Low-Temperature Piping

Where piping systems are subjected to low temperatures, i.e. below 0 °C, materials and fabrication practices, e.g. post-weld heat treatment, should be selected to minimize the risk of brittle fracture. 

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Table 7-22 lists the thickness of conventional low temperature piping insulation for all the common materials except the foamed resins. Before beginning the piping design, the types and thickness of insulation must be firmly established so that proper line clearances may be set. Because the clearances required are much larger than conventional process piping, this aspect is... 

Figure 7-138. Insulation and clearance details for low temperature piping.

In Europe, asbestos -cement pipes were made by mixing fibers with Portland Cement and then allowing the cement to cure at ambient temperatures. In the United States the process was similar, except that rather than curing at low temperatures, pipes were cured in steam pressured autoclaves, at temperatures near 170 C, for about ten hours. This process makes very high quality pipes. I was astounded that this process could be operated economically, but it was and the product is superior. 

In the case of low temperature tar, the aqueous Hquor that accompanies the cmde tar contains between 1 and 1.5% by weight of soluble tar acids, eg, phenol, cresols, and dihydroxybenzenes. Both for the sake of economics and effluent purification, it is necessary to recover these, usually by the Lurgi Phenosolvan process based on the selective extraction of the tar acids with butyl or isobutyl acetate. The recovered phenols are separated by fractional distillation into monohydroxybenzenes, mainly phenol and cresols, and dihydroxybenzenes, mainly (9-dihydroxybenzene (catechol), methyl (9-dihydtoxybenzene, (methyl catechol), and y -dihydroxybenzene (resorcinol). The monohydric phenol fraction is added to the cmde tar acids extracted from the tar for further refining, whereas the dihydric phenol fraction is incorporated in wood-preservation creosote or sold to adhesive manufacturers. Naphthalene Oils. Naphthalene is the principal component of coke-oven tats and the only component that can be concentrated to a reasonably high content on primary distillation. Naphthalene oils from coke-oven tars distilled in a modem pipe stiU generally contain 60—65% of naphthalene. They are further upgraded by a number of methods. 

A product used under the jurisdiction of the ASME Boiler and Pressure Vessel Code and the ANSI Code for Pressure Piping B31.1 is subject to any limitation of those codes. This includes any maximum-temperature limitation for a material or a code rule governing the use of a material at a low temperature. 

Changing the ambient temperature around the pipe to a temperature that 1 avoid low-temperature problems. Burying water pipes below the frost line or running them through a heated building are the two most common examples of this method. 

At very low temperatures with hquid air and similar substances, the tank may have double walls with the interspace evacuated. The weh-known Dewar flask is an example. Large tanks and even pipe hues are now built this way. An alternative is to use double walls without vacuum but with an insulating material in the interspace. Perlite and plastic foams are two insulating materials employed in this way. Sometimes both insulation and vacuum are used. 

Stocks which tend to sohdify on coohng require uniform coverage of the bottom or agitation. A maximum spacing of 0.6 m (2 ft) between turns of 50.8-mm (2-in) and larger pipe and a close approach to the tank wall are recommended. For smaller pipe or for low-temperature heating media, closer spacing should be used. In the case of the com-... 

The low temperature which, if the materials come into contact with the body, can cause severe tissue burns. Flesh may stick fast to cold uninsulated pipes or vessels and tear on attempting to withdraw it. The low temperatures may also cause failure of service materials due to embrittlement metals can become sensitive to fracture by shock. 

When applying low temperature requirements, one should consider safety valve and flare headers to be subject to "shock chilling" if they can be exposed to cold liquids released into the system. This includes flare headers from blowdown drums into which cold liquids are discharged. Where laterals of different piping material are combined, the material of the lower-temperature header is continued for the rest of the combined line, and is also extended back into the other lines for 6 m. 

When using hot-water pipes in the floor, the water temperature is usually low (30 °C - 40 °C), so the system is well suited for low-temperature heating. 

There are many design features and styles of safety relief valves, such as flanged ends, screwed ends, valves fitted internally for corrosive service, high temperature service, cryogenic service/low temperatures, with bonnet or without, nozzle entrance or orifice entrance, and resistance to discharge piping strains on body. Yet most of these variations have little, if anything to do with the actual performance to relieve overpressure in a system/vessel. 

Critical loads may be experienced for example, severe loads, impact loads such as jarring, pulling on stuck pipe, and/or operating at low temperatures. If in the judgment of the supervisor a critical load has occurred, or may occur, an on-the-job shutdown inspection equivalent to the periodic field inspection should be conducted before and after the occurrence of such loading. If critical... 

Pressure drops on the high-pressure side will be small enough to have little effect on the performance of the complete system. Pressure losses in the suction pipe and its fittings, especially if this is long, should be checked, and a correction made for the actual compressor suction pressure. For low-temperature applications, pipe sizes may have to be increased to avoid excessive frictional losses at these low pressures. 

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