CHAPTER 1: LIQUID DENSITY MEASUREMENT Objective Calculate density and specific gravity f fluids using ydrmeters, and investigate te dependence f tese variables it temperature. Intrductin Liquid density is an imprtant caracteristic used t prvide infrmatin cncerning cmpsitin, cncentratin, mass fl in fuels, and calric cntent. Density is expressed as mass per unit vlume but is ften expressed in terms f specific gravity ( ), ic is te rati f te uid density t te density f ater bt taken at te same temperature and pressure. P,T...(1-1) Petrleum industry frequently uses te specific gravity 60/60, ic means tat te densities f bt, uid and ater ere measured at 60 F ( 0.999041g/cm ) and atmsperic 60 F pressure. Te petrleum industry uses anter measure, degree API gravity, based n il specific gravity at 60/60: API 141.5 60/ 60 131.5...(1-2) Nte tat te ligter te fluid te iger te API gravity. API gravity is ne factr tat determines te price f il per barrel. Te Hydrmeter (Mdified frm Reference 1) 1-1
API gravity is measured using a ydrmeter, ic cnsists f a eigted flat it a small diameter indicatr stem at te tp as sn in Figure 1-1. Te stem is graduated in any density units (API used in petrleum industry). Accrding t Arcimedes principle, en a bdy is immersed in a fluid it lses eigt equal t te uid eigt displaced. Te ydrmeter element is a cnstant eigt bdy (cnstant buyant frce), ic ill displace different vlumes f fluid fr different fluid densities. Terefre te amunt f stem submersin is an indicatin f fluid density. Figure 1-1. Hydrmeter Hydrmeters are basic t density measurement. Tey are accurate, frictinless, direct indicating itut need fr mecanical linkages r external energy surces, and are cmpatible it mst crrsive fluids. Teir limitatins are in te fluid prperties tey can andle and in teir pressure and temperature ratings. Because fluid density suld be te nly factr affecting te flat psitin, frictin, turbulence, and viscsity effects must be minimized. Additinally, because te basis fr teir peratin is te cnstant eigt flat, material build-up n te flat cannt be 1-2
tlerated. Fr tese reasns, ydrmeters are ideal fr clean, nn-viscus fluids. Hever, tey can be used fr crude ils it gd accuracy. Hydrmeter scale is nn-linear as sn next; te buyant frce balances te ydrmeter eigt (See Figure 1-2): F b W V * g V *...(1-3) Were F b is te buyant frce, W is te eigt f te ydrmeter, V is te displaced vlume f uid it specific eigt, and g is te gravitatinal cnstant. Figure 1-2. Hydrmeter Frce Balance If te specific gravity f t different fluids is measured, te buyant frce f bt fluids must equal te eigt f te ydrmeter because te measurement is taken en te system (ydrmeter-fluid) is in equilibrium (i.e. F 0 ): W W V V * * * * Were is te specific gravity f te first uid and is te specific gravity f te secnd fluid. is te variable marked in te ydrmeter stem (Figure 1-2). By inspectin, 1-3
V * A. Slving fr te respective submerged vlumes, ten taking te difference f vlumes: V * W / * A W / * Slving fr (te increment in te mark): W A * *...(1-4) Equatin 1-4 ss tat te increment in te mark is a nn-linear functin f te specific gravity f te fluids. Density Measurement Using a API Gravity Hydrmeter Place te fluid sample in a cylinder and submerge it int a cntrlled temperature bat. Submerge a clean ydrmeter in te fluid cntained in te cylinder. Release te ydrmeter int te fluid slly. If te ydrmeter is nt itin te API gravity range f te fluid it ill tuc te bttm f te cylinder and crack. Tere suld be a clearance f 1 inc beteen te bttm f te ydrmeter and te bttm f te cylinder cntaining te fluid sample. Avid etting te entire stem f te ydrmeter, since te eigt f te uid n te stem ill affect te reading. Depress te ydrmeter t abut 2 scale divisins int te uid, apply a spin (tis ill elp te ydrmeter t remain in te center f te cylinder) and release it. Wen it cmes t rest, te scale in te ydrmeter stem suld be read at te principle surface f te uid (te bttm f te meniscus in mst cases). Te reading ill give te API gravity at te selected temperature and atmsperic pressure. Tis value is crrected using Table 5A f Reference 3. Te flling example illustrates te prcedure fr temperature crrectin. Example: If te API gravity reading bserved n a ydrmeter in an il at 77 F is 63.5. Wat is te API gravity at 60 F? (Taken frm Reference 3). 1-4
Slutin: Enter te table in te clumn API gravity at bserved temperature, ead t 63 API and recrd te API gravity at 60 F and d te same fr 64 API: API @ 76 F API @ 60 F 63 61.0 63.5 X 64 61.9 Using linear interplatin: X 61.9 61 * 64 63 63.5 63 61.45 61.5 API@ 60 F Once te API gravity at 60 F as been determined, te specific gravity f te fluid at te temperature f measurement is calculated using te flling crrelatin (frm Bear-72): 60/ 60 T...(1-5a) 1 a T 60 a exp 0.0106* API 60 F 8.05...(1-5b) Te density f te fluid is ten calculated by multiplying te specific gravity at measurement temperature by te density f ater at te same temperature. Te flling crrelatin can be used t cmpute te density f distilled ater at any temperature (Taken frm Reference 5): 8 3 6 2 6 1.30753*10 * T 5.44149*10 * T 6.00025*10 * T 1.00039...(1-6) Were is te density f ater in g/cc at desire temperature T C. Labratry Experiments 1-5
Part1: Hydrmeter. Place fluid sample in a cylinder. Measure te API gravity at rm temperature using te apprpriate ydrmeter. Make sure t fll te instructins f a ydrmeter given in tis capter. Use cntrlled temperature bats t raise te temperature f te fluid sample and take fur r five measurements at different temperatures. Part 2: Pycnmeter measurement Measure fluid density at rm temperature. Measure fluid density at same temperatures used in te ydrmeter measurements. References 1. Petrleum and Cemical Engineering Department, PETR 345 Lab Manual. Fall 1999. 2. Giles, Rnald V., Fluid Mecanics & Hydraulics, Scaum s Outline Series, Mc Gra-Hill, (1962). 3. Table 5A-Generalized Crude Oils D1298-85, Crrectin f Observed API Gravity t API Gravity at 60 F: Manual f petrleum Measurement Standards: Capter 11.1, Vlume 1, (ANSI/ASTM 1250-80)(API Std 2540), American Petrleum Institute, Wasingtn D.C. (1987). 4. Bear Jacb, Dynamics f Fluids in Prus Media, Dver Publicatins, INC. Ne Yrk 1972. 5. ttp://baervan.nmt.edu/petrpysics/grup/density-prcedure.pdf (ater density crrelatin). 1-6