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Pumps & Seals
ANSI dimensional requirements create unnecessary burden
Dipl. Eng. Laurens Janssen , Blackmer Product and Marketing Manager
The chemical industry today demands reduced maintenance costs through
greater pump reliability and improved mean time between failure (MTBF). A
new design for overhung impeller-type pumps accomplishes these
requirements through dramatic improvement in shaft stiffness ratio and
reduced bearing loads.
Some plants attribute up to 85% of all pump problems to premature seal and
bearing failure. Therefore, today’s pumps have to be built with seal and
bearing reliability as the foremost design criteria. However, most pump
manufacturers have been reluctant to concern themselves with seal problems
and extending their life through pump design. Recently, customer demands
for better reliability have forced these manufacturers to face the issue.
Pump operations and maintenance personnel are now looking for warranties
on the entire pumping system, including the seal. This will compel pump
manufacturers to offer seal warranties that will raise reliability to new
and appropriate levels.
Outdated specifications
The ANSI pump specification was written more than 20 years ago when
packing was common. The dimensional spec had to reflect the spacing
requirements for a minimum number of packing rings and a lantern ring.
However, in the chemical industry today, packing is becoming obsolete
because of stringent environmental standards that dictate zero product
leakage and near zero fugitive VOC emissions.
The burden created by the ANSI dimensional requirements is unnecessary.
The shaft overhang from the radial bearing to the impeller is far greater
than necessary for today’s sealing requirements, most new generation
mechanical seals can be fitted to a pump that is at least 2-in shorter
than the ANSI standard configuration requires. This reduction of overhang
lowers the shaft stiffness ratio, which is a measure of a shaft’s
resistance to deflection. Derivation of the formula for stiffness ratio is
shown in Fig. I. For the purpose of simple comparison, the deflection
formulae reduce to L3/D4, which is a pure function of the length of the
overhang (L) and the diameter of the section (D).
The lower the stiffness ratio, the less the shaft deflection under a
common load. Therefore, the amount of deflection is in direct proportion
to the increase in the stiffness ratio.
Reducing shaft deflection
Centrifugal pumps are designed to operate at a specific flow-a single
point on the pump curve. Most pumps do not operate at their design point
as a result of improper selection, changing process requirements, and
system head variations. Thus recirculation occurs, which causes uneven
pressure distribution in the volute, and radial forces on the impeller.
The radial loads from the impeller increase exponentially as the pump is
operated away from the design point (Fig. 2). These radial loads cause
shaft deflection, which is a constant irritant to the mechanical seal, and
dramatically, reduces seal life. When shaft motion is reduced, seal life
increases significantly.
Damaging shaft vibration can also arise from other sources related to
application, process and installation problems:
§ Insufficient net positive suction head (NPSH) available, causing
cavitation
§ Improper suction and discharge piping design, creating turbulence in the
pump suction and thus hydraulic imbalance
§ Poor pump and motor alignment at installation
§ Inadequate mechanical impeller balance
§ Hydraulic imbalance of the impeller caused by poor-quality sand castings
and improper machining set-up
§ Poor design and loose manufacturing tolerances of pump components.
Pipe strain, improper foundation, and operational problems all add to the
damaging forces on the pump. Mechanical seal faces are flat within 22
millionths of an inch, or two helium light bands. Yet, ANSI B73 and API
610 allow shaft runout in the mechanical seal area of 0.002 in. This means
that when a dial indicator is placed against the shaft in the seal area,
it will show a total reading of 0.002 in when the shaft is rotated one
revolution. Thus, the allowable shaft motion can be 1,000 times greater
than the flatness of the seal faces, a discrepancy that can damage the
seal and lead to premature failure.
The 0.002-in allowable runout is an easily attainable machining tolerance
for the manufacturing operation. However, today’s machine tools can
perform to tighter tolerances, yielding pumps with significantly reduced
shaft motion that will result in greater seal life.
The ANSI standard also states that the dynamic shaft deflection at the
impeller centerline will be less than 0.005 in. This value can translate
into significant shaft deflection at the seal faces, possibly 0.002 to
0.003 in. This dynamic deflection in addition to the allowable runout can
cause damaging motion and vibration on the seal faces.
Reducing bearing loads
A comparison of radial loads on the pump bearings due to impeller
forces shows that the pump with the lower LD design, that is, the pump
with less overhang will have reduced bearing loads. A reduction in shaft
overhang length from 8 in to 5 in will reduce the load by 22% on the
radial bearing and by 37% on the thrust bearing. The bearing rating life,
or L10 life (L10 life = the revolutions or hours that 90% of a group of
bearings will exceed), varies inversely as the cube of the applied load,
as demonstrated in the American Bearing Manufacturers Association (ABMA)
equation:
L10 = (C/P) ³ where:
C = basic dynamic load rating of the bearing
P = equivalent dynamic bearing load. As a result, the radial bearing
calculated L10 life more than doubles while the thrust bearing life
calculates at four times greater.
Eliminating shaft sleeves
Shaft sleeves in pumps result in two major design flaws:
§ They add virtually no stiffness to the shaft, so in the calculation for
stiffness ratio, the diameter under the sleeve is used.
§ The runout tolerance between the shaft and sleeve compounds the
aggravated motion on the seal faces. The mechanical seal must endure this
in addition to the shaft runout and dynamic shaft deflection already
present.
Packed pumps required shaft sleeves to save the expensive shaft from
constant wear and replacement. With today’s mechanical seals they serve no
purpose. Lowering of the shaft stiffness ratio by reducing impeller
overhang and eliminating shaft sleeves will result in the following
§ Improved mechanical seal life because of a decrease in shaft deflection
and vibration from impeller radial loading and other listed causes
§ Improved bearing life due to the reduction in bearing loads and
vibration
§ Overall improved pump reliability.
The importance of these design improvements for centrifugal pumps cannot
be overemphasized. They are based on simple design criteria that are easy
to explain and comprehend. The process of improving pump reliability
begins with a pump design that can withstand the inevitable destructive
forces associated with misapplication, poor installation and improper
operation.
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