Undersized Pipe

Up until about 15 years ago, most new pipe was manufactured at its defined ASTM wall thickness.  Prior to that, and going back 30 years or more, pipe was typically manufactured above its specification sufficiently that even today, we can provide examples where an ultrasonic examination of older pipe pipe still exhibits a wall thickness higher than defined.

Today, most pipe is produced substantially undersized due to what we believe are closer production tolerances enabling manufacturers to produce a lighter but still approved product within the tolerable limits of the ASTM code.  Extensive testing of new steel pipe has consistently found undersized material statistically beyond the possibility of being a random event.  With a permissible tolerance of + / – 12.5%, most new pipe we measure is toward the lower boundary of this limit, and very rarely at true ASTM factory specification or above any longer.  An FM or UL approval rating does not mean that the pipe wall is at true ASTM specifications, and neither ASTM, UL, or FM monitor all piping manufactured.

• Proof Beyond Random Coincidence

At left we show the test result for a section of new U.S. produced 8 in. carbon steel pipe of the ASTM specification A53 Grade B seamless.  Its ASTM specification calls for a wall thickness of 0.322 in., which is likely what design engineers are assuming for this addition to an existing condenser water system.  A pipe wall of 0.322 in. is stenciled onto the pipe wall.

Instead, we measure a wall thickness of 0.292 in. consistently throughout its length, which represents it being 9.1% undersized.  At a typical corrosion rate of 2 MPY, this reduction of 0.030 in. of wall thickness represents a potential 15 years of service lost.

In a second example at left, testing at a section of new 6 in. schedule 40 A53 pipe, with the ASTM specified wall thickness of 0.280 in., produced true wall thickness measurements over its entire length of 0.244 in., and 0.036 in. below specification.  For on-site skeptics in this project questioning the accuracy of the instrumentation, physical measurement by dial caliper produced the very same wall thickness dimension.

At 0.244 in., this new pipe is 12.9% under its ASTM factory specification prior to even being installed, and again will provide lower than expected service life.  Under higher corrosion conditions more common today, such undersized pipe will provide far less service.  Not only is this section of new pipe undersized, but it is undersized to 12.9%, and therefore technically unacceptable for installation.  Actual wall thickness dimensions are virtually never checked, however, leaving the mechanical design engineer, pipe installer, building owner, and facility engineers to all mistakenly believe that the stenciled wall thickness dimension or pipe schedule specification is accurate.

Further testing of two other sections of 6 in. pipe from the same HVAC condenser water expansion project, but from a different U.S. manufacturer, produced similar results with wall thickness dimensions consistently near 0.245 in. – precisely 12.5% under its ASTM specification and at the lowest limit of what is permitted by the ASTM code.  In this above example of 6 in. diameter schedule 40 pipe into a condenser water system where 2 MPY might be expected as a reasonable corrosion rate, the loss of 0.036 in. of material translates into an immediate loss of 18 years of service life.

At left, the installation of a new 18 in. condenser water pump header was nearing completion when the ultrasonic investigation at examples of older pipe allowed a quick evaluation to the new pipe still being welded into place.  With an ASTM stamp marking the pipe as STD, (standard), as well as a wall thickness dimension of 0.375 in. stenciled on the pipe itself, everyone associated with this piping project likely assumed that 0.375 in. was its true thickness dimension.

Instead, UT testing identified the pipe consistently between 0.332 in. and 0.336 in. across its entire approximate 20 ft. length – remarkably uniform, but lower.  At 0.332 in., this new pipe is precisely 12% under its defined ASTM thickness specification; consistent with most new pipe today.  Technically, this new 18 in. pipe has a wall thickness more closely associated with 8 in. schedule 40 pipe at 0.322 in.  Ultrasonic testing of the new 12 in. pipe to and from each condenser water pump, again defined by ASTM to have a wall thickness of 0.375 in., instead showed the same thickness values of near 0.330 in., and again being undersized to near its maximum allowable limit.

In another more critical example immediately left, we measure a new 2 in. schedule 40 threaded pipe nipple having an ASTM A 53 factory wall thickness specification of 0.154 in.  However, ultrasonic testing shows a wide variance in wall thickness along its unthreaded center of between 0.133 in. and 0.142 in. – significantly lower than its ASTM specification by 13.6 %, and technically below its ASTM minimum allowable limit for installation.

Similarly undersized 2 in. pipe nipples from the same lot were installed and placed into service – now having only 0.063 in. of usable wall before reaching the outer thread cut and a failure condition.

Unlike larger diameter pipe where a 10 % less thickness dimension might not present any immediate threat, this same loss at small threaded pipe having inherently thinner wall thickness has a significant impact.  With 0.070 in. cut away during threading, this unnecessary 0.021 in. loss of material can dramatically reduce service life at all but those systems with the lowest possible corrosion rate.

• Recycled Materials
  

While the use of recycled materials has its benefits on many levels, there is some element or characteristic of that recycled product, missing or present, that seems to be responsible for changes to the metal’s vulnerability to corrosion.  Various unproven claims that new steel pipe, now in its fifth generation of being recycled, has had an impact on its quality and corrosion resistance warrant further formal investigation.

• Unnecessarily Wide Tolerance

Steel pipe has an exceptionally wide manufacturing tolerance of 25% of its wall thickness – meaning that it can be manufactured + / – 12.% of its ASTM specification and still be considered suitable for sale and installation.  This extremely wide manufacturing tolerance, which was established in the early 1900s when steel pipe manufacturing was still in its infancy, was likely necessary given the tools and manufacturing methods of that day.  Now, more than 100 years later, manufacturing processes and the tighter tolerances maintained have dramatically improved across virtually all industries, with steel piping manufacturing obviously included.  Yet this wide 25% wall thickness tolerance remains.

With unquestionably greater control over manufacturing practices, pipe can be reliably produced to any wall thickness desired, and from our perspective, all pipe manufacturers, foreign and domestic, have taken advantage of this excessive manufacturing allowance to their financial benefit and competitive needs.  For the building property owner, however, it represents a hidden 12.5% loss, or even greater.  Given the many other negative influences driving higher corrosion rates, new building properties need all the pipe wall they can get – not less.

We can very accurately measure wall thickness using ultrasound technology.  We can also calculate the minimum acceptable wall thickness permissible for different piping systems and pipe sizes under the operating conditions which exist.  Until recently, the once easiest component of any piping assessment had been in defining its original or beginning wall thickness since such values were well defined by ASTM.

• A New Variable Influencing Its Evaluation

In evaluating any older building property, the wall thickness of 12 in. schedule 40 pipe was always known to begin at 0.406 in. or slightly above, 8 in. schedule 40 pipe at 0.322 in. etc., with most pipe produced decades ago being at or above ASTM specifications.  The issue was so common that our earlier computer program by which we produce our ultrasonic based piping assessments even had an entry field for percentage over ASTM specifications.  In fact, we still document 60 year old pipe at older New York City properties exceeding its new ASTM specified wall thickness – the combination of low corrosion activity acting against high quality oversized pipe!

At left, we document 12 in. extra heavy Bethlehem Steel wrought iron condenser water pipe installed in 1933, used for 30 years, then cut and left exposed at a roof location for over 52 years.  Yet this old pipe still shows a higher wall thickness than its original ASTM specification; significantly more, in fact, than the 0.338 in. undersized pipe installed with its new cooling tower!

In the above title photos, all taken at new pipe not yet installed, 18 in. standard pipe is actually measured at 0.334 in. rather than 0.375 in., new 6 in. schedule 40 at 0.244 in. rather than 0.280 in.  A section of 8 in. schedule 40 pipe specified at 0.322 in. is actually 0.278 in. and even 2 in. thinwall schedule 10 pipe at 0.109 in., already substantially below schedule 40 specifications, has been reduced to 0.088 in. – a massive 19.3 % below its ASTM specification.  Destined for a dry fire sprinkler system already subject to a high corrosion environment, this heavily undersized pipe section has likely already failed and been replaced with new undersized pipe – with property owners undoubtedly questioning why.

• Misleading Pipe Stamps

A very common misunderstanding is that stenciled ASTM, FM, or UL pipe stamps citing pipe diameter and wall thickness actually define the pipe they are printed on, when in fact they only suggest what dimensions the pipe should be manufactured.  A 0.375 in. pipe stamp for any pipe manufactured today means that it might just be 0.328 in., or any value in between.  With outside diameter defined, any wall loss will only show internally, and disappear from sight upon installation.

• Varying Impact

The impact of undersized pipe varies greatly.  For 12 in. schedule 40, still high wall thickness remains at even 12.5% below specification.  As pipe size decreases so does its wall thickness, and therefore the same undersized factor produces a greater loss of service.  For threaded or cut-grooved pipe, removing half the wall thickness of inherently thinner material can reduce already low service life estimates significantly.  For dry and pre-action fire sprinkler systems already under higher corrosion stress, the use of thinwall schedule 10 and even schedule 7 pipe, already at a low 0.109 in. for 2 in. schedule 10, dramatically reduces its expected service.

Combined with less effective corrosion controls, lower quality pipe, poorer galvanizing, ERW seamed pipe, and greater operating demands – undersized pipe is just one more issue guaranteeing less service to all new building properties.

• Substituting Copper Pipe

In response to such observed increases in corrosion, many property managers, operating engineers, and plant managers have turned to the use of Type L copper tube (commonly called pipe) for all smaller diameter run-out distribution lines, and in some cases even for the main risers.  This, however, may only be a short term solution to the often more complex corrosion condition, and in some cases may actually complicate an existing corrosion problem with additional and unseen threats.

Important to consider in the substitution of copper pipe for carbon steel is the significant difference in initial wall thickness.  Common Type L, ASTM B 88 copper for 3 in. diameter pipe has a standard wall thickness of only 0.090 in., whereas 3 in. A 53 schedule 40 carbon steel has a wall thickness of 0.216 in.  Compared to steel pipe having a stress efficiency (a strength rating, not pressure rating) of 15,000 lb./sq. in., B 88 copper pipe only offers 6,000 lb./sq. in. – a physical decrease in strength of 60%.  Copper pipe has less than half the tensile strength of steel, and quickly loses that strength as temperature increases.

It is generally recognized that the minimum acceptable wall thickness for copper tubing, under any conditions, is approximately 0.020 in. At higher pressures, this minimum value increases – thereby allowing for less physical wall loss to occur before being judged as unsuitable for further reliable service.  Copper pipe, since it exhibits far less physical strength than steel, will fail sooner than steel at the same wall thickness dimension and under the same internal pressure – making it obvious that the greatest threat for any copper piping or components installed in a high rise building property exists at the higher operating pressures of the lower floors.

While the corrosion rate against copper is commonly believed to exist well below 0.5 MPY under all conditions, CVI has well documented that the same corrosive environment responsible for raising corrosion rates against carbon steel past 10 MPY, will greatly increase the copper corrosion rate above its normal value as well.  Copper corrosion rates exceeding 5 MPY are therefore not uncommon where an already high steel corrosion rate has been documented.

• Same Undersized Threat

A further concern relates to the seemingly similar decrease in quality of copper pipe to steel, as well as its often undersized wall thickness dimensions.  In the example of new American made 2-1/2 in. pipe shown at left, a field demonstration of ultrasonic testing procedures to project engineers at a construction site revealed dramatically different wall thickness measurements within the same length of new pipe.

With heavier Type K copper tube specified due to a known corrosion condition, and having an ASTM defined wall thickness of 0.095 in., our testing identified a wide range in wall thickness from between 0.065 to 0.089 in, and lowest wall thickness a significant 32% below its specification.  Measurements were so dramatically low and uneven that observers, as well as the pipe fitters installing the new pipe, questioned the accuracy of the ultrasonic test method.

The pipe was cut and a physical measurement made by standard dial caliper produced the same low wall thickness measurements – with a difference of 0.024 in. at opposite walls.  This represents a 25% variance in wall thickness alone; with measurements showing the pipe generally below Type L specifications and approaching thin wall Type M dimensions.  A close visual examination, again shown by this photo of the subject pipe with substantially lower wall thickness at left, further supported the original ultrasonic findings.  Subsequent dial caliper measurement of the remaining copper pipe stock produced similar results.

For new copper pipe being added to a condenser water system plagued with a high corrosion condition, the design engineer’s intent to extend service life by specifying heavier (and more expensive) Type K copper pipe in reality accomplished nothing.  With the service life of all piping being defined by its lowest wall thickness, thinwall Type M copper pipe was instead installed into this project.  In the above actual event, the contractor argued that the pipe was acceptable and it was installed.

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