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

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How to Trap Flash TanksInstallationCondensate return lines contain both flash steam and condensate.To recover the flash steam, the return header runsto a flash tank, where the condensate is drained, and steamis then piped from the flash tank to points of use, Fig. CG-57.Since a flash tank causes back pressure on the steam trapsdischarging into the tank, these traps should be selected toensure their capability to work against back pressure andhave sufficient capacity at the available differential pressures.Condensate lines should be pitched toward the flash tank,and where more than one line feeds into a flash tank, eachline should be fitted with a swing check valve. Then, any linenot in use will be isolated from the others and will not befed in reverse with resultant wasted flash steam. If the trap isoperating at low pressure, gravity drainage to the condensatereceiver should be provided.Generally, the location chosen for the flash tank shouldmeet the requirement for maximum quantity of flash steamand minimum length of pipe.Condensate lines, the flash tank, and the low pressuresteam lines should be insulated to prevent waste of flashthrough radiation. The fitting of a spray nozzle on the inletpipe inside the tank is not recommended. It may becomechoked, stop the flow of condensate, and produce a backpressure to the traps.Low pressure equipment using flash steam should beindividually trapped and discharged to a low pressurereturn. Large volumes of air need to be vented from theflash tank; therefore, a thermostatic air vent should beused to remove the air and keep it from passing throughthe low pressure system.Figure CG-58. Flash Steam Recovery from an Air Heater BatteryFlash is taken from the flash tank and combined with livesteam, the pressure of which is reduced to that of the flashby a reducing valve.FlashTankLowPressureSectionHeaterBatteryHighPressureSteamAirFlowFlash Tank DimensionsThe flash tank can usually be conveniently constructed froma piece of large diameter piping with the bottom ends weldedor bolted in position. The tank should be mounted vertically.A steam outlet is required at the top and a condensateoutlet at the bottom. The condensate inlet connection shouldbe 6''-8'' above the condensate outlet.The important dimension is the inside diameter. This shouldbe such that the upward velocity of flash to the outlet is lowenough to ensure that the amount of water carried overwith the flash is small. If the upward velocity is kept low, theheight of the tank is not important, but good practice is touse a height of 2'-3'.It has been found that a steam velocity of about 10' persecond inside the flash tank will give good separation ofsteam and water. On this basis, proper inside diametersfor various quantities of flash steam have been calculated;the results are plotted in Chart CG-22. This curve givesthe smallest recommended internal diameters. If it is moreconvenient, a larger tank may be used.Chart CG-22 does not take into consideration pressure—onlyweight. Although volume of steam and upward velocity areless at a higher pressure, because steam is denser, there isan increased tendency for priming. Thus it is recommendedthat, regardless of pressure, Chart CG-22 be used to findthe internal diameter.Chart CG-22.Determination of Internal Diameter of Flash Tankto Handle a Given Quantity of Flash SteamFind amount of available flash steam (in pounds per hour)on bottom scale, read up to curve and across to verticalscale, to get diameter in inches.INTERNAL DIAMETER OF FLASH TANK IN INCHES30252015105Condensate0 1,000 2,000 3,000 4,000 5,000 6,000POUNDS FLASH STEAM PER HOURArmstrong 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.com47CG-42
How to Trap Absorption MachinesAn absorption refrigeration machine chills water for airconditioning or process use by evaporating a water solution,usually lithium bromide. Steam provides the energy for theconcentration part of the cycle and, except for electricpumps, is the only energy input during the entire cycle.A steam trap installed on a steam absorption machineshould handle large condensate loads and purge air at lowpressure, modulated conditions.Trap Selection and Safety FactorDetermine the condensate load produced by a low pressure(normally 15 psig or less) single-stage steam absorptionmachine by multiplying its rating in tons of refrigeration by20, the amount of steam in lbs/hr required to produce a tonof refrigeration. This represents consumption at the ratedcapacity of the machine.EXAMPLE: How much condensate will a single-stagesteam absorption machine with a rated capacity of500 tons produce?Multiply the 500-ton machine capacity rating x 20 lbs/hr toget the condensate load—10,000 lbs/hr.A 2:1 safety factor should be applied to the full capacitycondensate load, and the steam trap must be capable ofdraining this load, at a 1/2 psi differential. In other words,the machine in the example would require a trap capableof handling 20,000 lbs/hr of condensate at 1/2 psi, andthe capability of functioning at the maximum pressuredifferential, usually 15 psi.In comparison, two-stage absorption machines operate at ahigher steam pressure of 150 psig. They have an advantageover single-stage units in that their energy consumption perton of refrigeration is less (12.2 lbs steam/hr/ton of refrigerationat rated capacity).On two-stage steam absorption machines, a 3:1 safetyfactor should be used. Therefore, the example requiresa steam trap with a capacity of 9,000 lbs/hr. At pressuresabove 30 psig, the trap capacity must be achieved at1/2 maximum pressure differential, which in the exampleis 75 psi. At pressures below 30 psig, trap capacity mustbe achieved at 2 psi differential pressure. However, thetrap must still be capable of operating at a maximum inletpressure of 150 psig.The F&T trap with an integral vacuum breaker is ideallysuited for draining both single- and double-stage steamabsorption machines. It provides an even, modulated condensateflow and energy-conserving operation. An invertedbucket trap with an external thermostatic air eliminator mayalso be acceptable.InstallationMount the steam trap below the steam coil of the absorptionmachine with a drip leg height of at least 15" (Fig. CG-59).This ensures a minimum differential pressure across thetrap of 1/2 psi. Whichever trap is used, a standby trappingsystem is recommended for this service. In the event thata component in the drainage system needs maintenance,the absorption machine can operate on the standby systemwhile the repairs are being made. This ensures continuous,uninterrupted service.In some cases, very heavy condensate loads may requirethe use of two traps operating in parallel to handle thenormal load.Figure CG-59. Generally approved method of piping steamabsorption machine with standby trapping system.EXAMPLE: How much condensate will a two-stage steamabsorption machine with a rated capacity of 300 tons produce?H 2 O VaporSteam Supply15 psig Single Stage150 psig Two StageMultiply the 300-ton machine capacity rating x 10 lbs/hr toget the condensate load—3,000 lbs/hr.SteamLB Mixture & H 2 O15''Chart CG-23. Recommendation Chart(See Page CG-2 for “Feature Code” References.)Equipment BeingTrappedSteam AbsorptionMachine1st Choice andFeature CodeF&TA, B, GNOTE: Vacuum breaker and standby system should be provided.*With external thermostatic air vent.Alternate Choice*IBF&T Trap w/Integral Vacuum BreakerDraining to Gravity Return48CG-43Armstrong 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
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