Recommendations on the measuring of the pressure drop

Transcription

1 ECAI International Conference 8th Edition Electronics, Computers and Artificial Intelligence 30June -02July, 2016, Ploiesti, ROMÂNIA Analysis of a filter used in the natural gas gathering system Recommendations on the measuring of the pressure drop Faculty of Mechanical and Electrical Engineering, Petroleum and Gas Uniersity, Abstract The article presents some results of a research work carried out at the request of a company producing natural gas filters. The main requirement was to erify the recipient of the pressure drop of the filters in arious constructie shapes, in a range of flow rates, pressures and temperatures at which the gas passing through the filter. The paper highlighted a number of issues that could affect the filtration process and durability of the filters. There are indicated solutions for a better measuring of the pressure drop on the filter element. Keywords filters; natural gas; numerical analysis; measurement solutions; I. INTRODUCTION The principal relationship that describes the filtering process presents in the books / articles deoted to this subject, expressing the connection between the pressure drop and the elements inoled in fluid flow through the filter is [1-3]: Q h Q h p (1), H S S where Q is the natural gas flow through the filter, h filter width, η dynamic iscosity, S filter surface; p pressure drop across the filter and ψ iscous permeability. Also in (1) was used the filter resistance which is the inerse of the iscous permeability H measured in m -2. The last form of (1) is a practical one (often the iscosity is not measured separately), as it indicates the range of filtration with the term η α H that is between: mpa s m -2 (filtering ery rapidly) and mpa s m -2 (nearly unfilterable). Because (1) is alid only for an incompressible media and laminar flow it has been proposed a uniersally applicable formula [4-6]: Q h Q p (2). S S i The second term on the right side of (2) considers the fluid density ρ and contains the inertial permeability ψ i (in m). The inertial permeability ψ i is equal to the olumetric flow rate for a fluid with a density equal to the unit, passing through an area equal to the unit, under a pressure drop equal to unity, resistance to fluid flow is gien only by the inertial losses. Also for a compressible medium, the left term Ploiești, Romania, ion.pana@upg-ploiesti.ro, flgdinu@yahoo.com, gbucur@upg-ploiesti.ro of (3) has a better expression, which includes explicit alue of the pressure p 2 : the p Qh Q p 1 (3), 2 p2 S S i p p 1 p 2 (4), where: p 1 is the upstream pressure and p 2 the downstream pressure of the filter element. In many cases, the second term in (3) may be neglected without affecting (in a significant measure) the accuracy of the calculations. The recommended flow field is characterized by a linear change in the pressure drop ersus the flow of fluid, inoling a laminar flow regime. The turbulent flow changes hydraulic characteristic and the pressure drops are larger. The iscous permeability ψ by numerical transformations can be expressed as [3]: 3 2 dm (5), relationship which is obtained theoretically for a porous medium with a particular structure, wherein the solid medium is composed of a certain number of spherical particles of mean diameter d m, known as Kozeny - Carman equation. It works satisfactory for a porous media with a more complicated structure too. Figure 1. Recommended maximum gas flow, filter ALN HFC, Natural Gas Filters Low Pressure Natural Gas Filters, HFC Media Max. Rated Flows Nm3/hr at Various Operating Pressures, Source:

2 2 In (5) ε is the superficial porosity, which is defined as the ratio of the area of openings S g to the total area of the flow S: S g (6). S This amount does not exceed the alue of 0.6 at natural materials [6]; for comparison topsoil superficial porosity is between and iscous permeability between m 2. It should be noted that generally the porous enironments are inhomogeneous and anisotropic, so the permeability is a second order tensor (9 components) that depends on point [7-10]. Numerical modelling of flow through the filters is an issue of interest to researchers, yielding interesting model results and quite close of the practical determinations [10, 11]. The relations (1-6) are implemented into specialized program and allow an understanding of the numerical model used and direct actions related to an efficient filtration. II. MODELLING OF FLOW THROUGH THE FILTER Gien the practical aspect of research, for filter selection are useful the diagrams indicating for a particular type of filter, the maximum gas flow through the filter at a certain inlet pressure Fig. 1 [12-16]. Thus a manufacturer indicates the maximum flow rate at the filter can be used, for an inlet pressure. It is chosen always a filter that would accommodate more flow than required a practical situation. So, it is known the filtration surface. Knowing the actual flow rate through the filter, can be established from the equation of continuity (at the pressure and temperature of the gas mixture), the elocity rate through the filter element. The filter area A f is related to the area of the A r nozzles (input / output) into a n area ratio that determines a elocities ratio (differences between densities are insignificant): r A f r (7). Filter manufacturer recommends a maximum flow elocity through the filter in order to obtain efficient filtration. This speed is 0.16 m/s for filters with a pressure drop (a new filter) of 0. 1 bar at a remoal limit between 2-10 µm, and 0.1 m/s for filters with a pressure drop (a new filter) of 0.02 bar at a remoal limit of 160 µm. The areas ratio r is on aerage alue 200:1, and at the nozzles the elocity is 32.5 m/s respectiely 20 m/s keeping the imposed areas report. Simplifying the aboe conditions, the user of a filter takes the maximum flow recommend by the manufacturer in m 3 N/ h, conert it into m 3 /h at the operating temperature and pressure of and checks if the elocity through the nozzle is less or equal than the recommended alue. So filters are selected by the calculus of the elocity through the nozzle aspect that leads us away from their functional role. A r f Figure 2. The circuit arrangement of the gas filter. After separating of the liquid fraction of the gas extracted from the gas well, in the filter liquid droplets continue to be retained with the remaining solids. In this way they are protected gauges and compressor elements. If this condition of elocity ( m / s through the nozzle, it supposes that the areas ratio is around 200) is not ensured, the appropriate filtration effect isn't obtained, the particle deposits on filter are growing faster and the filter is quick out the operation. These aspects are consistent with the indications of [1, 2, 6]: optimum filtration elocity 0.16 m / s; elocity which enhances the rate of deposition 0.25 m / s and the maximum filtration elocity 0.66 m / s. Filter element manufacturer in the research work mentioned in the preamble, indicated at the calculus of selection an uniform elocity of 0.26 m / s, ery close to the limit to boost deposits on filter. If we look at (1), (5) and (6) note that while deposits are growing, the superficial porosity is reduced so iscous permeability decreases rapidly and the pressure drop across the filter increases. The maximum limit of the pressure drop indicated by the filters producers [12-16] is Δp = 0.5 bar. The manufacturer of the filters into the expertise (that was the support for this paper) required to be indicated only the pressure drop between the measuring points. Location diagram of the filter is shown in Fig. 2. It protects the meter. Modelling was performed with a software package donated to the Uniersity of Petroleum and Gas by Schlumberger Company (Petrel, Eclipse, Pipesim, DHCAE Tools, Fluid Flow) and Parametric Technology Corporation (Creo, Flow EFD) products marked with italics used for this application. For the analysis was conducted a tridimensional model of the filter Fig. 3. There hae been traced the elocities to the model of the filter element. The data of the filter used for this example are: the maximum input pressure of 4.01 bara, maximum gas flow rate m 3 N / day, the filtered product is natural gas, the aerage working temperature 15 C; two layers into the filter: first felt 6 mm width, for the separation of particles of water (een upstream in used an plan of drying), Fig. 4; second paper 0.9 mm width; outside diameter 216 mm; inside diameter 153 mm; length 600 mm. Speeds in the entry area of the paper zone hae not been represented since there are small differences compared to those of exit. The elocities are increasing towards the base of the filter, where the flow diameter is reduced. At the top of the filter cartridge is a zone between m, Fig. 5, with slow elocities.

3 Analysis of a filter used in the natural gas gathering system 3 Figure 3.Tridimensional model of the filter. Simulation parameters used are: input pressure 4.01 bara, gas temperature 15 C, gas flow m 3 /s. There are used two measurement planes (at 2/3 of the nozzle end) with 4 points at 90 : In PM1, In PM2, In PM3, In PM4 (input) and Out PM1, Out PM2, Out PM3, Out PM4 (output). An axial and a transersal sections are used to represent the elocity and pressure distributions. Figure 4. A detail with the filter cartridge. Two layers were considered: first a felt layer with coalescer effect, and the second a paper layer. The third layer mechanical support (a steel cylinder with holes) wasn t represented. Distribution of the elocities / pressures was made on the lines 1, 2 and 3. The prescriptions about maximum filtration elocity (remember it is 0.66 m / s) are not respected and thus filter will block quickly. The pressure is about the same on the three lines followed, except the area at the base of the filter, where the pressure on the central area is slightly smaller Fig. 6 (due to the elocity increase). The pressure differences on filter cartridge height are up to 1800 Pa. The elocity rates along the filter are quite irregular Fig. 7. After completing the input nozzle, the elocities drop when the natural gas enters into filter body. Velocities ary on the height of the filter element and circumferentially, the filter shape haing a decisie role in this distribution. Partly on the upper filtration zone is carried out in conditions close to the normal Fig. 8. The bottom area is critical. Passing the entire olume of gas through the central hole and with high elocities causes a non-uniform distribution that is maintained at the outlet. The form of the current lines indicate the same thing as the filter is not suitable for filtering under normal conditions fig. 9. There are special filters that hae this in mind and manufacturer soughs to eliminate the disadantage mentioned. The shape of the flow lines indicate the same thing: the filter is not suitable for filtering under normal conditions Fig. 9. There are special filters with an improed construction and their manufactures sought to eliminate the disadantage mentioned [13-16]. Regarding the beneficiary s requirement: pressure difference between the measuring points below the maximum alue of 0.5 bar, in the Table 1 are the alues of the elocities and pressures into eight points (s. Fig. 3). Here it is noted that the pressure differences depends on the measurement points used and there is not the pressure drop oer the filter, which is much less. They were used for analysis four points located at 90 to the measuring section on the inlet connection (about 1/3 of the length of the filter input nozzle). Figure 5. Velocity distribution on the filter height. The gas elocity aries in the ertical direction and reaches the highest alues at the bottom of the filter cartridge section, in which the gas discharge takes place. Figure 6. Pressure distribution on the filter height. The gas pressure has the same alue in the ertical direction until the lower part of the filter cartridge. Here starts a rapid ariation of the alues and in the central hole of the filter cartridge, pressure falls.

4 4 Figure 7. Velocity distribution in the axial section of the filter. Can obsere high alues of gas elocity, far aboe the recommended limits on filtering. Figure 10. Pressure distribution on the transersal section of the filter. Variations in pressure are reduced along the filter. Figure 8. Velocity distribution in the filter transersal section of the filter. Velocities increases at the bottom of the filter cartridge. When passing through the elbow supporting cartridge, maximum speeds are obtained. Figure 11. Pressure distribution on the axial section of the filter. The most important pressure ariations occur in the nozzle zones. Howeer slight ariations in gas pressure on the filter cartridge causes rapid changes in elocity. TABLE I. TABLE II. Measure point SIMULATION VALUES IN THE MEASURE POINTS A Pressure in the measure point, Pa Velocity in the measure point, m/s PM1, in PM1, out PM2, in PM2, out PM3, in Figure 9. Flow lines. Velocities ary on the height of the filter element and circumferentially, the filter shape haing a decisie role in this distribution. It is a slow ortex moement in the top right part of the filter. PM3, out PM4, in PM4, out

5 Analysis of a filter used in the natural gas gathering system 5 TABLE III. Measure point PRESSURE DROP IN THE MEASURE POINTS A Input Pressure, Pa Output Pressure, Pa Pressure Drop, Pa PM ,32 PM ,98 PM ,26 PM ,77 A. See fig. 3 for details; Pressure drop on the filter element is between Pa At the outlet of the filter were used four points located at 90, at one third of the nozzle length. It is noted that the pressure differences Table 2 are similar regardless of points in the axial direction (placed on the same generators) where the difference between pressure is calculated. The matters relating to the oerall situation of the pressure reflect the aboe obserations, the pressure distribution on the filter element being almost constant Fig. 10, and the biggest differences are between the two nozzles Fig. 11. By using of the new filter characteristic (link experimentally determined by the manufacturer of flow rate through the filter and pressure drop), the analysis says that a new filter has a pressure drop of almost 10 times more at the flow through the filter body, compared to the flow through the filter element. The further behaiour of the filter (probable) is as follows: the filters are loaded ery quickly at the bottom, and then elocities increase on the generator of the filter, from the bottom up, causing plugging of the pores of the filter further. Finally the pressure drop between the measuring connections will reach 0.5 bar, maximum alue imposed and the filter will be changed. The research carried out under the conditions of the deposits on the filter leading to a mechanism similar to that set forth aboe [17-21]. III. MEASUREMENT CONSIDERATIONS In order to aoid frequent filter change, by analyzing the pressure diagrams (Figure 11), the pressure points positions are distributed at the filter base, on both sides of it (Figure 12). The measurement is done by using a differential pressure gauge. The gas meter is positioned after the filter it is an MZ type [22]. The gas flow turns the turbine blades meter, so that the speed of rotation of the turbine is proportional to the linear speed of the gas. The moement is mechanically transmitted to the counter through a mechanical coupling. The meter is equipped with a temperature sensor to measure the gas temperature. The pressure losses on MZ counters are done by the following relationship [23]: p (8) gm p r n Q Pb Q max Tb Figure 12. Filter pressure drop measurement. Pressure drop near to the filter cartridge is 1,360 Pa (points A-B). Pressure drop between the measurement points is around 3800 Pa, s. Table 2. where Δp gm is the pressure loss in terms of computing, Δp r pressure loss in reference conditions; ρ n the gas density (kg/m 3 ) from 0 o C and 1013 mbar; P b the working pressure; Q the flow (m 3 /h); Q max the maximum flow (m 3 /h); T b the gas temperature ( o C). IV. CONCLUSION The using of filters in normal conditions means ensuring a elocity of filtration in the laminar domain. It will aoid the rapid growing of filter deposits and blocking an area with a cascading effect. This condition is not insured for the analysis performed. Filter selection is made by the user based on the nozzle elocity that entails (is belieed) a proper filter filtration rate. As it was shown the filter shape and aspect of moement of gas through can change the elocity distributions, aspect that the user is not aware. Basically the user will change the analysed filter more often. An analysis of the distribution of elocities on the filter element should be requested from the manufacturer as a guarantee of efficiency of the filter. The pressure drop at the measurement points is 7 to 16 times the pressure drop on the filter cartridge, and 3 times the pressure drop near the filter cartridge (s. Fig. 12). It is proposed a measurement solution which includes a differential pressure gauge with measurement points placed nearer the filter element, Fig. 12. The gas circulation through filter must be studied carefully in the design phase. Lower elocities in the cartridge area and their uniformity are desirable. The possible measures are: the increase of filter size; the change the filter shape with the gas input from the inside, and gas exit into the annular space of the filter; increasing of the number of filter cartridges; changing the filter construction for uniform distribution of elocities on the filter cartridge, Fig. 13.

6 6 a. Analysed filter cartridge: 0.9 mm paper width; outside diameter 216 mm; inside diameter 153 mm; length 600 mm. High elocities and non-uniform distribution. b. A larger filter cartridge: 0.9 mm paper width; outside diameter 380 mm; inside diameter 320 mm; length 600 mm. Central rod was remoed. The elocities are reduced. c. A new type filter: 0.9 mm paper width; outside diameter 216 mm; inside diameter 153 mm; length 600 mm. A helix chicanery conducts the gas. Gas input from the inside, and gas exit into the annular space of the filter. A better distribution of the elocities on the filter cartridge Fig. 13. The improement of the elocity distribution. References [1] K. Vafai. Handbook of porous media. Taylor & Francis, [2] N.P. Cheremisinoff. Handbook of Chemical Processing Equipment, Butterworth-Heinemann Woburn, MA, Boston, pp , [3] S. Ripperger, W. Gosele and C. Alt, Ullmann s Encyclopedia of industrial chemistry - Filtration fundamentals. Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, [4] Y.H. Cheng and H.J. Tsai, Factors influencing pressure drop through a dust cake during filtration, Aerosol Sci. Technol., 29, pp , [5] T. C. Dickenson, Filters and Filtration Handbook, Forth Edition, Elseier, Kidlington Oxford UK, [6] K. S. Sutherland, G. Chase, Filters and Filtration Handbook, Fifth edition, Butterworth-Heinemann Elseier, Burlington MA, USA, [7] A. Naboati and A.C. Sousa, Fluid flow simulation at open-porous medium interface using the lattice Boltzmann method, Int. J. Numeric Method Fluids 56(8), pp , [8] A. Koponen, D Kandhai and E. Hellen, Permeability of three-dimensional random fiber webs, Phys Re Lett, 80(4), pp , [9] J.R. Weitzenbock, R.A. Shenoi and P.A. Wilson, Measurement of three-dimensional permeability, Compos Part A: Appl Sci Manuf, 29(1 2), pp , [10] A. Naboati, E. W. Liewellin, and A. Sousa, A general model for the permeability of fibrous porous media based on fluid flow simulations using the lattice Boltzmann method, Composites: Part A Elseier, pp , 2009 [11] F. Jiang and A.C. Sousa. Smoothed particle hydrodynamics modelling of transerse flow in randomly aligned fibrous porous media, Transport Porous Med 75(1), pp 17 33, [12] ***. Gas filtration and coalescent products, Kelburn Engineering Limited, Filters catalog, [13] ***. Improed Natural Gas Filter/Separator Performance, Velcon Filters, LLC, [14] ***. Natural gas filter type FGN, Armax Gaz, Gas and Oil Equipment, catalog [15] ***. Solutions for filtration applications, Petrogas filtration, Steenoen 5626 DK, Eindhoen, The Netherlands, [16] ***. The Basics of Coalescing Compressed Air & Gas Filtration, Catalog Parker, Parker Hannifin Corporation Filtration and Separation Diision, [17] D. Láička and P. Koařík, Numerical Simulation of DPF Filter for Selected, Regimes with Deposited Soot Particles, EPJ Web of Conferences, 010, pp , [18] D. Laicka, J. Knourek and J. Polansky, CFD Simulation in the Indiidual Channels of the Particle Filter, in 3.rd European Automotie CFD Conference, Frankfurt, Germany, pp , 5-6 July [19] J. Chunga, Y. G. Seob and C. Kanaokac, Numerical simulation of fluid flow in porous filters for a particulate remoal facility, Adanced Powder Technology, Volume 9, Issue 1, pp , [20] K.L. Tung, J. Shiau and C. Chuang, CFD analysis on fluid flow through multifilament woen filter cloths, Separat Sci Technol, 37(4), pp , [21] Q. Wang, B. Maze and H.V. Tafreshi, On the pressure drop modelling of monofilament-woen fabrics, Chem Eng Sci, 62(17), pp , [22] ***. Gas Filters, accesed on [23] ***. Products and Serices - Gas, Itron, accesed on