GB2289108A - Piping - Google Patents

Abstract

Plastics pressure piping comprises at least two layers of plastics material, of different composition and/or properties, such that a first layer provides the main pressure-bearing ability, and a second layer provides resistance to fracture. In one form there are three layers of which the intermediate layer is a pressure-resistant layer, between inner and outer layers which provide mechanical protection for the pressure-resistant layer, and resist the generation and propagation of cracks. Alternatively there are three layers of which the intermediate layer provides resistance to rapid crack propagation, and inner and outer layers provide resistance to impact damage and to slow crack growth. The outer layer is tough enough to allow the attachment to it of means which may apply a tension load to the pipe, for example a grab ring of a pipe fitting.

Description

PIPING This invention relates to plastics piping and has particular but not exclusive reference to pressure piping capable of being used for example for mains water supply and distribution.

Traditionally, cast iron pipes were used for pressure piping, but these were difficult to handle, heavy and brittle. More recently, plastics pressure piping has been used, particularly in smaller diameters. Possible materials include polyethylene, polypropylene and PVC-U.

Conventional PVC-U materials have the positive attributes of high strength, high fracture toughness, high stiffness and low weight. However, despite and because of these attributes, there are certain product properties that are not ideal. They are:i. Susceptibility to failure from point loads due to imperfect bedding practices particularly in larger sizes.

ii. Susceptibility to impact damage during handling particularly at lower temperatures ( < 0 C).

iii. Tendency to catastrophic failure (i.e. the complete pipe fractures when there is a failure).

Existing PVC-U pressure pipe systems are connected through push-fit joints which in general are not capable of supporting substantial end load forces. Within the pipe industry in general, there are many push fit joints which are end load capable. A common feature is a grab ring with teeth which are hard compared to the pipe material.

PVC-U is considered too brittle for such a grab ring mechanism to be applied. The indentations and loads caused by a grab ring are liable to act as stress raisers and initiate failure of the pipe material.

According to the present invention piping, suitable for use as pressure piping, comprises at least two layers of plastics material, of different composition and/or properties, such that a first layer provides the main pressure-bearing ability, and a second layer provides resistance to fracture.

The piping may comprise only two layers in which case the preferred arrangement is that the inner layer provides pressure strength and the outer layer provides crack resistance, mechanical protection, and the ability to absorb a tensile end load, for example from a grab ring.

There is preferably a distinct discontinuity between the layers. Such a discontinuity inhibits the propagation of cracks from one layer to another, this reduces the risk of catastrophic failure even if a crack is initiated in one of the layers.

The pressure-resistant layer should have high strength and ductility.

In one suitable construction the pipe has three layers, of which an intermediate layer provides resistance to rapid crack propagation, inner and outer layers provide resistance to impact damage and to slow crack growth, and the outer layer is tough enough to allow the attachment to it of means which may apply a tension load to the pipe, for example a grab ring of a pipe fitting.

In a preferred construction the piping has three (or more) layers of which the or an intermediate layer is a pressure-resistant layer, between inner and outer layers which provide protection (in particular mechanical protection) for the pressure-resistant layer, and resist the generation and propagation of cracks.

In a preferred construction, the pipe consists of three layers with relative thickness as follows:a. An inner layer typically between 0% to 30% of the total thickness.

b. A central layer typically between 60% to 90% of the total thickness.

c. An outer layer typically between 0% to 30% ofthe total thickness.

These thicknesses are for illustration only and the actual values will depend upon the particular balance of properties required in any pipe design.

The inner and outer layers may be of the same material or can be of different materials.

The three layers, in combination, provide improved performance related to the areas listed previously. In one preferred construction the pipe functions as follows:a. The central layer provides the main pressure bearing capability; the inner and outer layers make a limited contribution to this, reflecting the thickness and material properties.

b. The inner layer has three functions as follows: (i) improves the resistance to slow crack propagation - as slow crack propagation usually initiates on or close to the inside bore of the pipe.

(ii) improves the impact strength-impact failures usually initiate on the bore of the pipe, particularly at lower temperatures (OOC).

(iii) contributes to fast fracture resistance in the ultimate case of the propagation of a crack.

c. The outer layer also has three functions as follows: (i) to provide a tough outer layer through which a grab ring or other end load capable joint can function.

(il) as with the inner layer, contributes to fast fracture resistance in the case of catastrophic crack propagation.

(iii) resists external surface damage during the distribution, handling and installation ofthe pipes.

The physical attributes ofthe three layers differ substantially, reflecting the particular functions the layers perform. A typical application for a water pressure pipe for the UK market is as follows:a. The central layer is composed of a PVC blend incorporating 6phr of CPE modifier. Typically such a material would have a long term (50 yrs) stress of > 23MPa, a notched Charpy level of 5-10 KJ/m2 with a brittle/ductile temperature transition of 150C. The material will show sufficiently good resistance to slow crack propagation that a design safety factor of 1.3 can be applied.

b. The inner and outer layers are modified PVC, incorporating 30 + 10 phr of ABS modifier. A particularly suitable modifier is available commercially under the trade name Baymod A50 from Bayer AG. Typically, such a material would have a short term failure stress ofl30MPa, with a long term failure stress (50 yrs) of < ISMPa. The toughness measured in a notched Charpy mode is 70KJ/m2 with a brittle/ductile transition temperature of-100C. An important feature is that the material maintains good toughness at OOC, preferably 40/50 KJ/m2 or better. The outer layer must also show a sufficiently high strength and thickness to allow a grab ring mechanism to function.

The above structure is for example only and is dependent upon the particular balance of properties required. The structure may change with pipe diameter and pressure class of the pipe.

As mentioned previously, there is another preferred construction in which the intermediate layer provides the resistance to rapid crack propagation. A typical application for a water pressure pipe for the UK. market is as follows: (A) A central layer incorporating 30+1- 10 phr of ABS modifier, (trade name Baymod A50, supplied by Bayer). Typically, such material would have a short term failure stress of greater than 30 MPa with long term failure stress, (50 years), of greater than 50 MPa. The toughness measured in a Charpy mode is 70 KJ/m2, with a brittle/ductile transition temperature of-100C. Importantly, the material maintains a good level oftoughness at OOC, typically 40/50 KJ/m2.

(B) The inner and outer layers are modified PVC, incorporating 6 phr. of CPC modifier. Typically, such a material would have a long term, (50 years), stress of greater than 23 MPa, a notched Charpy toughness level of 5 - 10 KJ/m2, with a brittle/ductile transition temperature of 15"C. The material will show sufficiently good resistance to slow crack propagation that a design safety factory of 1.3 can be applied.

The particular construction will depend upon the size of pipe, (diameter, wall), and will be chosen for ease of processing and upon the type/construction ofthe gripping mechanism of the end load joint.

Other materials which may be used in a three layer structure are as follows: (a) Central Laver i. PVC-U with a design strength (50 yrs) of > 26MPa.

ii PVC-T; blends of PVC incorporating a range oftoughening agents, with a long term strength (50 yrs) in the range 15-23MPa.

iii CPVC.

iv The range of rigid thermoplastics shown in Table I.

v. The range of alloys/blends/co-polymers shown in Table II.

(b) Outcr/lnncr Lovers The toughness required ofthe inner/outer layers may be achieved in many ways; a list of possibilities is included in Table m.

Table I Rigid Thermoplastic/Blends PVC-U ABS ASA SAN PMMA CPVC POLY CARBONATE Table II Polvmer Alloys/Blends/Co-Polymers PVC/ABS PVC/ASA PVC/SAN PVC/MBS PVC/NBR PVC/CPE PVC/VA PVC/EVA PVC/PV PC/ABS PC/ASA PVC/ABS/MBS PVC/MMA PVC/PMMA Table fit Materials Potentially Sultable for Inner/Outer Lavers PVC/ABS PVC/MBS PVC/ASA PVC/SAN PVC/NBR PVC/CPE PVC/VA PVC/EVA PVC/PV PC/ABS PC/ASA PVC/ABS/NBR PVC/MMA PVC/PMMA The piping is preferably pre-fitted with seals at the factory. Jointing is preferably by means of integral push-fit spigot and socket joints, for example similar to the wellknown Loc-Ring (trade mark) joint.

The jointing system preferably includes means for end load retention, such as a grab ring, arranged to engage primarily the outer layer ofthe pipe. This avoids any need for welding or adhesive jointing of the pipes. Provided that the materials and pipe section have been suitably designed, end loads can be absorbed without any significant risk of crack propagation.

Claims (8)

CLAIMS:

1. Plastics pressure piping which comprises at least two layers of plastics material, of different composition and/or properties, such that a first layer provides the main pressure-bearing ability, and a second layer provides resistance to fracture.

2. Piping as claimed in claim 1 comprising only two said layers in which inner layer provides pressure strength and the outer layer provides crack resistance, mechanical protection, and the ability to absorb a tensile end load.

3. Piping as claimed in claim 1 comprising three said layers of which the or an intermediate layer is a pressure-resistant layer, between inner and outer layers which provide protection (in particular mechanical protection) for the pressure-resistant layer, and resist the generation and propagation of cracks.

4. Piping as claimed in claim 1 comprising three said layers of which an intermediate layer provides resistance to rapid crack propagation, inner and outer layers provide resistance to impact damage and to slow crack growth, and the outer layer is tough enough to allow the attachment to it of means which may apply a tension load to the pipe, for example a grab ring of a pipe fitting.

5. Piping as claimed in claim 3 or 4 in which the pipe consists of three layers of which the intermediate layer is 60-90% ofthe total pipe wall thickness and the inner and outer layers are each not more than 30% ofthe total pipe wall thickness.

6. Piping as claimed in claim 5 in which the intermediate layer is a PVC blend incorporating 6 phr of CPE modifier, and the inner and outer layers are modified PVC incorporating 30 + 10 phr of ABS modifier.

7. Piping as claimed in claim 5 in which the intermediate layer is modified PVC incorporating 30 t 10 phr of ABS modifier, and the inner and outer layers are modified PVC incorporating 6 phr of CPE modifier.

8. Piping as claimed in any preceding claim in which there is a distinct discontinuity between adjacent layers.