Running Steam Turbines Above Rated Speeds

Are you on a grid or island?

Problems

- Minimum - unbalance, vibration

- maximum disntegration

On grid:

Minimum : Trip

maximum: Black-out.

- Why do you want an equipment to run at that range ? (or is it a 50Hz alternator to be used for 60Hz?) - don't even try. There are so many factors involved and that includes system natural frequencies etc these are all calculated and balanced based on the operating RPM.

And you should not neglect added centrifugal force on the blades, blade profiles, crossetion (strength), tip speeds etc

Both guests have assumed that the steam turbine is driving a generator, this is not stated in the OP and I can see no reason why one would wish to run such a machine at a higher speed than design, however based on their assumptions their comments are valid, especially guest two's comment "- maximum disintegration".

However, what the turbine is driving is irrelevant, one may not run a steam turbine above its maximum continuous rating without getting the express agreement of the OEM. You do not have enough design information to calculate all the stresses involved at higher speeds than design, only the OEM can do this for you and must supply the new rating in writing.

In most countries you will end up in jail if the turbine disintegrates and kills someone if you have knowingly been running it above design speed (and or any other design limit).

I suspect that this is a mechanical drive turbine (not power gen), and I assume that the OP is asking about running the machine continuously at a speed above max continuous, but below trip speed.

The answer is don't do it. The machine is designed to handle short duration excursions to trip speed, but max continuous means just what it say that is the maximum speed the turbine is designed to run at on a continuous basis. Running above that will induce forces, moments, resonance excitation, and thermal conditions that the machine was not designed for. The likely result is fatigue failure, blade breakage and machine destruction.

It may actually be possible to do, but and lot of serious engineering evaluation and consultation with the OEM would be necessary first.

Thankx for sharing yes i do mean that operating mechanical drive turbine above its maximum continuous speed but below its critical speed. Is there any safe range of operation above maximum continuous speed?

No! Read both my and StevS's posts again. You do not have enough design information to make a call on whether you can exceed the maximum continuous rating specified by the OEM.

Do not do it without the express written approval of the OEM to change the MCR.

Back in my Navy days we never exceeded 110%. Once you reach and pass ideal blade speed there isn't much point as the turbine won't make any more power and you run the risk of the thing flying apart due to the increased forces involved!

"According to API trip limit is at 15% above the rated speed. Any one with practical experience of running turbines more than that limit"

Yes, I've practical, bitter experience.

On a PM check, for turbine trip speed, it did not trip!!! In a fraction of second, the turbine cover ripped off and broken parts of rotor blown out. This accident disabled on of our men permanently.

I know that you are not intended to run above 15% limit. But when the safety margin is lesser, the accident is nor farther.

In addition one needs o know if operating at that speed will bring the turbines blade tip speed to super sonic! Strange things happen when you move stuff that fast!!!

This reminds me of a situation about thirty years back where a customer with a huge design force and a penchant for "CYA" had purchased new turbines from an OEM, the driven element had been designed improperly and could not attain rated HP without running at 105% rated speed.

The customer went to the OEM and posed the question "Can we run this machine at 105% speed of rated, sustained speed?" I am glad they didn't ask me because that would have seemed like the ultimate "No Brainer". At any rate the OEM reviewed their calculations and advised the customer that it shouldn't be a problem. The customer probably wouldn't have asked if there were not warranty considerations.

As it turned out both the customer and the OEM missed the fact that at 105% rated, the last stage blades were operating in a blade critical speed. There were a large number of units involved and the last stage blades started to fail in a large number of the units. This of course led to the quandary of who the hell was really at fault here.

Both the customer and the OEM had deep pockets, they also both had little short arms so there was quite a battle over responsibility.

At any rate any OEM or adviser of whatever sort would be reluctant to tell you to run a turbine outside their original design limits without a huge engineering study. Why not just upgrade?

Well running the piece of machinery above the rated RPM can lead to un-predicted vibrations .The Design is based on the Factory of Safety of machinery of say 1.5 times the rated.So this means that the machinery can run at higher rpms but for limited time..It can induced unwanted vibrations causing failure of rotors,seals.etc..

Secondly in API if read 15 % doesnt mean u can run throughout at continous at 15 % rated.This can be down for short durations and based on OEMs design factors..

So u have to be very careful that u dont run the machine for long durations of time...

Well running the piece of machinery above the rated RPM can lead to un-predicted vibrations .The Design is based on the Factory of Safety of machinery of say 1.5 times the rated.So this means that the machinery can run at higher rpms but for limited time..It can induced unwanted vibrations causing failure of rotors,seals.etc..

Secondly in API if read 15 % doesnt mean u can run throughout at continous at 15 % rated.This can be down for short durations and based on OEMs design factors..

So u have to be very careful that u dont run the machine for long durations of time...