Intelligent monitoring of industrial gas tanks optimises logistics: Technical gases are an indispensable production resource, running out of gas can cause serious losses and down time.
For many companies in industry, technical gases are an indispensable production resource. Therefore, any shortage in their supply must be avoided at all costs. This forces operators of gas tanks into sophisticated logistics planning when it comes to the resupply of their tanks, which are often distributed rather than centralised. Their efficiency is therefore crucially dependent on the measurement technology installed on site. The solution in the 'Industry 4.0' era: a combination of mechatronic level control and remote data transfer.
The need for cryogenic liquefied gases, and thus the necessary storage capacity, has risen sharply worldwide in recent years. This development has been influenced, among other things, by the demand for liquefied natural gas (LNG). In this aggregate state, the fuel can be transported efficiently and stored in a space-saving manner, remote from distribution networks. One litre of LNG expands to a volume of approximately 600 litres in the gaseous state and it also has an energy density which is approximately twice as high as conventional compressed natural gas (CNG). As well as LNG, other gases stored as liquids play a crucial role in a wide variety of applications and processes. The metal industry, for example, requires argon as a protective gas for welding, the food industry requires nitrogen or carbon dioxide for shock-freezing. Oxygen, on the other hand, is an important medium in the medical industry.
Measurement under cryogenic conditions
Liquid gases for industrial consumers are stored in suitably sized tank plants. Their monitoring, in terms of measurement technology, is not simple, due to the extremely cold environmental conditions (also known as cryogenic, from the Greek word for frost): Inside the special double-walled, vacuum-insulated tanks there are temperatures down to -250 °C.
For the process control, there are two main parameters in focus: working pressure and level. The working pressure is constantly monitored for safety purposes. If it increases above a specific level through fluctuations in temperature or too little demand, the excess pressure, and thus a correspondingly large volume of gas, must be vented to atmosphere through a safety valve. The level, the second relevant parameter, is also monitored using pressure measurement. For this, the differential pressure between the liquid phase and the gaseous phase in the vessel is determined. For the accurate calculation of the contents, in addition to the hydrostatic pressure, the tank geometry must also be considered in the calculation (horizontal or vertical vessel, various covers and bases, etc.), as must be the specific density of the medium.
Transformation of a business model:
In the past, level control for gas tanks relied upon a mechanical measurement − the value determined needed to be read locally. These days, measuring instruments with electrical signal outputs are generally used, transmitting all the information to a control room or control station of the operator. There, the re-ordering for all in-house tanks can be coordinated or forwarded to external suppliers.
Therefore, the measurement technology must increasingly be aligned to a different business model for the liquid gas supply: More and more, the operators of the tank plants are putting the organisation of the resupply completely in the hands of their suppliers. In order to enable the suppliers to deliver gas to a significant number of tanks at different locations efficiently, all measurements must be brought together into a single system or database. So any measuring arrangement that should correspond to this model must cope with the following tasks:
- Accurate measurement of working pressure and level
- Electronic provision of measured values (local signal outputs)
- Remote data transfer of measured values to the supplier
- Local indication of both measured values as a back-up
In line with these demands, WIKA has developed the modular Cryo Gauge measuring instrument concept. It is based on a tank-specific, mechatronic differential pressure gauge which displays the measured values for the level locally and converts them into an electronic format via an integrated transmitter. Coupled via a valve manifold, there is a mechanical working pressure indication with a pressure transmitter connected which, as for the transmitter of the level measuring instrument, delivers a standard 4 ... 20 mA signal. Both outputs serve as the input for the system's remote data transfer module. This unit digitises the analogue measured values and transmits them using GSM technology - SMS or GPRS mode is selectable - to an online data centre, to which the appropriate supplier has access. In addition, the module is equipped with two cable outputs which also offer the operator the parameter values locally.
The wireless transmission of the information to the data centre, either daily or hourly, is defined by the user depending on the tank size, the particular gas and its intended usage. An additional monitoring function is immediately triggered as a warning message in the event of an unexpectedly rapid drop in the level, for example, due to a leak in the tank.
The online data centre, which is accessible through a password-protected website, does more than just record and visualise all tank data. The platform also enables convenient setting of the transfer module. Core elements such as the configuration of the user-defined alarms and the forwarding of alert notifications are also included. Interfaces to third-party systems are also possible.
To monitor a tank's level, typically two or three alarm thresholds are set (see Fig.). The first mark signals to the logistics staff responsible that a resupply should be planned. The second threshold notifies that the safety reserve limit has been crossed and escalates the notification to perhaps a colleague or a supervisor. A third mark will share, for example, when the maximum level has been reached.
Furthermore, all messages can be forwarded on to a specific person or group of people in a targeted manner. Thus, for example, the regular information on resupply goes to the logistics department via e-mail, while technical disruptions are reported directly to the service team.
The Cryo Gauge measuring system is not only for tank control on fixed locations. With mobile and/or temporary tank plants, its electronics can be supplied via a battery pack. Thus, reliable monitoring is even possible in areas without any proper infrastructure.
The application example described here makes it clear that measuring systems with telemetry offer advantages to both sides. They enable suppliers to carry out cost-optimised route planning for the resupply of tanks via road transport. Tank operators no longer have to take responsibility for the procurement of gases themselves, and they also benefit from enhanced operational safety. The potential within the combination of traditional measurement technology and modern communications electronics is still far from being exhausted.