquid and protect the pump from damage due to contamination or loss of lubricant. Mechanical seals have replaced traditional packing seals due to their superior performance and reliability.
Several types of mechanical seals are available on the market, each with advantages and limitations. The appropriate seal selection depends on the pump’s operating conditions, the pumped liquid’s characteristics, and the application requirements.
Proper installation, operation, and maintenance of mechanical seals are critical to their long-term performance and reliability. Failure to follow the manufacturer’s guidelines can result in premature seal failure, costly downtime, and safety hazards.
