CODE ACCEPTANCE
Plumbing Systems
When selecting a piping material for any component of plumbing-related systems, the applicable plumbing code is of primary importance. Here, the allowable piping materials will be listed as well as any restrictions for their use. In addition, the code will also stipulate various accepted standards that govern the manufacture, tolerances, and installation of all components.
Plumbing Systems
When selecting a piping material for any component of plumbing-related systems, the applicable plumbing code is of primary importance. Here, the allowable piping materials will be listed as well as any restrictions for their use. In addition, the code will also stipulate various accepted standards that govern the manufacture, tolerances, and installation of all components.
When renovating an existing facility or where circumstances require a unique design, it may be necessary to request a deviation from the accepted list of materials in order to match existing piping or obtain special design characteristics. The authorities reviewing such requests require enough information to determine that the intent of the applicable code provisions is followed in terms of safety and suitability of materials for the purpose intended.
Applicable plumbing and model building codes are discussed in Chap. 9,
‘‘Plumbing Systems.’’
Other Code Requirements
Codes required for systems other than plumbing are generally the following:
1. ANSI B31.l-9 series
2. FDA requirements, such as cGMP
3. Sanitary 3-A Standards for the food and pharmaceutical industries
PRESSURE AND TEMPERATURE RATINGS The pressure rating for a wide range pipe and other components is provided by the ANSI Pressure Classification System, which publishes qualitative performance standards. The design pressure is the maximum sustained (steady-state) pressure that a piping system must contain without exceeding its code-defined stress limits.
The design temperature of a component must be equal to or greater than the maximum sustained temperature that will be experienced during any normal or abnormal mode of operation. Normal operating temperature is the temperature maintained by the system while it is operating at steady-state, full-load, non transient operation. It is the temperature at which the allowable stress is calculated.
COST The total installed cost of the network includes the piping material cost, assembly of the joint selected, handling due to the weight of the material, physical damage allowance, and the support system required.
CORROSION RESISTANCE Corrosion is the thinning of a pipe wall (usually) caused by a chemical reaction from a corroding fluid or agent and is limited almost exclusively to metal products. Corrosion resistance is the ability of a pipe to resist the internal corrosive effects of the fluid flowing through it, as well as the external corrosive forces on the pipe, for example, soils (if underground) or surrounding atmospheric conditions (when installed above ground). Corrosion can be reduced or eliminated with the application of suitable coatings, linings, and cathodic protection, depending on the nature of the problem. Since there is no universal corrosion-resistant material, the selection of a specific grade or class of pipe material with specific alloys to resist expected corrosion resulting from specific fluids within a pipe is a matter of tradeoffs unique to each project.
Corrosion is separated into two basic types: general and localized. General corrosion describes the potential dissolution of pipe over its entire exposed surface. Localized corrosion affects only a small area of the pipe surface.
General Corrosion
This is a breakdown of the pipe material at a uniform rate over its entire surface by direct chemical attack. It is caused by the loss of the protective passive film that forms on the surface of the pipe coupled with a chemical reaction occurring between the pipe material and the chemical in the fluid.
Galvanic Corrosion. This type of corrosion occurs in a liquid medium (called an electrolyte) when a more active metal (anode) and a less active metal (cathode) come in contact with one another and form an electrode potential. When this occurs, the more active (noble) metal will tend to dissolve in the electrolyte and go into solution. This is shown in the galvanic series of metals (Table 2.1).
Inter granular Corrosion. This type of corrosion occurs in the pipe wall when material in the grain boundary of some alloys is less resistant to the corroding agent than the grains themselves, and the bond between the grains is destroyed.
Erosion Corrosion. This is caused by a wearing away of the pipe wall, usually as a result of excessive fluid velocity or impingement by suspended solids.
Localized Corrosion
This takes place on small areas of the surface, usually at high rates, and takes various
Stress-Corrosion Cracking. This type of corrosion is the physical deterioration and cracking of the pipe wall caused by a combination of high operating temperature, tensile stress on the pipe, and chemicals in the fluid stream.
Pitting. This is characterized by deep penetration of the metal at small areas of the surface, concentrating in small cells, without affecting the entire surface.
Crevice Attack Corrosion. This occurs at junctions between surfaces (often called crud traps) where a crack exists that allows an accumulation of a corroding agent.
Corrosion failure occurs if any pipe material is reduced to a minimum thickness mandated by code.
Manufacturers of pipes and fittings publish corrosion-resistance tables concerning chemical resistance and compatibility with fluids carried by the piping they produce. These relate to temperature, concentration, and specific chemicals. These tables should be used for selection of a specific pipe to transport any specific chemical.
PHYSICAL STRENGTH Physical strength is the capability of any pipe to resist the damage that might occur either during the construction phase or after the pipe is placed in service.
FIRE RESISTANCE Fire resistance is the capability of a piping system to simply remain intact and not fall during a fire or, in some cases, to also retain the ability to carry water. Where this is a factor, pipes, joints, and supports strong enough for this purpose should be selected.
AVAILABILITY Any pipe is considered available when it can be supplied at a competitive price to the area where the project is under construction. In addition, obtaining the me- chanics and special tools needed to assemble the piping system must also be possible.