2000 Hook-up Book

Flash Steam

Flash Steam Utilization In an efficient and economical steam system, this so called Flash Steam will be utilized, on any load which will make use of low pressure steam. Sometimes it can be simply piped into a low pressure distribution main for general use. The ideal is to have a greater demand for Low Pressure steam, at all times, than available supply of flash steam. Only as a last resort should flash steam be vented to atmosphere and lost. If the flash steam is to be recovered and utilized, it has to be separated from the conden sate. This is best achieved by passing the mixture of flash steam and condensate through what is know as a “flash tank” or “flash vessel”. A typical arrange ment is shown in Fig. II-76 (page 120). The size of the vessel has to be designed to allow for a reduced velocity so that the sep aration of the flash steam and condensate can be accomplished adequately, so as not to have car ryover of condensate out into the flash steam recovery system. This target velocity is ten feet per second per ASHRAE standards to ensure proper separation. The condensate drops to the bottom of the flash tank where it is removed by a float and thermo static steam trap. The flash steam outlet connection is sized so that the flash steam velocity through the outlet is approximately 50 ft./sec. The condensate inlet is also sized for 50 ft./sec. flash velocity. A number of basic require ments and considerations have to be met before flash steam recov ery is a viable and economical proposition: 1. It is first essential to have a sufficient supply of conden sate, from loads at sufficiently higher pressures, to ensure that enough flash steam will be released to make recov ery economically effective.

The steam traps, and the equipment from which they are draining condensate, must be able to function sat isfactorily while accepting the new back pressure applied to them by the flash recovery system. Particular care is needed when attempting to recover flash steam from condensate which is leaving equipment controlled by a modulating temperature con trol valve. At less than full loads, the steam space pres sure will be lowered by the action of the temperature control valve. If the steam space pressure approaches or even falls below the flash steam vessel pressure, con densate drainage from the steam space becomes impractical by a steam trap alone, and the equipment becomes “stalled” and water logging will most definitely occur. 2. The second requirement is a suitable use for low pressure flash steam. Ideally, low pres sure load(s) requires at all times a supply of steam which either equals or exceeds the available flash steam supply. The deficit can then be made up through a pressure reducing valve set. If the supply of flash steam exceeds the demand for it, the surplus may have to be vented to waste through a backpressure control valve (see Fig. II-77, page 120). Thus, it is possible to utilize the flash steam from process condensate on a space heat ing installation - but the savings will only be achieved during the heating season. When heating is not required, the recovery system becomes ineffective. Whenever possible, the bet ter arrangement is to use flash steam from process condensate to supply

process loads, and that from heating condensate to supply heating loads. Supply and demand are then more likely to remain “in step”. When all else fails, in many facilities there is always a need for hot water, especially in the boiler house. This can be supplied via a heat exchanger and the use of flash steam. 3. It is also preferable to select an application for the flash steam which is reasonably close in proximity to the high pressure condensate source. Piping for low pressure steam is inevitably of larger diameter. This makes it somewhat costly to install. Furthermore, the heat loss from large diameter pipes reduces the benefits obtained from flash steam recovery and in the worst cases could outweigh them. Flash steam recovery is sim plest when being recovered from a single piece of equipment that condenses a large amount of steam, such as a large steam to water converter of a large air han dling coil bank, but we cannot forget that flash steam recovery systems by design will apply a backpressure to the equipment being utilized as the flash steam source. How To Size Flash Tanks And Vent Lines Whether a flash tank is atmos pheric or pressurized for flash recovery, the procedure for deter mining its size is the same. The most important dimension is the diameter. It must be large enough to provide adequate separation of the flash and condensate to mini mize condensate carryover.

SYSTEM DESIGN

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