Power limiting


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I havw a steam turbine driven compressor. There is one power limiting controller which receives input from exhaust pressure transmitters & chest pressure transmitters. Also power limiting controller is giving output to the Woodward speed controller.

Please guide me on what is the need of "power limiting" in steam turbine driven compressor & how it is done.
Let us start from a matter of principle. No prime mover is designed to operate to infinite power output. All prime movers have an upper power output limit to protect their internals from undue damage. Your steam turbine driven compressor is no exception.

In the case of your steam turbine, like any other steam turbine for all that matters, the shaft power output of the steam turbine is proportional to the steam flow through the steam turbine, the steam temperature and the steam pressure differences across the steam turbine. Assuming that the steam temperature from boiler is constant (which seems to be the case in your steam turbine) then the steam flow is proportinal to the difference in steam pressure across the turbine, or to put it conversly, the steam pressure difference across the steam turbine will be directly proportional to the steam flow in the turbine.

When the steam turbine is running at its maximum allowable continuous rating, you will have the design steam temperatures, design steam flow and design pressure difference. Thus by measuring the pressure difference, one can deduce the steam flow and thus the power output.

If you try to get more output from your steam turbine, you can only do this by increasing the inlet steam pressure and this in turn increases the steam flow.

At the end of the day it is the woodward governor that is controlling the steam flow to the steam turbine through the steam admission valves. Your load limiter is sending a signal to the governor, such that if it detects too high a steam pressure difference it will lower the setpoint of the governor. This in turn will send a signal to the steam admission valves to close and thus lower power output.
Thanx for the help. I would appreciate if you can clear few more queries on the subject. Does the power limiting controller receive any feedback from the governer like speed feedback? Do we always calculate the power generated from the pressure differential across the turbine or we also take into consideration turbine speed? Can you please send me the equation used to calculate the equilant power from the steam pressure?
It is not normal that the power limiting controller receives speed feedback, although it may be used to derive a better indication of the shaft power output. There are several ways of measuring the power output of a steam turbine, but it must be clearly stated that ALL depend on the design of the steam turbine.

As I stated in my previous posting, when one is using the differential pressure to measure he/she is assuming that a number of other parameters are constant or vary in direct proportion to the steam pressures and so can be derived. E.g. if the steam turbine is a fully condensing turbine (i.e. all steam exhausted from the turbine is returned to the liquid state in a condensor) then the exhaust pressure and the exhaust temperature can be assumed to be directly propostional. In your setup, it is also assumed that the inlet steam temperature is constant, as turbine shaft power output is also proportional to steam inlet temperature.

So summing up, your setup is a relatively simple approach to your load limiting requirements, which I believe works fine for its intentions (as long as the steam turbine is operated with all other parameters as per design). If you need the equations, you have to get hold of the complete design of your steam turbine.