Problem Of The Month
August 1997--Pump Practices And Component Life Multipliers

 A survey on how the pump installation and use practices effect the life of pump components has revealed interesting information. Survey participants included knowledgeable rotating equipment experts from around the world. This survey is one of the few efforts to link how a given practice effects the quantification of component life. Preliminary survey results are described below and refinements to these findings will be added, as additional results become available. Results from this survey are intended for use in making life cycle cost calculations.

Pump manufactures such as Duriron (now know as Floserve) have details in their catalogs listing good practices that should be followed, but quantifiable effects on component life are not listed. Well known text book authors such as Heinz Bloch and Fred Geitner have details of good practices identified in their 1994 book: An Introduction To Machinery Reliability Assessment, 2nd Edition, published by Gulf Publishing Company in Houston, TX--they provide some quantification of effects on component life.

Costs for a good practice are readily identifiable. Results of the practice and the effects on inherent life resulting from the practice have not been readily available. Now a central tendency is available from knowledgeable individuals in the field of rotating equipment (from chemical plants, refineries, and consultants) showing the effects of practices on the life of components. This World Wide Web publication makes the details readily available for common use. It's important to quantify the cost of practices and the results of component life and how that fits into the overall life cycle costs.

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Background
Respondents to the 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).

Data was screened, and central tendencies established for each element. Smaller variances were noted in the best practices. Larger variances were observed for the lower grade (good) practices.

The survey results have been converted into MIL-HDBK-217 type-models. MTBF models are used rather than failure rate models since many practicing engineers involved in pumps and their life seem to relate to MTBF better than with failure rates.

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The Problem
What are good practices for centrifugal pumps, how do the practices effect component life, and how are the results put into a reliability model? These questions are summarized in the three tables below. The component life multiplier reduces the inherent life of the components.

The best practices for each category are described in the percentage of inherent life that would result. The component life multiplier is obtained from the product of the elements. Because of rounding errors for the charts shown below, the multiplier of inherent life may not literally equal the products obtained from this presentation table.

For the best practices, note the worst multiplier for loss of life may not be destroyer of overall life for the pump--it depends upon the inherent reliability of the component. Best practices result in the highest grade of installation and highest grade of operation. Of course the price paid for this high-grade installation varies as multiples of the lowest grade installation (described as "good" but meaning bad). The multipliers have practical meaning when absolute cost numbers are used.

Best Practices (Highest Grade): Installation & Use

Highest Cost Installation & Superb Operations

Updated
3/31/98
Using Latest Info.

Pump Curve % Off BEP

L/D Suction Straight Runs

Rotational Shaft Alignment

Piping Alignment

Rotational Balance

Foundation Design

Grouting

Component

Component Life Multiplier

+5% to
-10% of BEP

L/D = 10 to 12

0.001 inches/inch error

0.003 inch error

Smooth at 0.0198 inches/sec.

5 Times Equipment Mass

Monolithic & Adhesive Epoxy

Impeller

0.9726

98%

100%

100%

100%

100%

100%

100%

Housing

0.8547

86%

100%

100%

100%

100%

100%

100%

Pump Brgs

0.8719

98%

100%

100%

100%

99%

100%

100%

Seals

0.9533

98%

99%

100%

100%

100%

100%

100%

Shafts

0.8719

98%

100%

100%

100%

99%

100%

100%

Coupling

0.9801

99%

100%

99%

100%

100%

100%

100%

Motor Brgs

1.0000

100%

100%

100%

100%

100%

100%

100%

Motor Wd'gs

1.0000

100%

100%

100%

100%

100%

100%

100%

Motor Rotor

1.0000

100%

100%

100%

100%

100%

100%

100%

Motor Starter

1.0000

100%

100%

100%

100%

100%

100%

100%

Cost As % Of Lowest Grade

--------->

225%

130%

150%

150%

168%

400%

300%

Life cycle costing will tell if the extra cost paid for the best practice is worth the benefits received for the extra life as compared to the life from the lowest grade [cheapest] installation. 

 

 

Better practices are the in-between grade of installation and use practices. More loss of life occurs (as the multiplier gets smaller) from these practices than with the best practice as shown in the table below.

Better Practices (Middle Grade): Installation & Use

Mid-Cost Installation & OK Operating Practices

Updated
3/31/98
Using Latest Info.

Pump Curve % Off BEP

L/D Suction Straight Runs

Rotational Shaft Alignment

Piping Alignment

Rotational Balance

Foundation Design

Grouting

Component

Component Life Multiplier

+10% to
-20% of BEP

L/D = 6 to 8

0.003 inches/inch error

0.010 inch error

Smooth at 0.0448 inches/sec.

3.5 Times Equipment Mass

Slightly Porous But Adhesive

Impeller

0.6583

88%

95%

95%

94%

95%

95%

98%

Housing

0.5163

73%

95%

95%

92%

95%

95%

95%

Pump Brgs

0.3950

79%

90%

88%

88%

90%

90%

90%

Seals

0.4314

88%

90%

90%

84%

90%

90%

90%

Shafts

0.3950

79%

90%

88%

88%

90%

90%

90%

Coupling

0.5705

92%

95%

90%

94%

95%

90%

91%

Motor Brgs

0.6036

94%

93%

94%

97%

95%

90%

90%

Motor Wd'gs

0.9776

100%

100%

100%

100%

100%

99%

99%

Motor Rotor

0.6036

94%

93%

94%

97%

95%

90%

90%

Motor Starter

1.0000

100%

100%

100%

100%

100%

100%

100%

Cost As % Of Lowest Grade

 

181%

120%

120%

120%

138%

200%

200%

 

The good practices (meaning the lowest grade) show the greatest loss of inherent life. Generally speaking, most pumps do not operate continuously so far off the BEP point. Thus the calculations can be misleading in finding the overall component life multiplier---thus the pump curve component for actual installations needs to be a weighted average. Other items (excluding Pump Curve % Off BEP) do operate continuously and thus the life of components operating at the extremes continuously destroys inherent component life.

Rarely does a single pump installation incorporate all of the 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.

 

Good Practices (Lowest Grade): Installation & Use

Lowest Cost Installation & Inferior Operations

Updated
3/31/98
Using Latest Info.

Pump Curve % Off BEP

L/D Suction Straight Runs

Rotational Shaft Alignment

Piping Alignment

Rotational Balance

Foundation Design

Grouting

Component

Component Life Multiplier

+15% to
-30% of BEP

L/D = 1 to 3

0.009 inches/inch error

0.125 inch error

Smooth at 0.248 inches/sec.

0.5 Times Equipment Mass

Cementitous& Low Adhesion

Impeller

0.1949

68%

75%

90%

69%

81%

88%

88%

Housing

0.1438

70%

80%

83%

64%

79%

78%

80%

Pump Brgs

0.0151

65%

60%

58%

40%

61%

50%

55%

Seals

0.0095

51%

60%

40%

40%

64%

55%

55%

Shafts

0.0151

65%

60%

58%

40%

61%

50%

55%

Coupling

0.1149

76%

80%

65%

71%

78%

70%

75%

Motor Brgs

0.0737

78%

80%

55%

80%

75%

60%

60%

Motor Wd'gs

0.8625

97%

100%

100%

100%

95%

96%

98%

Motor Rotor

0.0737

78%

80%

55%

80%

75%

60%

60%

Motor Starter

1.0000

100%

100%

100%

100%

100%

100%

100%

Cost As % Of Lowest Grade

--------->

100%

100%

100%

100%

100%

100%

100%

Perhaps if you have pumps requiring monthly repairs, you are operating under the conditions described above for the "Good" Practices.

 Questions:
1) Why do you need to know the component life multipliers?

2) How do installation costs and demonstrated component lives effect life cycle costs?

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Solutions:

Answer to Question 1): Why do you need to know the component life multipliers?

The components mentioned above plus other components in a pump (such as pump shaft and motor shaft) form a series reliability model. You need to know the MTBF for the items to make an estimate of the MTBF for the mechanical system and the electrical system--which in turn form the pumping system. Many people are surprised at how small the MTBF for the system becomes when the inherent life of components are reduced by the component life multiplier--see the September '97 problem of the month. Of course other people see why they make so many maintenance visits to their pumping systems each year because life is lost from the grade of installation and use--see the October '97 problem of the month.

Answer to Question 2) How do installation costs and demonstrated component lives effect life cycle costs?

The issue of life cycle cost is to find the alternative, which will result in the least long term cost of ownership. The cost issues are generally computed as a net present value (NPV) using a discount technique. The NPV is sensitive to the initial capital cost and the calculation also includes the yearly cost of maintenance actions, which become less sensitive as the assembly ages. Of course the costs are comprised of a series of elements which vary in time and amount. The only way to determine a specific case is to make the calculations---and don't forget the cost of electricity for pumping systems as this is frequently the largest annual cost item for spared pumps. Of course the final question involves life cycle costing to find cost-effective actions to arrive at a solution most effective for the stockholders.

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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     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

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