PHILADELPHIA FIGHTS WATER WASTE
Third of a Series of Articles Telling Story of Successful Campaign on Broad Lines— Pitot Tube Most Valuable as Means of Measuring Pump Discharge—High Velocity Measurements Most Difficult—Data Must be Used to be of Value
AS a means of measuring pump discharge the Pitot Tube has proved itself the most valuable and acurate, also the cheapest tool available to the water works man. It can be set up, read and removed without disturbing in the least the continuous operation of the pump. It interposes no perceptible obstacle to the flow. It is easily portable, and on this account can be used where a Venturi meter would be too expensive, or even impossible.
In fact, it has been often used as a means of checking Venturi meters whose accuracy was questioned. As an instance in point, may be cited the use of the Venturi in connection with the acceptance test of a recently installed 25,000,000 gallon centrifugal pump. The pump was bought and installed under a guaranteed discharge rate of 25,000,000 gallons per day under the specified conditions of head, suction, and speed. The pump’s capacity, it was specified in the contract, was to be determined by pitometer, but it happened that on the two 48″ discharge lines there were already installed two recording Venturi meters. So by mutual agreement, between the manufacturer and the bureau, the clause in reference to the pitometer was waived and the Venturi meters already in place, decided upon as the means to be employed during the test for measuring the discharge.
But disappointment was in store for the pump people, for as often happens in these cases the pump failed to come up to expectations on the first few tests, and its friends blamed the Venturi meters.
Pitot Tube Establishes Venturi Meter Accuracy
At this stage, enter the pitot tube. The accuracy of Venturi meters, and the dependent results of the test, were absolutely established by the following report:
December 28, 1918.
Mr. H. R. Cady, Mechanical Engr. Bureau of Water.
Dear Sir: In accordance with your instructions, we have carefully checked the Venturi Meters at Queen Lane Pumping Station, and find that their results agree with those of thePitometer, so closely as to amount to an absolute check upon, their accuracy.
The above report shows that even in connection with a Venturi meter, the pitometer may serve a useful purpose. And further, let it be said that while the object in the above case was to check the Venturi meters, the check was a mutual one, the accuracy of the pitometers being established, as well as that of the Venturis, it being a matter of coincidence of results, through a series of determinations.
Of course it must be admitted that hydraulic conditions, in the above case were very favorable to accurate pitometer work—namely, long straight stretches of fairly smooth 48″ pipe—with no obstacles such as gate valves and check valves for a long distance up stream from the pitot orifices.
But on the other hand, many examples of very accurate work might be described, in which the test was per force made under the most unfavorable conditions, and where a Venturi meter was out of the question. A striking instance of this kind is afforded in the acceptance test of what is now known as No. 7 pump at Lardner’s Point Pumping Station.
Another Test Under Less Favorable Conditions
In this case the location of the pitot tap was limited by practical considerations to a point just down stream from a double bend in the vertical plane of the discharge pipe. Further upstream, were also a number of 90° bends, and in spite of these unfavorable conditions, a successful determination of the pump discharge was made.
The new pump was a DeLaval turbo-centrifugal guaranteed to deliver 35,000,000 gallons a day under specified conditions and here, curiously enough, the pitot test showed a discharge of over 40,000,000 gallons per day. That this result was correct is demonstrated daily by the fact that two of the 20,000,000 gallons reciprocating pumps are always kept out of service while this pump is running, and are again thrown into service when it is shut down. Suffice it to say, that the results of that test have in the course of a year been indirectly verified in numerous ways on many occasions, and were certainly correct at the time of the test within two or at the worst 2½ per cent.
In Fig. 1 are shown a family of traverse curves taken on the vertical diameter of the pipe in connection with this test. Their similarity, at different velocities testifies to the accuracy of the constant. And their curious shape is characteristic of the conditions.
Below is printed the report submitted on the determination of the pipe coefficient. It is offered chiefly because it describes briefly the operation and methods involved in a test of this kind.
Contract 305, Test, Pito.
L-65 Lardners Point.
The object of this test was to determine the rate of discharge of the new centrifugal pump, installed at Lardner’s Point Pumping Station under Contract 305.
Location of Tap.
The tap was located on the East Building line of Nos. 2 and 3 houses. Although this places it on the down stream side of a double bend in the vertical plane, this point was preferred to points further upstream because of the impeller disturbance and to any further down stream because of the increasing depth of the pipe and if taken on the header further on, because of the possible leakage through closed gates.
Two taps were drilled as shown in sketch and are hereafter designated as L-55-V for the vertical and L-55-45 for the 45° one.
Determination of Diameter.
The diameter was calipered with a special “Simplex” ⅜” brass calipering rod having a one-inch hook.
The diameter was found to be 48 1/4″ for L-55-V and 48″ even, for L-55-45, an average of 48⅛”. This value was taken as the average diameter of the pipe and used in the calculations of discharge.
The traverses were made by the 24 point method, in which the deflections are read at the centers of 12 annular rings of equal area, the velocities then calculated, regarded as the average velocity through each ring. The values of the velocities in eacn ring are then plotted to scale, a smooth curve drawn through them and the values from this curve taken as the true values of the velocities. It being assumed that the points must fall on a smooth curve and that variations from the smooth curve are due to difficulties of reading.
The 24 Point Method.
The distance iro.n the center of the pipe of the 24 points which are the centers of the 12 rings of equal area are found by the formula R = √22n-l/n wherein in this case equals 24 and R
These distances were laid off on a suitable length stick and with the rod orifices on the center of the pipe the index pointer on the rod was set at “center” on the stick. The successive setting of the index pointer of the rod on the various graduations on the stick thus sets the pitot orifice on the corresponding point in the pipe. Readings are then taken at each of the 24 points and the center is read as often as it seems necessary to detect any change of rate of discharge in the pipe, it being necessary, of course, to complete the traverse under uniform velocity conditions.
Reading the Tube.
In order to prevent violent fluctuations of the manometer liquid the tube was “choked” by “nearly closing” one of the admission pinch cocks. This also tended to give an average reading, all readings (except L-55-45 number 3 which was very rapidly read) was the mean value of an oscillation. The highest and lowest points of a surge were read and one-half their sum was taken as the value of the deflection.
The manometer liquid was mixture of carbon tetrachloride and non-crystallizable benzol having a specific gravity of 1.50 at 15° C.
Calculation of Velocity.
The velocities were calculated from the formula V=1.179 d. Where d= deflection in inches and V = velocity in feet per second. The constant 1.179 being a product of the rod co-efficient .72, the specific gravity factor √10.50 the constant 2g=√64.4 and the factor √12, to reduce from ft. to inches.
Attached herewith are the following: Four sheets of field notes of eight traverses, four on each tap. Two sheets of traverse curves—four to each sheet.
The average value of the pipe co-efficients obtained was 0.962.
The large value of this co-efficient is due to the effect of the curved pipe upstream from the taps, and as shown in the curves it is a normal effect and characteristic of the conditions. It undoubtedly is constant condition, under all practical velocity values.
No Corkscrew Motion.
Careful rotation of the pitometer rod showed no corkscrew or helical flow in the pipe.
There was no evidence of pulsating flow.
John Coates & Thos. Lambert, Pitometer Operators.
Win. M. Crowe, Engineer in Charge.
After the acceptance of No. 7 pump described above, the pitot rod was left in the pipe, and connected up to a U-tube or differential manometer inside the station.
The manometer reads in inches deflection of carbon tetrachloride and is observed constantly by the pump attendant. Any falling off in the deflection serves to warn of something amiss, such as, for example, loss of suction of breaking of the water seal. It serves therefore as a valuable accessory in the operation of the pump. In addition, readings of the deflection are taken every hour and noted in the station log. They are referred to a table of capacities against deflections, and worked up into daily discharge in gallons by one of the station clerks.
The pitometer tap on the 48″ discharge main being located outside the building it was necessary to drill through the 18″ brick and concrete foundation wall of the stall wall to get the piping connections into the building. The connections are all of 1/4″ brass and substantially hung and connected.
In connection with the reading of the manometer it might be added that this is done by the attendant who happens to be on duty. No great difficulties were en countered in securing the hearty co-operation of the pump attendants in the rather tedious task of reading the height of the pulsating liquid. Only one refine ment was introduced, namely a correction for specific gravity variation due to changing temperature. A table of correction factors against temperatures is fur nished to the clerk who works up the daily reports and he takes care of this element without trouble.
The above is a typical example of what may be ac complished with an ordinary commercial pitometer rod and an ordinary U-tube. Indeed, its field of usefulness may be greatly extended if a skilled operator, acquainted with its underlying principles is available. Not only water measurements but air also may be measured with equal facility with the ordinary pitot rod. In Fig. 2, is shown part of the results of a pitometer test of a blower discharging into a 20″ pipe.
High Velocity Measurements Most Difficult
Contrary to the general belief, the most difficult pitot measurements are not those of low velocities but those of high velocities. This is so because high velocities are generally accompanied by disturbances of flow whose effect if they can be reckoned at all, can only be discounted at their true worth by a careful and searching analysis of the traverse curves and the conditions accompanying the compilation of the data from which they are platted. This is particularly true of pump discharge when the pitometer is located near the pump. A so-called “50 million gallon” pump at Torresdale, when properly tested with the pitometer showed only a 28,000,000 maximum rate. This pump had been credited with that amount for a long period on the strength of the manufacturers rating and a previous pitot test with inferior instruments. This highly important revelation of the pitometer was afterward checked absolutely and the pump is now often spoken of in the department as the “so-called 50 million pump.”
Yet this test was made under the most unfavorable conditions possible. The pump which is a double centrifugal driven by one steam turbine, discharges into a surface condenser, and thence into a 42″ cast iron pipe which, after passing through the foundations of the station, hooks into an eleven-foot header only a few feet below the condenser. Unfavorable location was, therefore, unavoidable.
The hydraulic conditions were very complicated and it seemed for a time that good results were impossible of attainment. It was found, as was to be expected, that in the pipe the water was in a greatly disturbed state. In the first place there were a helical or corkscrew motion of the water. See Fig. 3, where the shaded portion under the velocity curve on the right represents the Vector effect of this helical flow.
In addition, there were eddies and “pockets,” so that the liquid in the tube fluctuated in a remarkably violent manner. Nevertheless the data was susceptible of analysis and a coefficient was found which was correct within reasonable limits.
It is such conditions as outlined above, that when encountered by the amateur, discourage him, and discredit the pitot tube. There are many of these, and the pitfalls into which he may trip are innumerable. Horse sense in a pitometer man is much more to be prized than much technical knowledge. When the two are combined with manipulative skill the materials from which a skillful pitometer man can be made are at hand.
Important to Remember Everyday Facts
It is important to remember the common everyday facts when analyzing pitot data and avoid getting lost in a maze of mathematical formulae. For instance, it is easy to forget that traverse curve, which is merely the calculated velocity plotted against the pipe diameter, does not by any means represent actual conditions within the pipe. It is merely a method of plotting data and must not be regarded literally. Water in pipes, except under rare conditions does not flow in straight lines. In fact, just how it gets through the pipe is a good bit of a mystery. We do know, however, that it doesn’t follow the straight line or the annula tube flow upon which Weisbach’s formulae and, indeed, most formulae are based. This is the first and greatest menace to the amateur pitometer man, that he will accumulate such a mass of mathematical rubbish that he will forget how to apply common sense—the greatest of aids to the problem before him.
As an instrument of precision the pitometer takes high rank. Given good conditions, and good instruments, results may be obtained of any required degree of precision.
In Figs. 4 and 5 are shown pitot data obtained in straight 48″ pipe, under reservoir flow. The traverses were carefully made, with calibrated rods, and the results are such as to leave no doubt that the pitometer is a precision instrument of the highest degree.
Only One Unavoidable Failure
Its application in the Philadelphia Bureau of Water has been very broad, yet it has never failed to give results, except in one case, namely, a 48″ discharge main at the Mingo Creek Drainage Station, in which air was so churned up with the water in the pipe as to be practically nothing but foam and bubbles. This was a physical condition having no reference to the instrument. In all other cases, it has proved a great and often an indespensable aid to the operation of the plant.
For instance, the chart shown in Fig. 5, is compiled from ink line records, taken from two Simplex pitot recorders installed on each of the two 48″ effluents from the clear water basin of Queen Lane Filters, Philadelphia.
These recorders are of the integrating, recording and indicating type. The integrators calculate the total quantities on each of the instruments in gallons, the rate of flow in gallons per day is recorded on a chart in red ink as shown in Fig. 6 and a needle pointer moving across a scale indicates the rate of flow at any instant. The charts are changed every three or four days, then carefully averaged up with a planimeter, and the results platted as shown in Fig. 5. This is a monthly record of the daily performance of the plant. The figures in the first column marked “Pumpage” are taken from the Venturi meters whose reliability was established, as described at the beginning of this article. Any great discrepancy between the figures in this column and those in the third column entitled, “Total,’’ which is the sum of the quantity registered on the two pitot recorders on the filter effluent, is immediately investigated. For instance a break, even a small one, in the approximately four miles of 48″ pipe between the “pumps” and “filters” shows up on comparing these two columns.
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Philadelphia’s Fight Against Water Waste
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