Pressure Vessels Testing

Pressure vessels are industrial containers designed to hold vapors, liquids or gases.

Because pressure vessels usually hold materials at a high pressure they can be dangerous if not properly maintained. Regular inspections are a critical part of the maintenance process for pressure vessels, helping to reveal potential problems before they develop further.

Because pressure vessels usually hold materials at a high pressure they can be dangerous if not properly maintained. Regular inspections are a critical part of the maintenance process for pressure vessels, helping to reveal potential problems before they develop further.

Here is a list of the topics covered in this resource to help you find the information of most interest to you:

If a pressure vessel is operated beyond the pressure or temperature it was designed to handle, the result could be the catastrophic failure of the unit. In the worst case scenario, operating outside of a vessel’s design could lead to fires, poisonous gas leaks, or even explosions, all of which could pose an extreme danger to anyone working in the surrounding area.
Common Characteristics of Pressure Vessels

Here are some common characteristics of pressure vessels:

• ➣ Shape. While a pressure vessel could hypothetically be made in several different shapes, the most common shapes used are cones, spheres, and cylinders.
• ➣ Design. Regardless of the vessel’s shape, the minimum mass of a pressure vessel scales along with the volume it contains and pressure it will be put under. This mass is inversely proportional to the strength and weight ratio of the vessel’s construction material, and the minimum mass required goes down as strength goes up.
• ➣ Construction materials. Since they have to withstand extremely high pressures, pressure vessels must be incredibly strong. For this reason, the most common material used to make them is steel. Pressure vessels can also be made from composite materials, like filament wound composite, or concrete with cabling wrapped around or within the vessel to provide the tension needed to resist internal pressure.
• ➣ Corrosion resistance. If a pressure vessel will be used in a scenario that may produce corrosion, this must be taken into consideration in choosing the material.
• ➣ Pressure. Most pressure vessels are designed to operate at 15 psig* or above.

Note: Psig is a measure of pressure that stands for Pounds Per Square Inch Gauge. Learn more about psig on the Energy Education website.

It’s important to note that the term “pressure vessel” can refer to a variety of storage containers used in industrial settings.
Here are some of the most common types of pressure vessels.

• ➣ Safety valves. Also called a relief valve, these valves allow for the quick release of pressure so that the desired pressure is not exceeded to a dangerous extent during use.
• ➣ Leak before burst. This feature refers to a design element in pressure vessels that allow them to crack instead of exploding. Most pressure vessel standards throughout the world, including the ASME Boiler and Pressure Code and the AIAA metallic pressure vessel standard require that pressure vessels have this feature, which allows the vessel to crack and leak fluids rather than suddenly burst, allowing for a safer option for releasing pressure in the event of failure.

Most pressure vessel regulations provide specific requirements for the frequency of inspections. As a general rule of thumb, pressure vessels should be inspected at least once every five years. An inspection should also be conducted once the vessel is installed, prior to it being put into service.

What is done during and Inspection
Pressure vessel inspections can refer to an inspection of the vessel’s condition externally, internally, or both.

In these inspections, inspectors may :

• ➣ Collect visual data regarding the condition of the vessel, including the condition of insulation, welds, joints, or structural connections
• ➣ Collect thickness data to determine whether the vessel has changed due to use
• ➣ Conduct a stress analysis to determine whether the vessel is still OK for use
• ➣ Inspect the vessel’s pressure release valves to make sure they functioning properly
• ➣ Conduct a hydrostatic pressure test.

Types of Pressure Vessel Testing
There are five common types of tests inspectors perform during pressure vessel inspections :

1. VISUAL TESTING Visual testing is the most common type of non-destructive testing (NDT) an inspector might perform. The goal of a visual inspection is to visually review both the interior and exterior of the vessel to look for any cracks or flaws (see the checklist just below for more details). Learn more about visual inspections.
2. ULTRASONIC TESTING Ultrasonic testing uses sound waves to measure the thickness of a material’s surface in order to detect any defects that may have arisen. This kind of testing is volumetric, meaning it can detect flaws inside the vessel as well as on its surface. Learn more about ultrasonic testing.
3. RADIOGRAPHIC TESTING Radiographic testing uses radiography to detect defects near the surface or on the surface of a vessel. This testing method is also volumetric. Learn more about radiographic testing.
4. MAGNETIC PARTICLE TESTING Magnetic particle testing uses magnetic current run through the pressure vessel to identify deformations or defects on the surface of the vessel, which will interrupt the flow of the magnetic current and appear as a “flux leakage field.” Learn more about magnetic particle testing.
5. DYE PENETRANT TESTING Dye or liquid penetrant testing uses liquid (i.e., the penetrant) sprayed onto the vessel to identify defects or flaws on its surface. A fluorescent chemical can be added to the penetrant to make flaws visible under U.V. light. Learn more about dye penetrant testing.

Pressure Vessel Inspection Checklist

Here is a checklist of the things inspectors usually look for during a pressure vessel inspection.

EXTERNAL INSPECTIONS — WHAT TO INSPECT
Here's what inspectors look for when inspecting the exterior of a pressure vessel:

• ➣ External coverings, including insulation and corrosion resistant coatings, inspected for defects
• ➣ Entire vessel exterior inspected for any kind of leakage of gas, vapor, or liquid
• ➣ Mountings inspected to see if they allow for appropriate expansion and contraction
• ➣ Vessel and vessel connections inspected for deformations, cuts, cracks, or gouges, including on nozzles, manholes, and reinforcing plates
• ➣ Nuts, bolts, flange faces, vessel surface inspected for corrosion or other defects)
• ➣ Shell surfaces and heads inspected for blisters, bulges, or other deformations
• ➣ Welded joints and adjacent areas inspected for cracks or defects

INTERNAL INSPECTIONS — WHAT TO INSPECT
Here's what inspectors look for when inspecting the interior of a pressure vessel

• ➣ Interior of vessel inspected for cracks, blistering, corrosion, deformation, or any other defects.
• ➣ Threads inspected to ensure the adequate number of threads are engaged on threaded connections.
• ➣ Openings leading to any external fittings or controls inspected to ensure they are free from obstruction.
• ➣ Special closures inspected to ensure they are adequate.
• ➣ Areas of high stress concentration inspected for cracks or other wear.