The autonomous process loggers from the firm AIRVALVE operate with pressure sensors from STS in monitoring critical points of the gas grid owned by SWK Netze GmbH. The principle applied here affords planning reliability at a comparatively low outlay in its implementation.

SWK Netze GmbH performs extensive measurements on its gas distribution grid for calibration of its pipeline program. To this end, continuous pressure measurements are to be made at fifteen critical points as part of its project “Grid Monitoring of the Gas Distribution Network.” Besides expectations of the most precise of measured values, it was also crucial upon realization of this project that the measurement instruments performed both reliably over a longer time span and simultaneously had sufficient signal strength to regularly transmit measurements even when mounted below ground. To reduce underground and pipe installation work to an absolute minimum, pressures were instead to be measured at already existing ventilation fittings. For this purpose, the measurement equipment was to be installed in size 3 street caps.

To fulfill this task, the selection went to process loggers of the type LS-42 produced by AIRVALVE. During extensive testing, it previously emerged that the products of this process logger series were the only to avail of an integrated high-performance antenna, which could provide for an undisturbed signal transmission even in underground shaft workings.

Long-term stability and user-friendliness are key factors

In addition, this measurement instrument, thanks to its high-performance, interchangeable battery, functions free from electrical and telephone connections over a duration of 10 years and more. This easily mounted process logger, which is also remotely configurable, ensures a secure transmission of the measured readings due to freely selectable SIM cards or multi-network with a private VPN tunnel (see Fig. 1 about design of the process logger). It is therefore perfectly suited to remote or poorly accessible facilities, which have to be monitored over a longer timeframe without arduous maintenance requirements.

These requirements in terms of durability and operational performance were, of course, also placed upon the sensors used for pressure measurement. AIRVALVE opted here for the ATM.ECO/N pressure transmitters from STS.  These 100 mbar sensors are provided with power from the interchangeable battery of the process logger, have a resilient stainless steel housing and deliver precise results to an accuracy of ≤ ± 0.70 % over a temperature range from -5 to 50°C. In terms of long-term stability, the ATM.ECO/N registers < 0.5 %.

Assembly of the measuring system on the gas distribution grid

The entire measuring system for monitoring the gas distribution grid is housed in street caps (see Fig. 2). By using already existing ventilation fittings, the work necessary could be performed without major outlay. To implement pressure measurements, the ventilation riser plug was replaced with a reducer fitting (1). Using a stainless steel ball valve, the measurement connection can be shut-off (2). Calibration of the pressure sensor is facilitated by a Minimess coupling (3). The pressure sensor (4) is connected via a pressure-equalizing junction box (5) to the AIRVALVE process logger (6). This is then fixed to a ground anchor (7) by a click fastener.

Measurements are performed every 5 minutes. This measuring interval is fundamentally selectable between one and sixty minutes. The measured values are transmitted several times daily to the control center. Transmission of the readings can take place over VPN-secured multi-network cards or basic agreement SIM cards. Communications are possible using internet control centers or also with SCADA systems. In this example application, SWK Netze GmbH opted for the “Web-LS” internet control center to manage the obtained data through highly secure servers.