Sizing Condensate Return Lines

Calculation of condensate piping by two-phase flow techniques is recommended however, the tedious work per line can often be reduced by using empirical methods and charts. Some of the best are proprietary and not available for publication however, the Sarco method [42] has been used and found to be acceptable, provided no line less than VA is used regardless of the chart reading. Under some circumstances, which are too random to properly describe, the Sarco method may give results too small by possibly a half pipe size. Therefore, latitude is recommended in selecting either the flow rates or the pipe size. 

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Since a biphase mixture is possible in most condensate return lines, their correct sizing becomes essential. 

Example 2-19 Sizing Steam Condensate Return Line... 

The accompanying nomograph quickly sizes the recommended condensate return-line. 

Sizing of flashing steam condensate return lines requires techniques that calculate pressure drop of two-phase flow correlations. Many correlations have been presented in the literature [15,16,19,23]. Most flow patterns for steam condensate headers fall within the annular or dispersed region on the Baker map. Sometimes, they can fall within the slug flow region however, the flashed steam in steam condensate lines is less than 30% by weight. 

Figure22.14 Condensate line sizing chart where pressure at traps is above 4bar (SI units). 1. From pressure upstream of trap move horizontally to pressure in return line (A). 2. Drop vertically to condensate load in kg/h (B). 3. Follow curve to RFI scale and across to same return line pressure (C). 4. Move upward to return line flash velocity - say, 25 m/s maximum (D). 5. Read return line size.

The pickup, transport, and redeposition of corrosion debris and deposits can happen anywhere in steam distribution and condensate return systems and are not confined to any particular boiler plant size or pressure rating. For example, deposit pickup may occur in a superheater with redeposition taking place perhaps in a pressure reducing station, steam trap, or condensate line. The starting point for transport mechanisms is often a combination of BW carryover and condensate line corrosion. 

Due to the much greater volume of flash steam compared with unflashed condensate, sizing of the return line is based solely on the flash steam. It is assumed that all flashing occurs across the steam trap and that the resulting vapor-liquid mixture can be evaluated at the end-pressure conditions. To ensure that the condensate line does not have an appreciable pressure-drop, a low flash-steam velocity is assumed (50 ft/s) [i]. 

Oil Release to the Columbia River. On February 6, 1987, approximately 19 L (5 gal) of turbine oil was discharged to the Columbia River through the 182-N tank farm raw water return line. A small (pin-hole size) leak in a lube oil line in the No. 2 drive turbine allowed oil to enter the secondary steam system. Steam condensate from this system returns to the lOO-N steam condensate system that drains to the river. The leak in the lube line was repaired (DOE-RL 1990). 

An evaporator is condensing 5,500 Ib/h of steam at 150-psig supply pressure. During normal operation, a control valve maintains a pressure of 85 psig upstream of the steam trap. The condensate is returned to a vented tank. What line size is recommended downstream of the trap ... 

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