One-Shot Rotor Balancing of a Nuclear Steam Turbine Generator Set

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"Turbine engineers use the system to monitor the health of the turbine generator sets through vibration data and other relevant parameters. The system provides live monitoring, analysis, and comparison of historical data, as well as a powerful multiplane rotor balancing module."

- Stephen Gillon, Elstar Elektronik

The Challenge:
Performing a one-shot balance solution (without performing any trial runs) on a nuclear turbine generator set during a refuelling outage using only influence data recorded in the 1980s, with a different vibration acquisition system, with different sensors, and on a different machine.

The Solution:
Using vibration data recorded by the permanently installed Cabstar_LTS monitoring system, developed with LabWindows™/CVI software and NI PXI hardware, and the historical vibration data for the original influence runs on sister unit TG11 to select a balancing plane, correction weight, and angle for a successful one-shot balance.

Author(s):
Stephen Gillon - Elstar Elektronik

High-vibration amplitude was present at several bearing locations on TG22, one of four steam turbine generator sets at the Beznau nuclear power plant in Switzerland. To reduce vibration amplitudes, and thereby minimise mechanical wear on the bearings, we planned in-situ rotor balancing during a scheduled refuelling outage—a process involving the application of balance weights at various planes and angular positions on the rotor.

Under ideal circumstances, the in-situ balancing process involves the application of a trial weight in one axial plane, and a run-up to nominal speed and run-down to measure the vibration response. We repeat this process for each balancing plane. We calculate influence coefficients for each balancing plane, then calculate an optimised multiplane balancing solution.

The Beznau units have 10 planes available for application of balance weights, so ideal circumstances would require 10 separate run-ups and run-downs of the machine before the final balance solution could be calculated—a very costly exercise.

Introduction

Rotor dynamics is a specialised field of vibration analysis that deals with rotating machinery, often large turbomachinery. Elstar Elektronik’s rotor dynamics acquisition and analysis system, Cabstar_LTS, is an in-house software solution created using LabWindows/CVI software and NI PXI platform acquisition hardware.

In early 2015, Cabstar_LTS was permanently installed on two of the four units at the Axpo nuclear power plant Beznau. The installation on the remaining two units was completed in 2015 and set to be commissioned in early 2016. Turbine engineers use the system to monitor the health of the turbine generator sets through vibration data and other relevant parameters. The system provides live monitoring, analysis, and comparison of historical data, as well as a powerful multiplane rotor balancing module.

The Plant

The Beznau nuclear power plant in Switzerland has operated since 1969, making it the world’s oldest operating nuclear power plant. The plant directly employs more than 500 people and subcontracts another 300 specialists during maintenance and refuelling revisions. The plant comprises four similar turbine generator units, divided into two blocks. Each unit consists of a 190 MW generator driven by one high-pressure steam turbine and two low-pressure steam turbines, giving a total plant capacity of 760 MW. Six journal bearings support each shaft train. We installed the first stage of the Cabstar_LTS system on the two units in Block 2 and commissioned in late 2013.

Vibration Sensors

We measure radial vibration at eight planes along the shaft train, close to each bearing location, with two planes on either side of bearings two and three. Each vibration measurement plane consists of a vertical and horizontal piezoelectric velocity transducer to measure absolute bearing vibration (ABV), and a vertical and horizontal non-contact eddy probe displacement sensor to measure relative shaft vibration (RSV). In addition, the DC component of the displacement sensors is used to track shaft centreline movements. Integrated ABV and raw RSV are summed to deliver absolute shaft vibration (ASV).

PXI Hardware

We route vibration signals for each unit, 35 total, through signal conditioners to three NI PXIe-4497 modules mounted in an NI PXIe-1082 chassis in the plant electronic room. We route speed and phase trigger signals to an NI PXIe-6251 module used as a counter card. We route approximately 30 slow sampled process signals, including axial expansion, active and reactive generator load, reactor feedwater pump vibration, as well as various operating temperatures and pressures, to a PXI-6255 module. We use an NI PXIe-8381 module to provide remote control from the Cabstar_LTS computer, mounted in the same rack, which is in turn operated remotely from the engineering offices via a keyboard, video, and mouse (KVM) extender.

Figure 1. PXI Hardware Installed in Cabinet

We used the LabWindows/CVI development environment to create Cabstar_LTS, a vibration monitoring and analysis system, designed to be permanently installed on continuously operated critical machinery. It monitors vibration and process signals, performs data storage, generates amplitude and vector alarms, provides online and offline rotor dynamic analysis tools, and includes a multiplane balancing module. The balancing module is thanks to Elstar Elektronik’s experience delivering complete data acquisition and software solutions to large rotor balancing facilities worldwide, and can be used to calculate multiplane, multispeed balancing solutions.

Figure 2. Screenshots From Cabstar_LTS

Figure 3. Screenshots From Cabstar_LTS Balancing Centre

The Vibration

During 2014, RSV (and therefore ASV) amplitude, particularly at bearing three, on unit TG22 was high enough to cause occasional alarms on the Beznau plant protection system. The plant protection system, part of the control system, monitors many hundreds of signals and operational parameters, and is constantly monitored by operations personnel. Alarm conditions always trigger action by responsible specialists, in this case turbine engineers.

Vibration alarms and trip values for the Beznau protection system are based on ISO 7919: Mechanical vibration: Evaluation of machine vibration by measurements on rotating shafts, Part 2: Land-based steam turbines and generators in excess of 50 MW with normal operating speeds of 1500 r/min, 1800 r/min, 3000 r/min and 3600 r/min. This standard categorises vibration amplitude into four zones as follows:

Zone A        The vibration of newly commissioned machines would normally fall within this zone.

Zone B        Machines with vibration within this zone are normally considered acceptable for unrestricted long-term operation.

Zone C        Machines with vibration within this zone are normally considered unsatisfactory for long−term continuous operation. Generally, the machine may be operated for a limited period in this condition until a suitable opportunity arises for remedial action.

Zone D        Vibration values within this zone are normally considered to be of sufficient severity to cause damage to the machine.

It is common practice to set a warning alarm to indicate when vibration amplitude falls into Zone C, and automatically trip the machine when vibration amplitude reaches Zone D.

Detailed analysis of the vibration behaviour using Cabstar_LTS, by turbine engineers with support from Elstar Elektronik, resulted in the recommendation that in-situ rotor balancing, that is applying balance weights to the rotor, could be an effective strategy to reduce vibration amplitude to acceptable levels.

A refuelling outage in August 2014 provided an opportunity to perform in-situ balancing. The customer asked Elstar Elektronik to provide technical support in both rotor dynamics expertise and to demonstrate the application of the advanced capabilities of the Cabstar_LTS system. Due to time constraints, not only had the customer requested a one-shot balance, meaning the solution should work the first time, the customer also specified that a correction weight could be applied in one balancing plane only (of the 10 available).

Balancing

The only relevant historical influence data available was recorded on sister unit TG11 in the 1980s, with a different vibration acquisition system and sensors. We received this data on paper, and it included trial runs for all 10 balancing planes. We used the Cabstar_LTS balancing module to enter this data manually after first correcting for differences in sensor orientations and phase calculation methodology.

Using vibration data recorded during the run-down at the start of the refuelling outage by the permanently installed Cabstar_LTS monitoring system and the historical trial run data, Cabstar_LTS’s powerful rotor balancing module could be used to deliver prognoses for the expected vibration response to the application of various correction weight combinations. The prognoses included expected vibration response at all measurement locations and at all operating conditions of interest (first critical speed, second critical speed, full speed idle, full speed base load). We selected a balancing plane, correction weight, and angle, and placed the weight during the outage.

The vibration data recorded during the run-up, loading, and heat soaking phases at the end of the outage showed that the applied balance solution successfully reduced vibration amplitude to acceptable levels at all measurement locations. Figure 4 shows a Bode plot comparison of bearing three absolute shaft vibration, between the pre-outage run-down (blue) and the post-outage run-up (green). The upper half of the plot shows vibration amplitude versus rotor speed and a significant reduction.

Figure 4. Bearing 3 Vibration, Before (blue) and After (green) Balancing

If the one-shot balance had not been successful, we would have had to deload and run the unit down to apply further correction weights. Each cycle of deload, run-down, weight application, run-up, and loading requires approximately 4–6 hours, an expensive exercise in terms of lost generation revenue when the unit could otherwise be selling 190 MW to the Swiss grid.

The success of this exercise helped us prove the value and effectiveness of the Cabstar_LTS system, and reinforces the case to install the system on the remaining two units.

Author Information:
Stephen Gillon
Elstar Elektronik
Switzerland
Tel: +41564271888
mail@elstar.com

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