Some specifications for pressure gauges restrict design and manufacturing methods and require a specialized Bourdon tube construction. How the Bourdon tube is constructed can impact gauge life, and if it isn’t constructed in a suitable way for your application, you could face equipment damage.
There are many ways to construct a Bourdon tube for your pressure gauge. Which is best for your application?
This article will discuss the types of Bourdon tube construction and in which applications they work best.
Bourdon Tube Requirements
Several tests are recommended in ASME B40.1 (gauges, pressure indicating dial type – elastic element), which is a document established by a committee representing both gauge manufacturers and users. One of the more important criteria is the fatigue life of the Bourdon tube when subjected to pressure pulsations of 20 to 80 percent of scale range.
A Bourdon tube for the process industry should withstand at least one million cycles without failure. There are several requirements for making a robust Bourdon tube, starting with a low-stress design, and forming without scratches, sharp corners or other stress intensifiers.
Bourdon Tube Construction Types
There are several ways of making the tubing for Bourdon tubes, and good or bad material can result from any of them.
Seamless Construction
A white-hot billet is extruded or pierced into a 2- to 3-inch diameter “tube shell,” which must be reduced to the size needed in a long and cumbersome series of drawing and annealing operations. Each one of these steps provides an opportunity to create defects and to make eccentric tubing.
The casting and extrusion operations are the most common cause of defects but drawing and annealing present opportunities for oxidation and excessive grain growth if proper procedures are not followed. Some specifications require seamless tubing, which is no guarantee of a satisfactory life.
Welded and Drawn Construction
This tubing starts by continuously forming strips into a round tube, which is then welded under inert shielding gas. It is then given a few drawing and annealing operations, which homogenize the weld so that it is virtually indistinguishable from the parent metal.
Since the tube starts with a strip of consistent thickness, the resulting tube is more concentric than the seamless tube. Strip material is inherently less prone to defects than tubing. Also, both strip surfaces are visible for inspection before forming into the tube.
As-Welded Construction
This tubing is made from annealed strip, similar to welded and drawn, but is not subjected to any further operations. The welded tubing is formed directly into the finished Bourdon tube. The weld is oriented on the lowest stressed area of the Bourdon tube cross-section on the flat, instead of on the edges. Ashcroft, Inc., holds a patent on this method of manufacture.
Ashcroft manufactures the majority of our tubes with the as-welded method (see Figure 1 below). This allows us to place the weld in the lowest stress region of the tube and allows better control over the wall thickness of the tube. The result is longer and more consistent tube life.
Figure 1: Ashcroft Bourdon Tube Construction
We don’t like to pressure you, but we have more information.
Now that you better understand the construction of Bourdon tubes for your pressure gauge, you can choose the right solutions for your application. Regardless of which manufacturing method is used, it is important that the tubing be dimensionally consistent and free of defects. One of the basic requirements for good gauge performance is a Bourdon tube with long life.
As part of the design process, Ashcroft gauges are rigorously tested for the ability to maintain accuracy and pressure boundary integrity while withstanding vibration, pulsation and other conditions encountered in the field.
If you want to learn more about pressure gauges, here are some related articles:
- Choosing the Right Pressure Gauge Dial Size
- How Can I Order Custom Dials for Pressure Gauges?
- How To Identify Your Pressure Gauge and Get the Proper Replacement
- How Does Temperature Affect Pressure Gauge Performance?
- How Do I Select the Right Pressure Gauge Range?
- How Often Should I Check the Calibration of My Pressure Gauge?
Feel free to contact us with any questions or concerns. Our industry experts at Ashcroft can help answer your questions and get you back on track!
