90 SMALL WATER SUPPLIES. 



of severe shock, and altogether improves the working 

 of the pump. They are sometimes placed on the 

 suction side, but only usually in large pumps. For 

 the delivery side their cubic capacity is generally 

 five times the total displacement of the ram or 

 rams, as the case may be ; those on the suction side 

 have about half this capacity. The shape of an air 

 vessel does not matter much, generally the cylindrical 

 or inverted pear shape is found in practice. They 

 must be smooth inside. 



An important problem relating to pumps for estate 

 water supply and other purposes is the power re- 

 quired to drive them. So many factors depend on 

 this. For instance, knowing the required duty of the 

 pump it might be necessary to ascertain if the power 

 required to give this duty can be had in the form of 

 water or other natural source, or if not, what size of 

 engine will be wanted and its consequent capital cost 

 and cost of running. 



For instance, let us take the following example. 

 A three-throw pump is to be installed to pump 3000 

 gallons per hour to a height of 300 ft. The size of 

 the rams is 4 in. and the stroke 6 in. At each revolu- 

 tion of the pump the theoretical amount of water dis- 

 placed will be equal to 

 3 x 4 x 4 x 7854 x 6 _ 7854 __ 



1728 6 



and '1309 x 6*25 = *8 1 8 gallons per revolution 



3OOO 



= say 60 revolutions per minute 



818x60 



as a fair speed, but bearing in mind that the theoretical 

 discharge will only be about 80 per cent of the actual, 

 it will be well to provide for the pump running at 



