Don't blow your money on a steam trap
Condensate piping systems design guidelines
Sizing of the condensate return piping system requires careful analysis and depends on system pressure, type of condensate piping system, and slope of the piping. The most common type of condensate piping is a gravity drain, which relies on condensate being drained to a condensate receiver that is vented to the atmosphere. Flow in the condensate piping is two-phase, consisting of flash steam and condensate, and should be sized to keep velocities below 4500 fpm for flash steam and below 7 fps for condensate, according to an article in Plant Engineering by Kelly Paffel. Piping should be pitched downward in the direction of the condensate flow at 0.5 in. per 10 ft to ensure adequate condensate removal.
Piping in condensate systems is recommended to be schedule 80 steel piping in most cases, due to heavier wall thicknesses to extend the life of the pipe. Where possible, weld piping connections and avoid threaded connections to limit leak points due to continuous thermal expansion and contraction. Condensate should also be drained into the top of the main condensate header similar to steam takeoffs to prevent hot condensate from mixing with cool condensate, causing flash steam and water hammer, according to the article by Paffel. Figure 4 demonstrates an installation with the appropriate top takeoffs for condensate piping.
Piping design should be in accordance with ASME B31, which covers the standards for pressure piping. ASME B31.1 includes the design, fabrication, erection, testing, and inspection of high-pressure power piping for systems exceeding 15 psig. Steam systems are considered high-pressure above 15 psig and low-pressure below 15 psig. Although building services can use high-pressure steam piping, the most common locations for ASME B31.1 are within industrial plants or central/district steam heating plants. ASME B31.9 covers similar piping requirements typical of low-pressure steam systems for building services for those systems not directly covered by ASME B31.1. In any case, steam traps and systems must be rated for the temperature and pressures of the system.
Testing steam traps
Unfortunately, in many buildings and campuses, steam trap maintenance is typically ignored unless a larger problem occurs at the boiler plant or distribution system. As steam leaks through steam traps, the steam is condensed by conductive losses in the piping system as it returns to the condensate receiver or is vented out the receiver and lost, many times without detection. A typical steam trap maintenance program should include at least yearly testing of all steam traps to find leaking traps and replace failed traps. This test-and-replace strategy is subject to two main costs by the owner: testing and steam loss.
The most difficult item in a steam system survey is to determine if steam traps are operating correctly or are faulty and wasting energy. Many times steam system surveys are not implemented due to the cost or staffing required, but the energy savings observed as a result of a properly implemented program will typically pay back the cost to implement the program in less than one year. If a yearly investigation of all the traps can’t be completed, the focus should be the larger traps that can waste significantly more energy than smaller traps.
For example, based on the size of the steam trap and orifice size, one can estimate the amount of live steam lost through the trap. The worst-case scenario would assume the entire orifice is open, but typically condensate simultaneously flows through the trap so approximately 1/3 to 1/2 of the orifice will contain lost steam, according to the DOE. Figure 5 shows a comparison of various orifice sizes in steam traps and the amount of steam wasted based on a blown trap.
Test methods to determine steam trap operation can be segmented into four categories: visual, ultrasonic, temperature, and conductivity.
Visual: Visual testing involves a test valve arrangement or inline sight glass to visually determine if the steam trap is malfunctioning. Inline sight glasses should generally be avoided due to reliability issues. Testing valves provide a visual observation of the fluid downstream of the steam trap by allowing a momentary discharge of the downstream fluid. Visual indication requires that the personnel observing the visual cues be knowledgeable enough to determine the condition of the steam trap as blow-through steam and flash steam can both escape the test connection with only one indicating a failure. Flash steam is created when condensate flashes to vapor upon expansion to atmospheric pressure and is typically a billowing plume, compared to live steam which is a sharper, higher velocity plume that may not be immediately visible as it exits the test valve, according to the DOE.
Ultrasonic: Ultrasonic measuring allows the operator to listen to sonic and supersonic sounds as the steam and condensate flow through the trap. Devices on the market today are able to compare the sound of a known condition to the tested trap to accurately determine if the trap is functional or blowing steam.
Temperature: Temperature testing of steam traps can help identify issues, but must use the pressure/temperature relationship to accurately assess the condition. The most common method of temperature measurement involves infrared testing to determine the surface temperature of an object at the inlet and outlet of the steam trap. The inlet condition should correspond to saturated steam pressure, and the outlet temperature should correspond to condensate pressures. However, care must be taken with the temperature method since saturated steam and condensate can coexist at the same temperature. This method can still be used to provide a general assessment of the trap condition.
Conductivity: A fourth method of testing relies on the use of fluid conductivity to indicate if the steam trap is operating properly. Testing is conducted with a sensor located either inside of the steam trap or in a sensing port upstream of the trap to detect the physical state of the fluid. Under typical conditions, the probe would be immersed in condensate, but if the steam trap has failed, condensate would not be present and the probe would be sensing the conductivity of the steam. To get the most accurate diagnosis, maintenance personnel should integrate conductivity readings with temperature.
Many factors influence the success of a steam and condensate system. Good system design coupled with the understanding of a correctly sized steam trap for the duty is a critical first step. Ultimately, steam traps require a maintenance program to determine if they are working to their full potential. Failed steam traps can have a spiraling cost impact above and beyond wasted steam, including increased feedwater costs, additional chemical treatment and make-up water, and higher blowdown requirements.
David Grassl is a mechanical engineer at Ring & DuChateau, and an adjunct professor in the civil and architectural engineering and construction management department at the Milwaukee School of Engineering. Grassl has designed various high- and low-pressure steam systems including steam-to-hot-water heat exchangers, steam humidifiers, steam heating coils, and kitchen equipment.