Pressure Vessels

Abstract

Pressure vessels are formed on a hollow aluminum mandrel having an external surface of a configuration corresponding to the desired internal shape of the vessels. An impervious vessel inner liner is formed on the mandrel as by electro-deposition, and the vessel wall is then formed by winding filamentary reinforcing material on the inner lining, impregnating such material with a resinous material, curing the resinous material, and then removing the hollow aluminum mandrel chemically by use of an aqueous solution of caustic soda.

Description

This invention relates to improved filament wound pressure vessels incorporating very thin impervious metal linings.

A method of making a fluid-tight pressure vessel according to the invention consists essentially in preparing a hollow aluminum mandrel which presents an external surface which corresponds in external configuration and dimensions with the required internal configuration and dimensions of a pressure vessel to be made, building up by electro-deposition of nickel on to said external surface of the mandrel an impervious nickel inner liner of the vessel, forming the vessel wall by winding filamentary or fibrous reinforcing material on to said inner liner and mandrel and impregnating said reinforcing material with resinous material, during said winding thereof and curing the resinous material, and the reafter removing the hollow aluminum mandrel chemically by use of an aqueous solution of caustic soda. In place of impregnating the reinforcing material during the winding operation, it may be done before winding or after winding, but the impregnation is believed to be more effective if carried out during the winding operation.

In general, the purpose of the inner layer of a vessel according to the invention is mainly, if not entirely, to provide reliability of impermeability over a wide range of temperatures even as low as those of liquified gases and its thickness may be small, e.g. in the range of some two to five thousandths of an inch, except where it may be of locally changing or quite substantial thickness, e.g. one-tenth of an inch, for example where the main body is shaped progressively into a neck-like formation at an outlet or inlet opening thereof.

It has been found very satisfactory to use a copper coated mandrel of pure aluminum and to nickel plate this in a nickel sulphamate bath as follows:

Nickel Sulphamate 300 gm./liter. Boric Acid 35 gm./liter. pH (adjusted with Sulphuric acid) 4.0 Sodium Lauryl Sulphate -- Sufficient to reduce surface tension to 28 dynes/cm. Temperature 50.degree. C. Current Density 30 amps per sq. ft.

After plating as above and applying the composite reinforcement and resinous material to the nickel inner liner while still on the mandrel, the whole of the material of the mandrel may conveniently be removed from the interior of the vessel by simply pumping hot aqueous caustic soda solution, 10 percent w./v. through it at a temperature of not more than about 80.degree. Centigrade. The copper coating may then be removed by means of a 50 percent w./v. or stronger solution of nitric acid. The expression w./v. is used herein to represent "weight in volume concentration".

One example of the manufacture of a pressure vessel in accordance with the invention is described below.

The vessel is of hollow cylindrical form with externally convex ends of which one is closed and the other is formed with an integral open neck extending co-axially from it.

The method included the following steps:

1. A hollow aluminum former was copper plated externally to a depth of about 0.003 inches by a standard plating process.

2. The copper plated mandrel was nickel plated to the required thickness in a nickel sulphamate bath as indicated above to form a vessel of the configuration indicated above.

3. Two layers of glass fibre rovings were wound onto the surface of the nickel liner at a helical angle of about 27.degree. to the axis of the vessel to cover the circular cylindrical centre section and the two end sections with the exception of the end connection.

4. During winding these layers were impregnated with a polyester resin mix having a methyl ethyl ketone peroxide catalyst (60 percent by weight solution in dimethyl phthalate) and a cobalt naphthenate accelerator (a solution in styrene containing the equivalent of 1 percent cobalt) and cured at room temperature.

5. Two and a half layers of glass fibre rovings were hoop wound over the first fibre layers on the cylindrical portion of the vessel and impregnated with the same resin mix as used on the first layer, and cured at 100.degree. C. for 16 hours.

6. The aluminum former was dissolved away by pumping a hot 10 percent w./v. caustic soda solution at up to 80.degree. C. through the hollow former until all the aluminum was removed.

7. The copper coating and any residual matter was removed in a nitric acid solution of at least 50 percent w./v.

At and near the junction of the main body with the neck the thickness of the nickel inner layer is of progressively increasing thickness to a constant greatly increased thickness to provide ample strength at the neck for the fixture of a charging and closure valve assembly.

With this process a pressure vessel with impermeable walls is formed, having a high strength/weight ratio and good insulating and corrosion resistant properties.

In one example a pressure vessel manufactured in accordance with the invention was pressure tested to 1,500 lb. per sq. inch with liquid nitrogen, the pressure being raised and lowered repeatedly. Subsequent inspection of the vessel showed that the nickel liner remained unwrinkled and gas tight.

In a further example according to the invention a vessel constructed in the manner described was filled with liquid hydrogen pressurised to 200 p.s.i. and held at this pressure for 10 minutes and this cycle of operations was repeated many times with no deleterious effects to the vessel.

While particularly useful for the production of pressure vessels, the invention is also applicable for the production of tanks and pipes for handling gases, corrosive and cryogenic fluids and in fact to any reinforced plastic structure which is required to be impervious to liquids or gases at low and high temperatures.

Claims

We claim:

1. A method of making a fluid-tight pressure vessel comprising copper plating a hollow aluminum mandrel which presents an external surface which corresponds in external configuration and dimensions with the required internal configuration and dimensions of a pressure vessel to be made, depositing an impervious nickel liner on to said copper plating in a nickel sulphamate bath as follows:

winding filamentary or fibrous reinforcing material on to said nickel liner and mandrel, impregnating said reinforcing material with resinous material during said winding thereof, curing the resinous material on said inner liner to form the vessel wall, and thereafter removing the hollow aluminum mandrel chemically by use of an aqueous solution of caustic soda.

2. A method of making a fluid-tight pressure vessel as claimed in claim 1 in which the aluminum mandrel is removed by pumping hot aqueous caustic soda solution, of about 10 percent weight in volume concentration through it at a temperature of not more than 80.degree. C., and the copper plating is then removed by means of a solution of nitric acid of at least 50 percent weight in volume concentration strength.

3. A method of making a vessel of hollow cylindrical form with externally convex ends of which one is closed and the other is formed with an integral open neck extending radially from it comprising the following steps:

copper plating a hollow aluminum mandrel which presents an external surface which corresponds in external configuration and dimension with the required internal configuration and dimensions of the vessel to be made,

depositing an impervious nickel liner on said copper plating in a nickel sulphamate bath as follows:

winding on to the nickel plated mandrel at least two layers of glass fibre rovings at a helix angle of about 27.degree. to the axis of the vessel to cover the circular cylindrical centre section and the two end sections with the exception of the end connection,

impregnating these glass fibre rovings during winding with a polyester resin mix having a methyl ethyl ketone peroxide catalyst, 60 percent by weight solution in dimethyl phthalate, and a cobalt napthenate accelerator, a solution in styrene containing the equivalent of 1 percent cobalt, and cured at room temperature, hoop winding on two and a half layers of glass fibre rovings over the first fibre layers on the cylindrical portions of the vessel and impregnating them with the same resin mix as in the step above which is cured by subjecting it to a temperature of approximately 100.degree. C., for a period of substantially 16 hours, removing the aluminum mandrel by pumping hot aqueous caustic soda solution, of about 10 percent weight in volume concentration into it at a temperature of not more than 80.degree. C., and,

removing the copper plating by means of a solution of nitric acid of at least 50 percent weight in volume concentration strength.