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Pipe Specifications

The following tables offer a good visualization of the differences which exist for various common size carbon steel pipe at different ASTM thickness schedules.  The dimensions of the blue thickness bars are technically to scale, although they may display and print out differently depending upon your specific computer and monitor resolution.

The tables are meant to serve as a general guide in illustrating the different relevant thicknesses between different pipe schedules, and their importance to reliable system operation – not as an absolute dimensional template.

This table of wall thickness dimensions clearly illustrates the significant difference which exists between standard and extra strong pipe.  A much greater difference exists at the larger diameter piping, although the additional pipe wall is usually most needed at the smaller and typically threaded pipe sizes.

For 10 in. and under, schedule 40 is equal to standard pipe, and schedule 80 is equal to extra heavy – although for larger sizes, the thickness of schedule 80 defined pipe will increase significantly.

Table A

• Thinner Pipe Substituted

Today’s common use of schedule 20 and schedule 10 pipe for various services is based upon the operating pressure rating it carries, and not its thickness value.  The use of schedule 40 pipe, considered as “thin wall” to the use of schedule 80 decades ago, is now reduced another 50% in thickness by choosing to install schedule 10.

The below table well illustrates the limited amount of wall thickness offered by such extremely thin wall pipe.  In most applications, this allows little or no room for corrosion to occur before reaching minimum acceptable thickness limits, and failure.

Table B

• Threads A Significant Factor

Very often, an examination of pipe, whether by ultrasound or metallurgical means, will provide a range of remaining wall thickness values.  Where threaded pipe is involved, that measured wall thickness value, minus its thread cut, will show the true remaining thickness at the pipe ends, valves, elbows, tees, and other threaded fixtures.

In many examples, remaining thickness at the threads of 0.025 in. and below is not unusual – and offers an easy explanation of why thread failures are more common.  CorrView International, LLC strongly recommends the use of heavier schedule 80 steel pipe in all threaded condenser or open process water applications.

The below table of relative wall thickness well illustrates the threat existing where pipe has been deteriorated to such lowest limits.

Table C

• A Self Inflicted Problem

To put it in perspective – an 8 in. condenser water or open piping system constructed in 1940 would have typically used schedule 80 pipe having a wall thickness of 0.500 in., and still be in service today.  That same system constructed in 1980 would have used schedule 40 pipe with a wall thickness of 0.322 in.  Remaining service would be generally dependent upon corrosion activity during that time.

Today, schedules 20 and 10 are sometimes found; having original thickness values for 8 in. pipe of only 0.250 in. and 0.188 in respectively.  This offers very limited room for any corrosion to occur, and even under good corrosion control of 2-3 MPY, nowhere near the service life expectancy of older piping systems.

• Various Reasons Exist

Ultrasonic investigations performed by CorrView International, LLC have routinely documented the advanced failure of more current piping installations using schedule 40, 20 or 10.  Surprisingly, such failures even exist where the original pipe within the same system can be shown having decades of remaining service life.  This is due to a complicated and interrelated set of factors which have developed over the past 30 years.

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