Steam Turbine Shutdown Procedure

[Rreach]

Hi,

This is regarding normal shutdown of a steam turbine.

For one of our steam turbines, for a normal shutdown, once it reaches minimum governor speed, the logic will close the governor valve first to 500 rpm before sending a signal to close the steam inlet valve. And according to the engineer on site, he said the turbine will slowly be ramped down in a controlled manner and this is industrial practice. However, the block diagram from the IOM indicate to close both the governor and steam inlet valve concurrently during a normal shutdown sequence.

I could not understand what the rationale is for closing the governor valve first and ramping down in a controlled manner.
Does anyone have any experience on this?
What is the industrial practice on this?

Thanks

 

[TugboatEng]

For steam in general, it is wise to close the downstream valve first and let it take the wire-drawing as the flow is pinched off. This way the upstream valve is more reliably available for isolation.

One could argue that it's best to shut the turbine down rapidly to prevent wire drawing of valve seats.

 

[georgeverghese]

In general, shutdown and startup operations on any rotating machine place heavy loads on the shaft bearings. And dynamic loads on these bearings are also higher when running at higher speeds. So it is good practice to slow the machine down, (and bring it down to low loading from a process operating load perspective also if applicable) as much as possible before shutting it down so bearing loads are minimised.

The block diagram may not show some details such as this.