A logical, step-by-step process, from data collection to detailed engineering
A typical university campus can have as many as several hundred buildings. While these buildings may vary greatly in terms of type and utilization, they all need thermal energy for space heating, humidification, domestic-water heating, and the like. With most university-campus buildings in close proximity to each other, district steam systems are a viable thermal-energy source, as confirmed by the hundreds of universities across North America that utilize them.
A district steam system consists of at least one steam boiler, a fuel system, a feedwater system, a flue-emissions-control system, a boiler-control system, a condensate-return system, and a steam-distribution system. This article will discuss steam-distribution-system design outside of a boiler plant, focusing on data collection, preliminary engineering, and detailed engineering. A university replacing building- dedicated steam boilers with a campuswide district steam system will be used as an example.
The long-term success of a district steam system is dependant upon the gathering of all relevant site physical data (site survey, soils report, environmental testing, current steam usage) and owner requirements (future steam usage, steam requirements).
Site survey. For a district steam system, hiring a state-licensed land surveyor to develop survey drawings is essential. These drawings must include:
- Topography profiles, with contours every 2 ft of elevation, for the entire steam path.
- Building locations, with main- and basement-floor elevations.
- All roads and walkways.
- All road and utility easements.
- All underground utilities (water, sewer, electrical, teledata, steam, etc.). The surveyor's contract should include pot holing so that exact utility inverts can be determined.
- All underground man-made obstructions (culverts, tunnels, mines).
- All power/telephone poles, street lights, signs, and monuments.
- Grave sites.
- Historical and archaeologically significant sites. * Trees and shrubs.
- Underground springs or streams.
Soils report. In plotting a steam-distribution-piping path, a good understanding of subsurface soil conditions (rock, silt, sand, previously disturbed earth, water level) is vitally important. With a couple thousand of extra dollars spent on soil borings--and the subsequent moving of a planned steam-distribution path by 30 or 40 ft or the raising of buried steam-distribution piping by a foot--the expense of several hundred thousand dollars on the blasting of rock, the replacement of bad fill material, or construction dewatering can be avoided.
In addition to information on subsoil conditions, a soils report should provide information on soil compressive strength.
Environmental testing. Most mechanical-engineering firms do not perform environmental testing, their liability insurance not covering environmental claims. Therefore, universities are encouraged to hire an independent environmental consulting firm for all environmental testing and remediation-plan development. Designers should discuss with these firms a campus' history. Does a proposed steam-distribution path go through an area from which a chemistry building or boiler fuel-oil tank was removed? Does the soil need to be tested for contaminants? Does existing steam piping have asbestos insulation or fittings?
Current steam usage. Utilize available steam-flow data. If individual-building steam-flow data is not available, install temporary flow sensors. If temporary flow sensors are impractical, the university's facility department may have steam-flow values it wants used, or it might be necessary to perform heating-load calculations to determine each building's maximum steam-flow requirement.
Future steam usage. Most universities have a 10-, 20-, or 30-year strategic capital plan noting the location, size, and utilization of future facilities. Based on existing-building steam usage, determine future-building steam usage on a square-foot basis. Beyond that, determine with the university the excess steam capacity. (Typically, excess steam capacity is in the 10-to-30-percent range.) Once all steam usage is determined, develop a table with building name, maximum steam usage (pounds per hour), and required steam pressure (Table 1). Note any maximum steam usage that does not occur during normal high-steam-usage months (December, January, and February). Do this on a monthly or quarterly basis.
As can be seen in Table 1, maximum steam usage on our example campus is 258,500 lb per hour.
Steam requirements. What is the required steam pressure for each building? What boiler chemicals and feedwater-treatment system are being used? What is the required system reliability and maintainability? What is the quality of the steam? What metering is required?
The steam-distribution-system design criteria for our example campus included the ability to shut down a portion of the system for expansion or maintenance without interrupting steam service, minimal system maintenance, and project completion within a restricted capital budget.
Pipe routing. The most reliable pipe routing proved to be a loop configuration around the campus (Figure 1). The design engineer wanted to route the steam main just west of buildings 301 and 303; however, an ancient Indian burial site was discovered in the proposed steam-main path. The steam main then was rerouted farther west, through dormitories.
Steam pressure. According to Table 1, 100-psig pressure is required for 40 percent of the steam load, 50-psig pressure is required for 5 percent, and 15-psig pressure is required for the remaining 55 percent. One possible solution was to provide two steam-distribution systems: one at 125-psig pressure and one at 75-psig pressure. Because of restricted funding, a single 125-psig system was chosen. The higher distribution pressure was seen as a way to reduce steam-main pipe size.
Burial method. Steam piping can be buried three ways:
- In a crawl tunnel.
- In a walking tunnel.
Although it is the most economical option, directly buried piping is inaccessible for inspection and maintenance, except at valve pits. A crawl tunnel allows restricted access, while a walking tunnel, which is the most expensive option, allows full access.
District-steam-system piping on our example campus is being buried directly (Photo A).
Although they may look relatively simple, district steam systems are complex, requiring a substantial amount of engineering effort.
Pipe-material selection. District steam and condensate piping typically is made of seamless carbon steel meeting ASTM A53/A53M-07, Standard Specification for Pipe, Steel, Black and Hot-Dipped, Zinc-Coated, Welded and Seamless, or ASTM A106/A106M-06a, Standard Specification for Seamless Carbon Steel Pipe for High-Temperature Service (Grade A or B). These standards establish chemical-composition, manufacturing-process, tensile-stress, and yield-stress requirements for piping. (For more information on these standards, see the sidebar below.) Design, fabrication, and installation should follow ASME (American Society of Mechanical Engineers) B31.1, Power Piping, which has a more stringent fabrication and maximum-allowable-stress requirement than ASME B31.3, Process Piping, and ASME B31.9, Building Services Piping.
Directly buried steam piping can be either:
- Prefabricated and preinsulated. A basic prefabricated piping system has a steam-service pipe, insulation (most often, mineral wood or cellular glass), an air gap, a conduit pipe, and an outer protective coating (fiber-reinforced plastic or coal tar). Enhanced prefabricated piping systems include larger air gaps between the service-pipe insulation and conduit pipe and feature polyurethane insulation on the exterior of the conduit pipe (Photo B). When steam enters the service pipe, the service pipe expands, while the conduit pipe does not. With underground installations, it is important that the conduit pipe handle maximum service-pipe expansion. Prefabricated piping is produced in many different lengths, with 20 ft and 40 ft the most economical.
- Encased in poured calcium carbonate. This method involves the installation of steam piping, the installation of form work around the piping, and the pouring of calcium carbonate to a prescribed depth around the piping. The steam piping expands and contracts within the calcium carbonate. Serious consideration should be given to providing a sacrificial-anode or rectifier cathodic-protection system for steam piping.
Most directly buried steam piping is prefabricated and preinsulated.
Crawl- and walking-tunnel-installed steam piping can be prefabricated or field-fabricated. Typically, it is carbon-steel piping with field-applied fiber-glass, cellular-glass, or calcium-silicate insulation and an aluminum outer jacket. Although prefabricated piping costs more than field-fabricated piping, it can be installed in a shorter period of time.
Steam-pipe design. Steam-pipe design includes:
- Steam-pipe sizing. Steam pipes are sized according to steam velocity and steam pressure drop. District steam velocities typically are in the 6,000-to-10,000-fpm range. The allowable total steam-pressure drop within a steam main is based on the difference between the initial supply pressure and the minimum allowable user pressure. In long distribution systems, steam-pressure drops around 10 to 20 percent of initial supply pressure are not unusual. On our example campus, the maximum allowable steam velocity is 8,000 fpm during normal operation and 10,000 fpm with a segment shut down. This requires 18-in. Schedule 40 pipe with a steam-pressure drop of 0.4 psig per 100 ft. The longest system run (2,325 ft) occurs when segment VP-15 to VP-1 (Figure 1) is shut down and results in a 9.3-psig, or 7.4-percent, steam-pressure drop. The 18-in. Schedule 40 pipe has a steam velocity of 8,900 fpm, which is below the owner's maximum-allowable standard, while the lowest steam-main pressure of 116 psig is above the minimum requirement of 100 psig.
- Steam-pipe slope. If condensate is allowed to pool at the bottom of a pipe, a condensate slug can be picked up and hurled down the pipe, causing water hammer and, possibly, catastrophic pipe failure. Condensate can be moved to a steam trap by sloping a steam pipe in the direction of flow. Sloping district steam piping is challenging because of the elevation changes that occur throughout a distribution system. Providing a constant steam-pipe slope from the beginning to the end of a distribution system is not practical. The district steam-distribution system on our example campus would drop more than 23 ft if a constant 1-percent slope were maintained. One sloping method is to saw-tooth steam pipe from valve pit to valve pit (Figure 2). Our example system would have a maximum 3.5-ft elevation drop from valve pit to valve pit if a 1-percent pipe slope were maintained.
Sloping steam pipe in a district steam system is not always possible. Steep topographic elevation changes can have steam pipe sloping sharply up a hill. In such cases, extra attention needs to be paid to steam-trap location. In a looped system, in which steam can flow in either direction, which way should pipe slope?
Automatic air vents at steam-piping high points are important to removing air and carbon dioxide from pipe during system warm-up.
- Steam-pipe condensate removal. Thermodynamic and inverted-bucket steam traps typically are used on steam mains. With crawl and walking tunnels, steam traps can be placed every 100 to 200 ft. With buried steam mains, steam traps can be placed up to 300 ft apart. Placing steam traps in valve pits reduces the number of dedicated steam-trap manholes.
- Steam-pipe-expansion compensation. The two types of district-steam-pipe-expansion compensation are expansion loops and mechanical joints. Expansion loops utilize the spring characteristics of carbon pipe. Configurations include 90-degree elbows, 90-degree "Z" elbows, and loops (Figure 3). Mechanical joints include externally pressurized bellows (Photo C), slip joints (Photo D), and flexible ball joints (Photo E).
For crawl- and walking-tunnel-located piping, mechanical joints are utilized because of their relatively small size and ability to absorb significant pipe expansion in the axial direction. Expansion loops would be difficult to use in tunnels because of space constraints. For directly buried piping, the primary form of expansion compensation is the expansion loop.
Prefabricated, preinsulated, double-wall piping must be designed for the carrier pipe to expand within the conduit pipe. Where there is a straight run from a pipe anchor to a valve pit or steam-trap manhole, a mechanical expansion joint could be installed in the valve pit or steam-trap manhole to absorb carrier-pipe expansion.
Steam-pipe expansion should be designed for the maximum pressure a steam pipe will experience during a system failure. On our example campus, the boilers serving the steam system have two pressure-relief valves set at 175 psig. Steam-pipe expansion is 2.676 in. per 100 ft of pipe length. The maximum distance between anchors is 225 ft, resulting in total pipe expansion of 6.0 in.
Mechanical expansion joints with expansion-compression capacities of 4 to 12 in. are available. When designing a steam-pipe anchor using mechanical-joint expansion compensation, try not to anchor at elbows. Take advantage of the natural expansion capacity of elbows, or use ball joints at elbows.
- Steam-pipe supports and anchors. Steam piping typically experiences maximum pipe-support and anchor forces during hydrostatic tests. The steam system on our example campus needs to be hydrostatically tested to 150 percent of the design steam pressure. During this test, the steam pipe is full of water. As a result, the pipe needs to be supported every 28 ft instead of every 37 ft.
Steam-pipe forces can be substantial. For loop expansion joints and mechanical expansion joints, pipe-anchor forces typically are in the 5,000-to-15,000-lb-force range and 15,000-to-55,000-lb-force range, respectively.
- Steam-pipe isolation valves. Steam valves should provide adequate isolation of district-steam-system sections for routine maintenance, emergency repairs, and system expansion. In our example district steam system, isolation valves are installed at every valve pit--on building branch pipe and either side of tees--to isolate the main between valve pits, as shown in Figure 4.
Valve-pit/manhole design. Valve pits/manholes need to be big enough for piping, expansion joints, isolation valves, pipe anchors, condensate pumps, and sump pits with sump pumps. For maintenance-staff members to be able to access both sides of a pipe, a second access ladder/manhole may be needed. A 10-ft-wide-by-20-ft-long valve pit is not unusual.
A valve pit/manhole should be able to withstand HS-20 highway loads and should not be used to anchor steam and condensate piping. All annular spaces around pipe penetrations need to be sealed with a mechanical sealing mechanism to keep water from leaking into the valve pit.
There should be two ventilation pipes, with one terminating a foot above the floor and the other terminating a foot below the ceiling/lid. These pipes need to be at least 18 in. above grade (adjust height for local snow conditions) and terminated with a bird-screen-covered gooseneck.
Tunnel design. A dedicated district steam tunnel is desirable; however, tunnels often are used for other utilities (electrical, teledata, domestic water, chilled water, etc.). The number and size of the utilities installed will affect the overall size of a tunnel. Tunnels should include a 24-in.-wide maintenance-access aisle. Wall-mounted tunnel lighting should be used to prevent maintenance-staff members from hitting their heads on light fixtures. Tunnels also should include material-installation/removal hatches, drainage, and space for piping expansion.
Tunnels should be able to withstand HS-20 highway loads and reinforced in areas handling steam-pipe-anchor loads. It is important that a tunnel meet this structural requirement in its entirety, not just under roads. Tunnels should be made of field-formed concrete or prefabricated concrete sections.
Hire a professional waterproofing contractor to waterproof the top and sides of your tunnel. Pay extra for premium waterproofing materials, as their performance will be the most significant factor determining your tunnel's useful life.
Tunnel ventilation design. Providing a small amount of ventilation in a tunnel helps remove moisture and heat generated by steam and condensate piping and provides better working conditions for maintenance-staff members. A base 0.10-cfm-per-square-foot ventilation rate will help remove heat and moisture from a tunnel. If the 2,325-ft-long district steam system on our example campus were installed in a 5-ft-wide tunnel, it would require 1,165 cfm of ventilation. To prevent positive pressurization, exhaust fans with intake louvers are used to ventilate the tunnel.
A well-engineered and constructed district steam system can provide safe, reliable, and efficient steam service, while a poorly engineered and/or constructed one can be expensive to operate and maintain, fail frequently, and, possibly, cause a fatality. This article's intent was to provide a logical step-by-step process for engineering a district steam system and to highlight the different components that need to be evaluated. Sidebar: ASTM Piping Standards
ASTM A53/A53M-07, Standard Specification for Pipe, Steel, Black and Hot-Dipped, Zinc-Coated, Welded and Seamless. The two grades of A53 steel pipe are A and B. The primary difference is the percentage of carbon and manganese, which affects tensile and yield strength. The three types of A53 steel pipe are E (electric-resistance-welded), F (furnace butt-welded), and S (seamless).ASTM A106/A106M-06a, Standard Specification for Seamless Carbon Steel Pipe for High-Temperature Service. The three grades of A106 steel pipe are A, B, and C. The primary difference is the percentage of carbon and manganese, which affects tensile and yield strength.
A senior mechanical engineer with Merrick & Co., Vincent A. Sakraida, PE, LEED AP, has 25 years of experience designing mechanical systems for high-technology facilities, as well as extensive experience designing central-plant, laboratory-utility, and HVAC systems.