Protection of Pipe Systems

Selection of buried pipelines coatings is based on the following requirements  

Residual stresses introduced into pipe weldments by fabrication processes are aggravated by hot dip galvanizing. 

Pipes up to a 5 cm (2 inch) hore can be lined, normally in lengths up to 3 m (10 ft), with rubber where required larger hores can he lined in lengths up to 6 m (20 ft). Standard flanged-bolted-type pipework can he used for ruhher hning screwed joints are generally not satisfactory. Elbows, bends, and tees can also be hned. Normally the substrate surfaces should be clean, metallized or primed prior to apphcation of a rubber lining. 

For design of mortar-lined-and-coated steel pipes. 

Where no coatings are used on the cathode surface of a closed system, the area of optimum protection should cover a 2.5 1 spacing. If the closest anode to cathode spacing is 0.3 m (1 ft) and a potential of 1000 mV is obtained, a potential of at least 850 mV will be achieved at 0.75 m (2.5 ft) distance from this anode. 

---

Gorbunov, N. S., Glovatskii, V. G., Sukhomlin, A. I., et al. (1977). Corrosion protection of pipe systems functioning in coke gas purification media. Koks Khim. (6), 43-45 (in Russian). 

Steel Construction Institute. Protection of piping systems subject to fires and explosions, UK HSE research report 285, Her M gesty s Stationery Office, Norwich, UK ... 

The International Marine Organization (IMO)/American Bureau of Shipping (ABS) has fire test performance requirements for the protection of piping systems (primarily for shipping that utilize a fire exposure very similar to UL 1709 for durations of 30 and 60min. Four different level ratings are used. There has been some limited apphcation of these in the industry ... 

For optimum protection of pipe systems, a combination of resistant materials, low contents of corrosive gases in liquids, chemical tieatment for passivation of metal surfaces, low erosion and impingement geometries, suitable velocities, and cathodic protection may be required. 

T. Arai and M. Ohkita. Application of polypropylene coating system to pipeline for high temperature service. In Proceedings Volume, pages 189-201. 8th Bhra Internal External Protect of Pipes Int Conf (Florence, Italy, 10/24-10/26), 1989. 

Steam—Primarily utilized for systems composed of closed vessels, with interconnecting piping. It has some similarity to empty chamber studies in steam sterilizers. Important parameters to confirm are appropriate time-temperature conditions throughout the system. Emphasis is placed on the removal of air and condensate from the system, strict adherence to the defined sequence for the sterilization procedure and inclusion of methods for the protection of the system between sterilization and use. 

Corrosion Control External pipe corrosion is normally controlled by coating the exterior surface of the pipe and by installing cathodic protection equipment on the line. In addition, pipelines must be electrically insulated at all interconnections with foreign systems unless provisions are made for mutual cathodic protection of the systems. 

Menon, E. S. 2005. Piping Calculations Manual. New York McGraw-Hill. Nine chapters cover different types of piping systems water, fire protection, wastewater and stormwater, steam, compressed air, oil, gas, fuel gas, and cryogenic and refrigeration. Appendices cover units and conversions, pipe properties, and viscosity corrected pump performance. 

Feedwater treatment is designed to protect the feedwater system and, to some extent, the boiler. Most systems contain carbon steel piping. Carbon steel corrosion (Fig. 23a) is considerably slower at a pH between 9.0 and 11.0. In aH-ferrous feedwater systems, the preferred pH range is therefore 9.2 to 9.6, although some systems are operated at a pH as high as 10. In systems where copper alloys are present, high concentrations of ammonia accelerate corrosion of the copper alloys. In those systems the preferred pH is 8.8—9.2. 

Cathodic Protection This electrochemical method of corrosion control has found wide application in the protection of carbon steel underground structures such as pipe lines and tanks from external soil corrosion. It is also widely used in water systems to protect ship hulls, offshore structures, and water-storage tanks. 

Examples of the sacrificial-anode method include the use of zinc, magnesium, or aluminum as anodes in electrical contact with the metal to be protected. These may be anodes buried in the ground for protection of underground pipe lines or attachments to the surfaces of equipment such as condenser water boxes or on ship hulls. The current required is generated in this method by corrosion of the sacrificial-anode material. In the case of the impressed emf, the direct current is provided by external sources and is passed through the system by use of essentially nonsacrificial anodes such as carbon, noncor-rodible alloys, or platinum buried in the ground or suspended in the electrolyte in the case of aqueous systems. 

API RP 1632, Cathodic Protection of Underground Petroleum Storage Tanks and Piping Systems, 3rd ed.. May 1996. 

The Guidelines for Process Equipment Reliability Data with Data Tables covers a variety of components used in the chemical process industry, including electrical equipment, analyzers, instrumentation and controls, detectors, heat exchangers, piping systems, rotating equipment (pump, compressor, and fan), valves, and fire protection systems. 

Cement coatings are usually applied as linings for water pipes and water tanks, but occasionally also for external protection of pipelines [7]. Cement is not impervious to water, so electrochemical reactions can take place on the surface of the object to be protected. Because of the similar processes occurring at the interface of cement and object and reinforcing steel and concrete, data on the system iron/ cement mortar are dealt with in this chapter taking into account the action of electrolytes with and without electrochemical polarization. To ensure corrosion protection, certain requirements must be met (see Section 5.3 and Chapter 19). 

The 1997 edition of the API RP 521 extends the two-thirds rule to include the upstream and downstream system. At a minimum, the inlet and outlet piping up to and including isolation valves must be designed for the two-thirds rule to be able to block in the exchanger. If the upstream and downstream equipment is not designed for the two-thirds rule, relief devices may be required on both the inlet and outlet piping to protect the piping and adjaeent equipment. 

Piping systems should be designed for an economic flow velocity. For relatively clean fluids, a recommended velocity range where minimum corrosion can be expected is 2 to 10 fps. If piping bores exist, maximum fluid velocities may have a mean velocity of 3 fps for a 3/8-in. bore to 10 fps for an 8-in.-diameter bore. Higher flow velocities are not uncommon in situations that require uniform, constant oxygen supply to form protective films on active/passive metals. 

Presence of crevices, dead-ends, etc. Effective protection by inhibitors relies on the continued access of inhibitor to all parts of the metal surface Aeration and Movement of the Liquid). It frequently happens that this condition is difficult to achieve due to the presence of crevices at joints, deadends in pipes, gas pockets, deposits of corrosion products, etc. Corrosion will then occur at these sites even though the rest of the system remains adequately protected. 

---