HYDRAULIC DREDGES 99 



cast in one piece of round cross section, the dimensions being deter- 

 mined by the required capacity of the dredge. Owing to the fact 

 that the upper part of the shell tends to break, especially when 

 dredging through hardpan and gravel, it is found more convenient 

 to have the shell cast in two separate parts, bolted together so as to 

 renew the upper part when damaged without being compelled to 

 change the lower one. The runner or impeller consists of a cast-steel 

 disk with four cast-steel vanes tapering in thickness and sharply 

 curved in the back as shown in Figs. 27 and 28, which is an 

 outline of one of the pumps used for sand dredging on the Missis- 

 sippi River. The runners can also be made with 5 blades, as in the 

 U. S. hydraulic dredge " Delta," or even with seven blades, as in 

 dredge " Epsilon," used on the Mississippi River. 



The pumps are often lined with steel plates for the purpose of 

 removing the lining when damaged and substituting new plates. 

 Mr. Robinson states that the delay and expense of renewing the 

 linings amount to more than the occasional renewal of the entire 

 pump shell, as the latter can be replaced in less time than the linings. 

 Besides the bolts or fastenings of the linings tend to produce abrasion 

 and wear at those particular points. Wherever there is a crack 

 or joint in the interior of the pump it is liable to produce an eddy 

 or change of direction in the flow, and a stream of gritty material 

 acting in this way will soon cut out the fastenings and joints of the 

 linings. 



Mr. Robinson says that the interior wear of the pump can be 

 greatly reduced by careful design. It is better to allow ample 

 clearance for the flow at all points, especially the periphery of the 

 pump, and this can be so proportioned that the abrasion is compar- 

 atively slight and evenly distributed. If experience shows that 

 there is undue wear at one point, it means that there is a stream 

 of gritty material flowing or impinging against the surfaces at 

 that point and at a high velocity. This can be remedied by placing 

 those surfaces further away, thus giving the stream more room 

 at that point. Many pump designers are of opinion that the throat 

 or cut-off of the interior of the pump should be as close as possible to 

 the periphery of the pump runner in order to prevent any flow 

 past this point. Careful experiments have shown that this makes 

 very little difference, and for dredging pumps especially it is better 

 to allow great clearances at this point, otherwise it will cause great 

 wear. 



