Who Should Test the Fire Engine?
It’s Time That Fire Departments Should Do Their Own Testing—Fully Competent For This Work.
ONE of the tasks of the Bureau of Fire Prevention and Engineering Standards of the National Board of Fire Underwriters, when making an engineering survey of the fire protection facilities, is the testing of fire apparatus in service. Such tests have been necessary, and still are necessary, to determine the capabilities of pumping engines in particular in service in the various fire departments. A pumping engine, after it has been in service for several years, may be found to have a much reduced capacity over that which it possessed when first delievered. This is natural, for the vigorous service to which fire engines are submitted is bound to have its effects. Because the National Board tests all engines in service when making surveys of cities, this organization is usually called upon to test new pieces of motor fire apparatus prior to being placed in service.
Early Tests Were Uncertain
In the early days of the motor fire engine there was considerable uncertainty as to just what the motor pumping engine could give in the way of performance, and the National Board gladly offered its engineering services to assist fire departments in making acceptance tests to determine the capabilities of the apparatus being installed. From the standpoint of the Underwriters, such tests were necessary, for upon the fire protection facilities of a city depends the making of insurance rates. During the first few years of the motor pumper considerable grief was encountered by fire departments through the failure of motor pumping engines to stand up under severe service. And this co-operation of the National Board of Fire Underwriters was welcomed, and in many cases was urgently needed.
Present Day Motor Apparatus Dependable
Motor apparatus as built at the present time is dependable. The large manufacturers of fire apparatus, building their own motors, have reputations which they jealously guard, and they can be depended upon to put out only the finest of apparatus, both from the standpoint of power plant as well as pumping equipment.
The small manufacturer, not having sufficient capital nor business to warrant the building of his own motors, purchases them from commercial truck motor builders. In this latter case the use of proven commercial motors is beneficial to the department putting such apparatus into service. The small manufacturer could not afford to employ high grade automotive engineers, but by using commercial motors of established reputation he can offer his customers machines which can be expected to stand up in a satisfactory manner in the fire department service. Thus both the large and small builders of apparatus are today turning out machines which are reliable from the power end standpoint.
As far as pumping equipment goes, the small manufacturers of motor apparatus confine themselves to rotary pumps, which are now so well understood that errors in design are few and far between. Failures of present day pumping engines, whether manufactured by large or small concerns, are very infrequent occurrences. Rarely are they the result of poor design.
Purposes of Tests
Two types of tests are commonly conducted, namely, the acceptance test, and the annual, or semi-annual test, to determine the condition of the apparatus.
The purpose of the acceptance test is to definitely determine whether or not the engine lives up to its specification.
Each of these tests is conducted in the same manner, and the objects are precisely the same. With old apparatus, however, allowance is made for wear, so that if a pump, after serving several years, does not give its initial rated capacity at specification pressure, it is not necessarily considered defective or unserviceable.
Can Fire Departments Test Their Own Apparatus?
Many fire departments, particularly those in small cities, hesitate to make tests of apparatus, fearing that such work requires the training of an engineer. This is an erroneous impression. The testing of a motor pumper is a comparatively simple problem, once proper instructions are at hand, and are faithfully followed. The only special apparatus that is required to satisfactorily conduct a test includes the following: revolution or speed counter; a stop watch and wrist strap; a small pitot tube; an air chamber on pitot; two or more pressure gauges; a hydrant or engine discharge cap; appliance for attaching counter; for convenience it is also desirable to have a pitot bracket for supporting the pitot tube in proper position to make nozzle pressure readings.
A brief outline of the methods of making various tests are given later in this article. From this outline the reader can secure a good idea of just the work entailed in testing a motor fire engine. Incidentally, the National Board of Fire Underwriters have available a pamphlet entitled “Fire Engine Tests and Fire Stream Tables” which outlines fully the methods of testing fire engines. This little pamphlet also includes, in addition to instructions for making tests, very useful tables on effective reach of fire streams, friction loss in fire hose, discharge from nozzles, nozzle and engine pressure on various layouts, etc.
There is not a motor fire apparatus operator who cannot, with a little instruction, conduct a satisfactory test of his apparatus. Of course, testing the apparatus is one thing and making the computations after the test has been completed i_____ another.
While any member of the department with a good knowledge of arithmetic can make his own computations. some departments might prefer having these calculations made by others than themselves. In such case, the engineers of the National Board of Fire Underwriters, or of rating organizations whose departments conduct fire engine tests, will gladly make the computations if data sheets of tests are submitted to them. Of course, the Underwriters’ engineers have not time to compute engine tests results where engine is tested several times during a year. On the other hand, they will gladly check over figures made on acceptance tests of new apparatus, or tests where the results do not seem to be correct and the advice of someone conversant with testing is desired.
For acceptance tests, the National Board will furnish fire departments with data sheets on which the data secured in tests are to be entered. These sheets, after the figures have been entered, may be sent directly to the National Board whose engineers will make the necessary computations and submit a report to the fire department thereafter.
What Tests Include
For acceptance, engines are required to deliver their full rated capacity at 120 pounds average net pressure for 2 hours; 50 per cent of their rated capacity at 200 pounds average net pressure for one half hour; and 33 per cent of their rated capacity at 250 pounds average net pressure for one-halt” hour. Tests should preferably be made when drafting with about 10 feet of lift, especially if the engine may be required to draft water from a river, canal or cistern when in service.
For a two-hour run, nozzle and engine pressures may be recorded every five minutes, after the engine is brought up to speed, as with any reliable motor the speed and pressure will lie nearly uniform. If it is inconvenient to attach a continuous reading counter to the pump the readings may be taken for one minute, during each five minutes interval, using a speed counter. On some makes of pumps it is possible to take the pump speed directly from the end of the pump shaft by removing a foot board or bracket; on other makes, speed may be taken at the engine on the end of the circulating pump shaft, timing shaft, or possibly on the fan drive wheel shaft; if pumping speed is obtained, the engine speed may be computed from the gear ratio. Some makes of pumpers have tachometers attached which can be used for speed reading, but they should be checked by a counter reading. In many of the smaller towns, the test is advertised in a similar manner to the coming of a circus or a traveling minstrel show. Promptly at the specified hour, about half of the town’s population will be at the appointed spot arrayed in their finery waiting to see something spectacular. The novelty of pushing the crowd aside every time it is necessary to take a reading or make some hose adjustment will soon wear off, and of far greater importance is the hazard of the hose breaking loose and injuring some of the crowd.
In selecting a site for the test, the best plan is to try and locate one away from the accessible part of the city.
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Who Should Test the Fire Engine
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The body of water from which the pump is to draft should have a bottom that is free from mud and an abundance of foreign material which will act as an abrasive should it be sucked into the pump. The lake or creek should also be of sufficient depth to insure sufficient immersion of the strainer, which is attached to the end of the hard suction. The depth should be such that the top of the strainer will be about two feet beneath the water surface and still be clear of the bottom of the body of water.
Material Necessary for the Test
In addition to the equipment listed previously the following items are needed: if the apparatus to be tested is of 600 gallon capacity or larger, a two-way Siamese; smooth bore nozzles of 1, 1 1/4, 1 1/2, and either a 1 5/8 or 1 3/4-inch nozzle; when a 1000 gallon pumper is tested, a 2-inch nozzle.
A crow bar driven into the ground should be used to secure the nozzles so that they will not get away. Provisions should be made for a tender with gasoline to bring the supply to the test about one hour after the test has started. On the average about thirty-five gallons of gasoline are burned during the standard threehour test.
Construction of Pitot.
The pitot blade consists of a brass plate of semicircular shape with a knife edge on the straight section as shown in Fig. 1. Around the circumference is soldered a fine copper tube which projects slightly beyond the knife edge. The other end of the tube passes through a brass fitting which is cut with a standard pipe thread. The balance of the pitot can be made from pipe parts as shown in the (Fig. 2) accompanying drawing. An air chamber is put on to steady somewhat the fluctuations of the gauge needle. The pitot gauge is designed to read velocity pressure, and contrary to the popular conception, is not static pressure. In themselves the pitot gauge readings mean nothing, but must be interpreted by means of tables into gallons per minute.
The gauges which are used should not have a pin to hold the needle at zero. It is only by allowing the needle to come to an absolute rest that one can tell if the gauge is recording properly.
The apparatus to be tested should be so placed that the suction side of the pump is nearest to the body of water. If the pumper is to draft from a wooden bridge, do not place the entire weight of the machine on the frame structure—either the front, or better, the rear wheels should be resting on the ground.
Some of the pumpers are so equipped that the cooling water from the engine is discharged through a section of garden hose, and in others it is sent through the exhaust pipe. Should the pump be of the latter type, place a board under the exhaust. The force of the gas and water issuing from the exhaust port will, in three hours time, wear a depression in the road. W here there is a dirt road, the rear wheels should be run up on boards or other material which will enable the driving wheels to get a firm grip on the road after the test is over and the machine is to be returned to quarters. Where this precaution is not provided, it is not uncommon for the rear wheels to cut their way up to the hubs into the soft dirt.
Figuring the Layout.
This is a very simple matter. The gauge reading on the pitot should not be below forty pounds, due to the inaccuracy of the gauge in low readings; and for convenience, it should not be too high. Preferably, the gauge needle should point in the vicinity of sixty to eighty pounds.
Discharge tables for smooth bore nozzles and a table of friction losses in fire hose may be obtained from the pamphlet referred to above, or from charts which a number of the manufacturers issue. Knowing the gauge reading that is desired, one can look up the discharge table and see what size nozzle should be used to get the rated discharge for the pump.
Let us assume that a 750-gallon pumper is to be tested, and that the pitot reading will be about seventy pounds. A 1 3/4-inch nozzle at this reading will discharge 761 gallons per minute—this is near enough to the desired discharge and therefore a 1 3/4-inch smooth bore nozzle should be used.
The standard test for new pumpers, as noted previously, requires the discharge of rated capacity at a pressure of 120 pounds per square inch. In other words, the gauge at the engine must show a certain reading which, when the figure for lift is added, will show a net pressure of 120 pounds.
The tables show that for 1 3/4-inch nozzle to discharge 750 gallons, the pitot should show 68 pounds. Subtracting 68 from 120 pounds leaves 52 pounds to be lost in friction. The next problem is to find the hose layout which will give the required friction loss. Were this quantity of water sent through a single line, the friction loss would be extremely high. For that reason, the discharge is divided into two equal parts by running one line from each side of the pumper and then joining them a short distance from the nozzle by means of a siamese. If no Siamese is used, two nozzles of the same diameter will be required, and instead of reading one nozzle, it will be necessary to read two nozzles.
When two lines are used, half of the discharge from the pump is sent through each line. The friction loss per 100 feet of 2 1/2-inch cotton rubber lined hose for a flow of 375 gallons (half of 750) is 33 pounds. A line ,150 feet long, will give a friction loss of 49 pounds which is close enough for the problem. No layout can be computed exactly. The friction loss in smooth, new hose is considerably less than that in old hose, and on this account it may be necessary to make a few trial layouts, after the calculations, in order to find the suitable hose layout.
Readings Affected by Hose Layout
Should the pitot reading be too low and the reading at the engine too high, it is an indication that there is too much hose used. The remedy is to take out a length of hose. On the contrary, if the pitot gauge shows too high a reading and the pressure reading is low, it is caused by not enough hose to build up the proper friction loss, and a length of hose should be inserted.
Sometimes, in order to strike a happy medium, it may be necessary to partially close or throttle the discharge gate on the pumper. This should only be used as a last resort, as the gates have a tendency to open up during the course of test, and the reading will fluctuate from the desired average.
Taking the Pitot Reading.
The air chamber of the pitot should be grasped in one hand the section near the blade with one finger and thumb of the other; the remaining fingers of that hand grasp the nozzle so as to steady the pitot. The blade is held parallel to the nozzle to eliminate splashing, and the end of the blade is inserted in the center of the stream and at a distance away from the nozzle of not less than half the diameter of the nozzle; if held closer to the nozzle the reading will be too high. The pitot should be held in the stream for a few seconds to allow the air chamber to fill up before the reading is taken. It is very necessary for all joints in the pitot to be tight. A small leak at a coupling gives too low a reading.
Observations at the Engine.
At the engine the following readings are necessary: pressure gauge should be read to the nearest pound, and the speed of the pump or engine should be taken every ten minutes or whenever the readings fluctuate from the apparent average. The shaft of the counter used for taking the readings should be inserted in the countersunk place on the pump motor; on some machines, it is more convenient to take the motor speed. Note the counter reading before the instrument is inserted in the rotor, hold the counter there for exactly a minute and note the new reading. By subtracting the two, the revolutions per minute will be found. About five readings should be taken; readings above and below the apparent average should be discarded. It is only necessary to take this number of readings at the beginning of the test and whenever the motor speed has changed appreciably and the motor can not be brought back to its normal speed.
Recording the Readings.
A record should be kept of the readings of the pump pressure, and the nozzle reading taken every five minutes, and of the motor or pump speed taken every ten minutes. A new pumper is tested or rated for capacity at the pressure of 120 pounds for two continuous hours. Then it is followed by two pressure tests of one-half hour each one, one-half capacity at a pressure of 200 pounds and the other at one-third capacity at 250 pounds.
It will be necessary to change the hose layouts for each of the pressure tests, although in some cases the layout of the second test may be retained and the discharge gate throttled.
Calculating the Results.
After the test the readings should be averaged. From the discharge tables, the pitot readings can be converted into gallons of water discharged per minute. The vertical distance of the water surface, from which the pump is drafting, to the center of the line of the suction port of the pump will give the vertical lift in feet. A good way to get this distance is to lower a weight attached to a cord until the weight just touches the water, and note that the point where the cord reaches the center of the suction port. This distance is measured in feet. Roughly, one-half of the distance in feet is equivalent to the lift in pounds, and this correction for lift should be added to the pump pressure reading. Most manufacturers state their guarantees in terms of lift not over ten feet, and for this reason it is best not to draft water from a greater distance.
Knowing the pump r.p.m. to get the engine r.p.m. multiply this figure by the gear ratio from engine to pump and vice versa. On centrifugal pumps, the engine reading is multiplied by the gear ratio to obtain the pump speed, since the centrifugal pumps operate at a speed greater than that of the motor.
Calculations for Slip.
It is not practical to calculate slip for centrifugal type pumps. On the other types, divide the discharge in gallons by the r.p.m. of the pump. This will give the actual displacement per revolution. The manufacturers furnish the theoretical or nominal displacement.
Nom. displacement — act. displacement / Nominal displacement = % slip
Testing Old Apparatus.
In testing apparatus now in service it is only necessary to test the pumpers for capacity at 100 pounds pressure for twenty minutes, taking readings every minute, and to run the pressure tests for ten minutes.
Some chiefs who believe that their pumpers are in good operating conditions are surprised at the poor showings made at these tests. In one case the company officers sung the praises of their pumper which in spite of its age could, they claimed, still deliver 100 per cent of its rated capacity. The test revealed the true state of affairs—it was exceedingly difficult to obtain 50 per cent of its rated capacity.
The chief should test all the pumpers in his department at regular intervals so that any mechanical defect may be detected. The tests and calculations for determining the pumper’s efficiency are not difficult, and special apparatus needed is not at all costly.
Note should be made of any faults of construction, such as clutch slipping, absence of locks or stops for control levers, inadequate gasoline or lubricating oil supply, inaccessibility of parts, magneto and sparking circuits not waterproof, etc. The feed of lubricating oil is especially important in an automobile pump, where many parts are enclosed or not quickly accessible, and should be carefully looked into by the person making the test.