Despite the progressive digitisation of measurement and control technology, mechanical pressure gauges continue to assert themselves in the market with rising sales figures. Customers particularly appreciate them for their reliable local display of process pressure, without any need for external power, and also for their manageable initial and subsequent costs. For those applications with a need for electronic control and monitoring, mechatronic variants also exist. However, due to their mechanical measuring principle, physical limits are set on pressure gauges, e.g. in overload situations. To fulfil the high demands of accuracy, durability and also the wide range of applications, users frequently use a combination of pressure gauges and fittings. There is a wide selection of these which, if appropriately designed, enable an efficient optimisation of the entire measuring point.
Selection of the measurement technology
In the design of pressure gauges with fittings, a precise knowledge of the application-specific influencing factors is essential. The matching of the components should ideally take place during the selection of the pressure gauge. Guidance for this is given in part 2 of the EN 837 European standard: Selection and installation recommendations for pressure gauges.
After completing the basic specifications (type of pressure, scale range, wetted materials, nominal size/indication accuracy, process connection), one should check whether any ‘special conditions’, which are also listed in the standard, apply to the application. Included in these conditions, which have a critical effect on mechanical pressure measurement, are, among others, dynamic load cycles, temporary overpressure and extremely high process temperatures.
Reduction of dynamic load cycles and pressure surges
Particular caution should be exercised with respect to pressure spikes, ie, with short, but very intense pressure surges at a multiple of the operating pressure. These are principally caused by the opening and closing of fast acting valves in fluid flows.
Dynamic load cycles often result from pressure surges from a pump and are visible in the form of pointer fluctuations on the measuring instrument. They impede the readability of the display and can significantly increase the wear of the elastic pressure element. In principle, each surge corresponds to a pulsating load cycle, which stresses all mechanical components strongly, thus having a detrimental effect on the service life of the pressure gauge.
This wear can be reduced to an ordinary level by using a snubber or orifice.
The orifice can already be incorporated in the pressure inlet port during the manufacture of the pressure gauge, or later connected as an instrument accessory.
The external solution (snubber) offers the major advantage of a variable adjustment of the volume flow of the medium. The pressure port is narrowed by an adjustment screw, any pressure surge is damped optimally, and thus an unnecessarily high load on the pressure element is prevented. Through this mechanism, any change in the viscosity of the medium can be conveniently ‘compensated for’. But beware: A snubber only protects the system from the effects of dynamic pressure cycles, not against a consistently increasing static pressure.
For the reduction of pulsations, one can also consider using syphons or compact syphons, particularly when hot media such as steam are present in combination with pressure surges.
Prevention of overpressure
In piping systems, under certain conditions, temporary overpressure can occur for which the installed pressure measuring instruments have not been designed. As an example: Within a process, different gases are required consecutively. Before each change of medium cleaning must be carried out, this is generally done at a pressure that is considerably higher than the normal process pressure. The connected measuring instruments must be protected so they are not damaged by this situation nor is a danger point created for the environment.
An overpressure protector protects all of the subsequent components in the process from overpressure. The required setting of the pressure range is made via a screw: If the process pressure in the pipeline rises above the defined threshold value (spring force), the fitting closes the pressure port independently and thus protects the connected pressure measuring instrument. Once the pressure drops below a defined level, the device opens and the pressure gauge will once again show the current pressure in the pipeline.
As an alternative, the use of a manual shut-off valve is offered. This is closed and re-opened by hand as and when it is needed.
Reduction of high medium temperatures
A medium temperature that is too high degrades the indication accuracy of pressure measuring instruments. Exceeding a maximum value leads to irreversible damage to the measuring device. As an example: For stainless steel measuring systems, measures for cooling the medium must be taken for process temperatures above 200°C. For such cases, a pressure gauge fitted with a syphon is generally suitable, mostly with this installed as a primary process-side instrument fitting. Syphons extend the ‘cooling element’ and provide an effective temperature reduction at the measuring instrument through heat convection. In accordance with the DIN 16282 standard, they are available in two geometries: in a U-shape for horizontal pressure taps and in a C-shape for vertical ones. Their basic function is the formation of condensate, which protects the connected measuring equipment from direct contact with the hot medium. Before the first use, therefore, they should always be filled with a liquid protection medium.
Furthermore, it may make sense to use a diaphragm seal fitted with a capillary. The additional separating diaphragm, combined with an extended pressure line, also enables a very good reduction in the medium temperature present at the measuring instrument.
Safe process isolation
Shut-off devices such as shut-off valves and needle valves control a variety of tasks within a process, eg, during commissioning, or calibration of measuring instruments. Shut-off valves are generally only used for the simple turning on or off of the pressure supply. A typical application for needle valves is the support in the measurement of differential pressures, eg, in the monitoring of filters and pumps. In order to fulfil the measuring task optimally, specific valve manifolds exist.
The one-way valve manifold for differential pressure gauges only serves for the pressure compensation between the two pressure inlet ports. This design is used for zero point checking or adjustment of the measuring instrument. With three-way valve manifolds, each process line can also be shut off separately, e.g. for the removal and exchange of an instrument. Five-way valve manifolds offer two additional outlets in order that both pressure connections can be isolated from each other for venting.
Conclusion
The selection of fittings undoubtedly exerts an influence on the service life, reliability and application range of pressure measuring instruments. For the design and assembly of the measuring point, the knowledge of the critical process parameters and the know-how of the design of the individual components are of the highest importance. For anyone who wants to avoid a modular system build, they can fall back on what is known as a ‘hook-up’. This means that manufacturers such as WIKA offer a complete design of the measuring point including all instrument fittings, fully assembled. For the customer, the hook-up offers a qualified measuring system from tested individual components that can be integrated into the process directly and with minimal installation effort.
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