COLUMN ENGINEER'S NOTEBOOK source, or with an increase in the speed with which it is started or stopped. Examples of situations where hydraulic shock can occur are valves that are opened or closed quickly. Surge pressures should be calculated for plastic piping systems. The surge pressure is added to the maximum operating pressure and should not exceed the short-term pressure rating of the pipe. Pipe Insulation Materials Insulation provides the primary thermal resistance against heat loss or gain in distribution systems. Insulation of piping in underground utilidors that could be subject to water intrusion should use closed cell insulation. Thermal properties and other characteristics normally used in thermal distribution systems can be found in Chapter 12, "District Heating and Cooling," of the 2012 ASHRAE Handbook.6 Insulation used in most preinsulated chilled water and heating hot water applications is polyurethane. No insulation system is completely vapor tight. The best way to minimize corrosion is to make the system highly water resistant by using closed-cell insulation material coupled with a good vapor retarder. Preinsulated ferrous piping should have polyethylene heat shrinkable end caps or similar method to prevent moisture from penetrating the preinsulated casing. Carbon steel welded exposed joints should be protected with a polyethylene tape coating system on the pipe prior to bridging the preinsulated casings with a polyethylene sleeve and insulating the joint. Do not assume that the preinsulated jacketing system will provide complete protection against corrosion of ferrous materials. Copper wires can be installed during fabrication to aid in detecting and locating liquid leaks in the preinsulated piping system. These systems can monitor the entire length of the underground piping system by looking for a short in the circuit using Ohm's law or monitor the impedance change using time-domain reflectometry. The copper wires can also be used as a tracer wire for locating the buried pipe in the future using a metal detector. www.info.hotims.com/49808-21 56 ASHRAE JOURNAL ashrae.org SEPTEM BER 2014