The question occasionally arises, in connection with Bourdon tube pressure gauges, about the overload safety of the instruments. This article deals with exceeding the measuring range by up to 30 %. What happens with the pressure gauge? Does it still indicate precisely afterwards? Can it start to leak? As a next step, we should shed further light on these questions through explanation and discussion of the common standards.
To start with: If the pressure to be measured slightly exceeds the measuring range of the pressure gauge, it will not “explode”. Generally, there is only a risk of this if the measuring range is significantly exceeded. For such cases, the gauges have safety features that minimise the effects of this hazard.
As a rule, pressure gauge manufacturers comply with the specifications for overload safety in accordance with the 2014/68/EU directive (pressure equipment directive) and EN 837-1: 1996. Pressure gauges ‒ Part 1: Bourdon tube pressure gauges. On the dial, this is indicated by the CE marking and the inscription EN 837-1. The specifications contained in these documents vary, since EN 837-1 is a non-harmonised standard. Both documents are independent and the manufacturer of Bourdon tube gauges is required to comply with them.
Non-full-scale loadable and full-scale loadable Bourdon tube pressure gauges
Before summarising the overload specifications, an important classification of the pressure gauges from EN 837-1 needs to be explained: Under point 9.4, EN 837-1 defines “non-full-scale loadable” (1) and “full-scale loadable” (2) pressure gauges. Since this can lead to confusion, the differences are briefly explained here: Usually, the pressure elements (Bourdon tube) of non-full-scale loadable pressure gauges consist of copper materials. In contrast, the pressure elements of full-scale loadable Bourdon tube pressure gauges are made of stainless steel. Since full-scale loadable pressure gauges have an identifying mark (black triangle at the end of the scale), the two types of instrument can also be distinguished from the outside.
The main difference between the instruments is that “non-full-scale loadable Bourdon tube pressure gauges” should not be operated permanently in the range above 75 % of the full-scale value (FS). Otherwise, under continuous pressure loading, irreversible deformations can occur with the measuring system, which can, for example, be seen as a zero point offset. Example: A non-full-scale loadable Bourdon tube pressure gauge, which should indicate the pressure in a 300 bar gas cylinder, must be designed so that the full scale value is at least 400 bar.
Requirements for the overload safety of Bourdon tube pressure gauges
The requirements of the directive 2014/68/EU and the European standard 837-1 differ on the overload safety of Bourdon tube pressure gauges. EN 837-1 specifies requirements for indication accuracy and strength (sealing) following overloading. In particular, the requirements with respect to sealing, with up to 2.5 times the overload over at least 24 h, are demanding and significantly higher than with the pressure test (3) in accordance with the 2014/68/EU directive. The pressure equipment directive requires only 1.43 times the maximum permissible pressure (PS). As can be seen from the table, instruments in accordance with EN 837-1 thus automatically fulfil the requirements of pressure testing (3) in accordance with the directive 2014/68/EU. The reverse is not valid.
In the following table, the different requirements are contrasted against each other:
PFSV = Full scale value (pressure at the end of the pressure gauge scale)
PS = “Maximum permissible pressure” – defined by the manufacturer, the highest pressure for which the pressure gauge has been designed. With non-full-scale loadable pressure gauges: PS = 0.75 * PFSV, otherwise: PS = PFSV
(a) Dependent on pressure range
(b) The change of the class accuracy is measured after an additional load cycle test (not explained here). A specification about the allowed class accuracy change after overload only (without load cycle test) does not exist. The table values must therefore be viewed as the absolute maximum.
(c) For non-full-scale loadable instruments, the following applies: Overload = 1.43 * PS = 1.43 * 0.75 * PFSV = 1.07 * PFSV
Clarified by example
Pressure gauge 0 – 10 bar, pressure element from copper material: This instrument is not full-scale loadable and must meet the following requirements:
- In accordance with EN 837-1: 12 h static pressure load of 10 bar (PFSV) and 15 min static pressure load of 12.5 bar (1.25 * PFSV). After a rest time of 1 h following this overload, the display deviation must not exceed 1.2 times the error limits. In addition, the instrument is exposed to a pressure of 25 bar (2.5 * PFSV) and an external temperature of 60 °C for 24 h. After that, it must continue to be leak-tight in accordance with EN 837-1 (leak rate < 5 * 10-3 mbar l s-1).
- In accordance with the 2014/68/EU directive: The instrument must survive a leak test with an overload of 10.7 bar (1.43 * PS = 1.43 * 0.75 * PFSV), and still be leak-tight after the test.
Pressure gauge 0 – 600 bar, pressure element from stainless steel: This instrument is full-scale loadable and must meet the following requirements:
- In accordance with EN 837-1: 12 h static pressure load of 780 bar (1.3 * PFSV). After a rest time of 1 h following this overload, the display deviation must not exceed 1.2 times the error limits. In addition, the instrument is exposed to a pressure of 1,500 bar (2.5 * PFSV) and an external temperature of 60 °C for 24 h. After that, it must continue to be leak-tight in accordance with EN 837-1 (leak rate < 5 * 10-3 mbar l s-1).
- In accordance with the 2014/68/EU directive: The instrument must survive a leak test with an overload of 858 bar (1.43 * PS = 1.43 * PFSV), and still be leak-tight after the test.
When selecting a Bourdon tube pressure gauge, overload safety is an important parameter to take into consideration. Depending on the structural design of the selected instrument, the overload safety will be different. Pressure gauges can cope with slight and short-term overloads. The indication accuracy generally is maintained or only worsens slightly. A high overload with up to 2.5 times FSV manifests itself in pressure gauges through an increasing deterioration of the indication accuracy and a clearly visible zero point error. However, the pressure-bearing components do not burst and the instrument remains sealed.
Full-scale loadable and non-full-scale loadable pressure gauges can be found on the WIKA website.
(1) EN 837-1, sect. 9.4.1: Pressure gauges for a max. static pressure load of 75 % of the full scale value
(2) EN 837-1, sect. 9.4.2: Pressure gauges suitable for maximum steady pressure load equal to the full scale value
(3) Directive 2014/68/EU, annex 1, point 7.4 Hydrostatic test pressure.