The Pressure’s On
Considerations for selecting pressure and vacuum gauges
By Fred Workley
Hazards if the gauge fails
There are several modes of gauge failure including: a loss of accuracy, clogging of the pressure port, damage to the internal mechanism, a leak in the pressure containing parts or joints, a crack or fatigue of the Bourdon tube, rupturing of the Bourdon tube due to overpressure, or an explosion within the system due to a chemical reaction of the pressure medium with contaminants causing the Bourdon to explode.
Pressure gauges have an excellent record of safety. In most instances, the cause of failure has been misuse or misapplication. The following systems are considered potentially hazardous: compressed gas systems, oxygen systems, systems containing hydrogen or free hydrogen atoms, gas or liquid corrosive fluid systems, non-steady pressure systems, systems where a high overpressure could accidentally be applied, and pressure systems containing a flammable or explosive mixture or medium.
The range of the gauge should be approximately twice the intended operation pressure. Avoid applying pressure beyond the top of the scale. By design, the gauge should not have to continuously operate above 75 percent of its indication span. Bourdon tubes are necessarily highly stressed in ranges over 1,000 psi, and continuous operation at full scale will result in early fatigue failure and subsequent rupture. High internal stress could lead to a catastrophic failure. Pressure regulators, chemical (diaphragm) seals, pulsation dampers or snubbers, syphons and other similar devices are available for use in potentially hazardous systems. Generally the hazard increases with increases to very high operating pressures.
Excessive pressure pulsation, usually evidenced by continuous rapid pointer movement, could result in fatigue failure of the pressure element, or rapid movement-wear. A pulsation-dampening device installed between the pressure source and the gauge can eliminate cyclic pressure pulsations.
Some installations use a liquid-filled gauge. The liquid used is a viscous fluid, and these gauges have a sealed case. Some means of venting the case must be provided. In some cases, the vent is sealed to prevent the loss of fluid during shipment, and must be released prior to installation. Be sure to follow the installation instructions for proper venting of the gauge after installation. The liquid most commonly used is a mixture of glycerin and water. Be aware that when glycerin is combined with strong oxidizing agents including (but not limited to) chlorine, nitric acid, oxygen and hydrogen peroxide; potential hazards may result from the chemical reaction, such as ignition or explosion. Completely chlorinated or fluorinated fluids may be suitable for these applications calling for liquid filled gauges.
Excessive vibration could cause the movement to wear resulting in loss of accuracy. Vibration may also cause early failure of the Bourdon tube. Vibration concerns can be addressed with liquid-filled gauges. In addition, the gauge might be isolated by remotely mounting it on a non-vibrating surface and connecting with flexible tubing.
The Bourdon tube will fail after extended use, releasing the pressurized medium. The larger the numbers of applied pressure cycles, and the greater the extent of the pressure cycle, the earlier the premature failure may occur. Fatigue failure may be explosive. Precautions must be taken to direct or contain the release of the pressurized medium, such as having a box with a window over the gauge, or installing a restrictor. A restrictor will not reduce the immediate effect of failure, but it will help control the flow of escaping fluid following a rupture.
Safety and Scheduled Maintenance Protect Your Welding Assets By Mike Pankratz, Miller Electric Manufacturing Company October 2000 You are faced with the task of removing the engine from...
The ABC's of Engine Mount Inspection & Repair By Tom Heid October 2000 You are faced with the task of removing the engine from your customer's aircraft. It might be routine, scheduled...
A critical factor in tire life By Joe Escobar There are many factors that can shorten the life of an aircraft tire. One of these life shortening factors is operating at improper inflation...