A pressure measuring point operates reliably at 6 bar under normal conditions. However, when the system starts up, a valve closes rapidly or an operating error occurs, the pressure briefly rises much higher. Afterwards, the pressure gauge no longer returns to zero, the pressure sensor delivers a shifted output value or the measuring cell is permanently damaged.
Such damage is often caused not by the normal operating pressure, but by infrequent pressure spikes, an insufficient measuring range or an unsuitable protective device. The key question is therefore not only which pressure is to be measured. The highest possible process pressure, the dynamics of the pressure rise, the permissible overload of the measuring instrument and the behaviour of the entire measuring point after the protection device has responded are equally important.
This article explains how pressure gauges, digital pressure gauges and pressure sensors can be protected reliably by selecting an appropriate measuring-range margin, an adjustable overpressure protector, a pressure snubber and a suitable shut-off device.
Table of Contents
- Why overpressure is so critical at pressure measuring points
- Distinguishing between measuring range, overload limit and burst pressure
- Typical causes of overpressure and pressure spikes
- Selecting the correct measuring range and safety margin
- Practical example: 6 bar operating pressure and a 16 bar pressure spike
- How an overpressure protector works
- Correctly defining the closing pressure and reset behaviour
- Distinguishing between overpressure protection, pressure snubbers and safety valves
- The role of shut-off valves and pressure gauge cocks
- Considering the process connection, materials and seals
- Special considerations for different pressure measuring instruments
- Commissioning and functional testing
- Common design errors
- Checklist for a robust pressure measuring point
- Suitable pressure measurement technology from ICS Schneider
- Conclusion
- FAQ: Protecting pressure measuring instruments against overpressure
Why Overpressure Is So Critical at Pressure Measuring Points
A measuring instrument is designed for a defined measuring range. Above the full scale value, it may no longer display the pressure correctly or its output signal may saturate. This does not necessarily mean that the instrument will be damaged immediately. Whether it can withstand an excursion without a permanent change depends on the overload limit specified in the data sheet, as well as the duration, frequency and dynamics of the load.
Repeated overloads can fatigue the elastic measuring element. In a Bourdon tube pressure gauge, the Bourdon tube can become permanently deformed. In an electronic pressure sensor, the diaphragm, measuring cell, joints or seals may be damaged. Typical consequences include:
- permanent zero-point shift,
- altered sensitivity or characteristic curve,
- poorer repeatability,
- a mechanically blocked pointer,
- leakage at the process connection or measuring element,
- failure of the electrical output signal,
- and, in extreme cases, leakage of the process medium.
A high overload limit is therefore a safety margin for the measuring instrument, but it is not an additional measuring range and does not permit continuous operation above the full scale value.
Distinguishing Between Measuring Range, Overload Limit and Burst Pressure
| Term | Meaning | Important for sizing |
|---|---|---|
| Measuring range | Range within which the instrument measures or displays in accordance with its specification | The normal operating pressure must lie within this range with a sufficient margin. |
| Full scale value | Upper limit of the measuring range or scale | It is not automatically identical to the permissible overload. |
| Overload limit | Maximum pressure that the instrument can withstand under the manufacturer’s defined conditions without an impermissible permanent change | Observe the duration, frequency and type of load specified in the data sheet. |
| Burst pressure | Pressure above which pressure-retaining components may lose their tightness or mechanical integrity | It is not an operating or design limit; burst pressure must never be treated as a usable safety margin. |
| Pressure spike | A very brief increase in pressure that can often be detected only with fast measurement technology | It can be significantly higher than the pointer movement visible on a pressure gauge. |
| Closing pressure | Pressure at which an overpressure protector shuts off the connection to the measuring instrument | It must be above the normal operating range and below the permissible load on the instrument. |
Some data sheets use terms such as overload safety, permissible overpressure or “proof pressure” instead of “overload limit”. The exact definition provided by the respective manufacturer is always decisive. A high burst pressure also provides no indication of whether the instrument will still measure accurately after an overload.
Typical Causes of Overpressure and Pressure Spikes
The maximum pressure that can occur cannot be derived solely from the set operating pressure. Critical loads can result from:
- rapidly opening or closing valves,
- water hammer in liquid lines,
- starting and stopping pumps or compressors,
- pulsating positive-displacement pumps,
- pressure intensification caused by pistons, diaphragms or hydraulic area ratios,
- thermal expansion of an enclosed liquid volume,
- blocked lines or closed shut-off devices,
- failure of a pressure regulator,
- incorrectly connected test or calibration pumps,
- changes of medium and cleaning processes at a higher pressure,
- operating errors during maintenance or commissioning.
Very brief pressure spikes are particularly difficult to detect. Due to its mechanical inertia, a mechanical pressure gauge may show little or no visible indication of them. A sensor with a slow sampling rate or strong digital filtering can also underestimate the actual peak value. In the event of unexplained damage, the pressure profile should therefore be recorded using sufficiently fast measurement technology.
Selecting the Correct Measuring Range and Safety Margin
The measuring range must meet two opposing requirements. It must be large enough to ensure that normal operation and expected fluctuations are not constantly close to the full scale value. At the same time, it must not be so large that the resolution, readability or measurement uncertainty is inadequate for the application.
Rule of Thumb for Mechanical Pressure Gauges
For steady pressure, the normal maximum operating pressure should generally not be significantly higher than approximately three quarters of the full scale value. For fluctuating or pulsating loads, a more conservative design limiting the pressure to approximately two thirds of the full scale value is often recommended. However, the load specifications for the specific pressure gauge remain decisive.
The following formula can be used for an initial estimate:
required full scale value ≥ maximum operating pressure / permissible utilisation factor
For a maximum operating pressure of 6 bar and a dynamic utilisation factor of no more than 0.67, the result is:
6 bar / 0.67 ≈ 9 bar
A measuring range of 0 to 10 bar would therefore be plausible for the normal indication. Whether it is also suitable for malfunctions and pressure spikes must be assessed separately on the basis of the overload limit and protection concept.
Assess Electronic Pressure Sensors Individually
There is no universal percentage rule for pressure sensors. The measuring principle, diaphragm, process connection and measuring range have a significant influence on the overload capability. The maximum process pressure must therefore be compared with the specified measuring range, the permissible static and dynamic overload and the required accuracy.
If accuracy is specified as a percentage of span, an unnecessarily large measuring range increases the absolute error. A sensor with an accuracy of 0.5% of span has a span-related error component of 0.05 bar for a range of 0 to 10 bar, but 0.5 bar for a range of 0 to 100 bar. An extremely large measuring range is therefore not automatically the best protective measure.
Practical Example: 6 Bar Operating Pressure and a 16 Bar Pressure Spike
A pneumatic system normally operates between 4 and 6 bar. A brief pressure of 16 bar was measured when a valve was switched. A clearly readable indication of the normal operating pressure is required.
| Solution | Advantage | Disadvantage or point to check |
|---|---|---|
| Pressure gauge, 0 to 10 bar, without additional protection | Good readability during normal operation | Permissible only if the specific overload limit safely covers the magnitude, frequency and duration of the 16 bar spike |
| Pressure gauge, 0 to 25 bar | The pressure spike lies within the measuring range | 4 to 6 bar lies low on the scale; readability and accuracy may be poorer |
| Pressure gauge, 0 to 10 bar, with overpressure protector | Good readability and protection against prolonged overpressure | The closing pressure, overload capability until closure and reopening pressure must be compatible |
| Pressure gauge, 0 to 10 bar, with a pressure snubber and overpressure protection | Protection against rapid pressure changes and limitation of the maximum pressure applied to the measuring instrument | Consider the response time, medium, viscosity and risk of contamination |
If 16 bar is a permissible process condition and must be measured, a larger measuring range is the correct solution. If, however, only the range up to 6 bar is relevant and 16 bar represents a rare fault load, a correctly sized overpressure protector may be appropriate. The underlying cause of the pressure spike should nevertheless be eliminated if it also places a load on other system components.
How an Overpressure Protector Works
An adjustable overpressure protector is installed between the process and the pressure measuring instrument. The passage remains open within the normal pressure range. If the pressure exceeds the set closing pressure, a spring-loaded piston moves and shuts off the connection to the measuring instrument.
The measuring instrument therefore remains exposed to approximately the pressure that was present when the valve closed. The device does not reduce the process pressure and does not discharge any medium into a relief line. It protects only the downstream measuring instrument.
After the pressure has fallen sufficiently, a self-resetting version opens again. The design creates a difference between the closing and reopening pressures. For example, the adjustable WIKA model 910.13 overpressure protector reopens after the pressure has fallen to approximately 25% below the closing pressure. This value must not be applied generally to other designs.
An overpressure protector is particularly suitable for:
- pressure gauges with a small measuring range installed on systems with a higher possible fault pressure,
- digital pressure gauges and test gauges,
- pressure sensors with limited overload capability, provided the dead volume and dynamics are suitable,
- maintenance and test connections with changing pressure ranges.
It is not intended for use as a pressure regulator, process shut-off valve, safety valve or protective device for the entire system.
Correctly Defining the Closing Pressure and Reset Behaviour
The closing pressure must be sufficiently above the highest normal measured value so that the protective device does not close unintentionally during normal operation. At the same time, it must close before the measuring instrument is subjected to an impermissible load.
At least the following values are required for the selection:
- highest normal operating pressure,
- required full scale value,
- permissible overload of the measuring instrument,
- maximum possible process or fault pressure,
- rate of pressure rise and duration of the spike,
- setting range and reopening behaviour of the protective device,
- temperature dependence of the setting.
For combinations assembled at the factory, the closing pressure can be set to suit the measuring instrument. For the model 910.13, WIKA specifies a setting of 1.1 times the full scale value when the protector is factory-mounted to a measuring instrument. This is a product-specific requirement and not a general rule for all protective valves or measuring instruments.
After the valve closes, the pointer may initially remain near the closing pressure because a volume of pressurised medium is trapped between the protective device and the measuring instrument. The pressure equalises only when the device reopens. This behaviour is normal and must be taken into account for alarm contacts, electronic evaluation and troubleshooting.
Distinguishing Between Overpressure Protection, Pressure Snubbers and Safety Valves
| Component | Main function | What it does not do |
|---|---|---|
| Overpressure protector | Shuts off the measuring line at a set pressure | Does not relieve the process or regulate the system pressure |
| Pressure snubber | Restricts rapid pressure changes and stabilises the indication | Does not safely limit prolonged static overpressure |
| Pulsation dampener in the main line | Reduces the actual pressure or flow pulsation in the process | Does not automatically replace an individual overload assessment of the measuring instrument |
| Shut-off valve or pressure gauge cock | Manually isolates the measuring instrument from the process and enables maintenance or testing | Does not automatically protect against overpressure when open |
| Safety valve | Protects the pressure-bearing system through controlled pressure relief | Is not merely an accessory for protecting the measuring instrument |
For rapid pressure spikes, a combination of a pressure snubber and an overpressure protector may be appropriate. The pressure snubber reduces the rate of pressure rise at the measuring instrument, while the overpressure protector shuts off a high pressure that persists for longer. With contaminated or viscous media, however, it must be checked whether the restrictor orifice could become blocked. Further information is available in the article Pressure Gauges on Pulsating Pumps: Correctly Selecting Damping, Pressure Snubbers and Case Filling.
The Role of Shut-Off Valves and Pressure Gauge Cocks
A shut-off valve enables replacement, calibration and depressurised removal of the measuring instrument. Opening it slowly can provide a more controlled pressure build-up at the instrument. However, this operating procedure is not an automatically acting form of overpressure protection.
The following combination is often useful for maintenance measuring points:
- process-side shut-off valve,
- test or vent connection,
- pressure snubber if required,
- overpressure protector,
- pressure measuring instrument.
The sequence must be selected so that the components can be tested, vented and safely depressurised. Dead spaces containing trapped pressure must be avoided or depressurised in a controlled manner.
Considering the Process Connection, Materials and Seals
A protective device is reliable only if the process connection and materials are also suitable for the application. The following must be checked:
- thread type and sealing principle, for example a G thread with a defined sealing face or a tapered NPT thread,
- pressure rating of all adapters, valves, seals and lines,
- chemical resistance of all wetted parts,
- temperature limits of the body, spring, piston and seal,
- viscosity, particles and crystallisation tendency of the medium,
- permissible mounting position and mechanical load,
- cleanability and possible blockage of small orifices,
- risk of leakage from additional threaded connections.
A seal can become the weak point even before the actual sensor element. At high pressure, unsuitable O-rings can extrude, age or be forced out of their groove. Adapters with a lower pressure rating must not be used merely because the thread fits mechanically. The lowest permissible pressure and temperature limits always apply to the entire measurement chain.
Special Considerations for Different Pressure Measuring Instruments
Bourdon Tube Pressure Gauges
Bourdon tubes are suitable for many industrial pressure ranges, but they are sensitive to sustained overload and severe load cycling. A shifted zero point after depressurisation is a clear warning sign. A liquid-filled case stabilises the pointer and movement under vibration, but it does not automatically increase the permissible static overload.
Diaphragm Pressure Gauges
Diaphragm measuring systems can provide greater overload capability by design because the diaphragm can support itself against a backing surface. Nevertheless, the actual permissible overload depends on the model, measuring range and maximum system pressure.
Electronic Pressure Sensors
For sensors, the diaphragm, process connection, seal and internal joints must be considered in addition to the measuring cell. A 4 to 20 mA output signal does not limit the process pressure. A programmed alarm or switch contact protects the measuring cell only if an external measure reduces or shuts off the pressure in time.
Differential Pressure Measuring Instruments
For differential pressure sensors, the permissible static line pressure and one-sided overload must also be considered. A small differential pressure range can operate at a high static pressure level if the instrument is designed for it. Incorrect valve operation, however, can briefly apply the full line pressure to only one side of the measuring cell.
Commissioning and Functional Testing
An adjustable overpressure protector should not be installed without testing. The following procedure is recommended:
- Compare the data: Check the measuring range, overload limit, setting range, pressure rating, temperature and materials.
- Check the installation: Observe the flow direction, threads, sealing faces and tightening torques.
- Perform a leak test: Check the complete measurement chain using a suitable test medium and safe test pressure.
- Increase the pressure slowly: Determine the response or closing pressure using a suitable reference instrument.
- Check reopening: Reduce the pressure slowly and document when the connection reopens.
- Assess the dynamics: For rapid pressure surges, check whether an additional pressure snubber or process dampener is required.
- Check the measuring instrument: Verify the zero point and indication after several switching cycles.
- Secure the setting: Prevent unintended adjustment and document the setpoint.
The test must not exceed the permissible overload of the connected measuring instrument. If the closing pressure still needs to be set, a sufficiently pressure-resistant reference instrument or suitable test bench may initially be required, depending on the configuration.
Common Design Errors
| Error | Consequence | Better solution |
|---|---|---|
| Using burst pressure as the permissible overload | Measurement error or mechanical failure well before bursting occurs | Assess the measuring range and defined overload limit separately |
| Selecting a measuring range that ends exactly at the operating pressure | No margin for control deviations, pulsation or start-up | Place the normal operating range within the scale with an appropriate margin |
| Confusing overpressure protection with a safety valve | The system remains at a hazardous pressure even though the pressure gauge is protected | Design the system protection independently of the measuring-instrument protection |
| Using a pressure snubber against sustained overpressure | After a short delay, the full pressure is applied to the measuring instrument | Design pressure-spike damping and pressure limitation separately |
| Setting the closing pressure too low | The measured value is lost or freezes during normal operation | Provide sufficient separation from the highest normal measured value |
| Setting the closing pressure too high | The measuring instrument is subjected to an impermissible load before the protector closes | Verify that the closing pressure is below the permissible instrument load |
| Ignoring the reset behaviour | The indication appears to remain stuck after the event | Document the reopening pressure and trapped volume |
| Checking only the sensor | An adapter, valve or seal fails first | Size the complete measurement chain according to its weakest component |
Checklist for a Robust Pressure Measuring Point
- Define the normal minimum and maximum operating pressures.
- Determine the maximum possible fault pressure and pressure spikes.
- Assess the duration, frequency and rate of pressure rise.
- Select the measuring range according to resolution, accuracy and operating margin.
- Do not confuse the overload limit with burst pressure.
- Determine whether high pressures must be measured or merely kept away from the instrument.
- For overpressure protection, select the appropriate setting range and closing pressure.
- Consider the reopening pressure and the behaviour of the trapped volume.
- Provide suitable damping for rapid pressure spikes.
- Plan shut-off, venting and safe removal.
- Check the pressure rating, material, seal, temperature and medium for all components.
- Test and document the installation, tightness, closing pressure and reset behaviour.
- Check the zero point and measurement deviation after an overpressure event.
Suitable Pressure Measurement Technology from ICS Schneider
ICS Schneider offers components for the complete design of a protected pressure measuring point:
- Pressure measurement technology for industrial measurement, monitoring and testing tasks,
- pressure gauges and digital pressure gauges with different measuring ranges, designs and overload capabilities,
- analogue pressure sensors and pressure transmitters for automation and process monitoring,
- adjustable overpressure protectors for pressure gauges,
- pressure snubbers for damping rapid pressure changes,
- shut-off valves and pressure gauge cocks for maintenance, testing and safe isolation.
In addition to the measuring range, the maximum process pressure, medium, temperature, required accuracy, pressure dynamics and connection design should always be specified when selecting the components. This enables the entire measurement chain to be matched correctly.
Conclusion
A pressure sensor or pressure gauge is not safe merely because its burst pressure is above the maximum system pressure. The usable measuring range, permissible overload and actual pressure load during operation and in the event of a fault are decisive.
Select the Measuring Range with a Safety Margin
The normal operating pressure should not remain at the full scale value continuously. The margin must suit the pressure dynamics and load capacity of the specific instrument.
Treat Overpressure and Pressure Spikes Separately
An overpressure protector closes at a set pressure. A pressure snubber slows rapid pressure changes. Both functions may be required in dynamic applications.
System Protection Remains a Separate Task
Protecting the measuring instrument does not replace a safety valve or a reliable means of limiting the system pressure. Critical pressure spikes should be reduced at their source wherever possible.
In Practice
Determine the operating and fault pressures → record the pressure dynamics → define the measuring range and accuracy → check the overload limit → select the closing pressure and setting range of the overpressure protector if required → additionally damp rapid pressure spikes → provide shut-off and venting facilities → verify the process connection, materials and seals → test the complete measurement chain → document the closing and reopening behaviour → check the measuring instrument after overpressure events.
FAQ: Protecting Pressure Measuring Instruments Against Overpressure
What is the difference between the measuring range and the overload limit?
Within the measuring range, the instrument performs its specified measurement function. The overload limit describes a higher, limited load that the instrument can withstand under defined conditions without an impermissible permanent change. A correct indication is not guaranteed above the measuring range.
Is burst pressure a permissible operating limit?
No. Burst pressure describes the limit of mechanical pressure resistance or tightness and must not be used as an available operating margin. Measurement deviations or permanent damage can occur at significantly lower pressures.
How large should the measuring-range margin be?
This depends on the instrument and the pressure dynamics. For mechanical pressure gauges, a common guideline is that the highest constant operating pressure should not exceed approximately three quarters of the full scale value, while a fluctuating pressure should not exceed approximately two thirds. The specifications of the specific manufacturer take precedence.
How does an overpressure protector for pressure gauges work?
Within the normal range, it allows the process pressure to reach the measuring instrument. At the set closing pressure, a spring-loaded piston shuts off the passage. A self-resetting version opens again after the pressure has fallen sufficiently.
Can an overpressure protector replace a safety valve?
No. It protects only the downstream measuring instrument and does not relieve the process. The safe maximum system pressure must be limited by devices designed for that purpose.
Why does the pressure gauge remain at a high value after the protector closes?
A pressurised volume can be trapped between the protective device and the measuring instrument. The indication therefore remains near the closing pressure until the device reopens or the trapped pressure is released in a controlled manner.
What is the difference between an overpressure protector and a pressure snubber?
The overpressure protector shuts off at the set pressure. A pressure snubber restricts rapid pressure changes but does not limit a sustained static overpressure. A combination may be useful for brief spikes and a higher fault pressure.
Does a liquid-filled case protect a pressure gauge against overpressure?
A case filling dampens pointer movement and mechanical vibration. It does not automatically increase the permissible static overload of the measuring element.
Can I simply select a significantly larger measuring range?
This increases the pressure margin, but it can impair the resolution, readability and absolute measurement accuracy. If the high pressure does not need to be measured, a smaller measuring range with suitable protection is often the better solution.
What must be checked after an overpressure event?
At a minimum, check the zero point, indication or output signal, tightness and any visible damage. For safety- or quality-critical measuring points, an additional comparison measurement or calibration should be performed.
Can a switch contact on the pressure gauge protect the measuring point?
The contact can trigger an external shutdown, but it does not limit the pressure itself. The response time, switching logic and downstream actuator must operate quickly enough. The mechanical overload capability of the measuring instrument must still be observed.
Where is the overpressure protector installed?
It is installed in the measuring line upstream of the pressure measuring instrument to be protected. The mounting direction, connection design, seal, pressure rating and accessibility for adjustment depend on the specific version.