IGH BELOW FREEZING
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Cryogenic tankers are among the most technically advanced bodies in the field of road haulage — with cargo temperatures of approaching —200°C they have to be. The trick is to maintain that temperature by insulation alone without sacrificing payload.
• In the world of cryogenic liquefied gas distribution, familiar terms take on new meanings. Operating temperatures are frequently down to -196°C, demanding very special kinds of insulation, and the product has to be carried under pressure. A figure of 3bar is typical, but much higher pressures can be involved during loading and unloading.
One problem with cryogenic liquids is that at atmospheric pressure their boiling point is lower than -150°C. Above this level they turn to gas, but they need to be kept liquid if they are to be dense enough to make their transport and handling an economic proposition.
The equipment may be exotic, but the products include everyday products such as oxygen, nitrogen and argon, all of which can be obtained from the atmosphere by air separation. A good gauge of the economics of transporting them is given, somewhat tongue in cheek, by Jon Ostle, engineering manager of BOC Gases, who points out that in liquid form, oxygen costs less than an equivalent amount of beer!
It is not surprising that bodybuilders have generally steered clear of tackling bulk cryogenic tankers. Instead, marketleading UK producers BOC and Air Products run their own manufacturing divisions.
Not that either company refuses to buy elsewhere: "If other manufacturers could
offer reliable and competitive products I would be happy to look at them," says Ostle. "But historically the economics of building such specialised tankers have not been very attractive.
A maximum-length cryogenic tanker semi-trailer could run to around 130,000, making it among the most expensive pieces of transport equipment around.
A similar picture has developed abroad. In France. for instance, L'Air Liquide builds its own cryogenic tankers; in Scandinavia. AgaCryo does the same. It is perhaps only in the United States, with its large and unified marketplace, that a more conventional user/supplier relationship exists.
REFURBISH
Fortunately it has proved possible to refurbish these tankers, and sometimes to extend their lives up to 20 years or more. They can then be stripped and if necessary stretched to meet changed legislation.
Even so, BOC is currently turning out three or four new vehicles and tankers every week to meet demand.
The two UK factories do occasionally cross-quote each other on specific jobs, although it is very rare in practice for business to pass between them. However, BOC (which has the larger of the two fleets, including 300 cryogenic tankers) does sometimes buy from outside suppliers. Recently, for instance, it has taken some equipment from AgaCryo.
Air Products prides itself on being entirely self-sufficient in terms of cryogenic tankers. Indeed, its factory at Acrefair. near Wrexham in North Wales, is the official supplier to Air Products' bases throughout Europe — a role which has won it five Queen's Awards in recent years.
The manufacturing approach of the two big British suppliers has been refined somewhat in recent years. BOC, for instance, no longer produces its own running gear in its Cryoplants factory at Edmonton, North London. It concentrates on building the vessels themselves, and subcontracts the chassis equipment and pipework to outside specialists. Air Products also buys in running gear to its own designs for fitting to its tanker bodies.
There are no on-board refrigeration units on these bodies: the product is expected to retain its temperature for up to a week — much longer than would normally be needed. Double pressure relief valves are fitted as standard; in theory, if left long enough the entire load would eventually vaporise and be vented back to the atmosphere. Obviously efficient insulation is critical. Normally a space of between 125 and 150mm is left between the inner vessel and the outer casing, and this is evacuated to a near vacuum which users expect to be sustained without attention; BOC, for instance, is confident enough to restrict checks to four-monthly intervals.
The gap between tanks (or 'interspace') is filled with insulation. Perlite beads made from volcanic ash are the most common material, but Air Products, in particular, has moved towards 'super-insulated' designs, which use a multi-laminar aluminium-based wrap. This has reflective qualities that improve performance and it is slightly lighter than perlite; but it is more expensive and requires a higher degree of vacuum.
Lately BOC has been investigating an alternative American approach, which is to use a glass fibre-based material that is marginally less effective than perlite but can save as much as 500kg in weight and, unlike perlite, does not settle and solidify over the years.
The inner and outer vessels are usually built of the same materials to make welding of the complex pipework easier. Aluminium and stainless steel are both commonly used — the price differential between them is small.
Aluminium offers a weight advantage of up to 2.000kg, but also poses the problem of greater internal contraction at low temperatures (longitudinal movement of up to 50mm is possible), and generally requires more careful design to reduce fatigue problems.
Design of the inner vessel is generally dictated by pressure requirements, while the other vacuum jacket carries road stresses. However, the inner tank is usually so stiff that mountings are needed only at two points: sometimes just at the very ends. Top-quality welding is essential, and joints have to be X-rayed to meet international standards.
Suiting the equipment to the product is a matter of compromise. Argon is much denser than the other liquefied gases, but designing a tanker to capitalise on this in terms of payload would rule out • economic operation with nitrogen or oxygen. The tendency is to optimise for one product when possible.
REDUNDANT
With Britain's 38-tonne weight limit, cargoes are constrained by payload. Fiveaxle outfits can carry up to about 22 tonnes of liquid oxygen or 22.5 tonnes of argon.
Avoiding redundant tare weight has therefore become a preoccupation rivalling that in the brewery trade. Ostle's break-even point for expenditure is a cost of £15 per kilogram of weight saved.
Lack of any harmonised international pressure-vessel standards is currently the greatest frustration for manufacturers. A British code of practice on construction has already been drawn up and has been welcomed by the main contenders, who say they will have no problem meeting it; indeed, they already exceed most of its demands (BOC, for example, insists voluntarily that its vehicles meet Petroleum Regulation standards). However, this standard may not be tough enough for some European countries, which tend to demand testing to their own national requirements. These local variations make life hard for Air Products' internationally-orientated output, and could become an irritant for BOC now that a vigorous European expansion program is under way there.
However, BOC has now started building "oblique-cone" trailers, which have two sections with different diameters. The theory is that either section could be varied in length later without the need for extensive international re-testing, re-coding and rejigging. Stability is said to be improved too, so this could prove to be the shape of tanks to come.
E by Peter Rowlands