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A Boiler: The Explosive Potential of a Bomb
Acoustic Emission Examination of Metal Pressure Vessels
Anatomy of a Catastrophic Boiler Accident
Austenitic Stainless Steel
Basic Weld Inspection - Part 1
Basic Weld Inspection - Part 2
Black Liquor Recovery Boilers - An Introduction
Boiler Efficiency and Steam Quality: The Challenge of Creating Quality Steam Using Existing Boiler Efficiencies
Boiler Logs Can Reduce Accidents
Boiler/Burner Combustion Air Supply Requirements and Maintenance
Carbon Monoxide Poisoning Preventable With Complete Inspection
Combustion Air Requirements:The Forgotten Element In Boiler Rooms
Creep and Creep Failures
Description of Construction and Inspection Procedure for Steam Locomotive and Fire Tube Boilers
Ensuring Safe Operation Of Vessels With Quick-Opening Closures
Environmental Heat Exchangers
Factors Affecting Inservice Cracking of Weld Zone in Corrosive Service
Failure Avoidance in Welded Fabrication
Finite Element Analysis of Pressure Vessels
Fuel Ash Corrosion
Fuel Firing Apparatus - Natural Gas
Grain Boundaries
Heat Treatment - What Is It?
How to Destroy a Boiler -- Part 1
How to Destroy a Boiler -- Part 2
How to Destroy a Boiler -- Part 3
Identifying Pressure Vessel Nozzle Problems
Inspection, Repair, and Alteration of Yankee Dryers
Inspection, What Better Place to Begin
Laminations Led to Incident
Lay-up of Heating Boilers
Liquid Penetrant Examination
Low Voltage Short Circuiting-GMAW
Low Water Cut-Off Technology
Low-Water Cutoff: A Maintenance Must
Magnetic Particle Examination
Maintaining Proper Boiler Inspections Through Proper Relationships
Microstructural Degradation
Miracle Fluid?
Organizing A Vessel, Tank, and Piping Inspection Program
Paper Machine Failure Investigation: Inspection Requirements Should Be Changed For Dryer Can
Pipe Support Performance as It Applies to Power Plant Safety and Reliability
Polymer Use for Boilers and Pressure Vessels
Pressure Vessel Fatigue
Pressure Vessels: Analyzing Change
Preventing Corrosion Under Insulation
Preventing Steam/Condensate System Accidents
Proper Boiler Care Makes Good Business Sense:Safety Precautions for Drycleaning Businesses
Putting a Stop to Steam Kettle Failure
Quick Actuating Closures
Quick-Actuating Door Failures
Real-Time Radioscopic Examination
Recommendations For A Safe Boiler Room
Recovering Boiler Systems After A Flood
Rendering Plants Require Safety
Repair or Alteration of Pressure Vessels
Residential Water Heater Safety
School Boiler Maintenance Programs: How Safe Are The Children?
Secondary Low-Water Fuel Cutoff Probe: Is It as Safe as You Think?
Short-Term High Temperature Failures
Specification of Rupture Disk Burst Pressure
Steam Traps Affect Boiler Plant Efficiency
Steps to Safety: Guide for Restarting Boilers After Summer Lay-Up
Stress Corrosion Cracking of Steel in Liquefied Ammonia Service - A Recapitulation
Suggested Daily Boiler Log Program
Suggested Maintenance Log Program
System Design, Specifications, Operation, and Inspection of Deaerators
Tack Welding
Temperature And Pressure Relief Valves Often Overlooked
Temperature Considerations for Pressure Relief Valve Application
The Authorized Inspector's Responsibility for Dimensional Inspection
The Effects of Erosion-Corrosion on Power Plant Piping
The Forgotten Boiler That Suddenly Isn't
The Trend of Boiler/Pressure Vessel Incidents: On the Decline?
The Use of Pressure Vessels for Human Occupancy in Clinical Hyberbaric Medicine
Thermally Induced Stress Cycling (Thermal Shock) in Firetube Boilers
Top Ten Boiler and Combustion Safety Issues to Avoid
Typical Improper Repairs of Safety Valves
Wasted Superheat Converted to Hot, Sanitary Water
Water Maintenance Essential to Prevent Boiler Scaling
Water Still Flashes to Steam at 212
Welding Consideration for Pressure Relief Valves
Welding Symbols: A Useful System or Undecipherable Hieroglyphics?
What is the Best Welding Process?
What Should You Do Before Starting Boilers After Summer Lay-Up?
Why? A Question for All Inspectors

Liquid Penetrant Examination

Jim Worman
Senior Staff Engineer
National Board

Category : Design/Fabrication

Summary: This article was originally published in the Winter 2011 National Board BULLETIN. (4 printed pages)



Liquid penetrant examination is one of the most popular Nondestructive Examination (NDE) methods in the industry. It is economical, versatile, and requires minimal training when compared to other NDE methods. Liquid penetrant exams check for material flaws open to the surface by flowing very thin liquid into the flaw and then drawing the liquid out with a chalk-like developer. Welds are the most common item inspected, but plate, bars, pipes, castings, and forgings are also commonly inspected using liquid penetrant examination.

Over the years, liquid penetrant examination has been called many names: penetrant testing (PT), liquid penetrant testing (LP), and dye penetrant testing (DP). The American Society for Nondestructive Testing (ASNT) uses the name liquid penetrant testing (PT). The American Society of Mechanical Engineers Boiler and Pressure Vessel Code (ASME B & PVC) and the National Board Inspection Code (NBIC) use the name liquid penetrant examination (PT).
The first documented use of PT was in the railroad industry. Cast railroad wheels were dipped in used oil, dried off, and then coated with powder chalk or suspension of chalk in alcohol.  Once the wheels were dry, any oil stored in the flaw would bleed out into the chalk and be detected. This was called the oil and whiting method.
The ASME Boiler & Pressure Vessel Code recognizes six different techniques of PT. They vary by type of penetrant and method of cleaning before applying a developer. The two penetrant types are either fluorescent or color contrast (dye) penetrant. They can then be used with any of the three methods of cleaning – water washable, post-emulsifying, and solvent removable. The most popular is dye penetrant that is solvent removable. This method is referenced throughout the article.
The dye penetrant solvent removable method is most popular because it is low cost and very versatile. It typically comes in three aerosol cans – cleaner, penetrant, and developer. The cans can be purchased from welding supply distributors for typically $5 to $15 a can. For less than $50 you can have all the equipment you need to conduct liquid penetrant examinations. The aerosol cans are very versatile which allow them to be taken up ladders, inside boilers, down into pits, and into very tight places. Most nonporous materials (steel, stainless steel, cast iron, aluminum, brass, bronze, titanium, rubber, plastics, and glass) can be examined using PT. Porous materials (concrete, wood, paper, cloth, and some types of fiberglass if the fibers are exposed to the surface) should not be examined using PT. 

Dye penetrant solvent removable aerosol cans

There are several advantages and disadvantages to using liquid penetrant examination.
  • High sensitivity to small surface discontinuities
  • Easy inspection of parts with complex shapes
  • Quick and inexpensive inspection of large areas and large volumes of parts/materials
  • Few material limitations (metallic and nonmetallic, magnetic and nonmagnetic, and conductive and nonconductive can all be inspected)
  • A visual representation of the flaw are indicated directly on the part surface
  • Aerosol spray cans make the process portable, convenient, and inexpensive
  • Indications can reveal relative size, shape, and depth of the flaw
  • It is easy and requires minimal amount of training
  • Detects flaws only open to the surface
  • Materials with porous surfaces cannot be examined using this process
  • Only clean, smooth surfaces can be inspected. (Rust, dirt, paint, oil and grease must be removed.)
  • Metal smearing from power wire brushing, shot blasting, or grit blasting must be removed prior to liquid penetrant examination
  • Examiner must have direct access to surface being examined
  • Surface finish and roughness can affect examination sensitivity. (It may be necessary to grind surfaces before PT.)
  • Multiple process steps must be performed and controlled
  • Post cleaning of parts and material is required, especially if welding is to be performed
  • Proper handling and disposal of chemicals is required
  • Fumes can be hazardous and flammable without proper ventilation
It is important to remember penetrant is a very thin liquid designed to seep into the smallest crack. Consequently, if an assembly has stitch welds or material not sealed by a weld, the penetrant will travel behind the welds and between layers of unfused material. Penetrant can be nearly impossible to remove from these areas. Trapped penetrant will cause defects in welds if further welding is done or will bleed out over time and contaminate paint and process fluids.
For PT to be used on ASME Code construction or NBIC repair or alterations, a written procedure must be followed. This must comply with ASME Boiler and Pressure Vessel Code, Section V, Article 6, and address all essential and nonessential variables. Many liquid penetrant examinations are done for informational purposes only, and do not follow a written procedure. For instance, a written procedure does not need to be followed if a welder grinding out a weld crack for repair is using PT to ensure removal of the entire crack. However, if the PT is being done to comply with Code, the written procedure needs to be followed by qualified NDE personnel.
There are six basic steps to follow when using the dye penetrant solvent removable method.
  1. Pre-clean part. 
    This can range from grinding and wire brushing to merely wiping the part with a rag moistened with the cleaner/ remover. The surface needs to be free of dirt, rust, scale, paint, oil, and grease, and be smooth enough to wipe off the penetrant without leaving residue.


  2. Apply penetrant. 
    This is generally done by spraying penetrant from the aerosol can or applying it with a brush. A dwell (soak) time needs to be observed to allow for the penetrant to permeate into cracks and voids. This is typically 5 to 30 minutes but should never be long enough for the penetrant to dry. The penetrant manufacturer’s recommendations and written procedure should be followed.


  3. Remove penetrant. 
    All penetrant should be removed with clean, dry, lint-free rags until thoroughly clean. The part or material should be rubbed vigorously until the penetrant is not visible on the dry rags. Next, cleaner/ remover should be sprayed on another clean, dry, lint-free rag and used to vigorously rub the part again until there is no penetrant visible on the rag.


  4. Apply developer.
    A thin, light coating of developer should be sprayed on the part being examined. A dwell time needs to be observed to allow time for the dye to exit the flaws and create an indication (flaw) in the developer. The dwell time for developer is typically 10 to 60 minutes. The developer manufacturer’s recommendations and written procedure should be followed closely.


  5. Evaluate indications.
    It is critical to examine the part within the time frame designated in the written procedure.  Length of an indication can grow over time as penetrant bleeds out, causing an acceptable indication to be a rejectable defect.  Length of indication is measured for evaluation, not length of the flaw.  Here, the two linear indications are rejectable defects.  The round indication is nonrelevant. 


  6. Post-clean part.
    The part needs to be cleaned to remove all developer after it has been evaluated.

ASME Section V also requires the dye penetrant solvent removable method be evaluated with a minimum light intensity of 100 foot candles on the part surface. Proper quantity of light must be verified using some type of light meter.

Light meter showing 109.9 footcandles of light

In the ASME B & PV Codes of Construction, magnetic particle examination or liquid penetrant examination is called out many times to detect the possibility of surface defects. If material is nonmagnetic the only choice is PT. Some typical examples of ASME Code required examinations include:
  • castings for surface defects
  • plates for laminations in corner joints when one plate’s edge is exposed and not fused into the weld joint
  • head spin hole plug welds
  • weld metal build-up on plates
  • removal of defects before welding repair
Once boilers and pressure vessels are in service, PT can be a very valuable tool. The NBIC recommends PT for examination of: firetube boiler tube sheets to find leakage around tubes, external inspection of weld joints, evaluating components subjected to fire damage, historical boilers, fiber-reinforced thermosetting plastic pressure equipment, Yankee dryers, and pressure vessels in liquefied petroleum gas (LPG) service. 
During inservice inspections, PT should also be used in areas suspected of defects. These include, but are not limited to, nozzles (see Figure 1), tubesheets (see Figure 2), knuckles of heads (see Figure 3), and head spin hole plug welds (see Figure 4). To effectively use liquid penetrant on the tubesheet in Figure 2, an extensive amount of work would need to be done. All rust and scale would need to be removed so the penetrant could be cleaned off. Rolled, unfused tube ends would also bleed out dye and cause false indications. The head spin hole plug weld looks acceptable to the naked eyes, but shows many defects once it has been liquid penetrant examined (see Figures 5 and 6).

In conclusion, PT can be a very valuable tool during new construction and inservice inspections. PT does have limitations and is not the best method for all applications. However, for quick, low cost examinations in any location, PT is often the best choice of NDE methods.


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