How to Trap Steam Distribution Systems - Armstrong International, Inc.

How to Trap Superheated Steam LinesAt first glance, this may seem confusing due to the ideathat superheated steam produces no condensate; therefore,the steam lines carrying superheated steam should not haveany condensate in them. This is true once the system is upto temperature and pressure, but condensate removal isnecessary up to this point. This section will explain whatsuperheated steam is and the applications for its use.The specific heat of any substance (using Btu standards)is the quantity of heat required to raise the temperature of1 pound by 1 degree F. With this definition, the specific heatof water is 1, and the specific heat of superheated steamvaries according to temperature and pressure. Specific heatdecreases as the temperature rises but increases as thepressure goes up.Superheated steam is customarily made by the addition ofan extra set of coils inside the boiler or in the exhaust areaof the boiler so as to use the “waste” heat from the boiler.Or, by the addition of a superheat chamber somewhere afterthe boiler, attached to the steam main. A schematic diagramof a steam generator with a superheated section of coil isshown below.Properties of Superheated SteamSuperheated steam has several properties that make itunsuitable as a heat energy exchange medium yet idealfor work and mass transfer. Unlike saturated steam, thepressure and temperature of superheated steam are independent.As superheat is formed at the same pressure asthe saturated steam, the temperature and volume increase.In high heat release boilers with relatively small drums,separation of steam from water is extremely difficult.The combination of the small volume of water in the drumsand rapid load swings produces severe shrink and swellconditions in the drum, which promotes water carryover.Figure CG-39. Steam GeneratorPreheatedAirStackGases OutletAirInletThis water can be removed with separators and trapsin the steam outlets, but they are not 100% efficient. Inapplications where dry steam is a necessity, additionalsuperheating coils are placed in the boiler furnace asconvection passes. More heat is added to the steam tovaporize the water carryover, which adds a small amountof superheat to guarantee absolutely dry steam.Because superheated steam can give up so little heatbefore it converts back to saturated steam, it is not a goodheat-transfer medium. Some processes, such as powerplants, require a dry heat in order to do work. Whateverthe type of power unit, superheat helps reduce the amountof condensation when starting from cold. Superheat alsoincreases the power output by delaying condensation duringthe expansion stages in the equipment. Having drier steamat the exhaust end will increase the life of turbine blades.Superheated steam can lose heat without condensingwhereas saturated steam cannot. Therefore, superheatedsteam can be transported through very long steam lineswithout losing sufficient heat to condense. This permits thedelivery of dry steam throughout the entire steam system.Why Trap Superheated Systems?The primary reason for traps on superheat systems is thestart-up load. It can be heavy because of the large sizeof the mains. On start-up, manual valves will most likely beused since time is available to open and to close the valves.This is known as supervised start-up. A second reason forsteam traps is to handle emergencies such as superheaterloss or by-pass, which might require operation on saturatedsteam. In these unscheduled events, there is no timeavailable for manually opening valves; therefore, steamtraps are a necessity.These are the situations for which proper trap sizing is amust. Condensate must be removed as it forms in any steamsystem to keep efficiency high and to minimize damagingwater hammer and erosion.Superheated Steam(High Pressure)WarmAirHot WaterTurbineGeneratorVaporSteam(Low Pressure)FuelSuperheaterLow-TemperatureWater(High Pressure)CondenserPumpCool WaterPump28CG-23Cool Water(From Tower orLake/River)Armstrong Steam and Condensate Group, 816 Maple St., P.O. Box 408, Three Rivers, MI 49093 – USA Phone: (269) 273-1415 Fax: (269) 278-6555www.armstrong-intl.com