262 HYDRAULIC EQUIPMENT 



a large cylinder containing a balancing piston, the purpose of 

 which is to equalize the load on the valve, allowing, however, 

 a small residual force tending to close the relief valve. Above 

 the balancing cylinder is a smaller cylinder containing a piston 

 for operating the valve. The two pipe connections shown at the 

 ends of this small cylinder are joined by pipes to the two ends 

 of a jack cylinder mounted on the tailrod on one of the operating 

 cylinders of the turbine. The jack cylinder and the operating 

 cylinder of the relief valve displace equal volumes when their 

 respective pistons move through the full stroke. The relief valve 

 is thus forced to move by an incompressible fluid column, and the 

 operation is similar to that which would be obtained by a direct 

 mechanical connection between the turbine gates and the relief 

 valve. 



The slow-closing feature of the valve operation is obtained by 

 by-pass connections joining the two ends of the operating cylinder. 

 A needle valve permits the rate of closing to be adjusted. The 

 method of operating this relief valve has several advantages. 

 One of these is the positive action obtained, the effect of which is 

 to prevent the turbine gates moving at a rapid rate, if for any 

 reason the relief valve should fail to move owing to any accidental 

 cause, such as lodging of obstructions in the relief valve. Thus, if 

 the relief valve is unable to open, the turbine gates will be auto- 

 matically prevented from closing, except at a slow rate which will 

 not endanger the penstock. 



For relief valves used with impulse wheels see section on " Tur- 

 bines," page 221. 



4. WATER-FLOW METERS 



One of the most convenient means of measuring the amount of 

 water taken by a hydraulic station and for ascertaining the 

 efficiency of the turbines is the Venturi meter. 



Venturi Meter. It consists of a meter tube, which is inserted 

 in the pipe line similar to a section of pipe, and of a register which 

 is piped to the tube and which can be located at any convenient 

 place in the station, as shown in Fig. 142. 



The interior contour of the meter tube is shown in Fig. 143, 

 and the accuracy of the meter greatly depends upon its proper 

 design. As the water flows from A toward the throat B, its 

 velocity rapidly increases and the pressure at B becomes materi- 



