Next, we will develop a Logic Diagram for our example project. Please refer to
Figure 2-21 in Chapter 2, P&IDs. There are three diamonds with an internal 1
on the P&ID. One is on the output of LSL-301, the low-level switch on 01-D-
001. The second is on the start-stop scheme (HS/HL-401 and 402) of 01-G-
005, the condensate pump. The third is on HV-400, the hand operated on-off
valve. The diamond is on the output of ZSH-400, the position switch high
which signifies that HV-400 is fully open.