Proof ring selection, calibration and operation basics
Proof rings are very simplified constant load devices. They are used in various stress corrosion cracking (SCC) applications but especially as quality assurance tools for sulfide stress cracking (SSC) in pipeline and oil & gas industry related applications. Below I write a few lines about my experience related to the proof ring selection and calibration.
Cormet applies NACE TM0177-2016 as a reference for the proof ring design. TM0177 defines the minimum and maximum load levels of a proof ring design.
The applied load must produce a ring deflection more than 0.6% of the ring diameter but no less than 0.51 mm. I guess that the testing standard writer has been concerned about very low proof ring deflection that easily create high errors.
Obviously, the proof ring should not ever deform plastically in the allowed load range. Cormet’s proof ring geometry has been modelled using FEM. TM0177 checks the elastic operation as every proof ring must be preconditioned with 110% maximum allowed load at least ten times before the calibration.
A proof ring with a testing cell, a heating coil, a temperature sensor, gas inlet / outlet and drain valves. A micrometer is attached on the top of the proof ring. A timer box is on the right.
TM0177 defines that the “Loading device shall be constructed and maintained to minimize bending and torsional loads”. This is a self-evident and legitimate requirement, but not easy to realise if the load is applied using a conventional nut that rotates the specimen holder nut when tightened. Cormet uses a special multi-jackbolt tensioning systems. These tensioning systems are easy to use and most importantly, they do not create any torsional loads.
A proof ring calibration is performed using an external loading device that deflects the proof ring. The deflection is read using a digital micrometre attached to the proof ring. The applied load is read using the load cell of the loading device. Obviously, both the digital micrometre and the load measurement system must be traceable calibrated. Cormet’s engineer takes the measurement data to Excel and creates a calibration chart with a linear fit and R2 value.
A calibration certificate for a proof ring showing the line, the linear regression equation and numeric dependence for certain load - deflection pairs.
Normally, constant load tests are performed in a load range within 70% - 90% of the yield stress. First define the applied stress and calculate the corresponding load as you know the specimen diameter. Install the specimen, fill the testing cell following all the TM0177 rules and create the needed chemical environment. As you stress the specimen, use the calibration chart to define the deflection that corresponds the applied load. Install the micrometer on the proof ring and apply the deflection using the multi-jackbolt nut. Easy!
Alternatively, one could instrument a proof ring with a load measurement system: a load cell and a load cell amplifier. One would not have to mind the calibration chart, but monitor the load amplifier display during the tightening of the proof ring multi-jackbolt nut. Cormet is using both single-channel load cell amplifiers as well as portable multichannel instruments. It is important that each load cell and amplifier (channel) form one unit that has been calibrated together.
The most recent TM0177 version defines that the applied load, i.e. indirectly the deflection, must be monitored during the test or after the test. The proof ring deflection readjustment is not required. The deflection testing during the test would require a continuous deflection measurement. A digital micrometre would be an optimal, but an expensive choice if several proof rings are operated. Fortunately, there are analogue dial gauges in the market with reasonable prices. The load deflection monitoring after the test is relevant only if the specimen has not failed.
The proof ring testing results are common with all the constant load tests. Either the specimen fails or not, when immersed to the environment and stressed. If the specimen fails, one has to record the Time-To-Failure (TTF). Conventionally, a proof ring has a microswitch that switches off a timer as the proof ring relaxes as a result of the specimen failure. If a continuous load measurement is available, the TTF is indicated by the load drop.
Personally, I have an ambiguous relationship with proof rings. As a scientist, I consider a proof ring as an impoverished tool that can do only one task and even that not very well because it is difficult to maintain the initial stress. On the other hand, as an engineer I appreciate the simple and inexpensive proof ring design and operation. Maybe proof ring is not the best solution for SCC/SSC R&D work but very suitable for massive quality assurance tests in the pipeline factories.