Dear Sirs,

We are using bently nevada 3500 system for our turbine supervisory system. Now the problem is the analog output we are taken from Bently Nevada to our DCS (yokogawa) is showing wrong value for a second oftenly. For example, While turbine running we are getting 3000 rpm but for a second the speed output suddenly fluctuate to 2600rpm and resume to earlier point i.e 3000rpm. we have another speed output from the Woodward governor 505E, that's working properly doesn't have any problem.

Please explain what we have to do to clear the problem, because we have critical interlock with this speed like JOP Auto start at 1200 rpm)

for your information i have already checked cable shield that was found ok and i have changed the DCS channel also but still the problem persist.

Please kindly give us a solution.

 

Usually BN system detect speed via the key phasor probe which is only 1 notch per revolution. BN do/did have dedicated speed cards but they are not common. One notch per revolution gives a course speed indication. I would check the probe gap and alignment and the accumulation of rubbish on the face of the probe.

The 505E would be looking at a multi tooth gear and hence give a high resolution.

Sounds like an application problem. Why don't you use the 505E analog output. 505E can also be installed with redundant speed sensor inputs. The 505E also has related outputs which can be configured as speed switches.

 

Dear 309Eguy,

Thanks for your post,In our bently System we have a keyphasor module and tachometer module. I have checked the configuration the speed output is configured from tachometer monitor only. and about the probe gap and alignment i have ensured that in last shutdown also. everything is perfect but still the problem persist. we have supplied the same BENTLY system for all our projects But this is the first time we are facing this kind of problem.

 

Looking at your query and Woodward Tachometer along with Yokogawa DCS it suggest me that you may be having Mitsui Gas Turbine. I have faced this problem some 20YRS back with Bently 7200 system along with Woodward and YOKOGAWA DCS. We discovered it to be inappropriate heat dissipation of Tachometer monitor and corrected. I am eager to know if it happens to 3500 based system how you fixed it.