Abbotts & Co. has been building pressure vessels since it began back
in 1870.
It began by making boilers for the British admiralty, and continues
today manufacturing a diverse range of vessels for a wide range of
industries.

Back in the beginning pressure vessels and boilers were manufactured
from riveted steel plates (boilerplate), today electric welding has
made that process obsolete in all but the most traditional restoration
workshops.

I had a look at a Wikipedia reference for pressure vessels and got
this:
A pressure vessel is a closed container designed to hold gases or
liquids at a pressure different from the ambient pressure. The end
caps fitted to the cylindrical body are called heads.

The legal definition of pressure vessel varies from country to
country, but often involves the maximum safe pressure (may need to be
above half a bar) that the vessel is designed for and the pressure-
volume product, particularly of the gaseous part (in some cases an
incompressible liquid portion can be excluded as it does not
contribute to the potential energy stored in the vessel.) In the
United States, the rules for pressure vessels are contained in the
American Society of Mechanical Engineers Boiler and Pressure Vessel
Code.

( Abbott and co website gives more info on current methods
http://www.air-receivers.co.uk/vessels.htm

Pressure vessels are used in a variety of applications. These include
the industry and the private sector. They appear in these sectors
respectively as industrial compressed air receivers and domestic hot
water storage tanks, other examples of pressure vessels a diving
cylinder, recompression chamber, distillation towers, autoclaves and
many other vessels in mining or oil refineries and petrochemical
plants, nuclear reactor vessel, habitat of a space ship, habitat of a
submarine, pneumatic reservoir, hydraulic reservoir under pressure,
rail vehicle airbrake reservoir, road vehicle airbrake reservoir and
storage vessels for liquified gases such as ammonia, chlorine,
propane, butane and LPG.
Steel Pressure Vessel

In the industrial sector, pressure vessels are designed to operate
safely at a specific pressure and temperature, technically referred to
as the "Design Pressure" and "Design Temperature". A vessel that is
inadequately designed to handle a high pressure constitutes a very
significant safety hazard. Because of that, the design and
certification of pressure vessels is governed by design codes such as
the ASME Boiler and Pressure Vessel Code in North America, the
Pressure Equipment Directive of the EU (PED), Japanese Industrial
Standard (JIS), CSA B51 in Canada, AS1210 in Australia and other
international standards like Lloyd's, Germanischer Lloyd, Det Norske
Veritas, Stoomwezen etc.

Pressure vessels can theoretically be almost any shape, but shapes
made of sections of spheres, cylinders and cones are usually employed.
More complicated shapes have historically been much harder to analyse
for safe operation and are usually far harder to construct.

Theoretically a sphere would be the optimal shape of a pressure
vessel. Unfortunately the sphere shape is difficult to manufacture,
therefore more expensive, so most of the pressure vessels are
cylindrical shape with 2:1 semi elliptical heads or end caps on each
end. Smaller pressure vessels are arranged from a pipe and two covers.
Disadvantage of these vessels is the fact that larger diameters make
them relatively more expensive, so that for example the most economic
shape of a 1,000 litres (35 cu ft), 250 bars (3,600 psi) pressure
vessel might be a diameter of 914.4 millimetres (36 in) and a length
of 1,701.8 millimetres (67 in) including the 2:1 semi elliptical domed
end caps.

(Abbott air receivers are a basic form of pressure vessel - web site
link for more info below

http://www.air-receivers.co.uk/receivers.htm

Generally, almost any material with good tensile properties that is
chemically stable in the chosen application can be employed.

Many pressure vessels are made of steel. To manufacture a spherical
pressure vessel, forged parts would have to be welded together. Some
mechanical properties of steel are increased by forging, but welding
can sometimes reduce these desirable properties. In case of welding,
in order to make the pressure vessel meet international safety
standards, carefully selected steel with a high impact resistance &
corrosion resistant material should also be used.

This is a very general outline according to the contributors of
wikipedia - and pretty much covers the topic.

What we make in Newark today includes:
Pressure Vessels, Expansion Vessels, Surge Tanks, Industrial
silencers, Nuclear Industry, Oil industry, Process Tanks, Vacuum
Vessels, Blowdown Tanks, Diesel Engine Starter Bottles, Fabrication,
PD5500, ASME, u stamp pressure vessel, Lloyds, DNV, Bureau Veritas,
BS5169, EN286, ISO9001, Dished ends, cylinder, coded welding, 97/23/
EC, ASME, PD 5500, DNV, AD-Merkblatter, EN13445

If you need any further information please do contact the author.

Abbott & Co.(Newark) Ltd
Newark Boiler Works
Northern Rd
Newark
Notts. NG24 2EJ
Tel: 01636 704208
Fax 01636 705742
www.air-receivers.co.uk