I. CHEM. E. SYMPOSIUM SERIES NO. 85 THE SMALL-SCALE RELEASE RATE OF PRESSURISED LIQUEFIED PROPANE TO THE ATMOSPHERE

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1 THE SMALL-SCALE RELEASE RATE OF PRESSURISED LIQUEFIED PROPANE TO THE ATMOSPHERE M.R.O. Jones* and M.C. Underwood* Pressurised liquefied propane has been released to the atmosphere down open-ended pipes of diameter up to 4.5mm. The mass flowrate was measured for a variety of pipe lengths, and the results compared with a semi-empirical predictive model. In addition, the flowrates of propane and several Freons down pipes terminated by apertures of smaller area were recorded, and common behaviour was observed. INTRODUCTION Liquefied gases, such as propane and ammonia, are often stored at ambient temperatures. When the corresponding saturated vapour pressures exceed atmospheric, loss of containment results in release to the atmosphere. A knowledge of the possible release rate is important where a toxic or flammable material is concerned, in order to assess the hazard. We consider here the sudden release of such material, from the liquid phase within a container. As such liquid is depressurised, it becomes superheated, and boiling can occur. When this boiling, or "flashing off", begins in pipework two-phase vapour/liquid flow results. Under these circumstances, the flowrate will be less than that of a single phase liquid, partly because the vapour component effectively reduces the average density, and possibly because of a critical flow phenomenon, which can maintain the pipe exit pressure at a value exceeding atmospheric. A recent reanalysis of published experimental data () has shown that the expression often used to predict the two-phase mass flowrate, for open-ended horizontal pipes, is not satisfactory. An alternative expression was given which predicted the mass flowrates of water and several Freons with reasonable accuracy. This expression was based upon a simple annular flow model, with an empirical correlation describing the data presented. * Applied Physics Branch, BP Research Centre, Sunbury-on-Thames, Middlesex, TW6 7LN 57

2 In this paper, the results of an experimental investigation involving the release of propane down open-ended pipes are presented. In addition, findings involving the release of propane and several Freons down pipes terminated by apertures of smaller area are described. The latter observations are of considerable practical interest, as leakage from damaged valves or flanges is more common than "guillotine" breaks of pipework. EXPERIMENTAL MEASUREMENTS The experimental apparatus consisted of a pressure vessel containing any of: propane (90-95 percent purity), Arcton (Freon) 2, Arcton (Freon)22, or Freon 4. The container was connected via 4.3 or 4.5mm internal diameter (I.D.) copper tubing, or 4mm I.D. nylon tubing, to a 6mm I.D. ball valve. The pipe diameter constituted the narrowest constriction to flow up to the ball valve. Different apertures could be inserted after the ball valve, comprising:- a set of circular apertures with diameters, d a, in the range 0.5 to 4mm, and a/da 3; a circular aperture of d a = 0.5mm, and a /d a = 0; and three short rectangular apertures with height to width ratios of 5.5, 0.2 and All experiments were performed outdoors, and the ambient temperature was used to determine the SVP. The mass flowrates were measured using a calibrated load cell or strain gauge to determine weight change of the pressure vessel as a function of release time. Using the copper pipes, p /d p could be varied over the range 7 to 767. In addition, by using the 0.5mm I.D. aperture, an effective p /d p of 0 was obtained, with the pipework upstream behaving as a reservoir. Measurements were made in this way for propane, with temperatures in the range 5 to 20 C. Data obtained for Freons 2, 22 and 4 have already been presented (). The propane data was processed in the same manner as before (). This involved calculating a theoretical flowrate from an annular theory: A modified discharge coefficient, C DM, was then evaluated by dividing the experimentally observed mass flowrate by W T. The range and mean of C DM 58

3 values so calculated for propane are presented in Figure. In addition, the empirical correlation obtained before, for d p in the range 0.5 mm to ~6.5 mm, is presented. This is: The agreement between the required and predicted values of C DM is extremely good. No simple method for assessing the effect upon two-phase flowrate of terminating a pipe of area A p with an aperture of area A a, where A a < A p, is known to the authors. One might expect the flowrate to scale linearly with aperture area, A a, provided the flow remains two-phase. This was investigated using the apertures described above to terminate pipes of large p /d p, down which the flow was two-phase when unterminated. The aperture length, l a, was kept small (ie for circular apertures a/da 3) so that the length of the aperture did not itself constitute a significant constriction to flow, as it follows from the above that an increase in aperture length results in a decrease in two-phase mass flowrate for a given aperture area. Measurements were made for Freons 2, 22 and 4, as well as for propane, varying p /d p from 24 to 67. Mean flowrate values, normalised to the unterminated mean flowrate, are presented in Figure 2, plotted against (A a/a p) ½. The point spread was such that no flowrate datum differed from the mean by more than 0 percent. A surprisingly consistent behaviour is evident, regardless of aperture shape and liquid type. 59

4 Interestingly, the two-phase mass flowrate does not scale linearly with A a/a p, as might be expected for a single phase liquid, but with a p 2 (A /A ). The data may be conveniently divided into three regions:- (i) 0 (A 2 a/a p) < In this region, metastable liquid flow was observed. The flowrate was adequately described by a single phase, incompressible liquid expression using the aperture area only, and a pressure drop to atmospheric. (ii) a p (A /A ) < 0.9. In this region the data obeyed a linear 2 relationship with (A a/a p). A least squares fit to all the data in this region was obtained, shown by the line in Figure 2, giving a straight line of gradient.5. (iii) 0.9 (A 2 a/a p).0 close, or equal, to the maximum mass flowrate.. In this region the flowrate was very CONCLUSIONS The experimental mass flowrate results for open-ended pipes presented here, and in reference, are well described by equation, when the empirical term C DM is included to account for pipe geometry. However, it must be noted that no data relate to releases down pipes of diameter greater than a few millimetres, and so application of the equations at a scale likely to be of practical interest requires substantial extrapolation. It remains to be demonstrated that such extrapolation is valid. However, caution is advisable because the findings of other authors (2,3) suggest a variation of C DM 60

5 with l p only, whereas a variation with l p/d p is used here. Should mass flux depend solely upon pipe length, then tne C DM value given by equation 6 overpredicts mass flowrate as d p is increased. Experimental data for a significantly larger value of d p (ie > 0mm), and a wide range of liquids and l p/d p ratios, would be useful in clarifying this issue. For a pipe within which two-phase flow occurs when terminatd by an aperture, the associated mass flowrate does not scale linearly with the aperture to pipe area ratio. However, on the basis of results obtained for pipe diameters up to 4.3mm, the flowrate does scale linearly with the square root of this ratio, for 0.25 < (A a/a p) ½ < 0.9. No marked dependence on aperture shape or liquid type was noted. Again, the use of this finding at a scale likely to be of practical interest requires substantial extrapolation, and caution should be exercised. Nevertheless, the result is interesting, and serves to illustrate a marked difference between single and two-phase flow. ACKNOWLEDGEMENT We thank The British Petroleum Company p.l.c for permission to publish this paper. REFERENCES () Jones, M.R.O, and Underwood, M.C., Chem Eng J, 983, 26, p.25. (2) Fletcher, B., paper presented at AIChE Loss Prevention Symposium, Denver, 983. (3) Kevorkov, L.R., Lutovinov, S.Z., and Tikhonenko, L.K., Thermal Engng (Trans of Teploenergetika), 977, 24(7), p.58. NOMENCLATURE A a Area of aperture terminating pipe (m 2 ) A p Pipe internal area (m 2 ) C DM d a d p Modified discharge coefficient Diameter of circular aperture terminating pipe (m) Pipe internal diameter (m) I.D. Internal diameter l a l p Length of circular aperture terminating pipe (m) Pipe length L Average latent heat of vaporisation between T and T C(Jkg ) P Pressure within containment vessel, equal to liquid SVP at T (Nm 2 ) P c Pipe critical exit pressure (Nm 2 ) R g Pipe fractional area occupied by vapour 6

6 S Average specific heat capacity of liquid between T and T c (J kg k ) SVP Saturated vapour pressure T T 2 T c Ambient temperature (K) Liquid boiling point at atmospheric pressure (K) Liquid boiling point at P c (K) W T Theoretical flowrate, from equation (kgs ) x c Fraction of total mass which vaporises for pipe exit conditions ρ g Density of vapour at P c and T c (kgm 3 ) ρ l Density of liquid at P c and T c (kgm 3 ) ρ n Average liquid/vapour density for annular flow (kgm 3 ) 62