Problem Of
The Month
September 1997--Demonstrated Life Of Pump
Components From Pump Practices
In the August '97 Problem Of The Month, pump practices and the life of components resulting from the practices were discussed. Now the question is: How do the practices change the inherent life of a pump?
Details of this analysis will appear in an Excel spreadsheet for calculating
life cycle costs using
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Background
Respondents to a pump practices survey were asked to define good, better, and
best practices--each practice is a higher grade of installation and use. Each
expert provided their estimates of the resulting component life resulting from
each practice. Also the respondents gave their estimates of costs for the
practice based on the lower grade practice as 100% (i.e, the cost datum). The
results were summarized in the August
'97 Problem Of The Month.
Based on survey results, models can be built to show the effect of practices on demonstrated life of the system. The problem will make use of Weibull statistics.
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The Problem
What deterioration of inherent life occurs for an ANSI pump based on use of
good practices, better practices, and best practices?
Best practices result in the highest grade of installation and highest grade of operation. Best practices have the highest installation costs but preserves the large amount of inherent reliability.
|
|
Inherent Component Reliability |
|
Best Practices Installation & Use |
Effects Of Practices on Component Life |
|||||
|
beta |
eta |
|
beta |
eta |
|
||||
|
ANSI Pump |
shape factor (no dimensions) |
location factor (hrs) |
MTTF (yrs) = |
Life |
shape factor (no dimen-sions) |
location factor (hrs) |
MTTF |
Updated |
|
|
Impeller |
2.5 |
300,000 |
30.39 |
0.9726 |
2.50 |
291,784 |
29.55 |
||
|
Housing |
1.3 |
300,000 |
31.63 |
0.8547 |
1.30 |
256,412 |
27.03 |
MTBF For All Mech.--> Items |
|
|
Pump Bearings |
1.3 |
100,000 |
10.54 |
0.9712 |
1.30 |
97,119 |
10.24 |
||
|
Seals |
1.4 |
100,000 |
10.40 |
0.9677 |
1.40 |
96,773 |
10.07 |
2.50 |
|
|
Shafts |
1.2 |
300,000 |
32.21 |
0.9712 |
1.20 |
291,357 |
31.29 |
|
|
|
Coupling |
2 |
100,000 |
10.12 |
0.9801 |
2.00 |
98,010 |
9.92 |
|
|
|
Motor Bearings |
1.3 |
150,000 |
15.81 |
1.0000 |
1.30 |
150,000 |
15.81 |
MTBF For All Elect.--> Items |
|
|
Motor Windings |
1 |
150,000 |
17.12 |
1.0000 |
1.00 |
150,000 |
17.12 |
||
|
Motor Rotor |
1 |
300,000 |
34.25 |
1.0000 |
1.00 |
300,000 |
34.25 |
5.50 |
|
|
Motor Starter |
1.2 |
300,000 |
32.21 |
1.0000 |
1.20 |
300,000 |
32.21 |
|
|
|
|
1.76 |
=~Mean time between system failures= |
1.72 |
|
|||||
|
or |
15,443 |
hours |
D = Loss |
or= |
15,039 |
Hours |
|||
|
|
3% |
loss= |
404 |
Hours |
|||||
Better practices are the in-between grade of installation and use practices. The loss of life from these better practices is lower than the best practice as shown in the table below.
|
|
Inherent Component Reliability |
|
Better Practices Installation & Use |
Effects Of Practices on Component Life |
|||||
|
|
beta |
eta |
|
beta |
eta |
|
|||
|
ANSI Pump |
shape factor (no dimensions) |
location factor (hrs) |
MTTF |
Life Multiplier * eta From Practices |
shape factor (no dimen-sions) |
location factor (hrs) |
MTTF (yrs) = |
Updated 3/31/98 Using Latest Info. |
|
|
Impeller |
2.5 |
300,000 |
30.39 |
0.6583 |
2.50 |
197,484 |
20.00 |
||
|
Housing |
1.3 |
300,000 |
31.63 |
0.5163 |
1.30 |
154,897 |
16.33 |
MTBF For All Mech.--> Items |
|
|
Pump Bearings |
1.3 |
100,000 |
10.54 |
0.3950 |
1.30 |
36,501 |
4.16 |
||
|
Seals |
1.4 |
100,000 |
10.40 |
0.4314 |
1.40 |
43,143 |
4.49 |
1.21 yrs |
|
|
Shafts |
1.2 |
300,000 |
32.21 |
0.3950 |
1.20 |
118,503 |
12.72 |
|
|
|
Coupling |
2 |
100,000 |
10.12 |
0.5705 |
2.00 |
57,054 |
5.77 |
|
|
|
Motor Bearings |
1.3 |
150,000 |
15.81 |
0.6036 |
1.30 |
90,544 |
9.55 |
MTBF For All Elect.--> Items |
|
|
Motor Windings |
1 |
150,000 |
17.12 |
0.9776 |
1.00 |
146,644 |
16.74 |
||
|
Motor Rotor |
1 |
300,000 |
34.25 |
0.6036 |
1.00 |
181,089 |
20.67 |
4.10 yrs |
|
|
Motor Starter |
1.2 |
300,000 |
32.21 |
1.0000 |
1.20 |
300,000 |
32.21 |
|
|
|
|
1.76 yrs |
=~Mean time between system failures= |
0.93 yrs |
|
|||||
|
or |
15,443 |
Hours |
D = Loss |
or= |
8,188 |
Hours |
|||
|
|
47% |
loss= |
7,255 |
Hours |
|||||
Good practices are the lowest grade--least costly to install but low cost results in a short life. Rarely does a single pump installation incorporate all minimum conditions and these extremes cause engineers to use them as examples of why they can adopt these minimums as a way of life--only to incur high operating costs and high maintenance costs. In general, do not use all of these minimums simultaneously unless cost is no object and you really want to build a monument to less than desirable practices--do what you can afford as you will pay a price in significant loss of life.
|
|
Inherent Component Reliability |
|
Good Practices Installation & Use |
Effects Of Practices on Component Life |
|||||
|
|
beta |
eta |
|
beta |
eta |
|
|||
|
ANSI Pump |
shape factor (no dimen-sions) |
location factor (hrs) |
MTTF (yrs) = h*G(1+ 1/b) |
Life Multiplier * eta From Practices |
shape factor (no dimen-sions) |
location factor (hrs) |
MTTF (yrs) = h*G(1+ 1/b) |
Updated 3/31/98 Using Latest Info. |
|
|
Impeller |
2.5 |
300,000 |
30.39 |
0.1949 |
2.50 |
58,458 |
5.92 |
|
|
|
Housing |
1.3 |
300,000 |
31.63 |
0.1438 |
1.30 |
43,141 |
4.55 |
MTBF For All Mech.--> Items |
|
|
Pump Bearings |
1.3 |
100,000 |
10.54 |
0.0151 |
1.30 |
1,511 |
0.16 |
||
|
Seals |
1.4 |
100,000 |
10.40 |
0.0095 |
1.40 |
949 |
0.10 |
0.05 |
|
|
Shafts |
1.2 |
300,000 |
32.21 |
0.0151 |
1.20 |
4,533 |
0.49 |
|
|
|
Coupling |
2 |
100,000 |
10.12 |
0.1149 |
2.00 |
11,494 |
1.16 |
|
|
|
Motor Bearings |
1.3 |
150,000 |
15.81 |
0.0737 |
1.30 |
11,048 |
1.16 |
MTBF For All Elect.--> Items |
|
|
Motor Windings |
1 |
150,000 |
17.12 |
0.8625 |
1.00 |
129,372 |
14.77 |
||
|
Motor Rotor |
1 |
300,000 |
34.25 |
0.0737 |
1.00 |
22,097 |
2.52 |
0.74 |
|
|
Motor Starter |
1.2 |
300,000 |
32.21 |
1.0000 |
1.20 |
300,000 |
32.21 |
|
|
|
|
1.76 |
=~Mean time between system failures= |
0.05 |
|
|||||
|
or |
15,443 |
hours |
D = Loss |
Or= |
416 |
hours |
|||
|
|
97% |
loss= |
15,027 |
hours |
|||||
Remember--seldom does a pump operate for long periods of time under all of these minimum conditions as described by the lowest grade of installation and use and called "good" practices!!
Questions:
1) How much of the inherent life is lost by use of best pump practices?
2) How much of the inherent life is lost by use of better pump practices?
3) How much of the inherent life is lost by use of good pump practices?
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Solutions:
Answer to Question 1): How much of the inherent life is lost by use of
best pump practices?
3% of the inherent life is lost.
Answer to Question 2) How much of the inherent life is lost by use of
better pump practices?
47% of the inherent life is lost.
Answer to Question 3) How much of the inherent life is lost
by use of good pump practices?
97% of the inherent life is lost.
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Comments:
Refer to the caveats on the Problem
Of The Month Page about the limitations of the following solution.
Maybe you have a better idea on how to solve the problem. Maybe you find where
I've screwed-up the solution and you can point out my errors as you check my
calculations. E-mail your comments, criticism, and corrections to: Paul
Barringer by 
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