In-situ balance of an Open Cycle Gas Turbine

A vibration problem arose at an Open Cycle Gas Turbine. Following an outage on the four gas turbines and two SSS[1] clutches, the machine returned to service with unacceptable vibration levels on some generation runs resulting in a significant loss of commercial operation.

The OCGT consists of two sets of two Rolls Royce ‘Avon MK 1533’ gas generators (A & B side) with each side feeding into its associated Power Turbine. The Power Turbines are connected via a clutch arrangement to the exciter on the ‘B’ side and to the Generator on the ‘A’ side.

The unit can be operated in ‘Generation Mode’ where either ‘A’ and/or ‘B’ sides are utilised to generate and export MW’s (full load ~ 55MW). Because of the station’s remote location it is often requested to run in ‘Synchronous Compensation Mode’ where the unit is run to speed by either of the gas generator sets to rated speed and synchronised. The gas generator set is then shutdown in order to perform MVAR[2] compensation for the transmission grid.

The complexity of this arrangement becomes evident when we consider that the overall imbalance forces through-out the system change dependent on the clutch engagement angles for both clutches. It was found that at certain engagement angles the resultant forces were significant enough to make the vibration levels unacceptable on the generator and exciter bearings.

In order to analyse these conditions, temporary velocity transducers were fitted and the signals analysed and recorded using a Protor Mobile vibration monitoring system which is capable of measuring up to 16 channels of dynamic data with a bandwidth of 1 kHz per channel. Data is captured and analysed relative to a once-per revolution phase marker to allow extraction of the amplitude and phases of the harmonics of the shaft speed.

To assess the resultant balance states the generator is held at 3000 rpm under synchronous compensation mode and each clutch disengaged and re-engaged a number of times until vibration data at a full spread of engagement angles was collected.

From this spread of engagement angle data, the first order amplitude and phases readings were displayed. Note that at some engagement angles the measured 1st order vibration exceeds 9 mm/s RMS.

Figure 1: Pre-Balance Vibration Data

After careful analysis an in-situ balance was performed which resulted in a state where the vibration data for all clutch angles was acceptable (all below 6 mm/s RMS).

Figure 2: Post-balance vibration data




[1] SSS – ‘Synchro-Self-Shifting’ clutch. A clutch mechanism where the clutch teeth are phased and then automatically shifted axially into engagement when rotating at the same speed. The clutch disengages as soon as the input speed slows down relative to the output speed. The video below (or at shows an SSS clutch in action

[2] MVAR – Mega Volt-Amperes Reactive. VAR is the unit for reactive power. Power in an AC circuit is a complex quantity consisting of the real and imaginary components; Real Power and Reactive Power. Real power is the energy which can be used, Reactive power is energy temporarily stored in inductive and capacitive elements on the network. Reactive power flow strongly influences the voltage levels across the network and must be carefully controlled within a transmission system.




The PROTOR Vibration Condition Monitoring System provides reliable on-line condition monitoring of Turbine Generator, Main Boiler Feed Pumps and other auxiliary plant equipment. Without timely health information, impending faults in turbines, pumps and associated equipment go unnoticed and develop into a condition that results in failures. Failures may lead to unscheduled outages or more serious equipment damage with consequent loss of revenues. Find out more at

About Vibration Diagnostics

From over 25 years of experience within the commercial power industry Vibration Diagnostics are acutely aware of the importance of being an informed operator, able to make effective investment and operational decisions affecting commercial plant performance. Recognising this, we have created a range of Diagnostic Services providing information from Turbine Generator rotating plant data analysis, essential in enabling operators to improve commercial, engineering and environmental plant performance. Find out more at


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Chris, of Vibration Diagnostics Ltd, worked for RWEnpower and its predecessors for over 25 years specialising in large steam and gas turbine vibration and rotor dynamics. Within this role he was responsible for providing an expert engineering consultancy and trouble shooting service for turbine generators and auxiliary plant, to ensure the highest levels of flexibility, availability and efficiency. This included rotor dynamics and vibration, shaft alignments, bearing performance, rotor repair requirements, modal analysis and in-situ balancing, on over 20 GW of plant.

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Terry Nichols
Terry Nichols
8 years ago

Very interesting. I wonder why the problem occurred after an outage? Perhaps I should have expected it. Those SS clutches are generally a pain in the proverbial. Where were the balance weights fitted?

Christopher White
Christopher White
8 years ago

I believe there are 3 potential issues, imbalance of the generator and exciter, imbalance of the two halves of the SSS clutch and mis-alignment of the SSS clutch. The biggest component being mis-alignment within the clutch. The mass additions are made to the exciter coupling as there is no balance plane on the SSS clutch itself.

Mal Stein
Mal Stein
6 years ago

Chris Many years ago we produced a relative phase device to measure the relative phase of the rotors either side of the clutch. This ensured a constant position within the engagement and improved the repeatability and accauracy of balancing attempts. Do you measure this ?

Malcolm Stein

Christopher White
Christopher White
6 years ago

As a temporary measure we measured the phase angle so that we got a good spread of engagement angles for me to work out the best balance position for all conditions. I do not think the stations that have SSS clutches measure angle as a routine though but i do know of one large Siemens CCGT station that does as it has so many vibration issues.

Neil Brinkworth
Neil Brinkworth
6 years ago

Chris I have also worked on a similar machine in Ireland. To aid with the balancing and optimisation I also fitted proximity probes to measure the eccentricity at both ends of the generator bearings. There are lots of different operating modes for this type of machine A end drive, B end drive, A and B end start-up together, generating and synchronous compensation. I also had problems with the balancing as the generator due to thermal bend on-load.

Would love your thoughts, please comment.x