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Threaded Pipe Limitations

Over the life-span of any building property or plant operation, the replacement, renovation or addition of various piping systems is a frequent occurrence.  Due to the fact that operating pressures of most high rise buildings or process plants rarely exceed 250 PSI, internal pressure does not often factor in the selection of pipe schedule.

Today, schedule 40 black pipe is almost automatically chosen for most small diameter piping needs – with little thought given to the physical wall thickness limitations of the pipe itself, or long term service life.  High pressure steam, steam condensate, and other critical services are the exception.  As a result, it is not uncommon to find the premature failure of relatively new condenser or process water installations, as well as examples of properties which have replaced such small diameter piping on a regular basis every 4, 5, or 6 years.

• Fewer Problems Decades Ago

In contrast, piping systems installed decades ago often provide significantly longer service life.  This is in part due to the previous use of schedule 80 or extra heavy pipe for all threaded applications.  Other factors such as higher quality steel and more effective chemical corrosion protection decades ago also play an important role, but it is the thinner pipe wall thickness used at installations within the past 30 years which often becomes the most important limitation to long service life.

Allegations of poor or unsatisfactory chemical water treatment is almost immediately cited in such piping failures, although for most cases, testing will show an acceptable to moderate corrosion rate.  In fact, the problem commonly exists because more than 50% of the original pipe wall is removed in the threading process, leaving little material remaining.  Joint compounds and sealants, and/or the degree to which the connection is tightened, are rarely capable of holding back water once the threaded wall area is penetrated by corrosion.

Thread Leaks

Imminent Failure

• Thread Loss Significant

The below table well illustrates the degree to which the threading process weakens schedule 40 pipe – a loss rarely considered by building engineers, plant operators, and many mechanical contractors in design and construction.  A consulting engineer or mechanical designer will often initially specify schedule 80 or extra heavy pipe as soon as threading is involved, but such plans then change at some point along the way toward installation.  Cost cutting, or reducing the pipe thickness based on pressure requirements alone, are typically the reasons.

At the smallest sizes, the amount of wall lost during threading actually equals approximately 65% of the original pipe wall.  Such high initial wall loss, coupled with a corrosion rate anywhere exceeding 2 MPY, will inevitably produce a premature pipe leak.

• Minimum Thickness Clearly Defined

The long established standard piping formula, known as the Barlow formula, (Piping Handbook, Nayar, 6th Edition, C.138) is used to calculate the pressure that a section of pipe of known thickness will tolerate, and is represented as:

tm = PD/2SE + A

This formula takes into account variances in:

Pipe diameter
Internal pressure
Material strength efficiency factor
Temperature
Corrosion allowance
Mill tolerance
Material strength
Joint preparation and efficiency

The minimum wall thickness derived by this calculation is relevant for all types of pipe, as well as for all materials, and is the standard by which design engineers specify the material wall thickness for new building or process piping construction.

In general, schedule 40 steel piping satisfies the engineering requirements of most building applications.  However, under certain pressure conditions and pipe sizes, special consideration must be taken to ensure that threading or grooving does not reduce the pipe wall thickness past the permissible minimum dimensions.  With threaded schedule 40 pipe used in condenser water or open process applications, that minimum acceptable wall thickness standard is not met for most small pipe sizes on the first day of installation.

• Schedule 40 Often Not Acceptable

For illustration, CorrView International has prepared the following table showing a series of calculations for carbon steel black pipe in sizes 3/4 in. through 3 in.  Different configurations of schedule 40 vs. schedule 80, open vs. closed, and welded vs. threaded pipe are presented.  A corrosion factor of 0.065 in. is applied as defined by Barlow in estimating permissible wall loss.

At the two far right columns comparing both new original pipe wall thickness and calculated minimum acceptable wall dimensions, we can easily illustrate the threat of using schedule 40 pipe under certain operating conditions.  While corrosion activity is still the major factor in piping failures, in fact, even new schedule 40 pipe does not meet minimum thickness requirements in threaded open water applications based upon the minimum value calculated according to Barlow and considering a reasonable 0.065 in. corrosion allowance.  In such applications, therefore, the use of schedule 80 is recommended.

We can see by both above tables that the primary cause of failure to meet minimum specifications is the amount of wall thickness removed in the threading process.  Secondary to that is the higher corrosion rate factor specified for open water condenser systems – a 0.065 in. lifetime corrosion allowance we have well documented as being far too low for today’s operating conditions.

This 0.065 in. wall loss was factored into the Barlow formula based upon anticipation of a low 1 MPY corrosion rate over the 65 year estimated service life for a typical building property.  With 3-5 MPY being the average condenser water corrosion rate today, and with 10 MPY corrosion rates not uncommon, it is in fact possible to exceed this previously anticipated lifetime corrosion loss of 65 mils in only a few years.

CorrView International, LLC has long used the Barlow formula in all ultrasonic piping investigations, and has found the resulting minimum acceptable wall thickness and remaining life prediction estimates accurate and reliable.  It is the firm recommendation of CVI that schedule 80 black carbon steel pipe should be used exclusively under such threaded condenser water conditions – regardless of operating pressures.  Its use in other services where small diameter piping exists provides an added level of protection.

• Dramatic Wall Loss

The below sample of new 2 in. threaded ASTM A53 pipe very well illustrates the high degree of wall loss caused when pipe is threaded.  Such thread loss is unavoidable in most circumstances, but where installed in more corrosive environments, must be compensated for by using heavier materials if long service life is desired.

This above cross sectional close-up view of a threaded end of schedule 40 pipe shows the original wall thickness at the left having a micrometer and ultrasonically measured wall thickness of 0.156 in. – just slightly above the ASTM factory specification of 0.154 in.  This cross section of a standard NPT taper thread shows the deep loss of steel at the right side or forward edge of the thread.  We provide another close-up view of the same pipe sample below, and again illustrating the dimensional loss due to threading.

Here, a micrometer measurement at its solid wall thickness dimension shows 0.156 in. of available pipe wall, while the below photograph taken at the lowest cut area of the threads provides only 0.079 in. of pipe wall.  This represents a 0.077 in. or an approximate 50% loss which occurs on the first day of installation.

It is this high initial wall loss, ranging from 35% to 68% of the original schedule 40 pipe wall, which typically limits the service life of any threaded piping system.  Schedule 40 pipe typically does not provide reliable condenser water service much beyond 20 years except at the most well maintained systems – where corrosion can be confirmed at 1 MPY or below.  With 0.079 in. of material at its leading edge, and having a more typical corrosion rate of 3-5 MPY, it is easy to demonstrate that a service life of 20 years or less is all that should be expected from schedule 40 condenser water pipe.

• Schedule 40 Condenser Water Leaks Inevitable

Failures at threaded joints represent the most common of all corrosion related problems.  The appearance of a leak is rarely an isolated event, but simply the first indication of a larger and system wide problem.  While spot repairs may extend the service life of the system, at some point the increased frequency of failure will demand total pipe replacement.  As the piping system is allowed to operate under such conditions, the level of threat from a major piping failure also increases.

We provide multiple examples of this problem in our Corrosion Photo Gallery illustrating the typical evolution of a threaded joint failure at steel to steel connections.  Under the same corrosion characteristics, threaded joints will always fail prematurely.  In many examples, this loss is amplified by galvanic activity caused by the direct connection of threaded black pipe to a brass valve or copper fitting.  We also provide examples of galvanic induced failures in our Corrosion Photo Gallery.

• Exterior Deposits A Certain Sign Of A Problem

The first sign of a thread failure is the appearance of rust and brown colored encrustation around the threads.  While often misinterpreted as corroding from the outside, this is an internal corrosion product.  Dissolved iron oxide, fine particulates, calcium carbonate and other water soluble elements are carried through the microfine penetration of the pipe to the outside where it evaporates.  These dissolved elements then precipitate out and accumulate at the immediate area.  As long as the rate of evaporation exceeds the rate of the leak, the problem remains localized.

Such small leaks are often not noticed or simply ignored.  Once the leak rate exceeds the evaporation rate, however, water droplets travel elsewhere to cause additional problems and attract notice – thereby prompting repairs.  Ignoring such small tell tale signs of a problem allows a greater amount of the pipe to wear – thereby possibly creating a much greater leak once it does eventually fail.  The common argument that pipe leaks seal themselves up with internal rust simply does not hold.

The use of schedule 40 threaded pipe at fixtures such as temperature wells, pigtails, pressure gauges, and control sensors also presents the same threat.  If located before a shut-off valve, the failure of even such small piping components can require the shut down of the entire piping system for repair.

• Dependent Upon Corrosion Rate

Under controlled corrosion conditions of 1 MPY or less, such as occurs typically at chill water systems and other closed conditions, threaded schedule 40 pipe offers generally acceptable service life.  Yet, under extremely high corrosion conditions, the installation of threaded schedule 40 pipe can produce failures in as little as two years.

It should be noted that while schedule 80 pipe does provide greater wall thickness and therefore greater pipe life, it does not offer substantially longer service life under high corrosion conditions exceeding 10 MPY.  Brass valve to carbon steel connections lacking a galvanic coupling or insulator, a common installation scenario, simply accelerate the corrosion process.

In short, a high corrosion condition is a problem – but a high corrosion condition where threaded schedule 40 pipe has been installed represents significantly greater threat, and far more difficulty and expense to correct.

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