Operating principle

SITRANS SL is a gas analyzer employing single-line molecular absorption spectroscopy. A diode laser emits a beam of infrared light which passes through the process gas and is received by a detector unit. The wavelength of the laser diode output is tuned to a gas-specific absorption line. The laser continuously scans this single absorption line with a very high spectral resolution. The degree of absorption and the line shape are used for the evaluation.

Basic design of the SITRANS SL

The field design of the SITRANS SL in-situ gas analyzer consists of a transmitter unit and a detector unit. The light which is not absorbed by the sample is detected in the receiver. The concentration of the gas component is determined from the absorption.

The SITRANS SL analyzer measures a single gas component by means of the absorption capacity of a single fully resolved molecular absorption line.

Absorption spectrum of measured signal and reference signal with SITRANS SL

SITRANS SL is designed for measuring oxygen (O₂) and carbon monoxide (CO) at high sensitivity.

Typical application specifications:

The measuring performance of the SITRANS SL depends, among others, on the actual, individual process conditions with regard to concentration ranges, pressure and temperature.

An internal reference cell is used to constantly check the stability of the spectrometer.

The self-calibration of the analyzer is therefore valid for at least one year without the necessity for external recalibration using calibration gases.

Typical spectral bandwidth of an absorption line compared to the bandwidth of the laser light.

Configuration

A feature of the in-situ analytical procedure is that the physical measurement takes place directly in the stream of process gas and directly in the actual process gas line. All process parameters such as gas matrix, pressure, temperature, moisture, dust load, flow velocity and mounting orientation can influence the measuring properties of the SITRANS SL and must therefore be investigated for each new application.

The standard applications listed in the ordering data for the SITRANS SL are distinguished in that the typical process conditions are adequately well-known and documented. If you cannot find your application among the standard applications, please contact Siemens. We will be pleased to check your possible individual application of the SITRANS SL. You can find an application questionnaire on the SITRANS SL product site on the Internet.

Typical cross-duct arrangement of the SITRANS SL

The SITRANS SL can be optionally purged on the process side using appropriate purging gases to prevent contamination of the sensor optics on the process side. Purging tubes on the sensor heads, which slightly extend into the process gas stream, define the effective measuring path length.

Influences on the measurement

Dust load

As long as the laser beam is able to generate a suitable detector signal, the dust load in the process gas does not influence the analytical result. By applying a dynamic background correction, measurements can be carried out without any negative impact. Under optimal conditions, the SITRANS SL can cope with dust loads up to 20 g/Nm? and up to a measured path length of 8 m. The influence of a high dust load is extremely complex, and depends on the optical path length and particle size. The optical damping increases exponentially at longer path lengths. Smaller particles also have a very large influence on the optical damping. With high dust load, long path length and small particle size, the technical support at Siemens should be consulted.

Temperature

The influence of temperature on the absorption line is compensated by a correction file. A temperature signal can be fed into the instrument from an external temperature sensor. The signal is then used for mathematical correction of the influence of the temperature on the observed line strength. If the process gas temperature remains constant, a static correction can be carried out as an alternative. Without temperature compensation, the relative error caused by changes in the gas temperature has an extensive effect on the measurement (e.g. up to 0.24 %/K with the O₂ application). An external temperature signal is therefore recommended in most cases.

Pressure

The process gas pressure can affect the line shape of the molecular absorption line. For known pressure values, the SITRANS SL uses a special algorithm to adapt the line shape. Additionally, an external pressure signal can be fed to the instrument to provide complete mathematical compensation for the pressure influence including the density effect. Without compensation, the relative error caused by changes in the process gas pressure is approx. 0.1 %/hPa. An external pressure signal is therefore recommended in most cases.

Effective optical path length

As a result of Beer-Lambert’s law, the absorption of laser light depends on the optical path length within the sample gas. Therefore the precision of the effective optical path length measurement can have an effect on the precision of the total measurement.

Since the sensor optics on the process side usually has to be purged to keep it clean for a longer period, the extent of the mixed zone between the purging medium and the process gas as well as the latter's concentration distribution must be considered. In a typical in-situ installation with an optical path length of several meters, the influence of the purging gas on the effective path length can be ignored.

The path length and dust load are mutually influencing: the higher the dust load in the process, the shorter the max. possible path length.

Design of the non-Ex version of the SITRANS SL system

Design of the ATEX version of the SITRANS SL system

Design of the FM version of the SITRANS SL system

The transmitter and detector units are mounted on process flanges provided by the customer. Correct alignment of these flanges must be guaranteed, e.g. by using the optional sensor alignment kit.

Adjustment of the pair of sensors

The flange connection plates (process interface) of the SITRANS SL to the process flanges on the customer side must be correctly aligned so that the laser beam generated by the transmitter hits the photodetector in the detector unit This is guaranteed in that the transmitter and detector units have a curved surface integrated in the connection plates. The adjustment is carried out by shifting the flanges on these surfaces, through which the symmetry axis is aligned. The axis can be offset by ± 1 degree, which means that the process flanges must be welded onto the process wall with at least this accuracy - see following figure.

Installation/adjustment requirements for the pair of cross-duct sensors

Purging

The easiest way to avoid condensation and dust deposits on the sensor windows or excessively high thermal load of the windows and the sealing material as well as the sensor electronics is to purge them (with O₂ application: nitrogen). Purging must be selected depending on the application. The transmitted-light sensors can therefore be configured for the respective situation. The application reference table provides recommendations for suitable purging for the standard applications.

If oxygen is to be measured with the SITRANS SL - which is also present in measurable quantities in the ambient air - oxygen-free purging gases must be used, such as nitrogen. It is equally necessary to purge the inside of the sensor heads, since the ambient air must also be displaced here out of the laser beam path. A differentiation is therefore made between purging on the process side and purging on the sensor side.

Arrangement for purging on the sensor side of the SITRANS SL

Purging on process side

For purging on the process side, the flow of purging gas can be adjusted between 0 and approx. 50 l/min at each sensor head using a needle valve (included in delivery).

Purging on sensor side

This can be combined with the purging on the process side, if required. Purging with nitrogen on the sensor side is almost always necessary for O₂ applications to avoid an offset caused by the oxygen of the air present in the unit. The cells in the sensor head are then continuously purged with nitrogen. Particularly when (re)starting the SITRANS SL O₂, a sufficiently high flow of purging gas of approx. 3 to 5 l/min must be provided for several minutes to ensure that all residues of oxygen are displaced. The flow of sensor purging gas can subsequently be set to a lower value using the needle valve (included in delivery).

Note:
With purging on the process side, it may be necessary to use non-return valves to ensure no process gas can enter the purging gas line in the event of failure of the purging gas supply. This applies especially in the case of cascaded process and sensor purging where there is otherwise the danger that, for example, corrosive process gases could enter the sensor enclosure.

Purging tubes

The purging media used on the process side flow through purging tubes into the process gas stream. The tubes extend into the process area by a few centimeters, usually perpendicular to the process gas stream. This means that an exactly defined optical path length is defined through the sample gas. The effective measuring path in the process gas is therefore defined as the distance between the ends of the two purging tubes. The standard length of the purging tubes is 340 mm. To enable sufficient pivoting, the process wall should be max. 150 mm thick.

Measurement of the optical path length between the ends of the purging gas tubes

Maintenance and fault messages

The SITRANS SL carries out continuous self-monitoring, and outputs alarms and warnings to indicate maintenance requirements or a system fault. The information is output as plain text on the LUI display, where symbols identify the category and the severity of the fault.

Alarm categories:

• Maintenance (system must be cleaned or repaired)
• Process value (problem with external sensor, or process conditions outside the permissible range for SITRANS SL)
• Configuration (SITRANS SL is not correctly configured)

Severity:

• Fault (measurements could not be carried out)
• Warning (measurements may be inaccurate, or the system will soon shut down measuring mode if an intervention is not made)
• Advanced warning/information (measurements are carried out)

The two binary (relay) outputs can be configured freely for the alarm output.

The response of the analog outputs in the event of an alarm is configurable; possible actions are:

• Off (current measured value is displayed)
• Last measured value (freezing of last value displayed)
• Standard level (setting to predefined value)
• 3 mA (NAMUR NE43 fault status)

In addition, the transmission is available as an output variable.

Note

Specific requirements for the measuring point can make the utilization of special sensor equipment necessary. The possibilities for adapting the sensors are:

• Special materials for purging tubes (on request)
• Various types/sizes of sensor flanges
• Explosion-protected sensor configurations

Essential characteristics

• Long-term stability through use of an internal reference cell; calibration interval at least one year
• Dynamic background correction for varying dust loads
• Isolated signal outputs of 4 to 20 mA
• User-friendly, menu-driven operation
• Selectable time constants (response time)
• Password-protected user interface
• I/O operation in accordance with NAMUR recommendations
• Monitoring of overall optical transmission
• Sensor enclosure resistant to wear and corrosion
• Simple local operation using remote-control unit with numeric keypad and menu prompting