Electricity + Control June 2018

PRESSURE + LEVEL MEASUREMENT + INSTRUMENTATION

Overload safety of Bourdon tube pressure gauges

Dipl.-Ing. (FH) Mark Syguda, WIKA Alexander Wiegand SE & Co. KG

The question occasionally arises, in connection with Bourdon tube pressure gauges, about the overload safety of the instruments.

Take Note!

The pressure elements of non-full-scale load- able pressure gauges consist of copper mate- rials. In contrast, the pressure elements of full-scale loadable pressure gaug- es are made of stainless steel.

1

T his article deals with exceeding the meas- uring range by up to 30 %. What happens with the pressure gauge? Does it still indi- cate 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 pres- sure gauge, it will not 'explode' Generally, there is only a risk of this if the measuring range is sig- nificantly exceeded. For such cases, the gauges have safety features that minimise the effects of this hazard. As a rule, pressure gauge manufacturers com- ply with the specifications for overload safety in ac- cordance 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-har- monised standard. Both documents are independ- ent 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 load-

able 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.

2

Full-scale loadable Non-full-scale loadable

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, ir- reversible deformations can occur with the meas- uring system, which can, for example, be seen as a zero point offset. Example: A non-full-scale load- able 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. 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 particu- lar, 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 Requirements for the overload safety of Bourdon tube pressure gauges

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.

34 Electricity + Control

JUNE 2018