To ensure cylinders continue their safe use, there are a number of regulations which demand that a periodic examination or test has to be performed, with a satisfactory outcome, by the cylinder’s owner.
The frequency of the test is a variable from one set of regulations to another and even varies within a particular regulation. The test interval depends primarily on the gas contained and the cylinder material (or materials in the case of composites – see Part 4), though a number of other risk factors, often covered by the owner’s quality system, are also relevant (see Test Intervals below). To support the regulations there are both national and international standards which give details of the work which has to be undertaken. The most widely used of the International Standards is probably ISO 6406 revised in 2006, while EN 1968 is also in common use.
It will be very difficult to cover all the various combinations of tests that are required for a successful retest programme. The much specialised area of acetylene cylinders is not covered here and will be the subject of a separate article. What follows is a useful summary of some key aspects and the way events are likely to progress in future. I will be pleased to expand any points in this complex field, which the readers may have.
The traditional test
The periodic inspection, often referred to as a retest, or a requalification, has been performed for over a century. It has key components which all need to be performed and passed. The work relies heavily on the competence of the inspector since some aspects can be subjective. The subjectivity is reduced and a more objective outcome can result with increasing experience on the part of the inspector. The key components which need to be covered for seamless and composite
a) Internal and external inspection
b) A pressure test
c) Check on neck threads
d) Check on cylinder mass
e) Check of fitted accessories.
However, before any of the activities are undertaken, it is essential that the valve fitted to the cylinder is removed in a safe manner. When the latter procedure is not meticulously followed serious accidents have occurred, even fatalities in some cases. To help operators there are specific instructions in various standards such as ISO 6406. A vital aspect included is ‘the whisper test’ whereby nitrogen at around 5 bar is forced through the valve and ‘heard’ leaking back out through the valve.
a.) In a little more detail, the internal and external inspections essentially are looking for physical or corrosion defects. To help the inspector it is often necessary to shot blast the internal/external surfaces (though other less onerous techniques are needed for cleaning aluminium alloy cylinders on account of their low strength). Detailed data are provided in the standards to identify unsafe combinations of defects categorised by size, extent and distribution. Nevertheless it is always a difficult task to assess unsafe defects when one considers that the view is through a small opening (usually 25mm) and the defect is 1.5m away (in the case of 50 litre water capacity cylinders). Hence even skilled inspectors are trained with the slogan: “If in doubt, throw it out.” The result of this conservatism, which is the correct approach, is that the internal inspection is the highest source of failures, accounting for around 90% of all rejections.
b.) The pressure test is usually a hydraulic test performed at the cylinder’s test pressure. There are two main variants of the hydraulic test, proof pressure test and the volumetric expansion test. During the former, the cylinder is pressurised to its test pressure and the externals of the cylinder are scrutinised during the ‘hold period’, typically 30-60 seconds, for any leakage or even a burst (in rare cases) or a drop on the recording pressure gauge. Meanwhile those organisations conducting the volumetric expansion test, measure the permanent expansion suffered by the cylinder after taking the cylinder to its test pressure, completing the hold period and depressurising the cylinder.
c d & e.) These 3 operations are not always obligatory though in most cases necessary to perform. Weighing for loss of cylinder weight, as an indirect means of measuring corrosion, is now only needed for liquefied gas cylinders. Valve thread checks are often a visual examination in some standards, while gauging is required in others to ensure that the threads are still worthy for a further term of service. All fitted accessories such as guards and foot rings are also checked to ensure that they are integral and not damaged.
The twenty-first century option
In recent years regulators and cylinder users alike have resorted to more meaningful and productive tests. Some of the change is driven by the users of gases who are now demanding ever cleaner products. For example, gas users in the electronics industry require parts per billion of a minority gas in the carrier product. So it makes no sense to introduce water into a cylinder, as is the case for the hydrotest, and then expend a lot of energy drying the cylinder’s internals.
Regulators have also promoted tests which are more reliable in predicting cylinder failure than the hydraulic test which essentially is a go/no-go test: the cylinder either leaks and bursts or not. Not getting a leak or burst, does not ensure that the cylinder is safe.
However it has been noted on several occasions that a recently hydro-tested cylinder has ruptured on the filling line. This happens when a large flaw harboured within a cylinder is extended by taking the cylinder to its test pressure. So the hydro-test has in fact rendered the cylinder more unsafe than it was before the test.
Enter the era of a truly non-destructive test. Of the various options available, by far and away the most meaningful and productive test is ultrasonic examination (UE). With its roots in the 1970’s following some serious accidents involving stress-corrosion cracking of steel cylinders in carbon monoxide service, UE has been championed for cylinder applications by the Swiss authorities. Acknowledged by Switzerland in 1990 and then by International legislation in the form of ADR / RID in 1997, UE is now accepted as an alternative to the hydraulic pressure test. A typical modern rig is shown in Fig. 1.
However, organisations need to seek approval from their competent authority to make the substitution. This has not been a problem in countries such as Germany, United Kingdom, Belgium, Switzerland etc, where the competent authorities have granted approval, in some cases, for over a decade.
UE relies on comparing the size of a defect in a cylinder against artificial flaws introduced into a calibration test piece. In the case of ISO 6406 it is possible to reject a cylinder which has a flaw greater than 5% of the design wall thickness. During a UE, water, which acts as a coupling agent, is dribbled onto the external surface of the cylinder which rests on a bed of rollers (see Fig.2).
With the cylinder rotating on its axis, typically 5 probes (4 for detecting flaws and a fifth for wall thickness) traverse the entire parallel length of the cylinder. The helix thus formed ensures that every point along the length of the cylinder is examined. Cylinders as short as 100mm and with diameters up to 325mm can be tested using an automatic system. The workhorse 50 litre type cylinder, commonly used in the industrial gases sector, can be tested in around 30 seconds.
Greater safety, high productivity and maintaining internal purity are major considerations currently converting many users to UE. In fact it is now permitted to UE a cylinder with the valve in-situ and up to 5 bar gas retained within the cylinder. This ensures that integrity of the internal cleanliness is fully maintained.
Mention must also be made of acoustic emission testing (AET). However, to detect defects this technique relies on the presence of moving flaws, unlike in UE where all flaws (greater than 5% of wall) are detected. Also AET is unable to measure the cylinder’s wall thickness. Nevertheless, progress is being made here and AET’s strength lies in the fact that jumbo tubes mounted onto trailers need not be dismantled in order to test them. Watch this space!!
Test intervals for various gas applications and cylinder materials are stipulated in the regulations. The latter state the maximum permitted periods while the supporting standards identify operational criteria which legitimise the maxima. These criteria include ensuring the gas contained is free from moisture; pre-fill checks are performed to confirm the “dryness” of the cylinder; quality of the gas to be filled is checked, etc.
Whilst the maximum test period for gas cylinders in inert service is 10 years, corrosive gases are on a 5 year cycle and certain underwater cylinders can be on an even more stringent frequency. Regrettably some cylinder owners often resort to the maxima, without fulfilling the operational requirements.
Post the completion of a successful test, the cylinder needs to be stamp marked and finally painted. Here too, some critical areas to watch are the temperature and time the cylinder spends in an oven, if the paint has to be cured, particularly for aluminium alloy cylinders which can be permanently damaged if they experience excessive heat.
In this 21st century vision there is a remarkable irony that whilst an inexpensive seamless steel cylinder can be retested using an elaborate UE technique, the sophisticated composite cylinder still has to resort to a century old hydraulic test for its requalification. Still, the 21st century has only just begun and a new affordable non-destructive technique for composites will be established