What is Pressure Testing?
Pressure testing is a way to provide quality assurance of any pipeline or pressure vessel and helps identify damaged or defective pipe fittings, joints, valves, hydrants, and thrust restraint systems.
There are many advantages of pressure testing such as decreasing expense risks and decreasing health and safety risks.
We can divide pressure testing into 2 methods:
- Hydrostatic Pressure Test.
- Pneumatic Pressure Test.
What is Hydrostatic Testing?
What is hydrostatic head pressure? Let’s examine the words to better understand the meaning of hydrostatic.
• Hydro meaning liquid
• Static meaning unchanging.
• Pressure is a force exerted over an area.
Which of leads us to the following;
It is a pressure that is generated by the weight of the liquid due to gravity. The taller the height of a liquid column the greater the force, which is expressed as pounds per square inch (psi) for our purposes. The Hydro (liquid) of interest on the exam is water since it is the primary liquid we use for Hydrostatic testing. Other liquids can be and are used.
This is a nondestructive test used to check the quality of welds on closed containers such as pressure vessels and tanks. The test usually consists of filling the vessel with water and applying a pressure greater than the working pressure of the vessel. Sometimes, large tanks are filled with water that is not under pressure to detect possible leakage through defective welds. Another method is to test with oil and then steam out the vessel. Back seepage of oil from behind the liner shows up visibly. Hydrostatic testing is the most common procedure used to qualify newly manufactured cylinders, spheres, and tubes used for the transportation of dangerous goods. Hydrostatic testing is also required periodically to re-qualify these pressure vessels for continued service.
What is Hydrostatic Pressure Testing?
Hydro pressure testing is a way in which leaks can be found in pressure vessels, pipelines, and plumbing .using this test helps maintain the safety standards and durability of a vessel over time. newly manufactured pieces are initially qualified using the hydrostatic test, they are then continually re-qualified at regular intervals using the proof pressure test which is also called the modified hydrostatic test. hydrostatic testing is also a way in which a gas pressure vessel, such as a gas cylinder or a boiler, is checked for leaks or flaws. testing is very important because such containers can explode if they fail when containing compressed gas.
Some pressure vessels may be re-qualified by means of a proof-pressure test. This method, also known as a modified hydrostatic test, consists of subjecting a pressure vessel to a specified internal pressure and inspecting the pressurized container for leaks, bulges, or other defects. This method is permitted only when the applicable regulations do not require the determination of volumetric expansion measurements (e.g. for certain types of low-pressure cylinders such as those used on many portable fire extinguishers).
Hydrostatic Test Marking
Pressure vessels that have been hydrostatically tested and re-qualified must be marked by the retest facility with the facility retester identification number (RIN) and the retest date. Depending on the age of the container, it may contain none or many sets of retest markings.
The oldest date marked on the container is the date of the original hydrostatic test carried out by the manufacturer and is considered the manufacturing date. Any subsequent dates marked on the container are a record of the hydrostatic tests that the container has undergone.
Hydrostatic Test Intervals
The hydrostatic test interval and service life of various pressure vessels are generally determined by the specification or exemption to which the pressure vessel was manufactured.
What are the checks before Hydro Test
Activities before pressure vessel hydro-static testing, which need to be checked by the manufacturer quality control team and a third party inspector:
1. Checking all welding already finished and fully accepted by the NDT examination per the project Inspection and test plan.
2. Making sure the inner part of the vessel is clean and free of remaining slag or other elements.
3. Making sure the external surface is dry for the correct execution of the visual inspection during the vessel pressure testing.
4. Checking the pressure gauges’ calibration tag and certificate and the range of the lower limit and upper limit of the gauges. It needs to be between 1.5 and 4 of the pressure test value.
5. Controlling testing equipment such as the test pump and housing for soundness and tightness.
6. Making sure the test temperature will not violate the Hydro test procedure values.
Hydro Testing Procedure
During a hydrostatic test, a pressure vessel is placed inside a closed system, usually, a test jacket filled with water, and specified internal water pressure is applied to the container inside this closed system. The applied internal pressure causes an expansion of the container being tested, and the total and permanent expansion that the container undergoes are measured.
These volumetric expansion measurements, in conjunction with an internal and external visual inspection of the container is used to determine if a pressure vessel is safe for continued use, or has suffered from a degradation in its structural integrity and must be condemned.
Precautions after Hydro Testing- Drying
Equipment left wet after testing is at risk because the chloride levels of pools of water will rise as the water evaporates: PCC may initiate at these high chloride levels.
To prevent chloride concentration after hydro testing, the equipment should be thoroughly dried and sealed if it is to be left for more than a day or two before being put into service. This can be achieved by draining the water out (properly designed equipment will allow for complete drainage), then blowing dry air through the wet spaces. When the humidity of the air is high, this process should be completed when the work is at its coolest (i.e. at night), to prevent condensation when the work cools.
Drying the equipment also stifles any microbiological infections, which will not progress on a dry surface.
Important precautions during Hydro Testing
- Min. Test temperature= MDMT + 30°F
- Max. Test temperature = 120°F
- MDMT is the pressure vessel minimum design metal temperature, and it is stated in the pressure vessel design document.
- This reduces the risk of a brittle fracture during the test.
- Making sure which reinforcement pads are already soap tested.
- Controlling the testing water quality and using corrosion inhibitor if it is necessary or when the vessel metal is sensitive material.
- Checking of vents. It is required to be placed at high points of the vessel in a position where it is possible to purge air pockets while the vessel is filling.
- Making sure the vessel is immediately and carefully drained after the test and dried by air.
- Activities after the pressure vessel hydro-static testing that need to be checked by the manufacturer quality control team and a third party inspector.
- The pressure vessel hydro-static testing report needs to be prepared by the manufacturer quality control team and signed by the third party or authorized inspector.
- . If the test failed by leaking from weld joints or any other kind of defect, it is necessary that the vessel is drained and dried and repaired based on approved repair procedure. pressure vessel hydro-static testing needs to be repeated.
- Activities during the pressure vessel hydro-static testing that need to be checked by the manufacturer quality control team and a third party inspector:
a. Making sure the filling and pressurizing are done from the lowest point and venting from the highest point.
b. Witnessing water overflow through the venting in order to assure that no air bubbles remain in the vessel.
c. Controlling and witnessing which pressurizing is done in three stages as follow:
1.First Stage: Raise the pressure to 40% of the final pressure, stop pressurizing, keep it for 5 minutes, and then make a fast visual inspection of the external surface.
2. Second Stage: Restart pressurizing up to 70% of final pressure, stop the operation, keep for 5 minutes and make a fast visual inspection on external surface.
3. Third Stage: Restart pressurizing up to 100% of the final pressure, stop the operation, and keep for 45 minutes.
- When 45 minutes elapse, making sure the depressurizing is started and the pressure is dropped to the “Inspection Pressure.”
Calculation of Hydro Testing Pressure
This inspection pressure can be calculated as follows:
1. Inspection Pressure = Hydrostatic Test Pressure / 1.3
2. Making sure a detailed and comprehensive inspection is carried out over the whole body of the vessel and in the welding joints and attachments.
3. Making sure the pressure vessel hydro-static testing pressure calculated correctly as follows:
a. Hydrostatic Test Pressure = 1.3 x MAWP X Stress ratio
b. Stress Ratio = (Allowable Stress at Test Temperature)/(Allowable Stress at Design Temperature)