Consider chemicals, temperature, pressure, expansion and more.
by Rodney Van
May 15, 2019

In 1988, the Environmental Protection Agency (EPA) issued regulations for underground storage tanks (USTs) and piping systems that handle petroleum products and hazardous chemicals. In 2015, regulations that affected owners and operators installing secondarily contained tanks and piping were revised. Owners and operators must use interstitial monitoring as release detection for these new or replaced tanks and piping. EPA regulation 40 Code of Federal Regulations (CFR) 280 lists more than 800 chemicals that are considered hazardous and have to be double-contained. The Resource Conservation and Recovery Act 40 CFR 280 defines USTs as any tank system, including any piping connected to it, with at least 10 percent of its volume underground.

State environmental guidelines and enforcement may exceed those of the EPA. States with an approved UST program have the lead in UST/piping enforcement where states without an approved program work with the EPA to coordinate enforcement actions.

image 1 large diameterImage 1. Large diameter double-containment polypropylene system (Images courtesy of Asahi/America Inc.)

EPA regulation 40 CFR 280 requires that secondary containment must be able to hold leaking media for a minimum of 30 days. Drainage or suction systems can be monitored manually while pressurized systems require automated monitoring so the flow of the media can be restricted to avoid potential ground water contamination. Aside from the EPA regulating and enforcing 40 CFR for below-ground installations, many corporations have internal safety and environmental policies stipulating double-containment systems for underground and overhead piping systems. The Occupational Safety and Health Administration (OSHA) does not have a specific double-containment regulation for above-ground piping systems. The OSHA occupational chemical database lists hazardous chemicals and their permissible exposure limits (PELs). OSHA recommends isolating or enclosing the process as a way to control exposure of personnel to toxic substances.

image 2 butt weldImage 2. Example of a simultaneous butt weld

Design Criteria/Operating Parameters

Materials

Reasons why thermoplastics should be considered when designing a piping system include: excellent flow characteristics, corrosion/chemical resistance, energy efficiency, light in weight, fast installation time and long life expectancy. A wide assortment of materials exist that offer long-term performance where there is little or no degradation after 25 years. Little maintenance is required because there are no pitting, scaling or rusting corrosion. Polyvinyl chloride (PVC) and chlorinated polyvinyl chloride (CPVC) solvent-cemented joining systems, polyolefins—including polyethylene (PE) 100-RC, high density polyethylene (HDPE) and polypropylene—and fluoropolymers, such as polyvinylidene fluoride (PVDF), ethylene tetrafluoroethylene (ECTFE) and perfluoroalkoxy (PFA), use a variety of thermal fusion joining methods. These can be assembled using simple hand-held welding tools, hydraulic equipment or automated equipment that verifies proper welding procedures. Fusion joining methods have become more common in process applications due to their mechanical strength, chemical resistance and ease of installation for most field conditions. Pressure testing can be done within minutes of a final weld.

Chemicals & Temperature

All thermoplastic materials are resistant to a variety of organic and inorganic chemicals up to high concentrations with temperature capabilities ranging from cryogenic to 400 degrees F. Chemical resistance and temperature tolerances vary from one plastic to another. Many thermoplastic manufacturers provide chemical resistance guides. Consult the manufacturer for applications where there may be many chemicals or ones that are aggressive.

Pressure

Thermoplastics have varying pressure ratings at different temperatures. Pressure ratings for thermoplastics are reduced at elevated temperatures. Knowing the system’s standard operating pressure, temperature and heat gain from sun exposure is the starting point. Spikes in temperature may reduce the pressure rating. It is necessary to examine other conditions that may affect the operations of the piping, like heat radiating from a roof. If pressure spikes occur, determine for how long and how high.

image 3 restraint fitting optionsImage 3. Restraint fitting options for a buried system

Temperature

The design for a double-containment piping system is similar to single-wall piping design. Chemical, temperature and pressure data are important for the carrier line and the containment secondary line. Temperature plays a key role, affecting the pressure rating of the pipe system and expansion rate. With regards to expansion: it is the rule, not the exception when engineering a piping system using thermoplastic materials. As temperatures rise and fall, thermoplastic materials expand and contract to relieve stress. Stress analysis can be calculated to determine the amount of growth and flexibility the system will require. 

Expansion

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