Articles & Case Studies

End-load Resistant Material Transitions in Pipe Systems

Posted: Wednesday 4th October 2006

Requirements and Practical Experience

Synopsis

This article deals with the problems that typically occur in material transitions used in pressure pipe systems and shows various approaches to handle them. Different types of end-load resistant material transitions are described briefly and it is derived from practical experience how they comply with the requirements. Thus it appears that only a few of the products available on the market can meet these requirements. Only products that combine the highest degree of all-purpose usage, user-friendly fitting and operational safety provide a quick and cost-efficient solution for the problems connected with material transitions.

Material Transitions in Pressure Pipe Systems

Material transitions gain more and more importance in modern pipeline construction. The reasons are incrementally deployed new pipe materials on one hand; on the other an increase of repairs needed for existing pipelines at the expense of laying new ones. The integration in existing networks also implies the realisation of material transitions in most cases.

Besides the classic solution of using flange connections, there has been a wide range of couplings available on the market for several years, which can be deployed as end-load-bearing material transitions in the shape of pipe couplings. This article tackles the requirements for such couplings and points out the advantages and disadvantages that may arise from the common practice of laying pipes.

Since end-load resistance his rarely required in sewage pipe systems, this article solely expands on the implementation of pressure pipelines in the supply of gas and water.

The Pipeline Network

Historically, the pipeline network in Germany and most other European countries predominantly consists of materials that have been complemented or replaced throughout the years by stretches and new insertions made of modern materials. One frequently comes across networks in which pipelines were originally made of grey cast iron, then upgraded by ductile iron pipes and eventually complemented by pipes made of steel, asbestos cement, PVC and polyethylene.

Maintenance and restoration of such a mixed material pipeline network proves to be problematic, as transitions between the individual materials frequently occur. In case of pipe burst or integrating and replacing stretches of pipes, it is essential to consider materials suitable for the transition. The biggest problem here often is the bridging of different outside diameters of the old and new pipe (figure 1).

Why end-load resistant material transitions?

In many fitting cases the connector does not have to be end-load resistant. There are however several reasons that make the deployment of end-load resistant couplings reasonable:

· unknown course of the pipeline

· transition to plastic pipes with substantial length variation exposed to temperature differences

· warranty problems in case of outsourcing

· no thrust blocks necessary (figure 2)

· cost efficiency through standardisation of materials

Types of material transitions

There is a wide range of end-load resistant materials available on the market. They can be distinguished by several criteria:

transitions for specialized pipe types Ű all-purpose transitions

end-load-bearing system installed on site Ű integrated end-load resistant system

gripping system integrated in gasket Ű gripping system functionally separated

Thus, there are various solutions, according to the usage; guidelines for the choice of the suitable product can be derived from the practice of construction sites as follows:

· all-purpose usability

· easy handling

· high operational safety

· reliable long-term performance

The following points need further explanation:

All-purpose usability encompasses all those couplings which can comply with more than only one specialised material transition. Ideally, the outside diameter range of the coupling has been chosen to fit all the outside diameters within one nominal size (table 1). It also implies that the couplings can be used for gas as well as for water without awkward changes of the gasket and also that the system which guarantees end-load resistance performs as safely on metal pipes as on plastic pipes.

Insert: Table 1
Problematic: outside pipe diameter

In many cases the outside diameter of a pipeline can only be determined by uncovering, due to incomplete maps or unstandardised pipelines. Steel pipes frequentlyused to be manufactured in company-specific dimensions. Many nominal sizes which were used withgrey cast iron and steel pipes in the old days (e.g. DN60, DN70 or DN90) are not customary any more and have been replaced by preferential nominal sizes. Then again, grey cast iron pipes are not dimensionally stable up to two metres in front of the fitting, but conically tapered. This makes it difficult or even impossible to mount screwed union fittings.

These difficulties show that a network provider is well advised to always have suitable connecting and mending products at hand. These couplings should not only be distinguished by the correct outside diameter range, but also be easy to use in order to make the user's life easier and not to burden him with additional tasks. This can be achieved by the following attributes of the component:

· As little preparation for the installation as possible, i.e.:

- no chamfering or bevelling of pipe ends

- no exchange of gaskets

- no choice of specialized systems which guarantee end-load resistance

· Installation with standard tools

· Clear testing criteria for the correct installation

The points mentioned above are fulfilled by only few end-load resistant material transitions available on the market. Many products have to be fitted with various gaskets or gripping systems by the user on site due to the materials that are to be connected; this is unreasonable under the predominantly rough conditions down in the trench and constitutes a great source of errors. This is where products with an integrated gripping system display their strengths, as they can be installed without further preparation.

When installing the couplings, it is a crucial advantage if they can be fitted with the most basic tools such as open-end wrenches or box wrenches. Screwed union technique however is not up-to-date any longer due to immense space requirements as well as due to the forces emerging during the installation. Especially in cases of old grey cast iron pipelines fitted with lead joint sockets there is the risk that the lead joint sockets of the neighbouring sleeves fail due to the impact of forces caused by the heavy tools necessary for the installation of screwed union fittings. (figure 3a/b).

Just as important as an easy installation is the possibility to determine the final state of the installation and to check it. Using connectors with standardised bolts makes it possible to tighten them as much as necessary by using a torque wrench or to check the already tightened connections according to the required torque respectively. This is a priceless advantage for the user, the client and the producer of the connector, since the correct pretension of the bolted connection ensures impeccable operation as well as a corresponding life span of the connection. Ideally the required torques are adjusted to the dimensions of the bolts and independent of the materials present due to operational safety.

Bolts in the ground?-– A never-ending discussion (in Germany…)

Few topics are being discussed as widely in pipeline construction in Germany as bolts in the ground. Each pipe network provider has certainly gained his own experience with corroded bolts. It is known that there are in fact individual cases of soil in which all kinds of metal components corrode within very short time, which results in massive damages. Nowadays corrosion can be eliminated by deploying austenitic materials for bolts, shims and nuts. Then again, there is the problem that nuts and bolts may seize, especially under building site conditions. A package of measures that conforms with up-to-date technology can work as a corrective:

· Bolt threads rolled without cutting action

· Dry lubricant coating of bolts

· Different materials of bolts and nuts (e.g. 1.4301 und 1.4501)

· Slide coating of nuts, e.g. with aluminium alloy

After application of these measures, seizing of the bolts can only be effected by inappropriate application of force.

Once the end-load resistant material transition is installed, it is supposed to guarantee a high degree of operational safety. The connection must be leak-proof on the long run and be able to absorb tractive forces that may crop up without damage.

Excursion: End-load-resistance vs. Pull-out-resistance

In the literature, couplings which connect pipes to form a pipeline operating under working pressure without externally applied locking devices (e.g. restrainers, concrete thrust blocks) are called end-load or pull-out resistant connectors. These terms appear to be slightly ambiguous and are often used synonymously or even confused. The body of technical rules and the standards distinguish two cases:

End-load resistantconnections are those described in DIN 3387-1 [1] for metal pipes and those described in VP 600 [2] for polyethylene pipes. The capability of the connector to bear longitudinal forces is constituted in both cases by applying a defined, axially directed test force.

The term pull-out resistant is clearly defined in VP 600 [2]. A pull-out resistant connection thus guarantees to be at least as strong as the polyethylene pipes connected with it. Therefore the expression "pull-out resistant" is only defined for polyethylene pipe systems.

It can be noted as an easy rule:

End-load resistant = connector collapses before pipe

Pull-out resistant = pipe collapses before connector

Up to now there is no basis for testing in the standards and technical rules, which would define the requirements for both metal and plastic pipes concerning end-load resistantconnectors. Therefore, end-load resistant material transitions suitable for metal and plastic pipes can currently not gain approval. Only components which get into direct contact with the pipeline material, for instance the gasket material and the coating, can and have to be tested and approved according to the effective standards.

Thus, in practice only the producer's specification for the maximum internal pressure of the material transition is relevant. It is easy to decide for or against the use of a product according to the required nominal pressure and the specified data.

As pipeline systems constitute investments, which have to comply with high standards in terms of operational safety and serviceable life, only those products which display a safe long-term behaviour should come into operation. This applies especially to end-load resistant material transitions, which have to adjust the various imbalances of the connected pipelines. Apart from ensuring the necessary pre-stress of the gaskets by observing the required tightening torque as already mentioned, this predominantly applies to two points:

· Retaining of different loads

· Long-term operation of the gripping system

The loads which affect the connections and are caused by the pipeline itself, the surrounding earth or other environmental factors (e.g. traffic loads) have to be compensated by the connecting element. This means that beside possibly accruing axial tractive or shearing forces, axial misalignment or bending of the pipeline also have to be absorbed to a certain degree.

Modern products permit up to 4° of angular deflection on first sight per pipe end; yet, this specification usually applies to the actual state at the point of connection, i.e. this bending has to exist before the actual installation of the connector. Only in individual cases there are material transitions that can also absorb a bending of the pipe ends after they have started operating under working pressure. Yet this case strictly is the norm, as a bending is effected at the connection point by a subsidence of the pipeline after the filling and compaction of the pipe trench. This effect gets even aggravated by traffic loads, which account for the biggest part of damages of older pipelines.

An intelligent construction of the system that guarantees tensile strength is necessary for the absorption of bending as well as for the safe and enduring transmission of tractive forces.

Focus on the Gripping System

There is a multitude of end-load resistant material transitions available on the market. Two systems of all-purpose connectors for metal and plastic pipes can be distinguished:

· Power transmission by a metal element

· Power transmission by a mineral-coated element (figure 4)

Here it is crucial which material comes into operation in the connection. As is generally known, metal and plastic pipes differ considerably as surface hardness is considered. Since the pipe surface constitutes the contact area between connector and pipe, the realisation of the gripping element should be carried out with the utmost attention.

A fundamentally necessary quality of an end-load resistant material transition is the functional independence of the gasket and the gripping system. Only this independence ensures that the gripping system, can operate faultlessly even if the outside diameters of the pipes vary considerably. If the grippers are for instance vulcanised into the rubber of the gasket, the application area of the connector is restricted to a certain external pipe diameter.

Back to pipe surfaces: as mentioned above, there is a great variety in surface hardness. Grey cast iron pipes are an extreme case, in which a high proportion of carbon in the pipe surface results in an enormous hardness. If the transmission of power is supposed to be ensured by a metal gripping element, the limits of the materials are easily reached. In this case hardened steels which are able to penetrate the surface of grey cast iron would have to be used.

There are a few products available with grippers made of stainless steel. Stainless steel is a comparatively soft material which can not penetrate most metal pipe surfaces. Frequently, load resistanceis only achieved because the claws of the grippers get stuck in the wavy surface of metal pipes. Heightened strain on the gripping system endangers it to lose its marginal grip on the rough pipe surface and then slips off abruptly. Furthermore, stainless steel grippers often require differentially shaped gripping teeth for metal and plastic pipes, which results in confusion on the user's side.

If the hardness of the pipe surface is taken as a guideline, it appears to be reasonable to assign the transmission of the tractive forces to a material which is even harder than the pipe surface. For this, there is a choice of different mineral materials, whereas aluminium oxide is especially appropriate. It is also called corundum and has been used as an abrasive for metal surfaces for a long time. In order to be used with gripping systems a few basic requirements need to be fulfilled:

· The corundum has to be available in a certain grit size distribution.

· The thickness of the corundum layer has to be defined.

· The corundum requires an appropriate carrier material, e.g. made of high-strength plastic.

· Since the tractive forces are predominantly transmitted by friction between the corundum and the pipe surface, the contact pressure of the gripping systems on the pipe surface needs to be ensured.

If these conditions are fulfilled, a highly efficient gripping system with a high degree of safety can be realised by an appropriate construction. Corundum qualifies for the application on metal pipes as well as on plastic pipes and transmits the tractive forces predominantly through friction onto the pipe surface. This means that the connection is not – as in the case of other systems – form-locking so that the full flexibility of the connection is maintained. This way it is possible to construct anend-load resistant connector, which can even compensate bending and subsidence while in operation without becoming leaky.

Moreover, a connector with a corundum gripping system is superior in case of excessive load: the friction on the pipe surface decreases evenly. In this case, the connector starts sliding slowly, until the gasket protrudes over the pipe end. Then the excess pressure can exhaust, in which case the connector remains on the pipe.

Conclusion

The changes in pipeline construction and pipeline operation demand new solutions. This applies especially to material transitions which are realised more and more frequently inend-load resistant form. The selection of a suitable product should be the result of pragmatic consideration, so that the pipe connection is accomplished without any difficulties and all possible malfunction sources are eliminated. Only few products can meet the demand for all-purpose use, user-friendly fitting and operational safety.

Mature in all those aspects and well proven in daily building site use is the UltraGrip® series, which grants the user a high degree of efficiency at the highest profitability.

The UltraGrip® series can be used for gas and water pipelines ranging from DN40 up to DN300. Itnowoffers the new Pecatadaptor for the easy connection of polyethylene pipes to existing pipe networks in addition to couplings, flange adaptors, reduced couplings and end caps.

Table captions

Table 1: Pipe OD (mm) of pressure pipes at the example of DN90/100

Captions

figure 1: Material transition from steel to grey cast iron with anUltraGrip® coupling DN250

figure 2: Changes of direction do not demand a thrust block if end-load resistant material transitions are used.

figure 3a/b: Non-violent installation on delicate older pipelines... in comparison to traditional technology

figure 4: A closer view at the UltraGrip® gripping system

References

[1] DIN 3387-1: Removable pipe connections for metal gas pipes – plain ended pipe connections.

[2] VP600: Material transition connector made of metal for pipes of polyethylene pipes.

For further information please contact;

Tim Gardiner
Viking Johnson
46 - 48 Wilbury Way
Hitchin
Hertfordshire
SG4 0UD

Tel - 01462 443 322
Fax - 01462 443 311

E-mail - tim.gardiner@vikingjohnson.com
Web - www.vikingjohnson.com




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