Taking the Mystery Out of Fire Apparatus Tests
Methods of Making Tests—How to Figure on Proper Hose Layouts and Calculate Results—Instruments Needed
TESTING various types of fire apparatus, it has been observed, is sometimes mystifying to fire department officials and the object of this article is to outline the proper method of performing these tests and how to make the necessary calculations. It is only by conducting tests on the apparatus at regular intervals, that any defect in the mechanical parts can be detected, and a true conception of the worth of the apparatus realised:
Tests and purchase of fire apparatus may be considered synonomous, but while fire chiefs may acquaint themselves with all the salient features of the various types of fire engines which they are considering, very few of the fire-fighting fraternity ever give a thought to an understanding of the matter of making tests of the apparatus which they will receive. They know that at their call, they can have an engineer sent from any of the underwriters’ organizations, and this may in a measure be responsible for the laxity in learning the correct methods of testing pumpers.
Select a Good Site for the Test
An honest interest in the fire department by the laymen is a valuable asset for the chief. Most of the lure comes from the glamor of nickel plate, red paint, sirens and bells. And so whenever a pumper is tested there are few who have a bona fide interest in the results of an apparatus test, or can interpret the meaning of the results. For that reason, in selecting a site for a test, the best plan is to try and locate one away from the accessible part of the city. In many of the smaller towns, the test is advertised in a manner similar 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. No wonder then that after waiting an hour or so, they go home disappointed —there are few indeed who stay and see the test to the finish.
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.
The body of water from which the pump will draft should have a bottom that is free from mud or 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 covering the strainer, which is attached to the end of the hard suction, to a depth so that the top of the strainer will be about two feet from the water surface and still be free of the water bottom.
Material Necessary for Test
In addition to the equipment which can be found in the average fire department, the following items are needed: a pitot which can be constructed in the city machine shop in accordance with the accompanying plan, a gage which is graduated in pounds from 0 to 200 pounds, and a counter for getting the speed of the engine or pump.
If the apparatus to be tested is of 600-gallon capacity or larger, a two way Siamese will be needed. Smooth bore nozzles of 1-, 1 1/4-, 1 1/2-, and either a 1 5/8-, or a 1 3/4-inch nozzle are necessary. Whenever a 1,000-gallon pumper is tested, a 2-inch nozzle should be added.
A crow bar driven into the ground should be used to tie the nozzles so that they will not get away. Provisions should be made for a tender with gasoline to bring a 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 three hour test.
Construction of Pitot
The pitot blade consists of a brass plate of semi-circular shape with a knife edge on the straight section as shown in Fig. 3. 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. 4) accompanying drawing. An air chamber is put on to steady somewhat the fluctuations of the gage needle. The pitot gage is designed to read velocity pressure and contrary to popular conception is not static pressure. In itself the pitot gage readings mean nothing but must be interpreted by means of tables into gallons per minute.
The gages 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 gage is recording properly.
Placing the Apparatus
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 your 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 away a section of the road. Where 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 heeded, it is not uncommon for the rear wheels to cut their way up to the hubs into the soft dirt.
Figuring on the Layout
“What layout shall I use?” Even some of the seasoned delivery men who are sent out from the factories to train novices in the handling of pumpers have a hazy idea of how this is figured. There are very few problems that are simpler. The gage reading on the pitot should not be below forty, due to the inaccuracy of the gage in low readings and for convenience, it should not read too high. Preferably, the gage needle should point in the vicinity of sixty to eighty pounds.
A copy of the discharge tables for smooth bore nozzles and a table of friction losses in fire hose may be obtained from the various fire apparatus concerns or underwriter organizations and they have been published in the numerous books on fire department subjects. Knowing the gage 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 he about seventy. A 1 3/4-inch nozzle at this reading will discharge 761 gallons per minute—this is near enough to the desired conditions and therefore a 1 3/4-inch smooth bore nozzle should be used.
The standard tests for new pumpers requires the discharge of the rated capacity at a pressure of 120 pounds pressure per square inch. In other words, the gage 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 a 1 3/4-inch nozzle to discharge 750-pounds, the pitot should show 68 pounds. Then subtracting 68 from 120 equals 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 exceedingly high. For this reason, the discharge is divided into two equal parts by running one line from each side of the pumper and then connecting 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 pitot two nozzles.
When two lines are used, half of the discharge or capacity of 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 accurately. The friction loss in smooth new hose is considerably less than the resulting loss 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,
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 gage 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 the 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 the test, and the readings will fluctuate from the desired average.
Taking the Pitot Reading
The air chamber of the pitot should be grasped in one hand and the section near the blade with one finger and thumb of the other; the remaining fingers of that hand grasps 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 a distance away from the nozzle not less than half the diameter of the nozzle. 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.
Observations at the Engine
At the engine the following readings are necessary: pressure gage 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 rotor; 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 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 or 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.
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 for rated capacity at a pressure of 120 pounds for two continuous hours. Then it is followed by two pressure tests each 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 line of the suction port of the pump will give the vertical distance in feet. A good way to get this distance is to drop a weight attached to a cord over until the weight just touches the water, and note 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 lifts 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. The manufacturers will furnish you with 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.
A number of fire chiefs through the country 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 but sad conditions of their idol—it was exceedingly difficult to even obtain fifty per cent of its rated capacity.
The chief should test all the pumpers in his department at regular intervals so that any mechanical defect can be detected. The tests and calculations for determining the pumper’s efficiency are not difficult, as was explained, and any apparatus needed is not at all costly.