Hydrostatic Strength of NEXOR HDPE Pipes – Test Procedure

NEXOR Pipes

June 6, 2006

ABSTRACT

The procedural standard of hydrostatic HDPE pipe test performed at NEXOR PIPES is detailed below.

Introduction

Hydrostatic pipe test measures the resistance of pipes to internal pressure. It is one of the easiest ways to assess long term performance if piping materials. The methodology performed at NEXOR PIPES, which closely follows ISO 1167, is detailed below.   

Procedure

Preparation of Test Pieces

Three test pieces are required. The length of the pipe between the end caps must be at least the length listed on table 1.

Table 1 Length of Test Pipe

PIPE OD (mm)	Length of Test Piece from end cap to end cap(mm)
20	250
25	250
32	250
40	250
50	250
63	250
75	250
90	270
110	330
125	375
140	420
160	480
180	540
200	600
225	675
250	750
280	840
315	945
355	710
400	800
450	900
500	1000
560	1120
630	1260
710	1420
800	1600
900	1800
1000	2000

Conditioning

A pipe after being extruded must be allowed to sit for a minimum period of 24 hours before performing a hydrostatic test. A conditioning period must also be followed to ensure that the pipe walls achieve thermal equilibrium. Conditioning is done by placing the pipe inside the test bath at the specified test temperature for a period indicated on table 2.

Table 2 Conditioning Period

Thickness (mm)	Conditioning Period (h)
<3	1
>3 and <8	3
>8 and <16	6
>16 and <32	10
>32	16

Calculation of Test Pressure

The test pressure is calculated based on the design stress of the pipe material and using the following formula.

P = 10σ(2)/(SDR-1)…………………………..1

P = test pressure in bar

σ = is the stress to be induced by the applied pressure, in MPa.

SDR = standard dimension ratio of the pipe

The design stress is based on the Minimum Required Strength (MRS) at 50 years and 20 ⁰C and the design coefficient.

PE 80 has an MRS of 8 MPa and the minimum design coefficient is 1.25; both of these lead to a maximum design stress of 6.3 MPa.

Using a design stress of 6.3 MPa and formula 1, table 3 lists some of the test pressures at various SDRs.

Table 3 Sample test pressures for PE 80 with a Design Stress of 6.3 MPa

SDR	Test Pressure (psi)
9	228 +/- 1
11	183 +/- 1
13.6	145 +/- 1
17	114 +/- 1

Application of Test Pressure

Pressure is applied in a steady incremental manner at the rate of 30 psi per minute.

Measuring Hydrostatic Strength according to ISO

Following ISO 4427 and ISO 1167, the three test pieces are to be subjected to different test conditions. Each are listed on table 4 for PE100 and PE 80.

Table 4 Three different Test Conditions - Test Stress, Temperature, and Time

Pipe Material	Test Stress (MPa) of Condition 1 (100 h, 200C)	Test Stress (MPa) of Condition 2 (165 h, 800C)	Test Stress (MPa) of Condition 3 (1000 h, 800C)
PE 100	12.4	5.5	5.0
PE 80	9.0	4.6	4.0

Based on the test stresses, the following test pressures are to be applied for a given SDR. The values given in tables 5 to 8 have a tolerance of +/- 1 psi.

Table 5 Test Pressures for SDR 9

Pipe Material	Test Pressure (psi) of Condition 1 (100 h, 200C)	Test Pressure (psi) of Condition 2 (165 h, 800C)	Test Stress (psi) of Condition 3 (1000 h, 800C)
PE 100	450	199	181
PE 80	326	167	145

Table 6 Test Pressures for SDR 11

Pipe Material	Test Pressure (psi) of Condition 1 (100 h, 200C)	Test Pressure (psi) of Condition 2 (165 h, 800C)	Test Stress (psi) of Condition 3 (1000 h, 800C)
PE 100	360	159	145
PE 80	261	133	116

Table 7 Test Pressures for SDR 13.6

Pipe Material	Test Pressure (psi) of Condition 1 (100 h, 200C)	Test Pressure (psi) of Condition 2 (165 h, 800C)	Test Stress (psi) of Condition 3 (1000 h, 800C)
PE 100	285	126	115
PE 80	207	105	92

Table 8 Test Pressures for SDR 17

Pipe Material	Test Pressure (psi) of Condition 1 (100 h, 200C)	Test Pressure (psi) of Condition 2 (165 h, 800C)	Test Stress (psi) of Condition 3 (1000 h, 800C)
PE 100	225	99	90
PE 80	163	83	72

Pipe Failure

In case of pipe failure, the mode of failure is recorded and a picture of the pipe is taken.

A failure is brittle if there is no visible yield deformation.

If a break occurs ain the test piece at a distance of less than 10 % of the length of the test piece from the end cap, the results must be disregarded.

Test Reporting

Please see attached sample report.

References

ISO 1167 -1 Thermoplastics pipes, fittings, and assemblies for the conveyance of fluids – Determination of the resistance to internal pressure. Part 1 : General Method.

ISO 1167 -2 Thermoplastics pipes, fittings, and assemblies for the conveyance of fluids – Determination of the resistance to internal pressure. Part 2 : Preparation of Pipe Test Pieces.

ISO 4427 Polyethylene Pipes for water supply - Specifications

Chasis, David. Plastic Piping Systems. 2^nd Ed. Industrial Press Inc. USA. 1988

Willoughby, David et al. Plastic Piping Handbook. Mcgraw-Hill Handbooks. USA. 2002