One approach to eliminating seal problems in the standard centrifugal pump is to install some type of a non-contacting seal and a “repeller”. There are a lot of different designs but they all share the same basic concept:
- As the pump is rotating a secondary impeller (referred to as a repeller) takes over to pump the fluid going to the stuffing box back to the pump discharge. To be effective the repeller must be the same diameter as the impeller.
- If there is packing in the pump the repeller will prevent leakage to the atmosphere.
- If the pump is sealed most designs have the seal faces in contact when the pump is stopped ands then open when the shaft is at operating speed. This opening of the lapped seal faces can be done by centrifugal force or an air system operating an expensive metal bellows.
When asked why the pump engineers felt it was necessary to come up with this complicated concept the answer was always the same, “To restrict leakage and prevent premature wear of the mechanical seal faces”
One look at the mechanical seals that had been removed from conventional centrifugal pumps would have proved that this concept is flawed. There is plenty of wearable carbon face left on 90% of the mechanical seals that were removed when the pump began to leak. A simple inspection would show that for a variety of reasons the faces have opened and solids have penetrated between them causing face damage. These solids penetrate into the softer carbon face where they are trapped. The impregnated carbon face will then act like a grinder causing wear and damage to the hard face.
There are a couple of problems associated with repeller pump designs:
- Most of the designs open the faces on purpose allowing the solids to penetrate between the faces as the pump slows down at shut off.
- If the stuffing box is packed, the repeller can pull air into the product
All is not bad however. To install these special mechanical seals most manufacturers need an oversized stuffing box. This means that you can throw these “funny seals” away and there is usually plenty of room to install a real seal with plenty of clearance.
To design a good seal for pumps or any other piece of rotating equipment you only have to follow a couple of simple rules:
- Select designs that generate the least amount of heat.
- Design as many non-clogging features into the seal, as you can. Removing the springs from the sealed fluid is a good example
- Keep the lapped seal faces together. Good seals are lapped to less than a micron of flatness meaning that dirt or solids cannot penetrate until they open.
- Choose seal designs that have a short axial length and install them as close to the bearing as you can.
- Choose designs that are easy to install. Split seals and cartridge seals are the obvious choices.
- If the product is dangerous use back up seals.
- When needed, apply the correct environmental control.
- Be careful to select materials that will be chemically compatible with the fluid you are sealing and any cleaners or solvents that might be circulated throughout the system.