The steam trap is a small, inexpensive component in steam plants. What they lack in size, they make up for in numbers. In a typical steam plant their numbers range from several hundred in a large industrial steam plant to more than 20,000 in refinery or chemical complexes. Because of their ubiquitous nature, steam traps never received sufficient attention from most energy managers or system designers until the energy crisis of the 1970s.
According to the American Society of Mechanical Engineers (ASME), the first steam trap, a floating head design, was invented in 1835. This early steam trap separated valuable, live steam from condensate, thus reducing the danger of water hammer in building heating systems. The modern float, fixed orifice and bucket steam traps were developed in the early 1900s. Though an improvement over their early ancestors, they were bulky, heavy and expensive. These traps operated on the principle that steam is a gas and condensate is a liquid. Traps like these are still used today and have changed little from their original design. Major changes in steam trap design started with the development of the thermostatic and thermodynamic traps. One of the first patents for the thermostatic design was registered by C.A. Dunham around 1900.
The evolution continues today with the advanced features to improve energy efficiency. A new generation of traps is available combining the characteristics of bimetallic, thermostatic and thermodynamic designs. The selection of a trap for a given service must consider many factors that require the knowledge of trap manufacturers, researchers and other power plant engineers. What features make the perfect steam trap? There are several key design features to look for when selecting the steam trap best suited for each application:
- No tolerance for lost steam under all operating conditions;
- Rugged construction with low maintenance costs;
- No disruption of operations if the trap fails;
- Self-draining to prevent damage from freezing during plant shutdown;
- Minimized valve seat wear;
- “All-in-one design,” with a strainer, check valve and air vent;
- Efficient temperature control through effective condensate discharge; and
- Fast plant start up with continuous venting of air and CO2.
Some of these features are critical such as no steam loss during process operation. Others, such as an all-in-one (integral) trap, usually fall in the category of nice-to-have. This isn’t always true. Such a characteristic may become extremely important if the trap must fit into a tight corner. The ranking of these features is part of the selection process.
In the process of trying to simplify their choices in steam traps, many plant engineers are making the selection based only on the purchase price of the actual trap, not the overall cost. The cost of the steam trap will include the additional parts required for piping and the cost of wasted energy (steam loss) during its operating life. Traps that address these issues will reduce the overall cost of installation and maintenance.
There is more to choosing a steam trap than calculating the load. Conditions vary, but it’s common for traps to be exposed to stresses of a chemical, thermal and mechanical nature. Corrosion from the action of oxygen and carbon dioxide in the condensate attacks the piping interior surfaces. Erosion from blast effect in intermittently discharging traps can speed up corrosion in return lines. Thermal effects include damage from freezing in cold weather and wide ranges in temperature changes during trap operation. Contamination from solids can restrict the movement of mechanical linkages or block orifices and vent holes. Mechanical vibrations transmitted through the piping or slugs of water traveling down the pipeline at high speeds (water hammer) can easily damage certain types of traps.
Which type of trap is better at meeting these ideal performance goals, resisting harmful plant conditions and providing an overall lower cost? To help make that decision, consider the advantages and disadvantages of the most common traps available today. Understanding the operating principle of steam traps will help.
Float and thermostatic
The F&T trap follows the condensate level in the chamber to open the discharge valve (Figure 1).
Figure 1. The float and thermostatic steam trap is sensitive to freezing and water hammer.
A modulating action is produced by the increased opening due to the rising level in the chamber, but the hot and cold capacities are the same. A built-in thermostatic element purges air and gases and closes off when steam enters the trap. Rapid response to changing load is possible, but as operating pressure increases, valve size must be reduced because the buoyancy force from the float is fixed.
The volume of condensate necessary to operate this trap is sensitive to freezing and the float can be damaged by water hammer. This type of trap is large and heavy and must only be mounted in the horizontal position. Main applications are process and space heating where condensate loads are light-to-medium at lower pressures. No strainer or check valve is required, but steam loss can result if internal air vent fails.
Condensate flows around the bucket to discharge through the valve (Figure 2).
Figure 2. The inverted bucket trap is the workhorse of large process units.
The steam following this condensate fills the top of the bucket, which rises to close the valve due to its buoyancy. Steam escaping through a bleed hole in the bucket and condensing in the body causes loss of buoyancy allowing the bucket to fall and overcome the pressure valve. The action is cyclic with steam loss from the condensing action each cycle.