Boiler Facts... George R. Carey, Jr. Vice President, Fluid Industrial Associates gcarey@fiainc.com twitter: @Ask_GCarey Float and Thermostatic Traps (Understanding how this trap does what it does…) If you have had the chance to work on a two-pipe steam system with a condensate pump and receiver then you have probably encountered float and thermostatic (F&T) traps. That is because an F&T trap is required at the end of each supply main and at the base of any dripped risers to prevent steam from reaching through to the dry returns. If steam is allowed to flow from the supply mains directly into the return mains, the pressures will equalize between the two, causing the steam to stop dead in its tracks. The result is cold radiators, high fuel bills and unhappy customers. If the system happens to use a boiler feed tank, the steam will pour out of the vent pipe creating other problems. I once walked into a boiler room that had a new boiler and boiler feed tank installed. The customer was complaining about no heat throughout most of the house, yet very high fuel bills. When the service man took me over to the receiver, he beamed his flashlight to the end of the vent pipe that was capped off! I said, “I think we found the cause of the problem!” By plugging the vent pipe, no steam was allowed to pour into the basement. Unfortunately, no air was allowed to leave the system either. Besides, receivers are not rated to withstand any pressure. Float and thermostatic traps are supposed to vent air, stop steam from passing through and drain condensate as soon as it forms. How it does all these things is quite interesting… An F&T trap is a normally open and a normally closed trap at the same time. Sounds confusing? The “T” part of F&T stands for thermostatic. There is a thermostatic element in the trap body, which is just like the element found in a radiator trap. The element is normally open and its function is to pass air and other non-condensable gases into the return line. Once the steam arrives, the element snaps shut, preventing steam from passing into the return lines, and its job is complete until the next cycle. The “F” part stands for float. The job of the float is to drain any condensate that forms when the steam turns back to water. The float is attached, by a lever, to a plug that seats against the discharge orifice. It is normally in a closed position due to three factors: first, the pressure difference that exists across the trap. There should always be higher pressure on the inlet side of the trap than what is found on the outlet side. In addition, this higher pressure helps keep the plug up against the traps discharge orifice, keeping the trap closed. Second, the float has a specific weight to it and when attached to the lever, because of its length, this weight is magnified. Last, the diameter of the discharge orifice plays an important role in opening and closing the float part of the trap. Trap manufacturers will vary the diameter of this orifice depending upon the steam pressure. With high pressure, the diameter is very small because there is a lot of “push” behind the condensate that is passing through the trap. On a low-pressure application, the orifice is larger because the differential across the trap is much lower. Naturally, the temperature of the condensate makes no difference to the float, so as soon as condensate enters the trap body, the float lifts the plug from its seat and starts to drain. It handles light and heavy loads very well by having the float modulate as the load changes. Fast Air Removal Because the trap is great for handling large volumes of air, it is very beneficial for the quick distribution of steam. Air is such a great insulator that it can greatly reduce the rate of heat transfer of heating equipment and slow the distribution of steam throughout the system. Applications such as dripping the ends of mains for heating systems, unit heaters, heat exchangers and anything else that requires air to be removed quickly are perfect for this trap. If you think about the operation of a typical heating system, it is constantly cycling on and off. In addition, every time it shuts down, all the leftover steam in the system condenses. This condensing action forms vacuums all over the system until the traps and main vents re-open. Then air is allowed to rush in, breaking the vacuums and re-filling the entire system with air—air that needs to be vented on the next call for heat. That is why F&T traps are the perfect choice for heating system applications. 22 ICM/March/April 2016
ICM March-April 2016
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