Notes on grinding out cracks in the area of pipe connections
The employees of IBL and LES GmbH are very often asked by operators of power plants and chemical plants what should be done if cracks appear in the vicinity of pipe connections. Given the immense costs associated with downtime, they almost always seek solutions that can be implemented quickly. Even though detailed assessments – which may be more or less conservative depending on the size and depth of the crack – or detailed (FEM) analyses may be required to evaluate the strength behaviour, and depending on the results, complex reinforcements or even a costly replacement might become necessary (which can be carried out by IBL and LES GmbH), there is a surprisingly effective measure that provides immediate relief and significantly improves the (fatigue) strength behaviour considerably – the targeted grinding out of the crack root.
This blog post was written in response to requests from a number of customers. The description has been deliberately simplified so that it remains easy for production engineers to understand. I would ask structural engineers to bear with me regarding the simplifications necessary for technical clarity.”
Whether you decide in favour of or against grinding, please contact us so that we can let you know what information we require to carry out strength analyses – in particular (creep) fatigue analyses – to determine maximum permissible operating times and load cycles, in order to establish the remaining service life and the associated inspection intervals.
Why crack reduce fatigue strength so significantly
Nozzles are usually among the most heavily stressed components in pressure-bearing systems. They combine local notch effects, welded joints, unfavourable stress gradients and frequently changing loads.
Particularly at risk are:
• necked-out nozzles (necked-out sections)
• weld seam transitions between the shell and the nozzle
• internal edges with an insufficient radius
• nozzle subject to eccentric loads or bending moments
The combination of notch effect, tensile stresses in the weld seam and cyclic loading makes these areas classic hotspots for fatigue cracks.
If cracks develop here – whether due to cyclic loading, corrosion or weld imperfections – the strength, particularly the fatigue strength, decreases dramatically.
Why?
Cracks act like extreme notches.
Even small cracks measuring just a few tenths of a millimetre can:
• increase local stress by a factor of 3–10 Cracks act like extreme notches.
• reduce the remaining service life by orders of magnitude due to the logarithmic relationship between notch stress and number of load cycles shown in the ‘Wöhler diagram’
• accelerate crack growth with every load cycle, ultimately leading to
• unstable crack propagation when pressure peaks occur
Grinding out cracks – the underestimated service life booster
Grinding out cracks is one of the most effective and, at the same time, most cost-effective methods for improving fatigue strength.
What happens during grinding out?
During grinding out:
• the crack root is completely removed
• the notch effect is reduced
• a harmonious stress distribution is restored
• the surface is made smooth and low in stress
This reshapes the critical area so that it behaves once again like a virtually crack-free component.
The main advantages of grinding
- Immediate elimination of the crack root
As long as a crack exists, it will continue to grow under cyclic loading.
Grinding completely removes the crack root – the most important prerequisite for reducing crack growth and thus improving fatigue strength. - Reduced notch effect through optimised geometry
A cleanly ground transition with a defined radius:
• reduces notch stress
• reduces local stress peaks
• improves load distribution
This leads to a massive increase in the permissible number of load cycles. - Restoration or improvement of weld quality
Often, the grinding process simultaneously:
• removes weld bead excess
• eliminates weld defects or porosity
• smooths transitions
The result may even be a higher-quality weld than in its original condition. - More cost-effective than replacement or reinforcement
Compared to:
• manufacturing new nozzles
• welded-on reinforcements
• complete vessel repairs
grinding is significantly faster, cheaper and less invasive. - Ideal for combining with NDT and service life assessment
Following grinding, non-destructive tests such as:
• PT (Dye Penetrant Testing)
• MT (Magnetic Particle Testing)
• UT (Ultrasonic Testing)
can be carried out to confirm the absence of cracks.
Subsequently, staff from IBL or LES GmbH could reassess the remaining service life in accordance with ASME VIII/2, EN 13445, PD5500 or AD 2000.
When is grinding particularly advisable?
• Cracks < 3–5 mm deep
• Cracks in weld bead ridges
• Notch-sensitive transitions at throated nozzles
• Fatigue cracks caused by vibrational stress
• Cracks with no signs of hydrogen-induced damage.
In many cases, the service life can be increased by a factor of 5–20 following grinding.
Limitations of grinding
Of course, grinding is not always sufficient.
It is not suitable for:
• deep, branched cracks
• cracks caused by corrosion (e.g. SCC)
• cracks in highly stressed compression zones without sufficient wall thickness
• materials with low toughness
In these cases, additional measures such as build-up welding, sleeves or nozzle replacement are necessary.
Conclusion: Grinding is an immediately viable way
of improving remaining service life
For operators of pressure vessels, pipework and plant, grinding is a technically founded, cost-effective and rapidly implementable method of improving the fatigue strength of nozzles with cracks.
It combines:
• Safety
• Cost-effectiveness
• Compliance with standards
• Demonstrable increase in service life
and is therefore an indispensable tool in the maintenance strategy for modern plants.
Whether you decide in favour of or against grinding, please contact us so that we can inform you of the information we require to carry out strength analyses – in particular (creep) fatigue analyses – to determine maximum permissible operating times and load cycles, in order to establish the remaining service life and the associated inspection intervals.