Page 209 - Improving Machinery Reliability

180 Improving Machinery Reliability

ciency should be referenced to the pumpage-to-motor input power ratio, not input
power to the pump rotor, which might not accurately account for eddy current or
other magnetic losses.
Look for experience. Some zero emission pump manufacturers are trying to make
inroads on price alone. Be sure to buy pumps that come with protective instrumenta-
tion. Be aware that reliability and rock-bottom price are often mutually exclusive.
Write a specification or acquire a specification, but by all means invoke a specification.
Pay particular attention to the type of containment shell proposed by a particular
vendor (See Table 3-15 for a comparison of typical materials.) Verify vendor experi-
ence and suitability for fluid conditions.

Table 3-15
Comparison of Containment Shell Materials for Zero Emission Pumps
Magnet Dimensional Chemical Resistance Tensile* Abrasion
Material Losses Stability Cost Acids Solvents Safety Strength Resistance
316s High Excellent 5 Fair Excellent Good 82 Good
Hastelloy
C276 Medium Excellent 7 Excellent Excellent Excellent 1 I I Good
Peek Low Fair 6 Good Poor Fair 23 Poor
FiberGIass
Re. Vinyl Est Low Good 3 Good Poor Fair 39 Poor
FiberGIass
Re.Epoxy Low Good 3 Good Fair Fair 30 Poor
PVDF Low Fair 5 Good Poor Poor 8 Good
Ceramic Low Excellent 10 Excellent Good Good 3-10 Excellent
*ksi
Courtesy of Goirlds Pimps, Seneca Fulls, N. Y.

Develop operating procedures, commissioning procedures, and repair procedures.
And never forget: If you allow zero emission pumps to be abused the way process
plants sometimes abuse conventional pumps, neither magnetic drive nor canned
motor pumps will give long-term reliable service.
Study our attempt at tabulating the pros and cons of the two principal types of
zero emission pumps by assigning a maximum of three points to positive attributes
and one point to somewhat less advantageous features. (See Table 3-16.) Take issue
if you wish, or assign your own numbers based on in-house experience, but study it
nevertheless.
Finally, resist the inclination to simply total up the above numbers. Assign special
value to vendor experience in a given size or application. Remember to summarize
your requirements in a well laid out specification. Look at the vendors’ response and
make an experience check. Then choose wisely.