Pressure Gauge Safety Design Under EN 837

Standards and safety mechanisms for Bourdon tube pressure gauges

Mechanical pressure gauges are simple instruments, but their failure modes can be energetic when the pressure medium is a compressed gas or steam. In Bourdon tube gauges, the sensing element is a curved metallic tube that straightens slightly as internal pressure rises. If that tube cracks or ruptures, the process medium can discharge directly into the gauge case.

Pressure gauge safety design therefore focuses on controlling the direction and effect of that release. Safer constructions use relief paths, non-splintering windows, rear venting panels, and internal barriers so that case pressure is relieved away from the operator side of the instrument.

EN 837 as the pressure gauge safety framework

Pressure gauges are commonly specified against recognized standards so that dimensions, pressure ranges, accuracy classes, tolerances, testing, packaging, and marking are not left to individual interpretation. In Europe, the main standard family for industrial mechanical pressure gauges is EN 837.

The EN 837 series is divided into three main parts:

• EN 837-1 covers Bourdon tube pressure gauges, including dimensions, metrology, requirements, and testing.
• EN 837-2 provides guidance for pressure gauge selection and installation.
• EN 837-3 covers diaphragm and capsule pressure gauges, including their dimensions, metrology, requirements, and testing.

This article focuses on Bourdon tube instruments because they are widely used in medium- and high-pressure service. Their safety design is especially important where the process medium is gas or steam. Liquids are generally less compressible, so they store less expansion energy at the same pressure. Compressed gases, by contrast, can release stored energy rapidly if the Bourdon tube fails.

The safety objective under EN 837-style construction is not to make rupture impossible. Instead, it is to reduce the danger to personnel if a rupture occurs. A safe gauge case should relieve pressure in a controlled direction, limit the chance of window fragmentation, and prevent the front of the gauge from becoming the main failure path.

Blow-out devices for case pressure relief

A blow-out device is a small relief element, usually a plug or similar part, fitted into the rear or top of the pressure gauge case. Its purpose is to provide a weak, intentional pressure-relief path.

If the Bourdon tube ruptures, pressure inside the case rises. The blow-out device is designed to eject or open so that gas can escape from the case rather than forcing the window out toward the operator. This is one of the simplest forms of pressure gauge safety design and is associated with S1 construction under EN 837-1.

The device must remain free to operate. It should not be blocked by:

• dirt, paint, corrosion, or debris;
• the mounting panel;
• a wall, pipe, or bracket behind the gauge;
• adjacent instruments or fittings;
• adhesive labels, covers, or insulation.

Installation matters as much as the component itself. A rear blow-out plug fitted tightly against a panel cannot relieve pressure properly. Adequate clearance must be left behind or around the relief device so that it can move and vent freely.

Liquid-filled pressure gauges also require a blow-out device. In filled gauges, the case contains damping liquid, commonly used to reduce pointer vibration and protect the movement. If the sensing element fails, the case still needs a relief path so that pressure is not trapped inside the filled housing.

Safety pattern pressure gauges

A safety pattern pressure gauge is a gauge built with enhanced protection for severe Bourdon tube rupture. It is used where a simple case with an ordinary window is not considered sufficient for the stored energy and service conditions.

Typical safety pattern features include:

• laminated safety glass or non-splintering plastic for the window;
• a blow-out back or other rear relief construction;
• an internal baffle wall between the Bourdon tube and the window.

Safety pattern gauges are often distinguished by whether they include an internal baffle wall. A design without a baffle can still provide improved protection through safer window material and case relief. A design with a baffle adds a physical barrier between the rupturing tube and the front window, directing the pressure release toward the rear of the instrument.

The value of a safety pattern gauge is greatest when the medium is compressible, the pressure range is high, the gauge is large enough to contain more energy, or the operator is likely to stand directly in front of the dial.

Laminated safety glass and non-splintering windows

The front window is one of the most important parts of pressure gauge safety design because it faces the reader. If the internal case pressure rises suddenly, an ordinary glass window can break into sharp fragments or be expelled from the case.

Laminated safety glass is designed to reduce this hazard. It consists of glass layers bonded with an interlayer so that, if cracking occurs, fragments are more likely to remain attached rather than separating into loose splinters. It also offers greater resistance than ordinary gauge window glass.

The purpose is not only to keep the dial visible during normal use, but also to reduce personnel injury if the case is internally pressurized. A safer window should resist breakage for long enough to allow rear relief paths to operate.

Where permitted by the relevant specification and service conditions, non-splintering plastic can be used as an alternative to laminated safety glass. Plastic windows may be suitable where chemical compatibility, temperature exposure, ultraviolet exposure, and mechanical durability are acceptable for the installation.

Blow-out backs as large relief panels

A blow-out back is a large rear relief panel, often forming most or all of the back of the gauge case. Instead of relying on a small plug, the entire rear area is designed to separate or open when internal case pressure rises.

During a severe Bourdon tube rupture, the blow-out back allows the pressure release to travel away from the operator side of the gauge. This rearward venting limits pressure buildup inside the case and reduces the likelihood that the front window becomes the failure path.

A blow-out back requires free space behind the gauge. If the rear panel is installed against a flat mounting surface, structural member, wall, or pipe, it may not be able to open. This can defeat the safety function. Panel-mounted gauges therefore need careful attention to rear clearance and case type.

Some blow-out back designs use an O-ring to retain the rear panel while still allowing the case to be liquid filled. The O-ring helps seal the filled case during normal service, but the rear panel remains able to relieve pressure if the Bourdon tube fails.

Internal baffle walls between tube and window

An internal baffle wall is a solid barrier positioned between the Bourdon tube and the gauge window. In many safety pattern gauges, the baffle is located behind the dial and fixed inside the case. The Bourdon tube and movement are behind the barrier, while the pointer and dial remain visible from the front.

If the Bourdon tube ruptures, the baffle reduces direct pressurization of the window. Instead of the released gas acting immediately on the front glass, the barrier redirects the flow toward the rear relief path, typically a blow-out back.

This design is especially important in higher-energy failures. Without a baffle, the front window may be exposed to the pressure wave and any fragments from the ruptured sensing element. With a baffle, the gauge case is divided so that the operator side is better protected.

Openings in the baffle should be kept as small and as few as functionally possible. Some openings are necessary for pointer movement, fastening, movement alignment, or liquid-fill equalization. However, every opening is also a potential path for pressure to reach the window. Good safety design balances mechanical function with minimum flow area toward the front.

Energy release testing for severe tube rupture

Safety pattern pressure gauges are subjected to an energy release test. The test is intended to simulate a severe Bourdon tube rupture, where high-pressure gas is suddenly discharged into the case.

The purpose of the test is to evaluate how the gauge responds when the normal pressure boundary inside the case fails. The case, window, rear relief, and baffle arrangement must work together so that pressure is released in a safer direction and the front of the instrument does not become the primary hazard.

This type of test is important because static inspection alone cannot fully demonstrate failure behavior. A gauge may appear robust in normal operation, but rupture safety depends on how quickly pressure is relieved, how the window behaves, and whether internal barriers direct the release toward the intended relief area.

EN 837 safety design classifications

EN 837-1 uses safety design classifications for Bourdon tube pressure gauges. These classifications should not be confused with accuracy classes. Accuracy class describes permissible measurement error. Safety classification describes how the gauge is constructed to reduce risk during internal failure.

The three main safety design levels are S1, S2, and S3. They represent increasing protective construction, from a basic blow-out device to full safety pattern construction with a baffle wall and blow-out back.

S1: blow-out device construction

S1 pressure gauge safety design requires a blow-out device. This is the basic safety construction level and is intended to provide case pressure relief if the Bourdon tube fails.

An S1 gauge does not necessarily include the full set of safety pattern features. It should therefore be distinguished from S2 and S3 gauges, which use safer window materials and additional relief or barrier arrangements. S1 is commonly relevant where some case relief is required, such as for liquid-filled gauges or specified lower-risk services.

S2: safety pattern design without a baffle wall

S2 is a safety pattern design used for certain Bourdon tube pressure gauges without an internal baffle wall. The described nominal diameter range for S2 gauges is 40 to 80 mm.

An S2 gauge should pass the energy release test. It also requires a safer front window material, such as laminated safety glass or non-splintering plastic. These materials reduce the risk of dangerous splintering if the window is stressed during an internal failure.

For pressure relief, an S2 gauge uses either:

• a blow-out device, or
• a blow-out back.

Because S2 does not include the baffle wall used in S3 construction, its protection depends heavily on the window material and the effectiveness of the relief path. It provides a higher safety level than S1, but it is not the most protective safety pattern arrangement.

S3: safety pattern design with a baffle wall

S3 is the higher safety pattern classification. It applies to gauges with an internal baffle wall and is described for nominal diameters from 40 to 250 mm.

S3 includes the S2-level safety requirements, including the use of safer window material and successful energy release behavior. It adds two important elements:

• an internal baffle wall between the Bourdon tube and the window;
• a blow-out back for rearward pressure relief.

The baffle wall and blow-out back work as a system. The baffle reduces direct loading of the window, while the blow-out back provides the preferred escape path for gas released into the case. For higher-pressure gas or steam service, or for larger gauges where more case volume and stored energy may be involved, S3 construction provides the most complete personnel protection among the EN 837-1 safety classifications.

Selecting a safety design for the pressure service

The required safety design depends on the pressure medium, gauge filling, pressure range, and nominal gauge size. A gauge used on low-pressure liquid service does not present the same failure energy as a large gauge on high-pressure gas. EN 837-2 provides more detailed selection and installation guidance, and it separates recommendations for liquid service from those for gas and steam.

When added pressure gauge safety features are needed

Dry pressure gauges at lower measuring ranges may not need added safety features. However, liquid-filled gauges generally require some safety construction because the case is sealed or semi-sealed and must still relieve pressure if the Bourdon tube fails.

For selection purposes, the major distinction is between:

• liquid pressure applications, where the process medium is relatively incompressible;
• gas and steam pressure applications, where stored energy and rapid expansion can be much greater.

The following guidance summarizes the safety design logic commonly associated with EN 837-2 selection practice. It should be applied together with the actual standard, plant safety requirements, chemical compatibility, temperature limits, and local regulations.

Liquid pressure applications

For liquid pressure measurement with dry gauges, additional safety features are generally not required according to the reference guidance. Because liquids are relatively incompressible, the stored expansion energy released by a Bourdon tube rupture is usually lower than for compressed gas at the same pressure.

For liquid pressure measurement with liquid-filled gauges, S1 design is recommended regardless of gauge size or measuring range. The key reason is that the filled case still needs a controlled relief path. If the Bourdon tube ruptures, the blow-out device allows internal pressure to escape rather than building in the gauge housing.

This does not remove the need for normal compatibility checks. The wetted parts, case filling liquid, seals, and window material must still be suitable for the process fluid, ambient temperature, and installation conditions.

Gas and steam pressure applications

Gas and steam service requires more careful safety classification because compressed media can release energy rapidly when a pressure boundary fails.

For dry gauges on gas or steam at 25 bar or below, the required safety design depends on gauge diameter. For gauges below 100 mm diameter, no additional safety features are described in the summarized guidance. For gauges of 100 mm diameter and above, S1 construction is described.

For dry gauges on gas or steam above 25 bar, the safety level increases. S2 construction is described for gauges below 100 mm diameter. S3 construction is described for gauges of 100 mm diameter and above.

For liquid-filled gauges on gas or steam at 25 bar or below, S1 construction is described regardless of diameter. Again, the presence of case filling means a reliable blow-out device is needed.

For liquid-filled gauges on gas or steam above 25 bar, S2 construction applies below 100 mm diameter, while S3 construction applies at 100 mm diameter and above.

A simplified view is shown below.

This table should be treated as a selection aid, not a substitute for reviewing the applicable standard and site safety rules. Special media, pulsation, vibration, temperature, fatigue, and user exposure can justify a more protective gauge than the minimum described category.

Exceptions for oxygen, acetylene, and oxidizing agents

Oxygen and acetylene pressure gauges require special attention. Gauges for these services are stated to require safety pattern construction using S2 or S3 safety features. The reason is not only pressure energy, but also the specific hazards of the gases. Oxygen can greatly intensify combustion, and acetylene has strict pressure and decomposition-related safety considerations.

The gauge must also be cleaned, marked, and built for the specific gas service. For oxygen, materials and lubricants must be suitable for oxygen use. Oil, grease, or incompatible organic residues can create a serious ignition hazard.

Glycerin-filled gauges should not be used with strong oxidizing agents. Glycerin is an organic compound and can react dangerously with certain oxidizers. Incompatible examples that should be checked against authoritative chemical safety data or compatibility references include acetic anhydride, calcium hypochlorite, chromium trioxide, potassium chlorate, potassium permanganate, potassium peroxide, silver perchlorate, and sodium hydride.

For oxidizing or reactive service, the gauge specification should confirm:

• process wetted material compatibility;
• case filling liquid compatibility, if a filled gauge is considered;
• oxygen-clean or gas-specific cleaning requirements where applicable;
• correct safety pattern classification;
• suitable markings for the intended gas or chemical service.

In these applications, pressure gauge safety design is only one part of safe selection. Chemical compatibility, cleanliness, temperature, pressure cycling, installation orientation, and venting direction all affect whether the installed gauge can fail in a controlled and predictable way.