Steam Turbine Operating Cost Calculation 1

[bk19702]

I'm working on some cost justifications and looking at the existing operating costs of some seal oil pumps. One pump is motor driven, one turbine driven. What do I need to calculate the operating cost of the turbine?

What I have:

Turbine HP
Inlet / exhaust conditions
Cost of steam ($/1000lb)
Steam Flow

 

[dcasto]

The simple answer is Cost $/1000lb times flow lb/hr = $/hr. What you do not have is info about the turbine and can the steam out be used or is used. In any case, the turbine should have a credit coming back from the equation above from the steam oulet.

A typical turbine with 600 psi @ 700 F inlet, exhausing into a 40 psi header will have superheated steam at about 440 F (based on 45% isentropic effiency). The enthalpy goes from 274 btu/lb to 179btu/lb (net 95btu/lb). If the exit steam is used as a heat source and condensed and cooled to 250F, an additional 1035 btu/lb is removed. So in reality, the turbine only used 95/1130 or 8.4% of the steams energy which means, only 8.5% of the value in the first paragraph.

Now, if you have a condensing steam turbine, you'll need to go through the same calculations and you can get 30% or more of the energy is used.

 

[stgrme]

As decasto said, keep it simple. If you know the cost of steam generation in $/1000lb, the steam consumption of the turbine in 1000lb/hr and the number of operating hours, the cost is simply the product of these three numbers. This assumes that the exhaust steam from the turbine is not used for other purposes.

As for decasto's example, the enthalpy of 615 psia (600 psig), 700 deg-F steam is approximately 1350 Btu/lb. Assuming the same efficiency of 45% and 55 psia (40 psig) exhaust pressure, the exhaust enthalpy is about 1249 Btu/lb.

 

[rmw]

We would need to know more about your system in order to give a real answer. For example, if you are condensing, then that to me is expensive steam. But, if you would have to let down the same amount of steam from a high pressure to a lower pressure for some purpose via a let down station, then the work to drive the pump is essentially free. The steam costs would be charged against what ever its use was at the reduced pressure.

Give us some more information.

rmw