Gas-liquid Two-phase Flowmeters With Extracting And Separating Method

Gas-liquid two-phase flows are common in the petroleum,chemical and power industries,but the measurement of their flowrate are still a difficult problem in engineering.In recent years,with the rapid development of modern engineering project,this problem become more urgent to solve,a great deal of investments throughout the world have been made to develop this kind of flowrate instruments.The objective of this thesis, financially supported by the National Natural Science foundation of China key project(59995460),is to research and develop new measurement methods of two-phase flow, and the methods invented in this paper have been granted to be Chinese patents. In the first part of the thesis, a review of the prior arts of two phase flow measurement was made, and these arts can be classified as separation method and Non-separating method, depending on whether the two phase mixture is separated in the process of measurement. In traditional separation method, a large separator is always employed to separate the gas-liquid mixture first, and then meter the flowrate of each phase with conventional single phase flow meters. This solution offers definite advantages, such as metering reliability and flow regime independent. Usually the separation equipment is very large and expensive, and some special pipe lines and metering stations also have to be built. The partial flow separation method, developed in recent years, partially separate the two phase mixture into predominantly liquid and predominantly gas streams before measuring with two-phase flowmeters. Although the separator can be reduced in size and each flow stream only needs to be measured over a limited range of component fraction, the improvement in measurement precision is limited and the cost of flowmeter may increase, for the reason that the gas-liquid mixture is not fully separated, and the number of sensors and instruments needed is doubled. The major disadvantages of Non-separating method lie in the facts that all the sensors and instruments are directly disposed to the two-phase flow, although some of them are non-intrusive. The violent fluctuations and flow pattern changes of two phase flow will make the sensors and instruments be far less stable,reliable and precise than that in single phase flow, and even can not work at all. Except for Nuclear Magnetic Resonance (NMR) and Radio type techniques such as PNA (pulsed neutron activation), other non-separating meters are lack of reasonable theoretical foundations. So far, none of two-phase flow meters can meet the commercial stand: measuring error less than ±5%, flow regime independent, and suitable for use in industrial environment. NMR and PNA are hopeful, but they are too expensive (cost～$105-106) to be used in engineering. The new method proposed in this thesis is called "Extracting and Separating Method or abbreviated to ESM" which break through the prior models of gas liquid two-phase flow measurement (separating or non-separating). The process is that:first,proportionally extracting a stream of gas-liquid mixture(5%～20%)from the two-phase flow to be measured with a distributor,then separating it into single-phase gas and liquid with a small compact separator,after that,measuring each phase with conventional single phase flowmeters and converting the measured values to the two-phase flowrate to be measured according to the extraction ratio,at the end, returning the extracted stream into the two-phase flow pipe again.Because only a small portion of the total flow is extracted, the separator can be reduced 5～20 times in size compared with the traditional separation technique in which all the two-phase mixture is separated, and the volume of the two-phase flowmeter may nearly reach the bulk of a common single phase flowmeter.Furthermore, the ESM is also much better than the most non-separating methods in reliability ,stability ,precision ,measuring range and so on. Because the instruments utilized in ESM system work in the single phase flow, the measurement results would not be influenced by the flow patterns and the transit nature of the two-phase flow. How the "proportional extraction" can be approached to assure that the extracted stream is the representative of total fluid stream or the extraction ratio is stable, is the major task of the thesis. How to construct such distributors is the key problem to be solved in the thesis. Four basic types of distributor were proposed and studied in the thesis: 1. The T-junction type By utilizing the phase split phenomena in T-junctions, a single-phase gas stream is extracted and separated from the total fluid stream, and a conventional flow meter is provided to meter the gas flow rate which then is used to calculate the mass flow rate or mass quality of the two-phase mixture to be measured. The junctions not only act as a distributor but also a separator. Based on a series of experiments, the basic equations on the extraction loop and the main loop were proposed and the equation of extraction ratio was also derived consequently which show that the extraction ratio is not a constant, it varies with the mass quality according to a linear function. Only one of the parameters (the flow rate or mass quality) can be determined by the metered value of extracted gas flow, another parameter has to be metered with other means i.e. this is a single parameter two-phase flow meter. 2. The sample tube type Having tested the extracting behavior of conventional sample tube systematically, a new reformed one was constructed, which has a "S" front port, and a mixer is also added in front of the sample tube. Quite stable extraction ratio were appeared over a big span of flow rates. 3. The rotational drum type. In order to avoid the influence of flow regimes on extracting effectively, the thesis broke through the conventional manner of flow distribution, which divide flow only from "space", and tried to extract flow by "time" sharing for the first time. The extraction stream is proportionally obtained by accurately control the extraction time fraction in which total flow is completely conducted into the extraction loop. The extraction ratio is equal to the time fraction and independent of flow patterns. Because the time fraction can be easily controlled, so the task of assuring extraction ratio stable becomes much simpler. The distributor comprises of a rotational drum and shell.The shell is to support the drum and has not any moveable part. The outline of the drum is a cylinder, the inside space is equally segregated into a series of small flow channels which twisted around the drum axis.The outputs of the most channels are directed to the down stream of the pipe, only a few of the extraction channels are connected to the separator.As two-phase mixture passes through the channels, the drum will be forced to run at a high speed around its axis by the fluid.With the running ofthe drum, the entrance of each channel will continuously scan over every point on the cross section of the pipe, and so the fluid at any point on the cross section of the pipe will have the same possibility to enter each channel. In a rotation period of the drum, the time fraction of fluid at any point on the cross section of pipe flowing into each channel is equal, so the drum seemingly acts as a time controlled switch which equally directs the total flow into each channel, the extraction time fraction and extraction ratio is proportional to the number of extraction channels and equal to the ratio of extraction channel number to the whole channels in the drum.An ingenious proof was made in the thesis, and a further modified equation to consider the effect of liquid leakage through the gap between drum and shell was established too. 4. Swirler type. Contrasting to the rotational drum distributor, there is no any running parts in a swirler distributor that is very suitable for use in engineering.By using two or three swirler assembles and conditioners, the two-phase mixture itself is guided to pass through a special routing to complete the equal distribution in a multi-channel assemble in which each channel has the same geometrical shape but is not connected each other.The outputs of the most channels is directed to the down stream of the pipe, only a few of extraction channels is connected to the separator.Therefore, the flowrate of the stream (the extracted stream)entering the separator is only dependent on the number of channels(the extracting channel) connected to the separator and is independent of the flow pattern and other factors, so the extraction ratio remain constant. Experiments for the four types of ESM two-phase flowmeters mentioned above were conducted in an air-water two-phase flow loop.The inner diameter of the loop was 30 mm and the test section was horizontally placed.The pressure and temperature were near that at room conditions.The air superficial velocity ranged from 4 to 40m/s, and the water superficial velocity varied between 0 and 0.28m/s.The flow patterns observed during the tests included stratified flow,wave flow and annular flow.The experiment results showed that all the four types of ESM two-phase flowmeters mentioned above have stable extracting ratio or the extraction ratio change with definite manners.The flowmeters were able to work steadily and accurately over a wide range.The average error was less than 5%, and the best results ever reached was within 3% . The ESM with Chinese intellectual property provide a new way to solve the difficult problem of two-phase flow measurement in engineering.By extracting, the size of separator can be reduced greatly; by separating, the measurement of two-phase flow is translated into that of single phase flow and by choosing an appropriate distributor very stable extraction ratio can be obtained.All these would make it possible for the two-phase flowmeters to approach or even reach the precision of single phase flowmeters.The cost of a ESM type two-phase flowmeter is about 30000 RMB (a NMR two-phase flowmeter with the precision of ESM, costs 105 ～ 106 US dollar). So two-phase flowmeters would also be as largely manufactured and widely used as single phase flowmeters.