P SL CLTBW D EA X 3 b07 | 2008-09-14

P SL CLTBW D EA X 3 b07 | 2008-09-14
Application Note
SKF Multilog On-line System DMx
and temperature measurements
By Oscar van Dijk • SKF Reliability Systems and Marcel de Boer • SKF Reliability Systems
Introduction
The SKF Multilog On-line System DMx was introduced as an
intrinsically safe, vibration based Machine Protection System.
However, there have been requests for the device to include
temperature measurements.
This is a common requirement to upgrade obsolete analog API
670 monitoring systems. These would often measure (for example)
16 channels of vibration and axial position, and 6 channels of
temperature. Each temperature sensor chain would involve direct
connection of a Resistive Temperature Detector (RTD) or
Thermocouple to the monitor.
This application note describes how to address this situation with
an SKF Multilog DMx.
RTDs and thermocouples
These are industry standard devices for sensing temperature. These
devices operate on low currents and voltages. Thermocouple
sensors (such as J or K type) use a very low voltage, and typically
require special cold junction compensation. Both RTDs and
thermocouples require linearization curves to convert the measured
voltage (or current) to an accurate temperature reading.
With the exception of directly connected Eddy Current Probes (for
radial vibration and axial position) the SKF Multilog DMx requires a
linearized input. Hence, to handle RTDs and thermocouples, the SKF
Multilog DMx requires an external temperature-current converter or
“temperature transmitter.”
The SKF Multilog On-line System DMx module.
However, the selected model must still have the correct “entity
parameters” for IS use with the SKF Multilog DMx. This is discussed
in more detail later.
Multiple vendors for these types of transmitters exist. SKF
Condition Monitoring Center – San Diego has tested the SKF
Multilog DMx with the PR533x range of modules from PR
electronics – see Figure 2.
This model has entity parameters compatible for use with the SKF
Multilog DMx in a hazardous area. The model can, of course, also be
used in nonhazardous area applications.
Connection to the SKF Multilog DMx
Figure 1. Examples of industry standard devices such as RTDs and
thermocouples.
Temperature Transmitters
By using an external temperature transmitter, temperature sensor
signals can be linearized and converted to a current signal. This
current signal is then sent to the SKF Multilog DMx and handled as a
DC input.
Temperature transmitters are commonplace on the general
instrumentation market. They are small, low power devices that use
a two wire connection. Over these two wires, the devices are
powered and the output signal is returned as a 4 to 20 mA signal to
the SKF Multilog DMx.
These transmitters are available for use with both RTDs and
thermocouples. Some models are programmable for the sensor
type and measuring range used with the transmitter. This
programmability makes these devices an ideal addition to the SKF
Multilog DMx as a front-end temperature transmitter.
For Intrinsically Safe (IS) use, care must be taken to maintain the
safety of the loop. By powering the transmitter from the SKF
Multilog DMx, you are making sure that the power to any
transmitter does not exceed the allowed parameters on the
certification.
Figure 2. PR533x, example of a temperature to current transmitter.
2
The transmitter is connected directly to the SKF Multilog DMx.
Power for the transmitter and current loop is supplied by the SKF
Multilog DMx. In this case, a model CMMA 9920 must be used.
Load
Resistor
Transmitter Sensor
RTD or OHM or TC
Figure 3. Temperature transmitter to SKF Multilog DMx connection.
An extract from the SKF Multilog DMx manual illustrating a typical
connection to the SKF Multilog DMx is shown in Figure 3. The
transmitter receives an RTD or thermocouple as its input, and
provides a 4 to 20 mA current signal as its output. A load resistor on
the SKF Multilog DMx is required to convert the current output from
the transmitter to a voltage.
For each channel, the SKF Multilog DMx can supply up to 14 mA
current at 20 volts. This power is enough for the selected
temperature transmitter to work. However, you cannot create
current, so this 14 mA supply is insufficient to give a 20 mA full scale
range. This limitation can simply be overcome in practice by only
using 50% of the full scale (such as, a 4 to 12 mA range). Since the
temperature transmitter is programmable, the full scale can be
adjusted to fit the requirements.
Temperature transmitter setup
This section describes a typical setup of a PR533x series
“transmitter,” using programming software that must be supplied
with the device.
Temperature channel example
Sensor type: RTD 100 Ω input
Measuring range: 0 to 150 °C
OK Detection: Yes. Downscale.
Load resistor: 1 000 Ω, 1%, 0.5 W
Transmitter programming
To match the required range, and the 14 mA limitation above, the
transmitter is programmed for a full scale of 300 °C, which is
illustrated in Figure 4.
Figure 5. Output screen, PR533x series.
Load resistor on SKF Multilog DMx input.
As show in Figure 3, a load resistor is required across the SKF
Multilog DMx input terminals. The current (I) from the transmitter
generates voltage (U) over the load resistor (R).
The voltage is calculated as U = I ¥ R.
For a 1 000 Ω resistor, at 4 mA current, the voltage over the load
resistor is 4 volts. At 12 mA, the voltage is 12 volts.
SKF Multilog DMx sensitivity
SKF Multilog DMx sensitivity is set using the following formula:
mV/EU = R · 16/Programmed Transmitter Range
Figure 4. Input screen, PR533x series.
In this case:
Figure 5 shows the associated output of the transmitter.
The programmed output is 4 to 20 mA and the action upon a
defective sensor or cable is set to “downscale.” This means that:
• at 0 °C, the current output is 4 mA
• at 150 °C, the current output is 12 mA
• In the case of a bad sensor or cable, current is 3.5 mA (or lower)
mV/EU = 1 000 · 16/300 = 53.33 mV/°C
Since the transmitter output is 4 mA when the temperature sensor
measures 0 °C, an offset must be calculated to allow the SKF
Multilog DMx to provide the correct result. The offset is calculated
as:
Offset = Offset Current · R/Sensitivity:
In this case:
Offset = 4 mA · 1 000/53.33 = 75 °C
3
SKF Multilog DMx setup
This section describes how a CMMA 9920 module channel is
configured using SKF Multilog DMx Manager software for the above
example.
First, the sensor type is configured as a “Temperature C” sensor,
see Figure 6.
Next, the sensor power is configured for constant voltage, see
Figure 8.
Figure 8. SKF Multilog DMx measurement channel – transducer power
Tab.
Then the Transducer OK settings are used to generate a sensor
failure indication. A current below 3.8 mA, or a current above 12.5
mA, generates a sensor NOT OK, see Figure 7.
Finally, in Figure 9, the measurement defaults to a “deg C” unit and,
as temperature is a DC measurement, the AC filtering is set to “Off,”
both AC and DC smoothing are set to 1 second.
The 75 °C offset calculated, is entered in the “Offset” field as
shown in Figure 9.
Any alarms required from the SKF Multilog DMx are set using the
normal alarm settings pages.
Figure 7. SKF Multilog DMx measurement channel – transducer tab.
Figure 9. SKF Multilog DMx measurement channel – measurement tab.
Figure 6. SKF Multilog DMx measurement channel – general tab.
4
Use in an Intrinsically Safe environment
Cable check
Since the SKF Multilog DMx is certified for use in ATEX Zone 1, it is
required to check if the combination of any temperature transmitter
and the SKF Multilog DMx are a safe solution.
To do this, we must look at the IS parameters for the SKF Multilog
DMx and the transmitter. If these match, then the combination of
the two devices is a safe system. The necessary calculations are
explained in the SKF Multilog DMx manual. For this example, the
most relevant calculations are shown below.
To comply with ATEX rules, a cable evaluation for the use of the
temperature transmitter must also be made. This is necessary even
if the transmitters are in same enclosure as the SKF Multilog DMx.
The calculation is based upon the safety descriptions of both
devices, indicating the maximum allowed capacitive and inductive
load. The following cabling requirements apply for meeting the
Group II C requirement:
PR533x entity parameters
These are found on the device’s ATEX certificate. For the model
PR5333D, the maximum supply and input parameters are:
Ui = 30 V; Ii = 120 mA; Pi = 0.84 W; Ci = 1nF; Li = 10 µH.
(From ATEX certificate KEMA 03ATEX1535X issue 2, dated 12 January 2009)
SKF Multilog DMx entity parameters
The maximum output parameters for the SKF Multilog DMx are:
Uo = 26.5V; Io = 90.9 mA; Po = 0.432 W; Ci = Negligible;
Li = Negligible; Cmax = 95 nf; Lmax = 3 mH
Capacitance: Cmax = Cext–Ci
where
Cmax= Maximum permissible cable capacitance
Cext = Maximum permissible capacitance that may be connected to the SKF Multilog DMx channel
Ci = Internal capacitance of the temperature transmitter
Hence, in this case: Cmax = 0.095 – 0.001 = 0.094 μF
This means that, using a typical 120 pF/m cable, the distance
between the transmitter and the SKF Multilog DMx module is
limited to:
(From ATEX certificate BVS07ATEXE075X dated 4 July 2007)
0.094 μF/120 pF = 0.094/0.000120 = 94/0.120 = 780 meters
Entity parameters check
Therefore, if the transmitter is located remotely, the SKF Multilog
DMx – perhaps located in the RTD sensor head on the bearing cap
– then the SKF Multilog DMx needs to be within 780 meters. In
practice it would be only a few meters away in a junction box on the
machine skid.
The above lead to the following Parameter Evaluation:
Uo (26.5 V) ≤ Ui (30 V)
† Comply
Io (90.9 mA) ≤ Ii (120 mA) † Comply
Po (0.432 W) ≤ Pi (0.84 W) † Comply
Intrinsically Safe conclusion
The above calculations show that the SKF Multilog DMx CMMA9920
Module can be used in combination with the PR electronics model
5333D Transmitter for classification Ex ia IIC.
The same combination can also be used, of course, in a non-IS
application, as shown in Figure 10.
Note
For any IS installation, the system must be checked with the newest certificates for all
connected equipment. The calculation above is an example using the certificates valid at
the time of writing.
5
Summary
Use of external, programmable, temperature transmitters can
provide an easy and flexible means of supporting temperature
measurements with the SKF Multilog On-line System DMx,
enhancing the value proposition of the system.
A transmitter model has been identified and tested and shown to
be safe for use with the SKF Multilog DMx in both IS and non-IS
applications.
Figure 10. SKF Multilog DMx and PR5333D transmitters (in non-IS
application).
Please contact:
SKF Reliability Systems
SKF Condition Monitoring Center – San Diego
5271 Viewridge Court · San Diego, California 92123 USA
Tel: +1 858-496-3400 · Fax: +1 858 496-3531
Web: www.skf.com/cm
®SKF and Multilog are registered trademarks of the SKF Group.
All other trademarks are the property of their respective owners.
©SKF Group 2009
The contents of this publication are the copyright of the publisher and may not be reproduced (even extracts) unless prior written permission
is granted. Every care has been taken to ensure the accuracy of the information contained in this publication but no liability can be accepted
for any loss or damage whether direct, indirect or consequential arising out of the use of the information contained herein.
PUB CM3120 EN · October 2009
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