Environmentally Sound Piping Systems

Double Containment Piping Prevents Leaks

Two infamous names—-Exxon Valdez and Deepwater Horizon—bring to mind the catastrophic damage man has inflicted on the environment. From the Exxon Valdez tanker spill the oil industry learned the hard way that reconstructing tankers to a double hull design was the best solution for preventing another devastating accident. Today’s oil tankers have an inner carrier hull for carrying the oil as well as an outer hull to serve as a container in case the carrier hull leaks.

When an application calls for a risk-free, no-leak piping system, the clear answer is double containment piping (DCP), known also as “dual-containment.” This principle of dual containment has now been used for nearly three decades.

Most typical DCP systems have three basic components: carrier pipe, containment pipe, and spacers or centralizers (attachments placed every 4 to 5 feet and used to keep the containment pipe concentrically supported to the carrier pipe). The gap between the inner and outer pipes is called the interstitial or annular space (figure 1). To locate, pinpoint and assess a carrier pipe leak, automatic leak detection sensors or visual sight glasses are employed.

In any DCP system there must be room enough inside the containment piping to maneuver the carrier pipe and fittings. Table 1 lists the most common inner and outer pipe diameters. Let’s look in closer detail at the three basic types of DCP systems: pre-assembled, custom-fabricated and retrofit.

Fig. 2:Pre-assembled PP DCP piping system

Pre-assembled DCP system: These systems are manufactured with stock lengths (usually 20 feet) of standard piping materials and pre-assembled fittings. Its advantages lie in the speed of shipping and lower overall cost (figures 2 and 3).

Fig.3:Pre-assembled PVC DCP fitting

Most pre-assembled systems are limited to the same material for both the carrier and containment piping, such as PVC x PVC, CPVC x CPVC, or PP x PP. Typical applications for pre-assembled DCP systems are gasoline and jet fuel handling, chemical waste systems, chemical process lines, landfills, and the transportation of other harmful fluids in critical areas.

Fig. 4:Pre-assembled copper insulated piping

A variation of a pre-assembled DCP system uses foam-filled insulation in the interstitial area instead of spacers. This system is not used to contain leaks but rather to encapsulate the insulation from external corrosive, chemical and impact damage. A system like this can maintain non-toxic fluid temperatures at a desired level for applications such as hot water distribution, steam, cryogenics and condenser and chilled water. Although the initial material cost for this system is high, the real savings comes from eliminating costly external labor-intensive insulation fabrication (figure 4).

Fig. 5:PVC x stainless steel DCP waste system for pharmaceutical firm

Custom-fabricated DCP system: By far the dominant DCP market is for custom systems. In this category, carrier and containment components can be specified in any combination of piping material. Carrier and containment piping combinations consist of plastic and/or metal such as PVC x PVC, PVDF x PE, PP x PP, Copper x PVC, Steel x PVC, Stainless Steel x CPVC, and PVDF x FRP. The possible piping combinations are almost endless (figure 5). Applications here are the same as those for pre-assembled DCPs.

Fig.6:Clear PVC retrofit system

Retrofit DCP system: Conditions of service in manufacturing plants are continually revised and changed, as are government codes pertaining to the protection of the environment and the safety of personnel. If codes and/or service conditions require DCP, it is usually simpler and more cost effective to retrofit an existing single line piping system than it is to replace it with a completely new DCP system. Retrofit systems consist of split piping and fittings used to contain existing single piping systems. Presently, retrofits are limited in their containment materials and containment pipe diameters. But, if conditions and budget warrant, it would be prudent to investigate the use of retrofitting existing piping systems (figure 6).

Fig.7:PP DCP valve box

Although dual piping is the heart of the DCP system, there are other critical ancillary products that ensure system integrity. Valve boxes (see figure 7), double-contained tanks (figure 8), and leak detection devises (see figure 9) may be required to create a total leak-free system.

Fig.8:PE double-contained tank for landfill application

Though there are just a limited number of experienced DCP fabricators in North America, their products and systems are proven. For that reason it is recommended for engineers and end-users to use the piping systems and resources of these reliable companies rather than, in an attempt to save costs, pay a piping contractor to reinvent the wheel.

Fig.09:PVC x PVC DCP with visual & electrical sensor leak detection - courtesy of IPEX

As potable water continues to be a critical concern for the public welfare, additional codes and resources will be required to ensure the protection of the billions of gallons in underground water aquifers. OSHA, also, will continue to insist nothing is more important than protecting plant workers. Clearly DCP systems will be necessary and will certainly play a growing and more important role in delivering efficiency and safety.

In applications where leaks are not an option, double contained piping systems is the best and only option.

David A. Chasis

Chasis Consulting Inc.

Table I
Common Size Range of Double Containment Piping Systems

Special thanks:
Figures 2 & 7 courtesy of Asahi-America
Figures 4 & 8 courtesy of PPFA
Figure 3, 5 & 9 courtesy of IPEX
Figure 6 courtesy of George Fischer-Sloane