Steam Turbine Control 1

[brainstorming]

I got a question with respect to the control of a condensing/extracting steam turbine (about 65MW) connected to a process compressor and driven by VHP steam (510 C and 105 barg).
From my little experience in the control scheme of such application, the usual design I have seen is that the governor system is controlling the various need of the process by the compressor 1st stage suction pressure by controlling the turbine speed via the throttling of the steam admitted to the turbine.
I have got a scheme for a new plant design for the same application and applying different control:
Governor system is controlling the various need of the process by the compressor 1st stage suction pressure by controlling the turbine speed via the throttling of the steam exhausted via pass out.

I have doubt if this control in terms operability and economically. The later is not of concern as the former.

Cheers

 

[TD2K]

It's not possible to give you a definitive answer without a better understanding of your system so bear with me on a more general discussion level.

The 'traditional' way of controlling a turbine that you described is one that I've also seen. If you think through it, it easily handles process changes. Extraction steam flow, in the ones I've seen, are typically controlled to supply steam to some pressure level maintaining steam pressure or some other process variable. The balance of the steam flowing into the turbine (through the inlet governor steam valve) then flows through to the surface condenser. The amount of steam flowing to both the extraction and condenser sets how much power the turbine produces. The total inlet steam is being set, in your case, by the compressor suction pressure or some other process variable.

As the compressor Hp requiements change, the suction pressure will rise or fall causing the steam flow to the turbine to change to maintain setpoint.

If the extraction flow changes, that means more or less steam will go to the surface condenser affecting the total Hp produced and therefore affecting compresssor suction pressure which then adjusts the turbine inlet flow to bring the suction pressure back to setpoint.

The point is that the control system handles changes to the process that you can 'what if'.

With your proposed system, you are going to control the extraction flow to maintain the compressor suction pressure. How are you going to control the total steam flow to the turbine? What is the effect on the process downstream of the extraction point if you change that to maintain compressor suction pressure? How much flexibility do you have on the extraction steam flow, how much effect does this have on total compressor Hp and does this give you enough response for the expected operating range for your process?

To develop a control system, you need to understand how your system works and responds to various changes to ensure your proposed system will work. I'm assuming there's a problem with the traditional control setup for your system where you need to change it to another mode of control.

 

[sshep]

Like both of you I have only seen the conventional scheme where: suction press is controlled by steam to turbine (turbine speed); and extraction steam make controlled by extraction steam valve (usually to maintain lower pressure steam header pressure).

You new scheme seems to be missing a handle because if extraction steam make is used to control the process, how will the total steam to the turbine be controlled? I must assume that the total steam is fixed or otherwise related to the extraction steam in some manner, but this is not described. This additional control requirement may be internal to the turbine.

If the extraction steam can be used for useful heat, then maximizing extraction steam is the most economical means of driving the compressor (assumes condenser discharges waste heat to cold utility). Usually however there is a limit to how much exhaust steam can be soaked up by the process heat requirements. The typical optimization control (in my experience) is therefore to have extraction steam control the lower pressure (LP) steam header pressure. My fear with your system is that the extraction steam will either be too low in which case a supplemental letdown may be required to keep the LP steam header pressure, or too high in which case LP steam must be vented. Unless there are other LP steam generators that can take the swing, your proposed control system seems like a bad idea. Even if there are other generators to control LP steam pressure, the scheme seems likely to introduce additional disturbances to that system.

You are definitely right to be asking these type pointed questions to the proposer of this new scheme.

best wishes, sshep