PRINCIPLES OF NAVAL ENGINEERING 



Most area meters are classified as (1) ro- 

 tameters, or (2) piston-type meters, depending 

 upon the type of device used to vary the area of 

 the orifice. 



Figure 7-31 shows a simple rotameter of a 

 type used in some shipboard distilling plants. 

 A tapered glass tube is installed vertically, 

 with the smaller end at the bottom. Water flows 

 in at the bottom, upward through the tube, and 

 out at the top. A rod, supported by the end fit- 

 tings of the tube, is centered in the tube. A 

 movable rotor rides freely on the rod and is 

 positioned by the fluid. Variations in the pres- 

 sure of the fluid lead to variations in the posi- 

 tion of the rotor; and, since the glass tube is 

 tapered, the size of the annular orifice between 

 the rotor and the tube is different at each posi- 

 tion of the rotor. An increase in flow is thus 



FLOW SCHEMATIC 



75.290 

 Figure 7-31.— Area meter (rotameter type) for 

 measuring fluid flow. 



indicated by the rotor rising on the rod. The 

 glass tube is so calibrated that the flow may be 

 read directly, with the reading being taken at 

 the top of the rotor. 



In a piston-type area meter, a piston is 

 lifted by fluid pressure. As the piston is lifted, 

 it uncovers a port through which the fluid flows. 

 The port area uncovered by the lifting of the 

 piston is directly proportional to the rate of 

 fluid flow. Therefore, the position of the piston 

 provides a direct indication of the rate of flow. 

 The means by which the position of the piston is 

 transmitted to an indicating dial varies accord- 

 ing to the design of the particular meter and 

 the service for which it is intended. 



MEASUREMENT OF LIQUID LEVEL 



In the engineering plant aboard ship, it is 

 frequently necessary for operating personnel to 

 know the level of various liquids in various 

 locations. The level of the water in the ship's 

 boilers is a prime example of a liquid level that 

 must be known at all times, but there are other 

 liquid levels that are also important— the level 

 of fuel oil in service and stowage tanks, the level 

 of water in deaerating feed tanks, the level of 

 lubricating oil in the oil sumps of main and 

 auxiliary machinery, and drains in various drain 

 tanks, to name but a few. 



A wide variety of devices, some of them 

 simple and some complex, are available for 

 measuring liquid level. Some measure liquid 

 level quite directly by measuring the height of a 

 column of liquid. Others measure pressure, 

 volume, or some other property of the liquid 

 from which we may then infer liquid level. 



The gage glass is one of the simplest kinds 

 of liquid level measuring devices and one that is 

 very commonly used. Gage glasses are used on 

 boilers, on deaerating feed tanks, on inspection 

 tanks, and on other shipboard machinery. Ba- 

 sically, a gage glass is just one leg of a U-tube, 

 with the other leg being the tank, drum, or other 

 vessel in which the liquid level is to be meas- 

 ured. The liquid level in the gage glass is thus 

 the same as the liquid level in the tank or drum, 

 and the reading can be made by direct visual 

 observation. Gage glasses vary in details of 

 construction, depending upon the pressure, tem- 

 perature, and other service conditions they must 

 withstand. 



The measurement of liquid level in tanks 

 aboard ship may be accomplished by simple 

 devices such as direct-reading sounding rules 



146 



