By: Michelle Knight on May 14, 2019 9:00:00 AM

ARE YOU OVERENGINEERING YOUR PIPING SYSTEM?

Industrial piping design engineers strive to install systems that will operate safely and reliably over the long term. To that end, more than one set of specified conditions for the piping system’s temperature and pressure are often considered. These may include standard operating conditions, maximum operating conditions and design conditions. To create a wide margin of safety that ensures the system will continue to operate reliably, engineers usually choose a very aggressive set of circumstances regarding temperature and pressure conditions.

It’s important for engineers to keep in mind that manufacturers share this concern for reliability and incorporate a significant safety factor into the piping systems they produce. Thus, if industrial piping design engineers increase the safety factor to account for extra-aggressive design conditions, the system may require expensive add-ons such as exotic alloys or FRP-reinforcement when ordinary plastic pipe alone would meet and exceed the application requirements for safety and reliability.

A Piping System Application Scenario

Lubrizol was recently approached with a chemical resistance inquiry regarding whether Corzan® CPVC can handle sodium hypochlorite at a concentration of maximum 12 percent. The maximum working conditions for this application were 106°F at 188 psig (41°C at 13 bar), with the design conditions specified for 130°F at 226 psig (54°C at 15.6 bar).

Corzan CPVC piping has sufficient chemical resistance for the above application scenario (please see the Corzan Pipe Chemical Resistance Chart). The following pipe sizes were calculated as having the necessary pressure rating:

According to ASTM F441 for the design conditions:

Schedule 80 pipe only in sizes 2.5” (63.5 mm) and smaller

Schedule 40 pipe only in sizes 1” (25 mm) and smaller

According to ASTM F441 for the operating conditions:

Schedule 80 pipe in sizes 8” (203.2 mm) and smaller

Schedule 40 pipe in sizes 3” (76.2 mm) and smaller

With both options, the pipe’s pressure rating already includes a safety factor of 2.0. No specific life span is stated in the ASTM F441 standard.

For cost efficiency, pipe design engineers should consider the safety factor that is already built into the pipework for different standards when deciding if it’s necessary to increase the safety factor through additional design requirements. Designers must decide whether the safety factor included in the pipe’s pressure rating provides a sufficient margin of safety, or

if more aggressive design conditions should be considered to increase the safety factor even further.

The Built-In Safety Factor for Plastic Piping

Plastic piping systems are pressure rated based on thousands of hours of test data. Detailed information about how piping systems are pressure rated may be found in the white paper “How CPVC Pipe Pressure Ratings Are Calculated,” which addresses topics such as testing methodology, calculating long-term hydrostatic strength, validating data, etc.

The calculations to determine the pressure rating of a plastic piping system already include a safety factor of 2 for pipes rated according to ASTM standards. This means the data actually predict the pipe will remain operational for approximately twice its rated pressure during its lifetime.

The Corzan® CPVC Comparison Burst Test below shows the minimum pressure required before failure of 1” schedule 80 pipe compared to Corzan CPVC.

Industrial piping design engineers should be aware of the inherent added safety factor that’s built into plastic piping based on ASTM standards for pressure and consider it when analyzing operating conditions versus design conditions. Additionally, it’s important to keep in mind that an additional safety factor is not built into the temperature rating for plastic piping. If maximum operating conditions or design conditions truly require performance at temperatures that exceed the material’s rated temperature, the design engineer should consider an alternate material.

Learning About the Safety Factor Incorporated into Plastic Piping Systems