The Pumping Engine; Its Many Points, Faults and Peculiarities.
There is a correction to be made in Part XV. In the fourth paragraph at the end of the fourth line the word “cocks” should read “covers.” Since writing the fifteenth article I have seen another case of bad casting; this was in some steel pumps carrying a working pressure of 2,500 pounds per square inch.
The working strain put upon the steel is only 4,500 pounds per square inch of section, but in spite of this very low strain a leak has gradually developed from a mere weep to a torrent of tears, and it is, of course, only a question of time for the pump to burst, as the metal is getting thoroughly saturated, and of course must eventually go.
There seems to be only one cure for such cases, and that is to keep on casting until sound castings are obtained, rejecting all castings that show any signs of leak when tested. It’s expensive, but it pays in the end. Another very important point in pump cylinders is to be careful in designing them, so that the fewest possible parts are to be machined. Leave the skin intact, both inside and outside, as much as possible. This can be done most readily in pumps of the ram type, as the valves may be placed in a separate chamber, so that the gland is the only part exposed to the water ; the foot and branch flange, of course, not being exposed to the water.
For those who wish to save a casting that shows signs of weeping a good thing is to warm the casting and then brush out with melted paraffine wax. Let it cool and harden, then test again. The rusting up process also sometimes does good but after using the sal-ammonia the casting should be thoroughly well washed. The solution of sal-ammonia should also be applied under pressure to reach into the pores of the metal. Of course this is only a makeshift at the best and in first-class work should not be allowed.
‘I’he only pump worthy of comment now left is the piston type as shown in cuts 12, 13 and 14. This is a form that is favored by makers all over the engineering world ; the principal reason and really the only reason of its favor is that it takes up little room and is of course somewhat cheaper to produce. The objections are many as liefore briefly touched upon. Firstly, the cooling strains are many and very bad ones at that; take the junction of the working barrel at A to the valve plate on scat also the form at B, then the bottom casting on suction (D) chamber invariably is made to form a part of the bed plate and is of bad form. One of the worst features is the impossibility of knowing when any leakage is taking place at K. A large amount of the useful effect may be and no doubt is lost here for months before the required new packing is put in.
The delivery valves are placed in such a manner that the only access to them is from manholes which are in most cases too small, and will not admit of the placing of the necessary tools for regrinding or scraping. The suction valves have the same serious fault although they may be reached much better than the delivery valves as the covers may be removed, of course, at much more expense and loss of time. When all these defects are taken into account it appears to me that any one requiring a first-class pumping engine would cheerfully spend a little more money and take up more room; they would have their pumps better looked after and kept in order as it would not be nearly as much trouble to the engineer to make repairs, and as they would have more room and more accessible parts t would not be such a dirty job.
‘I’he attending engineer is a much more important man than he used to be some years ago, and all good men like to have their engines and engine rooms clean and in good order and like all other men, he likes to obtain these results with as little trouble to himself as possible. An outside packed gland shows at once when it is leaking and the owner can see for himself, but with these internal pistons the pump may be leaking internally and he is at the mercy of his engineer until it gets so bad that there is a shortage of water, whereas the outside gland compels the engineer to keep it in order if he wants to have a clean engine room and cellar. Another most important objection is in the case of a breakdown of any part of the pump proper the whole engine must be shut down as it is impossible with such a pump to work one end while repairs could be made. It is also a much larger job to replace a pump barrel than what it is with the plunger type, especially when the valves are all carried on the same casting and consequently have all to be prepared for again ; whereas in the case of the ram a simple barrel with two facings and one boring is all that is requisite.