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District Energy System

DES System Descriptions

Steam

The DES contains approximately 22,100 ft of steam piping ranging in size from 20” to 1.25” diameter. The majority of this piping is buried beneath the streets and sidewalks of downtown Nashville, while the balance of the steam piping is located in the tunnels. Steam vaults are located throughout the system to provide access to valves, traps and expansion joints.

All of the steam piping is welded or flanged steel pipe designed for 150 psig and 366°F and is installed in accordance with ASME B31.1. The steam piping system remains in good condition even as portions are over 30 years old, which, by industry standards, is relatively young. The more recently installed steam piping has been hydrostatically tested at 150% of the design pressure and 100% of all the buried butt-welds have passed a radiographic examination prior to energizing the system. The extensive testing that we require on the direct-buried piping helps prevent costly steam leaks, excavations and unplanned shutdowns.

The steam system operates at 150 psig at 366°F saturated vapor. This pressure does not vary significantly throughout the system and all of the DES customers reduce the delivery pressure for their in-building systems through pressure reducing valves (PRV). The installation of all PRV’s at customer buildings must include an appropriately sized safety relief valve (SRV) or a tandem PRV must be used. In either case, the requirements of ASME B31.1 must be satisfied.

Expansion joints and buried expansion bends and loops are used in the steam system to compensate for thermal expansion. Pipe anchors are located throughout the system and are used to resist the forces generated by thermal expansion. Many of these anchors are direct-buried, but some are located in vaults and in the tunnels. Anchors must not be disturbed by unauthorized personnel or when the steam system is active. Damage to these anchors, the soil or vault structure around them may result in a failure of a portion of the steam piping.

Condensate

The DES contains approximately 23,000 ft of condensate piping ranging in size from 10” to 1” diameter. The majority of this piping is direct-buried and largely parallels the steam piping. Condensate piping is also located within the system steam vaults to collect the condensate from the steam traps.

All of the condensate piping is welded, threaded or flanged steel pipe designed for 150 psig and 300°F and is installed in accordance with ASME B31.1. Through several capital projects completed over the past few years, almost all of the original condensate piping has been replaced and is less than 10 years old. The more recently installed condensate piping has been hydrostatically tested at 150% of the design pressure and 100% of all the buried butt-welds have passed a radiographic examination prior to energizing the system. The extensive testing that we require on the buried piping helps prevent costly leaks, excavations and unplanned shutdowns.

The condensate system operates at varying pressures throughout the system. The DES customers typically collect their in-building condensate in atmospheric tanks and pump the water back to the DES. Condensate is also formed in the system’s steam piping and is removed via steam traps and piped into the condensate piping. Since the condensate flow rate from the customers is intermittent and essentially gravity-flows back toward the EGF, the system operating pressure varies from approximately 35 psig to 100 psig. The temperature of the condensate also varies, but it is seldom above 190°F. The in-building condensate systems must be designed and installed in accordance with ASME B31.9.

Expansion joints and buried expansion bends and loops are used in the condensate system to compensate for thermal expansion. Pipe anchors are located throughout the system and are used to resist the forces generated by thermal expansion. Many of these anchors are direct-buried, but some are located in vaults and in the tunnels. Anchors must not be disturbed by unauthorized personnel or when the condensate system is active. Damage to these anchors, the soil or vault structure around them may result in a failure of a portion of the condensate piping.

Chilled Water

The DES contains approximately 23,000 ft of chilled water supply piping and another 23,000 ft of return piping. This piping ranges in size from 42” to 3” diameter. The majority of this piping is direct-buried and some piping passes through the tunnels and various steam vaults. The isolation valves for parts of the chilled water system are located in the steam vaults and the tunnels to provide quick and easy access, but there are a number of direct-buried valves, too.

The chilled water piping is a collection of welded, threaded or flanged steel pipe and push-on ductile-iron pipe. In general, the chilled water piping system is designed for 150 psig and 42°F and is installed in accordance with ASME B31.1 or AWWA C600 (for ductile iron pipe). However, at some locations, the chilled water pressures can exceed 150 psig, especially during the summer months. Special care must be taken at these locations to ensure that all chilled water components that can experience these higher pressures are appropriately rated. All of the more recently installed chilled water piping has been hydrostatically tested at 150% of the design pressure and 100% of all the direct-buried butt-welds have passed a radiographic examination prior to energizing the system. The extensive testing that we require on the direct-buried piping helps prevent costly leaks, excavations and unplanned shutdowns.

The chilled water system operates at varying pressures throughout the system and through the year. However, the typical pressures experienced by the DES customers are approximately 120 to 140 psig. The chilled water supply temperature also varies between approximately 39°F to 43°F. The chilled water return temperature is dependent on the amount of heat the DES customers has removed from their facility. We would like to see as high a return temperature as possible from all of our customers, but the return temperature is typically 52°F to 54°F during the summer months and 48°F to 50°F during the winter months. The in-building chilled water systems must be designed and installed in accordance with ASME B31.9.

Vaults and Tunnels

The DES includes a vast underground network of piping through which steam, chilled water and hot condensate flows to and from the EGF and the DES customer buildings. The majority of this network is comprised of direct-buried piping but several tunnels are also located throughout the system to provide easier access to the piping and equipment located therein.

The steam piping operates at 366°F and loses heat to the surrounding soil. Even though the steam piping is insulated, the presence of a temperature difference between the hot steam pipe and relatively cold ground (~50°F) drives this heat loss. As the heat is lost from the steam travelling through the piping, liquid water forms and falls out of suspension with the steam vapor. This liquid water is referred to as condensate and must be collected and removed from the steam piping via devices called steam traps. The steam traps must be serviced regularly; therefore, underground steam vaults are constructed at critical intervals to provide a means to access these traps.

In addition, one method of compensating for the thermal expansion of the steam and condensate piping involves the use of devices known as expansion joints. These devices also require regular maintenance and inspection. Vaults are also used to provide access to steam and condensate isolation valves. The chilled water piping passes through a few vaults and isolation valves are installed on those lines as well.

Expansion joints and direct-buried expansion bends and loops are used throughout the steam and condensate systems to compensate for thermal expansion. Anchors are used to resist the forces generated by thermal expansion. Many of these anchors are direct-buried, but some are located in vaults and in the tunnels. Anchors must not be disturbed by unauthorized personnel or when the steam system is active. Damage to these anchors, the soil or vault structure around them may result in a failure of a portion of the steam or condensate systems.

Steam vaults and tunnels pose many significant dangers and must be accessed only by qualified personnel. In addition to the presence of hot surfaces, the confined space of these vaults can pose serious life safety issues, and these areas can also trap carbon monoxide and other toxic gases. To ensure the safety of DES personnel, extensive training is provided and the air quality is constantly tested whenever occupied and fans are used to circulate fresh air.

Energy Generating Facility

The DES Energy Generating Facility (EGF) is located at 90 Peabody Street and produces the steam and chilled water used to heat and cool the buildings for the 41 DES customers. The EGF is divided between the heating plant and the chiller plant with a shared area for water treatment and the space to expand the facility with additional equipment.

The chiller plant contains nine (9) dual compressor chillers each having a rated capacity of 2,600 tons. With one chiller maintained as a stand-by unit, 20,800 tons are available to be dispatched to the system. The chilled water is circulated through the EGF and the EDS via a combination of the six (6) variable speed centrifugal pumps. Five (5) centrifugal pumps circulate cooling water through the chillers and to eighteen (18) cooling tower cells which reject the heat from the chillers.

The boiler plant contains four (4) dual-fuel water-tube boilers each having a rated capacity of 65,000 pph. With one boiler maintained as a stand-by unit, 195,000 pph of steam production capacity is available. Each boiler is capable of burning natural gas or propane. The boiler plant also includes two de-aerators with a common feedwater storage tank and a condensate receiver.

The steam is dispatched from the EGF through a 20” main; condensate returns back from the customers through a 10” condensate return pipe. The chilled water supply leaves and returns to the EGF through 42” mains.