2000 Hook-up Book

Condensate Recovery Systems

Condensate Line Sizing Condensate recovery systems divide naturally into three sec tions, each section requiring different design considerations. a. Drain Lines to the traps carry pressurized high temperature hot water that moves by grav ity. b. Trap discharge lines that carry a two-phase mixture of flash steam and condensate. c. Pumped return systems uti lizing electric or non-electric pumps. Drain Lines To Traps In the first section, the conden sate has to flow from the condensing surface to the steam trap. In most cases this means that gravity is relied on to induce flow, since the heat exchanger steam space and the traps are at the same pressure. The lines between the drainage points and the traps can be laid with a slight fall, say 1” in 10 feet, and Table 13 shows the water carrying capacities of the pipes with such a gradient. It is important to allow for the passage of incondensibles to the trap, and for the extra water to be carried at cold starts. In most cases, it is sufficient to size these pipes on twice the full run ning load. Trap Discharge Lines At the outlet of steam traps, the condensate return lines must carry condensate, non-condensi ble gases and flash steam released from the condensate. Where possible, these lines should drain by gravity to the con densate receiver, whether this be a flash recovery vessel or the vented receiver of a pump. When sizing return lines, two important practical points must be consid ered.

The importance of effective con densate removal from steam spaces has been stressed throughout this course. If maxi mum steam system efficiency is to be achieved, the best type of steam trap must be fitted in the most suitable position for the application in question, the flash steam should be utilized, and the maximum amount of condensate should be recovered. There are a number of rea sons why condensate should not be allowed to discharge to drain. The most important consideration is the valuable heat which it con tains even after flash steam has been recovered. It is possible to use condensate as hot process water but the best arrangement is to return it to the boiler house, where it can be re-used as boiler feed water without further treat ment, saving preheating fuel, raw water and the chemicals needed for boiler feed treatment. These savings will be even greater in cases where effluent charges have to be paid for the discharge of valuable hot condensate down the drain. Condensate recovery sav ings can add up to 20 to 25% of the plant’s steam generating costs. One justifiable reason for not returning condensate is the risk of contamination. Perforated coils in process vessels and heat exchangers do exist and the cross contamination of conden sate and process fluids is always a danger. If there is any possibili ty that the condensate is contaminated, it must not be returned to the boiler. These problems have been lessened by the application of sensing sys tems monitoring the quality of condensate in different holding areas of a plant to determine con densate quality and providing a means to re-route the conden sate if contaminated.

First, one pound of steam has a specific volume of 26.8 cubic feet at atmospheric pres sure. It also contains 970 BTU’s of latent heat energy. This means that if a trap discharges 100 pounds per hour of condensate from 100 psig to atmosphere, the weight of flash steam released will be 13.3 pounds per hour, hav ing a total volume of 356.4 cubic feet. It will also have 12,901 BTU’s of latent heat energy. This will appear to be a very large quantity of steam and may well lead to the erroneous conclusion that the trap is passing live steam (failed open). Another factor to be consid ered is that we have just released 13.3 pounds of water to the atmosphere that should have gone back to the boiler house for recycling as boiler feed water. Since we just wasted it, we now have to supply 13.3 pounds of fresh city water that has been softened, chemically treated and preheated to the feedwater sys tem’s temperature before putting this new water back into the boil er. Secondly, the actual forma tion of flash steam takes place within and downstream of the steam trap orifice where pressure drop occurs. From this point onward, the condensate return system must be capable of carry ing this flash steam, as well as condensate. Unfortunately, in the past, condensate return lines Table 13: Condensate, lb/h Steel Approximate Frictional Resistance Pipe in inches Wg per 100 ft of Travel Size 1 5 7 10 1/2" 100 240 290 350 3/4" 230 560 680 820 1" 440 1070 1200 1550 1 1 / 4 " 950 2300 2700 3300 1 1 / 2 " 1400 3500 4200 5000 2" 2800 6800 8100 9900 2 1 / 2 " 5700 13800 16500 20000 3" 9000 21500 25800 31000 4" 18600 44000 52000 63400

SYSTEM DESIGN

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