Connecting plastic pipes with metal ones: analysis of the best methods and installation nuances

Sometimes when installing communication systems you have to combine elements made from different materials. Particularly common in construction practice is the connection of plastic pipes with metal ones, for which you can use different methods.

We will tell you how to connect a plastic section of pipeline to an existing steel or cast iron system with maximum reliability. We describe in detail the technologies that make it possible to ensure tightness during docking. The article provides the devices and tools necessary for the work.

Types and characteristics of pipes

To correctly connect pipes made from different types of materials, you should know the main types and properties of such products.

Option #1: metal

Such elements, characterized by hardness and mechanical density, are able to withstand considerable loads. A common disadvantage of metal products is their high cost.

Metal pipes made of copper, steel, cast iron have high rigidity and strength, but are not always able to resist corrosion and are also prone to blockages

Among the most common options are the following types of pipes.

Cast iron pipes. The most popular material, which is characterized by good resistance, durability, and a relatively budget price.

A negative factor of cast iron pipes is their fragility, due to which these products require careful handling during storage and transportation. Before starting work, it is important to inspect the cast iron elements for chips, cracks and other defects.

Steel pipes. Various product options are combined under this name:

• Pipes made of ordinary steel are quite easily covered with rust, and also have a tendency to overgrow the internal space, which leads to blockages.
• Galvanized steel is not prone to corrosion, but is quite difficult to install.
• Stainless steel has the highest consumer properties (resistance to aggressive substances, strength), but products made from this type of metal are expensive and require labor-intensive processing.

It should be noted that steel elements are more expensive than cast iron.

Copper pipes. A rather rare option, since copper pipes are expensive. However, such high-quality elements are sometimes used for communications (elevator units) in the construction of private houses.

Option #2: plastic

For laying communication systems, pipes are used, in the manufacture of which various types of polymers are used. All plastic products are characterized by such important advantages as inertness to chemical environments (even aggressive ones), resistance to corrosion, resistance to clogging, as well as affordable cost.

Polymer pipes are widely used in modern construction work due to their low cost and excellent resistance to aggressive substances.

At the same time, a number of characteristics of plastic pipes directly depend on the characteristics of the material from which they are made.

The most commonly used:

• Polyethylene : products made from this polymer have good elasticity and sufficient density. However, they are not able to withstand high temperatures (PET softens already at 80 degrees Celsius), which makes them undesirable for transporting hot liquids and gases.
• Polypropylene : pipes made from this plastic are most widespread, since this material ideally combines low weight with strength and durability. Polypropylene elements withstand elevated temperatures well, which allows them to be used in hot water supply systems even without reinforcement.
• Polyvinyl chloride (PVC) . The least common option, since products made from this plastic are quite fragile. Such pipes should be laid only in places where no loads will fall on them; most often they are used to create sewer systems.

All of the above types of pipes are used in everyday life.

Option #3: metal-plastic

The composite material, which has recently gained great recognition, consists of five layers, including two plastic shells (inner and outer), two layers of adhesive and aluminum foil.

Metal-plastic pipes have a combined design, which includes, in addition to two layers of polymer, a glued layer of aluminum foil. Such products are of high quality, combining aesthetics with strength and light weight.

Such products are distinguished not only by high consumer qualities (durability, reliability, lightness), but also by their beautiful appearance, which allows them to be used without additional finishing. Metal-plastic pipes can be used for laying various communications: water supply, sewerage, heating and drainage systems.

List of materials

First, let’s find in memory what specific types of pipes are used in the construction of water supply networks.

Dark steel. Now it has lost its former popularity due to the spread of polymer materials, but still holds its ground due to its great mechanical strength.

• Galvanized metal pipe. It compares favorably with dark steel due to its corrosion resistance.
• Corrugated stainless steel.
• Copper.

• Metal-plastic is a three-layer structure made of cross-linked aluminum and polyethylene, which provides mechanical strength.
• Polypropylene (without reinforcement; fiber reinforced; aluminum foil reinforced).
• Polyethylene, used only for cold water supply due to limited heat resistance.

How to connect water pipes in each specific case?

How to connect sewer pipes?

Depending on the purpose of the system, metal and polymer elements can be mounted in different ways. The simplest way is to connect sewer pipes made of different materials.

In order to combine them, special products are used (corrugated cuffs, plastic corner bends, seals), which can have different sizes and configurations.

A transition collar is an excellent option that allows you to join a polyvinyl chloride pipe with a cast iron neck. This method is usually used when installing sewer pipelines

Using a similar device, you can easily dock a polymer pipe with a cast iron neck. In order for these products to fit tightly to each other, it is important to choose a cuff or corrugation that matches the parameters. If for some reason this fails, you can use fine-pored rubber (even an old car mat).

A long narrow strip is cut out of the material, which is used to secure the fasteners between the pipes. This operation can be conveniently performed using a wide, unsharpened screwdriver, and you need to work carefully, but carefully, keeping in mind the possible deformation of the plastic.

It is strongly not recommended to seal a plastic element in a metal socket using caulking or to seal the joint with cement.

In the first case, you will not be able to achieve a tight connection, since the plasticity of the polymer will cause the pipe to deform. In the second case, the thermal expansion of the plastic will interfere with a good result: under the influence of a hot pipe, the fasteners will be damaged and the joint will lose its tightness.

Connections of heating and water pipes

This option is considered more difficult because the communication system is under pressure. In this case, you can use several methods of connecting pipes:

Threaded

For pipes whose diameter does not exceed 4 centimeters, the best option is to connect with fittings. Such convenient devices can have different configurations and parameters, but have a common design feature.

One end of the fitting ends with a smooth coupling designed for a polymer element, the other has an internal or external thread at the end intended for fastening a metal pipe.

Threaded fittings, designed to create reliable connections between plastic and metal pipes, can have a wide variety of shapes and sizes

For more complex connection options, a tee fitting is designed, using which you can join two plastic and one metal (usually steel) elements into a single system.

Flanged

For large diameter pipes (60 cm or more), it is better to use special detachable flange fasteners, which consist of two identical parts connected with bolts.

To connect dissimilar pipes, various types of flanges can be used (loose, shaped, supported by collars), which differ in size and design features

This allows you to avoid manually tightening the threads, which is difficult to do on large parts, but at the same time create a strong, reliable fastener.

Special types

Other types of fittings can be used to connect parts, for example, a special coupling or Gebo type fittings. The latter option is especially recommended for combining short pipes or systems located in difficult places (for example, close to the floor).

Metalsleeve

To combine plastic elements with metal pipes, you can also use a special device - a metal hose, which is designed to work in extremely difficult conditions (high pressure, exposure to aggressive substances).

Typically, such a device is used when laying gas pipelines or installing systems at chemical plants, but it can also be used in construction.

Using a metal hose you can create an elastic connection. This device is usually used in industry, including complex and dangerous work.

A metal hose, one example of which is a flexible connection for a mixer, is attached to a metal pipe using an ordinary thread (in the case of connecting large-diameter elements, a flange of the appropriate size can be used). An additional fitting with a polymer coupling is put on the second end of the metal hose, which connects to the plastic product.

A special advantage of this method is the ability to create a flexible connection, thanks to which the structure is able to “bypass” various obstacles.

Below we will look in detail at the installation of various types of connections.

Features of installation of threaded fittings

Thanks to various types of threaded fittings when installing a pipeline, the ability to perform the most complex bends and turns is achieved.

The most common method of connecting metal/plastic structures is an “American” fitting. Such an element, which has a coupling and thread at the ends, allows you to easily and quickly join dissimilar parts

The most popular device for joining polymer and metal elements is the “American” fitting, which is available in various sizes. A convenient device with a plastic coupling and metal thread is extremely easy to install and allows you to create a reliable sealed connection in a short time.

Step-by-step installation instructions

To connect a metal pipe with a polypropylene or other plastic pipe, you must:

• Using a special soldering iron, weld the fitting sleeve to the end of the polymer pipe, then wait for the joint to cool.
• Bring the metal part to the other end of the “American” and then tighten the thread. To seal the joint, it is advisable to additionally wrap it along the thread with one or two layers of FUM tape, tow or flax fiber (additionally, it can be coated with silicone).
• The fitting must always be tightened by hand: the use of tools is undesirable and even dangerous. Special equipment does not make it possible to fully control the applied forces, which can lead to damage to the part.
• After completing the work, it is important to check the strength of the resulting fastening. To do this, just turn on the water and make sure there is no leakage. If moisture still filters through the joint, you can try tightening the bolt a little tighter. If water continues to flow, it is necessary to unscrew the thread again and carry out all the manipulations again.

The shape of the finished connection can be changed by softening the plastic part using a hair dryer and then making the bend required by the project.

Metal pipe connection without thread

There are situations when a plastic pipe needs to be connected to a metal counterpart that does not have a thread.

Threads can be applied to a pipe using a special tool - a thread extender. In specialized stores you can find both mechanical and electrical models of such devices.

A similar problem can be solved by a special tool called a “thread maker” or “thread cutter”, with which you can apply grooves to a part made of steel or cast iron.

There are two adaptation options:

• Electric, which includes several cutters designed for pipes of different diameters. Such models are comfortable and easy to use, but have a high cost.
• A manual thread spreader is much cheaper, but working with it requires more physical strength and certain skills.

You can read in detail about how to cut threads on pipes in our article.

When using thread cutters, you should follow a few simple rules:

• The tool cannot be used if the pipe is located a short distance from the wall.
• The electric thread spreader heats up quickly, so you will need to rest periodically when processing multiple elements.
• When working with a hand tool, it is important to alternate movements, making half a turn forward and a quarter turn back, until you cut the thread to the desired length.

To thread a pipe, you must thoroughly clean the pipe, remove any existing paint if necessary, and grind off any metal threads. Then carve using an electric or carving tool.

Clean the cut grooves well and lubricate them with grease, oil or other lubricant, and then use them to connect to the fitting.

A few important points when welding

A tool such as a professional soldering iron is expensive. Therefore, it is more profitable to buy inexpensive options if you have to do a one-time job. “Amateur” models are usually equipped with several running attachments. An even better solution would be to rent a tool. The quality of the connections will not be affected in any way.

All soldering irons have a mandatory element - a stand. Even specialists make connections provided that the tool remains in a stable position. It should take 10 to 15 minutes for the work surfaces to heat up.

Features of flange mounting

With this option for fastening polymer and metal pipes, special detachable devices are used - flanges and clamps. They not only allow you to create a strong joint, but also provide access to the junction of the pipes (to do this, just unscrew the bolts).

The diagram presented shows the connection of a metal and plastic pipe using a clamp. Such fasteners can be used on large diameter pipeline sections

There are several options for such devices, among which the most common are:

• Loose clamps resting on a straight shoulder. Such parts are intended for the installation of light elements with a diameter of up to 30 cm, as well as for medium and heavy pipes, for which this figure does not exceed 15 centimeters.
• Loose clamps supported by a conical collar will help create structures from pipes with a diameter of no more than 20 cm.
• Blade fasteners with a shaped flange and a steel projection are considered universal; they can be used for joints of different elements, but it is important to pay attention to the size.
• A straight shoulder with a tapered transition can significantly increase the strength of the connection.

Regardless of the clamp model used, to create a connection you must adhere to the following work schedule:

• Before starting the manipulation, it is important to select an element of the required type, the size of which would correspond to the diameter of the metal part of the pipeline.
• You should carefully inspect the selected part: there should be no sharp burrs on the part that could scratch and damage the rather fragile plastic parts.
• The location of the intended insertion is determined on the steel (metal) pipe, then it is carefully cut (the edges must be smooth).
• After this, a clamp is put on the metal part. To ensure a tight connection, a rubber gasket is added to it, which should not extend beyond the cut line by more than one centimeter.
• Next, you need to connect all the elements of the fastening unit by fastening the clamp on the metal pipe with a similar element installed on the polymer part using bolts.

Tightening the bolts must be done calmly and evenly, making a full turn of the thread and avoiding excessive application of force, so as not to damage the rather fragile part.

Image gallery

Photo from

Weld Ring Flange Fitting

Repair of external pipeline with flange

Replacing a section of internal pipeline

Use of large diameter flanges

The use of clamps in connecting, repairing pipes and fixing openly laid pipelines is described in detail in this article.

Summing up

As can be seen from the above, the installation procedure in both ways is quite simple. First of all, it is important to pay attention to the correspondence of the sizes of pipes and joint elements. You should also carefully inspect all elements for flaws, cracks and burrs. Any damage will negatively affect the reliability of the structure, resulting in the need to re-repair the water supply after some time.

And the last point is to strictly follow the instructions. All work must be done step by step, without neglecting the stages and following the recommendations. Particular attention should be paid to sealing agents, and in no case should you forget about them, since they are an important component of the entire plumbing structure. You shouldn’t apply them in large quantities either – the pipes won’t thank you for it.

Special types of pipe connections

In some cases, it is advisable to use fasteners with other types of fittings. Let's consider two options for such work.

Fastening using a special coupling

A fairly simple part can not only be purchased at building materials stores, but also made independently.

The device consists of the following parts:

• body made of cast iron or durable steel;
• two nuts, which are located on both sides of the part;
• rubber gaskets to seal the connection;
• four metal washers, which must be located in the internal cavity of the coupling.

The parameters of all elements must correspond to the diameter of the pipes on which they will be used.

Instructions for use: insert the ends of the pipes up to the middle of the coupling, threading them through the gaskets, washers and nuts, and then tighten the latter firmly so that they compress the gaskets.

Installation using a clamping coupling (Gebo fitting)

Joining metal and polymer pipes using threads guarantees a sealed seam, but this method is not always possible.

If the length of the metal pipe is short or it is adjacent to a horizontal surface, it is advisable to use a clamping coupling, which is also called a Hebo-type fitting (for example, HEBO-QUIK).

The clamping coupling (Gebo fitting) is especially recommended for installing dissimilar pipes located in difficult-to-reach places (for example, close to a wall)

Such a device consists of the following parts:

• housings;
• fastening nuts;
• clamping, pressure and sealing rings.

The work does not require special skills, special equipment or long time.

Procedure:

• Clean the edge of the metal pipe from dirt and old paint.
• Unscrew the coupling completely.
• Sequentially place all fastening parts onto the cast iron or steel element, with the O-ring located at a distance of 1 or more centimeters from the end of the product.
• Insert the end of the metal pipe into the coupling body, push all the existing rings towards it, and then tighten the bolt.
• An American fitting should be attached to the thread located on the other side of the coupling, having previously soldered a polymer pipe to it.

To check the strength of the fastener, you need to carefully pull the element, holding the pipe to which it is attached with your hand.

Image gallery

Photo from

Crimp Connector for Steel Pipe

Expansion fitting disassembled

Extension of a metal branch with a plastic pipe

Installing a fitting for connecting pipes

The fitting should not move on the structure. If it reacts to your touch, you should tighten the thread tighter. Movement in this case will mean that you have chosen the wrong size, as a result of which the coupling does not sit tightly on the pipe.

All types of fasteners described above can be used to connect metal pipes with metal-plastic elements.

Types of flanges

Established standards provide for many types of flanges. Most often, the types of flange connections are divided according to the following criteria:

• material;
• type of contact surface;
• design;
• protrusion height.

Connecting parts are made of gray and ductile cast iron, carbon, stainless, alloy steel, polypropylene.

The types of sealing surface are designated by Latin letters and are distinguished as follows:

• A – plane;
• B – connecting protrusion;
• F – depression;
• E – protrusion;
• D, M – groove;
• C, L – spike;
• K – under the lens gasket;
• J – for a gasket with an oval cross-section.

Products with the “spike” and “protrusion” surface types are not widespread. They are made only upon special orders.

According to their design, flanged disks are divided into:

• flat;
• collar;
• loose on the welded ring;
• for vessels and apparatus;
• stubs.

The last ones to plug are the end holes of dead-end pipes or cut off sections of the pipeline to be repaired. In addition, they produce quadrangular connectors with four holes for fasteners.

The height of the projection determines the pressure at which the flange connection is guaranteed to operate without damage. To operate at pressures ranging from 1.03 to 2.07 MPa, a protrusion height of 1.6 mm is sufficient. If the maximum pressure is 2.76 - 20.68 MPa, then the protrusion height should be 6.4 mm.

The flanges are attached in the same way, regardless of the types of connectors.

Conclusions and useful video on the topic

The videos posted in this section clearly demonstrate the procedure for fastening metal and polymer pipes using an “American” fitting and a flange connection.

Practical application of flange:

The process of joining metal (cast iron, steel) and polymer pipes requires certain strength and skills. Regardless of the type of fastener, for perfect joints it is important to correctly select the parts of the required size, and also carefully follow the instructions.

Would you like to talk about how you connected a plastic pipe to a steel or cast iron pipeline during repairs? Would you like to share useful information with us and site visitors? Please write comments in the block below, ask questions and post photos on the topic of the article.

In what cases are two different pipes joined?

There are different situations, most often the connection is made during the renovation of buildings or premises with the installation of new networks. At the household level, this is the replacement of the internal wiring of the cold and hot water supply system, heating system and sewerage system. At the level of industrial facilities and highways, this is not only a major overhaul with replacement, but also a conversion to cost-effective materials. In this regard, plastic is several times cheaper than metal.

Plastic pipes are inferior to metal pipes in withstanding external loads. Therefore, external networks are assembled from the latter, especially in places where they are laid and installed under highways and railways. Therefore, the joint between two types of pipes is a simple necessity that occurs frequently.