Since being patented by Eugene Bourdon in 1849, the bourdon tube pressure gauge has played a crucial role in mechanical pressure systems.
Although the basic components have remained the same (bourdon tube, socket, movement and indication), subtle changes in bourdon tube pressure gauge design have allowed it to evolve with the changing requirements of industry.
The primary role of a bourdon tube pressure gauge (gauge or pressure gauge) is to provide local indication of how a process is performing. By checking the pressure gauge’s measurement of the pressure in the system, operators or maintenance personnel can quickly determine if equipment is operating at optimal efficiency.
A drop in pressure is an indication of leakage in the system while a pressure spike can indicate a blockage in the system, possibly in a filter or valve. While there are many other ways these problems can be identified, a pressure gauge usually provides the first intimation of a problem. The gauge’s value comes from its ability to show changes in system pressure, allowing technicians, engineers and operators to decide if there is an issue. A shut-down can then be scheduled for preventative maintenance rather than having an unplanned shut-down and costly repair after a breakdown.
Early pressure gauges were essential in many applications, but their effectiveness was hindered when vibration (the shaking of machinery) or pulsation (continuous pressure spikes of media) were present. Vibration and pulsation in the system cause the pointer to bounce or shake rapidly. This makes it impossible to get an accurate reading of system pressure, nullifying the effectiveness of the gauge. The increased cycling of the gauge movement causes premature wear, frequently requiring the gauge to be replaced.
Manufacturers began offering liquid-filled versions of pressure gauges for applications with excessive amounts of vibration and pulsation. A liquid-filled gauge is a standard gauge where the case is filled with a viscous liquid. The liquid selected for the fill is dependent on process and ambient temperatures and its chemical compatibility with the process media. Some common fill fluids include glycerin, glycerin/ water mix, silicone and halocarbon.
By filling the case with liquid, manufacturers can mitigate the effects of vibration and pulsation on pressure gauges. The liquid in the case provides hydrostatic drag (resistance) against continuous movement. The fluid also acts as a lubricant for the gauge internals, eliminating the threat of premature wear and reducing the probability of damage from vibration and pulsation.
The use of liquid-filled pressure gauges solves the issues of vibration and pulsation but introduces numerous other problems. The main issue involves the possibility of liquid leaking from the gauge.
While being shipped, or after being installed, there is the potential for liquid to leak out of the pressure gauge. This decreases the gauge’s ability to dampen vibration, negating its effectiveness. Also, depending on how much liquid has leaked from the gauge, the meniscus can be resting at a level that makes the gauge difficult to read.
The final and most serious leak-related problem occurs when the gauge is installed. The liquid that ends up on the floor is extremely slippery, resulting in a hazardous working environment. This can prevent liquid-filled gauges from being installed in facilities with strict safety guidelines.
If a liquid-filled gauge needs servicing or recalibration, the process becomes more involved than with a dry gauge. The gauge must first be emptied and dried before being serviced. Once it has been serviced or recalibrated, the gauge then needs to be refilled before being reinstalled. This increases the turnaround time of the pressure gauge.
The fill fluid added to a pressure gauge also affects the temperatures in which it can operate. Glycerin, which is often the standard fill in liquid-filled gauges, has a temperature range of approximately -20°C to 65°C (-4°F to 150°F). A specialized fluid is required for temperatures outside of this range which can be more expensive and result in longer delivery times.
Discolouration is another external environment issue associated with liquid-filling. Gauges that are installed in locations with prolonged exposure to sunlight, or gauges exposed to ambient or process media heat above fill fluid specifications, can experience discoloured liquid. This makes it difficult to read the pressure of the process.
Furthermore, a filled gauge (regardless of size), is twice the weight as a dry gauge. The added weight will result in higher transportation costs and fuel consumption.
As pressure gauges continued to evolve, a solution was engineered that provides vibration and pulsation dampening without the negative aspects of liquid-filling. Winters’ StabiliZR™ pressure gauges are dry gauges capable of eliminating pointer flutter caused by vibration and pulsation.
StabiliZR™ gauges are engineered with a pointer shaft that extends out the back of the movement with two paddles that are added to the shaft. This extended portion of the shaft is encased by a cap filled with dampening compound. The cap is then permanently sealed, preventing the dampening compound from leaking.
With the small amount of fluid being contained in a sealed cap, the StabiliZR™ gauge combines the best features of a dry gauge and a liquid-filled gauge. There is no risk of the gauge leaking, which means the gauge will always be easy to read and there is no risk of slipping caused by leaked fill fluid. Servicing and calibration can be completed faster by removing the need to drain, dry and then refill the gauge.
Because the case isn’t filled, StabiliZR™ gauges can operate over a wider range of temperatures (in some cases -40°C to 120°C (-40°F to 250°F)). The gauge can also be installed in direct sunlight without the risk of discoloured fill fluid obstructing the reading of the gauge.
Pressure gauges are often overlooked in a process. Their cost relative to other components can relegate them to an afterthought when designing, operating and maintaining equipment. However, the gauge’s role as an indicator of efficiency or potential failure, make it critical in a process’ overall health.
Its journey from dry to liquid-filled to StabiliZR represents continuous improvement as pressure gauges are adapted to the changing demands of industry.