Wear on Pressure Gauges In pursuit of a long-life pressure gauge Edited by Greg Rusznyak

Transcription

1 Press Release Pressure and Temperature Measurement ±750 words Wear on Pressure Gauges In pursuit of a long-life pressure gauge Edited by Greg Rusznyak Much has already been written on the durability of pressure gauges, especially when subjected to vibration. Vibration What happens to a pressure gauge during its lifetime? What happens, particularly when it is subjected to vibration? Typical wear, accelerated by vibration, consists of the following: Worn out bearings at the link (Figure 1). Broken adjustment U (Figure 2). Worn out segment shaft (Figure 3). Bourdon tube cracked at the top near the socket (Figure 4) Figure 1 Figure 2 C:\Documents and Settings\greg\My Documents\04 Marketing\02 Editorials & Press Releases\WIKA\ED_long life gauges\ed_longlifegauges doc Page 1 of 5 Created on: 21/07/2009

2 Figure 3 Figure 4 What is the best solution? Some manufacturers offer damped movements. In these designs, either the pinion shaft or the toothed segment (rack) is encased in a pot containing a viscous fluid. This provides the necessary damping forces to prevent or reduce movement damage. However, while the pointer is free of oscillation, the bourdon tube and connecting link are still subjected to severe vibration. As a result, the undamped parts wear very quickly. In this company s experience, the best solution is to fill the gauges. Aside from the obvious benefit of damping the amplitude of the oscillation both of the bourdon tube and the pointer, the filling fluid also lubricates the movement gear and the bearings. In a test conducted by WIKA (Julien & Schafer, 1993), according to a standard laid down by a leading petrochemical company, 4 gauges were subjected to the following conditions: Acceleration: 10g Frequency range: Hz Frequency change: 1 octave/minute Duration of test: 200 hours Page 2 of 5

3 Four instrument types were tested and the results are shown in the table below. Instrument Type Damped movement - single pinion viscous pot Damped movement - double pinion & quadrant arbour viscous pots. PTFE gear teeth Standard gauge - no damping Glycerine filled case - standard undamped movement. (86.5% glycerine 13.5% water) Zero Error After 50 Hrs 200 Hrs 2.3% 66% 3.9% 34% 2.3% 3.0% 0.6% 0.8% Instrument Condition after 200 Hours Inoperable, gear teeth & bearing necks severely damaged. Quadrant broke loose. Tube free to resonate Inoperable, severely worn connection linkage Still operable, but out of calibration Fully operational, still within calibration limits. No wear to the movement The results were conclusive. After 200 hours, the glycerine filled gauge is not only still operational, but also within calibration limits. Temperature effects When the gauge is used in ambient temperatures below 0 C, icing of the movement and other moving parts is common. Condensation inside the gauge housing may also ice up, obscuring the reading and even mechanically interfering with the operation of the instrument (Figure 5). Filled gauges are less susceptible to these conditions. Page 3 of 5

4 Figure 5 Movement improvements Once the movement of a pressure gauge is damaged or broken, the bourdon tube ruptures soon afterwards. This is due to the fact that once the movement is broken, it no longer has an anchoring effect on the end of the bourdon tube; leaving the bourdon tube free to oscillate more vigorously than before, resulting in its destruction. In September 2008, WIKA conducted tests to investigate various movement improvements. These include tests with lubricated movements, hardened movements and lubricated and hardened movements. During the test the gauges were subjected to an acceleration of 20g with a sweep of ±50 Hz around the natural frequency. An extract of the test results is shown in the table below. Instrument Type Movement intact Time (h) Bourdon tube breach WIKA standard movement 3 21 WIKA standard movement with only link lubricated WIKA hardened movement WIKA hardened movement with link lubricated It can be seen that the standard WIKA movement remained intact for 3 hours. After the movement Page 4 of 5

5 broke, the bourdon tube was breached 21 hours later. Contrasted to this, the hardened and lubricated movement remained intact for 110 hours. Summary Testing is conclusive: filled gauges offer the best solution to vibration, protecting not only the movement, but the connecting link and the bourdon tube as well. In addition to vibration damping, the filling fluid also acts as a lubricant. The filling fluid also prevents the icing of moisture inside the gauge. This icing often obscures the dial making reading difficult. In severe conditions the ice build-up may even interfere with the motion of the pointer. Improvements to movements, currently being investigated by WIKA, will significantly improve the ability of mechanical pressure gauges to withstand vibration. References Article: Safety and Long-term Accuracy of Bourdon Tube Pressure Gauges ; Julien, H & Shafer, K; 1993 Contact WIKA South Africa sales@wika.co.za Page 5 of 5