Steam turbine generator startups create fast temperature change and tight internal clearances. If the rotor and casing do not grow together, contact can occur. Good measurement lowers that risk.
Safety and scope: General maintenance guidance based on vendor datasheets and public standard summaries. It does not replace OEM manuals, site procedures, or a qualified engineer's instructions. Do not change setpoints or hardware without OEM/site approval.
Turbine Supervisory Instrumentation (TSI) for Safer Steam Turbine Generator Starts
Startup and load swings create the largest thermal movement. This section links common TSI growth channels to the practical decisions operators make, such as holding a start, changing ramp rate, or returning to turning gear.
Why Starts Are Higher-Risk Than Steady Load
The casing and rotor heat at different rates. Clearances can tighten quickly. A rub can damage seals and raise vibration. TSI channels give early warning while the unit can still be held or slowed.
Casing Expansion Sensors and Differential Expansion (DE) in Plain Terms
With TSI context set, focus turns to what each channel actually measures. Case expansion shows if the casing is free to move at its feet. DE shows rotor-to-case growth difference near the thrust area.
Case Expansion at the Sliding Feet
Casing Expansion Sensors measure casing thermal growth relative to the foundation. A dual arrangement (left and right sliding feet) is recommended because one foot can bind while the other moves, twisting the case and raising rub risk.
Differential Expansion (de) at a Collar or Ramp
Vendor datasheets describe DE measurement as two proximity probes observing a collar or ramp. Larger or faster DE changes raise clearance risk, so DE trends are most useful during warm-up and load ramps.
3300 XL Proximity Systems for Differential Expansion Channels
Selection starts with travel range, the installed probe gap, target material, and steam moisture at the DE location. This section keeps published numbers tied to the stated test conditions, so specs are used correctly.
3300 XL 25 MM Eddy Current Sensor System: Key Numbers and Conditions
The 3300 XL 25 mm Eddy Current Sensor System lists an average scale factor of 0.787 V/mm (20 mV/mil) nominal and a 12.7 mm (500 mils) linear range.
It defines that span as starting at about 0.63 mm from the target and running from 0.63 to 13.33 mm.
Stated measurement conditions: Unless otherwise noted, the datasheet states these specs apply with probe, extension cable, and Proximitor sensor at 0°C to +45°C, max altitude 2000 m, –24 Vdc supply, 10 kΩ load, a Bently Nevada supplied AISI 4140 steel target 61 mm diameter or larger, and a 7.0 mm probe gap. It also notes accuracy/interchangeability do not apply with a different target calibration.
The same datasheet gives an example temperature test case for DSL stability (probe at –35°C to +120°C with electronics/cable at 0°C to +45°C, DSL within ±0.92 mm).
3300 XL 50 MM Proximity System: Key Numbers and Conditions
The 3300 XL 50 mm Proximity System lists a 27.9 mm (1100 mils) maximum linear range and an average scale factor of 394 mV/mm (10 mV/mil) nominal.
It defines the span as starting at about 1.3 mm from the target and running from 1.3 to 29.2 mm.
Stated measurement conditions: Unless otherwise noted, the datasheet states its specs apply with probe/cable/electronics at 0°C to +45°C, max altitude 2000 m, –24 Vdc supply, 10 kΩ load, and a Bently Nevada supplied AISI 4140 steel target 102 mm diameter or larger; it also notes accuracy/interchangeability do not apply with a different target calibration.
The description also states the probe can operate and maintain accuracy in high temperatures up to 200°C and can withstand intermittent temperatures up to 250°C. A vendor brochure describes the 50 mm series as designed for differential expansion service in aggressive steam environments.
Linear Variable Differential Transformer (LVDT) Case Expansion Assemblies
DE channels show rotor-to-case growth difference, but case expansion channels show if the casing is free to move. LVDT assemblies are common because they measure direct travel through a rod and a protected enclosure.
LVDT Operating Principle and Dual-sensor Intent
The case expansion datasheet describes a Linear Variable Differential Transformer (LVDT) in a weatherproof enclosure, with a rod linked to the casing. As the casing grows, the rod moves and the conditioned output goes to a monitor for display and alarms.
It also recommends a dual transducer arrangement so a jam at a single sliding foot can be detected and alarmed.
Key LVDT Numbers with Operating Conditions
For the 24765 dc LVDT assembly, the datasheet specifies +24 Vdc input power (30 mA minimum). It lists model options such as:
- 24765-01: 0.346 V/mm with ±12.7 mm range
- 24765-02: 0.404 V/mm with ±25.4 mm range
It also lists ±0.5% full-range linearity, 0.125% full-scale stability, and operating temperature of –18°C to +71°C.
API Standard 670 Compliance Actions for Expansion Measurements
Sensors reduce risk only when installation and verification are controlled. Many plants use API 670 as a baseline for machinery protection system requirements, then add owner-specific rules for startup monitoring and audit control.
Version reference and audit-ready actions
API Standard 670 is published by the American Petroleum Institute. Industry listings and updates report a Sixth Edition with a publication date in July 2025.
API also provides a public scope summary sheet for the Fifth Edition (November 2014) that highlights installation, documentation, and testing as core areas.
Concrete actions that map to those themes:
- Document the chain: tags, scaling, datasheet doc number/revision, wiring and monitor slot.
- Record installed conditions: probe gap, target material/size, linkage free travel, photos.
- Verify with controlled motion: checks at two or three points inside the linear span.
- Keep acceptance evidence: signed SAT/FAT results and a saved "normal start" trend.
Illustrative Startup Trends and Quick Checklists
These examples and checklists help field teams spot patterns fast. They are illustrative only and must be adapted to OEM limits and site procedures. Use them as a shape check for symmetry, rate, and step changes.
Illustrative trend: stable symmetry
|
Elapsed (min)
|
Speed (rpm)
|
Case L (mm)
|
Case R (mm)
|
DE (mm)
|
|
0
|
0
|
0.0
|
0.0
|
0.0
|
|
60
|
1800
|
2.9
|
3.0
|
3.7
|
|
90
|
3600
|
4.5
|
4.6
|
4.6
|
Illustrative trend: binding at one sliding foot
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Elapsed (min)
|
Speed (rpm)
|
Case L (mm)
|
Case R (mm)
|
DE (mm)
|
|
0
|
0
|
0.0
|
0.0
|
0.0
|
|
60
|
1800
|
3.4
|
2.2
|
5.1
|
|
90
|
3600
|
4.8
|
2.4
|
6.7
|
Dual case expansion is intended to catch this kind of split.
Quick Decision Assets
25 mm vs 50 mm (4 checks)
- Compare expected travel to linear range (25 mm: 12.7 mm; 50 mm: 27.9 mm).
- Confirm installed gap stays inside the stated span window.
- Confirm the target and calibration match the datasheet conditions (AISI 4140, stated diameter, stated gap).
- If wet steam and contamination are common, weigh the steam-duty notes and sealing features described in vendor literature.
Alarm or abnormal trend (3 steps)
- Validate the signal (steps, dropouts, noise).
- Compare Case L vs Case R and compare DE rate to case growth rate.
- Follow the startup procedure and inspect likely mechanical points (sliding feet, brackets, target).
Conclusion: Verify Installation Conditions and Keep Compliance Records
Expansion monitoring is reliable only when the stated test conditions are respected at the installed gap and target, and when verification evidence is kept with the project documents. Use vendor revision-controlled datasheets and the plant's specified API 670 edition as the compliance basis.
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