Measurement of the Flow of Gases and Vapors I~AHNETJ E’. DODGE, Chemical Engineering Departmen!,, Yale Ihiversit y, Yea- Haven, Conn.
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?‘lie steadily gruujiiig demand by induslry lactory t o meet t h i s widt. froin tlrr: verystart that the for corg,rol of lms greatly variety of conditions. Iiencc, one finds several different types result of a gas measurestiniulated the study of metho& of measurement rnwt is desired in terms of a ,if meters in list' v ~ u n i eat smne standard conand control and has resulted in m a w i ~ ~ r o aud ~ ~many - variiitions of a givrn rnents in apparalrs urzd technic. I 1 is the genpral type. dit,ion PI in terms of weight if purposeofthispaper lo rt.oieuj so,ne cfthcmo&rrg There are two fuiidarnentd v x p o r is b e i n g m e a s u r e d . melhc,d$ ,for conlircuolA,s~y measuring and re- i.lenicnts in a l l g a s meters: licre is no one staridard condit.iun that has been agreed upon. ( I ) the prirnary element which cording gas J ~ O U J which are in extensine use in l’erhaps the most widely used is insertecl into the gas streair, standard to which gas volinrre industry. The tern1 “gus” W i l l ur&rsfood arid is acted upon by, or exerts is refmed is 60” F., 30 inches to include conderisu6le vapors, such us steam, ari influence on, the flow; but the discussion i d refer more ,$pecijjcally and (a) a sccondary element ur of mrreury (at 32” F.) absolute pressurf!, and complet,e saturnto (Irp not (!ondensed ut dcvice for r e g i s t e r i n g either timi vith water vapor. t h e r a t e o f flow or tho t o t a l atmospheric cmidiliriris. The conditions encountered flow. The classification giveii i n t hc measuremeiit of gas flow below is hnsed cntirdy o i i tlir arc rriany and varied. The voluin(: US liow iiuy vary E n i i i t B first element. Sex thousand cubic feet per hour or even less, to Peveral 1. Differential pressure metera (also illion euhic fet:t. TIle velocity of flow end tile pressur(!, meters) tmiperature, and composition of the gases to be metereti a. Orifice c. Pitot tube iiiay show great differences. Some ga.ses may he readily 1). Venturi d. Shunt metw 2. Volumetric or volume-displacement meters wndensablc or contain conchsable eomponcnts, they ma). a. Wet drum-type rotary meters wiitaiii solid or liquid particlcs in snspension, or they may b. Diaphragm or bellows type ciinlain corrosive constituent.s or sulfur compounds, etc. c. Rotary type The flow may be quite steady or it inay fluctuate over wide d. Gas holders or meter provers 3. Calorimetric or thermal meters limits, and the period of the fluctuations inay be quite diffcra. Thoma meter ent E r m one case to another. The temperature, pressurc 4’ Current type meters or composition of the gas being measured by a given meter a. Turbine meters b. Anemumetem will usually not he constant. There are difficultiesintroduced 5. ~ i ~Methods ~ ] l ~ ~ ~ ~ hy pulsations or by flow disturhances owing to proximity a. Dilution b. Pressure receiver e. Area meters uf the’ meter to fitt.ings in the line. In some cases a high The common feature of all the meters in class 1 (excluding degree of accuracy is essential, and in others a very moderate degree of accuracy is all that is needed. It may he iniportant il for the moment) is the measurement of two pressures whose t,hat the metering device does not offer appreciable obstruc- difference is related to the velocity of flow. Thus, they ditiorr to the iiow, or a considerable loss in pressure may be rectly measure rate of flow rather than total quantity. In readily perrnissible. Those are just a few of the prohiems the case of la and lb, a constriction is inserted in the flow encountered by the flow-meter manufacturer. It is to he line which produces R monxentary increase in the average vrpeeted that no one type of meter will be entirely satis- velocity of flow with a consequent decrease in static pressrut..
T 16 well to bear i n niiud
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A N 1)
E N G I N EER 1N G CH EM IST R Y
The differeiitial pressure or, in the case of tlie orifice, the difference between the static pressure just ahead of the orifice and that just after the orfice is related to the ratc of flow. In the case of the Pitot t.uhe, two pressure-measuring elements are inserted into the stream of fluid, and the differential pressure is the difference betwecn the so-called impact or kinetic pressure exerted on an opening whose plane is perpendicular to the direction of flow and the ordinary static pressure. The Pitot differential is related, not to the average flow in the pipe, but to the velocity a t a p a r t i c u l a r point. The shunt m e t e r is a rather special type of differciitial-I)ressurcmeter and will he treated separately. In the volumetric meters, conipartm e n t s of d e f i n i t e v o l u m e are nlCou,~~~y,B _,"= Instrummi Co. ternately filled with and emptied of F I G U I I E I . T H I N batches or isolated volumes of gas. I'LA'I'E: Om~lCr: Therefore quantity rathcr than rat? of flow is directly measured. Type 211, thougli once of considerable importancr, is now practically ohsoletc for large-scale measurement. of gases mid will not be discussed. Type 211 is used only for testing or calilxating otlicr types of meters and d l receive very brief (:onsidera.tim. The principle of t,hn thermal meters is very simple, cos)sisting merely in adding heat ttr the gps stream; the resulting increase in temperature is a measure of the flow if the rate of adding heat is constant. Conversely, the tempcrnturo rise may be maintained at a constant value, in which ease thc amount of energy dissipated to the gas stream is a measure of the flow. The latter scheme is used in the Thomas meter, the only representative of this class of meters that is used commemially. In the current meters, a fati wheel or turbine is insert.ed into the gas stream, and its speed or rotation is related to tlie velocity of gas flow. Small COWIOW oi ~ a i i M e ~B ICO. ~ ~ meters of this class, known as FIGURE 2. F w w NOZZLE anemometers, a r e useful f o r test purposes; but the principle has not been successfully applied to the continuous measurement of large volumes of gas. The methods in class 5 are not regularly used in rncteririg large volumes, but are mentioned for the sake of completeness and because they are sometimes very useful for special cases. In 5a, a foreign gas is added to the gas stream to he metered and, after complete mixing has occurred, a sample is taken for analysis. From the content of foreign gas and it,s krioivn rate of addition, the rate of flow of the main gas stream is readily calculated, As far as the author is aivarp, this method has not been applied to the commercial metering of gases. I t has proved quite useful for test and experimental purposes. To cite one example, it has been applied on a 1arp soale to determine the volumetric efficiency of blast-furnaet: blowing engines. It has also been used for the calibratiori of other flow meters, notably steam meters. Type 5b consists simply in determining the pressure increase of a constant-volume receiver as gas is pumped into it. I t is useful in testing comprcssors. Type 5c consists of a movable element suspended in the gas stream, the motion of which causes the area through wliich the gas must flow t,o vary. The element will take up an equilibrium position such that its weight is balanced by the differential pressure across the constricted area. Meters of this type have been used to measure the comgrossed-air consumption of drills and other pneumatic tools.
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Vol. 24. No. 3
Oair1i:E >ie.l.EIi
Ilecause of its wide range of application and its many advantages this meter is the most generally used and therefore most important of all the gasmeasuring devices, and onmequently will be dkcussed in greater detail than any other type. Several types of orifice may be used; but the concentric, circular, sharp-edged orifice in a thin plate is the most important, and practically all of the subsequent discussion refem specifically to this type, which will he called tlie "st.andard" type. A typical orifice plate of this type is shown in Figure 1. Another type of orifice, usually called a flow nozzle, is shown in Figure 2. Such a nozzle costs more than the standard orifice, especially in the larger sizes, and is used only in exceptional circumstances. I'racticaUy its only advantage over the standard type is the approximately 40 per cent greater flow capacity for a given pressure differential. Orifices shaped like the segment of a circle (segmental orifice), or circular orificesinstalled eccentrically in the pipe have some special applications to be ment,ioned Iatcr.
C""it0dY
FIGURE 3. DIAGIMM 01ACTIONOF
o/ Bailey Meter team meter. The two chambers beqide the pipe are condensers to aid in maintaining the water levels in the qame horizontal plan(. when the. flow is varying. The orifice plate for a steam meter is also generally provided n-ith a small drain hole that is located a t the bottom of the pipe and allows liquid to pass through the plate instead of accumulating behind it. I n order that the standard values of the orifice coefficients can be relied upon Tyith an accuracy of a few per cent, it is necessary that the orifice be a t certain minimum distances from any fittingP which are apt to cause disturbances of the flow.. This is especially true on the upstream side. It is not possible to give any hard and fast rule to cover this point, but in general the orifice should be not less than 10 to 15 pipe diameters from any fittings or obstructions on the approach side, and a t least 2 diameters on the downstream 4de. U'here the proper runs of straight pipe ahead of the orifice cannot be obtained, straightening vanes (Figure 9) are commonly installed. These are made in various forms, but consist in general of a "honeycomb" of some sort which eliminates the larger eddies or swirls. Corrosive gases introduce special problems. The majority of the differentidl gages are constructed of steel and contain mercury, and hence they would not k,e suitable
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for gases that attack these materials. The gages can be sealed with liquids to prevent the gas from coming in direct contact with the gage, but all gases are soluble in all ordinary liquids and this does not completely prevent attack. The use of absorbent chambers in the pressure lines can be used in some cases. Thus, for gases containing hydrogen sulfide, a purifier filled with a n active iron oxide should be suitable. If a flow indication without record is all that is needed, the orifice installation is very much simplified, and various special materials can be used to combat corrosion. L~DYASTAGES l i x ~ LIJIITATIOSS OF OR~FICE ~ I E T E R The S. orifice method has a very wide range of application. It can be used for almost any rate of flow and for a wide variety of pressures and temperatures. Orifices as large a3 io inches in diameter have been made, and differential gages for pressures as high as 5000 pounds per square inch are available. The first cost is relatively low; and, as the size of the pipe increases, the only part of the meter that increases is the orifice, which is relatively inexpensive. There are no moving parts in the line of flow, and chances of clogging from irnpurities in the gas are a t a minimum. The only part to get out of order is the differential gage, and that can be checked without great difficulty. iln orifice is simple to install and requires little change in existing piping. The space occupied
Courtesy o f .4merican S o c i e t y of lfechanacal E n g i r i e f r s
FIGURE 10.
CROSS SECTION OF V E N T U R I
hIErER
is small. -4given installation can easily be changed to meter different rates of flow merely by changing orifice plates. It is portable and therefore available for test purposes, and several orifices can be connected to one differential gage d i i c h is sometimes useful for plant tests. From the previous discussion of the factors affecting the orifice meter, it should be evident that any figure cited as to the accuracy of an orifice meter mould have but little meaning. Besides all the factors affecting the primary element, there are sources of inaccuracy in the secondary element also that must be included in any over-all estimate of the accuracy of a n orifice meter. When installed and operated under ideal conditions and when all known precautions are taken
March, 1932
the mtio 3 2 i s not accurately known for a wide range of Yai.r. conditiona. The Pitot tube is also more sensitive to disturbances of the Row than either of the other differentialpressure methods. Furthermore, since the Pitot tube does not produce any acceleration of the flow, the differential pressure produced is small except at high velocit.ies of flow,
CowIeav 01 American Me& Co.
FIGURE 13. R E L L ~ WTveE ~
OF
Dis-
P L I C E X ~ N TM ~ ~ t i n
a i d is difficult tu rrie&surt. awuratcly. Thus, air &tatinoe pheric pressure and 70" F.,flowing at a velocity at 20 feet per second, would produce 5 Pitot differential of only 0.14 inch of water. This coiilrl not he measured accurately by the commercial types of differential gage now available. With an orifice, the differential pressure can be brought to any desired value merely by proper choice of orifice diameter. For these reasons the Pitot tube has practically no application to the commercial metering of gases. It is very useful, however, as a portable instrument for test purposes where high accuracy is not required. A convenient form of tube for this service is one in which the two tubes are combined, 88 shown in Figure 11. The coefficient for such B tube is very clcse to unity. The insertion in the pipe line is very easily accomplished, provided the pressure is not far from atmospheric. For hig21cr pressures it would be better to use separate tubes securely fastened through the pipe wall. For low differential pressures some furm of inclined or other multiplying gage would be necessary.
In Figure 12 is shown a recently developed type of meter for small installations, known as a shutit meter, which is a true differentia,l-pressure meter, but which differs radically from the dierential-pressure types previously discussed. The primary and aerondary deviecs are comhinpd in B singlo unit. A rhord typo of nrificc i n t.he main line of tiow ruduces B differential premuro whirh cau8cs n Row through &e nozzles into the shunt circuit. The strram tlrmugh the noazlcs impiopes on small turbine wheel whove fipced of rotation is proprtioml to the square ro
