GB2160608A

GB2160608A - Flanged connections

Info

Publication number: GB2160608A
Application number: GB08416005A
Authority: GB
Inventors: Brian James Wickins; Frank George Groves
Original assignee: Individual
Current assignee: Individual
Priority date: 1983-09-03
Filing date: 1984-06-22
Publication date: 1985-12-24
Also published as: GB8416005D0; GB2160608B

Abstract

A method of protecting a connection between two metal parts, which may be of similar or dissimilar metals, which connection is formed between flanged surfaces (2) on the two parts and which is clamped together by at least one backing flange (3) formed of a metal of different electrode potential from the metal of one of the parts in the connection which it would normally contact in making the connection, in which there is interposed between the backing flange and the metal of different electrode potential an annular insulating collar (5) of substantial L-shaped cross-section having axial and radial dimensions sufficient to prevent any contact between the backing flange and said metal and a material thickness within the range of 0.1 to 2.5 mm, said collar being formed of a non-crystalline, high temperature engineering thermo-plastics material having a continuous working temperature of at least 90<o>C and being capable of withstanding the compressive forces in the connection without substantial creep and of isolating the two metals, thereby to prevent galvanic corrosion in the connection. Preferably the material of the collar has a thickness within the range of from .25 to 1.5 mm, a continuous working temperature resistance of at least 120<o>C desirably at least 150<o>C and can withstand a compressive force of at least 200 Kg/cm<2> without substantial creep. The material may be, for example, a polyether sulphone, a polysulphone, a polyetherimide or a polyphenylene sulphide.

Description

SPECIFICATION Flanged connections This invention relates to flanged connections in pipework and especially to such connections in which two dissimilar metals are used thereby giving rise to the possibility of galvanic corrosion of the connection in certain environments.

In the making of flanged connections in pipework, for example, between two pipes or between a pipe and a pump or valve or other section of the pipework it is frequently necessary to use a backing flange of a metal or metal alloy that has a different electrode potential from at least one of the other parts to be connected so that in certain environments an electric couple is set up between the two metals that are in contact, thereby resulting in galvanic corrosion at the connection.

When such a connection is used in an electrolytic environment such galvanic corrosion is particularly prevalent and the connection frequently has to be replaced after only a fairly short working life. Because of the cost and/or difficulty of replacing such connections it is desirable that a connection should have as long a working life as possible.

Connections between carbon steel and copper alloys or between carbon steel and stainless steel are especially prone to galvanic corrosion and considerable effort has been expended in the past with a view to reducing the extent of galvanic corrosion in such connections. It is, for example, common practice to paint a carbon steel backing flange with an insulating layer of a polymer-based paint, such as for example, polyamide or epoxy resin paint. Such paints provide adequate protection while the paint coating is a continuous layer but the paints themselves are bittle and can chip off during assembly of the pipework.

When this occurs, intense localised galvanic corrosion sets in very rapidly in the exposed portions of the backing flange.

This invention provides a method of protecting a connection between two metal parts, which may be of similar or dissimilar metals, which connection is formed between flanged surfaces on the two parts and which is clamped together by at least one backing flange formed of a metal of different electrode potential from the metal of one of the parts in the connection which it would normally contact in making the connection, which comprises interposing between the backing flange and the metal of different electrode potential an annular insulating collar of substantially Lshaped cross section of dimensions sufficient to prevent any contact between the backing flange and said metal, said collar being formed of a polymeric material capable of withstanding the compressive forces in the connection and of isolating the two metals thereby to prevent galvanic corrosion in the connection.

The collar is peferably formed of a high temperature engineering thermo-plastic material having a high resistance to creep when compressed in tightening up the joint. The material of the collar preferably has a thickness within the range of from .1 to 2.5 millimetres, especially .25 to 1.5mm.

The thermo-plastics material preferably has a continuous working temperature resistance of at least 90 C, more preferably at least 1 20 C and especially at least 1 50 C and should preferably be able to withstand a compressive force of at least 200 Kg/cm2 without substantial creep. It is moreover preferably a non-crystalline material.

As examples of materials which have been found to be especially suitable for use in the process because of their high compressive strength, their resistance to attack by water, especially hot water, steam and seawater and chemicals such as acids and bases, alcohols, oils and greases and other aliphatic hydrocarbons, their high temperature resistance and their low flammability with low smoke and toxic gas evolution, and especially their high dielectric strength there may be mentioned polyether sulphones such as those sold under the trade mark VICTREX, polysulphones such as those sold under the trade mark UDEL, polyetherimides, such as those sold under the trade mark ULTEM, and polyphenylene sulfide.

The collar may be made in one piece and may be slit axially to allow it to be distorted to be placed over a pipe or it may be formed in two or more pieces. When the collar is made in two or more pieces, the pieces are preferably made to form a lap joint to ensure complete isolation of the two dissimilar metals from one another and preferably in the region of the lap joint the material of the collar is thinner so that the thickness of the joint is not substantially greater than the thickness of the remainder of the collar. If desired, the portion of the collar that is compressed between the two flanges may be thicker than the portion that lies along the pipe and which is not subjected to a compressive force. This allows the backing flange to be a closer fit around the pipe.

The collar may be made, for example, by vacuum forming injection moulding or any other suitable forming technique.

By way of example two forms of connection according to the invention will now be described in greater detail with reference to the drawings.

Figure 1 shows a connection between two copper alloy or stainless steel pipes (1) each having a radial end flange (2). The end flanges (2) abut with an interposed conventional sealing gasket (6).

The connection is clamped together by compressing the end flanges (2) between annular backing flanges (3) made of carbon steel, the backing flanges being held together by bolts (4). In a convetional connection, the backing flanges (3) would act directly on the flanges (2) thereby giving rise to the possibility of galvanic corrosion of the backing flange (3) in the area of contact.

In accordance with the invention, a substantially L-shaped annular collar (5) is placed round each pipe (1) so that it abuts the flange (2) and extends a short distance along the length of the pipe (1). The collar (5) is made of high temperature engineering thermoplastic and has a thickness between 0.25 and 0.5mm depending on the diameter of the pipes to be jointed, the greater thickness being applicable to larger sizes of pipes.

The L-shaped collar (5) prevents any contact between the carbon steel of the backing flanges (3) and the pipe (1) and provides a very satisfactory bar to galvanic corrosion.

Figure 2 shows a connection between a copper alloy or stainless steel pipe (11) having a flanged end (12) and a carbon steel pipe (13). The carbon steel pipe (13) has a carbon steel flanged end (14) of greater diameter than the flange (12) on the pipe (11) and is drilled to receive clamping bolts (16). The joint is formed by clamping the flange (12) of the pipe (11) against the flange (14) of the carbon steel pipe using a backing flange (15) and bolts (16). Because the flanges (12) and (14) are of dissimilar metals capable of setting up an electric couple it is essential in this case to provide an insulating gasket (17) between the flanges (12) and (14). An L-shaped collar (18) is interposed between the backing flange (15) and the flange (12) on the pipe (11) in the same manner as described in connection with Fig. 1.

Claims

1. A method of protecting a connection between two metal parts, which may be of similar or dissimilar metals, which connection is formed between flanged surfaces on the two parts and which is clamped together by at least one backing flange formed of a metal of different electrode potential from the metal of one of the parts in the connection which it would normally contact in making the connection, which comprises interposing between the backing flange and the metal of different electrode potential an annular insulating collar of substantially L-shaped cross section having axial and radial dimensions sufficient to prevent any contact between the backing flange and said metal and a material thickness within the range of 0.1 to 2.5mm, said collar being formed of a non-crystalline, high temperature engineering thermo-plastics material having a continuous working temperature of at least 90 C and being capable of withstanding the compressive forces in the connection without substantial creep and of isolating the two metals, thereby to prevent galvanic corrosion in the connection.

2. A method according to Claim 1, wherein the material of the collar has a thickness within the range of from .25 to 1 .5mm.

3. A method according to Claim 1 or Claim 2, wherein the thermo-plastics material has a continuous working temperature resistance of at least 1 20"C.

4. A method according to Claim 3, wherein the thermo-plastics material has a continuous working temperature resistance of at least 150"C.

5. A method according to any one of Claims 1 to 4, wherein the thermo-plastics material is able to withstand a compressive force of at least 200 Kg/cm2 without substantial creep.

6. A method according to any one of Claims to, wherein the thermo-plastics material is a polyether sulphone, a polysulphone, a polyetherimide or a polyphenylene sulfide.