Select the Right Centrifugal Pump

Consider a wide variety of design and other issues.

By Jason G. Laws, Gulbrandsen Technologies

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Elastomeric O-ring selection also is important. Strive for a balance between chemical resistance and elastic memory. Typically, higher chemical resistance means lower elastic memory. O-rings with low elastic memory can set, which can lead to premature failure of the secondary seal and leaks.

Always use springs made of a high-nickel alloy that doesn't suffer from chloride stress cracking. Stainless steel springs can fracture from chloride stress cracking — and chlorides are everywhere. The cost difference between stainless steel and high-nickel alloy springs in seal service is negligible in the overall seal price.

API VERSUS ANSI
Now, let's consider the difference between API and ANSI frame pumps. API units cost about twice as much as ANSI ones and have lead times that can approach 22 weeks. However, spending more for an API pump often makes good economic sense. API pumps are more rugged in every aspect of design compared to ANSI ones (Figure 3). The shafts have smaller stiffness ratios, the bearings are larger and the stuffing box can accommodate larger seals. Preventing one or two seal failures and the accompanying cost of unplanned maintenance (labor, parts, discarded material, lost production) can more than make up for the price differential.

Another API benefit is the center mount of the volute to its mounting base. This forestalls thermal-growth-induced pipe strain at the flange of the pump.

In contrast, ANSI frame pump feet are attached to the bottom of the volute. Thermal expansion will cause the volute to rise in relation to the pipe and its previous alignment. This will induce pipe strain, which can lead to premature seal and bearing failure.

There are three ways to thermally align ANSI pumps:

1. Offset the "cold" alignment to account for the process temperature.

2. Raise the pump to process temperature, shut down quickly and "hot" align. (This doesn't obviate an initial cold alignment.) If the pump operates over a wide range of temperatures, you may not find a good temperature for hot alignment.

3. Use a C-Frame adapter (D-Frame in Europe) (Figure 4). It will eliminate thermal growth problems and the need for laser alignment. I urge you to use them on all new ANSI pumps and to retrofit existing pumps at your earliest opportunity. I have never regretted the changeover.

A WARNING
Good bearings also play a crucial part. Many foreign "bootleg" bearing companies exist. Their products may carry trusted companies' logos but are low quality knockoffs, like fake Rolex watches.

Beware of low cost discount-label pump vendors, too. The cost difference usually comes from somewhere (L3/D4, bearing selection, etc.). If so, you haven't found a bargain. You've found a costly headache.

Remember, make your selection based on lifecycle cost, not price.


JASON G. LAWS is plant manager for Gulbrandsen Technologies, La Porte, Texas. E-mail him at jlaws@gulbrandsen.com.

References
Bloch, H., and Budris, A., "Pump User's Handbook: Life Extension," 3rd ed., Fairmont Press, Lilburn, Ga. (2010).
Karassik, I., Messina, J., Cooper, P., and Heald, C., "Pump Handbook," 4th ed., McGraw Hill, New York (2008).
Mackay, R., "The Practical Pumping Handbook," Elsevier, Waltham, Mass. (2004).
McNally, W., "Centrifugal Pump & Mechanical Seals Manual," McNally Institute, Dade City, Fla. (2008).

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