Compressor end of life criteria 1

[MechDesign105]

Hi guys,

I am a young engineer with no particular experience in compressed air systems. We have about 300 5-stage air compressors compressing air at 3600 psig with a flow rate of about 29 cfm.

My employer has asked me to develop end of life criteria for these compressors. These end of life criteria would define when the compressors need to be replaced according to different parameters. My employer wants to minimize cost of maintenance and of replacement of compressors while keeping risk low. However, I find it hard to quantify risk...

I will contact the manufacturer but am afraid the criteria they give me will be very conservative..

How do you quantify or decide when a compressor is to be replaced?

Thank you for your help!

 

[zdas04]

Fundamentally, you replace this kind of kit when it costs more to keep it than to replace it.

You could look at when a combination of maintenance and energy costs (use a surrogate cost to mask price hikes) reach a threashold you toss it. Or you could set a downtime threshold (or a number of outages).

David

 

[BigInch]

Estimate maintenance and energy expense each year for any given unit. Convert that series of payments to an equivalent present value (Excel PV function) - salvage value, if any. When that is higher than the sum of the down payment plus the PV of the periodic payments to purchase, plus energy cost to install a new unit, it's time to replace the old one.

From "BigInch's Extremely simple theory of everything."

 

[MechDesign105]

Thanks for the tips!

When you calculate your Present Value with all the maintenance costs of the worn compressor, what time period do you consider?

Do you consider the foreseen compressor life as a present value for the maintenance costs?

Thanks!

 

[BigInch]

Yes you should evaluate alternatives over similar time cycles. To consider a worn compressor you must have some idea of the maintenance costs needed to keep it running, presumedly for a lifetime equal to the new unit to evaluate the alternatives over equal time periods, or you could allow it to expire early at minimal maintenance cost, then add the expense of a new unit at the time when it expires and estimate the costs of the two together against the new one you are evaluating now.

For example

Worn compressor requires a rebuild today: $ 10,000
and maintenance costs of 5,000 this year
7,000 next year,
9,000 in year 3
12,000 in year 4
20,000 in year 5
25,000 in year 6
35,000 in year 7

Take the PV of each of those maintenance expenses, plus the power cost too, if that's going to vary between alternatives, plus adding a new unit at the end of each of those years and the new unit's costs until it's life ends, or the time that the sequence reaches the end of the lifetime of a new unit added now today.

Compare the PV of a new unit with all of the alternatives of allowing the old one to continue with replacement option at the end of each of those years. Then if there are any cases with similar numbers, that would be the maximum time to replace.



From "BigInch's Extremely simple theory of everything."