Although not as common as they once were, air pressure systems do still exist, so you should prepare yourself for when you encounter one.
More reliable plant designs are now readily available for OSP cabling designs, but every once in a while you still run across the occasional air pressure system or two. Air pressure systems were intended to protect air-core cables from water and other liquid contaminants that may penetrate sheath cracks, breaks, or leaky splice closures. Their purpose was to keep the air-core sheath dry, which would in theory extend the life of the cable plant. You may run across these systems on large educational campuses or large commercial complexes.
In this seventh installment of EC&M's 12-part look at the Second Edition of BICSI's Customer-Owned Outside Plant Manual, we discuss the unique nature of air pressure systems and the steps you need to take to maintain them. This examination will help guide you through issues like placement locations, system components, and measured pressures to be maintained within these systems.
History of air pressure.
The concept of constructing cables and splice closures to hold air pressure goes back to the early days of OSP construction and design. In the early days of system design, paper (pulp) materials were introduced as an efficient and practical way of insulating cables. As a result, protecting the insulation from moisture and contaminants became critical.
System designers accomplished this by introducing air pressure into the cable from a main distributing point via large compressor units. These units were supported by additional feeds located at various intervals along the cable path. Alarms and pressure monitoring systems helped alert the plant owner to system failures and they isolated problem sections. With good mapping and record keeping, maintenance personnel could quickly locate and repair failed system components.
Most conduit routes that feed metropolitan areas still use air pressure systems to protect their cables. The high cost of replacing these large cables would make it difficult to completely eliminate such systems, so it's more cost-effective to maintain them. Advances in air pressure system technology have changed all of that, though. The introduction of plastic insulated conductor (PIC) insulation and filled cable designs has eliminated the need to build or add additional air pressure systems onto a network.
Today, new conduit systems within campus environments are constructed and planned in such a way so that the installation of an air pressure system won't be necessary. However, there are still several of these systems in use today that must still be maintained. Therefore, it's best to have a clear understanding of how the individual components of these systems work and how best to keep them working properly.
System components and specifications.
A pipe system method of cable pressurization is a configuration that applies essentially air pressure through an air pipe to cables in the cable entrance facility. Air pressure is also supplied at selected maintenance holes (MHs) or handholes (HHs) along an underground cable route by manifolding an individual cable to a paralleling air feeder pipe that carries pressurized dry air from an air source.
The following items are the most common components in a typical air system.
Air pipe (feeder, distribution).
Air dryer (compressor, dehydrator).
Automatic shutoff valve.
Pressure transducer unit.
Cable pressure monitoring system.
The Figure to the right shows a diagram of a typical air pressure system.
Air pipes are generally lined in aluminum and made of black polyethylene. They typically come in 2,000-ft to 3,500-ft reels and can accommodate a maximum pulling tension of about 150 lb. These pipes are generally placed to the last manifold location.
Air dryers come in various sizes, and their main function is to force air into pipes at a constant rate. Controls and monitoring devices maintain consistent generation of the air. They're also available in remote models to be pole- or pad-mounted at locations in the field where main piping sources have become isolated.
Manifold assemblies distribute air from the main air pumps to the individual cables. They're spaced along the route to maximize their ability to feed sections between monitoring points.
Automatic shutoffs are also in place to prevent backflow of air when low-pressure conditions exists. They're generally installed between the air pipe and the manifold assembly.
A well-designed system will have pressure transducer units located at monitoring points along the route that are capable of converting cable pressure to electrical resistance. These electrical resistance readings are then sent back to monitoring panels located at the centralized location via a cable pair placed along the route.
Today's monitoring systems can provide full-time surveillance of various system components like meter panels, pipes, and transducers. Using computer technology, a monitoring system can provide real-time reporting and alarm notification to pinpoint damaged sections, take measurements, and automatically alert you to problem areas on the system.
The Table to the right lists some commonly used minimum pressures among larger systems.
Closing the air gap.
As an OSP system designer, you must be prepared to encounter older system designs and outdated, but operable, technology on any future project. It's important for you to understand that any additions you make to these systems may impact the future reliability of the network. Your knowledge of these older technologies will go a long way toward maintaining the integrity of the system. And BICSI's Customer-Owned Outside Plant Design Manual is a good source for referencing this obsolete, but very important, method of OSP cable protection.
Next month we'll examine rights-of-way. Sometimes it may be necessary to acquire an easement or purchase a private right-of-way to “bridge the gap” between several pieces of property that make up the complex being served. As an OSP designer, you must be aware of how to secure these rights and gain approval from the proper AHJ.
Hite is a special projects engineer-OSP for CT Communications, Inc., Concord, N.C.
The material for this article was excerpted with permission from BICSI's Customer-Owned Outside Plant Design Manual, Second Edition.