Technical Bulletin # P-02


Why You Cannot Use Schedule 40
Threaded Pipe In Condenser Water Applications



THE PROBLEM:

     Over the life-span of any building property, the replacement, renovation or addition of building services piping is a frequent occurrence. Due to the fact that operating pressures of even the tallest buildings rarely exceeding 250 PSI, pressure is rarely a factor in the selection of pipe schedule.

     Installations of small diameter threaded black pipe generally use Schedule 40 automatically - with little thought given to the physical wall thickness limitations of the pipe itself. As a result, it is not uncommon to find premature failure of relatively new installations, as well as examples of properties which have replaced such small diameter piping on a regular basis of every four, five, or six years. Yet in contrast, other properties, decades old, often show long service life at their smaller diameter piping.

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



     The below Table 4A well illustrates the degreee to which the threading process weakens Schedule 40 pipe. At the smallest sizes, the amount of wall lost during threading actually equals approximately 65% of the original pipe wall. Such initial high wall loss, coupled with a corrosion rate anywhere exceeding 2.0 MPY, will inevitably produce a premature pipe leak.


Table 4A - Wall Loss After Threading



THE SOLUTION:

     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:

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

     In general, Schedule 40 steel piping satisfies the engineering requirements of most building applications. However, under certain 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.

     For illustration, ECI has prepared Table 4B 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, are presented. View Table P-02B.

     By reviewing the two far right columns comparing both new original pipe wall thickness and calculated minimum acceptable wall dimensions, you can easily understand the threat of using Schedule 40 pipe under certain operating conditions. While corrosion activity is still the major factor in piping failures, in fact, new Schedule 40 pipe does not meet minimum minimum thickness requirements in threaded open water applications, and should not be used.

     As can be seen in our table, the primary cause of failure to meet minimum specifications is the amount of pipe wall removed in the threading process. Secondary to that is the higher corrosion rate factor specified for open water condenser systems - a 65 mil lifetime corrosion allowance we have well documented as being far too low for current operating conditions.

     ECI has long used the Barlow formula in all ultrasonic piping evaluations, and has found the resulting minimum acceptable wall thickness and remaining life prediction estimates accurate and reliable. It is the firm recommendation of ECI that Schedule 80 black iron pipe should be used exclusively under such conditions - regardless of operating pressures.

     The below photograph illustrates the typical evolution of a threaded joint failure, and shows that under the same corrosion characteristics, threaded joints will always fail prematurely. In this investigation, failure of the pipe threads began within 4 years of installation. With 2 in. Schedule 40 pipe at 0.154 in. original wall, less 0.087 in. during the threading process, and having an identified a 7.2 MPY corrosion rate over a period of 3.5 years, wall thickness at the threaded areas was just under 0.042 in. or 42 mils at the time of testing.



     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 such a piping component can require the shut down of the entire piping system for repair. Shown below, leakage at the temperature well has developed past an encrustation or iron oxide at the threads, to an actual dripping leak.


     Even under controlled corrosion conditions, threaded Schedule 40 pipe does not offer long service life, as shown below by this evaluation of an 11 year old condenser water system. Here, a controlled 2-3 MPY corrosion rate still created multiple failures within a short period of time. The brass valve to carbon steel connection, a common installation scenario without benefit of a galvanic coupling or insulator, simply accelerated the corrosion process.



     Under extremely high corrosion conditions, the installation of threaded Schedule 40 pipe can present widespread operating problems. In the below example, a high corrosion rate of near 20 MPY produced leaks at many threaded fittings within 2-1/2 years of start-up. It should be noted that while Schedule 80 pipe would provide more wall thickness and therefore greater pipe life in the below example, it would not offer substantially greater service life under such high corrosion conditions.



     We therefore strongly recommend using Schedule 80 or Extra Strong pipe in all threaded applications serving condenser water or steam condensate locations, or where a high corrosion rate is anticipated. Establishing a strong building specification for such construction materials is also advised.



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