process feature | Pipe inspection & testing
sulphuric acid on PVDF pipe. The pipe had been used at a Swedish steel pickling plant, and was exposed to other acids such as hydrofluoric acid. The pipes had survived for 30 years without leaks. They were all in good condition, though showed some discoloration – and some socket joints used for sulphuric acid showed signs of cracking. Swerea researchers developed a special indicator
technique to study acid diffusion into the plastic, and found that the main cause of the cracking was when changing from 96% to 98% acid concentration.
Joint proposal At this year’s Antec conference in the USA, plastics consultant Jim Goddard gave his views on a proposed new test for pipe joints, which had been proposed in a report from the National Cooperative Research Program (NCHRP). NCHRP Report 190 attempted to address the
structural performance – largely based on shear failure at the joint, he said. “This project did a good job of analysing the shear
and rotation stresses for PVC and HDPE on a limited number of joints, but there are practical issues with the recommended test,” he said. The proposed test requires the construction of a
large loading frame that can test 24-60in (300-1500mm) diameter pipe. “It is unlikely that any State DOT [Department of
Transportation] would build or purchase such a frame,” said Goddard. “It might be fabricated for approximately $20,000, but the end plates would cost appreciably more.” He added that the test frame would be 10 by 20 feet,
Ultrasonic joint testing
A new ASTM International practice provides a standardised way to examine plastic pipe joints using sound waves. The new standard (E3044/E3044M, Practice for Ultrasonic Testing of
Polyethylene Butt Fusion Joints) provides a non-destructive way to identify flaws inside the pipe’s butt-fusion joints and to assess the joint’s overall quality.
It sets minimum requirements for coverage and sensitivity of
ultrasonic testing, helping identify problems such as lack of fusion and contamination by particulates. Due to advantages such as corrosion resistance, PE piping has been
increasingly used as an alternative to steel alloys in the petrochemical, power, water, gas distribution, and mining industries. ASTM member Jason Estabrook says the standard will be used by
engineers, procurement professionals and construction managers, as well as large scale fabricators that assemble butt fusion joints in the field. It may eventually have applications with nuclear components.
22 PIPE & PROFILE EXTRUSION | October 2016
and occupy a minimum of 600 sq ft. Plumbing, gauges and sensors would add further cost. “The loading of the pipe in this test does not replicate anything that has been reported or seen in the field,” he said.
In conclusion, he said that DOTs had not reported any joint shear failures – so that a test for it “was a test proposal in search of a failure, and not the other way round”. He said adopting such as test would add unjustified costs, and may eliminate smaller pipe producers.
Cooling simulation Also at Antec, Kenny Saul of SHS Plus told delegates how the company’s Chillware simulation program can be used to diagnose the problem of sagging or collaps- ing of pipe ends. The system is based on finite difference method and
finite element method and shows the capability to calculate the cooling process, the crystallisation time and the residual stress distribution in the product. Sagging occurs due a temperature difference
between the inner and outer walls of the pipe. In the calibration zone, the outer material layer is frozen but because of the low thermal conductivity of the material, it takes a long time until the inner material layer temperature is cold enough to avoid sagging. To analyse the situation, the cooling section of an extrusion line was measured, then built up as a virtual cooling section in the computational program. The program calculates the likelihood of where sagging will occur – and suggest answers. For instance, it can reduce the time period in which sagging can occur by making changes to the cooling water tempera- ture. Also, it can assess the effect of having a system to cool the inside of the pipe. In the case of collapsing (or deformation), this is
usually caused by inhomogenous cooling, which leads to warpage. Again, cooling parameters can be adjusted in order to minimise residual stresses. In this case, cooling water temperature and the lengths of annealing sections between different spray cooling tanks can have a huge effect, said Saul. He cited one case, in which cooling water temperature was increased from 12°C to 16°C, and an additional air cooling section was added.
Click on the links for more information: ❙
www.impact-solutions.co.uk ❙
www.betterpiping.com (Jana) ❙
www.nysearch.org ❙
www.swerea.se/en ❙
www.shs-plus.de ❙
www.astm.org
www.pipeandprofile.com
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