Switching from Steam Turbines to VFD drives on Boiler Draft Fans

[mjpetrag]

Wanted to get anyone's thoughts and experiences on switching from steam turbine driven boiler draft fans to VFD driven fans. We are looking at significant capital spend rehabing our existing boiler draft fan steam turbines, which take steam off the desuperheaters of the boilers and let down across the turbines to drive the fans. We use all the steam that is let down from these turbines, so we will also need to supplement our existing usage with a steam let down/desuperheating station if we decide to change out all these to VFDs. A loss of plant electrical power would result in a total steam shutdown regardless of using steam driven or VFD driven equipment in our plant, just the way we are set up. The maintenance costs of overhauling the turbines is much higher than VFD's, and we are looking to standardize all the VFD's to be the same so a spare can be kept on the shelf in the event of failure. Turbine overhauls also take considerably longer than a VFD replacement, especially if spare equipment for the VFD is available on the shelf. Starting up from a dead stop is challenging, as we have old eddy current drive equipment to restart a boiler until enough steam has been generated to restart the turbines, and these drives are very unreliable. The operating costs of the VFDs are obviously higher, but for the reasons above we are leaning toward changing to all VFDs.

I wanted to make sure we aren't missing something with this potential change.

-Mike

 

[EdStainless]

In most plants that HP for these pumps is so large that doing it electrical is a major job.
Big motors, very big wiring, and large VFDs.
At least with steam driven there is some spool down when the plant has emergency shutdown, not just dead off.
If your EC drives are questionable I have to ask when was the last time that they were worked on?
I have seen EC drives that are 40 years old still working perfectly.
These are very robust and should not be an issue.
If you need the steam that is now coming from the pump turbines could you take blead steam from some other place in the system?
In medium and small plants the electrical conversion is fairly common.
Just make sure that you have enough backup power to run these pumps also.

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P.E. Metallurgy, consulting work welcomed

 

[mjpetrag]

These are just for the FD and ID fans, about 150 hp each. The EC drives function and are PM’ed yearly but are very old with antiquated controls and parts availability are of concern so there would be capital investment needed to upgrade these drives also. The idea is to switch to VFD and eliminate the steam turbines/EC drives altogether as capital avoidance

I should mention this is not a power plant, but a chemical plant where the boilers purpose is to provide process steam.

The LP steam coming from the turbines is almost completely consumed by the process and the HP to LP makeup control valve is routinely at 80-90% open at full rates, so a reduction in the LP steam generation from removal of the turbines would require another let down to meet plant demand.


-Mike