Objective 11: Leak and Pressure Testing Flashcards
What is the purpose of Leak Testing?
To verify the integrity of a completed pressure vessel or fitting before it is placed into service or to determine if a vessel currently in service may continue to be used
What does Leak Testing identify?
Discontinuities in the vessel pressure boundary
What is the minimum test pressure to be used in Pressure Testing?
1.25 - 1.3x the MAWP
Is Pressure Testing required to be carried out on completed boilers, PVs, and pressure piping?
Yes
What are the 2 purposes of Pressure Testing?
- Prove that the vessel can withstand the pressure for which it was designed
- To ensure no leaks are present
Why is Pressure Testing used on piping and vessels that have already been in service?
To verify that they can still safely withstand the operating pressure with a proven margin of safety and that no through cracks or holes have developed
When is Pressure Testing especially useful to be carried out?
When the Inspector does not have adequate access to conduct a visual inspection
Why do many jurisdiction inspectors and owners require the organization performing the Pressure Test to provide written test procedures?
Reviewing and approving these procedures prior to the test averts potential problems
What is the first step in leak testing?
To determine whether leaks are present
What are the 2 types of pressure tests are used to aid in the detection of a leak?
- Hydrostatic
2. Pneumatic
What is the basic process of Hydrostatic Testing?
The vessel is filled with water and the fluid is pressurized to the required pressure. The vessel is then examined for leaks
How is a New Vessel prepared for Hydrostatic Testing?
- All connections such as flanges and couplings are closed off with blind flanges and plugs
- A drain valve is located at the low point of the vessel to allow removal of the water right after the test
- The filling connection must have an isolating valve and a vent valve is required at the top to allow all air to be expelled
How is an In-Service Vessel prepared for Hydrostatic Testing?
- If the fluid the vessel normally handles is toxic or flammable, the vessel is cleaned and purged
- The vessel is isolated from the test of the system. Existing valves are used if they’re in good condition; otherwise piping connections at the vessel are opened and blind flanges are installed
- Control line connections to the vessel are removed and closed off
- Procedures for New Vessel are carried out
How is a Vessel that isn’t designed to hold water prepared for Hydrostatic Testing?
Additional supports may be required for the vessel and the foundation must be inspected to ensure it can carry the additional weight
How is the test pressure monitored during Hydrostatic Testing?
On a calibrated pressure gauge; 2 gauges may be used as an additional precaution
What is the most common fluid used for hydrostatic testing?
Water
Why is water the most common fluid used for Hydrostatic Testing?
- Inexpensive
2. Readily available
When should a glycol/water mixture or methanol be considered for Hydrostatic Testing?
When the test is performed with ambient conditions near or below freezing temperature
What is the benefit of using a glycol/water mixture or methanol for Hydrostatic Testing at or below freezing temperature?
Avoids the possibility of frozen drain lines when the test is completed
What are the cons of using a glycol/water mixture or methanol for Hydrostatic Testing?
- More expensive than water
2. Disposal poses environmental problems
What test pressure must be used when performing Hydrostatic Testing on Used Equipment?
The owner or jurisdiction inspector may determine the test pressure but it should not exceed the test pressure used for testing when the vessel was new. If the inspector is only condernced with leaks that may have developed in service, a hyrostatic test at the operating pressure is acceptable
What should be the temperature of the water used for Hydrostatic Testing of boilers?
Not less than ambient temperatures but in no case below 20ºC
Why is temperature a concern in Hydrostatic Testing?
To avoid brittle fracture
What temperature should the metal be maintained at during Hydrostatic Testing?
At least 17ºC above the minimum design metal temperature
During Hydrostatic Testing, what should be done to prevent burn hazards?
If the test temperature exceeds 48ºC, inspection of the vessel should be delayed until the temperature falls to 48ºC or less
Once the vessel is filled, how is the internal pressure increased during Hydrostatic testing?
Pump
In how many steps is pressure increased during Hydrostatic Testing?
- For LP test, 1
2. For HP test, several with visual inspection conducted at the end of each step
At what test pressure is it safe for the Inspector to inspect the vessel for leaks during Hydrostatic Testing?
Below 3000 kPa
What is dangerous about inspecting a vessel at a test pressure greater than 3000 kPa during Hydrostatic Testing?
Water jets issuing from a pinhole or crack
How is a vessel with a test pressure greater than 3000 kPa during Hydrostatic testing safely inspected?
- The vessel is pressurized to test pressure and held there for a specified time while the pressure gauges are observed
- The pressure is then reduced to the normal operating pressure before inspecting is done
What does visual inspection of a vessel during Hydrostatic Testing entail?
- Examining the entire surface of the vessel for wet areas and small drops of liquid
- Looking for pressure drop during the Hydrostatic Test
Why is a lack of pressure drop during Hydrostatic Testing not proof the vessel is leak free?
- A very small leak may not produce a noticeable pressure drop
- An increase in the temperature of the test liquid can create sufficient expansion to compensate for any losses due to leakage
What is done after Hydrostatic Testing is complete?
Pressure is slowly released
Why are liquids used for Hydrostatic Testing?
They do not expand significantly when pressure drops. If a failure occurs, only a small, harmless flow of water will be released.
What are the advantages of Hydrostatic Testing?
- Inexpesive
2. Relatively easy to perform, provided the vessel is properly prepared
Pneumatic Testing
Pressurization of a vessel or piping system with a compressible gas such as air or nitrogen to determine if any leaks are present
Should Pneumatic Testing be a favoured NDE and why?
No because it may be hazardous
Why is Pneumatic Testing resorted to?
The air or nitrogen may be the only suitable test substance if water can damage the interior of the vessel, e.g. refractory linings or catalyst beds
What is the basic preparation for Pneumatic Testing?
Isolation of the vessel from the system and controls
What is a benefit of using gas as the test medium as in Pneumatic Testing?
It does not place a significant amount of static head in the vessel
When is air a suitable gas to be used in Pneumatic Testing and why?
For general applications due to it being inexpensive and readily available
When is nitrogen a suitable gas to be used in Pneumatic Testing?
When a possibility of combustion exists
Why do some codes allow Pneumatic Testing to use lower test pressures than Hydrostatic Testing?
The inherent danger in testing with compessed gases
When is a test pressure lower than operating pressure acceptable to be used in Pneumatic Testing?
- The Pneumatic Test is used as an additional test to the Hydrostatic Test
- The Pneumatic Test is used as a leak test prior to the Hydrostatic Test
How important is proper test temperature for Pneumatic Testing?
If there is any possibility of brittle failure of the test vessel, another method should be used
How is the testing gas for Pneumatic Testing usually delivered to the test site?
As a liquid or at a very high pressure
How is the required test pressure achieved for Pneumatic Testing?
Pressure reducing system
What effect can pressure reduction have on the Vessel or Piping for Pneumatic Testing?
It can cool them enough to affect the ductility of the metal
What is used to maintain a minimum gas temperature during Pneumatic Testing?
Heaters
What precaution must be taken if the supply pressure of the test fluid is higher than the test pressure for Leak Testing?
Overpressure Protection
How is test pressure increased during Pneumatic Testing?
In stages with inspection occurring at each stage
What is the primary safety concern in Pneumatic Testing?
The amount of energy stored in the compressed fluid during the test means that is a failure occurs, the results can be catastrophic
What are the advantages of Pneumatic Testing?
- At low pressures, the danger is reduced and leaks can be found by a soap test
- Does not create mass loading on the item being tested
- Doesn’t involve cleanup after the test because the gases can be vented to atmosphere
- May be the only acceptable test method in cases where the vessel interior is lined with material that liquids can damage
If leaks have been determined by Leak Testing, what are the 9 different methods used to pinpoint their exact location?
- Bubble Test - Direct Pressure Technique
- Bubble Test - Vacuum Box Technique
- Halogen Diode Detector Probe Test
- Helium Mass Spectrometer - Detector Probe Technique
- Helium Mass Spectrometer - Tracer Probe and Hood Technique
- Pressure Change Test
- Thermal Conductivity Detector Probe Test
- Helium Mass Spectrometer Test - Hood Technique
- Ultrasonic Leak Detector Test
What is another name for Bubble Test?
Soap Test
When is Bubble Leak Testing used?
The pressurizing fluid is a gas and access is available to the surface where the leak is suspected
What is the old procedure of the Bubble Leak Test?
- Liquid soap solution is brushed or poured over the pressurized vessel or piping system
- Escaping gas forms bubbles in the soap solution, providing a clealy visible indication of where a leak is occuring
- The number and size of bubbles indicates the size of the leak
What is problematic about the old Bubble Leak Test?
- The soap leaves a film where it dries
- If hard water is used, the soap tends to curdle rather than bubble
- Impurities in water can sometimes contaminate the material being tested
How does the new Bubble Leak Test differ from the old?
Instead of soap, special solutions with enhanced surface tension, viscosity, and film retention properties are used
What is the benefit of the new Bubble Leak Test?
The solution enables Inspectors to locate smaller leaks than is possible when using soap and water
Immersion Bubble Leak Testing
A more inolved method of bubble testing that involves placing the pressurized specimen in a water bath and the escaping gas forms a trail of gas bubbles as it rises to the surface of the liquid
Vacuum Leak Testing
Uses a vacuum to create the pressure difference necessary for leak detection
What is a disadvantage of Vacuum Leak Testing?
Because the best possible vacuum is 101 kPa below atmospheric pressure, the pressure differential is limited
What is a benefit of the limited pressure differential in Vacuum Leak Testing?
The danger of implosion is reduced
What is the procedure for Vacuum Leak Testing?
- The specimen is placed in a vacuum chamber or a vacuum chamber is attached to the side of the vessel
- A pump removes air from the chamber, causing the pressure to drop
- Because a pressure difference exists between the interior of the vessel or sample and the vacuum chamber, leakage occurs through any cracks or other openings
For what applications is Vacuum Leak Testing used?
- Testing electrical equipment
- Laboratory
- Floor plates of vertical tanks
For what application is Vacuum Leak Testing seldom used?
Pressure vessels
Dye Tracer Leak Testing
Fluorescent dye is added to the water inside a vessel undergoing Hydrostatic Testing to increase the visibility of very small leaks.
How does Dye Tracer Leak Testing make leaks more visible?
The dye leaks through the cracks or openings to the outside of the vessel and by examining the external surface of the vessel with an UV light, the dye is seen more readily than water
Halogen Leak Testing
Testing method that uses Freon or halogen gases to fill the vessel and a halogen detector probe to detect leaks
Why is Halogen Leak Testing being phased out?
The use of halocarbon gases poses environmental problems
What is the most sensitive leak detector available?
Mass Spectrometers
Helium Mass Spectrometer Leak Testing
Testing method that introduces a tracer gas such as helium into the test vessel and moves a detector “sniffer” hose over the surface to locate leaks
What are the benefits of using Helium as the testing medium for Leak Testing?
- Because helium atoms are smaller than most other atoms, they penertrate through smaller cracks, provifing a more sensitive test
- Helium is an inert gas that does not react with other gases or materials of construction
- The amount of helium in the atmosphere is not significant and does not interfere with leak testing
