Strainer And Footvalve

The total area of the holes in the strainer should be at least three times the area of the suction pipe. It is usually arranged as part of the footvalve and should be so fixed that the level of the water is a fair depth above the top of the footvalve or air will be drawn in due to the formation of vortices. Cases have been known where vortices have formed round suction pipes to a depth of four or five feet with fairly large pipes. A good rule for the minimum depth in inches of suction pipe inlet below the water level is 15 √D), where D = diameter of the pipe in inches. A bell-mouthed suction pipe is often an advantage in giving a smooth flow and preventing the formation of vortices and eddies, especially where the water velocity in the pipe is high. The holes in the strainer should be arranged to suit the size of pumps, as shown on our certified installation drawing.

Running

The bearings are lavish in length and should be kept clean and free from grit. Where grease cups or lubricators are provided, these should be kept well filled with grease or good lubricating oil, and beyond this little attention is necessary.

Where the pumps are dealing with clean water, the gland bushes are water sealed with pressure water from the delivery side of the pump, by means of a small external pipe. If the water is of a dirty or gritty nature, a separate water seal should be fitted from a clean source of supply, or if this is impossible a suitable filter should be supplied between the pressure side of the pump and the gland bushes so that the water may be filtered and used for sealing purposes.

The gland packing should not be screwed up too tightly but the glands should " weep " slightly when the pumps are running.

If the pump is not working satisfactorily, it is probably due to one of the following causes :

(1) ;Air leakage in the suction pipe or glands of the pump. If this is the case there is no remedy but finding out the leak and making it air tight.

(2) ;If the pump is full of water but does not deliver the right quantity to the proper height, see whether the valves are all open, and if so, the failure may be caused by the height of the delivery pipe being greater than that for which the pump is designed, or because the length of piping is so great that there is too much " loss of head," due to friction and bends. Unless this loss is excessive a slight increase in the speed of the pump may give the extra pressure.

(3) ;The strainer should be examined at regular intervals as it sometimes becomes choked. It is very important that foreign matter should not be allowed to obstruct the flow of water through the strainer; and an examination of the strainer should be the attendant's first duty if he notices any sudden falling off in the duty of the pump. A good method of testing this point is to fit a vacuum gauge to the suction flange of the pump. If the reading on this gauge appreciably exceeds the manometric suction lift as estimated from the static lift and friction losses, etc., then it is evident that there is a choke somewhere in the strainer or pipe line. (See Fig. 32.)