2000 Hook-up Book - Spirax Sarco

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SYSTEM DESIGN 20 Pressure Reducing Stations PRV Station Components A stop valve is usually needed so that the steam supply can be shut off when necessary, and this should be followed by a line size strainer. A fine mesh stainless steel screen in the strainer will catch the finer particles of dirt which pass freely through standard strainers. The strainer should be installed in the pipe on its side, rather than in the conventional way with the screen hanging below the pipe. This is to avoid the screen space acting as a collecting pocket for condensate, since when installed horizontally the strainer can be self-draining Remember that water which collects in the conventionally piped strainer at times when the reducing valve has closed, will be carried into the valve when it begins to open. This water, when forced between the valve disc and seat of the just-opening valve, can lead rapidly to wire-drawing, and the need for expensive replacements. Pressure gauges at each side of the reducing valve allow its performance to be monitored. At the reduced pressure side of the valve, a relief or safety valve may be required. If all the equipment connected on the low pressure side is capable of safely withstanding the upstream pressure in the event of reducing valve failure, the relief valve may not be needed. It may be called for if it is sought to protect material in process from overly high temperatures, and it is essential if any downstream equipment is designed for a pressure lower than the supply pressure. Steam Safety Valve Sizing When selecting a safety valve, the pressure at which it is to open must be decided. Opening pressure must be below the limitations of the downstream equipment yet far enough above the normal reduced pressure that minor fluctuations do not cause opening or dribbling. Type “UV” Safety Valves for unfired pressure vessels are tested to ASME Pressure Vessel Code, Section VIII and achieve rated capacity at an accumulated pressure 10% above the set-to- open pressure. Safety valves for use on boilers carry a “V” stamp and achieve rated capacity at only 3% overpressure as required by Section I of the Code. The capacity of the safety valve must then equal or exceed the capacity of the pressure reducing valve, if it should fail open when discharging steam from the upstream pressure to the accumulated pressure at the safety valve. Any bypass line leakage must also be accounted for. Figure 37 Typical Installation of Single Reducing Valve with Noise Diffuser Bypasses may be prohibited by local regulation or by insurance requirements Separator Figure 38 Typical Installation of Two Reducing Valves in Parallel Trap Set Pressure Sensing Line Reducing Valve Diffuser Safety Valve Downstream Isolating Valve is needed only with an alternative steam supply into the L.P. System Figure 39 Two-Stage Pressure Reducing Valve Station with Bypass Arrangement to Operate Either Valve Independently on Emergency Basis
Parallel Operation In steam systems where load demands fluctuate through a wide range, multiple pressure control valves with combined capacities meeting the maximum load perform better than a single, large valve. Maintenance needs, downtime and overall lifetime cost can all be minimized with this arrangement, Fig. 38 (page 20). Any reducing valve must be capable of both meeting its maximum load and also modulating down towards zero loads when required. The amount of load turndown which a given valve can satisfactorily cover is limited, and while there are no rules which apply without exception, if the low load condition represents 10% or less of the maximum load, two valves should always be preferred. Consider a valve which moves away from the seat by 0.1 inches when a downstream pressure 1 psi below the set pressure is detected, and which then passes 1,000 pounds per hour of steam. A rise of 0.1 psi in the detected pressure then moves the valve 0.01 inches toward the Parallel and Series Operation of Reducing Valves Case in Action: Elimination of Steam Energy Waste As part of a broad scope strategy to reduce operating costs throughout the refinery, a plan was established to eliminate all possible steam waste. The focus of the plan was piping leaks, steam trap failures and steam pressure optimization. Programs having been previously established to detect/repair steam trap failures and fix piping leaks, particular emphasis was placed on steam pressure optimization. Results from a system audit showed that a considerable amount of non-critical, low temperature tracing was being done with 190 psi (medium pressure) steam, an expensive overkill. It appeared that the medium pressure header had been tapped for numerous small tracing projects over the years. Solution Refinery engineers looked for ways to reduce pressure to the tracer lines. Being part of a cost-cutting exercise, it had to be done without spending large sums of capital money on expensive control valves. The self-con- seat and reduces the flow by approximately 100 pph, or 10%. The same valve might later be on a light load of 100 pph total when it will be only 0.01 inches away from the seat. A similar rise in the downstream pressure of 0.1 psi would then close the valve completely and the change in flow through the valve which was 10% at the high load, is now 100% at low load. The figures chosen are arbitrary, but the principle remains true that instability or “hunting” is much more likely on a valve asked to cope with a high turndown in load. A single valve, when used in this way, tends to open and close, or at least move further open and further closed, on light loads. This action leads to wear on both the seating and guiding surfaces and reduces the life of the diaphragms which operate the valve. The situation is worsened with those valves which use pistons sliding within cylinders to position the valve head. Friction and sticking between the sliding surfaces mean that the valve head can only be moved in a tained cast steel pressure regulators and bronze reducing valves were chosen for the job. In 1-1/2 years, approximately 40 pressure regulators and hundreds of bronze reducing valves have been installed at a cost of $250K. Annualized steam energy savings are $1.2M/year. More specifically, in the Blending and Shipping Division, $62,640 was saved during the winter of 1995, compared to the same period in 1994. Benefits • Low installed cost. The Spirax Sarco regulators and bronze reducing valves are completely self-contained, requiring no auxiliary controllers, positioners, converters, etc. • Energy savings worth an estimated $1.2M/year. • The utilities supervisor who worked closely with Spirax Sarco and drove the project through to successful completion received company wide recognition and a promotion in grade. series of discreet steps. Especially at light loads, such movements are likely to result in changes in flow rate which are grossly in excess of the load changes which initiate them. Load turndown ratios with pistonoperated valves are almost inevitably smaller than where diaphragm-operated valves are chosen. Pressure Settings for Parallel Valves Automatic selection of the valve or valves needed to meet given load conditions is readily achieved by setting the valves to control at pressures separated by one or two psi. At full load, or loads not too much below full load, both valves are in use. As the load is reduced, the controlled pressure begins to increase and the valve set at the lower pressure modulates toward the closed position. When the load can be supplied completely by the valve set at the higher pressure, the other valve closes and with any further load reduction, the valve still in use modulates through its own proportional band. 21 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 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 50 and 51: SYSTEM DESIGN 44 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
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SYSTEM DESIGN 70 Specific Heats and
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SYSTEM DESIGN 72 Specific Heats and
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SYSTEM DESIGN 74 Conversions To Con
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SYSTEM DESIGN 76 Flow of Water thro
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SYSTEM DESIGN 78 Moisture Content o
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SYSTEM DESIGN 80 ANSI Flange Standa
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HOOK-UP APPLICATION DIAGRAMS Sectio
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Figure II-3 Draining and Air Ventin
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Figure II-8 Draining Steam Main whe
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Figure II-12 Typical Pressure Reduc
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Figure II-16 Installation of Pressu
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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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