Figure 1-22.-Venturi flowmeter

Figure 1-22.-Venturi flowmeter. differential pressure (pressure drop) is sufficient, the microswitch will activate to indicate that the switch has the proper flow through it. You should be sure that the flow switch is defective before overhauling or replacing it. The problem could be a partially closed supply/return valve, obstruction in the coolant line, insufficient coolant pressure, or many other things. By using the coolant system pressure gauges and/or the installation of a permanent or a temporary in-line flowmeter, you should be able to correctly isolate the problem. In the secondary cooling system, a full-flow system flowmeter (figs. 1-6, 1-7, and 1-8) is provided to enable you to monitor the total system flow rate for troubleshooting purposes. There are three types of system flowmeters installed aboard ship. All of them serve the same functional purpose of monitoring coolant flow rate. You will encounter the venturi-type flowmeter, orifice-type flowmeter, and the rotameter flowmeter. Most systems incorporate one secondary coolant flowmeter and one or more smaller flowmeters to ensure that the electronic equipment is being supplied with an adequate flow of coolant. In the venturi-type flowmeter (fig. 1-22), as the coolant approaches the contracted portion (throat) of the meter, its velocity must increase as it flows through the contracted zone. The angle of approach is such that no turbulence is introduced into the stream. A pressure tap is located at the side wall in the pipe ahead of the meter, and another one is located at the throat. The increase in velocity of the coolant water through the throat results in a lower pressure at the throat. The flow rate is proportional to the difference in pressure between the two taps. The gradual tapering of the meter walls back to pipe size downstream of the throat allows the coolant water to slowdown with a minimum of lost energy. This allows a recovery of nearly 99 percent of the pressure on the approach side. To monitor the amount of flow through the venturi-type flowmeter, a differential pressure gauge is used to monitor the pressure difference between the two pressure taps. A calibration chart is usually supplied with the flowmeter to convert the differential pressure to gallons per minute (gpm), or the face of the meter may indicate readings in gpm. The orifice flowmeter works in the same manner as the venturi flowmeter, but its construction is much simpler and less expensive to manufacture. In place of the tapered throat, the orifice flowmeter uses a flat plate with a hole in it, which causes a considerable loss of pressure downstream. The efficiency of this type of flowmeter can be as low as 65 percent. The rotameter (fig. 1-23) is a variable area orifice meter that functions by maintaining a constant differential pressure with varying flow. The rotameter consists of a float positioned inside a tapered, tempered glass tube by the action of the distilled water flowing up Figure 1-23.—Rotameter. 1-18