Specifying pipe and piping materials
Piping is used within many building systems, including HVAC, plumbing, specialty chemicals and fluids, and fire protection. Knowing what type of piping to use in a specific application can help preserve the life of a system or avoid a catastrophic failure.
By Matt Dolan, PE, LEED AP BD+C, JBA Consulting Engineers, Las Vegas
1. Understand the challenges of low- and high-rise piping systems.
2. Learn about three piping system types: HVAC (hydronic piping), plumbing (domestic water, and waste and vent piping), and specialty piping for chemicals and fluids (saltwater systems and hazardous chemicals).
Pipe and piping systems are found within many elements of buildings. Numerous people have seen a P-trap below a sink or refrigerant lines routing to and from their residential split system. Fewer have seen the main utility piping routing from a central plant or the chemical treatment systems within a pool equipment room. Each of these applications requires a specific type of pipe to meet the requirements of the codes, physical limitations, specifications, and best design practices.
There is no simple piping solution to meet all applications. Provided that specific design criteria are followed and the right questions are asked of the owner and operational staff, these systems can meet all of the physical and code requirements. In addition, they can maintain the proper cost and lead times to create a successfully implemented building system.
HVAC piping encompasses many different fluids, pressures, and temperatures. This piping can be located above or below ground and route through the interior or exterior of a building. These factors must be taken into consideration when specifying HVAC piping within a project. The term “hydronic” refers to the use of water as a heat transfer medium for cooling and heating. In each application, the water is supplied at a set flow rate and temperature. Typical space heat transfer is completed using an air-water coil designed to return the water at a defined temperature. This results in a specified quantity of heat delivered or removed from the space. Hydronic chilled and heating water are the dominant systems used to condition large commercial facilities.
For most low-rise building applications, the expected system working pressure is typically less than 150 pounds per square inch gauge (psig). Hydronic systems (both chilled and heating water) are closed-loop systems. This means that the total dynamic head of the pumps takes into account the friction losses within the piping system, associated coils, valves, and accessories. The static height of the system does not affect the pumping capacities, but it does affect the required working pressure of the system. A 150 psig working pressure rating for chillers, boilers, pumps, piping, and accessories is common for equipment and component manufacturers.This pressure rating should be maintained within system designs whenever possible. Many buildings that are considered as low- or medium-rise will fall into the 150 psig working pressure category.
Maintaining the piping system and equipment below the standard pressure of 150 psig becomes more difficult when designing high-rise buildings. A static piping height above approximately 350 ft (with no pump pressure added to the system) will exceed the standard working pressure rating for these systems (1 psig = 2.31 ft of head). This system would most likely employ a pressure break (in the form of heat exchangers) to isolate the higher pressure requirements of the tower from the rest of the connected piping and equipment. This system design would allow standard pressure chillers to be designed and installed, while specifying higher pressure piping and accessories within the tower component.
When specifying piping for a large campus project, designers/engineers must be intentional when editing the associated specifications sections (ARCOM MasterSpec sections 23 21 13.23 and 23 21 13.13, respectively, for above- and below-grade hydronic piping) to be certain that the piping specified for the tower and podium are reflective of their individual requirements (or collective requirements if heat exchangers are not used to isolate the pressure zones).
Another component of the closed-loop systems is water treatment and purging of any oxygen from the water. Most hydronic systems are fitted with water treatment systems composed of various chemicals and inhibitors to maintain the water flowing through the pipes at optimal pH (approximately 9.0) and microbiological levels to resist bio-film buildup and corrosion within the piping. Stabilizing water within the system and removing any air helps provide the full life expectancy of the piping, associated pumps, coils, and valves. Any air left within the piping can cause cavitation at the chilled and heating water pumps and reduce heat transfer within chillers, boilers, or hydronic coils.