What is Pressure Testing
What is the purpose of pressure testing?
Want to know the basics of pressure testing? It’s always useful to start from the beginning to understand what the term means. Pressure testing explained follows.
Pressure testing or hydrostatic testing is the test which is carried out after the installation of any pipeline before it is put into use. The purpose of the pressure testing is to investigate the various limits of the pipeline which will test areas such as the reliability, maximum capacity, leaks, joint fittings and pressure. Without this information the pipe can not be put into service and the owner/operators have no knowledge if the pipe meets the set requirements.
The information found from pressure testing aids to maintain the safety standards and the upkeep of the pipeline. When there is a new manufactured piece of equipment the pipelines are initially qualified by using a pressure test/hydrostatic test and regularly re-qualified at different intervals, which is called, the “modified hydrostatic test” or “proof pressure test”.
When testing pipelines they are conducted under the industry’s or customer’s specifications, filling the vessel with an incompressible liquid, like water or oil. This examines the pipe for any leaks or changes in shape. Generally dyes are added to the water in order to detect leaks with ease. When pressure testing, the amount of pressure put onto the vessel is always considerably more than the normal operating pressure. This is to allow for any unexpected pressure levels on the vessel for maximum safety.
Information for the tester are stamped onto the vessel, this may include serial number, manufacturer and manufacturer date. There may be other information such as, REE (Rejection Elastic Expansion) and maximum expansion specified by the manufacturer for safety. This information is usually recorded on a computer system which will allow the owner to keep track when the tests have taken place or need to be scheduled in.
Why is Pneumatic Testing More Dangerous than Hydrotest?
Once you’ve successfully laid an underground pipeline or carried out all required hot work (such as welding or post-weld heat treatment), you’ll need to conduct a pressure test to ensure that the mechanical properties of the pipe haven’t been downgraded.
However, there are two primary methods used for pressure testing in this instance, namely hydrostatic and pneumatic. Whilst the former is performed using water as the test medium, the latter utilises air, nitrogen or any other form of non-toxic and non-flammable gas to complete the inspection.
There are other differences between these two testing methods that also need to be considered, as you look to choose the right option for any given application.
Interestingly, pneumatic testing is considered to be the more dangerous of the two options, as the amount of energy stored per unit volume of compressed air under test pressure is relatively high.
As a result, pneumatic testing is only recommended for low pressure applications, whilst all inspections must feature detailed safety precautions and be overseen by senior staff members.
You must also ensure that you have approval from the relevant authority to carry out a pneumatic test, otherwise you’ll find yourself in breach of the existing law. This will ensure that the appropriate conditions exists for this type of pressure test, as you can’t carry out such an inspection if the pipeline system is likely to fill with water or unable to tolerate trace amounts of the testing medium.
What is the Pressure for Test of a Pressure Vessel?
Whilst there are two distinct methods for pressure testing pipelines, in most instances you’ll be required to use the hydrostatic test method.
The reason for this is simple; as the pneumatic test is inherently more dangerous and can only be used when the design or function of a pipeline is incompatible with the notion of utilising water as the test medium.
When testing pipelines using this method, the vessel in question will be filled with water to help identify any potential leaks, mechanical deficiencies or subtle changes of shape when immersed. Dyes may also be added to the water to help detect leaks immediately, whilst you should ensure that all welding has been completed and verified before carrying out the hydrostatic test.
When it comes to gauging pressure, the amount of pressure placed on the vessel should always be considerably higher than the normal operating levels.
More specifically, it needs to be between 1.5 and 4 of the pressure test value, to allow for any expectedly high pressure levels that may be placed on the vessel during its normal operation.
What's pneumatic pipe testing?
A pressure test is always required when a new piping system has been completed, or instances where individual pipes have been altered. There’s good reason for this too; as it guarantees the safety of the system and the reliability of its operation, whilst also identifying any potential leaks.
Usually, pressure testing is carried out after all hot works and welding have been completed on a piping system, with post-weld heat treatment capable of downgrading the mechanical properties of individual pipes.
There are different types of pressure test that can be used, with pneumatic testing using air or an inert gas like nitrogen to pressurise the pipeline to 110% of its intended design capacity.
The gas is then kept in the pipeline for a sustained period of time, in order to gauge the system’s ability to operate safely and efficiently at its maximum capacity.
Whilst the results produced by pneumatic pipeline testing are highly accurate, however, this methodology is only used in markedly low-pressure applications where water or oil cannot be used as a viable test medium.
The reason for this is simple; as inert gases like nitrogen are capable of storing high levels of energy when they’re compressed during testing. This increases the risk of system failure and damage, whilst the nature of pneumatic testing requires formal approval from the local authority and the guidance of specialist engineers.
Want more information on Pressure testing equipment?
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