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Techniques for the installation and rehabilitation of water mains using plastics pipe systems
T e c h n i q u e s for the Installation •and Rehabilitation of Water Mains u s i n g P l a s t i c s Pipe S y s t e m s G.P. Marshall(~), D.L. Thomas (=), D. Pearson (=)
Abstract A large percentage of the U.K. water distribution network is currently in need of rehabilitation. The various techniques using both new.lay and 'no.dig' technology which have been considered for use by North West Water Ltd. are reviewed. A critique of the conventional processes is given with emphasis placed on the operational factors which dictate the selection process. The use of all welded MDPE systems is reviewed and a detailed account given of the development of new methods involving thin walled close.fit PE liners using die drawing techniques.
Introduction The Water Industry in England and Wales has responsibility for an approximate length of 295,000 km 3 of water main much of which is laid using cast iron. Subsequently, asbestos cement, uPVC and cement lined cast ductile iron have been used. It is known that in some areas customers receive a less than satisfactory water service due to low pressure and interruptions to supply. Part of the cause of this is tuberculation and/or a build.up of deposits on the inside of the cast iron mains, together with the problem of leakage through joints with ineffective seals. In many areas of the U.K. these problems are being addressed by many Companies. NWW has been active in surveying the present system and in examining new techniques. North West Water North West Water is one of the largest water companies in England and Wales. It serves a population of 7 million 3 and within its area there is over 38,000 km of water main. As well as the general problems mentioned above, FlWW has the specific added problem of transmiffing soft upland waters. Historically, water treatment has been via single stage plants which were unable to totally remove manganese and iron from the treated waters. Consequently, some quantities have been trans. mitted into the distribution system manifesting itself in dirty water at the customers tap or precipitation on the inside of the distribution system. The tuberculation caused by the aggressive upland waters and precipitation of manganese and iron create certain operational problems:. a) System failure caused by a weakening of the structural strength of the cast iron mains. b) Low pressures caused by excessive frictional losses on the system due to the build up of deposits on the inside of the mains. The deposits also reduce the flow area.
c) Reduced security of supply caused by an inability to re-zone supplies in the event of system failure else(1) Pipeline DevM,~ptmm~ L~., Manchester Polytechnic, John Dalton Building, Cl~asterSt., Manch#shPr, MI 5GD. (2) Nor~ W~t Water Ltd. (3) All statistics in this paper ere Koproximate and given as a guide only
CONSTRUCTION & BUILDING MATERIALS Vol. 4 No. 2 JUNE 1990
where. This is due to the reduction in pipeline diameter preventing transfer of available supplies. d) Dirty water caused by pressure fluctuations or changes in the direction of flow during re-zoning operations. When this occurs the deposits on the inside of the mains are removed and are carried in suspension into supply thereby leading to customer complaints. A further problem which exists in North West Water is that of leakage from the distribution system. It is estimated that approaching 30% of the water put into supply is "Unaccounted For" of which the major proportion is due to leakage. The reasons for this are: a) Pipe failures - largely caused by corrosion. b) The use of run-lead joints on the iron pipe systems laid in the late ]9th/early 20th century. These joints are now no longer watertight and pipe separation is particularly prevalent in areas of mining subsidence. c) Excessve pressures within the system. This has a significant effect on leakage since loss rates increase exponentially with pressure. Unless pressures are reduced, the leakage will remain high. In many cases it is not possible, even if topography permits, to systematically reduce pressures on a planned basis due to the excessive frictional losses and reduction in available carrying capacity of the system caused by the deposits on the inside of the mains. The need to renovate With the problems outlined above, it was obvious within NWW that a considerable investment strategy would have to be developed aimed at investigating the problems, identifying solutions, and the need to improve operational efficiency and customer service. Fundamental to North West Water's plan is the need to obtain sound, economic solutions. Consequently, considerable development has taken place into new pipe materials and several now established renovation techniques aimed at reducing both economic and social costs.
Laying of new pipes Although there have been many problems with uPVC
0950 - 0618/90/020074 - 05 © 1990 Buttetwotth-Heinernann Ltd
73
in the early/mid 1970's these have ceased with the introduction of new standards for demonstration of high fracture toughness. NWW have a policy of extensive use of such tough plastics pipe systems for situations involving new pipe. lines because of their durability and ease of handling in mainlaying.
range of HDPE materials being developed by Soivay with the intention of using such materials for thin-walled (SDR 26) higher pressure mains. Checks are being made to determine the limiting wall thickness which may be used to guarantee total resistance to failure and appropriate testing requirements will be incorporated in specifications for NWW.
Materials policy for PVC
Available techniques for renovation When considering assets in the distribution system many engineers and managers would consider that these refer to the pipes. In North West Water this is not the belief as the pipelines themselves have a limited life expectancy. It is the "hole in the ground" which is the asset and it is believed that economic efforts should be addressed at prolonging its life. This has involved using and developing techniques using minimum excavation although total replacement has, on occasions to be considered. Such 'no.dig' techniques have the advantages of reducing reinstatement (and hence cost) as well as significantly reducing social disruption. The techniques used in North West Water are:
The policy for Distribution sizes (up to 12" dia.) is to use uPVC wherever possible with MDPE being used where it is advantageous to save installation costs. Because thin walled plastics pipes have been demonstrated by in-house testing to have superior resistance to failure in a brittle manner, NWW specify use of 6 bar rating for MDPE and 9 bar for uPVC (a 6 bar size is not available at the moment). NWW have a policy of ongoing development of standards and specifications for plastics systems and currently require demonstration of far higher toughness properties than are required by B.S.3505. All standards are monitored on a continuous basis by contracts with Pipeline Developments at Manchester Polytechnic and testing of pipe being supplied is ongo. ing throughout the year. in the future, it is likely that NWW will be replacing uPVC with a 'modified' PVC (mPVC) which has been engineered to give only ductile fracture properties. These highly toughened systems will be available in blue material and will be used with reduced wall sections to not only enhance ductility but also save on cost.
Policy for MDPE At the present time, the most widely used type of poly. ethylene is the blue B.P. Rigidex 002-50 grade used by most of the U.K. Independent research work now being sponsored by NWW is being undertaken to examine the use of other types of polyethylene for small bore (to 63ram) and large diameter pipes (above 12"). There are frequent complaints from installation crews that the thick walled (SDR 11) MDPE service pipe is too stiff for easy handling on site. Consequently, monitored installation trials and testing (by PDL) is being undertaken on Linear Low Density Polyethylene pipes, currently in widespread use in Scandinavia. The LLDPE grades have the stiffness properties of the old low density PE with higher pressure ratings. Since almost no service pipe requires a 12 bar rating (as for SDR 11MDPE) it is believed that some 'strength' can be sacrificed to gain the advantage of lower stiffness to ease site handling. With the larger diameter pipes, the current WRc policy of derating MDPE pipes for sizes above 2 5 0 mm is found to be prohibitive. The NWW policy is to use MDPE at the standard ratings and to ensure that weld. ing quality is maintained by operation of continous onsite inspection and testing (welds are seen to be the crucial sections which could possibly initiate failure in PE). The on-site testing is backed up by a policy of certification of all welders by NWW via in-house training and assessment. In the future, research work is being undertaken to develop a new specification for the new
74
Replacement (open trench) This is often the most expensive solution both in terms of economic and social costs (ie: disruption to customers and road users). Hence replacement is only used as a last resort, although assuming new pipe is properly laid it offers the answer to resolving the problems identified earlier. With replacement being the "complete" solution, it is not a method of rehabilitation that can be ignored. A considerable amount of work is ongoing with:. i) narrow trench mainlaying (by means of mechanical excavators). ii) Main laying using trenching machines for excavation. iii) Main laying using steerable moles/directional drilling. iv) Moleploughing. All these methods to a lesser or greater degree reduce the permanent reinstatement costs of the highway and improve laying rates. Taken together or in isolation, both these factors reduce the unit costs of main laying and make the laying of new pipes more economically competitive when compared with some of the rehabilitation techniques referred to later.
Cement mortar lining Historically this has been the most frequently used technique, it involves the scraping of the deposits from the inside of the main and then applying a centrifugally spun cement mortar lining. It has been used in the UK for almost half a century and is well proven. It has the advantage of: (1) Minimal excavation (approx. every 100 metres). (2) Low cost - approx. 25.40% cost of replacement. (3) Minimum reduction to the available pipe bore. (4) There are sensibly no limitations on the size of pipe refurbished.
CONSTRUCTION & BUILDING MATERIALS Vol. 4 No. 2 JUNE 1990
But the disadvantages of: (1) No structural integrity - this factor limits its use to structurally sound pipelines. (2) The technique does not stop leaks (cement cracks at joints). (3) High residual PH values (10.11) may subsequently occur in soft waters for several months after lining. (4) A limited life expectancy. (5) Uncertain quality control - particularly with regard to lining thickness (improved now with use of
CCTV). (6) Ferrule blockages - via mortar being deposited in existing side connections. (7) Wash-back at ferrule connections.
Epoxy mortar lining Whilst initially used unsuccessfully some ten years ago, the technique has since been improved and is being used. The method is the same as for cement mortar lin. ing but with the application of an epoxy resin. It has the advantages of: (1) Minimal excavation - every 100 metres. (2) Low Cost - although more expensive than cement mortar. (3) Minimal reduction in available bore - less reduction than cement mortar. (4) Smooth finish - gives improved hydraulic capacity c.f. cement mortar lining. but the disadvantages of: (1) No structural integrity - this limits its use to structurally sound pipelines. (2) Does not stop leaks (does not cover or seal joints). (3) Limited life expectancy - unknown at present but possibly less than 50 years. (4) Need for high degree of quality control - linings are very thin (less than 2ram). (5) Cannot cope with displaced joints - insufficient thickness to "seal over". (6) Wash-back at ferrules.
Soft insertion liners At the present moment the UK Water Industry is carrying out various development studies of soft insertion liners involving the use of thermosetting resin reinforced tubes which are expanded and cured in contact with the original water main. There has been considerable site development of these techniques ("Paltem", "lnsituform", "Phoenix", etc.) within the Gas and Sewer Industries and these should give the Water Industry an estimation of the costs involved. First indications are that the cost per metre will be very high and this may preclude use of such tech. niques to large diameter pipes in heavily congested urban areas. Trials within the UK Water Industry will need to be carried out in the future when approval has been obtained on water quality for the resins involved. Potential advantages are: (1) (2) (3) (4)
Use for large diameter pipes (no size limitation). Ease of insertion in urban areas. Structural/reinforcement capability. Solving of leakage problems.
CONSTRUCTION & BUILDING MATERIALS Vol. 4 NO. 2 JUNE 1990
Disadvantages may include:
(1) Cost. (2) Guarantee of complete cure. (3) Clearance on toxity not yet obtained (may be also affected by degree of cure).
Pipe bursting~size for size This involves the displacement of the existing pipeline by inserting a percussion mole into the pipe. A PVC or thin MDPE sleeve is drawn through the line of the existing pipeline and a welded MDPE pipeline is then drawn through this sleeve. The advantages of the system are: (1) New pipe is structurally sound. (2) All.welded system - which overcomes leakage problems. (3) Cost - cheaper than replacement and competitive with sprayed linings in 3-6 inch range.. (4) Minimal excavation - every 100 metres and at service locations. but the disadvantages are: (1) Size limitations - only used up to 8" diameter. (2) Material limitations in pipes being lined - cannot be used on ductile iron or steel or where "piece-in" repairs have been undertaken. Development work has been undertaken with experienced contractors, aimed at assessing the viability of eliminating the need for the PVC sleeve. Initial trials have been very promising and the development work is continuing. If successful, the elimination of the sleeve will reduce unit costs for both labour and materials and may also allow the techniques to be used for replace. merit with larger diameter pipe.
Slip lining wit MDPE This is a technique that has been used to replace pipes which are structurally unsound and involves the scrap. ing of the deposits from the main and inserting a weld. ed MDPZ pipe into the old main. It has the advantages of: (1) Minimal excavation (although greater than sprayed linings due to need for a draw pit). However in favourable circumstances long lengths can be inserted. (up to 600m) (2) Provision of a structural lining which also overcomes leakage problem. (3) Cost-potentially cheaper than replacement. (4) Life expectancy greater than sprayed linings and expected to be much greater than 100 years. but the disadvantages of: (1) Reduction in hydraulic capacity of main - an annulus is needed to allow for insertion. (2) Some size limitations - only economically viable on larger diameter mains (10" and above) and those with few service connections. It can be ascertained from the above considerations that all the available techniques have their uses, advantages and disadvantages. Where engineering and finan-
75
cial assessments dictate, they will continue to be used. However, in general terms, sprayed linings have the advantage of lower costs and can be used over the full range of pipeline diameters. The major disadvantages are that they are non-structural and do not overcome the leakage problems. This is totally the opposite when considering MDPE linings. in view of the operational limitations imposed by most of these systems, NWW have undertaken trials and development of a technique using MDPE which potentially offers a complete pipeline renovation system for all sizes of main. This involves the use of pipe reduction and re-expansion techniques for both pressure MDPE pipelines and thin wall MDPE (with the advantages of lower costs) to form a structural composite pipeline with the existing main.
Die drawing of PE as a technique for relining Alhtough loose.fitting MDPE liners have been used to re.line high pressure oil pipelines in the USA and Canada for many years, these liners have been inserted via conventional slip lining and have relied on the service pressure to expand the PE into contact with the host pipe. For the rehabilitation of potable water and gas mains it is undesirable to have any gap between liner and host pipe - especially where many service connections are involved. Therefore, research has concentrated on the development of techniques which ensure complete contact of pipe and liner before application of service pressure. The impetus in development has initially been due to Bdtish Gas who have utJlised techniques involving HD and MDPE liners which have initial outside diameters greater than the bores of the pipes in need of rehabilitation. The PE pipes are pre-welded into long lengths and then are reduced in diameter by secondary processes. These involve either heating the PE liner followed by drawing through a hot die ("Swagelining"), or pass. ing the PE liner between specially profiled capstan rollers ("Rolidown"). With both systems, the PE liner maintains the redued diameter and may be inserted via a conventional slip lining process. With "Swagelining" the liner expands by natural reversion until it is in close contact with the host pipe. With "Roildown", internal pressurisation is used to effect expansion. Although there are other techniques for reducing the diameter of continuously welded MDPE pipe strings to allow for subsequent re.sizing following insertion into the host main, the swaging or die drawing process has the major advantage that it can be used on all sizes of pipes and was developed as a technique for rehabilitating gas mains by British Gas (North Western). Within the NWW region there are lengths of large diameter ( > 30") cast iron pipes which have severe joint problems and some unprotected ductile iron mains are suffering from external corrosion. Thus, it has been considered that it would be advantageous to have a new lining system developed which could be used without any limitation on size. Following a joint project with British Gas using ] 000ram MDPE pipe which was successfully reduced in size during a trial exercise, NWW have embarked on a series of laboratory and site installation trials to evaluate the potential of die drawing as a "universal" lining
76
technique. A stress analysis of the processes of reducing MDPE pipe by drawing through a conical die, has been made and this shows that the forces involved during the drawing are controlled by the die geometry, the coefficient of friction and the yield stress of the pipe. By suitable design of equipment, use of lubricants and choice of material, the frictional forces can be reduced to acceptable levels for site installation. In the case of the established British Gas process, the use of heating prior to drawing reduces the yield stress of the MDPE to allow for the reduction in applied force. The present development work has concentrated on utilisation of the viscoelastic recovery characteristics of MDPE which allow the drawn.down pipe to recover the imposed deformation without resort to internal pressurisation. It has been found from extensive laboratory and field trials that use of carefully profiled dies and lubricants which do not attack PE can allow liners to be drawn to a reduced diameter without the use of external heating. By maintaining tension on the liner following die drawing, sufficient reduction in diameter can be maintained to allow for insertion. After the re.lining has been effected, release of the applied tension causes the liner to contract in length and expand in diameter. Part of the recovery in size is elastic in nature and as such o o curs instantaneously. A secondary viscoelastic recovery component then proceeds over a period up to 24 hours following a release of the applied load. Unless the ini. tial reduction is very small, some permanent deformation is retained. By careful adjustment of the draw.down ratio, the pipe may be reduced sufficiently in size to allow for a large clearance for slip lining and the time for re.sizing into intimate contact may also be varied to suit site con. ditions. It is clearly undesirable to have instantaneous recovery since this would produce problems if there was a winch failure during insertion. There would be no time to allow the liner to be removed and allow a re-start. It is conversely unacceptable to have a very long recov. ery time as this increases the overall contract time and increases costs. Careful experimentation has shown that recovery may be controlled within close limits to give both adequate lining clearance with acceptable re-sizing times. Much of the development work has been carried out to investigate the effects of changing the die configura. tion and draw ratio to allow conventional sizes of iron main to be relined using standard PE pipe sizes and to develop a system which is not sensitive to substantial variations in pipe l.D's following scraping to remove tuberculations and other deposits. Test data from both full scale trials and site contracts have been obtained to allow for the prediction of Running Reductions; Permanent Deformation; Length Increases: Dimensional Recovery Rates and Pulling Forces all as functions of Reduction Ratios. The laws governing all these parameters have been determined and are now being used not only to predict behaviour during lining operations within NWW but also to model the behaviour analytically. It is hoped that the modelling of pipe performance during die drawing via -
CONSTRUCTION & BUILDING MATERIALS Vol. 4 No. 2 JUNE 1990
input of properties derived from laboratory tests will allow for the prediction of behaviour of other polymer systems which may prove to have benefit and also predict behaviour as a function of ambient temperature to cover different conditions world.wide. All tests caried out to date on PE sizes up to 500mm have been entirely successful and the analytical model which has been derived, has proved successful in predicting performance with acceptable accuracy. The site trials hve also been successful and the MDPE has been found to recover to an interference fit with the host pipe within a few hours (typically 1_-2 hours). Although there is a natural tendency for water engineers to regard this new technique as a potential soiution for the renovation of old pipes which are in need of structural support to enhance their service lifetime, it is believed that cold drawing of PE pipe could even. tually be most successfully developed to provide an al. ternative to cement and epoxy spray technique using thin wall liners - for all pipe sizes. As noted previously, the spray techniques have the major disadvantage that they do not solve problems with leakage and have a potentially limited life. "Swagelin. ing" was developed by British Gas to solve potential leak-
age problems and is currently being used with success in this role. Also, with the intensive R and D of the blue MDPE materials for pressure applications, there is a large database which gives assurance that for potable water applications, there is a life expectancy for MDPE which is beyond that for any conventional lining system. By employing novel methods of installation and reducing the time spent during insertion to a level competitive with that involved with centrifugal spraying, it is believed that costs for relining by cold drawing will be reduced to a level such as to make the technique the favoured method for mains renovation - for both structural and non-structural applications.
Conclusion NWW believe that by the judicious choice of renovation technique and pipe materials policy, it is able to ensure cost effective solutions to improve the pipeline network within the region. The overriding objective is to improve the service to customers by technical innovation and better use of nodig techniques which can give superior solutions at less direct and social costs.
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CONSTRUCTION& BUILDINGMATERIALSVol. 4 No. 2 JUNE 1990
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Abstract A large percentage of the U.K. water distribution network is currently in need of rehabilitation. The various techniques using both new.lay and 'no.dig' technology which have been considered for use by North West Water Ltd. are reviewed. A critique of the conventional processes is given with emphasis placed on the operational factors which dictate the selection process. The use of all welded MDPE systems is reviewed and a detailed account given of the development of new methods involving thin walled close.fit PE liners using die drawing techniques.
Introduction The Water Industry in England and Wales has responsibility for an approximate length of 295,000 km 3 of water main much of which is laid using cast iron. Subsequently, asbestos cement, uPVC and cement lined cast ductile iron have been used. It is known that in some areas customers receive a less than satisfactory water service due to low pressure and interruptions to supply. Part of the cause of this is tuberculation and/or a build.up of deposits on the inside of the cast iron mains, together with the problem of leakage through joints with ineffective seals. In many areas of the U.K. these problems are being addressed by many Companies. NWW has been active in surveying the present system and in examining new techniques. North West Water North West Water is one of the largest water companies in England and Wales. It serves a population of 7 million 3 and within its area there is over 38,000 km of water main. As well as the general problems mentioned above, FlWW has the specific added problem of transmiffing soft upland waters. Historically, water treatment has been via single stage plants which were unable to totally remove manganese and iron from the treated waters. Consequently, some quantities have been trans. mitted into the distribution system manifesting itself in dirty water at the customers tap or precipitation on the inside of the distribution system. The tuberculation caused by the aggressive upland waters and precipitation of manganese and iron create certain operational problems:. a) System failure caused by a weakening of the structural strength of the cast iron mains. b) Low pressures caused by excessive frictional losses on the system due to the build up of deposits on the inside of the mains. The deposits also reduce the flow area.
c) Reduced security of supply caused by an inability to re-zone supplies in the event of system failure else(1) Pipeline DevM,~ptmm~ L~., Manchester Polytechnic, John Dalton Building, Cl~asterSt., Manch#shPr, MI 5GD. (2) Nor~ W~t Water Ltd. (3) All statistics in this paper ere Koproximate and given as a guide only
CONSTRUCTION & BUILDING MATERIALS Vol. 4 No. 2 JUNE 1990
where. This is due to the reduction in pipeline diameter preventing transfer of available supplies. d) Dirty water caused by pressure fluctuations or changes in the direction of flow during re-zoning operations. When this occurs the deposits on the inside of the mains are removed and are carried in suspension into supply thereby leading to customer complaints. A further problem which exists in North West Water is that of leakage from the distribution system. It is estimated that approaching 30% of the water put into supply is "Unaccounted For" of which the major proportion is due to leakage. The reasons for this are: a) Pipe failures - largely caused by corrosion. b) The use of run-lead joints on the iron pipe systems laid in the late ]9th/early 20th century. These joints are now no longer watertight and pipe separation is particularly prevalent in areas of mining subsidence. c) Excessve pressures within the system. This has a significant effect on leakage since loss rates increase exponentially with pressure. Unless pressures are reduced, the leakage will remain high. In many cases it is not possible, even if topography permits, to systematically reduce pressures on a planned basis due to the excessive frictional losses and reduction in available carrying capacity of the system caused by the deposits on the inside of the mains. The need to renovate With the problems outlined above, it was obvious within NWW that a considerable investment strategy would have to be developed aimed at investigating the problems, identifying solutions, and the need to improve operational efficiency and customer service. Fundamental to North West Water's plan is the need to obtain sound, economic solutions. Consequently, considerable development has taken place into new pipe materials and several now established renovation techniques aimed at reducing both economic and social costs.
Laying of new pipes Although there have been many problems with uPVC
0950 - 0618/90/020074 - 05 © 1990 Buttetwotth-Heinernann Ltd
73
in the early/mid 1970's these have ceased with the introduction of new standards for demonstration of high fracture toughness. NWW have a policy of extensive use of such tough plastics pipe systems for situations involving new pipe. lines because of their durability and ease of handling in mainlaying.
range of HDPE materials being developed by Soivay with the intention of using such materials for thin-walled (SDR 26) higher pressure mains. Checks are being made to determine the limiting wall thickness which may be used to guarantee total resistance to failure and appropriate testing requirements will be incorporated in specifications for NWW.
Materials policy for PVC
Available techniques for renovation When considering assets in the distribution system many engineers and managers would consider that these refer to the pipes. In North West Water this is not the belief as the pipelines themselves have a limited life expectancy. It is the "hole in the ground" which is the asset and it is believed that economic efforts should be addressed at prolonging its life. This has involved using and developing techniques using minimum excavation although total replacement has, on occasions to be considered. Such 'no.dig' techniques have the advantages of reducing reinstatement (and hence cost) as well as significantly reducing social disruption. The techniques used in North West Water are:
The policy for Distribution sizes (up to 12" dia.) is to use uPVC wherever possible with MDPE being used where it is advantageous to save installation costs. Because thin walled plastics pipes have been demonstrated by in-house testing to have superior resistance to failure in a brittle manner, NWW specify use of 6 bar rating for MDPE and 9 bar for uPVC (a 6 bar size is not available at the moment). NWW have a policy of ongoing development of standards and specifications for plastics systems and currently require demonstration of far higher toughness properties than are required by B.S.3505. All standards are monitored on a continuous basis by contracts with Pipeline Developments at Manchester Polytechnic and testing of pipe being supplied is ongo. ing throughout the year. in the future, it is likely that NWW will be replacing uPVC with a 'modified' PVC (mPVC) which has been engineered to give only ductile fracture properties. These highly toughened systems will be available in blue material and will be used with reduced wall sections to not only enhance ductility but also save on cost.
Policy for MDPE At the present time, the most widely used type of poly. ethylene is the blue B.P. Rigidex 002-50 grade used by most of the U.K. Independent research work now being sponsored by NWW is being undertaken to examine the use of other types of polyethylene for small bore (to 63ram) and large diameter pipes (above 12"). There are frequent complaints from installation crews that the thick walled (SDR 11) MDPE service pipe is too stiff for easy handling on site. Consequently, monitored installation trials and testing (by PDL) is being undertaken on Linear Low Density Polyethylene pipes, currently in widespread use in Scandinavia. The LLDPE grades have the stiffness properties of the old low density PE with higher pressure ratings. Since almost no service pipe requires a 12 bar rating (as for SDR 11MDPE) it is believed that some 'strength' can be sacrificed to gain the advantage of lower stiffness to ease site handling. With the larger diameter pipes, the current WRc policy of derating MDPE pipes for sizes above 2 5 0 mm is found to be prohibitive. The NWW policy is to use MDPE at the standard ratings and to ensure that weld. ing quality is maintained by operation of continous onsite inspection and testing (welds are seen to be the crucial sections which could possibly initiate failure in PE). The on-site testing is backed up by a policy of certification of all welders by NWW via in-house training and assessment. In the future, research work is being undertaken to develop a new specification for the new
74
Replacement (open trench) This is often the most expensive solution both in terms of economic and social costs (ie: disruption to customers and road users). Hence replacement is only used as a last resort, although assuming new pipe is properly laid it offers the answer to resolving the problems identified earlier. With replacement being the "complete" solution, it is not a method of rehabilitation that can be ignored. A considerable amount of work is ongoing with:. i) narrow trench mainlaying (by means of mechanical excavators). ii) Main laying using trenching machines for excavation. iii) Main laying using steerable moles/directional drilling. iv) Moleploughing. All these methods to a lesser or greater degree reduce the permanent reinstatement costs of the highway and improve laying rates. Taken together or in isolation, both these factors reduce the unit costs of main laying and make the laying of new pipes more economically competitive when compared with some of the rehabilitation techniques referred to later.
Cement mortar lining Historically this has been the most frequently used technique, it involves the scraping of the deposits from the inside of the main and then applying a centrifugally spun cement mortar lining. It has been used in the UK for almost half a century and is well proven. It has the advantage of: (1) Minimal excavation (approx. every 100 metres). (2) Low cost - approx. 25.40% cost of replacement. (3) Minimum reduction to the available pipe bore. (4) There are sensibly no limitations on the size of pipe refurbished.
CONSTRUCTION & BUILDING MATERIALS Vol. 4 No. 2 JUNE 1990
But the disadvantages of: (1) No structural integrity - this factor limits its use to structurally sound pipelines. (2) The technique does not stop leaks (cement cracks at joints). (3) High residual PH values (10.11) may subsequently occur in soft waters for several months after lining. (4) A limited life expectancy. (5) Uncertain quality control - particularly with regard to lining thickness (improved now with use of
CCTV). (6) Ferrule blockages - via mortar being deposited in existing side connections. (7) Wash-back at ferrule connections.
Epoxy mortar lining Whilst initially used unsuccessfully some ten years ago, the technique has since been improved and is being used. The method is the same as for cement mortar lin. ing but with the application of an epoxy resin. It has the advantages of: (1) Minimal excavation - every 100 metres. (2) Low Cost - although more expensive than cement mortar. (3) Minimal reduction in available bore - less reduction than cement mortar. (4) Smooth finish - gives improved hydraulic capacity c.f. cement mortar lining. but the disadvantages of: (1) No structural integrity - this limits its use to structurally sound pipelines. (2) Does not stop leaks (does not cover or seal joints). (3) Limited life expectancy - unknown at present but possibly less than 50 years. (4) Need for high degree of quality control - linings are very thin (less than 2ram). (5) Cannot cope with displaced joints - insufficient thickness to "seal over". (6) Wash-back at ferrules.
Soft insertion liners At the present moment the UK Water Industry is carrying out various development studies of soft insertion liners involving the use of thermosetting resin reinforced tubes which are expanded and cured in contact with the original water main. There has been considerable site development of these techniques ("Paltem", "lnsituform", "Phoenix", etc.) within the Gas and Sewer Industries and these should give the Water Industry an estimation of the costs involved. First indications are that the cost per metre will be very high and this may preclude use of such tech. niques to large diameter pipes in heavily congested urban areas. Trials within the UK Water Industry will need to be carried out in the future when approval has been obtained on water quality for the resins involved. Potential advantages are: (1) (2) (3) (4)
Use for large diameter pipes (no size limitation). Ease of insertion in urban areas. Structural/reinforcement capability. Solving of leakage problems.
CONSTRUCTION & BUILDING MATERIALS Vol. 4 NO. 2 JUNE 1990
Disadvantages may include:
(1) Cost. (2) Guarantee of complete cure. (3) Clearance on toxity not yet obtained (may be also affected by degree of cure).
Pipe bursting~size for size This involves the displacement of the existing pipeline by inserting a percussion mole into the pipe. A PVC or thin MDPE sleeve is drawn through the line of the existing pipeline and a welded MDPE pipeline is then drawn through this sleeve. The advantages of the system are: (1) New pipe is structurally sound. (2) All.welded system - which overcomes leakage problems. (3) Cost - cheaper than replacement and competitive with sprayed linings in 3-6 inch range.. (4) Minimal excavation - every 100 metres and at service locations. but the disadvantages are: (1) Size limitations - only used up to 8" diameter. (2) Material limitations in pipes being lined - cannot be used on ductile iron or steel or where "piece-in" repairs have been undertaken. Development work has been undertaken with experienced contractors, aimed at assessing the viability of eliminating the need for the PVC sleeve. Initial trials have been very promising and the development work is continuing. If successful, the elimination of the sleeve will reduce unit costs for both labour and materials and may also allow the techniques to be used for replace. merit with larger diameter pipe.
Slip lining wit MDPE This is a technique that has been used to replace pipes which are structurally unsound and involves the scrap. ing of the deposits from the main and inserting a weld. ed MDPZ pipe into the old main. It has the advantages of: (1) Minimal excavation (although greater than sprayed linings due to need for a draw pit). However in favourable circumstances long lengths can be inserted. (up to 600m) (2) Provision of a structural lining which also overcomes leakage problem. (3) Cost-potentially cheaper than replacement. (4) Life expectancy greater than sprayed linings and expected to be much greater than 100 years. but the disadvantages of: (1) Reduction in hydraulic capacity of main - an annulus is needed to allow for insertion. (2) Some size limitations - only economically viable on larger diameter mains (10" and above) and those with few service connections. It can be ascertained from the above considerations that all the available techniques have their uses, advantages and disadvantages. Where engineering and finan-
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cial assessments dictate, they will continue to be used. However, in general terms, sprayed linings have the advantage of lower costs and can be used over the full range of pipeline diameters. The major disadvantages are that they are non-structural and do not overcome the leakage problems. This is totally the opposite when considering MDPE linings. in view of the operational limitations imposed by most of these systems, NWW have undertaken trials and development of a technique using MDPE which potentially offers a complete pipeline renovation system for all sizes of main. This involves the use of pipe reduction and re-expansion techniques for both pressure MDPE pipelines and thin wall MDPE (with the advantages of lower costs) to form a structural composite pipeline with the existing main.
Die drawing of PE as a technique for relining Alhtough loose.fitting MDPE liners have been used to re.line high pressure oil pipelines in the USA and Canada for many years, these liners have been inserted via conventional slip lining and have relied on the service pressure to expand the PE into contact with the host pipe. For the rehabilitation of potable water and gas mains it is undesirable to have any gap between liner and host pipe - especially where many service connections are involved. Therefore, research has concentrated on the development of techniques which ensure complete contact of pipe and liner before application of service pressure. The impetus in development has initially been due to Bdtish Gas who have utJlised techniques involving HD and MDPE liners which have initial outside diameters greater than the bores of the pipes in need of rehabilitation. The PE pipes are pre-welded into long lengths and then are reduced in diameter by secondary processes. These involve either heating the PE liner followed by drawing through a hot die ("Swagelining"), or pass. ing the PE liner between specially profiled capstan rollers ("Rolidown"). With both systems, the PE liner maintains the redued diameter and may be inserted via a conventional slip lining process. With "Swagelining" the liner expands by natural reversion until it is in close contact with the host pipe. With "Roildown", internal pressurisation is used to effect expansion. Although there are other techniques for reducing the diameter of continuously welded MDPE pipe strings to allow for subsequent re.sizing following insertion into the host main, the swaging or die drawing process has the major advantage that it can be used on all sizes of pipes and was developed as a technique for rehabilitating gas mains by British Gas (North Western). Within the NWW region there are lengths of large diameter ( > 30") cast iron pipes which have severe joint problems and some unprotected ductile iron mains are suffering from external corrosion. Thus, it has been considered that it would be advantageous to have a new lining system developed which could be used without any limitation on size. Following a joint project with British Gas using ] 000ram MDPE pipe which was successfully reduced in size during a trial exercise, NWW have embarked on a series of laboratory and site installation trials to evaluate the potential of die drawing as a "universal" lining
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technique. A stress analysis of the processes of reducing MDPE pipe by drawing through a conical die, has been made and this shows that the forces involved during the drawing are controlled by the die geometry, the coefficient of friction and the yield stress of the pipe. By suitable design of equipment, use of lubricants and choice of material, the frictional forces can be reduced to acceptable levels for site installation. In the case of the established British Gas process, the use of heating prior to drawing reduces the yield stress of the MDPE to allow for the reduction in applied force. The present development work has concentrated on utilisation of the viscoelastic recovery characteristics of MDPE which allow the drawn.down pipe to recover the imposed deformation without resort to internal pressurisation. It has been found from extensive laboratory and field trials that use of carefully profiled dies and lubricants which do not attack PE can allow liners to be drawn to a reduced diameter without the use of external heating. By maintaining tension on the liner following die drawing, sufficient reduction in diameter can be maintained to allow for insertion. After the re.lining has been effected, release of the applied tension causes the liner to contract in length and expand in diameter. Part of the recovery in size is elastic in nature and as such o o curs instantaneously. A secondary viscoelastic recovery component then proceeds over a period up to 24 hours following a release of the applied load. Unless the ini. tial reduction is very small, some permanent deformation is retained. By careful adjustment of the draw.down ratio, the pipe may be reduced sufficiently in size to allow for a large clearance for slip lining and the time for re.sizing into intimate contact may also be varied to suit site con. ditions. It is clearly undesirable to have instantaneous recovery since this would produce problems if there was a winch failure during insertion. There would be no time to allow the liner to be removed and allow a re-start. It is conversely unacceptable to have a very long recov. ery time as this increases the overall contract time and increases costs. Careful experimentation has shown that recovery may be controlled within close limits to give both adequate lining clearance with acceptable re-sizing times. Much of the development work has been carried out to investigate the effects of changing the die configura. tion and draw ratio to allow conventional sizes of iron main to be relined using standard PE pipe sizes and to develop a system which is not sensitive to substantial variations in pipe l.D's following scraping to remove tuberculations and other deposits. Test data from both full scale trials and site contracts have been obtained to allow for the prediction of Running Reductions; Permanent Deformation; Length Increases: Dimensional Recovery Rates and Pulling Forces all as functions of Reduction Ratios. The laws governing all these parameters have been determined and are now being used not only to predict behaviour during lining operations within NWW but also to model the behaviour analytically. It is hoped that the modelling of pipe performance during die drawing via -
CONSTRUCTION & BUILDING MATERIALS Vol. 4 No. 2 JUNE 1990
input of properties derived from laboratory tests will allow for the prediction of behaviour of other polymer systems which may prove to have benefit and also predict behaviour as a function of ambient temperature to cover different conditions world.wide. All tests caried out to date on PE sizes up to 500mm have been entirely successful and the analytical model which has been derived, has proved successful in predicting performance with acceptable accuracy. The site trials hve also been successful and the MDPE has been found to recover to an interference fit with the host pipe within a few hours (typically 1_-2 hours). Although there is a natural tendency for water engineers to regard this new technique as a potential soiution for the renovation of old pipes which are in need of structural support to enhance their service lifetime, it is believed that cold drawing of PE pipe could even. tually be most successfully developed to provide an al. ternative to cement and epoxy spray technique using thin wall liners - for all pipe sizes. As noted previously, the spray techniques have the major disadvantage that they do not solve problems with leakage and have a potentially limited life. "Swagelin. ing" was developed by British Gas to solve potential leak-
age problems and is currently being used with success in this role. Also, with the intensive R and D of the blue MDPE materials for pressure applications, there is a large database which gives assurance that for potable water applications, there is a life expectancy for MDPE which is beyond that for any conventional lining system. By employing novel methods of installation and reducing the time spent during insertion to a level competitive with that involved with centrifugal spraying, it is believed that costs for relining by cold drawing will be reduced to a level such as to make the technique the favoured method for mains renovation - for both structural and non-structural applications.
Conclusion NWW believe that by the judicious choice of renovation technique and pipe materials policy, it is able to ensure cost effective solutions to improve the pipeline network within the region. The overriding objective is to improve the service to customers by technical innovation and better use of nodig techniques which can give superior solutions at less direct and social costs.
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CONSTRUCTION& BUILDINGMATERIALSVol. 4 No. 2 JUNE 1990
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