Selection and Limitations of Piping Components

Pipe and Fittings

Furnace lap weld ferrous pipe, furnace butt-weld ferrous pipe, spiral-weld ferrous pipe, and fusion-welded steel pipe (made to ASTM A134, A53, Type F, API 5L furnace butt welded and A211) are not permitted for hydrocarbons or other flammable fluids within process unit limits or for hazardous fluids in any location. Non-ferrous pipe made by similar manufacturing process is similarly restricted.

The use of bell-and-spigot fittings is limited to water and drainage service. Also, pipe couplings made of cast, malleable, or wrought iron are not permitted for flammable fluids within process limits or hazardous fluids in any area. In addition, they cannot be used for flammable fluids outside process unit limits at design temperatures above 300°F or design pressures above 400 psig.

Comments on Selection and Limitations of Components

There are many Code mandated restrictions for the use of fittings, bends, intersections, and valves which are not universally applicable to all process plant services, and such components are not easily codified. Some guides, however, are of value to the designer and are listed:

Welding fittings are usually preferred to flanged fittings, not only for economic reasons but because the potential for leakage is reduced.

Pipe bends are preferred to butt welding elbows for reciprocating compressor suction and discharge piping, vapour relief-valve discharge piping, and piping conveying corrosive fluids (such as acid where turbulence in a fitting may cause excessive corrosion).

Bends or dead end tees should be used for piping which conveys pulverized abrasive solids suspended in gas in the dilute phase. Dead end tees (so arranged that the flow will impinge against the dead end) have a longer life than bends in abrasive service and should be used if the system can be designed to accommodate the resulting increase in pressure drop.

Bends should be used for dense phase flow of pulverized abrasive solids and for all piping which handles either pulverized or granular solids suspended in liquids or granular solids suspended in gases.

If the flow is through a branch into a header (or run pipe) in a piping system which transports pulverized abrasive solids suspended in gas in the dilute phase, a dead end cross (so arranged that the flow will impinge against the dead end) should be used.

In services with very high corrosion rates, butt welding fittings with the same inside diameter (ID) as the attached pipe (if not the same, consider taper boring the component with the smaller ID) are preferred to threaded and socket welding fittings.

Threaded cast iron fittings should not be used in pressurized process and utility piping.

Threaded plugs are preferred to pipe caps for threaded end closures to reduce dead end corrosion problems.

In most process plants, internal corrosion is a greater problem than external corrosion. Consequently, it is common practice that all 3/4 in and larger steel and cast iron (and all 2 1/2 in and larger brass) gate, globe, and angle valves (located above grade) be of the outside screw and yoke type.

PIPING GUIDE

Wednesday, November 18, 2015