November 1, 1918. THE COLLIERY GUARDIAN. 910 Sections 53 to 56, 58 and 59 are idential with Sections 31 to 44, 46 and 47 of the Directions of January 31, 1918. Section 45 of the old Directions is replaced by Section 57, which states that the present Directions shall not apply to sales of coal, coke-oven coke or patent fuel made before October 28, 1918. SHOCK IN HYDRAULIC SYSTEMS.* By Alfred Towler, M.I.Mech.E. Only a few years ago there were many who foretold that hydraulic power was on the wane, and would soon be obsolete. That forecast has now been proved to be utterly wrong. For certain work—such as forging, heavy pressing, gun-training, etc.—its position is unas- sailable and supreme. Although it does its work without apparent effort, at the same time it invariably impresses the beholder with the dignity of its power, occasionally mat red by disquieting manifestations, such as excessive pipe vibration and cracking sounds. These characteristics have recently lecome so increasingly prevalent, often attended by breakages, as to create a wide impression that they are inherent defects in the system. Whenever such symptoms appear in an aggravated form, it is obvious that there is something wrong which requires looking into—the cause traced, and either removed or rendered less acute. On investigation, it would probably be found that the hydraulic machinery had been supplied by makers of repute who had had nothing whatever to do with the layout, which had been entrusted either to an office boy or someone of equal experience who had left, died or gone to the front, leaving those behind wishing he had gone first. Some- times it is explained that there was such a shortage of solid-drawn steel hydraulic tubing at the time the plant was installed that they had to take what they could get, or, in reply to a remark that every other department had received consideration except the hydraulic system, which had been located straggling about odd extremities of the works, they might answer, That is true—we did not contemplate its installation in the first instance. Further, the works have grown far beyond our expecta- tion and provision, so that we had to dump down hydraulic plant in the spaces available. If we had to do it all over again, more consideration would be given £o the arrangement; but there it is—can anything be done to reduce our breakages ? The various makers inform us that their machines have been tested at their works to double the working pressure, but in spite of that when they are put to work here they fail.” If it is suggested that the installation is suffering from shock, one will be asked to define shock—its relative violence—zone of maximum intensity—effect— analogy between pressure and shock waves—syn- chronism— how shock is caused—how can it be ameliorated ? Shock. Shock may be defined as a violent collision, which is determined by momentum, i.e., the product of the mass by the velocity of the moving body usually expressed by MV. The object of this paper is to state broadly and simply cause and effect in principle, for, in order to advance further than this, it is obvious that each case would require to be considered on its merits. Relative Violence. To illustrate the relative violence of impact (a) by increasing the mass, (&) by increasing the velocity, let us assume that we have a stop block fixed at the bottom of a 5 per cent, incline—say 100 yards long on a railway line, at the top of which is released a 10-ton wagon that is allowod to run into the stop block. Now consider the increased force of impact if, running down the incline, there is a train of 10 wagons instead of one, in which case the velocity is the same, but the mass is increased tenfold. This is equivalent to two similar hydraulic presses operated in the same way on similar work, the power main of the latter being 10 times the length of the former. Now let us return to the single wagon on the incline and compare the relative force of impact with a similar wagon on an incline of the same gradient as the first but 10 times its length. In this case the mass is the same, but the velocity is increased and is like working similar presses out of relatively large and small mains. Further, one may have a combination of the two, namely, a long main of a small diameter, in which case the mass is considerable, the velocity is high, and the shock very violent. Zone of Maximum Intensity. This is the buffer, which may be the operating valve or the press itself going on to its work, stops, or resist- ance. Just as there are solid buffers and spring buffers, there is solid work, such as metal forging, and soft work, such as baling, where the materials are of springy or elastic nature. The latter need not be considered further, as such materials are not conducive to shock. Cause of Breakage. It is thus evident that sudden change in velocity is the common cause of breakages. In hydraulic pumps these changes take place every stroke, and are generally caused by an insufficient supply of overhead water, and too restricted and tortuous waterways, the effect being that the water does not flow sufficiently quickly into the pump, and therefore a vacuum is formed on the suction stroke between the plunger and incoming water, these colliding at the reversal of the stroke, or soon after, and causing repeated shock, abnormal wear and tear, and ultimate failure. Analogy between Shock and Pressure Waves. At the moment of impact there is a corresponding rise in pressure, or shock wave, which is transmitted with diminuating intensity throughout the whole system. Furthermore, every pump and every system or pipe circuit has a periodicity, precisely the same as there is n sound echo and re-echo. * From paper read before the Leeds Association of Engineers on October 31, 1918. Synchronism. Whenever the periodicity of a pump synchronises with that of the system or pipe circuit, there is trouble. Some years ago the author investigated a complaint that an engine, which was pumping into a closed service main, did not work well at its normal speed. He found that the complaint was justified, but proved to the satis- faction of the owners that the pumping engine did work equally well 6 revolutions per minute slower and 6 revolu- tions per minute faster than the normal speed, and to his own satisfaction that synchronism was the cause. Synchronism plays an important part in mechanics, and will, the author believes, receive more attention in the near futuie. Cause of Shock. It will be understood that one cannot operate hydraulic machines without change in the velocity of the flow in the pressure main from which it draws its power, and that a considerable variation is permissible without any serious effect so long as conditions are normal. If a hydraulic machine is remote from the accumulator, either the pipes should be extra large or the speed of the operation of the machine should be slow. In like manner, if the power of a hydraulic machine is great in relation to the diameter of the accumulator serving it, its operation must bj, slowed down, otherwise the arrest of the accumulator when rapidly falling will cause shock in itself. Ameliorating Shock. Speaking concisely, shock can be ameliorated by interposing some form of cushion or relief at the zone of maximum intensity. The term momentum valve is a misnomer, since it is not a valve in the strictest sense, and vould be better called momentum—or shock absorber or alleviator, as is now more common. These devices are usually spring-loaded, and in large sizes . there are several tiers of springs and several springs in each tier—this being for convenience of manufacture and to minimise risks in case of springs failing. Some- times the device is combined with a relief valve, so as to let a relatively small quantity of water escape when a predetermined pressure has been exceeded. The author’s works are situated at one end of Leeds waterworks service main, which are subject to the pulsations of pressure or shock waves already referred to ; but on opening the stop tap a very little, the pulsa- tions cease. As a general precaution in an hydraulic system, the author recommends a large combined momentum and relief valve being located near the accumulator and alleviator, near each pump and hydraulic machine; but the details of the installation should be in proportion, and the whole arranged with intelligence in every case to get the best results. Weight-loaded accumulators are not ideal on account of the loading or the mass, which is not sufficiently sensitive and often increases shock, particularly if the accumulator is on the small side. On board ship, where accumulators are subject to the rolling and pitching of the vessel, the loading is generally done by steam. On land, the author is strongly of opinion that compressed air is the best medium for leading hydraulic accumu- lators and alleviators, which really are accumulators in miniature. Where hydraulic plant has been arranged by amateurs it may be very difficult to set right without starting afresh, but in most cases installing alleviators of large size will remove or reduce the violence of impact. The movement of pipes under shock is easily accounted for by the fact that, owing to bends and blank ends, they are out of balance, and a sudden change of pressure causes them to have lateral move- ment which necessitates anchorage. Overhead hy draulic mains, though more accessible, are often more difficult to anchor. Air Vessels. It is un lesirable to employ air vessels on the delivery side of hydraulic pumps, because the air escapes when- ever the pressure falls. Not only is this an expense to replace, but air in hydraulic mains is a source of danger on account of the liability of air lo ?ks. It will be understood that in the air-loaded accumulators and alleviators recommended, the compressed air is con- fined in a separate vessel from the water, and should the pressure of the latter fail the former would not be released. Air vessels on the suction side of pumps are some- times objected to because they are alleged to have a habit of getting full of water, or empty. In the former case some means should be provided to replenish the air, and in the latter it is clear that there is an ineffi- cient water supply to the pump: either the pipes are too small, or there is not a sufficient head. All air vessels so used should be supplied with water gauges and air-charging devices, end need periodical attention. The correct water level generally indicates favourable suction conditions, which are essential to the satisfactory and efficient working of hydraulic pumps. Efficiency of Hydraulic Flant. The combined efficiency of a hydraulic plant is made up of power, mechanical and volumatic efficiency—the latter including not only the external or visible leakage, but also the internal leakage or slip (which is out of sight and often considerable). The total leakage is seldom less than 5 per cent., and in odd cases that the author has observed, be estimates it to be not less than 20 per cent. Water Saving* Devices. Turning to water saving, there are two kinds of water- saving devices—(1) voluntary; (2) automatic. It is com- monly said to be useless to apply the voluntary system where the operators are “ on piece,” because there is a generally well-founded belief that all water saving gears slow down the operation. In such cases something should be done to remove the grievance. Another source of leakage is the use of impure and gritty water. Frequently nothing is done to remove solid matter such as sand, scale and grit—these being left in continuous circulation. Closed suction straining boxes are nothing short of a calamity, since a joint generally has to be broken in order to examine the strainer, and therefore it invariably gets neglected. The strainers are much too small when clean, and cause the pump to become ‘‘ foot bound.” A better plan whenever the circuit is open is to have a large overhead tank in which a barrier is fixed between the inlet and outlet ends. In this barrier or partition are located duplicate sliding sashes to which wire copper gauze is attached. These can be withdrawn and cleaned one at a time while the pump or pumps are working. Overhead tanks of this kind should be covered to protect them from grit, etc., and a soluble lubricant, free from acid, should be added frequently. The author is of opinion that in order to keep hydraulic installations free from shock, and to work them with a minimum amount of maintenance, all such details as those set out above must be considered in laying out the plant; and he wishes strongly to emphasise this point. MINERS’ WAGE AGREEMENT PROPOSALS. A conference of representatives of the mining districts in England and North Wales comprised within the area of the Coal Conciliation Board con- sidered on Wednesday, in London, the renewal of the wage agreement for a further period. Mr. Stephen Walsh, M.P., presided. It was agreed to re-open negotiations with the coal owners for the renewal of the agreement on the following basis: — That the present war wage.of 18 per cent, be merged in the existing wage standard. That six days’ wage be paid for five shifts worked on the afternoon or night shift, with the addition of one-fifth to the wage for each shift to men who work less than five shifts in the week. That the new agreement apply to all men employed in and about the collieries who are members of the Federation. That an improved and more satisfactory system of ascertaining the selling price of coal at the collieries be adopted by the board. The new feature of the men’s proposals, apart from wage changes, is the extension of the Conciliation Board wage agreement to surface workers and to all members of the Miners’ Federation. THE AMERICAN COAL TRADE. No longer will it be possible for slackers in the mining ranks to lie down on their jobs (says the Coal Age, October 10). The Production Department of the Federal Fuel Administration has divided the coal- producing facilities of the country in such a manner that not even one single individual embraced therein will escape questioning if he fails to account for the full measure of the task allotted him. The necessity for an enlarged supply of bituminous coal is particularly pressing. The Fuel Administra- tion is no longer concerned about the coal situation except as it affects the output of iron and steel needed for munitions and other war purposes. The output required of the soft coal mines between now and March 31, 1919, is 12,234,000 net tons a week. If this average tonnage can be maintained for the next five months the bunker, naval and other oversea requirements will be met. For the week ended September 28 (the latest statistics available) the pro- duction of soft coal was 700,000 tons above the average mentioned, as the total quantity mined during that week was 13,043,000 net tons. The anthracite mines, too, have been asked to in- crease production. The hard coal mines are called on to furnish 2,030,000 net tons a week for the next five months. With a shortage of labour already exist- ing in the anthracite regions it will require strenuous efforts on the part of those engaged in this branch of the industry to meet the call. For the week ended September 28 the production of anthracite is esti- mated at 2,071,000 net tons, which is also well above the quantity demanded. Market conditions, in so far as domestic demand is concerned, are active. The coming of colder weather has witnessed an urgent demand for fuel from house- hold consumers. The influenza epidemic, too, has been responsible for starting fires in many homes earlier than .usual. The stove and chestnut sizes of anthracite are in strong demand, and calls for egg coal are also becoming more frequent. The outstanding feature of the market is the collapse in the demand for the steam sizes of anthracite, and several washeries are reported as about to close down for lack of orders. Cities located near the mines, such as New York and Philadelphia, for example, are receiving only light shipments of anthracite, as a concentrated effort is being made to take care of the outlying sections before winter ties up transportation facilities. It has been definitely decided that the Great Lakes shipping season will held open until the last possible moment this year. The lake shipments for the season to October 1 are, roughly, 800,000 to 850,000 tons behind schedule. At Hampton Beads there are still serious cases of over-detention. Pocahontas coal, because of its low price, is moving very largely to by-product plants and for coking purposes. Shipments from Baltimore and from Philadelphia have sloped off to such extent that movement to those piers has been curtailed quite radically. . The Fuel Administration continues to watch closely the amount of coke offered as foundry grade, and requires that this be kept down to the actual needs of foundies engaged in war work, all other coke to be offered as furnace, no matter if it has been in the oven 72 hours. These regulations have been helping the furnace coke supply for some time past. Sir James Dobbie, F.R.S., principal of the Government Laboratories, has been appointed a member of the Inter- Departmental Committee on Alcohol Motor Fuel.