STEAM TRAPS Introduction Steam Trap Selection Guidelines • PROCESS Applications
The Vacuum Breaker opens when the steam pressure drops below 0 psig. The ΔP across the trap is determined by the static head of the drip leg. (1/2 psig).
4 Using Safety Load Factors to size Steam Traps The largest condensate load occurs when the maximum steam pressure is present in the HX. However, if the steam trap is selected based on the maximum conden- sate load at maximum pressure, it will not be adequately sized at lower differential pressures. This occurs because the capacity of a steam trap decreases at a significantly faster rate than condensate load when the steam pressure drops. When temperature control valves are used to control steam flow to a HX, the pressure may reduce to 0 psig or less. The pressure available to discharge condensate would then be based on static head pressure of the drip leg. A drip leg length of 14" will produce a static head pressure of 1/2 psig. In this application, 614 lbs/hr of condensate is being generated at 0 psig steam pressure. See Load Chart. The appropriate safety load factor for this application is 2.5X the maximum condensate load of 855 lbs/hr. Therefore, select a steam trap based on: 2.5 x 855 = 2,138 lbs/hr at a differential pressure of 20 psi. .
40 psig
0 psig
OPEN Vacuum Breaker
½ psig Head Pressure
1 psig
2.31 ft. water column
Condensate flows through trap
0 psig
2.31 Ft. X 0.433 = 1 psig
14” = ½ psig
5 Selecting the Steam Trap
The steam trap should be sized for a condensate load of 2.5 x 855 = 2,138 lbs/hr at 20 psi differential pressure. The HX is assumed to have 20 psig steam pressure at the maximum water usage of 18 GPM . However, when selecting the PMO (maximum operating pressure) for the trap, assume the actual pressure could reach nearly the full line pressure of 40 psig . Therefore, select a trap with a PMO of at least 40 psig . Referring to the WFT Capacity chart below, we must select a WFT-075 model with a PMO of 75 psig. Enter the 20 psi column inside the WFT-075 section and scroll down until a condensate load greater than 2,138 lbs/hr is found. The capacity of 3,850 lbs/hr is the first condensate load greater than 2,138 lbs/hr . For this application, the WFT-075-15-N will be capable of discharging the calculated condensate loads of 2,138 lbs/hr at 20 psi Δ P and 614 lbs/hr at ½ psi Δ P .
CAPACITIES – Condensate (lbs/hr) PMO PipeOrifice Model Code (PSIG) Size Size 1/4 1/2
Differential Pressure (PSI)
1
2
5
10
15 20
30
40 50
75
15 3/4” 0.250 390 15
15 3/4” 0.250 1” 0.250 ” 0.312 15 1 ” 0.500 2” 0.625 30 3/4” 0.228 15
390 390 610
490 490 770
620 620 620 780 1050 1320 1500 620 780 1050 1320 1500 780 1050 1320 1500 780 1050 1320 1500 960 1210 1630 2040 2330 2570 3460 5120 6890 8190 3860 5040 7170 9360 10930
WFT-015-13-N WFT-015-14-N WFT-015-15-N WFT-015-16-N WFT-015-17-N WFT-030-13-N WFT-030-14-N WFT-030-15-N WFT-030-16-N WFT-030-17-N WFT-075-13-N WFT-075-14-N WFT-075-15-N WFT-075-16-N WFT-075-17-N
1” 0.250 390
15 1 1 / 4 ” 0.312 610
Up to 15 PSI
15 1 1 / 2 ” 0.500 1420 1910 2570 3460 5120 6890 8190
1420 2260
This 3/4” trap can discharge 855 lbs/hr at 20 PSI but is drastically undersized at 1/2 PSI
2” 0.625 2260 2950 3860 5040 7170 9360 10930
30 3/4” 0.228 330 30 1” 0.228 30 1 ” 0.228 30 1 ” 0.390 930 2” 0.500 1420 30 1” 0.228 330 30 1 1 / 4 ” 0.228 330 75 3/4” 0.166 175 330 330 330
420 420 420
530 530 530 670 930 1180 1350 1500 1720 530 670 930 1180 1350 1500 1720 530 670 930 1180 1350 1500 1720 1650 2190 3210 4280 5060 5700 6750 2570 3460 5120 6890 8190 9260 11020 670 670 930 1180 1350 1500 1720 930 1180 1350 1500 1720 530 670 930 1180 1350 1500 1720
Up to 30 PSI
30 1 1 / 2 ” 0.390 930 1240 1650 2190 3210 4280 5060 5700 6750 30 2” 0.500 1420 1910 2570 3460 5120 6890 8190 9260 11020
225 225
295 295
385 385
545 545
705 705
825 825
920 1075 1200 1305 1525 920 1075 1200 1305 1525
75
1” 0.166 175
Up to 75 PSI
75 1 1 / 4 ” 0.312 640 75 1 1 / 2 ” 0.312 640
850 1130 1500 2180 2900 3420 3850 4540 5110 5600 6610 850 1130 1500 2180 2900 3420 3850 4540 5110 5600 6610
75 2” 0.422 1020 1340 1760 2310 3330 4380 5140 5760 6770 7590 8290 9730
1 1 / 4 ” trap is able to handle the condensate load of 614 lbs/hr at 1/2 psi. (see Load Chart to left)
The actual load is only 855 lbs/hr. However, with 2.5X SLFs applied, this larger trap was selected since it has adequate capacity at lower pressures.
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Tel: 610-495-5131 • Pottstown PA • USA •• www.watsonmcdaniel.com
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