Tubes Pipes (steel)
A pipe is a tubular section or hollow cylinder, usually but not necessarily of circular cross-section, used mainly to convey substances which can flow — liquids and gases (fluids), slurries, powders, masses of small solids. It can also be used for structural applications; hollow pipe is far stiffer per unit weight than solid members.
In common usage the words pipe and tube are usually interchangeable, but in industry and engineering, the terms are uniquely defined. Depending on the applicable standard to which it is manufactured, pipe is generally specified by a nominal diameter with a constant outside diameter (OD) and a schedule that defines the thickness. Tube is most often specified by the OD and wall thickness, but may be specified by any two of OD, inside diameter (ID), and wall thickness. Pipe is generally manufactured to one of several international and national industrial standards. While similar standards exist for specific industry application tubing, tube is often made to custom sizes and a broader range of diameters and tolerances. Many industrial and government standards exist for the production of pipe and tubing. The term “tube” is also commonly applied to non-cylindrical sections, i.e., square or rectangular tubing. In general, “pipe” is the more common term in most of the world, whereas “tube” is more widely used in the United States.
There are three processes for metallic pipe manufacture. Centrifugal casting of hot alloyed metal is one of the most prominent process. Ductile iron pipes are generally manufactured in such a fashion. Seamless (SMLS) pipe is formed by drawing a solid billet over a piercing rod to create the hollow shell. Historically seamless pipe was regarded as withstanding pressure better than other types, and was often more easily available than welded pipe. Advances since the 1970’s in materials, process control and non-destructive testing allow correctly specified welded pipe to replace seamless in many applications. Welded (also Electric Resistance Welded (“ERW”), and Electric Fusion Welded (“EFW”) pipe is formed by rolling plate and welding the seam. The weld flash can be removed from the outside or inside surfaces using a scarfing blade. The weld zone can also be heat treated to make the seam less visible. Welded pipe often has tighter dimensional tolerances than seamless, and can be cheaper if manufactured in the same quantities. Large-diameter pipe (25 centimetres or greater) may be ERW, EFW or Submerged Arc Welded (“SAW”) pipe.
Tubing, either metal or plastic, is generally extruded.
Metallic pipes are commonly made from steel or iron; the finish and metal chemistry are peculiar to the use, fit and form. Typically metallic piping is made of steel or iron, such as unfinished, black (lacquer) steel, carbon steel, stainless steel or galvanized steel, brass, and ductile iron. Aluminum pipe or tubing may be utilized where iron is incompatible with the service fluid or where weight is a concern; aluminum is also used for heat transfer tubing such as in refrigerant systems. Copper tubing is popular for domestic water (potable) plumbing systems; copper may be used where heat transfer is desirable (i.e. radiators or heat exchangers). Inconel, chrome moly, and titanium steel alloys are used in high temperature and pressure piping in process and power facilities. When specifying alloys for new processes, the known issues of creep and sensitization effect must be taken into account.
Pipe sizes are specified by a number of national and international standards, including API 5L, ANSI/ASME B36.10M and B36.19M in the US, BS 1600 and BS EN 10255 in the United Kingdom and Europe.
Many different standards exist for pipe sizes, and their prevalence varies depending on industry and geographical area. The pipe size designation generally includes two numbers; one that indicates the outside (OD) or nominal diameter, and the other that indicates the wall thickness.
Pipes/tubes come into two categories with regard to manipulation and carriage by sea: 1) large (unprotected/protected) diameter pipes, shipped in single pieces; 2) small diameter pipes/tubes, shipped in bundles.
1) Unprotected large diameter pipes are usually manufactured from low carbon mild steel which may, if the thickness of the plate forming the pipe is not too great, originate from a hot rolled steel coil, slit into stripes (referred to as skelp) to form the width of the plate intended to be fabricated into a pipe. The long edges of the sheet, or plate, are brought together and welded to each other so forming a pipe. Such pipes usually have a visible weld running the full length of the pipe. Another method of manufacturing such pipes is by spiral welding when a flat strip of plate is wound to form a hollow spiral tube. Adjacent edges are welded together and the pipe is formed with a spiralling weld visible along its length.
Protected large diameter pipes, made of mild steel and cast iron, are protected by the application of special coatings which may be fusion bonded epoxy coatings, polyethylene coating, neoprene, bituminous materials, coal-tar and asphalt mastics.
2) To manufacture small diameter pipes (welded pipes/tubes), a strip is cut from a hot rolled coil to the width required to form the walls of the pipe, the edges are planed to a ‘V’ shape after which the strip is curled to the form of the pipe as it passes through rolls. This brings together the two prepared edges which are welded. After fairing of the welding and clearing the inside of the pipe by forcing a plug through it, the pipe is passed through an annealing furnace, thereafter being stretch reduced to size. The pipes are then cut to length and put through an ultrasonic test. Depending on customers’ requirements the pipes may be given a protective coating, left with plain ends or threaded ends, or fitted with a sleeve, as the case may be.
Seamless pipes, sometimes referred to as ‘solid drawn pipes’ are a type of piping or tubing which is preferred for all purposes where internal pressure is high and service severe. The seamless pipe is achieved in three stages as described below. A round steel billet is removed from the furnace and the end is pierced in a rotary piercing mill. Thereafter, a mandrel is forced through the billet to transform it into a hollow cylinder. The cylinder is then put through a series of rollers so that it is elongated into a shell. There then takes place a final rolling, after reheating, to form the required size of tube. Again, depending on customers’ requirements, the pipes may be given a protective coating, left with plain ends or threaded ends as the case may be. These small diameter pipes and tubes are used in the manufacture of bicycle frames, furniture for offices and homes, gardening tools and equipment, vehicle shock absorbers and exhaust pipes, boiler and condenser tubes, frameworks for the roofs of sheds, to mention but a few.
Hollow steel bars are created by forcing a hole through the centre of a round bar in much the same way as described for solid drawn pipes. They are used for structural and machinery applications and also in the oil industry. They are considered stronger than the average pipe of comparable size.
Small diameter pipes, which might be distinguished by the fact that they are not shipped loose but in bundles, will usually have a protective coating applied. They may be greased for protection, varnished, painted or galvanised.
- Domestic water systems
- Pipelines transporting gas or liquid over long distances
- Structural steel
- As components in mechanical systems such as: rollers in conveyor belts, compactors (e.g.: steam rollers), bearing casing
- Casing for concrete pilings used in construction projects
- High temperature or pressure manufacturing processes
- The petroleum industry: Oil Well Casing, oil refinery equipment
- Delivery of fluids, either gaseous or liquid, in a process plant from one point to another point in the process
- Delivery of bulk solids, in a food or process plant from one point to another point in the process
- The construction of high pressure storage vessels (note that large pressure vessels are constructed from plate, not pipe owing to their wall thickness and size).
Shipment / storage
Very careful handling and adequate stowage, blocking and bracing is necessary to avoid damage. Exceeding the stacking limits may result in deformation of pipes in the lower parts of the stow(s).
Large diameter pipes
Damages to the bevelling on the end of a pipe may eventuate in the defective end having to be cut off and rebevelled. Such damage is usually in the form of a nick or score mark which has depth. Shippers often specify that any pre-shipment remarks, regarding this type of damage, should clearly state the depth of the defect. It is indicated that incisions in excess of 3 mm will result in reconditioning being necessary. On the other hand, minor incisions, score or chafe marks should always be considered as a defect which may or may not result in eventual loss to the end user of the goods.
Denting on the end of the pipe can result in the end being out of round to such an extent that it cannot be welded to the end of another pipe. The length of pipe which is so affected may have to be cut off and rebevelled. An appreciable dent on the body of the pipe may pull the ends off square to such an extent that the pipe is rendered useless and has to be scrapped. These types of pipes are usually shipped without any protection against the development of rust.
The coatings of protected large diameter pipes are applied on the understanding that they will remain intact. During manipulation, owing to the cumbersome nature of large pipes and their weight, the coatings often sustain some degree of damage.
Small diameter pipes
Owing to their thin wall structure, small diameter pipes will not withstand too much rust development before they become defective. Further to this, rain water, usually with a high PH in industrial areas, remains as free moisture within the bundles and can cause serious damage. Also, rust formation on small pipes can provoke claims in respect of commercial depreciation. Hence, small diameter pipes should always be kept dry. For carriage by sea, galvanised tubes should be given a passivating dip usually in a di-chromate solution. The packaging of small diameter pipes requires special consideration and it is recommended that when possible these pipes are bundled in such a manner that their ends form a hexagonal shape, being well strapped up with tensioned flat metal strapping bands.
Steel cargo is at risk of corrosion. There are two main causes of corrosion:
1) Pure oxidation
2) Electrochemical decomposition of the metal due to the presence of an electrolyte (.e.g. salts, acids, bases).
Pure oxidation means a combination of the ferrous metal with atmospheric oxygen. Oxidation is assisted by electrochemical (electrolytic) processes. The extent of electrolytic decomposition is determined by the conductivity of the electrolyte present. For example, salt water is more conductive than fresh water and therefore has a greater corrosive effect. The effect of sulfurous acid is even more extreme.
If corrosion damage is suspected, testing is performed using the silver nitrate method, to find out whether chloride solutions or fresh water are the cause.
Thin-walled pipes of a relatively small diameter are in particular highly sensitive to corrosion, for which reason such pipes are frequently provided with a protective coating.
In the case of pipes made from hot-rolled steel, it is usual to store them in the open and to transport them without protection, such that no protection is provided against rain etc.. Such pipes therefore generally exhibit a layer of surface rust (rust film). Since the rust is removed from the steel (by pickling) prior to further processing, the quality of the steel is not adversely affected. Hot-rolled material must also be protected from chloride solutions (e.g. seawater or fertilizers) as pickling cannot remove uneven local corrosion or pitting corrosion. Especially in the case of damage by salt water, the pipes should be rinsed off with fresh water as soon as possible after arrival with the receiver and then pickled because significant delay prior to pickling may have the above-stated consequences. For reasons of quality maintenance, the aim should always be to store, handle and transport the sheets in the dry.
Surface coated pipes (e.g. galvanized pipes) are more sensitive to corrosion than those made from hot-rolled steel, such that the former should additionally be packed in fiber-reinforced packing or plastic-coated kraft paper and plastic films. It is therefore important to keep moisture away at all times; unprotected storage in the open or unprotected cargo handling in wet weather should be avoided. Moisture may, for example, give rise to a white bloom on the zinc coating. If rain or condensation water penetrates between stacked galvanized pipes, the thin, protective zinc oxide layer does not form, but rather a thicker layer of pure zinc oxide.
Most steel products rust and bend easily, particularly when carried at sea. Shipowners and operators – and ultimately their P&I insurers – are therefore usually first in the firing line when bent and rusty steel arrives at destination.
As a result, shipowners, carriers, operators and P&I clubs now regularly commission independent pre-shipment surveys of steel cargoes to ensure that bills of lading and/or mate’s receipts accurately describe the apparent order and condition of the cargo when it was taken on board the ship. As a further precaution against rust claims, surveyors are also generally now asked to look at the watertightness and ventilation of the holds in which steel cargo is carried.
- Seawater damage
- Mechanical damage