Select the Right Centrifugal Pump

Consider a wide variety of design and other issues.

By Jason G. Laws, Gulbrandsen Technologies

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Operating to the left or right of the BEP imposes force loads on the shaft. These unbalanced loads cause shaft deflection, vibration and premature bearing and seal failure. Operating to the far right of the BEP also can induce cavitation.

The most effective way to deal with shaft deflection problems is to increase shaft stiffness, which is inversely proportional to L3/D4. The ratio should be less than 60.

Nearly all pump shafts are made of metal. As the pump runs across the curve, it will deflect if ever so slightly. The modulus of elasticity varies, but not much. So, changing material of construction is of little use to increase rigidity.

Shaft sleeves don't add stiffness to a shaft. They are a weight (force) riding on the shaft just like the wheel. They detract from the shaft's L3/D4 and reduce rigidity. Don't use the diameter of the sleeve in the L3/D4 calculation.

Shaft sleeves came into use for one reason — to combat fretting-induced leaks. Packing and lip seals damaged pump shafts. The sleeve was developed so the shaft didn't have to be thrown away. In the absence of packing it has no value. Actually, having a sleeve can increase the cost of a pump because standard shafts must be machined down to accommodate the sleeve.

No sealed pump should have shaft sleeves. Also, use labyrinth seals instead of lip seals.

Pumps with stiffness ratios below 60 provide resistance to: several types of cavitation-induced seal or bearing failure; shaft deflection from dead head or running out across the pump curve (empty running); and critical-speed-induced seal or bearing failure. Figure 2 shows fretting damage to seal sleeve caused by shaft deflection.

You must consider 1st critical speed, which is the rpm at which shaft vibration increases dramatically. Shaft deflection reduces the 1st critical speed. It normally significantly exceeds 1,750 rpm for most pumps. However, it can be close to the 3,600 rpm at which "high efficiency" centrifugal pumps run. This is one reason to avoid 3,600-rpm-and-higher pumps. They are vulnerable to critical-speed/deflection-induced premature failures. In addition, double-hung pumps have critical speeds half those of single-hung pumps. Avoid them unless absolutely necessary.

In principle 3,600-rpm pumps are more efficient than 1,750-rpm ones. However, typical centrifugal pumps operate at around 60% efficiency. The higher speed pumps maybe are 5 to 10% more efficient. This only is a big deal if you have a bunch of pumps that are running all the time.

Many pumps start and stop frequently; cranking is the biggest power draw. They run across their curves, reducing overall efficiency. Cranking power and increased maintenance costs can overshadow any perceived economy of high speed pumps. Seal faces will wear out eight times faster for every doubling of rpm. Heat is the enemy.

Whenever possible, specify 1,750-rpm or lower pumps (or their equivalent for 50 Hz). Only select high speed pumps for high discharge head, low flow service. Even then, investigate other pumping alternatives.

Consider how impeller clearance is altered. Some designs adjust the clearance from the volute casing backward, others from the stuffing box forward. Standardize on pumps that adjust the same way. Otherwise, you're setting yourself up for inevitable errors by maintenance technicians.

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