Some industrial processes involve the use of high pressure, i.e. the use of equipment that allows you to work at pressures higher than those of the environment. These procedures subject the materials to particular stresses that can compromise their mechanical characteristics, and often imply dangers for the user. For this reason, it is necessary to pay particular attention when carrying out processes with high pressure, and ensure compliance with the regulations that impose stringent restrictions for each sector of use.
High-pressure testing systems are a useful aid for those who wish to work safely, and allow the verification of components to ensure their suitability for working at high pressures, without suffering damage that would affect the functionality of the system itself.
Safety regulations are very stringent and updated on a regular basis, and require periodic checks on the entire system and its components to verify the total efficiency of the process, high levels of safety, and contamination-free work cycles.
The sectors of use vary widely, from petrochemicals and pharmaceuticals to naval and food.
Ideally we speak of all industrial fields where machinery requires testing, as:
- it involves direct or indirect contact with people (such as refilling cylinders, airbag testing, and the treatment of food containers, oil&gas tubes and plants),
- it must be used in an aseptic or high-risk environment (such as product R&D, food production, chemical-pharmaceutical processes, accumulator loading, gas transfer, and high pressure testing of valves, fittings, pipes, cylinders and tanks).
What is a high-pressure test?
Effectively, two methods exist to check the effects of pressure on a component: hydrostatic and pneumatic tests.
Hydrostatic Tests
A hydrostatic test is a set of procedures that uses a fluid to exert pressure on a system or component at higher levels than it normally operates, in order to determine the limits of the component itself and verify parameters such as reliability, maximum capacity, losses, maximum allowable pressure, and expansion.
To give a practical example, this kind of test is required before returning a plant to operation that has undergone maintenance operations.
To perform a hydrostatic test, it is necessary to fill the component with fluid, ensuring that the air present inside is eliminated, and then bring it to a pressure up to one and a half times higher than that for which it was designed. The pressure is then maintained for a certain period of time to check for any leaks of liquid.
When it is not advisable to perform hydrostatic tests
This type of test is not indicated if contact with water can be dangerous, such as due to problems related to corrosion (if the material used is not stainless steel, it is not possible to use water). In these cases, pneumatic tests with inert gases are recommended (such as nitrogen). Other times, it is legislation itself that prohibits the use of water, or the future use of components is not compatible with the use of water, as in the case of hydrogen (read our case history on our hydrogen compression system for 8 and 12 Nmc/h at a pressure of 1000 bar). It is for this reason that high-pressure tests must also be performed with gas. For tests with gas required by law, the gas used must be the process gas (e.g. hydrogen-methane), and not inert gas.
Pneumatic Tests
Pneumatic tests are normally considered to be more dangerous, as the amount of stored energy per unit volume of compressed air subject to testing is generally high. Consequently, a pneumatic test can be performed if the application pressure is low, or it has been ensured that the system is safe.
Types of pneumatic tests available
⇒ SEAL TESTS
They serve to identify any leaks in components brought to high pressures, such as valves or welded pipes. With the absolute drop method, the product to be tested is brought to the test pressure. After a settling period, the pressure variation over the testing period is measured. A product passes the test if the pressure drop is less than that specified by the manufacturer. For this type of test, it is also necessary to take into account the temperature, the variation of which in relation to the pressure is illustrated by the law of Gay Lussac: if the volume remains constant, the pressure increases as the temperature increases according to a linear relationship.
A particular type of leak test is performed with tracer gas (helium test), and can be used to identify the exact point where leaks occur in pipes or tanks, such as when testing heating systems, sanitary systems, aqueduct networks, underground gas lines, and fire-fighting systems.
⇒ FATIGUE TESTS
They are used to verify the behaviour of a material subjected to repeated fatigue cycles, to determine its duration and verify that it still has the mechanical characteristics at the end of the cycle that render it suitable for mounting on a specific system. These tests are often used to obtain the certifications required for the safety of materials, as in the case of tests carried out on the life cycle of hydrogen cylinders for automotive use.
To give a practical example: how many times would it be possible to refill an automotive cylinder from zero to maximum pressure? Leak tests have shown that cylinders mounted on a car would have a life cycle of 4.5 times longer than the hypothetical life of the car itself.
⇒ BURST TESTS
They serve to bring the material of a component to its breaking point (with gas up to a pressure level of 2000 bar, with liquids up to 5000 bar) to verify its behaviour.
To give an example, this type of test may be necessary to understand the tightness of PET bottles intended to contain sparkling water. If the material were to heat up, such as when exposed to sunlight, the bottles could burst: it is for this reason that the burst test includes a test up to 12 bar.
⇒ FLOW TESTS
Instead of checking the pressure drop, these tests verify the flow needed to maintain the tested product at the test pressure.ù
Performance of a high pressure test
The use of gas instead of compressed air during a high-pressure test serves to avoid contamination of the system, because gas is clean by its very nature. The use of gas also serves to avoid combustion reactions: a test is always carried out using an inert gas, most of the time with nitrogen (in some cases mixed with helium), also in cases where it is necessary to test systems normally intended for non-inert and dangerous gases. Only as a result of testing it is possible to use non-inert gases in the process.
Gases such as helium and nitrogen are indicated for use in high-pressure tests, as they are small molecules that have the ability to pass through even very small cracks in the material. Helium is a noble and inert gas, and does not bind to any molecule.
Helium test with mass spectrometer
In some cases it is not enough to know that there is a fracture and leak: it is necessary to know the exact location where the material is damaged. For this purpose, helium tests with a mass spectrometer are performed. After putting the component under pressure, it is tested with a mass spectrometer equipped with an electronic nose that detects any helium leaks. In a sense, the helium test is “the perfect test”, as it is able to detect very small fractures through which other larger molecules would not pass, thus making it impossible to detect the leak. Due to the cost of the mass spectrometer and the gas itself, this test is quite expensive, but is explicitly required in certain sectors due to its accuracy.
Usually this type of test is carried out on the joints of welded components (such as valve fittings, caps, etc.) to check their tightness. If problems are detected, it is therefore possible to fix the area where the leak is present, such as by welding. The portable spectrometer has a high cost that can be optimized in the event in which it is necessary to perform large batches of tests. A typical case is that of automotive: removing a machine from the assembly line to perform a check has a very high cost, so having a portable testing system with a mass spectrometer that allows the test to be performed on site saves time and money.
Why should a test be performed?
The information derived from high-pressure tests helps to comply with safety standards, and is a valuable aid in carrying out maintenance processes. Test results can be printed on the component, so that those who use it can have a trace of the tests performed, thus allowing the traceability of tests along the entire supply chain. Data relating to project or work pressure, test pressure, test time, and temperature are usually displayed.
- When the tests are performed in the laboratory, their purpose is mainly for research aimed at determining pressure values that would lead to the destruction of the component.
- Tests for high pressures in production are also carried out to verify operation and certain parameters, such as the internal or external water tightness of a product, as in the case of leak tests.
Although it is not possible to exhaustively list the industries in which high-pressure tests are required, it can be said that any product capable of containing gases or liquids can derive important benefits from these procedures.
In many cases, regulations and their continuous updates impose the need to carry out tests in order not to incur penalties. In other cases, the tests are not explicitly requested.
Why is it always recommended to perform a high-pressure test?
An initial answer is that safety should never be neglected. However, if we wish to evaluate the purely economic aspects of the matter, it can be helpful to think about the cost of recalling materials that have not been tested, which a customer returns to us because they are not compliant. We would be forced to send those pieces back into production, wasting time and resources, risking penalties, and we would certainly face economic damage as well as damage to our image. How difficult will it be to bring the customer back to buy from us?
High pressure testing systems
For every company that needs to perform testing on mechanical components, hydraulic tests, or reload operations for accumulators, it is vital to have available systems that comply with the most up-to-date standards for pressurized components.
These units allow to perform high pressure testing of valves, fittings, tubing, cylinders, tanks etc… and can be used in hazardous environments, if provided with Atex certification.
- All Interfluid gas booster units include a pressure gauge for the incoming gas pressure and a regulator and filtration unit for the compressed air and the pump (which is ATEX certified). Gas testing systems are also fitted with a gas filter for the gas booster.
They guarantee inherent safety, since they are furnished with multiple monitoring systems:
- a safety valve for compressed air,
- pilot valves to control minimum and maximum pressure (upstream and downstream) with automatic system stop,
- a relief valve for maximum downstream pressure, and finally,
- a pressure exhaust valve.
- Units for pressure testing with liquids ready for use allow to reach a maximum pressure of 5000 bar with 10 bar of compressed air. The units are equipped with controls and tools for managing the pressure and flow downstream of the liquid pump and can be mailny used with water and oil.
Typical applications of power units
- Hydrostatic tests (leak, burst, NDT) of cylinders, valves, flexible hoses, fittings, vessels, accumulators and well head control and gas cylinders;
- Pneumatic tests (leak, NDT) performed instead of hydrostatic testing, charging of airbags, accumulators, tyres, shock absorbers, cylinders, gas suspensions and diving cylinders.
Gas Booster, the core of a high pressure unit for gas
Compatible with nearly all gases, gas boosters are the core of the high pressure test units. Using air driven and electric gas boosters it is possible to convert compressed air into high pressure for transferring and pressurizing a wide range of gases such as nitrogen, helium, CO2, argon, hydrogen, oxygen. Outlet pressure is is obtained multiplying the value of the pressure set by the ratio of the booster itself.
Once the pre-set pressure is reached, the multiplier will automatically stop and maintain the pressure until there is a pressure drop downstream. It will then start again to reach the pre-set pressure once again. Air driven gas boosters can be single acting or double acting and two stages, with various compression ratios.
Single acting gas boosters have a single high pressure section, double acting boosters have two, with the same ratio and a superior flow. For applications with high compression ratio, low inlet pressure and high downstream pressure it is advisable to use two stages gas boosters that have two high pressure sections with different ratios.
Air amplifiers
Air driven amplifiers for air and inert gases convert compressed air into high pressure (up to 700 bar).
They are also known as pressure intensifiers and work with same functional principle as the ebove mentioned Gas Boosters but without the separation between the air motor and the high-pressure gas section. This is why they cannot be used with hazardous gases.
To learn more on how to choose an air driven pump you can read our article.
Flexibility and industry 4.0
A pressure multiplier unit must be as versatile as possible in order to be functional and usable for the greatest number of tests; today it can be used for a certain type of test, but in the near future it could have applications that were not planned at the time of purchase. Therefore, the same system can often be designed to be suitable for use with different fluids (air and gas). High-pressure testing units can become even more versatile if the expertise linked to the purely mechanical part is accompanied by the flexibility offered by information technology, as
they also become customized in the dedicated software designed ad hoc for the different application needs.
The possibility of integration with embedded software, such as data acquisition systems (PLC)
- allows the testing units to save all the data (pressure and temperature) generated,
- offers the possibility to create any graphics and certificates required.
In some cases, the testing units can be equipped with remote access to the software, allowing the parameters to be viewed and modified online, thus saving time and solving any issues in a few moments and without the need to be on site.
What’s inlcuded in Industry 4.0 plans?
It is interesting to keep in mind that the current national Industry 4.0 plan” involves all aspects of the life cycle of companies that wish to acquire competitiveness, offering support in investments, the digitization of production processes, the enhancement of worker productivity, the training of adequate skills, and the development of new products and processes”.
The most current Industry 4.0 Transition Plan has further expanded the economic benefits to support the digital transformation of companies, amplifying the country’s commitment to processes that facilitate automated and interconnected industrial production. High-pressure tests thus enter the world of Industry 4.0 by right, thanks to the increasingly effective interaction with other tools that facilitate the sharing of data, with positive effects on the timeliness of the intervention.
