Mechanical Pressure Gages in the CPI
In today’s digital age, there is still a need for mechanical pressure measuring instruments for safety, efficiency and economical advantages. Selecting the right type is described here
Whether at work or at home, more and more areas of life are, or will become, digital. Nevertheless, the manufacturers of mechanical pressure-measuring instruments continue to see rising sales of their gages. Customers in the chemical process industries (CPI) immediately mention two reasons for this continuing strong demand: the safety and efficiency of these instruments.
Pressure gages always provide a reliable measurement result that can be read at a glance, with no external power. Even if the power supply fails, they continue to fulfil their measuring task and display the value locally. The continued and widespread use of mechanical pressure-measuring instruments also has an economic basis. Mechatronic and electronic solutions require a higher investment. However, not all employees work in the control room or have a laptop in front of them in order to monitor the measurement processes. In most cases, the service and maintenance personnel are found within the plant and can thus read the pressure directly on site.
Types of gages
The basic decision to use pressure gages is easily understandable. In contrast, the answer to the question of which instrument suits which requirement is more complex.
Technologically, we distinguish three types of pressure gage (Figure 1): Bourdon-tube pressure gages work with a Bourdon tube as a measuring element, which expands with increasing pressure. This travel is transferred to the display through a link and a movement. With diaphragm pressure gages, the pressure acts on a diaphragm, which is either clamped or welded around its edge. The linear motion is transmitted via a link directly to a movement. Due to the large surface area of the diaphragm element, low pressure ranges can be measured (p= F/A). A special form of this type of gage is the capsule pressure gage. The measuring element is made of two diaphragm elements, welded together around their edges. The resulting double tube travel enables the measurement of even the lowest of pressures without reducing the material thickness.
All three technologies are equally suitable for monitoring gage, differential and absolute pressure. In general, gage pressure measurement is the most commonly used method. With this, the difference is measured to the currently prevailing ambient pressure, which is determined by the weather and the altitude above sea level. Compared to the other two methods, gage pressure measurement involves less effort and still meets most of the requirements within the CPI.
The EN 837 “pressure-gage standard” therefore covers mechanical measuring instruments with an elastic element for gage pressure up to a maximum of 1,600 bars. It is divided into three parts: “Bourdon tube pressure gages — Dimensions, metrology, requirements and testing” (EN 837 Part 1), “Selection and installation recommendations for pressure gages” (EN 837 Part 2) and “Diaphragm and capsule pressure gages — Dimensions, metrology, requirements and testing” (EN 837 Part 3). DIN, the German Institute for Standardization, has meanwhile published binding standards for pressure gages not covered by the European standard: DIN 16001 for high-pressure gages, DIN 16002 for absolute-pressure gages and DIN 16003 for differential-pressure gages. However, since EN 837 is applicable to more than 80% of all pressure gages, the information in the following selection criteria is based on this standard.
The first consideration after the type of pressure is the pressure range, whose limit values are defined in EN 837 according to the technology (Table 1). The range from 1 mbar to 600 mbar is covered with capsule element instruments. Models with multiple, cascaded capsule elements can detect even the smallest pressures. To display values between 2.5 mbar and 25 bar, diaphragm pressure gages are recommended. For ranges between 0.6 bar and 1,600 bars, Bourdon-tube pressure gages are predominantly suitable. Higher pressures, such as in the production of low-density polyethylenes, occur only occasionally in the process industries. Nevertheless, there are Bourdon-tube designs for pressures up to 7,000 bars, which have been developed on the basis of finite-element analysis (FEM) and using specific materials and geometries.
Alongside the pressure range, the measured medium plays a crucial role. If one takes this criterion as a benchmark, the diaphragm pressure gage presents itself as an all-arounder. Models with Bourdon tubes, however, should not be integrated into processes with highly viscous or crystallizing liquids, since the pressure connection and Bourdon tube have a small cross-section and thus can “clog.” The very sensitive capsule pressure gages are only suitable for use with gases or vapors: a liquid medium in the capsule would distort the measurement result due to its own weight.
Particularly the materials of the instrument components that come into contact with the medium also have to be suitable for this medium. For non-problematic substances, a copper alloy will suffice, while for aggressive or corrosive media, process connections and measuring elements from high-grade 316 stainless steel should be used. Depending on the requirement, special materials such as Hastelloy, Monel and tantalum can also be utilized. This is especially true for diaphragm elements, which can also be coated with polytetrafluoroethylene (PTFE), gold or platinum, for example.
Besides critical media, pressure gages in the CPI are also exposed to a high number of load cycles. The requirements for reliability and durability are correspondingly high. In addition to the quality of design and functionality, the display accuracy is another of the selection criteria. The EN 837 standard defines seven classes of accuracy from 0.1 to 4.0%, indicating the error limits as a percentage of the measuring span (Table 2). In the CPI, classes 1.0 and 1.6% are the most widely used.
The nominal size (NS) of a measuring instrument gives information about its readability. At the same time it relates to the display accuracy. The rule of thumb is that the better the accuracy class, the greater must be the diameter of the dial in order to resolve the pointer deflection precisely. An accuracy class of 1.0% requires an NS of at least 63 mm.
Working with critical media or in harsh environments can make it necessary to have a special design of pressure gage. Measuring instruments in a safety version (marked with an “S” in a circle on the dial in accordance with the standard) have a solid baffle wall between the measuring element and window as well as a blow-out back. In the event of damage (Bourdon tube bursting), the baffle wall ensures that any energy arising is dissipated through the back of the unit. The front window, in most cases made from safety glass anyway, remains intact, and any personnel who happen to be checking the pressure at that instant are protected.
Safety in critical situations
Where strong vibrations could damage or destroy a measuring instrument, a pressure gage with liquid filling is recommended. The liquid (usually glycerine) absorbs the vibration acting on the instrument and thus also the oscillations of the pointer, so that the measured value can still be read correctly. Moreover, the liquid acts as a lubricant between the mechanical components, which increases the durability of the pressure gage.
For other typical applications, measuring instruments can be exposed briefly to elevated pressures, such as when switching on a pump or opening/closing a valve. For this, diaphragm pressure gages are better suited than Bourdon-tube pressure gages due to the diaphragm being attached to the upper flange. Some diaphragm pressure gages, for example, come as standard with an overpressure safety of five times the full scale value. For the lowest measuring ranges (from 16 mbar) there are special versions that can withstand an overpressure of up to 400 bars. Bourdon-tube pressure gages can only be protected through a high design input to prevent plastic deformation of the measuring element. This is achieved by means of an additional link and an overpressure bracket. For pressures above the nominal value both components interlock, tube resistance increases and the motion of the measuring element is limited.
Due to their advantages, mechanical pressure-measuring instruments will continue to be indispensable for instrumentation in the process industries. Their only limitation: they cannot perform any control or regulation tasks. Their electronic “cousins” are necessary for this. Anyone who, for safety reasons, still needs an additional local display, free from external power, does not automatically have to operate with two measuring points. The dual function needed may be fulfilled by a mechatronic instrument; that is, a combination of pressure gage and electrical output signal or switch contact. Such two-in-one solutions save space and expense — which really assists in the cost-effectiveness of the process. n
Silvia Weber is a product manager at WIKA Alexander Wiegand SE & Co. KG (Alexander-Wiegand-Straße 30, 63911 Klingenberg, Germany; Phone: +49 9372 132-2862; Email: firstname.lastname@example.org). She has over 10 years of experience in the field of pressure measurement technology. Her area of expertise includes mechanical and mechatronic pressure measuring instruments, particularly in the chemical, petrochemical, oil-and-gas industries, as well as the food and pharmaceutical industries. Based on her train-the-trainer education, a further focus is the development of concepts for web-based training as well as further in-house education in the field of pressure measurement technology. In various publications she has already reported on the application, strengths and innovations of pressure gages. She holds a B.A. degree from the Duale Hochschule Baden-Wüttemberg in Germany.