2000 Hook-up Book

Condensate Recovery Systems

have been sized using water vol ume only and did not include the flash steam volume that is pre sent. The specific volume of water at 0 psig is .017 cubic feet per pound, compared to 26.8 cubic feet per pound for flash steam at the same pressure. Sizing of con densate return lines from trap discharges based totally on water is a gross error and causes lines to be drastically undersized for the flash steam. This causes con densate lines to become pressurized, not atmospheric, which in turn causes a backpres sure to be applied to the trap’s discharge which can cause equipment failure and flooding. This undersizing explains why the majority of 0 psi atmos pheric condensate return systems in the United States do not operate at 0 psig. To take this thought one step further for those people who perform temperature tests on steam traps to determine if the trap has failed, the instant we cause a positive pressure to develop in the condensate return system by flash steam, the con densate return line now must follow the pressure/temperature relationship of saturated steam. So, trap testing by temperature identifies only that we have a return system at a certain tem perature above 212°F (0 psig) and we can then determine by that temperature the system pressure at which it is operating. Elevated condensate return tem peratures do not necessarily mean a trap has failed. When sizing condensate return lines, the volume of the flash steam must be given due consideration. The chart at Fig. 51 (page 43) allows the lines to be sized as flash steam lines— since the volume of the condensate is so much less than that of the steam released. Draining condensate from

traps serving loads at differing pressures to a common conden sate return line is a concept which many find difficult. It is often assumed that the HP “high pressure” condensate will pre vent the “low pressure” condensate from passing through the LP traps and give rise to waterlogging of the LP system. However, the terms HP and LP can only apply to the condi tions on the upstream side of the seats in the traps. At the down stream or outlet side of the traps, the pressure must be the com mon pressure in the return line. This return line pressure will be the sum of at least three compo nents: 1. The pressure at the end of the return line, either atmos pheric or of the vessel into which the line discharges. 2. The hydrostatic head needed to lift the condensate up any risers in the line. 3. The pressure drop needed to carry the condensate and any flash steam along the line. Item 3 is the only one likely to give rise to any problems if con densate from sources at different pressures enters a common line. The return should be sufficiently large to carry all the liquid con densate and the varying amounts of flash steam associated with it, without requiring excessive line velocity and excessive pressure drop. If this is accepted, the total return line cross sectional area will be the same, whether a single line is used, or if two or more lines are fitted, with each taking the condensate from a single pres sure source. The return could become undersized, requiring a high pres sure at the trap discharges and restricting or preventing dis charge from the LP traps, if it is forgotten that the pipe has to

carry flash steam as well as water and that flash steam is released in appreciable quantity from HP condensate. While the percentage, by weight, of flash steam may be rather low, its overall volume in comparison to the liquid is very large. By determining the quantity of flash steam and sizing the return line for velocities between 4,000 and 6,000 ft/min, the two phase flow within the pipe can be accommodated. The information required for sizing is the conden sate load in lb/h, inlet pressure to steam trap(s) in psig and return line system pressure. Example: Size a condensate return line from a 160 psig steam trap dis charging to 20 psig. flash tank. Load is 3,000 lb/h. 1. Determine percent flash steam produced using Table 12 (page 41). With a steam pressure of 160 psig and a flash tank pressure of 20 psig read a value of 12.4%. 2. Next, multiply the condensate load by the percent flash from step #1 to determine the flowrate, of flash steam pro duced. 3,000 lb/h x .124 = 372 lb/h. 3. Enter Fig. 51 (page 43) at the flash steam flowrate of 372 lb/h at “A” and move horizon tally to the right to the flash tank pressure of 20 psig “B”. Rise vertically to choose a condensate return line size which will give a velocity between 4,000 and 6,000 ft/min, “C”. In this example, an 1-1/2” schedule 40 pipe with a velocity of approxi mately 5,000 ft/min. If schedule 80 pipe is to be used, refer to table within body of chart. Multiply the velocity by the factor to deter mine whether the velocity is within acceptable limits.

SYSTEM DESIGN

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