Tags: Steam Traps

Thermodynamic Steam Traps


A thermodynamic steam trap (see Figures 10-26 and 10-27) contains only one moving part, a hardened stainless-steel disc that functions as a valve. Because of its construction simplicity, this is an extremely rugged trap and is especially well suited for service on medium- and high-pressure steam lines operating under pressures up to 600 psig. The minimum operating pressure for some makes of these traps is as low as 3.5 psig.

Thermodynamic steam traps are small, unaffected by water hammer, and can be mounted in any position. The operating principles of a thermodynamic steam trap are illustrated in Figure 10-28.

thermodynamic steam trap Thermodynamic Steam Traps

thermodynamic steam trap 2 Thermodynamic Steam Traps

thermodynamic steam trap operating Thermodynamic Steam Traps

Installing Steam Traps


The installation of steam traps requires the following modifications in the piping:

1. Install a long vertical drip and a strainer between the trap and the apparatus it drains. The vertical drip should be as long as the installation design will permit. Exception: Thermostatic traps in radiators, convectors, and pipe coils are attached directly to the unit without a strainer.
2. A gate valve should be installed on each side of a trap, along with a valved bypass around the traps if continuous service is required. This permits removal of the trap for servicing, repair, or replacement without interrupting service.
3. A check valve and gate valve should be installed on the discharge side of a trap used to discharge condensation against back pressure or to a main located above the trap (as for lift service).

Always carefully follow the steam trap manufacturer’ s instructions for installing a steam trap. If these instructions are not available, call the factory or an authorized representative for information before attempting to install the trap. The following are offered as guidelines for installing steam traps:

1. All work must be performed by qualified personnel trained in the correct installation of the trap.
2. Installation work must comply with all local codes and ordinances.
3. Allow the boiler to cool down to approximately 80°F and the pressure to drop to 0 before attempting to do any work on the trap.
4. Wear heat-resistant gloves to prevent serious burns when opening and shutting steam valves.
5. Cap off the gate valves if they are not connected to a drain or not in use for test or pressure-relief purposes to prevent property damage, serious injury, or death.
6. Connect a temporary pipe between the steam pipe opening and a drain to prevent injury from steam pipe blow-down. In lieu of installing a temporary pipe, stand at least 100 feet from the pipe opening.
7. Open supply valves slowly after installing the trap.

Check the trap seat rating on the nameplate before installing it. The rating must be equal to or greater than the maximum pressure differential across the trap.

Sizing Steam Traps


Selecting the correct size steam trap for a system is an important factor in its operational efficiency. For example, an oversized trap will operate less efficiently than a correctly sized one, and will tend to create abnormal back pressure. Moreover, the installation cost will be higher and the operational life expectancy will be reduced.

Manufacturers of steam traps provide information in the form of capacity ratings and related data to make these selections easier. Data should be based on hot condensation under actual operating conditions rather than cold-water ratings. If possible, always try to determine the basis for a manufacturer’ s ratings. Table 10-1 and the sizing example were provided by Sarco Company, Inc., a manufacturer of steam traps.

table 10 1 Sizing Steam Traps

Steam Traps


A steam trap is an automatic valve that opens to expel air and condensation from steam lines and closes to prevent the flow of steam. The functions of a steam trap are:

• Remove (vent) air from the system so that steam can enter. (Air in the pipes will block the flow of steam into the radiators.)
• Prevent steam from leaving the system until all of its latent heat is removed.
• Remove (drain) the condensate from the system after the latent heat has been removed. (Draining the condensate from the system prevents corrosion and water hammer.)

All steam traps operate on the fundamental principle that the pressure within the trap at the time of discharge will be slightly in excess of the pressure against which the trap must discharge. This includes the friction head, the velocity head, and the static head on the discharge side of the trap. The steam trap cannot operate unless the excess pressure of discharge is greater than the total back pressure.

Each steam trap used in steam heating is designed for a specific range of applications that its operating characteristics best suit. Although there is no universal steam trap per se, the many different types can be grouped into the following three classes on the basis of their operating characteristics:

• Separating traps
• Return traps
• Air traps

Separating traps are designed to release condensation but close against steam. They are float-operated, thermostatically operated, or float-and-thermostatically operated. Thermostatic traps are designed to release air and condensation but close against them.

Return traps may be operated to receive condensation under a vacuum and return it to atmosphere or a higher pressure. Air traps are generally operated by a float.