The making of power from steam to drive industrial equipment has been going on for three centuries now.
Over the last few decades we have seen a drop off of steam turbine use because speed control on rotating equipment is now, more often than not, carried out by variable frequency drives (VFDs) which we talked about in the past two articles.
However even with the popularity of VFD’s there has been a resurgence in recent times of steam turbines in power generation, specifically related to combined cycle power plants. This is where, rather than just using a gas turbine to make electrical power, with the flue gases going up the stack, the waste combustion gases from the gas turbine are captured and go through a heat recovery steam generator (HRSG ) to create steam. This extra energy – the live steam – can now be harnessed to drive a steam turbine generator set to make additional electricity. The combined cycle technology yields a much higher overall thermal efficiency.
Today we will talk about some design considerations for steam turbines.
Steam turbines are known in the industry as a highly reliable piece of rotating equipment. And if the steam is clean, the steam turbine could run for four or more years straight without any maintenance required.
There are two main categories of steam turbines. Condensing and back pressure. (We consider an extracting turbine as a hybrid… and we will talk about extracting turbines in a later article).
With condensing we want to take all of the available energy out of the steam. To do this we use a condenser with circulating cooling water. The incoming cooling water condenses the steam and thereby creates a vacuum on the exhaust side. This negative pressure (vacuum) further pulls out the residual enthalpy from the steam for maximum conversion into electricity.
Back pressure turbines on the other hand are used when one has a need within an industrial plant, for example in process heating. Since steam is a good conveyor of heat, it is simple enough to design the boiler to provide a higher pressure and temperature that is required by the process. We can now use this higher temperature and pressure steam to drive a steam turbine and the outlet steam will be at the process steam requirements. This is also known as a Topping Turbine.
The advantage of this is the incremental cost in fuel is almost always more than offset by the added value of the electricity that can be produced. Note that you’re not getting something for nothing. There is energy lost in this process. However the BTUs of natural gas used costs less than the kWs of electricity produced.
If you have an existing boiler that has higher rating capabilities then it becomes a quick pay back project to add a steam turbine since much less capital is required — only for the turbine and alternator. If you have an energy wasting pressure reducing valve (PRV) then again this is a very quick pay-back to retrofit a steam turbine in place of a PRV.
Some people have enough steam to even sell it to the electrical grid, however it is often complicated and involves far more red tape to sell power to the grid. Therefore more companies are opting for “behind the meter” or “behind the fence” power generation. This simply means feeding power internally to offset the plant’s demands usually supplied from the grid.
If an industrial plant requires 3 MW of power and a steam turbine genset can deliver 2/3 of that and still have “a home for” the exhaust steam in the process, then the savings will be substantial.
Due to subsidies of power sometimes the payback is lengthened, or if there is a lot of hydraulic power in the area, such as in Quebec. There are many areas in North, Central and South America where electricity prices are high and these type of strategies have a very good ROI.
So how does one go about selecting the appropriate steam turbine technology for their industrial process? We will cover some of the main design considerations in the second installment.
Buffalo Turbines: Specializing in steam turbines for drive applications and steam turbine generators for power production.
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