An App..o~itnate P ..actical Co....elation of tlte Effect of P ..essu..e on tlte Dew Point T etnpe..atu..e of Pipeline Natu..al Gases

By G. W. GOVIER* and K. AZIZ**

(13th Annual Technical MeeNng, , May, 1962) Downloaded from http://onepetro.org/JCPT/article-pdf/1/01/14/2165694/petsoc-62-01-03.pdf by guest on 25 September 2021

ABSTRACT INTRODUCTION normal pipeline gases are influenced not only by the hydrocarbon com­ The contracts for the pipeline The dew point temperatures of position of the gas but also by the normal pipeline gases are influenced transportation of natural gas usu­ presence of traces of contaminants not only by the hydrocarbon com­ ally limit the higher hydrocarbon such as hydrate depressants and position of the gas but also by the content of the gas by specifying a compressor oil. For this reason, presence of traces of contaminants maximum hydrocarbon dew point. such as hydrate depressants and dew point temperatures calculated compressor oil. For this reason In contracts specify that from gas analyses are frequently dew point temperatures calculated the hydrocarbon dew point be lim­ lower than the observed dew points from gas analyses are frequently ited to 15°F. up to the max­ lower than observed dew points and and the effect of pressure on the imum operating pressure of the dew point temperature is not the the effect of pressure on dew point pipeline. In order strictly to comply temperature is not the same as for same as for a natural gas composed with this specification the dew point a pure hydrocarbon gas. entirely of hydrocarbons. temperature must be measured Actual dew point measurements It seems desirable then to base on 16 gases were compared with cal­ directly at the maximum operating culated dew points and differences pressure. Usually, however, the the estimation of the actual dew up to 50°F were observed. Not· gases are available only at pressures point at the maximum operating withstanding these serious differ· below the maximum pipeline oper­ pressure on measurement at some ences the shape of the actual and lower pressure and the general the calculated dew point tempera­ ating pressure and a direct measure­ ture-pressure curves were similar. ment is not practical. Under these shape of the dew point curves. This similarity has permitted the conditions the engineer must either The work reported in Reference development of a practical correla­ tion of the effect of pressure on the (a) calculate the dew point at the (1) was extended to check the semi­ dew point temperature of the gases. desired higher pressure dir­ theoretical correlation against the The correlation permits the esti­ ectly from the avaliable gas actual measured dew points of a mation of the dew point tempera­ analysis, or number of Alberta pipeline gases, ture at any pressure within the (b) estimate the dew point at the and to develop a method which range of 100 to 800 psi from its would allow the prediction of actual knowledge at anyone pressure higher pressure from dew within this range. The accuracy is point measurements at somf' clew point temperatures at any about -t-5°F for pressures in the lower pressure. pressure from a measurement at range of 400 to 800 psi. The corre­ some other pressure. lation, along with calculations of the A companion paper by Aziz and theoretical dew point, may also be Govier (1) has presented a semi­ used to estimate the degree of con­ theoretical correlation which facili­ FIELD MEASUREMENTS tamination of pipeline gases. tates the calculation, from gas A standard Bureau of Mines Dew analysis, of hydrocarbon dew points Point Tester, equipped with a dead of clean pipeline gases. The mea­ weight tester for pressure measure­ sured dew point temperatures of ments, was used for all dew point measurements. Dew point data for all gases were .. Professor of Ohemical Engineer­ (1) K. Aziz and G. W. Govier, "A ing, Dean of Faculty of Engineer­ Rapid Approximate Method for obtained, over a period of approxi­ ing, University of Alberta, Ed­ the Estimation of Hydrocarbon mately four months during the monton. Dew Points of Clean Pipeline summer of 1961, at the Alberta Gas Natural Gases," 13th Annual Trunk Line meter stations. Table 1 Technical Meeting, P. & N.G. ** Assistant Professor of Petroleum is a list of the meter stations where Engineering, University of Al­ Division, C.I.M., Calgary, May berta, . 1962. tests were conducted; their locations

14 Journal of Canadian Petroleum PROVOST •

2 Downloaded from http://onepetro.org/JCPT/article-pdf/1/01/14/2165694/petsoc-62-01-03.pdf by guest on 25 September 2021

...J ...J CD

II 10 MEDICINE• HAT FT: MACLEOD• • PINCHER CREEK ~7

NUMBERS IDENTIFY METER STATIONS.

Figure 1

Technology, Spring, 1962, Calgary 15 TABLE 1. VENT THE ALBERTA GAS TRUNK LINE __DEW POINT COMPANY LIMITED METER STATIONS METER APPARATUS 1 Provost North 2 Provost South 3 Sedalia North UPSTREAM DOWNSTREAM 4 Sedalia South ISOLATING ISOLATING 5 Oyen VALVE VALVE 6 7 North # 2 8 Bindloss North #1 DEAD WEIGHT 9 Bindloss South PRESSURE GAUGE-- 10 South 11 Medicine Hat North / 12 Atlee Buffalo 13 Princess Denhart Figure 2 14 Princess Iddesleigh 15 Princess 16 Enchant gary. In some cases a gas sample these gases but they were probably 17 Pincher Creek was analyzed by both laboratories. the glycol-hydrocarbon type of dew 18 Cessford Wardlow In the first stage of the field points. In these cases the calcu­ 19 Cessford East Downloaded from http://onepetro.org/JCPT/article-pdf/1/01/14/2165694/petsoc-62-01-03.pdf by guest on 25 September 2021 20 Cessford West measurements gases from sixteen lated dew points were as much as 21 Cessford Carolside different meter stations (Numbers 140°F lower than the measured 22 Cessford Burfield 14, 22, 36, 33, 18, 24, 1, 23, 15, 31, results. These gases were con­ 23 Countess Duchess 20, 6, 7, 8, 17 and 10) were investi­ sidered to be seriously contaminated 24 Hussar Makepeace gated. For some of these gases 25 Hussar Chancellor by extraneous materials such as 26 Wayne North several analyses were obtained and gycol or compressor oil which are 27 Three Hills Creek used for calculation of the dew not determined in the gas analysis. 28 Innisfail points. The total number of ana­ Because of the high degree of con­ 29 Prevo lyses considered from the sixteen tamination these four gases were 30 Gilby 31 Rimbey gases were twenty-eight. not used in the development of the 32 Nevis South The second stage of the experi­ final correlation. 33 Nevis North 34 Chigwell mental work included gases from The measued dew point tempera­ 35 Wood River eleven meter stations (Numbers 26, tures for the remaining twenty-one 36 Carstairs 27, 29, 30, 27, 31, 13, 10, 19, 25 and gases are presented in Figure 3. 37 Burstall 21). Two of the gases investigated, The data points have been omitted Numbers 10 and 31, were also in­ in the interest of clarity. At 800 vestigated in the first stage of the psia the range of dew points for are shown in Figure 1. The names operation. One analysis for each these gases is -13°F to +26°F. and numbers of the meter stations gas was obtained. No dew point are those used by the Alberta Gas calculations were performed for Trunk Line Company Limited. For CALCULATED DEW POINTS these gases. field measurements of dew points a Calculated dew point tempera­ section of the pipeline containing RESULTS OF DEW POINT tures for a few of the gases were the orifice meter (meter-run) was MEASUREMENTS determined both by manual calcula­ isolated in each case by two valves. Reference (1) contains thirteen tion and with the aid of an LGP-30 Gas samples were taken from this analyses of the ten gases considered. computer. Typical results appear isolated meter-run in carefully The measured and calculated dew in Figure 4. Full details are given purged sample containers. Figure point data for each gas are pre­ in the report to the Alberta Gas 2 shows a typical field installation sented in full detail in tabular and Trunk Line Company (2). The con­ for dew point determination. graphical form in a report submit­ vergence pressures for the gases considered were calculated to be Dew point data were obtained for ted to the Alberta Gas Trunk Line between 2,000 and 3,000 psi. The the gas in the meter-run at approxi­ Company (2). dew point calculations were per­ mately 100 psi intervals, from line No dew points were observed for formed for some of these gases for pressure down to about 100 psig. gases from meter stations 17 and these two convergence pressures To do this, the gas was vented to 21 down to a temperature of -30°F. and also for a convergence pressure the atmosphere thereby dropping This is to be expected of gases of of 5,000 psi. The results for the the pressure in the isolated meter­ this type which contain about 95% convergence pressure of 5,000 psi run at the required intervals. This methane were usually closer to the measured procedure for determining hydro­ For gases from meter stations 6, data than the other calculated re­ carbon dew points was found to be 7, 8 and 10 the dew points should sults. This does not mean, however, very satisfactory. Sufficient quan­ also be below -30°F as these gases that the effective convergence pres­ tities of gas were available to obtain contain about 95% methane. Dew sure of these gases was 5,000 psi. reliable results which could be points were observed, however, for It might rather be a compensating duplicated whenever necessary. For factor for the errors in gas analyses. some of the gases, data checks were The calculated dew points from also made in the University of Al­ (2) K. Aziz and G. W. Govier, "Report on the Effect of Pres­ the LGP-30 were not always the berta laboratory. sure on the Hydrocarbon Dew same as the results manually cal­ All gases were analyzed either at Point Temperature of Alberta culated. This was due to the slight­ Pipeline Gases," Report to Al­ the University or by the Alberta berta Gas Trunk Line Company ly different Kdata in the computer Gas Trunk Line Laboratory in Cal- Limited, September 1961. program and to the method of

16 Journal of Canadian Petroleum 900 800 700 600 500 q en 400 ci .. 300 w / 0:: / ::) / Downloaded from http://onepetro.org/JCPT/article-pdf/1/01/14/2165694/petsoc-62-01-03.pdf by guest on 25 September 2021 / (J) / / / ~ 200 / 0:: / / Cl. / / CD INDICATES GAS N2 I / / / / / / / 100 -40 -30 -20 -10 0 10 20 30 DEW POINT TEMPERATURE, OF Figure 3 the same gas resulted in differences 900 72 82 243 7j 8 1 181 242 31 1 182 in calculated dew points of up to 800 jj ;" i 43°F. The combination of differ­ 700 r \ i .\ ./ E·nces in analyses and calculation 600 11/ [\ il Ii procedure produced deviations in calculations of up to 50°F. This 500 iii .I.i 1.1 I I· shows the unreliability of dew point calculations for normal pipeline /?/JVI/ .. gases based on commercial gas ana­ 400 // /• •• i// lyses and conventional K-charts. .g 300 ••• • j .. . COMPARISONS OF FIELD MEASURE­ ~ MENTS AND CALCULATED 200 IJ//j // DEW POINTS Comparison of the calculated with the measured dew points revealed deviations ranging from -50°F to ~ loo~!lf!~~-! +27°F for the twenty-one gases considered. Deviations from the -80 -60 -40 -20 0 20 40 measured values are believed to be DEW POINT TEMPERA TURE. OF due largely to the presence of con­ taminants such as glycol and com· Figure 4 pressor oil, Some quick method of

handling the Cn + component. For lated results by different methods detecting and determining the ex· the computer calculations the pro­ was 24°F brought about mainly tent of these contaminants, and perties of the next higher hydro­ because of the manner in which finding the true hydrocarbon dew

carbon, C n + 1, were usp.d for the C n -c component was handled and to points in the presence of these con­

Cn + component. The manual cal­ a lesser degree because of the effect taminants, would be very desirable. culations were performed using the of different convergence pressures. No special methods were used in

properties of Cn for Cn + component. The difference in the analyses this investigation to detect the The maximum difference in calcu- obtained from different sources for effect of the contaminants.

Technology, Spring, 1962, Calgary 17 9 7 6 5 ~ 0::: 4 PRESSURE, p.s.i.a. W I­ w 3 ~

z 0.6 LEGEND Downloaded from http://onepetro.org/JCPT/article-pdf/1/01/14/2165694/petsoc-62-01-03.pdf by guest on 25 September 2021 I­ 0.5 EXPERIMENTAL DATA W PRE?SURE SYMBOL 0.4 (p.S.I,O,) 3= 100 -- - ... 0.3 200 --- • w 300 --- , 500 - -- .... > 600 --- .. l­ 0.2 800 - "- - .. t) W*= W AT 600 p.s.i.a. a ACTUAL DEW W 1.L POINT TEMPERATURE. 1.L W 0.1 '---J..._--'--_..l.....-----'-_---'-_.....L.---JC----l....._-'--_"'------'-_--'--_..L.------l_-.J -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 DEW POINT TEMPERATURE, of Figure 5

The general similarity of the for the theoretical correlation as a lines at the higher pressures. curves of Figures 3 and 4 suggested guide, a family of straight lines was the possibility of a practical correla­ drawn through the field data points, The final practical correlation, a tion of the effect of pressure on the one line for each pressure. Serious cross plot of Figure 5, is presented actual dew point temperature of the scattering of the data occurs at pres. as Figure 6. Here, practical dew pipeline gases - notwithstanding sures of 100 and 200 psia but the point pressure-temperature curves their contamination. In the com­ data are well represented by the are given for each of several values panion paper (1) theoretical dew point temperature-pressure curves were developed for various values of a "wetness parameter," W, de· EFFECTIVE WETNESS PARAMETER, W· fined by 0.25 0.30 0.40 0.50 O 1.00 0 ;( 800 , O'E rID 700 w' T y 600 A 500 ...... ", , ", ' .. , !J , 1 "', 0 400 where y=mole fraction hydrocar­ f .1 1< ," ".j j,,"" " • bon vi ., I ••• '.' c:i. / """ A = K value for the hydrocar­ 300 re,} I I ,.',.', '., ,,' bon at 600 psia, OaF J J .,,1 I. '. w I .' J The effective wetness parameter, J c:: • :::> 200 J W*, for each of the 11 gases con­ (f) (f) sidered in the first stage of the field Vt~ I " W J measurements was determined at c:: r ] 600 psia and at the measured dew a.. fl "',' 'I", point temperature from Figure 1 .. "I if- of Reference 1. The actual dew ., '_.' 100 point temperatures, at each of sev­ -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 eral pressures, were then plotted versus W* as in Figure 5. Using DEW POINT TEMPERATURE OF the corresponding figure developed Figure 6

18 Journal of Canadian Petroleum of the effective wetness parameter. ment with the actual 800 psia mea­ pipeline gases and is found to be The correlation may be used to esti­ surement. A scattering of some reliable to within about 5°F for mate the hydrocarbon dew point lOaF is observed. Figures 8 and 9, pressures changes up to 300 psi and temperature of a pipeline gas at one showing the reliability of 800 psia to within about 10°F for pressure pressure from its known value at dew point estimates based upon 500 changes up to 500 psi. another pressure. The procedure and 600 psia measurements indicate merely involves entering Figure 6 scattering of 4 to 6°F. ACKNOWLEDGMENT at the known dew point pressure and temperature and following the CONCLUSION The research reported here was effective wetness parameter lines to A practical correlation, based undertaken at the request of and the desired pressure. upon actual field dew point measure­ under the financial support of Al­ berta Gas Trunk Line Company The reliability of Figure 6 was ments of 11 pipeline gases, has been Limited. tested by comparing the dew point developed for estimating the effect temperature estimated at 800 psia of pressure on the hydrocarbon dew The authors wish to acknowledge from measured values at lower pres­ point. For normal pipeline gases the enthusiastic co-operation of Mr. sures, with the measured dew point the effect of pressure on the actual E. V. Hunt, Mr. C. T. McCall and temperature at 800 psia for each of dew point temperature is very Mr. J. Bulley, all of the Alberta Gas the 21 gases (the 11 on which the small in the range of 600 - 800 psia. Trunk Line Company.

Below 600 psia the effect varies Downloaded from http://onepetro.org/JCPT/article-pdf/1/01/14/2165694/petsoc-62-01-03.pdf by guest on 25 September 2021 correlation was based and 10 a others). The results of the check from less than 5 to nearly 20 F per 100 psi change in pressure. appear in Figures 7, 8 and 9. Figure (3) Katz, D. L. and Associates, 7 relates the 800 psia dew point The correlation has been tested "Handbook of Natural Gas En­ calculated from a 200 psia measure- with the original 11 and 10 other gineering," McGraw-Hill, 1959.

~

.9 40 :& 0 0

Figure 7 Figure 8 Figure 9

Technology, Spring, 1962, Calgary 19