Why do we need to provide HP TOP BOTTOM Casing Temp Protection for Steam turbine?

And why don't we provide the above protection for IP & LP Turbines casings?

 

I suppose each turbine vendor takes a different approach and I have by no means seen them all, nor am I an expert on the subject, but on our 200 Mw Alstom STs there are single TCs on the tops of the HP and IP turbines that are used in conjunction with the calculated rotor temps for the calculation of casing stresses.

Our 200 Mw Siemens uses quite a few more TCs on the HP and IP casings for the casing stress calcs. (I'm in different plant now so I don't recall exact quantity and position).

Prior to direct digital control, operators would stay within casing stress limits during turbine warm up using charts and graphs. Nowadays the calcs are done in the DCS, and in the Alstom situation, the stress is displayed in percent with 80% being the upper limit.

Casing stress is not a trivial thing, as it has a direct bearing on the machine's service life and if limits are exceeded, can cause rotor rubs, vibration trips, and in one case uneven heating during startup inherent to the initial design resulted in siezure of the HP rotor on our Siemens ST. That outage lasted several weeks.

There are no casing temp sensors on any of the LP casings I mentioned. I assume its just not an issue. Another important indicator of machine integrity would be differential expansion probes that motor relative growth between the rotor and casings. Exceeding those limits can cause rubs as well. Then there's casing absolute expansion, which is useful for telling if the casings are expanding and contracting evenly. If they're not, it will probably cause problems.