Steam-supply systems are commonly utilized in industrial facilities as a general heat source and also as a heat source in pipe and vessel tracing lines used to prevent freeze-up in non-flowing conditions. Inherent with the use of steam is the problem of condensation and the accumulation of non-condensable gasses in the system.
Steam traps must be utilized in these systems to automatically purge condensate and non-condensible gasses, like air, from the steam system. A steam trap holds back steam & discharges condensate under varying pressure or loads. However, a steam trap should never discharge live steam. Such discharges are dangerous as well as costly.
The three important functions of steam traps are:
1) Discharge condensate as soon as it is formed (unless it is desirable to use the sensible heat of the liquid condensate).
2) Have a negligible steam consumption (i.e., being energy efficient).
3) Have the capability of discharging air and other non-condensable gasses.
Types of steam traps: Based on working principle
In industrial applications 5 major types of steam traps are used, they are:
1) Inverted Bucket type
2) Float & Thermostatic
3) Thermodynamic
4) Bimetallic and
5) Themostatic
Each of these steam trap use a different method to determine when and how to purge the steam system. As a result each has a different configuration.
Mechanical Steam Traps:
Mechanical steam traps depends on the difference in density between steam and condensate in order to operate. They are able to continuously pass large volumes of condensate and are suitable for a wide range of process applications. Ball float and inverted bucket steam traps are some of the types of mechanical steam traps. This tutorial considers the operation and benefits of both types.
Inverted Bucket steam traps:
The inverted-bucket trap, which is shown in fig., is a mechanically actuated steam trap that uses an upside down (or) inverted bucket as Float. This bucket is connected to the outlet valve through a mechanical linkage (called lever).
Below fig., shows the method of usage. In (i) first the bucket hangs down, pulling the valve off its seat. The condensate enters from bottom of the bucket filling the body and flows away from the outlet. In (ii) when the steam enters it causes the bucket to become buoyant, it then rises and shuts the outlet. In (iii) The trap (valve) remains shut until the steam in the bucket has condensed (or) bubbled through the vent hole to the top of the trap body. it will then sink, pulling the main valve off its seat. Accumulated condensate is released and the cycle is repeated.
Inverted-bucket traps can handle a wide range of steam pressures and condensate capacities. They are economical solution for low-to medium- pressure and medium-capacity applications. For high pressure and capacity applications, these traps become large, expensive, and difficult to handle.
Each specific steam trap has a finite, relatively narrow range that it can handle effectively. For example, an inverted-bucket trap designed for up to 15-psi service will fail to operate at pressures above that value. An inverted-bucket trap designed for 125-psi service will operate at lower pressure, but its capacity is so diminished that it may back up the system with unvented condensate. Therefore it is critical to select a steam trap designed to handle the applications pressure, capacity and size requirements.
Float-and-Thermostatic:
The float-and-thermostatic trap shown in fig., is hybrid type. The float similar that found in a toilet tank operates the valve. These traps operates by sensing the difference in density between steam and condensate. As condensate collects in the trap, it lifts the float and opens the purge (or) discharge valve. This design opens the discharge only as much as necessary. Once the built-in thermostatic element purges non-condensable gases, it closes tightly when steam enters the trap. The advantage of this type of trap is that it drains condensate continuously.
The key advantage of float-and-thermostatic trap is their ability for quick steam-system startup because they continuously purge the system of condensate, air and other non-condensable gases. One disadvantage is the sensitivity of the float ball to damage by hydraulic hammer.
Float-and-thermostatic traps provide an economical solution for lighter condensate loads and lower pressures systems. However, when the pressure and capacity requirements increase, the physical size of the unit increases and its cost rises. It also becomes more difficult to handle.
Wear and dirt are particular problems that we face with a disk-type trap. Due to the large, flat seating surface, any particulate contamination such as dirt or sand, lies between the disk and the valve seat. This prevents the valve from sealing and allows live steam to flow through the discharge port. If pressure is not maintained above the disk, the trap will operate frequently. This wastes steam and may cause the device to fail prematurely.
The key advantage of these traps is that one trap can handle an entire range of pressures. In addition, they are relatively compact for the amount of condensate they discharge. In these type of traps disk is the only moving part, maintenance can easily be carried out without removing the trap from the line.
The main disadvantage is difficulty in handling air and other non-condensable gases. Thermodynamic steam traps will not work positively on very low pressures.
The key advantages of bi-metallic traps are (1) Compact size compared to load-handling capabilities and (2) Immunity to hydraulic-hammer damage.
Their biggest disadvantage is the need for constant adjustment (or) Calibration, which is usually done at the factory for the intended steam operating pressure. If the trap is used at a lower pressure, it may discharge live steam. If the trap is used at a higher pressure, condensate may back up into the steam system.
Thermodynamic (or) Disk type:
Thermodynamic, (or) Disk-type, steam traps use a flat disk that moves between the cap and the seat (See Figure). Initially, condensate flow raises the disk and opens the discharge port. Steam or very hot condensate that enters the trap applies pressure on the disc and seats the disk. It remains seated, closing the discharge port, until there is pressure is maintained above it. Heat radiates out through the cap, thereby diminishing the pressure on the disk, opening the trap to discharge condensate.Wear and dirt are particular problems that we face with a disk-type trap. Due to the large, flat seating surface, any particulate contamination such as dirt or sand, lies between the disk and the valve seat. This prevents the valve from sealing and allows live steam to flow through the discharge port. If pressure is not maintained above the disk, the trap will operate frequently. This wastes steam and may cause the device to fail prematurely.
The key advantage of these traps is that one trap can handle an entire range of pressures. In addition, they are relatively compact for the amount of condensate they discharge. In these type of traps disk is the only moving part, maintenance can easily be carried out without removing the trap from the line.
The main disadvantage is difficulty in handling air and other non-condensable gases. Thermodynamic steam traps will not work positively on very low pressures.
Bi-metallic type steam traps:
Bi-metallic steam trap works on the same principle as a residential-heating thermostat. As the name indicates, these traps are constructed using two strips of dissimilar metals welded together into one element. The element deflects when heated. A bi-metallic strip, or wafer, connected to a valve disk, bends or distorts when subjected to a change in temperature. When properly calibrated, the disk closes tightly against a seat when steam is present and opens when condensate, air, and other gases are present.The key advantages of bi-metallic traps are (1) Compact size compared to load-handling capabilities and (2) Immunity to hydraulic-hammer damage.
Their biggest disadvantage is the need for constant adjustment (or) Calibration, which is usually done at the factory for the intended steam operating pressure. If the trap is used at a lower pressure, it may discharge live steam. If the trap is used at a higher pressure, condensate may back up into the steam system.
