TABLE OF CONTENTS I am an anticommunist. That was not always so. But working in the former Soviet Socialist Republic of Lithuania altered my views about communism. Figure 32.1, drawn to scale, illustrates the reason for my revised political orientation. Really! That s the way the piping and control valve looked in the field. It happens because of bad design practices:
The mechanical engineer specifies a pump so that it produces excessive head and flow, so that he cannot be accused of industrial sabotage by undersizing the pump.
The piping engineer oversizes process lines to avoid being branded an enemy of the people.
The Soviet instrument engineer must then install a small control valve that will consume all the excess pressure without operating too far in the closed position.
Control valves only work properly within a certain range of positions. When a control valve is mostly closed (perhaps 5 to 10 percent open), opening it a little bit more will increase flow a lot. When a control valve is mostly open (perhaps 80 to 90 percent), opening it a lot more will hardly increase the flow at all. To summarize, a control valve can only respond to a change in position in a linear fashion, between 20 to 80 percent open. To avoid the control valve operating in the nonlinear, mostly closed position, the Soviet instrument engineer installed a small control valve that has a giant ? P at 50 percent open.
