Devil in the Details: Measuring Complex Pressure Vessels

The assurance of accurately placed features on complex units did not exist using conventional measurement tools and analog methods. But by moving into advanced metrology, this manufacturer now completes inspection in the desired detail and generates other benefits as well.

Cannon Boiler Works (CBW; New Kensington, PA) designs, manufactures, and services boilers and pressure vessels, including an extensive line of feed water heaters, condensing economizers, heavy duty economizers, vent condensers, steam accumulators and custom heat exchangers, as well as waste heat boilers, finned tubing products, inter- and after-coolers, lube oil coolers, air coolers, and other specialty products.

THE PROBLEM
For almost 40 years, CBW has been committed to helping produce energy in the most efficient manner possible. As a forward-thinking leader in the efficiency of industrial energy, their products reduce fuel costs and the emission of greenhouse gases.

For example, a wide variety of heat exchangers, both new and rebuilt, helps their customers solve many difficult corrosion problems by using sophisticated material selection and creative design features that greatly improve the performance and longevity of these exchangers.

The large air-cooled, high-temperature heat exchangers that are the subject of this examination are produced in three sizes: a “300 size” is actually 25 in x 25 in x 54 in, a “1500 size” measures 49 in x 80 in x 79 in, and a “3000 size” is 87 in x 100 in x 109 in.

To properly inspect the 300, approximately 62 different features and 80 constructions must be measured. The 1500 raises these numbers to 150 features and 280 constructions, while the 3000 increases even higher, to 200 features and 290 constructions.

All of these exchangers have structural bolting flanges on the shell side and finned tubes with pipe headers on the tube side. Technicians must measure and report the tube side flanges relative to the centers of the structural flange bolt hole patterns and report the integrity of the structural flange bolt hole patterns themselves as they tend to distort after welding.

Making these measurements requires a combination of levels, plumb bobs, tape measures, straight edges and a geometry-based metrology. By virtue of the construction of these units, many features cannot be measured using analog methods like these. Once completed, the largest unit weighs nearly eight tons and the shell-side passage is completely blocked with a finned tube array. This means that once completed, the only access to the bottom from the top is around the side of the unit.

Imagine the perimeter of the top of the unit is a rectangular bolting flange with 48 holes and the bottom is a bolting flange with 52 holes. These patterns must be measured and each hole located to within 3/32 in. Then the bolting pattern centers must be located within 1/4 in of one another in two dimensions.

The analog methods of measuring were a mixture of not capable, not accurate enough, too complicated, and had the potential for human error with the manual reporting required. These tools and methods also introduced many variables that could alter measurement accuracy.

For example, when CBW tried to find if a connection was centered between two rows of bolt hole centers 100 in apart, they first had to “fit” the rows to separate lines and measure toward the connection perpendicular to one of those lines. This proved extremely difficult, if not impossible, using traditional, analog tools and methods.

THE SOLUTION
As a solution to the measurement deficiencies of these traditional tools, CBW turned to FARO Technologies (Lake Mary, FL). Until the purchase and implementation of a FaroArm and CAM2 software, a number of features requested by customers for measurement were simply not possible using analog tools and methods.

In the example of the fabricated bolting flange patterns that are measured relative to the pattern center as well as other features relative to this same center, the FaroArm makes it possible to measure and report these and other features.

The CAM2 software and its iterative capabilities provide the shop with insight into fabrication methods and sequences that help them eliminate unit-to-unit variances in manufacturing. This yields a more consistent and precise product. The depth in quality that the new metrology offers cemented end customer confidence in the CBW product line.

This is an interesting application in that CBW calculates imaginary (non-measureable) center points of a rectangular bolt hole pattern and reports the centers of pipe header outlets relative to those points. The software must also perform an iterative fit of these structural flange bolt hole patterns and report any holes that fall out of “tolerance” in order to give a “real world” bolt-up acceptance or rejection.

In this particular application, it’s sort of like an advanced go-no go gauge on steroids. What makes all of this especially interesting is that this is done with five device positions while holding 1/8 in tolerance on a fully welded assembly with many feet of weld between measured features.

Because of the blocked shell-side passage on the heat exchangers, a number of device moves (leapfrogs) must be performed to construct each flange from data taken in four different positions.

Leapfrogging is a technique used with portable measuring devices like the FaroArm that extends their working volume by measuring reference points in one position and again after moving the device to a new position. The reference points link to the new device position, allowing for measurements to be taken and reported in the same reference frame.

The desired level of measurement of up to five device positions with so many features and constructions initially took some time and a healthy dose of help from FARO customer service to develop. Applications engineer Todd Wilson helped to develop a core programming and methodology for a single product size.