Graham Corporation Technical Article: Turbine Exhaust Surface Condenser Impacts on Turbine Operating Costs

Turbine Exhaust Surface Condenser Impacts on Turbine Operating Costs

July 2022

Greetings!

Does your plant have steam turbine driven equipment? This article is going to discuss turbine operation and the associated costs of operating the turbine, given varying steam flows. It’s important to optimize the turbine’s surface condenser and air removal system to insure the turbine is operating efficiently. Before getting into the calculations, keep in mind the transfer of energy is done through a phase change with the flow going around in a Rankine cycle. For additional information on a thermodynamic Rankine cycle for this application, please refer to one of Graham’s articles Ejectors Eject Energy Hogs. This article along with one of our other articles Condensing Costs includes additional examples as well.

The best way to show the impact on operating costs is through an example:

A plant operates a compressor that requires 20 MW of energy. Superheated steam is supplied to the turbine at 850 psig and 850˚F. The turbine efficiency is 75%. The condenser was originally designed to operate at 2.5” HgA is now operating at 6.0” HgA due to an unknown issue. At the turbine inlet conditions, a Mollier chart can be used to determine steam properties and find the associated enthalpy and entropy.

Inlet enthalpy (h-inlet) is 1425 Btu/lb

Inlet entropy (s-inlet) is 1.6101 Btu/lbm degrees R

Assuming ideal isentropic operation, ‘s’ should be kept constant to determine the enthalpy at the turbine’s design discharge pressure of 2.5” HgA: h-disch = 910 Btu/lbm.

Delta h = 1425-910 = 515 Btu/lbm.

The theoretical steam rate is now calculated based on delta h, remembering that this is the steam rate at 100%

turbine efficiency.

The theoretical steam rate = (3412 Btu/kW-hr) / (515 Btu/lbm) = 6.63 lb/kW-hr.

Next the turbine efficiency, representing the difference between entropy ideal and actual on the T-s diagram, is

corrected for. At 75% efficiency, the actual steam rate = 6.63/0.75 = 8.84 lb/kW-hr.

Thus for 20 MW output, with the condenser at 2.5” HgA, the turbine will require:

20 MW x 8.84 lb/kW-hr x 1000 kW/MW = 176,800 lbs/hr of steam

Now consider the effect with the condenser operating at 6” HgA. The same turbine efficiency of 75% will be used. It should be remembered that the actual turbine efficiency can be somewhat less at off-design conditions, making the calculation conservative. To evaluate cases where the turbine efficiency is not known, approximations can be made using 75% efficiency for units that are 20 MW and larger, and 65% for smaller turbines.

With ‘s’ constant at 1.6101 Btu/lbm degrees R and a pressure of 6” HgA:

h-disch = 956 Btu/lbm.

Delta h = 1425 - 956 = 469 Btu/lbm

This results in a theoretical steam rate of 7.28 lb/kW-hr and an actual steam rate of 9.71 lb/kW-hr. Thus, for the same 20 MW output, the turbine will require 194,200 lbs/hr steam, 9.8% more than at 2.5” HgA.

To put this in dollar amounts, we can make some assumptions. The actual cost of steam will depend on plant location, fuel type, environmental regulations, and other site-specific factors. The calculation is based on a cost of USD$4.00/1000 lbs of steam and should be modified as needed. Comparing operation at 6” HgA instead of operation at 2.5” HgA yields a difference of 17,400 lbs/hr steam.

Operating cost difference per year: 17,400 lbs/hr x US$4.00/1000 lbs x 8760 hrs/yr = US$610,000/yr.

In this example, improving the performance of the turbine exhaust surface condenser will save significant steam operating costs. One way to evaluate the system and how it is operating is to perform a pressure and temperature survey on the condenser and venting equipment to troubleshoot the system. Refer back to one of our previous articles on troubleshooting The Troubleshooting Process. Graham’s service department is available to provide troubleshooting assistance by phone, email, or web conference as well as offering service contracts for site troubleshooting.