March 14, 2018

Design considerations for steam turbines – Part 2

Generally, we start off by considering capital costs as they are a key determinant in the payback. Currently there are many companies that are willing to provide power purchase agreements (PPA). If companies have available waste steam, the PPA company will bring financial resources to bear if an industrial plant has a good payback scenario but is cash limited.

Another common design consideration is whether to select a single 100% capacity turbine or instead two 50% capacity turbines. This is a common question and something we will explore here.

Efficiency is always important because any designer or manager will want to maximize the power output using the available steam. Generally given the choice between one or two turbines the efficiency will be slightly higher with one dedicated turbine. However there are other considerations that we need to look at.

For instance, we would want to consider reliability and redundancy. When viewing from the perspective of reliability and redundancy the probability of a single turbine failure means that you will still make 1/2 the power. Or when maintenance is required again only 1/2 the power is lost during the outage.

Also most industrial processes have wide swings throughout.  So we have to look at the operating range that the turbine must encompass. There will be a peak load which is often in the summer when building space heating requirements are at the minimum. And within the different seasons there can be variation in flows between weekends and weekdays. With a turbine, like any turbomachine, there is one sweet spot of flow and pressure and any other point will be away from that sweet spot.

If one has four different operating points depending upon season and day of the week it is impossible to maximize all of these points.

However a choice must be made depending upon the dominant rating point – which is a function of the number of hours and the output.

This is where the turbines can really improve the operating scenario. The two turbines are designed to hit the highest rated point and one turbine can be shut down for the lower rating point.  By switching on and off one turbine we can more closely match up to maximum efficiency for all points.

In terms of capital generally capex will almost always be more for a two turbine scenario versus a one turbine scenario. However if the power required for instance necessitates a jump to the next model size, one may find that in some cases it is in fact no more expensive to use two turbines.

Another consideration for the use of one versus two turbines is turbine footprint and available real estate.

Depending upon the overall configuration two turbines may fit the available space better than one. For instance if the available space is long and narrow then two turbines would fit much better than if it is a square. If it is a square then one turbine would fit better.

For these and other questions when you want to turn steam into electricity, call your friendly neighborhood steam turbine guys.

In Buffalo Turbines we Specializing in steam turbines for drive applications and steam turbine generators for power production.

For a quote or for expert help regarding your power plant operations, please email today.

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