Chinese Journal of Chemical Engineering, 16(2) 320 324 (2008) RESEARCH NOTES

Performance of a Horizontally Mounted Venturi in Low-pressure Wet Gas Flow*

FANG Lide ( )** and ZHANG Tao ( )Key Laboratory for Process Measurement and Control, School of Electrical Engineering and Automation, Tianjin University, Tianjin 300072, China

Abstract The performance of a Venturi tube used in wet gas flow have been explored mainly under higher-pressure condition, but very often, low-pressure test exists in some oil and gas fields in Tianjin Dagang Oil and Gas Field in China. In this study, the performance of horizontally mounted Venturi meters in low-pressure wet gas flow is dis-cussed. Three 50 mm Venturi meters were tested systematically, with values of 0.4048, 0.55 and 0.70, the opera-tion pressure of 0.15 MPa, 0.20 MPa, 0.25 MPa, the gas densiometric Froude number from 0.6 to 2.0, the modified Lockhart-Maretinelli parameter from 0.0022 to 0.06, and the ratio of the gas liquid mass flow rate from 0.5 to 0.99. The effects of modified Lockhart-Maretinelli parameter, pressure, gas densiometric Froude number, diameter ratio, and gas-liquid mass flow rate ratio to the Venturi tube are analyzed with new independent data. Furthermore, low-pressure performance was compared with that under high pressure. Keywords gas-liquid two phase flow, wet gas metering, Venturi meter, over-reading

1 INTRODUCTION

Wet gas has been defined as 90% gas, 95% gas and even 98% gas by different technical papers [1].Normally, two ways are employed to meter wet gas, one is to use a multiphase flow meter in wet gas, while the other is to use a standard dry gas meter and apply corrections to the measurements [2].

As a mature single-phase flow measurement de-vice, the Venturi meter (or tube) is easily to be con-sidered for two-phase flow measurement [3]. Some tests have been done under high-pressure wet gas loop with high Lockhart-Maretinelli (LM) parameter to study the over-reading of Venturi meters in wet gas condition. In Steven’s paper [4], a 6˝ Venturi meter with diameter ratio ( ) of 0.55 was installed in NEL (National Engineering Laboratory UK) with pressure 2 to 6 MPa and Lockhart-Martinelli parameter 0 to 0.3. After that, NEL engineers Stewart and Boam pre-sented two reports and a paper [5 7], three 4˝ meters were chosen with different values (0.4, 0.60, 0.75) and tested over a range of pressure (1.5 6.0 MPa), gas velocity [gas densiometric Froude number (Frg)0.5 5.5] and liquid fraction (Lockhart-Martinelli pa-rameter X 0 0.4). Furthermore, Britton et al. [8],Kegel [9] did some tests in Colorado Engineering Ex-periment Station with pressure under 1.4 7.6 MPa, and Lockhart-Martinelli parameter between 0 and 0.25.

However, very few full tests were done under low-pressure wet gas condition, so the authors imple-mented a full test program at Tianjin University (TJU) multiphase flow loop to cover this range. The results add to the experimental database, and provide neces-sary reference for the application of Venturi meters in low-pressure wet gas metering.

2 OVER-READING THEORY

Defined over-reading as Eq. (1):

g tpR

g g

'm PO

m P (1)

T g gg 4

2

1

C A Pm (2)

T g tpg

4

2

1

C A P'm (3)

the real gas mass flow rate can be corrected to

gg

R

'mm

O (4)

Generally, the over-reading caused by two factors: one is the liquid presence in gas, which caused a blockage effect; the other is gas-accelerating effect to liquid, which caused a drag pressure drop.

3 EXPERIMENTAL APPARATUS

Each Venturi meter was calibrated in dry gas at each test pressure prior to the respective wet gas tests, to establish a dry gas baseline performance against which the wet gas results could be compared. Both the Venturi calibrated test and wet gas test were conducted on TJU multiphase flow loop at pressure from 0.15 MPa to 0.25 MPa across a range of gas velocity. TJU’s low-pressure wet gas test facility is a complete system with full automatic control and multiple functions, which is not only a multiphase flow experiment sys-tem, but also a multiphase flow meter calibration sys-tem. As an experiment system, the test can be con-ducted in horizontal pipe, vertical pipe and 0 90° in-clined pipe. As a calibration system, the test meter can be calibrated in standard meter method. Fig. 1 shows schematic of TJU multiphase flow loop.

Received 2007-05-08, accepted 2008-02-12. * Supported by the National High Technology Research and Development Program of China (2006AA04Z167, 2007AA04Z180).

** To whom correspondence should be addressed. E-mail: leed_amy@yahoo.com.cn

Chin. J. Chem. Eng., Vol. 16, No. 2, April 2008 321

Figure 1 Schematic of TJU multiphase flow loop 1 liquid tank; 2 air inlet; 3 compressor; 4 cooler system; 5 gas tank; 6 liquid pump; 7 9 Alicat mass flow controllers; 10 vortex flow meter; 11,13 roots flow meter; 12,15,16turbine flow meter; 14 electrical flow meter; 17 horizontal experimental pipe; 18 vertical experimental pipe; 19 watertower; 20 gas liquid separator; 21 gas liquid mixer

The facility makes up of six components, which are medium source, single phase flow pipe and stan-dard meters, horizontal pipe, vertical pipe, 0 90° in-clined pipe and computer control system. Gas is com-pressed air provided by two compressors, and it passes through cooling and drying units to access to two 12 m3 tanks. With these tanks, the pressure-regulating valve can hold a stable pressure 0 0.5 MPa for the test. The liquid used is water (oil or oil and water mixture also can be used) and a water pump pushes the water to a 30 m high water tower, which can hold a stable pressure for driving liquid flow. The uncertainty of the liquid standard meters is 5‰, and the gas standard meters is 1%.

Gas and liquid whose flow rates have been meas-ured through standard meters enter the mixer which makes the liquid become 100 1000 m mist, and then go through the 50 mm experimental pipe (Fig. 2). The ball valve installed 14D upstream of the meter is used when maintaining the metering system. Generally, the valve remains full open not to perturb the flow. An ball valve which adjusts the test pressure is installed at the outlet of the experimental pipe. The ball valve has a hole on the ball with the same diameter as upstream pipe.

According to ISO5167-1, 4 (2003), a classical Venturi tube with a machined convergent section must have its straight length and diameter ratio according with Table 1.

In this test, three Venturi tubes with value equal to 0.4048, 0.55 and 0.70 were fabricated, the length of Venturi tubes is 388 mm, diameter is 50 mm, the length of cylindrical throat is 20 mm, conical conver-gent angle is a constant of 21°, conical divergent angle is 12°, the pipe wall roughness is 0.06 mm, and stainless steel flanges used in connection [10, 11]. Type 1151 differential pressure transducers from Rosemont were used, and the uncertainty of pressure transducers is 2.5‰

The experimental parameters are listed in Table 1.

Table 1 Experimental parameters Diameter

ratioPressure/

MPa FrgLockhart-Martinelli

parameter

0.4048 0.15 0.8 1.5 0.0022 0.0338 0.20 1.0 1.88 0.0022 0.0472 0.25 0.67 1.81 0.0022 0.0495

0.55 0.15 1.04 1.78 0.0022 0.0572 0.20 1.09 1.85 0.0022 0.0431 0.25 0.92 1.73 0.0022 0.0514

0.7 0.15 1.04 2.0 0.0024 0.0480 0.20 1.08 2.0 0.0025 0.0525 0.25 0.87 1.66 0.0027 0.0576

4 RESULTS AND ANALYSIS

4.1 Effect of influencing parameters

It has been showed that under high-pressure the over-reading of a Venturi was dependent on the gas Froude number Frg, Lockhart-Martinelli parameter X,gas density g (or pressure P) and the Venturi value. Lockhart-Martinelli parameter is defined as

g g g

l l lP mX

P m (5)

The gas densiometric Froude number (Frg) is:

g gg

gl

FrgD

(6)

Besides these parameters, the slip ratio also has significant effect to the over-reading. Based on the momentum balance and separated flow theory [12 14],the over-reading can be expressed theoretically as

Figure 2 Horizontal experiment pipe

Chin. J. Chem. Eng., Vol. 16, No. 2, April 2008 322

g 2R

g

11 l

l

O X XSS

(7)

Equation (7) shows that slip ratio S is also a fac-tor influencing OR, but the slip ratio is hard to be de-termined accurately, so it needs to find another pa-rameter to substitute it. de Leeuw used the gas den-siometric Froude number to reflect gas liquid velocity ratio [15], but Eq. (6) can only reflect superficial gas velocity, not including liquid velocity. The parameter reflecting gas liquid velocity ratio is gas-liquid mass flow rate ratio, which can be shown as

g g

1 1l l

m xS

m x (8)

Equation (8) shows that gas-liquid mass flow rate ratio are related with gas-liquid density ratio, gas cross-section fraction and slip ratio, and it can be ac-quired with the ratio of the gas to total mass flow rate.

Next, the effects of the gas Froude number Frg,Lockhart-Martinelli parameter X, pressure P, the Ven-turi value and gas-liquid mass flow rate ratio

/(1 )x x to over-reading will be analyzed.

4.2 Effect of Lockhart-Maretinelli parameter

As expected, over-reading increases with in-creasing Lockhart-Martinelli parameter X, in agree-ment with almost all previous work under middle and high-pressure on this subject (Fig. 3). It is mostly due to the liquid, which reduces the gas passing area and brings about a blockage effect to the gas and an accel-erating effect to the liquid.

Figure 3 Venturi over-reading for all diameter ratio and pressure value: 0.4048; 0.55; 0.7

4.3 Gas Froude number

NEL (National Engineering Laboratory, UK) re-ported that some of the data seemed to tend to a value slightly above unity particularly at low X values [5 7].But this prediction was not the case under low-pressure and low Lockhart Martinelli parameter with X less than 0.05. It is found that OR differs for different gas Froude number (in Fig. 4), mainly be-cause that the gas liquid density ratio is lower under low-pressure. According to Eq. (6), for given Lockhart Martinelli parameter and gas Froude number, the gas velocity under low-pressure is higher than those under

high-pressure, so over-reading is affected obviously by the gas Froude number.

Figure 4 show the over-reading is less than 1 un-der low gas Froude number and low LM parameter as observed in many investigations. The over-reading less than 1 means that the differential pressure under two phase flow is less than it under signal gas phase flow when the gas velocity is small. The presence of a small amount of liquid may block the gas flow chan-nel, increasing the friction pressure drop, but, liquid has a lubrication action at the pipe wall, which de-creases the friction pressure drop. Under low gas Froude number and low LM parameter The over-reading less than 1 suggests simply that the blockage effect is weaker than the lubrication action.

Figure 4 Venturi over-reading in different Frg for 0.55 at 0.15 MPa Frg: 0.7; 1.1; 2.0

4.4 Pressure effect

Pressure also affects over-reading under low- pressure and low Lockhart- Martinelli parameter by affecting the gas liquid density ratio directly as shown in Fig. 5. It suggests clearly that the over-reading de-creases with increasing pressure. make the gas density decreasing, From Eq. (6), under the same gas Froude number, the low gas density under lower pressure means higher gas superficial velocity and thus the larger friction. So the over-reading under low pressure is higher than that under high pressure.

Figure 5 Venturi over-reading in different pressure for 0.55 under low-pressure (Frg 2.0)

0.15 MPa; 0.20 MPa

4.5 Effect of Venturi diameter ratio

Figure 6 show over-reading data from 3 Veturis for Frg 1.7 at 0.20 MPa. It is clear from Fig. 6 that the value shows definite effect on over-reading un-der low-pressure. In some condition, the over-reading decreases with increasing value with the exception of 0.70. The decrease of Venturi diameter ratio brings about the decrease of gas passing area in the Venturi

Chin. J. Chem. Eng., Vol. 16, No. 2, April 2008 323

throat, the increase of gas velocity and gas liquid ve-locity ratio, and finally results in the increasing of over-reading.

The over-reading of 0.70 is higher than 0.4048 and 0.55 because of differential pressure of 0.7 lower largely than the other , it can be acquired from Eq. (2),

24g

g 40 g

12

mP

C A (9)

4

g 41

P K (10)

For different , the differential pressure, which varies curvedly only with under the same pressure and gas Froude number, decreases rapidly with the increasing value of (Fig. 7). When the liquid exists in gas, the pressure drop is higher than gas flowing alone, for 0.70, small liquid fraction will make a large over-reading.

Figure 7 Differential pressure varied with diameter ratio in the same pressure and gas Froude number

4.6 Effect of gas liquid quality ratio

Figure 8 shows over-reading versus gas liquid quality ratio for Frg 1.7 at 0.20 MPa. At the same diameter ratio, the over-reading decreased with the increasing gas liquid quality ratio, the curves appeared exponential for all diameter ratio, and the over-reading decreased with the increasing diameter ratio. It is no-ticed that OR at 0.55 was the smallest than other diameter ratio. Fig. 6 shows the same phenomenon as that in Fig. 8. The reason of this behavior needs fur-ther investigation.

Figure 8 Over-reading plots on gas liquid quality ratio for Frg 1.7 at 0.20 MPa

0.4; 0.55; 0.7

ACKNOWLEDGEMENTS

The author would like to thank National Engi-neering Laboratory of UK for providing the reports and professional advice on the web.

NOMENCLATURE

Ag gas occupied cross-sectional area of pipe, m2

Al liquid occupied cross-sectional area of pipe, m2

AT area of the Venturi throat, m2

C discharge coefficient D inner diameter of upstream line pipe, m Frg gas densiometric Froude number defined by Eq. (6) g gravitational acceleration, 9.81 m s 2

mg modified gas mass flow rate, kg s 1

gm' apparent gas mass flow rate, kg s 1

OR over-reading Pg superficial gas differential pressure, kPa Pl superficial liquid differential pressure, kPa Ptp actual two-phase differential pressure, kPa

S slip ratio g( / )lS u uug real gas velocity, m s 1

ul real liquid velocity, m s 1

g superficial gas velocity, m s 1

X Lockhart-Martinelli parameter defined by Eq. (5) x ratio of gas to total mass flow rate g g[ /( )]lx m m m

void fraction g g[ /( )]lA A A diameter ratio ( /d D ) expansibility factor g gas density, kg·m 3

l liquid density, kg·m 3

Subscriptsg gas phase l liquid phase tp gas-liquid two phase

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Chin. J. Chem. Eng., Vol. 16, No. 2, April 2008 324

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