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لوله یا Pipe (Fluid Conveyance) – بخش سوم

Standards

The manufacture and installation of pressure piping is tightly regulated by the ASME “B31” code series such as B31.1 or B31.3 which have their basis in the ASME Boiler and Pressure Vessel Code (BPVC). This code has the force of law in Canada and the US. Europe and the rest of the world has an equivalent system of codes. Pressure piping is generally pipe that must carry pressures greater than 10 to 25 atmospheres, although definitions vary. To ensure safe operation of the system, the manufacture, storage, welding, testing, etc. of pressure piping must meet stringent quality standards.

Manufacturing standards for pipes commonly require a test of chemical composition and a series of mechanical strength tests for each heat of pipe. A heat of pipe is all forged from the same cast ingot, and therefore had the same chemical composition. Mechanical tests may be associated to a lot of pipe, which would be all from the same heat and have been through the same heat treatment processes. The manufacturer performs these tests and reports the composition in a mill traceability report and the mechanical tests in a material test report, both of which are referred to by the acronym MTR. Material with these associated test reports is called traceable. For critical applications, third party verification of these tests may be required; in this case an independent lab will produce a certified material test report(CMTR), and the material will be called certified.

Some widely used pipe standards or piping classes are:

The API range – now ISO 3183. E.g.: API 5L Grade B – now ISO L245 where the number indicates yield strength in MPa
ASME SA106 Grade B (Seamless carbon steel pipe for high temperature service)
ASTM A312 (Seamless and welded austenitic stainless steel pipe)
ASTM C76 (Concrete Pipe)
ASTM D3033/3034 (PVC Pipe)
ASTM D2239 (Polyethylene Pipe)
ISO 14692 (Petroleum and natural gas industries. Glass-reinforced plastics (GRP) piping. Qualification and manufacture)
ASTM A36 (Carbon steel pipe for structural or low pressure use)
ASTM A795 (Steel pipe specifically for fire sprinkler systems)

API 5L was changed in the second half of 2008 to edition 44 from edition 43 to make it identical to ISO 3183. It is important to note that the change has created the requirement that sour service, ERW pipe, pass a hydrogen induced cracking (HIC) test per NACE TM0284 in order to be used for sour service.

ACPA [American Concrete Pipe Association]
AWWA [American Water Works Association]
AWWA M45

Installation

Pipe installation is often more expensive than the material and a variety of specialized tools, techniques, and parts have been developed to assist this. Pipe is usually delivered to a customer or jobsite as either “sticks” or lengths of pipe (typically 20 feet, called single random length) or they are prefabricated with elbows, tees and valves into a prefabricated pipe spool [A pipe spool is a piece of pre-assembled pipe and fittings, usually prepared in a shop so that installation on the construction site can be more efficient.]. Typically, pipe smaller than 2.0 inch are not pre-fabricated. The pipe spools are usually tagged with a bar code and the ends are capped (plastic) for protection. The pipe and pipe spools are delivered to a warehouse on a large commercial/industrial job and they may be held indoors or in a gridded laydown yard. The pipe or pipe spool is retrieved, staged, rigged, and then lifted into place. On large process jobs the lift is made using cranes and hoist and other material lifts. They are typically temporarily supported in the steel structure using beam clamps, straps, and small hoists until the Pipe Supports are attached or otherwise secured.

An example of a tool used for installation for a small plumbing pipe (threaded ends) is the pipe wrench. Small pipe is typically not heavy and can be lifted into place by the installation craft laborer. However, during a plant outage or shutdown, the small (small bore) pipe may also be pre-fabricated to expedite installation during the outage. After the pipe is installed it will be tested for leaks. Before testing it may need to be cleaned by blowing air or steam or flushing with a liquid.
Pipe supports

Pipes are usually either supported from below or hung from above (but may also be supported from the side), using devices called pipe supports. Supports may be as simple as a pipe “shoe” which is akin to a half of an I-beam welded to the bottom of the pipe; they may be “hung” using a clevis, or with trapeze type of devices called pipe hangers. Pipe supports of any kind may incorporate springs, snubbers, dampers, or combinations of these devices to compensate for thermal expansion, or to provide vibration isolation, shock control, or reduced vibration excitation of the pipe due to earthquake motion. Some dampers are simply fluid dashpots, but other dampers may be active hydraulic devices that have sophisticated systems that act to dampen peak displacements due to externally imposed vibrations or mechanical shocks. The undesired motions may be process derived (such as in a fluidized bed reactor) or from a natural phenomenon such as an earthquake (design basis event or DBE).

Pipe hanger assembles are usually attached with pipe clamps. Possible exposure to high temperatures and heavy loads should be included when specifying which clamps are needed.

Joining

Pipes are commonly joined by welding, using threaded pipe and fittings; sealing the connection with a pipe thread compound, Polytetrafluoroethylene (PTFE) Thread seal tape, oakum, or PTFE string, or by using a mechanical coupling. Process piping is usually joined by welding using a TIG or MIG process. The most common process pipe joint is the butt weld. The ends of pipe to be welded must have a certain weld preparation called an End Weld Prep (EWP) which is typically at an angle of 37.5 degrees to accommodate the filler weld metal. The most common pipe thread in North America is the National Pipe Thread (NPT) or the Dryseal (NPTF) version. Other pipe threads include the British standard pipe thread (BSPT), the garden hose thread (GHT), and the fire hose coupling (NST).

Copper pipes are typically joined by soldering, brazing, compression fittings, flaring, or crimping. Plastic pipes may be joined by solvent welding, heat fusion, or elastomeric sealing.

If frequent disconnection will be required, gasketed pipe flanges or union fittings provide better reliability than threads. Some thin-walled pipes of ductile material, such as the smaller copper or flexible plastic water pipes found in homes for ice makers and humidifiers, for example, may be joined with compression fittings.

Underground pipe typically uses a “push-on” gasket style of pipe that compresses a gasket into a space formed between the two adjoining pieces. Push-on joints are available on most types of pipe. A pipe joint lubricant must be used in the assembly of the pipe. Under buried conditions, gasket-joint pipes allow for lateral movement due to soil shifting as well as expansion/contraction due to temperature differentials. Plastic MDPE and HDPE gas and water pipes are also often joined with Electrofusion fittings.

Large above ground pipe typically uses a flanged joint, which is generally available in ductile iron pipe and some others. It is a gasket style where the flanges of the adjoining pipes are bolted together, compressing the gasket into a space between the pipe.

Mechanical grooved couplings or Victaulic joints are also frequently used for frequent disassembly and assembly. Developed in the 1920s, these mechanical grooved couplings can operate up to 120 pounds per square inch (830 kPa) working pressures and available in materials to match the pipe grade. Another type of mechanical coupling is a Swagelok brand fitting; this type of compression fitting is typically used on small tubing under 0.75 inches (19 mm) in diameter.

When pipes join in chambers where other components are needed for the management of the network (such as valves or gauges), dismantling joints are generally used, in order to make mounting/dismounting easier.
Fittings and valves
Copper pipe fittings

Fittings are also used to split or join a number of pipes together, and for other purposes. A broad variety of standardized pipe fittings are available; they are generally broken down into either a tee, an elbow, a branch, a reducer/enlarger, or a wye. Valves control fluid flow and regulate pressure. The piping and plumbing fittings and valves articles discuss them further.

Cleaning

The inside of pipes can be cleaned with a tube cleaning process, if they are contaminated with debris or fouling. This depends on the process that the pipe will be used for and the cleanliness needed for the process. In some cases the pipes are cleaned using a displacement device formally known as a Pipeline Inspection Gauge or “pig“; alternately the pipes or tubes may be chemically flushed using specialized solutions that are pumped through. In some cases, where care has been taken in the manufacture, storage, and installation of pipe and tubing, the lines are blown clean with compressed air or nitrogen.

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