Piping systems, design temperature

N = equivalent number of full displacement cycles during the expected service life of the piping system.5 N shall be increased by a factor of 10 for all materials that are susceptible to hydrogen embrittlement (carbon and low alloy steels) when the system design temperature is within the hydrogen embrittlement range [up to 150°C (300°F)]. 

The code also excludes piping systems designed for internal gauge pressures at or above zero but less than 0.105 MPa (15 Ibf/inU provided the fluid handled is nonflammable, nontoxic, and not damaging to human tissues, and its design temperature is from -29 C (-20 F) through 186 C (366 F). Refer to the code for definitions of nonflammable and nontoxic. 

Thermal expansion and the resultant pipe stresses must be considered in any piping system design. For example, if the temperature changes from 50 to 600°F, the increase in length would be 4.9 in. per 100 ft for steel pipe and 7.3 in. per 100 ft for brass pipe. This amount of thermal expansion could easily cause a pipe or wall to buckle if the pipe were fastened firmly at each end with no allowances for expansion. The necessary flexibility for the piping system can be provided by the use of expansion loops, changes in direction, bellows joints, slip joints, and other devices. 

Piping systems designed for pressures less than 15 psi gage that handle fluids that are nonflammable, nontoxic, and not damaging to human tissues and are at temperatures between —29°C (—20°F) and 186°C (366°C) ... 

Design parameters as a function of temperature and design temperature limits are set forth in the ANSI/ASME B31 Piping Codes for a very broad range of materials. These codes, and the additional information available from manufacturers, vendors, and technical societies such as the National Association of Corrosion Engineers provide ample data for the selection of materials for piping systems (1—13). 

Design Conditions Definitions of the temperatures, pressures, and various forces applicable to the design of piping systems are as follows ... 

Design Pressure The design pressure of a piping system shall not be less than the pressure at the most severe condition of coincident pressure and temperature resulting in the greatest required component thickness or rating. 

Oil fuel pipeline systems transfer oil from storage to the oil burner at specified conditions of pressure, viscosity, temperature and rate of flow. There can be considerable variety in the choice of system, but its design (particularly correct pipe sizing and temperature control) is most important if it is to function satisfactorily. 

LTHW systems are usually under a pressure of static head only, with an open expansion tank, in which case the design operating temperature should not exceed 83°C. Where MTHW systems operating above 110°C are pressurized by means of a head tank, an expansion vessel should be incorporated into the feed and expansion pipe. This vessel should be adequately sized to take the volume of expansion of the whole system so that boiling will not occur in the upper part of the feed pipe. On no account should an open vent be provided for this type of system. MTHW and HTHW systems require pressurization such that the saturation temperature at operating pressure at all points in the circuit exceeds the maximum system flow temperature required. A margin of 17 K (minimum)... 

Liquefied hydrogen confined, for instance, in a pipe between two valves will eventually warm to ambient temperature resulting in a significant pressure rise. Standard storage system designs usually assume a heat leak equivalent to 0.5% per day of the liquid contents. 

Brazements included in a piping system that is subjected to a temperature 1,000°F (538°C) and greater shall require tests in addition to those of ASME BPV Code Section IX. These tests shall be considered a part of the qualification procedure for such design temperatures. Two tension tests on production type joints are required, one at the design temperature and one at 1.05T (where T is the design temperature in degrees Fahrenheit). Neither of these production-type joints shall fail in the braze metal. 

Code requirements for design and construction include service requirements, which affect selection and application of materials, components, and joints. Service requirements include prohibitions, limitations, and conditions, such as temperature or pressure limits. Code requirements for a piping system shall be the most restrictive of those which apply to any of its elements. 

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