3.3.3 Effect of pressure loss in pipework and valves
The effect of pressure loss in pipework and valves has been mentioned in previous sections and is now discussed in more detail. Simply, the effect of pressure loss anywhere in the steam path causes a loss in cycle efficiency by reducing the energy available for conversion into work.
Valve gear at the inlet to the HP and IP turbines is used to control the inlet mass flow and hence the load on the machine by a throttling process. Figure 1.41 shows a Mollier diagram comparing the condition lines of a reheat turbine at full load with all the valves wide open and one at part load, where the load has been reduced by 40% by throttling on the inlet valves to the HP turbine. Other pressure losses are neglected. The throttling effect is to reduce the inlet pressure by a constant enthalpy process (represented by horizontal line AB). The result is a loss in entropy and also a slight fall in temperature with some reduction in the available heat drop. This accounts for a small loss in efficiency. However, the major flow reduction which results is the main reason for the reduction of work done in the HP, IP and LP cylinders. In this example, the condenser pressure is assumed to remain constant and the pressure drop across the turbine cylinders is controlled by the Ellipse law relationship described in Section 2.1.2 of this chapter.
Pressure losses along pipe runs can be calculated using equations and charts . Pressure loss is associated with pipe diameter, of surface roughness, steam flow rate and the pipe geometry (number of bends, expansions, contractions). Simply reducing piping losses by redesigning pipe runs, or by adding to the number of pipes, may increase the capital cost of the plant in excess of the benefit gained in running costs.