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May 5, 2009 ~ Monday Morning Minutes

When do I need a flash tank?

Flash tanks serve a valuable purpose in condensate return systems with medium or high pressure steam. Flash tanks provide an area, in the return system, to allow high temperature condensate to flash to a lower pressure and, in the process, lower the temperature of the condensate to avoid damaging pumps in the system. Additionally, flash tanks allow the designing engineer to control where the flash occurs. So when do we need to add a flash tank to a system? Let’s start by looking at flash steam in the piping system. The condensate pipe must be sized large enough to handle the condensate water flow and flash steam with a reasonable pressure drop. When the hot condensate enters the low pressure pipe, the flash to steam does not occur immediately. The water flashes to steam over several feet. The pipe diameter and horizontal length might be considered a mini flash tank, with the vent being the end of the pipe entering the gravity return main.

Roy Ahlgren is the former instructor at the ITT Bell and Gossett Little Red Schoolhouse in Morton Grove, IL. His ASHRAE Journal article, “Flash Tanks for Steam and Boiler Systems” describes the formulae used to size the volume of a flash tank. If we assume the pipe is a flash tank with a small diameter (pipe size), a minimum length would be required to obtain the volume required to flash to zero PSIG. If the pipe is not long enough, the pressure will be greater than zero, causing a back pressure and possible damage to in low pressure systems with the same return.

Let’s look at an example of how long the return piping should be to bring the pressure to 0 PSIG:

Chart shows pipe sizes & lengths for 300 PPH drip line citing the Roy Ahlgren

article. In the chart to the left, we can see very long pipe lengths required for a relatively

small flow rate. Let’s assume there is a 30 PSIG drip trap. From the chart, the horizontal line would have to be 1” diameter and 27 feet long. If the pipe is only 15 feet long, to the gravity return line, we can see through extrapolation the pressure, at the end of the line will be 10 PSIG. If there is a low pressure trap off of a heat exchanger at this point, we would cause a back pressure and potential damage to the bundle due to condensate retention. You could size the drip pipe larger and make the pipe longer, but in general, we recommend using a flash tank whenever the system has low pressure and multiple medium or high pressure drips.

Click here for a reprint of the Ahlgren article

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