IEEES-1Proceedings of the First International Exergy, Energy and Environment Symposium

13-17 July 2003, Izmir, TurkeyPaper No.

PIPELINES AND PIPING NETWORK IN GEOTHERMAL DISTRICT HEATING SYSTEMS

A.Caner Şener1, Gülden Gökçen1

1 İzmirInstitute of Technology Mechanical Engineering Department Gülbahçe – Urla, 35437 İzmir, Republic of Turkey

Fax: +90.232.4986505 E-mail:acanersener@yahoo.com, güldengökcen@iyte.edu.tr

ABSTRACT In geothermal district heating project construction oftransmission pipeline system is required to transportthe hot water from source to user. In most of thegeothermal district heating projects piping constitutesthe biggest part of capital investment cost. Since theproperties of geothermal water, infrastructure of citiesand economic criterion are different for each country,standards and methods may change country tocountry. Factors such as pipe sizing, pipe material,insulation, installation, head loss, heat loss, and servicepoints should be considered before final decision. Inthis study, general methods and international standardsused in geothermal piping and district heating pipingwill be reviewed. Then suitable methods and standardsand their applicability will be discussed for the Turkeycase.

INTRODUCTIONTurkey is one of the richest countries in geothermalpotential which is mostly located at the western part ofthe country. Characteristics of the geothermalresources in Turkey are mostly suitable for heating. Theuse of medium temperature (30 to 150 oC) geothermalresources as a heat source in Turkish district heatingsystems and greenhouses has been accelerating since1980’s. Considering the geothermal district heatingprojects, which are in the phase of conceptual planningand construction, it is obvious that geothermal energywill be started to use widely for the heating purposes inthe next decade. [1]

Considering the fact that transmission and distributionpiping generally constitute the biggest portion in a costof geothermal system. “For district heating systems, thecost associated with the distribution network isfrequently 40 - 60% of the overall capital cost of theproject. [2]” One of the most important factorsinfluencing the success of geothermal project is properpiping from material selection to installation andmaintenance. Since characteristics of geothermalresources, weather conditions and infrastructure ofeach city are different from each other. Eachgeothermal project is unique. Therefore there is nostandard way of piping for all geothermal district heatingsystems.

In Turkish geothermal projects, piping has not beenconsidered with enough attention at the beginning.However, experiences in Turkish geothermal district

heating systems show that, pipes are usually primarysource of problems during operation.

In geothermal district heating systems piping may bedivided into two categories. Geothermal transmissionpipeline which carries geothermal fluid from wells toheat stations and city distribution network whichdistributes hot water from heat stations to customers.

This study investigates the basic principles of routeselection, sizing, material selection, installation andmaintenance in Turkish geothermal district heatingsystems.

ROUTE SELECTIONGenerally route of pipeline follows the shortest pathfrom geothermal well to customers. Although it is thebest solution most of the times, it may not be alwayspossible to follow the shortest path. Highways, landowners, military zones, rivers and other utility networksare the main obstacles in front of the pipeline route. Inmost of the times overcoming these obstacles requiresheavy bureaucracy and time. It is possible to gain timeif applicatios are made at the very beginning of theproject for a necessary permissions.

Geological structure of the area from where geothermalpipeline passes is also important. According to thegeological structure of the region pipes may be burriedor above ground pipeline system considered. In Turkishgeothermal district heating systems both geothermalpipeline and city distribution network are burried.Although city distribution networks have always beenburried in all over the world, it is not the case forgeothermal transmission pipelines. If there is no densesettlement between wells and heat station, then bothoptions underground and above ground piping shouldbe analysed. The primary decisive factor is cost ofinstallation for most of the cases, however externalcorrosion risk should also be analysed.

PIPE SIZINGThe pressure loss per unitary length is a common sizingparameter. If the pressure loss is high, then theinvestment in the pipe is well utilised, but the operatingcost is high. On the other hand, if the pressure loss islow, the investment is badly utilised, but the pumpingcost is low [3].

The target head loss in the design of a new pipelinesystem varies from 5 to 20 mm/m. While deciding target

head loss, future expansion scenarios of the systemshould be considered. If the geothermal reservoir andcustomer potential are promising for the future, then it iswisefull to lower the target pressure loss.

Pipe sizing is an optimisation problem. As stated beforeeach geothermal project has different conditions.Selection of target head loss directly influences thefeasibility of the all project. Figure 1 shows the variationpipe cost and pumping cost with target head loss for asample Turkish district heating system which is locatedat Aegean region and serves approximately 3000dwellings. However the scene in Figure 1 maycompletely change for different climates or pipe costs.

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Figure 1: Target head loss selection

PIPE MATERIALSThere are numerous types of piping materials whichgreatly vary in cost and durability for geothermalheating systems. The main classification is madebetween metallic and non-metallic pipes. Amongmetallic pipes steel pipes (slip-joint steel (STL-S) andwelded steel (STL-W)) are most commonly used. Typesof non-metallic pipes can be listed as: gasketedpolyvinyl chloride (PVC-G), solvent welded PVC (PVC-S), chlorinated polyvinyl chloride (CPVC), polyethylene(PE), cross-linked polyethylene (PEX), mechanical jointfibreglass reinforced plastic (FRP-M), FRP epoxyadhesive joint-military (FRP-EM), FRP epoxy adhesivejoint (FRP-E), FRP gasketed joint (FRP-S), andthreaded joint FRP (FRP-T). The temperature andchemical quality of the geothermal fluids, in addition tocost, are the decisive factors in the selection pipemateril. Both metallic and non-metallic piping can beconsidered for geothermal applications. Carbon steel isthe most widely used metallic pipe and has anacceptable service life if properly applied. [4]

Since metallic pipes used in higher temperaturescompared to non-metallic pipes, they are commonlyused in most of the geothermal applications. In addition,its properties and installation requirements are familiarto most installation crews. The advantage of non-metallic materials is that they are virtually impervious tomost chemicals found in geothermal fluids. However,the installation procedures, particularly for fibreglassand polyethylene are, in many cases, outside theexperience of typical labourers and local code officials.[4]

In Turkish GDHSs the most common selection iscarbon steel pipes (welded). Steel pipe is available inalmost all areas and manufactured in wide range of

sizes. Steel is the material most familiar to pipe fittersand installation crews. The joining method for smallsizes (<50 mm) is usually threading, with welding usedfor sizes above this level. For underground installations,all joints are typically welded when unlined piping isused.

The most important disadvantage of steel piping iscorrosion. In many geothermal fluids, there are variousconcentrations of dissolved chemicals or gases that canresult in corrosion. If the potential exists for this type ofattack, or if the fluid has been exposed to the air beforeentering the system, carbon steel should be thematerial of last resort. Steel piping is used primarily onthe clean loop side of the isolation heat exchanger,although in a few cases it has been employed as thegeothermal transmission line material. A distinctdisadvantage in using steel pipe is that the buried pipeis also subject to external corrosion unless protectedwith a suitable wrapping or cathodic protection [4]. Forinstance, the distribution system at Balçova-NarlıdereGDHS consists of carbon steel pipe with polyurethanefoam insulation wrapped with insulating coat (glassfibre) to provide a seal. The water seal degraded withtime (approximately 5 years) and allowed ground waterto contact the pipe. External corrosion resulted in anumber of failures, which cause significant amount ofleakage from the system.

Consideration of non-metallic pipes in Turkishgeothermal projects is rather new. Local manufacturershave just started to compete with steel pipemanufacturers in terms of price. However, non-metallicpipes cannot be used at temperatures higher than 90oCand it is still the main disadvantage. Also cost of non-metallic pipes is still higher than steel pipes.Considering the fast improvement of composite pipes, itis expected that the non-metallic pipes will be started touse more commonly in future geothermal projects.

INSTALLATION Installation and maintenance of pipelines can beinvestigated in 9 steps. [5]

1. Receipt, handling and storage2. Excavation of trench3. Installation of pipes and components4. Welds5. Leak and pressure tests6. Installation of joints7. As-built survey8. Related work9. Backfilling of trench

Typically preinsulated pipes and components aredelivered from the manufacturer’s store or plant by themanufacturer. The owner takes delivery of the goods ata temporary storage area or directly at the pipe trench.

The owner should carry out inspection on arrival of thematerials. An inspection on arrival should include.

• Inspection of bill of quantaties

• Inspection of quality of materials including check fortransport damage.

• Inspection of possible certificates

Any handling, transportation and storage of pipes andfittings, etc. must be done in consideration of theproperties of the different materials and current externalconditions, in order to avoid the components beingsubjected to harmful impacts and in order to avoidimpurities etc. in pipes and fittings. [5]

Figure 2 shows a typical cross-section of the trench withsafety fence between the work space and the traffic atone side, and a marked safety area to the other side ofthe trench.

Figure 2: Section of trench and surrounding speacerequirements [5]

When pipes and fittings are transported to the trenchsite, precautions must be taken to avoid damage of thepipes. At the trench site pipes and components arearranged and prepared for laying and welding. [5]

The contractor must monitor the quality of piping workby establishing and maintaining a quality assurancesystem with reference to EN 729-3 (Qualityrequirements for welding). Welding should preferablyonly take place in dry weather. Where the work isperformed during unfavourable weather conditions thework site must be covered with tents or equivalent. [5]

A complete visual inspection must be carried outthroughout the entire welding process by the contractor,the owner’s supervisors and sometimes by an externalspecialist consultant. [5]

In co-operation with the contractor the supervisorselects the welds for non destructive testing weldinginspection. The welds are divided into inspectionsections so that welds in the same section are unlikelyto have differences in qualty. [5]

The leak test is compulsory and the pressure test isoptional and can be specified according to localauthorities or the requirements of the owner. [5]

The main emphasis is laid on the leak test in order tofind any penetrating pinholes, which will result insubsequent failure. [5]

Welds can be subjected to a leak test by one of thefollowing methods.

• Test with water applied at 1.3 times maximumoperating pressure with simultaneous visualinspection of the welds for leaks.

• Test with air at 0.2 bar over-pressure or 0.65 barbelow atmospheric pressure where the tightness ofthe weld is checked by application of a suitableindicator fluid.

• 100% radiographic inspection of steel service pipewhere the weld seams are made up of a minimumof 2 passes and if start/end positions of the twopasses are mutually displaced.

A leak test with air must be performed before the pipeshave been water-filled, e.g. for flushing or pressure test.

Once buried the only information on the location ofpipes and components are the drawings and listingsbased on a survey of the executed work. [5]

This information is essential when it comes to later re-excavation of the trench in situations such as:

• Evidence of leakage into the pipes• Connection of new pipe sections or branches to the

pipes.• Planning of excavation for other underground utility

networks.

A high level of accuracy of the survey is essential forthe effective later use of the information. Each metre ofinaccurate excavation when accessing the buried pipesis costly and so are the costs of repair of pipes, whichhave been damaged due to incomplete and inaccurateinformation on their exact location.

The trend is to survey the co-ordinates of the pipes andcomponents in a common local co-ordinate system.However, in a transitional period co-ordinates to fixedobjects as mentioned above should also be measuredin order to calculate measurements for finding thepipeline. Within a few years it must be expected thenGPS (Global Positioning System) will be used. [5]

The objects of the pipe systems to be exactly identifiedand located are for example [5]

• Bends• Branches• Joints• Valves• Connections of electronic surveillance system• Other special components• Crossing of other utility cables or pipes

Installation of preinsulated bonded pipes often involveswork of another character than just fitting ofpreinsulated pipes. This could include concrete works,or work on other utilities in the trench. In many casesthe intended functioning and service life of preinsulated

pipes also depend on the execution and quality of therelated work. Related work may include valve chambersand fixpoints, drainage and sewerage work.

The assembled pipe system must be subjected to afinal inspection by the owner’s supervisor and thecontractor prior to backfilling of the pipe trench.

The excavation is to be backfilled in layers. Each layeris to be completely compacted before the next layer islaid. Where mechanical compaction is permitted,backfilling should be performed in layers of max. 300mm which are then compacted. In the case of manualcompaction the height of layers is to be max. 150 mm.[5]

Sand and gravel materials can be added directly intothe excavation using grabbing crane. If they are to betripped, the materials have to be unloaded at the side ofthe excavation. Materials must never be tipped into theexcavation.

CONCLUSIONIn this study higlights of the district heating pipingstandards were given. These procedures are acceptedas minimum standards in European district heatingsystems. Unfortunately, there is no standard pipingapproach in Turkish district heating projects. At somepoints like reception of pipes Turkish Standards arereferred, however especially at the installation levelthere is no standard approach.

In Turkey geothermal district heating projects aremostly financed by municipalities or local governments.In both cases the financial sources are very limited.This case affects the quality of piping work. In additionto the lack of money, geothermal district heatingprojects have limited time schedule, mainly because ofpublic and political pressure.

Experiences show that the vital points especially at theinstallation phase are generally neglected. For instance,

leakage is one of the traditional problems of Turkishgeothermal district heating systems, however, leak testhave never been applied in one of the Turkishgeothermal district heating projects.

Although skipping most of the standard pipingprocedures at the beginning may decrease theinvestment cost of the project, it definetely raises themaintenance cost and decrease the life of the system.

There is no question that the local contractors canprovide sufficient level of piping standards if they arerequired. The project owner should be aware of the factthat quality of piping determines the future of the allproject. Even the smallest deviations from standardsmay result in collapse of the all project. Geothermalproject owners which are generally municipalities andlocal governments should have ready list of conditionsto give to the contractors. These list of conditionsshould be prepared by geothermal specialists andaccepted by the contractor at the beginning of theproject.

REFERENCES1. A.C. Şener, Importance of load based automatic

control in geothermal energy systems, 3rd IFACWorkshop DECOM-TT 2003, 225-231 (2003).

2. K. Rafferty, Piping materials for geothermal districtheating systems, Geo-Heat Centre Publications.(1996).

3. T. Karlsson, Geothermal district heating the Icelandexperience, United Nations University GeothermalTraining Programme Report 1982-4, 11

4. K. D. Rafferty, Geothermal direct use engineeringand design guidebook, Piping, Geo-Heat Center,Klamath Falls, OR 97601, Editor in chief: J. W.Lund, 1998, p.241.

5. European District Heating ManufacturersAssociation, District Heating Handbook, pp. 215-290, 1997.