Despite the saturation of the market with many varieties of high-tech polymer pipes, metal for the installation of heating systems, hot and cold water supply is widely used today. Metal pipes for these purposes are mainly made from steel and copper, less often from aluminum. Each type of such pipes, depending on the material of manufacture, has a set of individual characteristics that provide the consumer with enough room for maneuver when choosing.
One of the most common types of pipes are zinc-coated steel products — a material that has many positive characteristics, but is specific in application. Consider what kind of material it is, and how to properly use galvanized pipes for heating.
Production methods and varieties of galvanized pipes
The main enemy of steel pipes is corrosion. One of the ways to protect against it is to apply a protective layer of zinc to the surface of the steel pipe, after which the material is classified as a separate subgroup — a galvanized pipe for water supply and heating systems.
The method is moderately expensive, so the cost of a steel pipe after galvanizing remains affordable — an approximate price ratio can be found according to the table:
| GWP pipe size
Zinc coating can be applied to a steel pipe made by any technology — straight-seam, with a spiral seam, seamless, therefore, galvanized pipes are classified in the same way as ordinary pipes by the factor of the presence of a seam.
There is no independent standard for galvanized products. Steel pipes with zinc protection are produced in accordance with the regulatory documents for electric welded products with a straight seam (GOST 10704) and material for gas pipelines (GOST 3262–75).
Zinc Coating Methods
Zinc coating, depending on the operating conditions of the pipe, can be carried out both on its outer surface and on its inner one.
There are 4 methods of galvanizing pipes, each of which is used depending on the size of the products and the requirements for the thickness and strength of the zinc coating:
- hot — the prepared part is immersed in molten zinc, which ensures high quality and durability of the coating, but the energy consumption of the process is high;
- cold — products are painted with zinc-containing substances by one of the painting methods, which does not provide high strength of the zinc layer (it is used to protect already assembled structures from corrosion);
- electro-galvanic — coating is carried out by electrolysis, that is, deposition of zinc dissolved in the electrolyte (cathode) on the part (anode), when an electric current passes;
- gas-thermal — the device of a protective coating by flame spraying on the surface of the zinc powder part, the disadvantage is porosity and low strength of the layer;
- thermal diffusion — parts are covered with a layer of zinc, which is after heating to 2500 degrees. in a vapor state, it is used for processing small parts in large quantities — screws, bolts, nuts, washers that require high adhesion of zinc to metal.
Parameters of zinc coated steel pipes
The main technical parameters of zinc coated steel pipes are:
- outer diameter (10.2 — 165 mm);
- weight of 1 m of pipe (0.4 — 22 kg);
- conditional passage (6 — 150 mm);
- wall thickness (1.8 — 5.5 m);
- length (4 — 12 m).
As with ordinary pipes, the walls of galvanized products can be light, reinforced or standard, the accuracy of execution is assigned to ordinary or increased.
Important! A protective zinc coating should be applied to the entire surface of the product and have a thickness of 30 microns. Peeling of the protective layer, peeling and swelling on it are not allowed (“Steel water and gas pipes”, Specifications, GOST 3262–75, rev. No. 4.6).
Advantages and disadvantages
Most of the characteristics of galvanized VGP pipes, both positive and negative, coincide with the parameters of black steel products, but there are also individual qualities.
- Strength (especially tensile strength).
- Fire resistance.
- Low coefficient of thermal elongation.
- Absolute tightness.
- Impact resistant.
- Possibility of use as a heat exchanger.
- Possibility of installation, including ongoing repairs, with your own hands.
- Two assembly methods (welding, threaded connection).
- Ease of disposal.
The fact that the external laying of the gas pipeline from the main to consumers is only allowed from steel material speaks in favor of steel GWP pipes.
- Significant share.
- Electrical conductivity.
- High thermal conductivity (the need to insulate pipes in heating and hot water systems).
- Susceptibility to corrosion if the zinc protection is damaged.
Mounting and assembly methods
Galvanized pipes that form a heating or hot water system are mounted in three ways:
- flange connection;
- threaded connection;
Each of the above methods has its pros and cons. Consider these installation methods and the nuances associated with them.
Welding of galvanized pipes
Zinc-coated water and gas pipes can be connected by electric or gas welding — both types of installation are convenient because they take little time. But there is one negative factor, the effect of which must be minimized, since it cannot be completely eliminated.
The fact is that the temperature of the weld reaches 1200 degrees, and zinc boils at 906 degrees and begins to evaporate from heating during the welding process. In doing so, the following happens:
- the harmful effects of zinc vapor on the welder, up to the onset of suffocation, as they are poisonous;
- evaporating zinc exposes steel and makes it vulnerable to corrosion;
- zinc vapor contributes to the formation of pores and cracks in the weld, reducing the strength of the joint.
For maximum localization of these processes, before starting welding, apart from the mandatory device for effective ventilation of the room, it is necessary to perform the following actions.
- prepare the joint edges, that is, make an external chamfer on them and remove the zinc coating by 25–30 mm on both sides of the joint;
- weld the joint with subsequent cleaning of the weld from slag and coating the bare section of the pipe with zinc-containing paint (zinc dust content — 94%, binder — 6%) — cold galvanizing.
To prevent the zinc layer from boiling, it is possible to treat the junction with hydrochloric acid 5 cm in both directions, but in this case, acid fumes will form during welding.
Important! According to paragraph 4.6 of SP (Building Rules) 73.13330.2012, the device of welded joints on pipelines made of galvanized steel is not allowed, since zinc is not removed from the inner surface of the pipe before welding, and zinc fumes, the formation of which cannot be avoided, cause the formation in the seam pores and shells. But this document is of voluntary use, and if the project does not contain a reference to the mandatory use of this paragraph, then the installation of galvanized pipes by welding is permissible.
In order to minimize the impact of zinc on the quality of electric welding, installation must be carried out in accordance with the following recommendations:
- perform welding at low speed, but avoiding burning the pipe, and with an increased current strength;
- use electrodes with rutile coating (electrode composition contains titanium oxide).
Electric welding of zinc-coated pipes requires certain skills from the welder. In addition to the composition of the outer coating of the electrode, the quality of the weld is affected by the thickness of its rod, which determines the power of the arc — an excessively thick electrode will burn through the wall, and a thin one will not provide the necessary strength of the welded joint. For welding galvanized pipes with a wall thickness of 1.5 — 5 mm, electrodes with a diameter of 2–3 mm are used.
Flange connection of galvanized pipes
This method is based on bolting together fragments of pipes, at the ends of which flanges are welded — steel rings with an inner diameter equal to the outer diameter of the pipe, and holes around the perimeter for mounting bolts. Two flanges of different fragments are applied to each other using an intermediate sealing gasket and tightened with bolt nuts or studs.
In this method of installation, there is the same negative factor as in the welded joint — in the process of welding the flanges to the pipes, zinc boils and the protective layer in the area of the weld is destroyed. Therefore, it is also necessary to take measures to localize the temperature effect on the zinc layer, and after welding, clean the seam and apply an anti-corrosion zinc-containing coating (cold galvanizing) on it.
The flange connection is not compact, therefore it is used in most cases when laying pipelines in utility rooms or outside. On the mating side of the flange there is an annular area called a mirror. Between the two mirrors of the joined flanges, before tightening them, a paronite gasket with a hole is installed, the diameter of which must match the inner diameter of the pipe. The outer diameter of the gasket is equal to the distance between the opposite fixing bolts.
This installation method eliminates the need for thermal connection methods and is performed using various types of fittings designed to connect individual pipeline fragments after threading them.
Threaded assembly also has its drawbacks:
- the process of threading is laborious and time consuming;
- a cutting tool (die) when cutting a thread removes a layer of steel of a certain thickness along with a protective zinc coating.
- the tightness of the threaded connection is ensured by wrapping the thread with FUM tape, sealing paste or tow with paint applied, which lose their properties over time and require replacement.
Soldering galvanized pipes
To mount a heating or hot water system from galvanized pipes without damaging the zinc layer, soldering is used, which is performed in the following sequence:
- the connected ends of the pipe are facing and, if the wall thickness is more than 3 mm, a chamfer is made on the outer edges;
- the joints are degreased by heating, after which the flux (HLS‑B composition) also heated to plasticity is applied in a thick layer to the surface adjacent to the planned joint;
- the ends are arranged with a gap of 2–3 m;
- the burner flame is set to excess oxygen.
The size of the burner is selected depending on the diameter and wall thickness of the galvanized pipe:
For high-quality soldering of a galvanized pipe, it is necessary to follow the rule: the size of the burner must be one unit smaller than when welding pipes of the same dimensions without zinc coating. During the soldering process, the flame must be concentrated on the edges to be joined and the joint gap in order to exclude heating and evaporation of zinc from under the flux layer.
The connecting seams of well-made soldering of galvanized pipes do not need additional anti-corrosion protection, but applying zinc-containing paint as an insuring operation will not be superfluous.
The specifics of the use of galvanized pipes in heating and hot water systems
Zinc-coated pipes in heating systems and water pipes are used taking into account operating conditions.
If the coolant temperature does not exceed 65 degrees, then the zinc coating successfully performs its functions. In the northern regions, where this parameter is much higher, the inner zinc layer reacts with water under the influence of high temperature:
Zn + H2O = ZnO + H2.
Both substances resulting from such an interaction are negative factors:
- ZnO is flakes that precipitate and clog the lumen of small diameter pipelines;
- H2 is hydrogen, which, when mixed with air in a certain proportion, is explosive or at least forms plugs in the system.
Therefore, in hot water and heating systems with a coolant temperature above 60 degrees, it is allowed to use pipes that have only an external protective zinc coating, which will protect the pipeline from corrosion during periods of downtime. However, when water gets on the surface of a hot pipe, the zinc shell begins to peel off from the base, therefore, in order to avoid this, it is necessary to paint such pipelines over zinc, which will lead to an even greater increase in the cost of the system.
Conclusion: the use of pipes galvanized on the outside in hot water and heating systems with a coolant temperature above 60 degrees is unreasonably expensive, and galvanized on the inside is harmful and dangerous.
A galvanized water and gas steel pipe is a material in demand today, but it must be used and installed taking into account the characteristics of the protective coating, so that the advantages of zinc are used rationally, and do not become a factor that only increases the cost of work.