STEAM TRAPS Introduction Steam Trap Selection Guidelines • PROCESS Applications
PROCESS Applications • Sizing a Trap for Heat Exchanger Application
Goal: Select appropriate model and size steam trap for Process Water Heating application using a Shell & Tube Heat Exchanger in which a maximum of 18 GPM of water is being heated from 45-140˚F. Steam Pressure to the control valve is 40 PSI. The trap is discharging to a condensate return line at atmospheric pressure (0 psig back pressure)
Steam Pressure in the HX varies directly with water flow rate
Example: Conditions of Service at Max Load • Maximum water load to be heated= 18 GPM • Temperature Rise: (140˚F - 45˚F) = 95°F
System Pressure
40 psig
Vacuum Breaker
20 psig
1 Calculate Maximum Condensate Load Using the formula below, to approximate steam load based on water flow rate (GPM) and temperature rise, a Steam Flow of 855 lbs/hr is required; therefore, 855 lbs/hr of condensate will likewise be generated.
STEAM
*
GPM x Temp. Rise (˚F) 2
Steam (lbs/hr) =
18 x 95˚F
=
= 855 lbs/hr
Hot Water 140˚F
0 psig
2
Cold Water 45˚F
Steam Flow (lbs/hr) = Condensate Load (lbs/hr)
Condensate (lbs/hr) = 855 lbs/hr
Max Flow Rate of Hot Water 18 GPM
Condensate Load (lbs/hr) equals steam load (lbs/hr) 0 psig back pressure
* For increased accuracy, multiply this formula by 1.1 or consult Page 47 for the Exact Formula.
2 Determining the Differential Pressure ( Δ P ) Across the Trap at the Maximum Condensate Load In order to size the steam trap, we must first know the pressure in the HX at the Max Condensate Load. This steam pressure is determined by the physical size of the HX. ( note that a larger HX uses lower steam pressure while a smaller HX requires a higher steam pressure to heat the same flow of water ). If the pressure of the HX is not known, assume 50% of the Pressure at the inlet of the control valve is required in the HX to heat the maximum flow of 18 GPM of water. We therefore have(40 psig x 0.5 = 20 psig) 20 psig steam pressure at 855 lbs/hr. 3 The Condensate Load at 0 psig Steam Pressure and Conditions at other Water Flow Rates The steam trap cannot be selected solely based the condensate load at the maximum steam pressure because it will be undersized at lower steam pressures (when there is much less force to push the condensate thru the trap). Lower steam pressures occur when less water is being heated. If the steam pressure and condensate load is known at the lowest pressure, the trap can be selected based on that operating point, and it would be adequately sized at higher pressures. In this particular application, we have a gravity return line at 0 psig back pressure.
Load Chart Based on HX Size to Heat 18 GPM of Water using 20 psig Steam Pressure
Flow Rate
Steam
Steam
Steam Actual
Trap
Flow Temperature Pressure Pressure Differential
Condensate
Water (required) (required) (required) in the HX Pressure
Flow
in HX (°F)
in HX (PSIG)
(GPM)
(lbs/hr)
(PSIG)
(PSI)
(lbs/hr)
At maximum Flow Rate of 18 GPM Flow Rate of Water at 0 psig Steam Pressure At Flow Rate of 10 GPM of Water
259
20
18
855
20
20
855
steam
212
0
1/2 psi
12.9
614
0
614
steam 14” drip leg
185
-6
1/2 psi
10
475
0
475
steam & air mixes together
vacuum
14” drip leg
For this size HX, when water flow rate is 12.9 GPM, the steam temperature required is 212˚F, therefore, the steam pressure is 0 psig. At flow rates below 12.9 GPM, the steam pressure would need to go into vacuum or mix with air drawn in thru the vacuum breaker in order to achieve the proper temperature.
3 3
www.watsonmcdaniel.com •• Pottstown PA • USA • Tel: 610-495-5131
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