Figure 1 – Typical layout of the main gas pipeline equipment

1 – crafts, 2 – gas gathering point, 3 – fishing collector; 4 – Installation of gas treatment; 5 – main compressor station (CS); 6 – main line 7 – Interim CS; 8 – linear closures;
9 – underwater passage from the backup thread; 10 – go under the railway; 11 – removal of the gas pipeline; 12 – gas distribution station (GDS); 13 – the final GDS;
14 – station underground gas storage (UGS); 15 – gas control point (GCP); 16 – Power Station, 17 – Gas Processing Plant (GPP)

Objects CS project in block–complete version. In most cases, the CS are equipped with centrifugal blowers driven by gas turbines or electric motors. Currently equipped with a gas turbine driven more than 80% of the CS, and the electric drive – about 20%.

By line facilities include proper header pipe, line closures, gas purification units, transitions through artificial and natural barriers, corrosion protection stations, drainage devices, and include a line of technological communication, branches off from the main pipeline to supply a portion of the transported gas to consumers and linear structures operational services (LES) [3 – 4].

The distance between the line shut – off devices (valves) should be no more than 30 km. Control of linear valves should be provided remotely from the premises of the operator compressor station, as well as manual in place. Linear valves should be fitted with automatic failsafe mechanisms.

With parallel laying two or more gas pipelines in a technological corridor provides connection with their jumpers valves. Jumpers must be placed at a distance of not less than 40 km and less than 60 km from each other in line valves, as well as before and after the compressor stations.

Distribution stations (GDS) are designed to reduce (reduction) of the gas pressure to the operating pressure of the gas distribution customers. GDS is also equipped units of accounting and purification plants and odorization (giving it a peculiar smell to facilitate the detection of gas leakage to prevent hazardous situations and poisoning of people).

After GDS gas enters the gas network of settlements, which supply gas to the place of consumption. The reduction and to the extent necessary to maintain the gas pressure in the gas distribution network is carried out on the gas control points (GCP). For large gas consumers also include thermal power plants and gas processing plants (GPP).

For smoothing uneven consumption of gas major cities and towns are built station underground gas storage (UGS). To pump gas into underground storage facility is equipped with its own UGS compressor station.

The auxiliary line facilities include the main gas pipeline link pipeline service road, helicopter pads, pad emergency stock of pipes etc.

Depending on the specific operating conditions of the main structures of the composition of the gas pipeline is subject to change. Thus, in the short–haul pipelines may not be interim COP. If the produced gas is not hydrogen sulfide or carbon dioxide, the need for plants for gas purification from them is eliminated. Stations are built underground gas storage is not always the case.

Based on the magnitude of the working pressure gas pipelines are divided into two classes [5]:

1st class – at an operating pressure in excess of 2.5 MPa to 10 MPa, inclusive;

2nd class – at an operating pressure of more than 1.2 MPa to 2.5 MPa inclusive.

The length of the gas pipeline is typically from a few tens to several thousands of kilometers, and the diameter – from 150 to 1420 mm, inclusive. Most of the gas pipeline has a diameter of 720 mm to 1420 inclusive.

In the operation of pipelines are subjected to different loads – internal pressure, the pressure of the soil backfill and movable assets, temperature extremes, the axial tensile or compressive stresses. These factors contribute to the development of corrosion on pipe walls – which, as known, is a major cause of accidents.

Protection of pipelines against corrosion

The pipeline, laid in the ground, exposed to soil corrosion and extends above the ground – atmospheric. Corrosion of metals called destruction or alteration of the properties caused by chemical or electrochemical processes in interaction with the environment. The nature of interaction of the metal with the environment are two main types of corrosion: chemical and electrochemical [6 – 7]..

Chemical corrosion refers to the cases of changes in the properties of the metal by chemical reactions without the occurrence and flow of electric current. This type of corrosion include gas corrosion and corrosion in non–electrolytes.

Gas corrosion occurs as a result of interaction of a metal with a highly preheated gas in the absence of moisture (e.g., corrosion of gas turbine blades, scale formation by thermal treatment of metal, etc.).

Corrosion non–electrolytes – destruction metal in liquid or gaseous media nonconducting (corrosion by reacting with oil containing sulfur).

Galvanic corrosion – is the oxidation of metals in conductive media, accompanied by the formation and the flow of electrical current. Thus different regions on the metal surface having anode and cathode portions. Corrosion damage are formed only on the anodic sites.

By electrochemical corrosion processes include:
– electrolytic corrosion (liquids, conducting electric current: river and sea water, salt solutions, acids and bases);
– soil corrosion – the destruction of the metal under the influence of the soil electrolyte;
– galvanic corrosion – corrosion of metal structures under the influence of stray currents;
– contact corrosion – corrosion of metals in the presence of water caused by the direct contact of two or more metals with different electrochemical potential;
– atmospheric corrosion – the destruction of the metal in the medium of air or any other wet gas;
– biocorrosion – corrosion, vital activity of microorganisms that produce substances that accelerate the corrosion process.

Process corrosion begins from the surface and extends inward (Fig. 2). Distinguish solid local corrosion.

When all the solid corrosion metal surface coated with corrosion products. Non–uniformity of uniform corrosion is directly proportional to the aggressiveness of the corrosive environment.

Local corrosion – the destruction of the metal surface in some areas. The following types of localized corrosion:
– spots (the thickness of the layer of corrosion products much less than the area of the spot);
– ulcer (depth of damage is significant and commensurate with its area);
– Point (pitting) – depth of damage is much greater than its diameter. Pitting into a through under favorable conditions, the corrosion processes.
– subsurface – swelling in the form of blistering and delamination of the metal;
– intergranular (extends along the boundaries of metal crystals and leads to loss of strength and ductility);
– structure–selective (broken any one alloying element);
– corrosion cracking (derived from a combination of corrosion and mechanical stress on the metal).