Pre-insulated polyurethane foam pipes

Consider the design features, production technology, advantages and applications.

Pre-insulated pipe structure

The structure of the product consists of a steel pipe, a layer of polyurethane foam (hereinafter referred to as PPU) and a waterproof shell. The pipes are equipped with a Rapid remote control system (RRCS): a system designed to monitor the state of the heat-insulating layer of polyurethane foam of pre-insulated pipelines and detect areas with high insulation moisture. The waterproof shell is made of polyethylene or galvanized steel.

• PE sheath
• PU foam layer
• Steel pipe

Production technology

There are five ways to produce pre-insulated pipes:

1. top filling
2. bottom filling
3. center filling
4. back filling
5. filling with passage

Top filling (fig. 1).

Figure 1

In the first stage, the steel pipe is wheel blasted, brushed or shot blasted. This treatment cleans the metal from rust and makes the surface rough, which enhances the adhesion of PU foam to the pipe and minimizes the risk of delamination. Then the pipe is heated to 30 °C. Centering supports are put on it, into which the conductor is inserted into the indicator of the remote control system, then a waterproof sheath and flanges on the edges follow. Special holes are made in the flanges for filling the pipe and for air outlet.

At the second stage, the pipe is installed at the desired angle from 1 to 15 degrees, on a special pouring table. A mixture of polyol and isocyanate components is fed through an opening in the upper flange. Under the action of gravity, the mixture flows down the pipe, the flow rate depends on the angle at which the pipe is lifted. As the foam begins to expand, the pipe fills from the center to the ends. Technological holes are closed, the pipe is kept for the time required for polymerization of the insulating layer. Then the flanges are removed, the pipe is ready.

Bottom filling (fig. 2).

The required amount of the mixture is fed through the technological hole in the lower flange, the foam begins to expand, filling the pipe. The exhaust air exits through an opening on the top cover, which is closed when the foam reaches it.

Figure 2

The advantage of the method is its simplicity. The downside is that the foam passes the entire length of the pipe, the consumption of components is higher. The density of PPU is different at the top and bottom of the pipe.

Center filling

The pipe is laid horizontally and the mixture of components is introduced into the hole in the center of the pipe, as shown in Fig. 3. The foam path is reduced to half the pipe, but there is a risk of air entrapment, which is expelled through the holes in the side flanges.

Figure 3

Back filling.

The pipe is installed horizontally. This method uses a casting machine with a mixing head located at the end of the stem (Figure 4). The stem is inserted with the head at the far end of the pipe. Foam filling begins and the stem extends continuously, distributing foam evenly along the length of the pipe. The size of the mixing head for passage into the annular space should be minimal. This limits the use of the method for small diameter pipes.

Figure 4

Filling with passage.

The pipe is installed horizontally. Foam is applied to the paper membrane and pulled through in the annular space. At the same time, the properties of PU foam will be the same along the entire length. The method allows filling long and narrow pipes. On the negative side, the membrane remains inside the pipe, negatively affecting adhesion.

Figure 5

Pipe characteristics

Outside diameter of steel pipes from 32 to 1420 mm. The length of steel pipes for diameters not exceeding 219 mm from 8 to 12 m, with a diameter of 273 mm and above - from 10 to 12 m.

Pre-insulated pipes must pass quality control inspection. Check the foam density, thermal conductivity, compression density, water absorption, shear strength and radial creep of the insulation. In the section, the PPU should have a homogeneous closed fine-mesh structure. Voids (cavities) larger than 1/3 of the thickness of the heat-insulating layer are not allowed.

Requirements for polyurethane foam in accordance with GOST 30732-2006 are presented in the table.

The waterproof casing also undergoes the necessary control.

Storage and transportation conditions for finished pipes

Mechanical damage, buckling, deformation, contamination are not allowed. Dropping, rolling, collision of pipes, dragging on the ground is prohibited. Pipes are transported by rail, water or road in accordance with the rules for the carriage of goods, which ensure the safety of insulation and exclude the occurrence of longitudinal deflection. Loading and unloading operations are carried out in the temperature range specified for the construction and installation works, but not lower:

• -18 °C - for pipes with a polyethylene sheath;
• -50 °C - for pipes with steel protective sheathing.

Scope of application

Pre-insulated PPU pipes are used for underground laying of heating networks (in a polyethylene sheath - by a channelless method, in a sheath of galvanized steel - in passageways and tunnels) and ground laying of heating networks (for pipes with galvanized steel coating). By agreement with the design organization, it is allowed to use insulated pipes in a polyethylene sheath in non-passable channels.

Advantages of PPU pre-insulated pipes:

• • thermal conductivity coefficient of polyurethane foam from 0.025 to 0.033 W/(m•K) minimizes heat loss of the pipe;
  • PPU insulation has no seams and gaps;
  • service life with preservation of the declared properties - 30 years;
  • water impermeability;
  • the steel pipe is reliably protected by a polyurethane foam layer from corrosion, aggressive media, fungi, mold and microorganisms;
  • the ability to use at temperatures up to 150 °C;
  • PPU refers to low-flammable substances, therefore, such pipes will burn only in the presence of a flame source;
  • easy maintenance;
  • the density of rigid polyurethane foam from 60 kg/m³ ensures the resistance of the product to mechanical stress.

The use of pre-insulated ones allows reducing heat losses by 2-2.5 times, in comparison with old models of pipelines, transferring heat over long distances without losses, as well as reducing capital, operating and repair cost categories.

Rostov Polyurethane Company offers 4 types of rigid polyurethane foam component systems for the production of pre-insulated pipes in accordance with GOST 30732-2006: