Description
Function of RTDs
The primary function of an RTD is to measure temperature by detecting the change in electrical resistance of its sensing element as temperature changes.
In a centrifugal compressor system, RTDs may be used to:
- Monitor compressor bearing temperature
- Monitor thrust-bearing temperature
- Monitor lubricating-oil temperature
- Monitor motor temperature
- Monitor gearbox temperature where applicable
- Monitor cooling-system temperature
- Provide temperature feedback to the compressor controller
- Generate high-temperature alarms
- Support protective shutdown functions
- Provide continuous temperature information to plant monitoring systems
The exact measurement points depend on the compressor design.
How an RTD Works
An RTD uses a temperature-sensitive resistance element whose electrical resistance changes predictably with temperature.
As the temperature of the sensing element changes, its electrical resistance changes. The associated instrumentation system measures this resistance and converts it into a temperature value.
The signal can then be used by the compressor control or monitoring system for:
- Temperature display
- Alarm generation
- Protective interlocks
- Trend monitoring
- Equipment-condition monitoring
Common RTD Types
PT100 RTDs
PT100 is one of the most commonly used industrial RTD configurations. Its nominal resistance is 100 ohms at 0°C.
PT100 sensors are widely used for industrial temperature measurement and may be supplied in different physical constructions and wiring configurations.
PT1000 RTDs
PT1000 sensors have a nominal resistance of 1000 ohms at 0°C and may be used where the instrumentation system is designed for this sensor type.
Other RTD Elements
Other resistance elements can be used depending on the original compressor instrumentation design.
The replacement sensor must match the existing control system and transmitter/input configuration.
RTD Wiring Configurations
RTDs may be supplied with different wiring arrangements.
2-Wire RTD
A two-wire configuration uses two conductors to measure the resistance of the sensing element. Lead resistance can influence measurement accuracy, particularly with longer cables.
3-Wire RTD
Three-wire RTDs are widely used in industrial applications because the wiring arrangement can compensate for a significant portion of lead resistance.
4-Wire RTD
Four-wire configurations provide a more precise resistance measurement by separating the current and measurement circuits.
The correct wiring configuration should match the compressor’s instrumentation system.
RTD Construction
A typical industrial RTD may include:
- Resistance sensing element
- Protective sheath
- Insulation material
- Lead wires
- Connection terminals
- Mounting thread or fitting
- Cable or extension leads
- Connector or terminal head
- Protective housing where required
The physical construction depends on the installation location and environmental conditions.
RTD Sensing Elements
Common RTD sensing elements are based on materials with predictable resistance-temperature characteristics.
Platinum is widely used for industrial RTDs because of its stable and repeatable temperature response.
Depending on the application, the sensing element may be housed inside a protective sheath or assembly designed to withstand vibration, temperature, moisture, and other environmental conditions.
RTDs for Compressor Bearings
Bearing temperature monitoring is particularly important in centrifugal compressor systems.
RTDs may be installed in or near:
- Radial bearings
- Journal bearings
- Thrust bearings
- Bearing housings
- Other critical bearing locations
An increase in bearing temperature can be associated with conditions such as inadequate lubrication, excessive loading, alignment problems, bearing deterioration, or other operating issues.
The RTD provides temperature information to the compressor monitoring and protection system.
RTDs for Lubricating Oil
RTDs may also be used to measure lubricating-oil temperature at selected points within the compressor lubrication system.
Possible measurement locations include:
- Oil reservoir
- Oil cooler inlet
- Oil cooler outlet
- Bearing oil supply
- Oil return line
- Other lubrication-system locations
Oil-temperature monitoring helps verify that the lubrication system is operating within its specified conditions.
RTDs for Electric Motors
Electric motors used with centrifugal compressors may use RTDs for monitoring winding or bearing temperatures.
Motor RTDs can provide temperature information for:
- Stator windings
- Motor bearings
- Other manufacturer-specified measurement points
These signals may be connected to the motor protection system, PLC, or compressor control system.
Importance of RTD Accuracy
Accurate temperature measurement is important because compressor protection systems may use temperature signals to initiate alarms or shutdowns.
Incorrect temperature readings may result from:
- Sensor degradation
- Incorrect RTD type
- Wiring problems
- Loose connections
- Calibration errors
- Damaged sensing elements
- Electrical interference
- Incorrect input configuration
- Incorrect installation
The RTD and its associated instrumentation should therefore be evaluated as a complete measurement circuit.
Common RTD Problems
Common RTD-related problems include:
- Open circuit
- Short circuit
- Unstable temperature readings
- Incorrect temperature display
- Sensor drift
- Damaged sensing element
- Broken lead wires
- Loose terminals
- Moisture ingress
- Insulation damage
- Incorrect wiring
- Incorrect sensor resistance type
- Physical damage to the probe or sheath
An abnormal reading does not necessarily mean that the RTD itself has failed. Wiring, input modules, transmitters, and connections should also be checked.
RTD Maintenance and Inspection
RTDs generally require limited mechanical maintenance, but their condition and measurement accuracy should be periodically verified.
Maintenance may include:
- Inspecting sensor wiring
- Checking electrical connections
- Checking insulation condition
- Inspecting the sensor sheath
- Checking mounting arrangement
- Comparing readings against a calibrated reference
- Checking sensor resistance
- Testing open- and short-circuit conditions
- Verifying PLC or controller input
- Checking alarm and shutdown functions
Maintenance and calibration intervals should be based on the compressor manufacturer’s recommendations and the criticality of the measurement point.
RTD Testing
An RTD can be checked by measuring its electrical resistance and comparing the result with the expected resistance for the measured temperature and sensor type.
Testing may include:
- Resistance measurement
- Continuity testing
- Insulation testing where appropriate
- Temperature comparison
- Calibration verification
- Wiring verification
- Input-channel verification
Testing procedures should be performed using suitable instrumentation and according to the applicable equipment and electrical-safety procedures.
Replacement of RTDs
A replacement RTD should be matched to the original compressor instrumentation configuration.
Important selection parameters include:
- Compressor manufacturer
- Compressor model
- Compressor serial number
- Measurement location
- RTD type
- Resistance value
- Number of wires
- Sensor length
- Probe diameter
- Sheath material
- Temperature range
- Accuracy class
- Process or mounting connection
- Cable length
- Connector type
- Terminal configuration
- PLC/controller compatibility
For bearing applications, the physical dimensions and mounting arrangement are especially important because the sensor must fit correctly into the original bearing or housing arrangement.
RTD Selection Specifications
| Parameter | Requirement |
|---|---|
| Component | RTD (Resistance Temperature Detector) |
| Equipment | Centrifugal Air Compressor |
| Measurement Location | Bearing / Oil / Motor / Cooling / Other |
| RTD Type | PT100 / PT1000 / As specified |
| Resistance at 0°C | As specified |
| Wiring | 2-Wire / 3-Wire / 4-Wire |
| Temperature Range | As required |
| Accuracy Class | As specified |
| Probe Length | As required |
| Probe Diameter | As specified |
| Sheath Material | As required |
| Mounting Connection | As specified |
| Cable Length | As required |
| Connector | As specified |
| Output | Resistance signal / As applicable |
| Control-System Interface | PLC / DCS / Controller / Monitoring system |
| Application | Temperature monitoring and protection |
Why Choose Aegis Projects?
Aegis Projects Technology provides industrial compressor spare parts and instrumentation components for centrifugal air compressor systems.
Our RTD solutions can be evaluated according to the compressor’s measurement point, sensor type, physical dimensions, wiring configuration, temperature range, and control-system requirements.
We support requirements for:
- Compressor bearing RTDs
- Thrust-bearing RTDs
- Journal-bearing temperature sensors
- Lubricating-oil RTDs
- Motor winding RTDs
- Motor bearing RTDs
- Cooling-system temperature sensors
- Replacement RTD assemblies
- Compressor instrumentation spares
- Replacement and retrofit requirements
For accurate selection, share the compressor make, model, serial number, existing RTD nameplate or part number, measurement location, PT100/PT1000 specification, number of wires, dimensions, photographs, or drawings wherever available.
Request a Quote
Looking for RTDs for a centrifugal air compressor?
Contact Aegis Projects Technology Pvt. Ltd. with your compressor and instrumentation details. Our team can assist with identifying a suitable replacement RTD for your compressor monitoring and protection system.
