While mechanical pressure gauges have been mainstays in the process industry, they have also brought some challenges with them. Still, when a situation calls for a simple and inexpensive device with a local display, a gauge can fit the bill in a variety of applications.
Traditional gauges have some serious drawbacks to consider in the selection process. Gauges operate using delicate mechanisms with springs and gears, making them vulnerable to shock and damage. Most operators have seen typical failures with broken glass, bent indicator needles or needles pointing straight down from broken gearing. In many environments, they are considered essentially disposable because of their low cost and frequent failures.
Yet using pressure gauges is still a great way to visually show what is happening in the process. But wouldn’t it be nice to get that information to a centralized location without having to be physically present at the gauge? Old-fashioned clipboard rounds with operators writing down readings take time and can lead to inaccurate information resulting from human error.
Selection considerations
The variety of traditional gauges continues to be massive, and key considerations require some analysis.
Ruggedness – Some models are designed for environments in which pipes vibrate or moving equipment may cause impacts. Cases can be armored with rubber covers and beefed-up mechanisms to survive tough environments, but these options add cost.
Material of construction – While the cheapest devices are mostly brass, industrial-grade gauges are usually made of stainless steel or other durable materials. Nonetheless, be sure to know what the wetted and nonwetted parts are made of. Brass or mild steel components can deteriorate in a humid or mildly corrosive atmosphere.
Inlet configurations – Most larger gauges have a male-threaded inlet, usually 1/2-inch national pipe thread (NPT) or M20. Smaller and cheaper devices may be 1/4-inch NPT. Usually the expectation is for screwing into an existing pressure port that has a female thread connection. Some offer more specialized options for more complex mounting situations, such as adding a siphon.
Overpressure and burst pressure limits – These designations can be confusing. An overpressure limit says how much a unit can withstand without damage. In other words, it can take a spike and continue working properly. Beyond that, the Bourdon tube may be permanently distorted or the mechanism pushed past its limits. Burst pressure is where some component fails, usually the Bourdon tube, blowing the case open and releasing process fluid to the atmosphere, which can often be a safety risk. In some cases, the process connection itself fails and it can “launch the gauge” as a flying projectile. In the case where extra isolation is needed, such as using a seal for instance, this can add additional protection for the gauge. However, in the case of a failure, some units will still release the process fluid. If the substance is flammable or toxic, a safety incident will follow. Others are designed to try to contain a release or direct the release, although limits exist.
