Pressure unlocks Compressed Natural Gas’ potential

Pressure unlocks Compressed Natural Gas’ potential

Thanks to its very high energy density, compressed natural gas (CNG) is well suited for use as an automotive fuel. CNG has an octane number of approximately 120 and combustion heat of 9,000 to 11,000 kcal/ kg or 38 to 47 MJ/ kg.

In addition, the combustion of CNG produces significantly less CO2 emissions than does the combustion of gasoline, for example. And because CNG is a particularly cost-effective fuel in many markets, manufacturers are showing a growing interest in developing vehicles that are capable of running on this alternative fuel source.

The primary challenge in optimizing an Internal Combustion Engine to run on CNG is regulating the injection pressure in the fuel rail.

Image 1: Example of a two fuel system for gasoline and CNG
Image Source: Bosch Mobility Solutions

CNG is stored at approximately 200 Bar, and is commonly injected at between two to nine Bar, depending on the engine requirements – low pressure for fuel efficient driving in the lower speed ranges, and higher pressures when greater power and torque are required.

The effectiveness of combustion within an engine’s cylinder is strongly influenced by the temperature and pressure of the CNG: An increase in pressure at constant volume will result in higher mass density of the gas, thus increasing its heating value.

However, even though the initial temperature and injection pressure can be varied, if not accurately calibrated during development, compressed natural gas vehicles can suffer from power loss and poor drivability.

Injecting CNG under pressure

Typically, CNG is fed from a high-pressure tank via a pressure regulator to the fuel rail. For efficient fuel combustion, the amount of natural gas injected must always be matched to the mass of air required by the engine. To achieve this the electronic engine management typically employs an air-flow meter to determine the exact amount of air required and subsequently the quantity of CNG to be injected.

With central point injection (CPI), the CNG is fed from a natural-gas distributor (NGD) into the intake manifold. A medium-pressure sensor measures the pressure and temperature in the NGD, allowing the natural-gas injectors to deliver the precise amount of fuel required.

Alternatively, the injection can also be implemented without the NGD, by aligning each injector with a corresponding cylinder. With this multi-point injection (MPI), the gas is injected under pressure at each cylinder’s intake manifold ‘runner,’ upstream of the intake valve.

Because changes in pressure have a significant influence on the engine’s performance when running on CNG fuel, engine torque and exhaust emissions (CO, CO2, NOx and hydrocarbons) all have to be recorded during engine testing.

Optimizing rail pressure for all driving conditions

To optimize the CNG system it’s important that during the design and testing phases the pressure within the rail is accurately measured at various throttle openings and cross referenced to engine torque and the corresponding exhaust gas emissions. Consequently high quality pressure sensors are demanded by most development engineers.

It’s important that these sensors deliver accurate readings across a wide range of pressures, while retaining their integrity at elevated temperatures.

Although an increase in CNG pressure reduces CO2, HC and NOx, CO in the exhaust gas increases, making it vital to accurately record the effects of modulating the CNG injection pressure.

During testing a pressure regulator is used to control injection pressure which is measured by an accurately calibrated pressure sensor located in the rail, while an analog flow meter, typically with a capacity of 2.5 m3/ h,is used to measure and control the inlet air flow rate. A chassis dynamometer is used to record engine torque.

For the duration of the test, gas temperature and flow rate are kept constant at 22°C and 0.1 SCFH, respectively. A high power blower is used to maintain engine temperature during the test, and emission test equipment is attached to the exhaust outlet to record CO, CO2, hydrocarbons and NOx content in the exhaust gases.

The process is quite complex and requires rail pressure, torque and emissions to be measured at hundreds of throttle opening points in order to create an effective map of the engine’s requirements for the engine ECU.

Measuring, recording and inputting all this data into the relevant tables is a time consuming task, therefore development engineers often turn to modeling tools to fast-track development. These tools commonly provide an environment for simulation and model-based design for dynamic and embedded systems, thereby reducing the number of hardware versions required to design the system.

The simulation model is coded with the information gained from the real-time testing and then built into an executable using C compiler to run on a real time operating system.

Once the baseline data is captured it’s possible to generate an infinite number of real-time simulations to be applied to any facet of the design cycle – from initial concept, to controller design, test and validation using hardware-in-loop (HIL) testing.

A well-developed test program using laboratory grade pressure sensors and test equipment unleashes performance and drivability from CNG fueled vehicles that is comparable to fossil fueled equivalents, while delivering cost and emissions benefits.

Mud Logging Requires High-Performing, Rugged Pressure Transmitters

Mud Logging Requires High-Performing, Rugged Pressure Transmitters

The term mud logging refers to the analytical methods that are performed on drilling mud during drilling operations. Powerful and, above all, rugged pressure transmitters are paramount to the process.

The words “mud” and “logging” already provide a good, albeit incomplete, description of the process involved: mud loggers (also surface-logging specialists) are tasked by drilling companies to create detailed records of a borehole. Mud loggers analyze the information brought to the surface during the drilling process, which is why many companies also use the term surface logging services (SLS). The drilling mud is the most important component of mud logging as it carries the information from the depth of the borehole to the surface, where the cuttings (i.e. pieces of formation rock) contained in the circulating drilling medium are examined.

These findings provide a depth-dependent protocol to determine the depth position of hydrocarbons, identify borehole lithology, and monitor natural gas that may enter the drilling mud. Further objectives of mud logging are estimating the pore pressure and porosity as well as permeability of the drilled formation, collecting, monitoring and evaluating hydrocarbons, and assessing the producibility of hydrocarbon-bearing formations as well as keeping a record of drilling parameters. This data is important to ensure safe as well as economically optimized drilling operations.

Mud logging takes place in real time in mobile laboratories that are set up at the drilling site. The real-time data is directly used for drilling control. Mud logging services are usually carried out by specialists contracted by the drilling company. STS is providing pressure transmitters to some of these providers of surface logging services.

Pressure sensors used in drilling processes: durability is key

In order to monitor the drilling process, mud loggers mount various sensors on the drilling apparatus. The detection of even minor losses of drill pipe pressure requires a very high degree of accuracy. Moreover, an immediate response is necessary as well to prevent fishing times, lost-in-holes as well as the risks and costs associated with abnormalities.

Drilling sites are rugged environments and as such can be very demanding on the sensor equipment. The two most important factors in this regard are the mud itself and the vibrations that are to be expected in drilling operations.

Image 1: ATEX certified pressure transmitter for Mud Logging applications

To deal with these harsh conditions, STS provides companies offering surface logging services with the ATM/ECO/EX with customized housing. The ATEX-certified pressure transmitter is optimized for high pressure ranges. The vibrations occurring during drilling processes largely affect the area between the tube and the process connection. STS solved the issue by double welding the connection. Moreover, the stainless steel tube is thicker than is usually the case (26,5 mm). Other than the high pressure ranges and the vibrations that have to be accounted for, the mud presents another challenge by potentially clogging the pressure channel. To prevent clogging, we made the channel a bit wider (10 mm). Normally, a wider pressure channel can put the pressure diaphragm at risk. However, since mud loggers largely work with static pressures, this is not an issue.

Mud Pulse Telemetry: MWD Data Transmission with Pressure Sensors

Mud Pulse Telemetry: MWD Data Transmission with Pressure Sensors

Hydraulic data transmission requires sensitive pressure sensors capable of enduring high pressures. This is particularly true when used in measurement while drilling (MWD) applications.

MWD has become a standard application, especially for offshore directional drilling. Real-time data collection is essential for measuring the trajectory of the hole as it is drilled. For this purpose, various sensors are mounted on the drill head to provide information about the drilling environment in real time. Inclination, temperature, ultrasound and also radiation sensors are used. These various sensors are physically or digitally connected to a logic unit that converts the information into binary digits. The downhole data are transmitted to the surface via mud pulse telemetry. In addition to monitoring and controlling the drilling process, the data are used for further aspects, including:

  • Information about the condition of the drill bit
  • Records of the geological formations penetrated by the borehole
  • Creation of performance statistics to identify possible improvements
  • Risk analysis for future drilling

Mud pulse telemetry is a binary coding transmission system used with liquids. This is achieved by a valve that varies the pressure of the drilling mud within the drill string and thus converts the recordings of the sensors mounted on the drill head into pressure pulses. The pulsations reach the surface via the drilling mud. The pressure pulses are measured on the surface by a pressure transmitter and converted into an electrical signal. This signal is transmitted to a computer and digitized.

STS provides offshore directional drilling companies with analog pressure transmitters optimized for mud pulse telemetry. The sensors have to meet high demands: They must be extremely sensitive in order to reliably register even the smallest pressure differences. At the same time, the sensors must withstand pressures of up to 1,000 bar. Very high pressures are required to power the drill head in very deep drill holes. The pressure transmitters used for mud pulse telemetry on the surface are also exposed to these forces.

In addition to the high sensitivity, very fast response times are required to ensure good data communication in real time. In order to exclude falsified measurement results, the measuring instrument should be low-noise. The mud pumps in particular can cause the most signal noise in drilling applications. The drive of the drill is another source of interference. For this reason, analogue sensors with a 4 – 20 mA output signal are the best solution for mud pulse telemetry.

Innovative solutions to pressure sensing in biogas production

Innovative solutions to pressure sensing in biogas production

Microbiological analysis is an important component of the biogas manufacturing process. In this instance, combined pressure and temperature transmitters from STS are employed.

The Institute for Agricultural Engineering and Animal Husbandry at the Bavarian State Research Center for Agriculture has been examining, amongst other things, the influence of activating or toxic substances on the process of biogas production. In contrast to the continuous flow process of a biogas plant, investigations of potential such as this are conducted in intermittent batch procedures. For these investigations, a mini-batch system has been specially developed, based upon combined pressure and temperature transmitters from STS.

Measuring microbial activity

To guarantee a reliable temperature control, which is essential to such investigations due to its vital role in microbiological activity, the mini-batch system is submerged in a water bath. Within this bath, some 33 measuring points are situated so that ten variants, as well as a control sample, can be tested for both parallel and statistical evaluation. The measurement of microbial activity takes place indirectly by a continuous determination of biogas production aided by the ATM/N pressure transmitter from STS.

To additionally calculate methane productivity, the gas composition is regularly analyzed using a gas chromatograph. After adding 100 ml of fermenter content to 300 ml Schott-Duran bottles, the ATM/N pressure transmitters are capable of exactly recording the pressure increase brought about by biogas production. From this, an exact statistical evaluation and assessment of the addition of substances is possible in the process of biogas production, as is a comparison between those individual variants.

Combined sensors are highly versatile

A substantial advantage of combined sensors for pressure and temperature is the recording of both process parameters from only one pressure port. The temperature probe here is submerged in the medium and provides a measurement range of – 25 … + 50 °C. All connections are welded and conform to the IP68 protection rating. This has the advantage that these sensors, apart from their industrial usage, can also be applied in the foodstuffs and pharmaceuticals industries. Other typical applications for the transmitters are in plant and machine engineering, in testing and calibration technology, process engineering and environmental technology, as well as shipbuilding. These sensors are also deployed in the industrial environment of biogas plants for determining the filling level inside fermenters.

The following characteristics set these pressure sensors apart: Measurement ranges from 0 … 50 mbar to 0 … 25 bar, high dynamic response and precision (< 0,1 % FS), mechanical and electrical adaptation to end user applications due to the manufacturer’s modular system. Upon request, intrinsically safe designs can also be supplied. It is through these technical properties that the pressure sensors are suited to various fields of application in measurement technology, as well as in equipping test beds and calibration facilities.

Original publication: INDUSTRIELLE AUTOMATION 2/2014 

Gas distribution grid monitoring by continuous pressure measurement

Gas distribution grid monitoring by continuous pressure measurement

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.

Figure 1: Datalogger construction (Source: AIRVALVE)

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.

Figure 2: Overview of the measuring system (Source: AIRVALVE)

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.

Density measurement in gas flow meters

Density measurement in gas flow meters

Gas consumption is calculated using gas meters measuring the flow volume. Since the density of gas, and thus its volume also, is both pressure and temperature dependent, the measured quantity can deviate due to the prevailing  pressure or temperature. The gas volume, depending upon pressure and temperature, can be described by the formula p · V/T = Constant (p: pressure, V: volume, T: temperature).

Whilst the pressure with which gas flows through the pipes can be relatively easily controlled and monitored, this is not the case with the temperature. The resulting differences in density have an influence on the measured flow rate. What remains negligible here to the normal consumer due to relatively light usage becomes an important cost factor to those major consumers.

With the Measurement Instruments Directive (MID), an EU-wide guideline for measuring instruments was issued to establish a uniform approval procedure for all EU states and some other nations. Further objectives of the directive include a one-time and unified test for the approval of measuring instruments, as well as a uniform and transnational regulation for initial calibration. With these designated, transnational regulations an even better product quality is striven for and a level playing field ensured. Ten types of measuring instruments in the sphere of legal metrology are covered by the MID, with the requirements for gas meters and volume converters laid out in Annex MI-002.

Pressure and temperature must be taken into account when calculating exact gas quantities. And this requires appropriate sensors in the gas meters. Instead of the volume, the gas mass must be indicated, since this is the more precise measure in light of fluctuating density. To reliably determine this, it is necessary to measure both pressure and temperature and thus determine the density.

High precision through computational compensation

There are two types of pressure and temperature sensors to be connected to gas meters. In the first variant, the pressure transmitter is screwed onto the gas-delivery pipe and connected to the gas meter by means of a cable. In variant two, however, the sensor is installed directly into the device (the specific example below describes variant two).

The pressure ranges used for gas metering generally fall between 0.8 and 3.5 bar (absolute) and 2.5 to 10 bar (absolute). The requirements in terms of precision are enormous: Demanded is 0.2% of the measured value at temperatures from -20 °C to 60 °C. This figure, however, cannot be achieved with conventional pressure sensors. To maintain this level of accuracy, computational compensation must be applied. For this reason, STS supplies its pressure and temperature transmitters not only functionality-tested, but also parameterized (coefficients for polynomial compensation).