№9 September 2012Table of contents Issue Archive
№9 September 2012Table of contents Issue Archive
№ 7 (July - August 2012)
Triple-flow vortex tubes (TVT) has become the technology of choice since the late 1990s in Russia for treating gas before transport. Whereas a typical double-flow design, separates cold and hot flows, TVT separates a third flow – that of gas condensate.
By Michael Zhidkov, Dmitriy Zhidkov, Konstantin Bunyatov, Rodion Ivanov, Aidar Gabdulkhakov
Industrial tests at vortex facilities have shown that the TVT cooling efficiency is higher in comparison with a throttle valve both by cold and mixed flow. This paper details results obtained when utilizing a TVT with high output, at Komsomolskoye field, in parallel with two TVTs and a throttle valve.
Vortex effect (Ranque-Hilsch effect) is the temperature decrease in the near-axis layers of a high-speed vortex at the expense of heat transfer to peripheral layers. Usually, this effect is implemented in a sufficiently simple device – double-flow vortex tube (DVT), in which a cold flow is taken through a membrane adjoining to the “volute” of a high-pressure gas inlet while a hot flow is taken in the opposite direction downstream the ratio flow controller µ.
On the basis of the energy conservation law, enthalpic balance of an adiabatic (i.e. without external heat exchange) vortex tube (VT) may be presented as the following equation:
iinlet = µ icold + (1 – µ) ihot (1)
Comprehensive assessment of the operating modes for a number of industrial three-flow vortex tubes, which were designed with a vortex flow fracture, has shown that the balance equation (1) is violated to its left side, which really increases the device cooling efficiency. In order not to contradict with the energy conservation law, a certain correction δi was introduced into theoretical equation (1). Then, the enthalpic balance looks like:
iinlet = µ icold + (1 – µ) ihot – δi (2)
Experiments with TVT [1-7] were revealed that δi depends on cold flow µ and tends to the maximum at µ → 1.0. Specifiс TVT cooling efficiency (qTVT = µ ∆Тcold = 1·∆Тcold or qTVT = ∆Тmix) was higher as compared with pressure reduction (qthr = ∆Тthr) in the extreme case, when µ = 1.0 (the whole cold flow was taken through a membrane) or when the temperature-separated (stratified) flows were mixed after TVT.
Certain researchers have yet noted the facts of the thermal balance violation during operation of adiabatic vortex tubes; however, they referred their observations to the experiment accuracy: inaccuracy during the measurement of flow rate, temperature, appearance of non-adiabatic conditions, etc. Subsequently, the impact-wave concept of the vortex effect has clarified the situation.
The previous studies, which have been carried out at industrial TVT in order to confirm the legitimacy of equation (2), had an essential disadvantage – temperature efficiency of a TVT and a throttle valve were defined separately rather than during the simultaneous operation of expanding devices. Moreover, the Joule-Thomson effect often was not measured in real conditions rather than computed. Both devices operated simultaneously in parallel, in absolutely equal operating conditions within the gas-treatment facility at Komsomolskoye field.
Fig. 1 illustrates the schematic layout of the system used for associated gas treatment at the above field. The layout shows compressor hardware providing the necessary pressure drop at expanders in addition to the equipment of the gas treatment unit.
The treatment system operates in a following way. Crude associated gas is fed to compression into CAGC unit (equipment designation is taken according to the valid diagram) and then into T1 and T2 recuperative tube-and-shell heat-exchanger via separate pipelines. Condensate is dropped from the chilled raw gas and separated into S1 tank. The gas after separation is split into two flows: the main flow is directed into TVT1/1 and TVRT1/2 three-flow vortex tubes while the auxiliary flow (for fine pressure tuning) is supplied into the pressure reduction unit to TV throttle valve. Gas is swirled in the vortex tubes during its expansion and division into two flows: cold flow (an outlet in the device upper part) and hot one (outlet at the device bottom). Simultaneously, gas components are condensed in TVT with further fluid separation. This liquid, according to the U-tube principle, flows from the TVT condensate collectors into ST-1 collection tank.
The flows stratified in TVT are joined at the outlet of the devices and enter S2 separators together with low-pressure gas supplied after TV throttle valve.
Then, the separated gas is fed into the low-pressure line of T1 heat exchangers, where it is heated by raw gas. Associated gas, treated and heated in T1, is compressed in TGC and is fed into a transport pipeline after GMS gas metering station.
Condensate from S1, S2 and ST1 storage tank is pressure reduced up to the preset pressure and is supplied into T2 heat exchangers: then the heated gas is directed to the condensate collection unit. Liquid phase is partially degassed in ST1 storage tank, particularly at the expense of the heating by a heat carrier (in a winter period). Flash gas is fed into CAGC compressor suction.
Regulating devices P1 and P2 are installed at the outlet pipes for hot flows from TVT in order to adjust the ratio of a cold flow µ. Methanol is fed into the inlets of heat exchangers and expanding devices to prevent hydrate formation.
Fig. 2 illustrates the layout of an adjustable TVT; this layout shows the general design of TVT1/1 and TVT 1/2 vortex tubes. Designing and performance characteristics of these devices considerably differ from the parameters of the TVT used at other fields. First of all, maximum design capacity of TVT1/1 and TVT1/2 vortex tube is 160,000 nm3/hour while the maximum output of the already used devices does not exceed 25,000 nm³/hour. Secondly, vortex device dimensions significantly increase (height is about 4.5 meters) as well as their weight (more than 3.0 tons). Design of TVT with high-flow rate comprises an unconventional inlet with two nozzles with two pneumatic actuators intended to adjust the gas flow rate as well as updated design of the vortex chamber with the characteristic dimension Dtube = 200 mm.
According to the data available there are no three-flow vortex tubes both in Russia and abroad with such high output, which are operated by stratified flow mixing mode.
General appearance of TVT1/1 and TVT1/2 vortex tubes with the servicing platform is given in Fig. 3. As it is seen, two actuators (red color) are installed on both TVT cases. Supply lines for high-pressure gas are seen at the right (covered by heat insulation). Heat-insulated pipes coming upwards from the TVT cases and then descending vertically are the cold flow discharge lines.
Before assessing the operation of LTS system containing vortex tubes, it is expedient to consider the temperature conditions attained by GFT at Komsomolskoye field, which uses only a throttle valve as a refrigeration generator. Such example recorded on October 6, 2011 is given in Table 1. One may see that the lowest temperature achieved by GTF using a throttle was Тthr = -15 С. Assessment of the whole data array on the GTF operation during the winter period has shown that the minimum gas temperature is within Тthr = -14 ÷ -17 С in this case. That, the water dew point is Тw = -10 С for the treated gas after its compression to 68-69 atm in TGC (hereinafter Тw is calculated at 40 atm according to Gazprom proprietary standard 089-2010). This value is considerably lower as compared with the required water due point during the cold period according to the above standard (Тw = -20 С).
As an example, Table 2 contains the data on GTF operating mode with one TVT1/1; the data were recorded on November 3, 2011 (experience has shown that TVT1/2 operated identically).
One may observe from Table 2 the usual thermal stratification of the initial gas into cold and hot flows. Measurement of the gas flow rate was not made by TVT flows; that’s why we should note that µ was calculated by the following equation both for this table and hereafter (flow nixing condition):
µ = (Тhot – Тmix) / (Тhot – Тcold) (3)
There was the following essence of the experiments at GTF with TVT and TV throttle valve operating in parallel: the difference between the temperature decrease in gas flows at two types of expanding devices, which is directly measured during the same operating conditions. In real conditions, this difference is 7 С for TVT1/1 that means the thermal efficiency by 35 percent for a three-flow vortex tube as compared with a throttle valve, which is the significant value for low-temperature separation process.
to be finished in the next issue