REGULATION OF TURBINES 



521 



the normal by a few pounds per square inch, and its area should be such 

 that, if the gates are closed suddenly due to a throwing-off of the entire 

 load, it is capable of taking the whole discharge. If A be the minimum 

 total cross sectional area of the guide passages when these are wide open, 



FIG. 250. Lombard Relief Valve. 



in order to allow for exceptional circumstances the total relief valve area 

 should not be less than *7 A. 



The ordinary spring-loaded relief valve is not usually very successful on 

 a large plant, one or other type of hydraulically operated valve being 

 commonly fitted. Such a one is the Lombard valve, illustrated diagram- 

 matically in Fig. 250. Here P is the penstock, with its relief valve V, 

 which is held up to its seat by the water pressure on the piston P. The 



