Application Scenarios
Picture a centrifugal air compressor skid in a nitrogen generation plant. The machine runs 24/7 on fluid-film thrust bearings, and while it is not classified as safety-critical, the maintenance team still needs to know when the thrust collar is migrating. A full 3500 rack would be overkill for a single measurement point—economically unjustifiable and physically too large for the OEM skid envelope. Yet relying on quarterly handheld readings leaves a three-month blind spot during which a thrust pad wear event or balance-piston seal failure could escalate into internal contact.
The OEM installs a Bently Nevada 991-01-XX-01-00 inside the local junction box. A 3300 NSv probe is targeted on the shaft collar, and the transmitter’s 4–20 mA output runs directly into the plant’s existing PLC analog input card. Six months later, the trend shows a steady drift from +12 mils toward +21 mils over four weeks. The team investigates and finds uneven thrust pad wear on the high-pressure side—caught before the rotor contacted the balance-piston labyrinth. The compressor is scheduled for a pad change during the next planned shutdown, and a potential rub is avoided.
This scenario illustrates the design intent of the 991 series: it is built primarily for OEMs of centrifugal air compressors and small pumps, motors, and fans who need to provide a simple 4–20 mA thrust signal to their machinery control system. It is not a substitute for a 3500-series monitoring system on large API 670 turbomachinery—but for balance-of-plant equipment where a full rack cannot be justified, it delivers Bently Nevada measurement integrity at a fraction of the infrastructure cost.
Parameter
| Main Parameters | Value/Description |
|---|---|
| Product Model | 991-01-XX-01-00 |
| Modification Code | MOD: 288865-01 (factory build variant) |
| Manufacturer | Bently Nevada (Baker Hughes) |
| Product Type | Two-wire, loop-powered thrust / axial position transmitter |
| Product Series | Bently Nevada 991 Thrust Transmitter |
| Full-Scale Option (01) | ±0.6 mm or ±25 mils (typical); confirm against ordering guide |
| Input | 1 × non-contacting 3300 NSv proximity probe + matching extension cable |
| System Length (XX) | 50 = 5.0 m, 70 = 7.0 m (matched to probe + cable total) |
| Mounting Option (01) | 35 mm DIN-rail clips |
| Agency Option (00) | None (general purpose, non-hazardous location) |
| Power Supply | +12 to +35 VDC at transmitter terminals (loop-powered) |
| Output Signal | 4–20 mA DC proportional to shaft axial position |
| Loop Accuracy | ±1.5% of full scale (TEST input to 250 Ω loop voltage) |
| Maximum Loop Resistance | 1,000 Ω including cable at 35 VDC |
| Current Limiting | 23 mA typical |
| Zero / Span Adjustment | Non-interacting external potentiometers |
| Not OK / Signal Defeat | Output < 3.6 mA within 100 μs; restored within 0.1 s after fault clears |
| Test Input Pin | Variable DC voltage source for quick loop verification |
| Power-Up Inhibit | Output suppressed to < 3.6 mA for 2–3 seconds after power-up |
| PROX OUT Impedance | 10 kΩ (calibrated for 10 MΩ load) |
| PROX OUT Linear Range | 0.25 to 1.65 mm (10 to 65 mils) |
| Scale Factor | 7.87 mV/μm (200 mV/mil) ±6.5% typical including interchangeability |
| Temperature Stability | Scale factor within ±10% from 0 °C to +70 °C |
| Minimum Target Size | 9.5 mm (0.375 in) diameter |
| Transmitter Operating Temp. | −35 °C to +85 °C |
| Probe Operating Temp. | −52 °C to +177 °C |
| Storage Temperature | −51 °C to +100 °C |
| Humidity | 100% condensing, non-submerged (with connector protection) |
| Transmitter Weight | 0.43 kg |
| Total System Weight | 0.82 kg typical |
| Dimensions (L × W × H) | 100 × 74 × 53.3 mm |
| Enclosure | Potted construction |
| Probe Cable | 75 Ω coaxial, FEP-insulated; optional stainless-steel armor with FEP jacket |
| Transducer Case Material | AISI 303 / 304 stainless steel |
| Transducer Tip Material | Polyphenylene sulfide (PPS) |
| Tensile Strength | 222 N (50 lbf), probe case to leadwire |
| Approvals | CSA/NRTL/C (North America), VDE (Europe), CE, FCC, RoHS |
| Maritime Approval | ABS Type Approval, Certificate 06-HS177078-3-PDA |
| Country of Origin | USA |