One in a series.
How to Select a Steam T r a p Big Enough for the Job Adequate capacity safety factors are essential to operating efficiency As promised to you in a previous Armstrong trap advertisement in this publication, here is some help ful information on steam trap safe ty factors. First, the definition. A trap safe ty factor is simply the ratio be tween actual continuous discharge capacity of the trap and normal condensate load. If the load is 500 l b / h r and the trap actually will discharge at the rate of 1000 lb/hr, the safety factor is said to be 2 to 1, and so on. Why a Safety Factor? You are not going to get maximum heat transfer efficiency from any steam heated unit unless your traps are sized with a generous ex cess of capacity over the normal condensate load. Remember that trap capacity is given in terms of continuous dis charge of condensate at a given pressure differential. If you don't employ an adequate safety factor, you make no provision for these requirements or conditions: 1. Venting of gas, Oa and CO2, would be impossible if a trap should discharge a full stream of water continuously. 2. Peak loads would back u p condensate in the line or unit if the trap were sized for average load.
3. Reduction in pressure dif ferential across the trap orifice would reduce capacity below re quirements. If steam pressure drops below normal, trap capacity is lowered more than condensing rate. An increase in back pressure for any reason cuts trap capacity. And, pressure differentials often drop substantially during warm ing-up periods. 4. "Group" trapping. Very oc casionally it is impractical to use an individual trap on each coil or condensing unit. Here, a generous ly oversized trap helps prevent backup of condensate or air from one unit to another. The frequent o p e n i n g of a b i g t r a p v a l v e "pumps" non-condensibles and condensate to the drain header. What Safety Factor? Fortunately, it isn't necessary to calculate safety factors. Experi ence is the best guide and the benefit of experience with tens of thousands of successful installa tions is available to you. The fol lowing table is taken from the Armstrong Steam Trap Book. Page Equipment Drained 24 Purifiers a n d Separators 25 Steam M a i n s or Headers 26 Steam H e a t i n g Pipes 29 30 32
Unit Heaters Submerged Coils Cylinder Dryers
Safety 2 or 2 or 2 to
Factors 3 to 1 3 to 1 to 1 up 6 to 1 3 to 1 2 , 3 or 4 to 1 4 to J up to 1 0 to 1
2)A £
3
INLET PRESSURE
"^r
BACK PRESSURE OR VACUUM
Pressure differential affects trap capacity. If "B" is back pres sure, subtract from "A"; if "B" is vacuum, add to "A". Safety factors compensate for drops in pressure differential.
Traps vent air. This requires a trap capacity safety factor.
The page numbers listed contain completely reliable data for any trapping job. CAUTION. The safety factors recommended in The Armstrong Steam Trap Book will have to be increased by an unknown factor for trap capacity ratings based on cold water tests, orifice tests or slide rule calculations. Armstrong trap steam temperature conden sate capacity ratings are based on actual operating conditions which take into account such capacityreducing factors as pipe friction and the choking effect of flash steam. If you'd like to have a copy of the 44-page Steam Trap Book, just call your local Armstrong Representative, or write the fac tory. There is no obligation. Armstrong Machine Works 9704 Maple Street Three Rivers, Michigan
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ARMSTRONG STEAM TRAPS
For furtner information, circle number 91 A on Readers' Service Card, page IIS A VOL. 49, N O . 5
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M A Y 1957
91
A
