Select the Right Centrifugal Pump

Consider a wide variety of design and other issues.

By Jason G. Laws, Gulbrandsen Technologies

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As a rule of thumb, a typical impeller-to-cutwater clearance is 4% of the actual wheel diameter for wheels under 14 inches, and 6% for larger wheels. Improper impeller/cutwater clearance can damage the pump and will degrade performance.

SEAL SELECTION
Someone at a plant invariably asks: "When environmental regulations and safety don't require the use of seals, why not just use packing?" After all, packing is cheap and easy to use.

However, properly tightened packing draws six times the power of a mechanical seal. Proper tightening requires a light feel of hand and experience. So, most packing is over-tightened. This causes excessive power requirements, and damaged shafts or shaft sleeves that together can waste thousands of dollars per year per pump in electricity and consumables (i.e., sleeves, lip seals, packing, labor, lost production).

A properly selected, installed and operated seal will last until a seal face wears out. This will save thousands of dollars over the life of the seal, even if the old packing was well maintained.

What you need from a seal vendor is progressive cutting-edge designs and responsive technical support. Often the brand isn't as important as the seal representative. Also, always maintain two seal vendors at any one site to service your needs.

Having 24-hour turnaround for seal repair isn't nearly as important as understanding why the seal failed and implementing corrective actions to prevent future failures. Seals always fail for a reason. So, strive to understand and correct all failures; if you don't perform a failure analysis, you'll be changing the same seal very soon. Lack of diligence can lead to unnecessary and expensive downtime.

Seals should last until the sacrificial face has worn out. Consider a seal leaking for any other reason as a premature failure.

Only use cartridge or cassette seals. They are relatively easy to install. Avoid stackable seals. They require the astute hand of an accomplished millwright.

Seal face selection is important. Choose seal faces made of monolithic materials. Pressed faces in metal retainers warp when heated. The difference in thermal expansion between the seal face and the metal retaining ring causes the faces to open and leads to premature seal failure.

When possible, pair sintered silicon carbide (SSC) and sintered carbon (SC) faces. Always specify the highest quality carbon available. This pairing offers the lowest coefficient of friction, which translates to the least heat generation. Again, heat is the enemy.

Using two SSC faces works well when required. The pairing has a higher coefficient of friction than SSC versus SC but provides better mechanical protection from erosion if hard material gets between the two faces. Solids can enter when the seal faces open (e.g., due to cavitation, misalignment and running to dead head), and cause "phonographing" on a face.

A pump that is installed and operated properly will not open seal faces. If inspection shows seal faces have opened, review the installation and operation of the pump to determine why. If SSC/SC faces can't provide sufficient mean time between failures, change to SSC/SSC.

Tungsten carbide (TC), another very hard material, is required from time to time. However, TC/SC face pairings have a higher coefficient of friction and cost slightly more than SSC. So, avoid them unless mandated by a failure analysis.

To extend seal life, specify a stuffing box that can accommodate the largest seal a vendor offers. The greater surface area of the seal faces dissipates heat better, so they will run cooler. Remember, heat is the enemy.

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