2000 Hook-up Book - Spirax Sarco

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SYSTEM DESIGN 44 Flash Steam Flash Vessel Configurations Flash vessels can be either horizontal or vertical. For flash steam recovery (pressurized receiver) the vertical style is preferred because of its ability to provide better separation of steam and water. Horizontal Flash Vessel Condensate inlet Length Length = 2 x diameter or 24" minimum Flash Steam outlet or vent pipe Condensate outlet Diameter For proper installation of flash vessels, controls and traps refer to Figures II-76, 77, 78, 79, 80, 81, 82, 83 (starting on page 120) The sour water condenser-reboiler is an important element in a refinery sulfur unit. Though the configuration/design will vary with the specific process used, the purpose and priority remain the same. Process water contaminated with ammonia and hydrogen sulfide gas (H2S) is stripped of those compounds for reuse. The remaining stream of contaminants goes to waste treatment. The process depends on accurate temperature control of the steam heated condenser-reboiler. 40 psig steam is supplied through a modulating control valve. Condensate is lifted 15 feet from the outlet at the bottom of the vertical condenser-reboiler to the overhead condensate receiver tank, which is maintained at 20 psig. Process load fluctuations and resultant turndown on the modulating steam control valve would have created a STALL situation, unacceptable process temperature control and reduced throughput. Figure 52: Flash Vessel Configurations Condensate inlet 35% of height Flash steam outlet or vent pipe Diameter Condensate outlet Case in Action: Sour Water Condenser-Reboiler Temperature Control Vertical Flash Vessel Height = 3 x Diameter or 36" minimum Solution A 3" x 2" PPF with 2-1/2" FTB 125 pump/trap combination was designed into the new project as was a VS 204 air vent. The installation was immediately successful. Benefits • With faster start-up, it came up to temperature faster than any other comparable unit, to date, at the refinery. This improves productivity. • The feed rate is higher than designed because the unit is able to operate efficiently at any degree of turndown. • Installation cost is several times less costly for the pump/trap combo than traditional level control system that would otherwise have been used. • Maintenance cost is lower, through elimination of electric/pneumatic controls and electric pumps used in a traditional level control system.
The importance of effective condensate removal from steam spaces has been stressed throughout this course. If maximum 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 reasons why condensate should not be allowed to discharge to drain. The most important consideration is the valuable heat which it contains 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 treatment, 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 savings 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 condensate and process fluids is always a danger. If there is any possibility that the condensate is contaminated, it must not be returned to the boiler. These problems have been lessened by the application of sensing systems monitoring the quality of condensate in different holding areas of a plant to determine condensate quality and providing a means to re-route the condensate if contaminated. Condensate Line Sizing Condensate recovery systems divide naturally into three sections, each section requiring different design considerations. a. Drain Lines to the traps carry pressurized high temperature hot water that moves by gravity. b. Trap discharge lines that carry a two-phase mixture of flash steam and condensate. c. Pumped return systems utilizing electric or non-electric pumps. Drain Lines To Traps In the first section, the condensate 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 running load. Trap Discharge Lines At the outlet of steam traps, the condensate return lines must carry condensate, non-condensible gases and flash steam released from the condensate. Where possible, these lines should drain by gravity to the condensate 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 considered. Condensate Recovery Systems 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 First, one pound of steam has a specific volume of 26.8 cubic feet at atmospheric pressure. 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, having 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 considered 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 system’s temperature before putting this new water back into the boiler. Secondly, the actual formation 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 carrying this flash steam, as well as condensate. Unfortunately, in the past, condensate return lines 45 SYSTEM DESIGN
Page 1 and 2: DESIGN OF FLUID SYSTEMS HOOK-UPS
Page 3 and 4: Spirax Sarco Spirax Sarco is the re
Page 5: Section 1: System Design Informatio
Page 8 and 9: SYSTEM DESIGN 2 The Working Pressur
Page 10 and 11: SYSTEM DESIGN 4 Sizing Steam Lines
Page 12 and 13: SYSTEM DESIGN 6 Sizing Superheated
Page 14 and 15: SYSTEM DESIGN 8 Draining Steam Main
Page 16 and 17: SYSTEM DESIGN 10 Draining Steam Mai
Page 18 and 19: SYSTEM DESIGN 12 Steam Tracing The
Page 20 and 21: SYSTEM DESIGN 14 Steam Tracing Sizi
Page 22 and 23: SYSTEM DESIGN 16 Steam Tracing Trac
Page 24 and 25: SYSTEM DESIGN 18 Steam Tracing A si
Page 26 and 27: SYSTEM DESIGN 20 Pressure Reducing
Page 28 and 29: SYSTEM DESIGN 22 Parallel and Serie
Page 30 and 31: SYSTEM DESIGN 24 How to Size Temper
Page 32 and 33: SYSTEM DESIGN 26 Temperature Contro
Page 38 and 39: SYSTEM DESIGN 32 Makeup Air Heating
Page 40 and 41: SYSTEM DESIGN 34 Draining Temperatu
Page 42 and 43: SYSTEM DESIGN 36 Draining Temperatu
Page 44 and 45: SYSTEM DESIGN 38 Steam Trap Selecti
Page 46 and 47: SYSTEM DESIGN 40 Steam Trap Selecti
Page 48 and 49: SYSTEM DESIGN 42 Flash Steam Flash
Page 52 and 53: SYSTEM DESIGN 46 Condensate Recover
Page 54 and 55: SYSTEM DESIGN 48 Condensate Pumping
Page 56 and 57: SYSTEM DESIGN 50 Clean Steam Printi
Page 58 and 59: SYSTEM DESIGN 52 Clean Steam Overal
Page 60 and 61: SYSTEM DESIGN 54 Clean Steam Figure
Page 62 and 63: SYSTEM DESIGN 56 Testing Steam Trap
Page 64 and 65: SYSTEM DESIGN 58 Spira-tec Trap Lea
Page 66 and 67: SYSTEM DESIGN 60 Steam Meters Densi
Page 68 and 69: SYSTEM DESIGN 62 Compressed Air Sys
Page 70 and 71: SYSTEM DESIGN 64 Compressed Air Sys
Page 72 and 73: SYSTEM DESIGN 66 Compressed Air Lin
Page 74 and 75: SYSTEM DESIGN 68 Typical Steam Cons
Page 76 and 77: SYSTEM DESIGN 70 Specific Heats and
Page 78 and 79: SYSTEM DESIGN 72 Specific Heats and
Page 80 and 81: SYSTEM DESIGN 74 Conversions To Con
Page 82 and 83: SYSTEM DESIGN 76 Flow of Water thro
Page 84 and 85: SYSTEM DESIGN 78 Moisture Content o
Page 86 and 87: SYSTEM DESIGN 80 ANSI Flange Standa
Page 89 and 90: HOOK-UP APPLICATION DIAGRAMS Sectio
Page 91 and 92: Figure II-3 Draining and Air Ventin
Page 93 and 94: Figure II-8 Draining Steam Main whe
Page 95 and 96: Figure II-12 Typical Pressure Reduc
Page 97 and 98: Figure II-16 Installation of Pressu
Page 99 and 100: Figure II-20 Typical Pneumatic Sing
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Condensate Return Steam Supply Stea
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Steam Supply Steam Supply Figure II
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Figure II-31 Temperature Control of
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Elevated or Pressurized Return P 2
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Figure II-39 Controlling Temperatur
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Page 113 and 114:
Page 115 and 116:
Figure II-50 Trapping Small Utensil
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Figure II-55 Draining and Air Venti
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Figure II-59 Air Venting and Conden
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Evaporator Pressure Figure II-64 Dr
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Figure II-68 Installation of Pump/T
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Figure II-73 Draining Small Condens
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Figure II-78 Flash Steam Recovery a
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Figure II-80 Heating Water using Re
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Figure II-82 Recovery of Flash Stea
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Figure II-84 Flash Steam Condensing
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Figure II-87 Tank Sterilization Cle
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Steam Supply Figure II-93 Typical S
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Figure II-97 Hand Operated Rotary F
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Figure II-102 Freeze Proof Safety S
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Figure II-106 Continuous Boiler Blo
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Figure II-111 Controlling Temperatu
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141
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PRODUCT INFORMATION Section 3
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Steam Traps • Balanced Pressure T
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147
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Subject Index Safety...............
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Subject Index Radiation—Baseboard
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Argentina Spirax Sarco S.A. Ruta Pa
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2000 Hook-up Book - Spirax Sarco

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