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 Monte Carlo techniques. So this is a small appetizer to get you thinking about inherent reliability and the loss of capability by use of various installation and use practices.

You can:
       1) Page down for this month's problem statement.
       2) Return to the list of monthly problems by clicking here.
       3) Bypass the background information and go directly to the problem statement by clicking              here.

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.

You can:
       1) Page down for this month's problem statement.
       2) Return to the list of monthly problems by clicking here.


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) =
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

0.9726

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
(yrs)

 

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
(yrs)

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
(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

0.6583

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?

Return to the list of monthly problems by clicking here. Return to top of this problem statement by clicking here.

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.

Return to the list of problems by clicking here. Return to top of this problem statement clicking here.

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     clicking here.

Technical tools are only interesting toys for engineers until results are converted into a business solution involving money and time. Complete your analysis with a bottom line which converts $'s and time so you have answers that will interest your management team!

Last revised 3/31/98
Barringer & Associates, Inc. 1997

Return to Barringer & Associates, Inc. homepage