Don't blow your money on a steam trap

02/01/2014


Locations, sizing of steam traps

With an overall understanding of steam traps, it is appropriate to discuss where steam traps are located within a steam distribution system and how trap sizing affects the system. Steam traps in HVAC applications are generally found in two locations: drip traps and at process equipment. 

Drip traps are located as part of the main steam distribution piping as the system radiates heat, loses energy, and creates condensate within the piping system. Drip traps consist of a short piece of vertical pipe called a drip leg on the bottom of the steam main with a steam trap. The drip leg must be adequately sized to collect and remove the condensate, with a recommended diameter the same size as the main piping up to 4 in. and half the diameter for piping above 4 in., but never less than 4 in. 

Figure 2: A steam drip leg and trap on a main high-pressure steam pipe upstream of a flow meter are shown. Courtesy: Ring & DuChateauThe recommended vertical length of the drip leg should be 28 in. to create 1 psi of head pressure on the steam trap according to the ASHRAE Handbook—HVAC Applications. These traps should be located at the end of mains; bottom of risers; ahead of pressure regulating valves, controls valves, and isolation valves; at pipe bends; and near expansion joints to allow the collection of condensate. Straight sections of piping should also receive drip traps at regular intervals depending on the pitch of the piping. Piping that pitches in the direction of flow should receive drip traps every 200 to 300 ft, and piping that pitches opposite the direction of flow should receive drip traps every 150 ft3 according to the ASHRAE Handbook—HVAC Applications. These traps only see a small amount of condensate from heat loss or start-up and are typically low-capacity traps that are not required to bleed air from the system. Figure 2 shows an example of a drip leg and trap.

Steam traps located at the outlet of process equipment serve the same general purpose as drip traps, but the overall intent is to confine the steam within the heat transfer equipment until the steam has released all its latent heat and condenses to condensate. At this point, it is acceptable to return the condensate to the boiler for reuse. These traps require large condensate and air handling ability to maintain efficient heat transfer.

All steam traps should be located below the device they serve to allow condensate to be removed by gravity and not rely on pressure or velocity. There are conditions when lifting condensate is acceptable, but it should be avoided wherever possible to reduce backpressure on the traps. At the bottom of the drip leg, a drain valve should be provided to remove condensate, and isolation valves and unions are recommended at the inlet and outlet of the steam trap to simplify trap removal. At the inlet to the steam trap, a strainer with a blowdown valve will provide the ability to remove any scale, dirt, and debris in the piping system and allow an operator to depressurize the steam trap for maintenance. Downstream of the steam trap, a check valve eliminates the potential for condensate to back up into the steam system. In all cases, the steam trap must be located with maintenance in mind in an accessible location, as an inaccessible steam trap may be forgotten for years. 

Figure 3: This schematic shows a steam-to-hot-water heat exchanger with a top takeoff at the main distribution piping and all necessary steam traps and accessories. Courtesy: Ring & DuChateauBecause proper steam piping design impacts condensate removal, steam takeoff to the process equipment should be a top takeoff to provide the highest quality dry steam in the distribution system, which reduces condensate from the bottom of the distribution main from getting into the equipment. At the takeoff, include a drip leg and trap as described above in addition to the heat transfer equipment steam trap (see Figure 3). 

Under- and oversizing steam traps can have adverse effects on the overall system; undersized traps will cause frequent cycling and potentially reduce heat transfer as condensate may back up into the equipment. Oversizing steam traps can be equally as problematic as oversizing traps can cool the condensate prior to discharge. A failed oversized trap has a larger orifice opening that can potentially blow through larger quantities of steam and waste more energy.

Steam traps should generally be sized two to three times the amount of condensate that will be produced under normal operation to account for varying pressures and condensate loads. This will provide additional capacity for cold start-ups when a great deal of condensate is produced and steam pressure is at its lowest value. Since steam pressure is used to move condensate throughout the piping and steam traps, start-ups require high capacity acceptance from the traps, according to the Watson McDaniel product catalog. In any case, the condensate piping to the steam trap should be no smaller than the designed condensate outlet of the process equipment, and the outlet of the steam trap should be the discharge pipe size of the branch piping to the condensate main to maximize gravity flow to the condensate return main piping.



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