The disadvantages of steel pipes include thermal conductivity, which is very poor as there is a difference in heat transfer. These types of pipes are usually bonded with aluminum or copper to increase thermal conductivity and improve heat transfer. Cost is another issue, as these pipes are expensive and this is guided by the misconception of being a one-time purchase. However, Carbon steel pipes are difficult to fabricate and lack the malleable qualities that other materials have, therefore repairs and replacements of steel pipes are extra difficult.
Steel pipe is required in the majority of oil and gas production and pipeline applications. ASME A53 and A106 and API 5L seamless, electric resistance welding (ERW), and submerged arc welding (SAW) steel pipe are commercially available and most commonly used in pipeline systems. PVC, fiberglass, polypropylene, and other materials may be used in low-pressure and utility applications. ASME B31.4 and B31.8 allow the use of alternate materials in very restricted applications. Seamless pipe is seldom used in pipeline applications because of the higher cost and limited availability. From a design and regulatory perspective, pipe made with ERW and SAW seams is equivalent to seamless pipe and is less costly. Note: This is not true for piping systems designed in accordance with ASME B31.3.
For high-pressure pipeline, high-grade pipe, such as API 5L grades X42, X52, X60, and X65, is selected because much thinner wall pipe can be used, which significantly reduces pipe costs. Construction cost savings also are realized, as the welding time is reduced and material shipping/handling costs are reduced.
Steel pipe is normally used for pipelines operating at a pressure of 100 psig or more. Steel pipe withstands high pressures, is durable, and has a long operating life cycle. Fiberglass, PVC, or high-density polyethylene (HDPE) pipe is used in certain instances for low-pressure gas gathering pipelines. Components of the pipeline include the following: pipes, valves, fittings, and equipment such as metering, pumps, and compressors.
In this book, we are concerned only with transporting hydrocarbons such as natural gas, refined petroleum products, crude oil, and liquefied petroleum gas in steel pipelines. Therefore, we will not deal with materials such as PVC pipe.
Hot rolled steel pipe is generally used aboveground for firewater lines. Underground piping systems can be constructed of steel, cement-lined steel, or high-density polyethylene (HDPE). Concrete is sometimes used but is seldom economical except in large diameters. HDPE does not corrode, resists accumulation of scale, and is very ductile and lightweight. But, because it fails when heated, it can only be used in buried installations, where it is also protected from mechanical damage and radiant heat.
High-performance butterfly valves and gate valves are recommended for block valves in firewater distribution systems. They should provide reasonably tight shutoff and use sealing materials that do not swell or deteriorate with age. Any valve that may be buried, and that may therefore not receive frequent maintenance, should be very durable.
Steel pipe should be used aboveground for firewater lines. Underground piping systems can be constructed of steel, cement-lined steel, or high-density polyethylene (HDPE). Concrete is sometimes used but is seldom economical except in large diameters. HDPE does not corrode, resists accumulation of scale, and is very ductile and lightweight. Because it fails when heated, HDPE pipe can only be used in buried installations. Burial also provides protection from mechanical damage.
The topic of equipment and piping layout is discussed in Chapter 11. Issues to do with the layout of a firewater system are described below.
In climates where freezing does not occur, above-ground installation of steel firewater distribution lines has the advantages of low first cost and ease of inspection and repair. In cold climates, distribution lines should be buried below the frost line.
When possible, firewater mains should be arranged in loops around process facility and tank farms. Shutoff valves should be located to allow isolation of system segments for maintenance while still providing water for all facilities. The minimum water rate with a section of pipe out of service should be at least 60% of the design rate at design pressure for that area. A firewater header should be provided in each process facility area to serve hose stations.