Abnormal vibration of COPT turbines on board VLCC.
Info Marine’s Service Engineer performed an annual vibration survey for one of our customers on board a VLCC. During measurements, there was noticed abnormal high vibration level of COPT turbines. The turbine speed was 6660 RPM (111 Hz). As you can see below, the dominant peak was at 1x turbine’s rotor rotation frequency:
The results indicated a problem with unbalance. Nonetheless, considering the cost and difficulty of inspection for such equipment, we (in agreement with the Superintendent) decided to perform additional vibration measurements. Since we found the root cause of excessive vibration, we wanted to check the velocity FFT spectrum depending on various turbine speed. The idea was to find the best working conditions for those turbines and above all to avoid failure until the planned dry dock.
Firstly, we measured the turbines at a speed of 4560 RPM. The analysis showed that those parameters are safe for the turbines. Secondly, to determine the maximum allowed speed and achieve the largest capacity possible – we have done additional tests. After analysis of collected data, we have set the critical value of speed equal to 5460 RPM (class C according to ISO standards). We highly recommend to do not exceed this value.
The superintendent has decided to implement a monitoring system to control the influence of excessive vibration on the machine. Meanwhile, he has brought the guaranty claim to the turbines’ manufacturer. After an investigation carried out by the producer, the diagnosis was the same as ours – unbalance of the main rotor. Moreover, after receiving the opinion Superintendent decided to balance the rotors – which nota bene confirmed the diagnosis. He also considers the implementation of a constant monitoring system. Please find the comparison graph:
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