Don't blow your money on a steam trap
Steam and condensate leaks cost buildings and industrial plants millions of dollars in lost energy, while increasing emissions from boilers due to increased operation, creating potential safety hazards, and lowering the reliability of operations. This article will review the many factors that impact the reliability, performance, longevity, and maintenance requirements for condensate return piping systems.
- Understand the variety of steam trap options and when to use each one.
- Learn to estimate the amounts of energy and money wasted from a blown steam trap.
- Understand how to pipe steam and condensate systems for safety and reliability.
According to the U.S. Dept. of Energy (DOE), approximately 20% of steam leaving a boiler plant could be lost due to leaking steam traps in steam systems without a preventative maintenance program. This represents a considerable amount of wasted dollars in energy production. A relatively simple maintenance program can help reduce losses by approximately half, while more sophisticated programs can virtually eliminate steam trap losses for improved building performance and reliability.
Unfortunately, loss of steam through a steam trap is virtually invisible as steam is lost into the condensate system—unless a program is in place to quickly and accurately identify these leaks. This bypassed steam provides no useful heating value to the system and effectively reduces the overall capacity of the system or requires additional capacity to make up for the system losses to meet the demand of the building.
This article will provide a holistic approach to steam and condensate systems by discussing the various types of steam traps, recommended locations for them, basic trap sizing, general steam and condensate design guidelines, and the various methods for testing steam traps to reduce wasted energy and dollars.
Purpose of steam traps
Steam is unique compared to hydronic systems in that the latent heat of steam contains the bulk of the energy and holds more energy per pound than water. As steam releases its latent energy, it converts back to water, typically called condensate, which must be separated and removed from the system immediately to prevent damage or reduced efficiency in the steam or condensate system. After condensing the steam, the best method to improve steam system efficiency is to return the maximum quantity of condensate to the boiler plant for reuse in the production of steam.
Steam traps are automatic valves that remove air, condensate, and noncondensable gases from steam piping or steam utilization equipment while preventing steam loss. The ability to separate these constituents from steam allows the steam system to reach the operating temperature quickly, and provides a safe and efficient system. Excess water in steam lines and poor condensate management can cause water hammer, which results from water being picked up by high-velocity steam and creates dangerous conditions that can damage piping and equipment. Air limits piping systems’ ability to carry their full capacity of steam and acts as an insulating agent within heat transfer devices. Noncondensable gases such as oxygen and carbon dioxide produce carbonic acid, scale, and corrosion, creating conditions that promote leaks within the distribution system.
Types of steam traps
There are a variety of steam traps on the market today for heating and process systems, and no single type of trap is appropriate for all applications. Steam trap selection varies based on system pressures and temperatures, capacity, trap function, piping orientation, and cost. As steam pressures, temperatures, and flow rates constantly vary, selection of the appropriate steam trap becomes more complicated. This is why there are multiple options for traps. Steam traps are classified based on the physical process that allows them to open and close; they generally fall into one of the categories described below, while some traps may utilize a combination of two categories.
Mechanical: Mechanical steam traps, also known as density steam traps, operate by using a float within the trap that will rise or fall based on the density of the fluid in the trap. The floating device is connected by mechanical linkage to a discharge valve that opens or closes based on the fluid level in the trap. For example, when condensate fills the trap, the denser fluid rests on the bottom of the trap and steam rises to the top. As the trap fills with condensate, the float will rise, actuate the valve, and discharge the condensate. The trap will then fill with less dense steam, which causes the float to fall and the valve to close for another cycle.
Mechanical traps discharge condensate at the same temperature as steam, which makes them great for areas of high heat transfer at equipment such as heating coils or heat exchangers. Mechanical traps are typically combined with a thermostatic valve to vent air out of the system, resulting in float and thermostatic (F&T) traps or various bucket traps such as the inverted or open bucket trap. Figure 1 shows an example of a float and thermostatic trap at a steam-to-hot-water heat exchanger.
Thermostatic: Thermostatic steam traps operate based on the temperature change of the steam and sub-cooled condensate to open or close the discharge valve. Within the steam trap, depending on the style of the trap, either a fluid is evaporated and condensed or two dissimilar metals expand and contract based on whether steam or condensate is located within the trap. Thermostatic traps will not open until condensate within the trap has been cooled below the saturated steam temperature. Because steam will not cool below the saturation temperature, the valve is normally open and closes in the presence of the hot steam or condensate. Examples of thermostatic steam traps include bi-metal and bellows type steam traps.
Thermodynamic: Thermodynamic steam traps operate due to the change in fluid dynamics of flash steam within the trap. As the fluid within the trap changes, static and dynamic pressures change based on velocity to operate the discharge valve. Condensate is a low-velocity fluid causing an increase in static pressure that lifts the valve and allows the condensate to be removed from the trap. Conversely, steam has a much higher velocity and dynamic pressure, so when steam approaches the trap, the decrease in static pressure and increase in velocity creates a pressure drop to close the valve, according to an article in Chemical Processing written by Tracy Q. Snow. The most common type of thermodynamic trap is a disc type where the disc is the only moving part. Thermodynamic traps discharge condensate close to saturated steam temperature and are very compact, simple, and rugged valves, making them ideal for steam main header drip traps.
Table 1 provides the recommended steam trap to use based on the location and function within typical heating and process systems.