Yankee Dryer Shell Thickness Measurement Techniques


For the most accurate result, a cold measurement of the dryer using measurement tape is recommended


This article discusses yankee dryer shell thickness measurement techniques and reviews the types of measurement differences (variances) that may occur depending upon the measurement method, the shell conditions (temperature and pressure), and the shell materials (gray cast iron and welded steel).

Information and calculations are provided to help quantify measurement differences (variances) at different shell conditions that help to provide guidance on adjustment of these measurements for review of different shell thickness reports in yankee dryer files.


The most accurate shell thickness determination is obtained by using a circumference measuring tape (or diameter measuring tape) of the yankee dryer when it is in a “cold” condition (yankee dryer at room temperature and no internal pressure).

  • Ultrasonic thickness measurement should never be used to record gray cast iron shell thickness due to the large variation in sound velocity of cast iron within the same casting. Data indicate a minimum-to-maximum sound velocity spread of about 6 percent in high-strength gray cast iron.

The purpose of this article is to share information on yankee dryer shell design thickness and ways that it can be measured. Basic equations are shared that show how shell measurement differences (variances) can be explained or accounted for so that the most accurate shell thickness can be recorded and reported. Recommendations for measurement techniques will help in improving the understanding of how yankee dryer shell thickness can be measured and how that shell measurement can be used to permit safe operation of the yankee dryer pressure vessel and protect papermaking personnel and the general public.


There are two types of yankee dryer shells – plain (smooth) bore and grooved (ribbed) bore.

Many years ago, all yankee dryers had plain (smooth) bores. In the 1950s and 1960s, the first dryers had their shell inner surface grooved so that heat transfer through-the-shell (drying rate) could be improved. Today, almost all yankee dryers are of the grooved (ribbed) bore design.

In yankee dryers with a plain (smooth) bore, the total thickness is the actual shell thickness. There is no adjustment necessary to the thickness when calculating the maximum allowable working pressure (MAWP), the pressure-retaining capability of the vessel.

In yankee dryers with a grooved (ribbed) bore, the effective thickness of the shell needs to be calculated (adjusted) and then used to calculate the MAWP of the grooved yankee dryer.


Calculating the Area if Solid

Grooved (Ribbed) Bore

Plain (Smooth) Bore

Calculating the Groove Area


Calculating the Effective Thickness of a grooved yankee dryer shell:

The effective thickness calculation looks at the shell as if it is solid, then the area removed by grooving is subtracted and the effective thickness is calculated. Start by calculating the area of a “repeating unit” of the shell. For every groove there is a rib on either side. For every rib there is a groove on either side. Thus, the repeating unit is one rib width and one groove width.

Step 1: Calculate the “Area If Solid.”

This is equal to the total thickness x total width (1 rib width + 1 groove width).

Step 2: Calculate the “Groove Area” (the area machined away).

This is the sum of the areas of the groove rectangle and the groove bottom. NB: The groove bottom can be simple or complex depending on vendor design. Some groove bottoms are “half-round” and some groove bottoms have complex (compound) curvature or corner radius with flat-bottom. Look at detailed vendor drawings or calculations to determine how to calculate the groove bottom of a specific yankee dryer.

Step 3: Subtract the groove area from the area if solid to calculate effective area.

This is the effective area of the grooved shell repeating unit (one rib and one groove).

Step 4: The effective area is divided by the total width (rib width + groove width) of the repeating unit and the result is the effective thickness of the shell.

NB: Typically, the effective thickness is about 80 to 85 percent of the overall total thickness.


The shell thickness as well as the shell material allowable stress are the primary inputs into the calculation of MAWP. The MAWP defines the pressure that can be retained by the pressure vessel. Typically, the operating pressure is several psi less that the MAWP to allow for some variance in the operation (protective release pressure) of safety relief valves (SRV).

The yankee dryer shell is designed to be ground during its papermaking lifetime to permit the occasional or as-needed refurbishment (regrinding) of the shell outer surface: the papermaking surface. When the outer surface is reground, the root thickness and the total thickness is reduced. The effective thickness must then be recalculated as the effective area is smaller (as a percentage) when compared with the original shell design calculation. Typically, the vendor supplies an operating curve that has MAWP rating as a function of root thickness or effective thickness.


There are two primary methods to measure shell thickness: circumference measuring tape;

and ultrasonic thickness meter. There are advantages and disadvantages to each technique.

Measuring Tape

  1. Tapes can be used for any shell condition, including measurements while the yankee dryer is under steam pressure and is at high temperatures.
  2. Tapes are the most accurate measuring technique for both gray cast iron and welded-steel shells.
  3.  Tape measurement technique results in an average shell thickness measurement; it cannot provide a thickness measurement in a specific local area or location on the shell.

Ultrasonics (UT)

  1. UT should only be used at cold yankee dryer conditions. High temperatures affect the shell material sound velocity. High temperature can also cause erratic UT measurements, or failure of the transducer due to over-heating. The UT transducer should not be allowed to reach temperatures higher than 122°F (50°C).
  2. UT is good at comparing thickness of local areas of steel shells to determine if there might be local thin areas (LTAs).
  3. UT is good for locating steel shell structural welds. Weld material normally has a different sound velocity compared to the base plate material. Thickness readings that are significantly different may indicate the transducer has found a circumferential or axial weld in the steel shell.
  4. UT should never be used to measure the thickness of gray cast iron shells due to the high variability of sound velocity in gray cast irons (approximately 6 percent variance in sound velocity within the same casting).


There may be differences in shell thickness measurements from time to time. The shell does not actually grow or shrink. However, the shell thickness may decrease over time due to abrasive/adhesive wear and after crown surface re-grindings.

The following conditions will result in shell thickness variances (differences) from previous inspection measurements:

  1. Exterior coating thickness differences on the outer surface of the yankee dryer.
  2.  Measurement tape reading and reporting differences.
  3.  Tape tension difference when using the circumference measuring tape.
  4.  Temperature change (thermal growth) of the shell and the measuring tape.
  5.  Pressure ballooning of the shell due to internal steam pressure.
  6.  UT measurement variances due to shell material velocity of sound variability.

These shell measurement conditions will be discussed and equations included so that theoretical measurement variances can be calculated for specific yankee dryer size and conditions.


There are two types of yankee shell coatings to be considered. There will always be films (chemicals, organics) to promote tissue production and creping on the papermaking surface. There may also be thermal-sprayed coatings (metallized coating) on the metal shell to promote uniform sheet drying and to minimize need for crown re-grinding. Typically, these hard coatings are metallic or ceramic in nature and vary between 0.020 in. and 0.040 in. (0.5 mm to 1 mm).

Steel yankee dryers are normally supplied with a thermal-sprayed coating. The hard coating is sprayed onto the yankee dryer after the papermaking crown has been completed.

As long as the hard metallized coating is on the yankee dryer outer surface, no wear of the base metal shell has occurred. Therefore, until the hard metal (or ceramic) coating is removed, the shell thickness is still at the original dimension as supplied.

To measure the yankee dryer shell with coatings on the surface, document the thickness of the thermal-sprayed coating and if any chemical/organic films are remaining on the surface. There are techniques to measure coating thickness; some are based on magnetic properties and electrical properties.

Techniques and methods are too numerous to discuss in detail. Ask the coating applicators for their recommendations. If the coatings are not magnetic, then a simple gauge based on magnetic “lift-off” can be used. This gauge has a magnet and spring that can be controlled by a dial marked in thousandths of an inch or in metric. The dial pulls on the spring until the magnet is pulled off the surface. At this instant, the dial reading is recorded and the total thickness of the coatings is reported.


The circumference measuring tape is a thin metal tape that is marked in either inches or in millimeters. It is provided in either spring steel or in stainless steel. The major markings on inch tapes are usually to the nearest 0.025 in. The actual reading is determined by looking at the nearest number and then by adding the vernier dimension (to the nearest 0.001 in.). The actual measurement is the sum of the main reading and the vernier gage reading. This addition step (main reading and vernier gage reading) may be where math errors occur.

NB: The circumference measuring tape marked in millimeters has major markings to the nearest millimeter. The actual reading is determined by looking at the millimeter reading and then by looking at the vernier gage to measure the dimension to the nearest 0.1 mm.

Diameter measuring tapes are also available with markings in pi inches or pi millimeters. Instead of measuring the circumference, measure the diameter of the yankee dryer. Regardless of the type of measuring tape, there may be reading errors when looking at the tape and the vernier to determine the actual measurement. Also, there may be errors in writing down the measurement results in the inspection report.

The measurement of the yankee dryer circumference or diameter is converted to a radius. Any change in the yankee dryer radius is a change in the yankee dryer shell thickness. If there is a range of circumference measurements (a difference of the readings) a typical practice is to delete the high and low values and then average the remaining values.


The following is a summary table of differences (variances) in measurements due to tape tension, shell temperature, shell pressure, and ultrasonics (UT).

The thickness differences (inches) – summary table (Fig. 5) is based on the following assumptions:

1) Yankee diameter (nominal) of 18 ft (216 in., 5,500 mm).

2) Shell total thickness of 2.8 in. (71 mm).

3) Temperature of saturated steam at 110 psi-g is 344°F (173°C).

4) Internal steam pressure of 110 psi-g (7.58 bar-g).

5)Tape tension of 15 lb-force (6.8 kg-f) and a spring steel measuring tape.

6)Materials properties used in these calculations are for alloys ASME SA-516 Gr 70 (steel) and ASME SA-278 CL 60 (cast iron).


Measuring tapes are usually supplied with a calibration certificate. This certificate lists the tape serial number and the result of measuring a cylinder with a documented circumference or diameter. The certificate should indicate the temperature of the tape and cylinder as well as the tension used when pulling the tape around the cylinder.

The temperature used in calibration is typically 68°F (20°C). The tension force used in calibration is typically 5 lb (2.27 kg-f).

When using the measuring tape for a yankee dryer, the tendency is to pull hard so that the tape lies flat against the shell and is properly aligned around the circumference. If too much tension force is applied, the tape will be stressed and will be stretched elastically. This stretch can be estimated by calculating the stress and then using that information to calculate the elastic strain (stretch) of the tape.

If the tape is stretched, the tape is actually longer than it was when it was calibrated, and the yankee dryer measurement will be smaller/shorter than it really is. The amount of tape stretch will be a function of the tension force that is greater than the calibration tension force, the tension force difference. In this calculation, assume that the field tension force used is 15 lb and the calibration tension force is 5 lb. Thus the tension force difference is 10 lb. This also assumes a spring steel measuring tape.

Tape Tension Stress = Tension Force Difference/Tape Area


Tension Force Difference = 10 lb

Tape Area = 0.5 in. X 0.010 in. = 0.005 in.2

Thus, Tape Tension (force difference) Stress = 2,000 psi

Tape Tension Strain

Tape Stress = Tape Modulus X Tape Strain

Tape Strain = Tape Stress/Tape Modulus


Tape Stress = 2,000 psi

Tape Modulus = 30,000,000 psi

Thus, Tape Strain = 0.0000666 in./in.

Tape Elastic Stretch = Original Tape Length X Tape Strain


Circumference – Tape Elastic Stretch = 678.583 X 0.0000666 = 0.045 in.

Diameter – Tape Elastic Stretch = 216 X 0.0000666 = 0.014 in.

Radius – Tape Elastic Stretch = 108 X 0.0000666 = 0.007 in.

Shell Thickness Variance = 0.007 in.

NB: The shell thickness is smaller than expected by 0.007 in. (0.18 mm).

This assumes a tape tension force difference of 10 lb. If the tape tension force difference was zero pounds, there would be 0-in. tape tension variance.


When the yankee dryer is measured hot it is usually during a crown grind or a refurbishing of the papermaking surface. The yankee dryer is heated by saturated steam at a pressure less than the normal operating pressure. Typically, the grinding pressure is 100 psig to 110 psig (psi-gauge). The inside of the shell would be at the saturated steam temperature of 344°F (110 psig). The shell outer surface should be at some temperature less than the inside. The measuring tape wrapped around the shell will be at some temperature slightly less than the shell outer surface.

The actual metal temperatures depend on the heat flow through-the-shell, heat losses to the machine room, time of contact of the tape with the shell, etc. Thus, measurements may change over time depending on heating (or cooling) effects of the environment and contact of the measuring tape with the shell.

The good news is that there is thermal growth of the yankee dryer shell and thermal growth of the spring steel measuring tape. Therefore, the overall effect of thermal growth is somewhat offset and not as bad as could be expected.

When calculating a thickness variance due to hot temperatures, estimate the various temperatures of the yankee dryer and of the measuring tape.

Know the thermal expansion coefficients of the shell material and of the measuring tape material. The thermal expansion coefficients for gray cast iron yankee, steel yankee, and spring steel measuring tape are all different.

Dimension at Temperature = COLD dimension +

dimension change

Dimension change = cold dimension X (temperature increase x thermal growth coefficient)


Shell temperature increase = 344-68 = 276°F

Tape temperature increase = 334-68 = 266°F

NB: This assumes a 10°F temperature difference between the yankee dryer shell and the measuring tape.

To calculate the temperature effect on the shell thickness variance, consider the two different combinations of shell materials and the spring steel measuring tape.

Case 1: Steel yankee dryer and spring steel tape

Steel thermal growth coefficient (ASME SA-516 Gr 70) = 0.0000069 in./in./deg.F

Spring steel growth coefficient (AISI 1095) = 0.0000065 in./in./deg.F

cold circumference dimension = 678.583 in.


Dimension at hot temperature

Steel shell circumference HOT = 678.583 + 678.583 x (276 x 0.0000069) = 679.875 in.

Circumference tape length HOT = 678.583 + 678.583 x (266 x 0.0000065) = 679.756 in.

Temperature (steel) variance = 679.875 – 679.756 = 0.119 in. (circumference)

Temperature (steel) variance = 0.038 in. (diameter)

Temperature (steel) thickness variance = 0.019 in. (thickness, radius)

NB: Steel yankee shell thickness is larger than expected by 0.019 in. (0.48 mm).

Case 2: Cast iron yankee dryer and spring steel tape

Gray cast iron thermal growth coefficient (ASME SA-278 CL 60) = 0.0000064 in./in./deg.F

Spring steel growth coefficient (AISI 1095) = 0.0000065 in./ in./deg.F

COLD circumference dimension = 678.583 in.

Dimension at hot temperature

Gray cast iron circumference HOT = 678.583 + 678.583 x

(276 x 0.0000064) = 679.782 in.

Circumference tape length HOT = 678.583 + 678.583 x (266 x 0.0000065) = 679.756 in.

Temperature (cast) variance = 679.782 – 679.756 = 0.026 in. (circumference)

Temperature (cast) variance = 0.008 in. (diameter)

Temperature (cast) thickness variance = 0.004 in. (thickness, radius)

NB: Gray cast iron yankee shell thickness is larger than expected by 0.004 in. (0.11 mm).


When the yankee dryer is measured hot it is usually during a crown grind. The yankee dryer is heated by saturated steam at a pressure less than the normal operating pressure. Typically, the grinding pressure is approximately 100 psig to 110 psig (psi-gauge). Pressure on the inside of the shell causes a ballooning effect (elastic tension stress).

The circumferential pressure tensile stress of a cylindrical pressure vessel can be expressed as:

Stress = Pressure x Diameter / (2 x thickness)


Pressure = 110 psi-g

Diameter = 216 in.

Effective thickness = 2.440 in.

Shell pressure stress = 4,869 psi

Pressure balooning of the shell







The shell pressure tensile stress increases with higher pressure and larger diameter. The amount of pressure ballooning is the elastic stretch of the wall due to the stresses. The stretch of the shell is related to the tensile stress and the stiffness of the shell material which is called the tensile modulus. The equation that relates stress, modulus (stiffness) and strain (stretch) is:

Stress = Modulus x Strain

Thus: Shell strain = shell pressure stress/shell modulus

Steel has a higher tensile modulus of elasticity than gray cast iron. Therefore the expected pressure ballooning of a yankee dryer steel shell will be less than the pressure ballooning of a yankee dryer gray cast iron shell if everything else is kept equal (same diameter, same pressure, same thickness).

Case 1: Steel Yankee dryer

Strain = Shell Pressure Stress/Shell Modulus


Shell pressure stress = 4,869 psi

Steel shell modulus = 30,000,000 psi

Pressure (steel) strain = 0.0001623 in./in.

Dimension change due to pressure ballooning

Pressure dimension = original dimension + (original dimension x strain)

Pressure (steel) circumference = 678.583 + (678.583 x 0.0001623) = 678.693 in.

Pressure (steel) variance = 678.693 – 678.583 = 0.110 in. (circumference)

Pressure (steel) variance = 0.035 in. (diameter)

Pressure (steel) variance = 0.018 in. (thickness, radius)

NB: The steel yankee shell thickness is larger than expected by 0.018 in. (0.46 mm).

Case 2: Gray cast iron yankee dryer

Strain = Shell Pressure Stress/Shell Modulus


Shell pressure stress = 4,869 psi

Cast iron shell modulus = 21,500,000 psi

Pressure (cast) strain = 0.0002264 in./in.

Dimension change due to pressure ballooning

Pressure dimension = original dimension + (original dimension x strain)

Pressure (cast) circumference = 678.583 + (678.583 x 0.0002264) = 678.737 in.

Pressure (cast) variance = 678.737 – 678.583 = 0.154 in. (circumference)

Pressure (cast) variance = 0.049 in. (diameter)

Pressure (cast) variance = 0.024 in. (thickness, radius)

NB: Gray cast iron yankee shell thickness is larger than expected by 0.024 in. (0.61 mm).


The speed of sound (sound velocity) varies greatly depending upon the material that transmits the sound. The speed of sound is slowest in air, moderately fast in liquids, and fastest in solids. Thickness measurements using ultrasonics are based on Sir Isaac Newton’s Laws of Motion:

Distance = velocity x time

The sound velocity must be known to calculate the distance. If the velocity is not constant within a material, the distance can only be estimated.

Cast iron alloys have a wide range of sound velocity. Lower strength cast iron alloys have a minimum sound velocity of 0.1400 in. per micro-second. Higher strength cast iron alloys have a maximum sound velocity of 0.2200 in. per micro-second. Thus the sound velocity of very different strength cast iron alloys varies by 60 percent (minimum-to-maximum).

In contrast, steel alloys have a smaller variation to their range of sound velocity. Steel alloys have a sound velocity of 0.2300 in. per micro-second (+/-) with a variation of about 1 percent for any specific alloy grade based on different composition, heat treatments, cold working, etc.

Low magnification of steel weld.

The speed of sound will vary in a material based on its temperature. Ultrasonic measurements at higher temperatures are not recommended due to transducer damage. Recommended usage cycle is 10 seconds maximum contact with hot materials followed by one minute of air cooling. However, the transducer itself should never be heated above 122°F (50°C).

The speed of sound in a metal material will also vary based on grain size and on micro-structural phase differences. For example, castings will have larger variations in sound velocity than rolled and forged materials. Weldments also have large grain size issues compared with base metal. Gray cast iron is known for the graphite flakes in its microstructure. (See photos for examples of microstructure differences.)

High magnification of gray cast iron graphite flakes.




Case 1: Steel sound velocity

Steel yankee dryers have a shell that is fabricated from rolled plates. The plates are purchased as flat rectangles which are rolled (cold worked) to the proper diameter/radius. The curved plates are then welded together to form the cylindrical shell. A longitudinal (axial) weld is needed to make the curved plate section into a shell segment or can. Depending on the face length of the yankee dryer shell, there may be two or three can segments. Shell cans are joined to other shell cans using circumferential welds. The completed shell is then joined to the heads of the yankee dryer by either bolting or circumferential welding. (See photos of cylindrical shells with several welds.)

Accuracy of steel shell thickness can be improved by using a steel calibration block that is similar in thickness to the shell. The block should be machined from excess shell material so that it is the same alloy and so that it is in the same strength and cold worked condition as the shell. Otherwise the sound velocity of the calibration block may be different than the shell sound velocity.

If there are some areas that tend to produce UT thickness results that deviate from most of the others, these variances should be investigated to determine if they are measurements taken in steel weldments. There are several circumferential and axial (longitudinal) welds in a cylindrical shell.

To summarize, steel alloys have a sound velocity of approximately 0.2300 in. per micro-second (+/-) with a variation of about 1 percent for any specific steel alloy grade based on different composition, heat treatments, cold working, weldments, etc.

NB: The steel yankee shell UT thickness may vary by 1 percent or 0.028 in. (0.71 mm).

Case 2: Gray cast iron sound velocity

Lower strength cast iron alloys have a minimum sound velocity of 0.1400 in. per micro-second and higher strength cast iron alloys have a maximum sound velocity of 0.2200 in. per micro-second. This is a very wide range in sound velocity; a minimum to maximum range of 60 percent. It is obvious that the sound velocity is needed for the yankee dryer specific cast material.

Typically cast iron yankee dryers are fabricated from ASME SA-278 CL 60 gray cast iron.

To determine a better assumption of sound velocity for yankee dryer shell cast iron a study was conducted. Calibration test blocks were machined from excess material from a yankee dryer shell casting. The material met the gray cast iron specification for ASME SA-278 CL 60. The design allowed easy access for measuring thickness locations with a micrometer and then taking ultrasonic measurements. When adjusting the ultrasonic meter to obtain the correct micrometer reading the sound velocity can be accurately measured at that specific location.

Each calibration block was machined to allow for several micrometer and ultrasonic measurements of the total thickness and root thickness. Eighteen calibration blocks enabled six measurements on each block for a total of 108 data points. This allowed known comparison of the actual thickness to the ultrasonic measurement so that the actual sound velocity can be reported.

The sound velocity in this gray cast iron shell casting per ASME SA-278 CL 60 varied from a minimum of 0.2010 in. per micro-second to a maximum of 0.2130 in. per micro-second. The range from minimum to maximum for all data (108 measurements) was 6 percent.

When looking at the individual calibration blocks, the maximum variation within any calibration block was 3.5 percent. In other words, in a small area of the yankee dryer shell with one groove and ribs on either side, the variation in the correct thickness would be 3.5 percent or 0.100 in. (2.54 mm). This is a huge error/variance.

NB: The gray cast iron yankee shell UT thickness may vary by 6 percent or 0.168 in. (4.27 mm).


There are many diameter sizes of yankee dryers and many different shell designs. The basic equations are shared so that differences can be calculated when measuring the yankee dryer under different conditions.

There are several circumferential and axial welds in a cylindrical shell.

The largest difference/variance to be expected is when using ultrasonics for thickness measurement of gray cast iron shells. The nominal variance of 6 percent is primarily due to graphite flakes and the cast microstructure.

For the most accurate measurement of the yankee dryer dimensions and shell thickness, cold measurement of the yankee dryer using measuring tape is recommended.

The tape tension effect is the same regardless if the shell measurement is cold or hot. When the yankee dryer is hot, the pressure ballooning and the temperature effects of saturated steam in the yankee dryer need to be considered. The expected sum of differences between cold and hot dimensional measurement for an 18-ft (5,500 mm) diameter yankee dryer is:

Steel yankee: 0.036 in. (0.91 mm) for steel shells

Cast iron yankee: 0.030 in. (0.76 mm) for cast iron shells.

NB: Refer to the Thickness Differences (in.) – Summary table. Fig. 5.

To understand the thickness information for your yankee dryer, it is recommended that you:

  • Use cold measuring tape calculations of shell thickness.
  • Use a written procedure that lists all of the requirements for shell inspection. Make sure the measurement procedure has the who-what-when-where-why questions addressed to ensure conformity in technique and data from one inspection to the next inspection.
  • Record the conditions of the shell when taking any measurements.


Shell temperature

Shell pressure

Inspector Name

Inspection measurements

Calibration information

Any adjustments that were used to compare cold with hot

Keep separate yankee dryer thickness record sheets to record cold and hot thickness measurements; otherwise, you may get confused when reviewing the information at some future date.


Conversion Units – US to Metric (SI)


US Units SI Units

1 Inch 25.4 Millimeter (mm)

1 PSI 689.5 Kilopascal (KPa)

1 PSI 0.06895 Bar

1 Degree F 0.556 Degree C (deg.C)

1 Pound-Force 0.454 Kilogram-Force (kg-f)


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