Pressure testing is performed to test the integrity and safety of pressure systems, and to ensure that they are reliable and leak tight. Whenever a new pressure system is built, it must be tested prior to use. In addition, if a pressure system is repaired or altered in any way, it must be re-tested for safety purposes. Both pneumatic and hydrostatic testing are employed to certify the safety of pressure systems.
When equipment is not designed to handle the loads required for a hydro-pressure test, pneumatic testing may be employed. Pneumatic testing uses air, nitrogen, or another non-flammable, non toxic gas. Pneumatic tests can be more dangerous than hydrostatic tests, mainly due to the higher level of potential energy used during the test.
For best results, pressure tests should always be performed under controlled conditions, and according to a verified and approved test plan. Pressure tests should also be documented carefully in a test record. A plan may be used for more than one test; however, each test should have its own separate test record.
As you might infer from the name, a hydrostatic test is conducted using water. Hydrostatic testing is considered to be the most accurate method for testing pressure vessels, such as pipelines, plumbing, and various chemical cylinders for leaks.
This type of pressure testing is known to be a non-destructive testing method. However, there is the potential for equipment to rupture and fail if the inspection is not done properly. Equipment failure can occur if an inspection exceeds the specified test pressure for the pressure system, or if a small crack expands quickly during testing.
Hydrostatic Testing Methods
There are three main hydrostatic testing methods which are used for testing small pressure vessels and cylinders. These methods include:
- The Water Jacket Method
The Water Jacket Method begins by filling the vessel with water and loading it into a “test jacket,” a sealed chamber filled with water. Then, the vessel is pressurized inside the test jacket for a set period of time. As a result, the vessel will expand within the test jacket, forcing water out of the test jacket and into a glass tube, which measures precisely how much the vessel expanded.The vessel is then deflated, returning it to its original size. As it shrinks, the water flows back into the test jacket. In some cases, the vessel will only partially deflate, and will not go back to its original size. Its new size is called a permanent expansion. Depending on the difference between the total expansion and the permanent expansion, the vessel may or may not be considered fit for service. The higher the expansion level, the more likely a vessel will be deemed unfit for service.
- Direct Expansion Method
The direct expansion method begins by filling a vessel with a specific amount of water and pressurizing the system. The pressure is then released, and the amount of water that leaves the vessel is measured. The amount of water that leaves the vessel is then compared to the amount of water originally forced into the vessel, and the test pressure is also considered.It is crucial that air does not get trapped inside the vessel, as this can create inaccurate test results. This can be avoided by filling the vessel completely with water, in order to eliminate any possibility of trapped air.It’s important to note that Department of Transportation Regulations do not allow the Direct Expansion Method to be used for qualifying cylinders for filling to 10% over service pressure. Therefore, this method is not acceptable for re-qualifying plus (“+”) stamped cylinders.
- Proof Pressure Method
The proof pressure method uses an internal pressure to find out if there are any leaks, or wall thinning within the vessel, which may result in failure. The sample is typically subjected to pressure that is two to six times its normal operating pressure. This is a less common pressure testing method, as it can only be used in the U.S. when the total expansion and permanent expansion are not required to be recorded.
Axenics Pressure Testing
We perform hydrostatic testing services for 4B 260 and 4B 300 low-pressure, stainless-steel chemical canisters. We also perform pneumatic pressure testing, up to 150 PSI, because as the pressure gets above 150psi, it becomes too dangerous to perform safely in our facility. To learn more about our hydrostatic testing equipment, procedures, and other testing services, contact Axenics today.