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How Long Should It Last?

Corrosion activity will vary greatly between different piping systems.  Dry fire pipe corrodes far more than the steel pipe at a wet fire system.  Steam condensate faster than the steam supply, and condenser water much faster than a closed piping system.

In addition, the schedule of pipe installed is a major factor in pipe service.  Schedule 10 pipe has approximately half the wall thickness of schedule 40 pipe, and under similar corrosion rate conditions, will last approximately half the service life.  The use of thin wall schedule 10 and even schedule 7 pipe in fire protection systems is a major factor in explaining their early failure.

• A Combination Of Factors

Simply stated, the service life of any piping system is a function of its corrosion rate or rate of wall loss, verses its pipe wall thickness.  Low corrosion activity acting against heavier pipe may provide 100 or more years of reliable service whereas high corrosion activity acting against thinner pipe may provide only a few years.  Few piping systems will survive a 20 MPY corrosion loss; with a much lower corrosion rate against schedule 7 pipe often producing the same results..

In evaluating any piping system for potential service life, it is first important to determine the true average corrosion rate of the piping.  As we have well commented upon in various articles based upon hundreds of ultrasonic investigations, corrosion coupons are virtually worthless as a measurement tool under most conditions at HVAC systems, and fail to report the true wall loss acting against the pipe itself.

Coupons may provide some accuracy under low corrosion conditions, and will show the relative corrosiveness of the water itself, but entirely fail to indicate actual pipe wall loss.  In all of the largest corrosion related disasters we have investigated, low corrosion coupon results were believed accurate over the most blatantly obvious evidence to the contrary – thereby allowing often excessive corrosion conditions to continue in many cases for years.  Our handout concerning Corrosion Coupons at left offers further information and a true case history.

For many piping systems such as fire protection, domestic water, steam service and others, or where there is no water circulation, corrosion coupons are not applicable and therefore no useful corrosion rate information may be available.

While certain assumptions may be made regarding the corrosion activity present at different piping systems based upon historical data, only ultrasonic testing can provide the hard evidence of how much pipe wall has been lost and at what rate.

Such corrosion rate information, in addition to knowledge about the original wall thickness and minimum allowable lower limit, can then be used to estimate useful remaining service life.

• General Service Life Estimates

In the below table, we offer service life estimates for different pipe sizes and schedules at different corrosion rates.  Although often found difficult to believe, we document that maintaining a low 0.5 MPY corrosion rate at a 12 in. schedule 40 chill water piping system will provide literally hundreds of years of service.  Left to reach a high rate of 8 MPY, and a far less 28 years of service can be expected.

For small diameter 2 in. threaded condenser water pipe, a low 1 MPY corrosion rate will provide about 60 years of useful service.  Yet at a 5 MPY corrosion rate, very common today, this same pipe will provide only 12 years of reliable service – thereby explaining the common failure of threaded schedule 40 condenser water pipe within 20 years.

Fire protection systems constructed of typically thin wall and small diameter pipe are highly vulnerable to any elevated corrosion levels.  When drained as required, new air and oxygen is then introduced into the system to guarantee higher levels of pipe corrosion.

Master Table of Pipe Service Life For Different Pipe Sizes And Schedules At Varying Corrosion Rates

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Our above table demonstrates the dramatic difference in service life that exists at the corrosion rates commonly seen today.  While the difference between a 1 MPY and 5 MPY corrosion rate may not be viewed as significant, it potentially plays a tremendous role depending upon pipe wall size and schedule as this table illustrates.  This table illustrates the benefit in using heavier pipe where corrosion rates are elevated, and especially for smaller diameter threaded pipe.

© Copyright 2023 – William P. Duncan, CorrView International, LLC