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1. 421 International Conference of Building Officials Uniform Building Code 1988 edition Lengqiust R and Hansen A L 1973 Geothermal steam piping at Big Geysers California U S A 1960 75 Veizades and Associates Inc 1981 Slope map from unpublished study for Occidental Geothermal Inc2. ICAL MUFFLER MAIN STEAM GATHERING LINE SEPARATOR F VENT l ISOLATION VALVE WELL LINE ISOLATION VALVE WELL LINE CONDENSATE COLLECTION __ POWER PLANT ay ear oan aera ATH M wy G ERING SYSTEM COND COLLECTION __ GENERATOR __ TURBINE STEAM GATHERING _ le uae CONDENSATE COLLECTION BASIN ELECTRICAL GRID a m WELL SITE lt 2 CONDENSATE RETURN CONDENSER e CF WELL LINE 6 gt gt COOLING TOWER BASIN _ jee WELL LINE 8 INJECTION CONDENSATE INJECTION SUMP REINJECTION WELL a NE pe Ie L E il E TYPICAL STEAM GATHERING SYSTEM DIAGRAM OESIGNEO FOR WELL OVERPRESSURE SHUT IN PRESSURE PROTECTION PROVIDED WELL BLEED ae STEAM GATHERING WELL LINE SEPARATOR HEADER CATCHER WIRE LINE VALVE WORKING VALVE WELL LINE ISOLATION VALVE MASTER VALVE TO CONDENSATE 5 COLLECTION BASIN TYPICAL WELL LINE DIAGRAM Figure 1 Simplified steam gathering piping and instrumentation diagram 246 Harry Veizades and William J Cain must be routed around these areas Removal of vegetation for construction must be minimized Removal of vegeta tion creates barren areas that erode and produce run off induced siltation of streams Access for construction is another important factor Many of the slopes in the area are very steep One un published study Veizades amp Associates 1981 of slopes at The Geys
3. NOTICE CONCERNING COPYRIGHT RESTRICTIONS This document may contain copyrighted materials These materials have been made available for use in research teaching and private study but may not be used for any commercial purpose Users may not otherwise copy reproduce retransmit distribute publish commercially exploit or otherwise transfer any material The copyright law of the United States Title 17 United States Code governs the making of photocopies or other reproductions of copyrighted material Under certain conditions specified in the law libraries and archives are authorized to furnish a photocopy or other reproduction One of these specific conditions is that the photocopy or reproduction is not to be used for any purpose other than private study scholarship or research If a user makes a request for or later uses a photocopy or reproduction for purposes in excess of fair use that user may be liable for copyright infringement This institution reserves the right to refuse to accept a copying order if in its judgment fulfillment of the order would involve violation of copyright law Geothermal Resources Council Monograph on The Geysers Geothermal Field Special Report No 17 1991 DESIGN OF STEAM GATHERING SYSTEMS AT THE GEYSERS A STATE OF THE ART REVIEW ABSTRACT Geothermal gathering system designs have evolved at The Geysers KGRA to meet ever changing operational and design parameters New technologies
4. analyses in a matter of hours on a personal computer Flow analyses in The Geysers were traditionally done by hand using the empirical Fritzche equation for pres sure drops For a large gathering system this could be come quite tedious when the designer wished to consider multiple scenarios of flow Today computer based flow analyses are performed using the Darcy Weisbach for mulation with steam properties automatically generated This process makes it possible to perform parametric studies to examine how velocity and pressure change at any desired point in the piping system This enables the designer to optimize the selection of pipe sizes and check system performance under various steam flow scenarios The advances in computer assisted stress analysis techniques have enabled the designer to develop a much better understanding of the behavior of piping systems Use of computer programs such as AUTOPIPE Engineer ing Design Automation 1987 enables the designer to 247 model pipe restraints at pipe supports and gaps at the guides and by iterative techniques provide an accurate determination of piping behavior pipe stress and pipe movements Several computer programs are available for analyzing piping systems Before any program is used however the designer must determine whether the pro gram accurately models the piping system and the pipe supports The proper modeling of incline supportsis very important in selecting computer prog
5. ble to cracking by INSULATION SADDLE PLATE WEB PLATE STRAP GUSSET U HOLT BEARING BAR SOLE PLATE STANCHION HOT POSITION COLD POSITION Figure 3 Strap on support shoe Harry Veizades and William J Cain high frequency vibrations produced by the control valves orifice restrictions and high velocities The use of carbon steel piping since the 1960s has been successful in geothermal steam service A silica coating of the pipe interior has protected the pipe from corrosion on systems with relatively clean steam Quite often a cor rosion allowance is not provided on such systems Recent ly however some areas of The Geysers steam fields have produced steam with high concentrations of chlorides These chlorides have made a devastating corrosive attack on the steam lines To combat this problem several me thods have been studied Among those are the use of more exotic piping materials acceptance of reduced use operat ing life with scheduled replacement of piping and scrub bing of the corrosive agents The most cost effective me thod of corrosion mitigation has been the installation of scrubbing systems to remove or neutralize the corrosive agents Arecent innovation in piping construction is the use of induction heat pipe instead of welding fittings for angle changes in pipe systems The induction bends minimize field welding expedite field installation minimize radio graphic inspection and reduce pipe stress d
6. c 249 Stress corrosion cracking at welding neck flanges and at welding fittings This cracking manifests as a hair linecrack The presence of H2S and arsenic compounds in the steam may contribute to this cracking The actual mechanism or cause of this cracking has not been conclusively determined This cracking while costly to repair is not catastrophic To combat the cracking problems the following me thods have applied in the design of piping systems e Minimize welding on the pipe Use strap on shoes and brackets rather than welding them to the pipe e Preheat and post heat when welding flanges Stress relieve fabricated spools downstream of control valves and other restrictions e Increase thickness of piping material downstream of control valves and other restrictions e Avoid control valve settings that induce high frequen cy vibrations e Use control valves with noise reduction times e Use of low hydrogen welding rods TYPICAL DESIGN CRITERIA Most steam gathering systems at The Geysers are de signed using the ANSI B31 1 piping code and the follow ing design parameters Well Steam Lines From wellhead to header Design Pressure 500 psig full wellhead pressure Operating Pressure 120 to 500 psig Temperature 490 F Saturated steam temperature Flow velocity 250 ft sec maximum System Test Performance 500 psig full wellhead pres sure Insulation 3 inch for smaller diameter pipes and 4 inc
7. ers revealed that in an area of about 15 square miles slopes exceeded two horizontal to one vertical over 75 percent of the area Many of these slopes are covered with highly fractured and friable shale We have found that route selection is the most critical aspect in the design process and should be assigned to an experienced engineer It is important that the engineer be well versed in geothermal piping construction techni ques geotechnical evaluation surveying behavior of pip ing under thermal movements condensate line hydrau lics and environmental matters Using maps produced by aerial photography several proposed alignments are stu died on paper to determine the controlling parameters in route selection With these results in hand the engineer then performs a field reconnaissance of the most promis ing alignments The final route selected is then marked for clearing and right of way construction Following right of way construction a precise survey of the proposed align ment is made This survey provides the basis for prepara tion of construction drawings and for piping construction Piping Design Techniques Lenggqiust 1973 set forth some basic design techniques and details that have been used in the design of several gathering systems at The Geysers Since then several aspects of piping design have advanced through the use of computer programs for flow and stress analyses It is now possible to make rapid cost efficient
8. f the Mayacmas Mountains requires careful selection of pipeline routes between wells and the power plants they supply Such factorsas access for construction and flexibility for thermal expansion must be taken into account in selecting the routing Condensate collection lines normally parallel the steam gathering lines In order to minimize pumping of conden sate it is important to minimize sags in the steam and condensate pipe alignment Ideally the steam and conden sate line alignment should have no sags allowing the collection of condensate by gravity to a single condensate storage facility for pumping Environmental constraints play a major role in route selection Most of the developments at The Geysers are located along ridge tops and they are highly visible for many miles Careful selection of piping routes is required to minimize the visual impact and noise transmission from the various system components to nearby communities This is particularly important in areas where communities are located within line of sight of the facilities In some areas the vegetation is very fragile and pipeline alignment Design of Steam Gathering Systems at The Geysers A State of the Art Review WELL SITE VENT TYPICAL CONDENSATE POT 1 m amp LIQUIO ORAINER eae SCRUB WATER wen une gt yA D INJECTION 74 rau ne gt SHH i perep t ce ee were ume S gt fx RUPTURE DISK TYP
9. ged by valving or drainers to the condensate collection system Recent practice has been to inject water into the steam upstream of the main separator to enhance the separator s ability to remove particulates and silica that are carried by the steam Careful control of this water injection has been shown to reduce the amount of silica deposited on the blades of the turbine The condensate system is normally designed to handle this additional load from the steam scrubbing operations at the separators CONCLUSIONS This paper has presented a broad overview of the developments in gathering system designs that have oc curred since Lenggiust s landmark paper in 1973 It should be noted that the design of geothermal gather ing systems at The Geysers has been and continues to be an evolutionary process The designer must keep abreast of the various operational and performance problems and develop piping designs to solve such problems Com munication between various operators and designers con tinues to advance the state of the art Only by the close cooperation and free flow of information between the various operators and designers can advancements be made in the design of steam gathering systems REFERENCES Engineering Design Automation 1987 AUTOPIPE User s Manual Version 4 1 Engineering Design Automation Berkeley CA Freeston D H 1981 Condensation pot design Model tests Geother mal Resources Council Transactions vol 5 p
10. h for larger diameter pipes Fiberglass with density of 3 lbs cu ft and aluminum jacketing Flange Rating ANSI Class 300 Overpressure Protection None Gathering Lines Design Pressure 200 psig Operating Pressure 105 to 165 psig Temperature 340 F Saturated steam temperature Flow velocity 50 to 250 ft sec System Test Pressure 240 psig Insulation 4 inch Fiberglass w density of 3 lbs cu ft and aluminum jacketing Flange Rating ANSI Class 150 Overpressure Protection Rupture disks at 190 psig When systems carry superheated steam the appropriate temperature should be used Design of Steam Gathering Systems at The Geysers A State of the Art Review CONDENSATE REMOVAL AND COLLECTION SYSTEMS Condensate generated in steam piping by heat losses and during warm up is collected in a series of condensate drop pots Freeston 1981 along the steam lines Conden sate is automatically removed from the drop pots with float controlled liquid drainers The use of various types of steam traps has been unsuccessful Presently the Arm strong and Nicholson drainers are the only successful devices for condensate removal Collection lines insofar as possible use gravity flow to convey the condensate to a collection tank or sump from where it is pumped or trucked to the cooling tower basin for reinjection The steam from wet wells is scrubbed to remove mois ture by the use of separators Condensate from separators is dischar
11. have enabled more efficient and economical designs to be developed This paper lists some of the developments that have occurred between 1973 and the present and presents the writers view of the current state of the art design and construc tion of the geothermal gathering facilities Topics covered are advances in pipeline route selection piping design techniques selection of piping materials and components development of details typical design criteria and condensate removal and collection systems INTRODUCTION The typical gathering system at The Geysers consists of the facilities necessary to convey geothermal steam from the steam wells to the power plant It includes facilities to collect condensate generated during start up and opera tion remove rocks and rock particles from the steam remove water and rock dust inject water for steam con ditioning convey excess condensate from the power plant to a well for reinjection into the reservoir and vent steam during start up and power plant outages A simplified piping and instrumentation diagram of a typical steam gathering system is shown in Figure 1 The 245 Harry Veizades and William J Cain Veizades amp Associates Inc 90 New Montgomery Street Suite 707 San Francisco California 94105 following sections discuss the state of the art of various aspects in the design of gathering systems at The Geysers Pipeline Route Selection The mountainous and steep terrain o
12. in thermal seis mic wind friction and thermal loads This approach in designing piping systems results ina balanced design that minimizes stress and support loads Supports for steam gathering lines are typically construc ted as a stanchion as shown in Figure 2 The bearing assembly is welded in the field This allows for final horizontal and vertical adjustment of the pipe bearing assembly to compensate for stanchion misalignments due to construction tolerances Loads on the stanchions due to thermal expansion friction seismic and wind loads are resisted by lateral bearing against the soil or rock Conservative loads for designing the supports are used to producea safe support design Design of Steam Gathering Systems at The Geysers A State of the Art Review amp COLD POSITION HOT POSITION i STEAM PIPE S PIPE BEARING BAR PIPE STANCHION CONCRETE CAISSON IN DRILLEO HOLE SUPPORT Figure 2 Typical pipe support and anchor stanchions The design of the supports and anchors is based on the Uniform Building Code U B C International Confer ence of Building Officials 1988 provisions for noncon strained pole type supports Section 2907 g 2 A The allowable soil bearing values are conservatively selected by the engineer based on observations made during re connaissance after construction of the pipeline right of ways The ultimate capacity of the supports is several times the design capacity de
13. rams for analyzing piping systems in the rough terrain at The Geysers The dynamic analysis capabilities of the computer pro gram allow the designer to investigate system perfor mance under seismic events and flow induced vibrations The concepts for designing steam gathering pipelines in mountainous terrain differ from piping designs within the confines of a power plant The typical power plant Piping is restrained to minimize loads on equipment noz zles Expansion is accommodated by expansion joints The steam gathering piping outside the power plant with its meandering layout as it follows roads trails ridges or topographic features is much more flexible and can accommodate large thermal movements without be coming over stressed The concepts we have followed in designing piping systems over the mountainous Geysers terrain are 1 Take advantage of the flexibility that the piping layout offers Often the piping layout has sufficient flexibility to accommodate expansion without the use of expan sion loops 2 Select supports and anchors to provide maximum flexi bility Anchors are spaced far apart bet ween 500 to 700 feet and located to control the pipe movements with out reducing piping flexibility for thermal expansion 3 Selectively restrain pipe for seismic and wind loads Such restraints are selected so that they do not unduly induce high support loads due to thermal expansion 4 Design supports and anchors to restra
14. termined under the U B C provisions This allows the supports to accommodate un expected loads resulting from construction sequencing differential earth movements and impact loads Although these loads may produce additional lateral deflection of the supports beyond the design values the flexible piping systems can accommodate them without over stressing The standard shoe design used at The Geysers in 1973 was a tee section welded to the pipe with or without a saddle This design experienced some hairline cracks in the pipe in the heat affected zones at the point where the shoes are welded to the pipe To correct this problem a strap on type shoe design has been adapted on recent installations Figure 3 This design has eliminated the cracking problem PIPING MATERIALS Materials for gathering systems must perform well under widely varying process conditions The presence of 248 ae PIPE N NG INSULATION ae REINFORCING PAD VENT HOLE N PIPE STANCHION CONCRETE CAISSON IN DRILLED WOLE ANCHOR HCl H2S and COz in the geothermal fluids as well as chemicals added in the various abatement processes re sults in fluids with highly corrosive to passive properties The primary piping material used at The Geysers is ASTM A53 Grade B The pipe is usually furnished seamless ERW or DSAW Seamless pipe is normally used for the steam lines from the wellhead to the root valve at the header Seamless pipe is less suscepti
15. ue to lower stress intensification factors The use of induction bent pipe with a bend radius from three to five times the pipe diameter has been very successful and cost effective The condensate transport piping systems handle geo thermal condensate that can be divided into two cate gories 1 Highly oxygenated fluids such as excess condensate at the cooling tower slated for reinjection and 2 Steam condensate collected for drip legs along steam lines and from separators This condensate is normally hot and contains some dissolved solids and noncon densable gases but has not been exposed to atmos pheric oxygen The oxygenated condensate is highly corrosive and piping materials used for its conveyance are stainless steel epoxy or cement lined carbon steel pipe or plastic pipe The nonoxygenated condensate is not as corrosive and piping material used for transporting it is normally carbon steel with a corrosion allowance Pumps used in condensate systems are specified with all wetted parts made from stainless steel PIPE CRACKING The piping systems at The Geysers have performed extremely well in 30 years of service There have been however occasions where cracking of pipes occurred The cracking can be attributed to the following e High frequency vibrations induced by steam flows downstream of orifice restrictions or control valves This cracking normally starts at a weld heat affected zone The cracking is sudden and catastrophi
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