Plastic pipework

Tolerance Whenever possible, equipment should tolerate poor installation or operation without failure. Expansion loops in pipework are more tolerant of poor installation than bellows are. Fixed pipes, or articulated arms, if rlexibihty is necessary, are fiiendher than noses. For most apphcations, metal is friendlier than glass or plastic. 

Provision must be made to drain off and dispose of the condensate, which is produced at a rate of up to 0.151/h for every kilowatt of input. This is slightly acidic, and plastic is the most suitable for the pipework. 

The water supply authorities normally insist that (for uses other than drinking-water taps) their main should discharge into a break-pressure vessel, after which the water quality becomes the consumers responsibility. The water tank should be covered against tramp dirt and access by birds, etc., and it must be shielded from sunlight to avoid the growth of algae. Nevertheless, access must be maintained for easy inspection. The distribution pipework is preferably all plastic and lead must be avoided altogether. The use of copper is doubtful with some corrosive waters, and soldered joints in it can lead to unacceptable concentrations of lead in the water. 

The metal lost from the inside of pumps, reaction vessels, pipework, etc. usually contaminates the product. The implications of this depend upon the product. Ppb levels of iron can discolor white plastics, though at this level the effect is purely cosmetic. Ppm levels of iron and other metals affect the taste of beer. Products sold to compositional requirements (such as reagent-grade acids) can be spoiled by metal pick-up. Pharmaceutical products for human use are often white tablets or powders and are easily discolored by slight contamination by corrosion products. 

Pipework and other components should be insulated after the safety pressure test, but usually before prolonged running of the plant, since it is very difficult to remove water and frost once it has formed. Only the low-pressure piping is insulated, where it does not form part of the evaporator, i.e. after the expansion valve, where this may be outside the cooled space, and from the evaporator back to the compressor. Basic materials are cork and the expanded plastics. These are sufficiently rigid to be moulded to the correct shape, remain firmly in place, and support the external vapour barrier which is essential to prevent the ingress of water vapour (see also Chapter 15). 

Equipment should tolerate maloperation, poor installation or maintenance without failure. E.g. expansion loops in pipework are more tolerant to poor installation than bellows. The construction materials should be resistant to corrosion and physical conditions. For most applications metal is safer than glass or plastic. 

Plastics such as polypropylene and acrylonitrile butadiene styrene are used for the construction of pretreatment tanks and pipeworks. 

WFI is quite corrosive, especially at 85°C, and it can promote leeching from even high-grade stainless steel piping. Addition of ozone to the WFI can alleviate this, as the ozone s microcidal properties facilitate prolonged storage/circulation of the water at 25°C. Other innovations in this field include replacement of stainless steel pipework with chemically inert plastics. However, extensive tests need to be undertaken in order to prove that WFI cannot leach potentially dangerous substances from these plastics before their use will become routine. 

Plasticized PVC can be extruded in various forms—including layflat film for bags and other containers with gusseted walls, flexible pipework, hoses, and continuous seals in many different configurations. 

Regenesys uses DuPont s Nafion (Section 6.1.7) as the perm-selective sodium ion transfer membrane, separating the two half cells. Figure 2.1. Diffusion of sodium ions in the concentration difference across the Nafion membrane is one of the irreversibilities of the system. The low-cost plastic (e.g. polyethylene) tanks and pipework are treated with fluorine to provide bromine resistance, and are able to operate with, and contain, both electrolytes at ambient temperature. 

PVC in the feedstock stream in limited amounts can thus be tolerated if a HCl removal process based on calcium hydroxide (slaked lime) injection is used. This is essentially a form of in scrubbing in the pyrolysis reactor itself. This method is used by a number of pyrolysis processes since the fuel oil end products need to be low in chlorine to minimize engine corrosion problems. In the pyrolysis of commingled plastics, lime can be added to the input material and the PVC content reduced down to 2-3% in order to avoid deposits of CaCl2 in pipework. The chlorine content of the final diesel should not exceed 10 ppm. 

Tank fittings, connection lines and pipes must also be fitted with specific regard to the strict guidelines. Tubing used for pipework should be manufactured to the most current defined standard and the pipework must also meet a minimum standard (for example, BS 1387 and ANSI B31.3 plus supplement EEMU Supplement 153). All pipework should be protected against corrosion as per the tank. Only under exceptional circumstances should pipework be made of plastic. 

Based on turnover value, the largest part of the plastic pipe industry is probably that part concerned with placing the pipes in their position of usage, above or below ground. But this is a very diverse industry and its pipe laying activities cannot be separated from other aspects of civil engineering and building construction. Whilst for some companies pipework may be a trivial proportion of their construction activities, for others it may be their main specialisation. 

Piping from plastic tanks may be in aluminium, stainless steel or rigid PVC. Metallic pipework should be used where there exists any serious risk of physical damage. Some fluorinated plastics and rubbers can be used in service with H2O2, for example, polytetrafluorethylene, polyvinylidene fluoride, and a co-polymer of vinylidene fluoride and hexafluoropropylene such as VITON. 

The behaviour of material conveyed in a non-suspension or dense phase mode of flow was investigated qualitatively in a small test rig. A conveying pipeline, constructed from glass pipework of 25mm bore, ineluding bends, was used. A binary mixture of salt and black plastic beads was chosen for its segregation characteristics and ease of observation. The salt had a mean particle size of 367pm and the beads had a diameter of approximately 5mm. 

Epoxy resins (epoxides) plastics available as liquid and solid resins and moulding compounds, into which large amounts of filler can be incorporated. Used for laminating, coating, adhesives, casting, encapsulation and compression moulding. Applications include cast and encapsulated electrical components, laminated plastics vessels, pipework and printed circuit boards. 

Non-metallic materials, such as glass reinforced plastics (GRP), may offer advantages for seawater pipework, particularly with respect to corrosion resistance and weight saving. However, speciahst expertise in design, fabrication, and installation techniques will be required for the evaluation of factors such as cost, susceptibihty to mechanical damage, and safety implications, before such materials are selected. Any proposal to use non-metaUic materials should be subject to approval. Also, it may be necessary to obtain waivers from the statutory authorities regarding the use of combustible materials. 

There are few guidelines for the use of adhesively bonded assemblies offshore as currently such structures are mainly welded steel. However, UKOOA (United Kingdom Offshore Operators Association) proposed a set of Guidelines for the use of glass-reinforced plastic (GRP) pipework offshore. Part 4 of which includes sections on installation and assembly (UKOOA 1994). For adhesively bonded assemblies, a quality program is proposed, which requires the recording of key values, namely ... 

In practice, there are problems with powders, and to some extent with fibre blankets, due to thermal contraction and expansion of the inner vessel and pipework. Over a period of time, the insulations settle or move so as to form voids. The presence of such voids are indicated by the appearance of cold spots or frost patches on the outer containing skin. These problems can be avoided wifli the use of PUF and other plastic foams. 

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