Advances in Filtration Using Sintered Metal Filters

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Key takeaways:

• Sintered metal filters offer high efficiency in particulate removal, with capabilities for backwashing and long service life.

• These filters are suitable for high-temperature applications and various industrial uses, including chemical and power generation sectors.

• The design and selection of sintered metal filters depend on their particulate holding capacity and the characteristics of the particles being filtered.

• They are advantageous for processes requiring high filtration efficiency, durability, and resistance to corrosive environments.

Abstract

Filtration technology utilizing sintered metal media provides excellent performance for separation of particulate matter from either liquid or gas process streams (i.e., liquid/solids and gas/solid separation) in numerous industrial liquid and gas filtration applications. Sintered metal filter media, fabricated from either metal fibers or metal powders into filtration elements, are widely used the in the chemical process, petrochemical and power generation industries. Applications require particulate removal to protect downstream equipment, for product separation, or to meet environmental regulations.

Sintered metal media provide a positive barrier to downstream processes. Sintered metal media have demonstrated high particle efficiency removal, reliable filtration performance, effective backwash capability, and long on-stream service. These filters can provide particulate capture efficiencies of 99.9% or better using either surface or depth media. Operating temperature can be as high as °C, depending on the selection of metal alloy. Along with the filtration efficiency consideration, equally important criteria include corrosion resistance, mechanical strength at service temperature, cake release (blowback cleanability), and long on-stream service life. These issues are critical to achieving successful, cost effective operations.

The life of such filter media (filter operating life) will depend on its particulate holding capacity and corresponding pressure drop. This accumulating cake can be periodically removed using a blowback cycle. The effectiveness of the blowback cycle and filter pressure drop recovery is a critical function of the properties of the accumulating particles in the cake and the filter media. Depth filtration media configured in a polishing filter may be utilized in those applications with light particle loading.

In addition to providing superior filtration in a single pass, clean-in-place backwashable media reducesoperator exposure to process materials and volatile emissions. While applications include high temperature and corrosive environments, any pressure driven filtration process with high operating costs has the potential for improvement using sintered metal filtration technology.

This paper will discuss filter-operating parameters of sintered porous metal media and filtration system design criteria for optimizing performance in a number of chemical process streams.

Introduction

The 21st century brings many economic and environmental challenges to the chemical industry. Major drivers for change include market globalization, demand for improved environmental performance, profitability, productivity and changing workforce requirements. Future competitive advantage in the chemical processing industry will come from patented technology and technical know-how. New economical high yield and high quality processes will characterize much of the industry&#;s production capacity with improved environmental impact and energy efficiency.

A high percentage of the chemical industry&#;s products and processes involve solids (particulate) handling. Filtration technology offers a means of reducing solids through mechanical separation via patented filter design and unique systems operation. Filtration can improve product purity, increase throughput capacity, eliminate effluent contamination (minimizing or preventing air and water pollution) and provide protection to valuable equipment downstream of the filter. Advances in filtration technology include the development of continuous processes to replace old batch process technology. Cost savings include less hazardous waste for disposal and labor savings from new technology. Fully automated filter systems can be integrated into plant process controls.

Solids reduction includes the removal of suspended solids from process effluent waste streams and cleaning solvents. The liquid product recovered is valuable for recycle to another chemical feed stream. Waste minimization includes the reduction of hazardous solids materials for recovery or recycle and solids reduction of non-hazardous materials to landfill. Filtration can reduce wastewater feed stream BOD (Biological Oxygen Demand), COD (Chemical Oxygen Demand), TSS (Total Suspended Solids), and TOC (Total Organic Carbon). These are the main parameters for which current emissions are measured with regard to local and international standards.

Filtration Fundamentals

Knowledge of filtration fundamentals is essential to ensure appropriate design of filter media and the optimum selection of appropriate media and filter design for each filtration application. Two main filtration modes can be considered, i.e., depth filtration and surface filtration. In the case of depth filtration, the particles are captured inside the media; while in surface filtration they are retained, as the term explains, at the surface where subsequently a cake of particles is formed.

Surface filtration is primarily a straining (sieving) mechanism where particles larger than the pore size of the filter media are separated at the upstream surface of the filter; their size prevents them from entering or passing through the pore openings. Subsequent particles accumulate as a cake that increases in thickness as more particle-laden fluid is forced into the filter medium. The cake, due to its potentially finer pore structure, may aid in the separation of finer particles than can be achieved by the filter media. However, the cake must exhibit sufficient porosity to permit continued flow through it as filtration proceeds. Processes can be run under constant flow/increasing pressure or constant pressure/decreasing flow. Because most surface filters are not perfectly smooth or have perfectly uniform pore structure, some depth filtration can take place that will affect the life of the filter.

Depth filtration is mainly used in applications where small particle levels have to be separated such as in the protection of downstream equipment against fouling or erosion, protection of catalysts from poisoning and in product purification. The particles penetrate into the media and are subsequently captured within its multiple layer structure. This multiple layer structure prevents premature blocking of the media and increases the capacity to hold dirt and on-stream lifetime. Because the particles are captured within the depth of the media, off-line cleaning will be required. This off-line cleaning can be accomplished with solvents, ultrasonic vibration, pyrolysis, steam cleaning or water back flushing. In addition, the media may be pleated, a configuration that minimizes housing size and cost.

Understanding of the ability of a filter to remove particles from a gas stream passing through it is key to successful filter design and operation. For fluids with low levels of particulate contamination, filtration by capturing the particles within the depth of a porous media is key to achieving high levels of particle efficiency. The structure of sintered metal provides a tortuous path in which particles are captured. Particles capture continues as a cake of deposited particles is formed on the media surface; however, particles are now captured on previously deposited particles. The life of such filters will depend on its dirt holding capacity and corresponding pressure drop. For fluids with high particle loading, the operative filtration mechanism becomes cake filtration. A particle cake is developed over the filter element, which becomes the filtration layer and causes additional pressure drop. The pressure drop increases as the particle loading increases. Once a terminal pressure is reached during the filtration cycle, the filter element is blown back with clean gas and/or washed to dislodge the filter cake. If the pore size in the filter media is chosen correctly, the pressure drop of the media can be recovered to the initial pressure drop. However, if particles become lodged within the porous media during forward flow, and progressively load the media, the pressure drop may not be completely recovered after the cleaning cycle.

Filtration rates are influenced by the properties of the feed particle concentration, viscosity and temperature. The filter operating mode can be constant pressure, constant flow rate, or both with pressure rising and flow rate dropping while filtering. Filtration cycle will be constrained if solids are fast blinding and allowable pressure has been reached, or for cake filtration, if the volume for cake buildup has been filled, even if the allowable pressure drop has not been reached. Permeability is expressed as flow rate against pressure drop. Permeability is influenced by filter type, fluid temperature and solids loading.

Sintered Powder Metal Media

Sintered metal media are manufactured by pressing metal powder into porous sheet or tubes, followed by high temperature sintering. A scanning electron photomicrograph of a typical sintered powder metal media is shown in Figure 1. The combination of powder size, pressing and sintering operation defines the pore size and distribution, strength and permeability of the porous element. Pore size of sintered metal media is determined using ASTM E-128. The media grade designation is equivalent to the mean flow pore, or average pore size of the filter. Sintered metal media are offered in grades 0.1, 0.2, 0.5, 1, 2, 5, 10, 20, 40 and 100. The filtration rating in liquid for media grades 0.2 to 20 is between 1.4 and 35 µm absolute. The filtration rating in gas ranges from 0.1 to 100 µm absolute.

Filter cartridges fabricated from sheet or tubes have an all welded construction. The filter media is designed and engineered with a stable porous matrix, precise bubble point specifications, close thickness tolerances, and uniformity of permeability, which assure reliable filtration performance, effective backwash cleaning and long on-stream service life.

Sintered Metal Fiber Media

Metal fiber filter media consists of very thin (1.5 to 80 μm) metal filaments uniformly laid to form a three-dimensional non-woven structure sintered at the contact points. A scanning electron photomicrograph of a typical sintered metal filter media is shown in Figure 2. These media are explicitly designed for either surface or depth filters. Either single or multi-layered construction are utilized with each layer comprised of potentially different diameter fibers to achieve optimal performance, e.g., pressure drop, filtration efficiency, particle loading capacity, and media strength. The multi-layered material has a graduated design, so the dirt holding capacity is much higher and consequently the life expectancy is longer. The final filter rating is determined by the weight per used layer, the fiber composition of the layer and the combination of several layers. The availability of a high porous structure (up to 85%) offers a very higher permeability and hence a low pressure drop.

The properties of metal fiber filters, fabricated from various metal alloys, for gas filtration applications allow the use in extreme conditions: high temperature, high pressure and corrosive atmospheres. The primary benefits of sintered metal filters are: strength and fracture toughness, high pressure and temperature capabilities, high thermal shock resistance, corrosion resistance, cleanability, all-welded assembly, and long service life.

Fiber metal media have a higher porosity than the powder metal media, thereby resulting in lower pressure drop. For high temperature or corrosive applications, Bekaert has developed fibres in other alloys besides AISI 316L. Inconel® 601 and Fecralloy® are used for high temperatures (up to 560°C and °C respectively) whereas Alloy HR can withstand temperatures up to 600°C and wet corrosive environments.

The inherent toughness of the metal filters provides for continuous, back pulsed operation for extended periods. For high temperature applications, additional criteria such as creep-fatigue interactions, and high temperature corrosion mechanisms need to be addressed. Filters with semi-permanent media are cost effective, since such units lend themselves to minimal downtime, closed and automatic operation with minimal operator intervention, and infrequent maintenance.

The proper selection of filter media with appropriate pore size, strength and corrosion resistance enables long-term filter operation with high efficiency particle retention. The filtration rating in liquid is between 2 and 35 µm absolute. The filtration rating in gas ranges from 0.1 to 10 µm absolute.

Filter Design

The filter design for liquid/solids separation is selected which produces the required filtrate, minimizes backwash or blowdown and maximizes throughput. Three types of filter configurations are described as follows:

1.) Outside-in filtration

Traditional liquid/solids barrier separation occurs on the outer perimeter of a closed-end tubular filter element (LSP). A gas assisted pneumatic hydro-pulse backwash has proven to be the most effective cleaning method for sintered porous metal filters.

2.) Inside-out filtration

Liquid/solid barrier separation occurs on the inside of a closed-end tubular filter element (LSI). LSI backwash modes include: a.) Full shell slurry backwash, b.) Empty shell slurry backwash, c.) Empty shell and empty element wet cake backwash and d.) Empty housing wet cake discharge.

3.) Inside-out Multimode filtration:

Liquid/solids (barrier or crossflow) separation occurs on the inside of open-ended tubular filter element (LSM and LSX). Elements are sealed within two tube sheets, thereby allowing for either top or bottom feed inlet. The LSM filter, with a feed recirculation feature, has proven itself in several continuous loop reactor systems. The downward velocity controls the cake thickness of the catalyst with the lower the velocity resulting in a thicker cake. Filter backwash modes are similar to LSI backwash modes and also includes a bump-and-settle type backwash that allows concentration of solids without draining the filter element or housing. Continuous loop reactor system may not require backwashing.

Scale-ability of the filtration systems allows for accommodating high flow rates and increased solids capacity. Filtration units are suitable for batch or continuous processes. Single housing filter systems are recommended where flow rates allow and flow can be stopped for a few minutes prior to backwash, or if off line periods can be tolerated for maintenance. Two filter dual systems are recommended where continuous flow is required and short periods of off line can be tolerated for maintenance. Three filter systems are recommended for continuous operation even during maintenance periods.

Benchscale and Pilot Testing 

A valid method of evaluating filter performance is through bench scale and pilot testing. Filter testing typically begins with a simple disc feasibility test to qualify media and obtain critical filtration characteristics. Successful feasibility studies usually progresses to more involved testing of pilot equipment. Pilot testing helps develop successful commercial separation practices. While bench scale tests produce reliable indication of filter performance, data obtained in pilot scale testing on a process line will show filter operating parameters with normal process variations. Development programs require direct access to suitable equipment over an extended period. Pilot testing of sintered metal backwashable filters can provide the following information:

• Verification of filtrate quality;
• Filter thruput per cycle at various flux rates;
• Rate of rise in pressure drop vs. thruput;
• Backwash volume and resulting solids concentration;
• Scale up data for full scale sizing;
• Accurate cost estimates;
• Demonstrate high product value;
• Reliable operation with high on-line time and low maintenance;
• Demonstrate new technology at a commercial scale.

In addition to verifying filter performance, pilot testing provides the opportunity for the operating engineer to learn to use the equipment and conduct experiments that optimize filter operation for their particular process. Pilot test trials address significant technical questions and problems prior to full-scale commercialization. The outcome of pilot plant operations verify:

• Filtration/reaction studies verified at laboratory and pilot plant scale;
• New technology demonstrated;
• High volume product consistently recovered;
• Product separation and recovery optimized;
• Capacity testing completed;
• Overall operating efficiency.

Media Selection 

Feasibility Case Study: Catalyst Solids Removal

A typical approach for feasibility testing and media selection is illustrated in the following test case. The objective was to evaluate the filtering characteristics of a new catalyst to support an existing LSI commercial filter installation. Filtration studies were conducted with a 70-mm disc test filter using both Grades 5 and 10 media to compare filter performance. Catalyst particle size distribution (PSD) was measured using a Horiba LA-910 Laser Scattering Particle Size Distribution Analyzer. The size range (based on volume %) was 0.51 to 60 µm with a mean size of 13.4 µm. SEM microscopy at - X magnification verified particle size distribution as shown in Figure 3. Catalyst slurry was filtered once through at a constant rate using Grades 5 and 10 media housed in the 70-mm disc filter housing shown in Figure 4. A particle size distribution comparison of feed and filtrates (Grade 5) sample is shown in Figure 5. Test results indicate that filtration using Grade 5 media resulted with a lower rate of rise pressure than Grade 10 media as indicated in Figure 6. Filtrate turbidity samples were similar. Filtrate from the Grade 5 media measured 2.9 NTU, while filtrate from Grade 10 media measured 2.3 NTU. The 1/8 inch thick filter cake backwashed effectively from the Grade 5 media surface. Some catalyst remained in the porous structure of the Grade 10 media, indicating that catalyst had blocked some of the surface pores.

Test results indicate that Grade 5 media is better suited for filtration of new catalyst sample using the HyPulse LSI filter configuration. Pilot testing at the commercial facility verified results of feasibility study and resulted in purchase of replacement cartridges for an existing filter vessel.

Commercial Applications 

Application 1:

Laboratory disc tests conducted in April indicate suitability of sintered metal filter for catalyst recovery application. Bench scale pilot filter tests were conducted at the customer&#;s lab facility to verify filter performance and filtrate quality. In November pilot testing with continuous catalyst filtration using 2% slurry demonstrated consistent flux rates of 0.2 gpm/ft2. A comparison of filter performance from disc testing through pilot testing is listed in Table 1. Axial velocity through the filter controlled cake thickness. The rate or velocity through the filter was optimized in bench scale testing. Optimal filter performance indicated that the filter could operate at pressures < 10 PSI without backwashing. Tests were conducted over about hours with no significant change in operating performance. The project gained approval to move to the final stage.

The objective of pilot test development program was to convert isomerization process from batch to continuous. The first commercial plant was scheduled for operation in . The process was started up in July in accordance with parameters established during the pilot testing. System dynamics experienced during start-up and initial operation exhibited performance similar to the pilot test studies. The filter operated successfully to recover and recycle precious metal catalyst after solvent wash and removal of 10% of the catalyst from the process after each batch. Process liquid is hazardous, however, because the filter system is completely enclosed, solvent could be used to wash and re-slurry catalyst back to the reactor.

The primary (larger) LSM catalyst filter is designed for bulk catalyst filtration and recycle. The filter design offers completely enclosed automated operation with minimal filter cleaning/regeneration. Fresh catalyst is added to each batch. The smaller LSP filter is designed for catalyst removal from the system. After 7 years of operation the filter bundle was replaced during a preventive maintenance schedule. The filtration system continues to operate since its initial installation in .

Application 2:

This catalyst filtration concept was proved in laboratory testing to confirm filter operating parameters and media selection. A development program utilizing pilot testing used a reactor equipped with filtration apparatus capable of separating product from catalyst, whereby the product can be removed from the

reactor while the catalyst is retained, thus permitting the reaction to be run semi-continuously or continuously. Testing utilized the HyPulse® LSM filter design.

By equipping a reactor with a means of maintaining catalyst in the vessel, the reactant can be pumped and the catalyst free product continuously removed. The hydrogenation process stops when the catalyst charge deactivates. The preferred method of filtration was to install a re-circulation loop onto the reactor,

as shown in Figure 7. For an extended batch or continuous process, a larger charge of catalyst is used to ensure sufficiently large commercially viable production quantities. This process allows up to a 50% reduction in total cycle time and an increase in over 65% in the amount of product run as indicated in Table 2.

Application 3:

The first use of sintered metal filters using inside-out (LSI) HyPulse® filtration technology for continuous slurry oil filtration was in . The installation demonstrated the suitability of sintered metal media for high temperature filtration of slurry oil for a carbon fiber development process. The filter operated reliably for many years producing clean oil with solids content of less than 20 ppm and was eventually shut down because of low product demand. Since then, refineries around the world have become aware of the benefits of filtration using sintered metal media for catalyst fines removal in slurry oil service.

Throughout the &#;s numerous LSI filtration systems have been installed for FCC slurry oil filtration. The largest continuous filtration systems utilizes (3) 66&#; LSI filters as shown in the schematic in Figure 8. Filtration cycle time ranges from 2 to 16 hours operating at 30 & 60 PSI, respectively in the filtration of ppm slurry oil. Extended cycle times were obtained by running two filters simultaneously, but staggered in cycle time, with the third being on stand-by for utilization when one of the other filter units is backwashed. The filter design uses a full shell backwash. Efficiency of the recovered product using two filters on line exceeds 99.8%.

Since there have been many refineries in China have installed LSI filtration systems for catalyst removal in resid fluid catalytic cracking (RFCC) units. A filtration system with (2) 24&#; LSI filters was installed in a RFCC unit with 1.4 million metric tons (mt) per year capacity and an output of slurry oil of 180 mt/day. The slurry oil has an average 3,000 to 5,000 ppm solids concentration. Cycle time varies from 2 to 8 hours. The filtrate solids content is under 50 ppm. The filter is controlled by local PLC that communicates with refineries distributed control system (DCS) to enable the operator monitor the filtration in the control room. The system is running continuously since then supplying a local company with clean filtrate to produce carbon black.

Application 4:

A process for producing Uranium Dioxide utilizes a HyPulse® gas/solids venturi pulse (GSV) blowback sintered metal filters, as shown in Figure 9, for the recovery of Uranium oxide fines from a process kiln. The sintered metal filters must withstand kiln off-gas stream temperatures of 300°F and be chemically resistant to the gaseous components. The primary risks associated with this conversion are chemical and radiological. The conversion process uses strong acids and alkalis that involve turning uranium oxide into soluble forms, leading to possible inhalation of uranium. In addition, the corrosive chemicals can cause fire or explosion hazards.

Successful field applications and laboratory support provided performance data that resulted in the first commercial filter installation put in service in . The completely enclosed GSV filter operates with 99.999% efficiency with a very low solids load to the filter and infrequent backpulsing. Key operating parameters include controlled approach velocity to the filter, high efficiency, and use of venturi for blowback for continuous operation. Today, one uranium conversion plant continues to operate in the United States using this patented process.

 

 

Application 5:

Cleanable sintered metal fibre filters offer an economical solution to processes with increased demand for higher particulate removal efficiency in extreme conditions. The development of metal fiber filter media such as Bekipor® contributed to an increased quality level through higher filter efficiency and a longer onstream

lifetime. Traditional separation systems such as cyclones, ElectroStatic Precipitators (ESP) and disposable filters are losing their appeal. Figure 10 compares emissions efficiency and relative cost of fiber metal compared to ESP and cyclones.

A highly porous structure, which is a characteristic of a sintered metal fibre medium, offers a high permeability and hence low pressure drop, even at high filtration velocities. This results in a low capital expenditure and low running costs. The cleanability for both on line cleaned surface filtration as for off line cleaned depth filtration is excellent.

This application used Bekiflow® HG for removal of alumina and alumina hydroxide dust having a particle size of 50% < 15 μm. Gas temperatures measured 842 °F. Dust concentration before the filter measured 250-800 mg/Nm³. Gas concentration after filtration was less than 30 mg/Nm³. Maximum pressure drop was 15 mbar. Total surface area of the filter was 830 m2. Fiber metal filters offers limited pressure drop and was tested for guaranteed lifetime of 27,000 operating hours. Customer benefits include less filter surface required, smaller bag house therefore less installation place required.

Summary 

Sintered metal media provides an effective means of filtering to remove particulate whether they are impurities or valuable by-product of a chemical process stream. These media are ideally suited for more demanding applications involving high temperatures, high pressures, and/or corrosive fluids. Chemical

companies are utilizing filtration to minimize waste products at the source rather than at the end of the line of the production process. Filtration improves product quality and protects downstream equipment in the production of chemical based products. Advances in filtration technology include the development of continuous processes to replace old batch process technology. Liquid/solids filtration using conventional leaf filters is messy and hazardous to clean and require extended re-circulation time to obtain clean product. Traditional gas/solids separation systems such as cyclones, ElectroStatic Precipitators (ESP) and disposable filters are being replaced by sintered fiber metal filtration systems.

Sintered metal filters should be operated within the design parameters to prevent premature blinding of the media due to fluctuations in process operations. Use of flow control assures the filter will not be impacted with a high flow excursion. Filter efficiency increases as the filter cake forms. The cake becomes the filter media and the porous media acts as a septum to retain the filter cake. Filter cakes can be effectively washed in-situ and backwashed from the filter housing. A gas assisted pneumatic hydropulse backwash has proven to be the most effective cleaning method for sintered porous metal filters. Sintered metal filters can be fully automated to eliminate operator exposure and lower labor costs while providing reliable, efficient operation.

 

Bekiflow and Bekipor are registered trademarks of Bekaert.

Hypulse is a registered trademark of Mott Corporation.

 

 

FAQs: Sintered Metal Technology

Q: What is sintered metal?

A: Sintered metal refers to a specialized material made by compacting and forming metal powder under heat and pressure, creating a solid, porous structure ideal for filtration and various industrial applications.

Q: How are sintered metal filters manufactured?

A: Sintered metal filters are produced by compacting metal powder in a mold and then heating it to a temperature below the metal&#;s melting point, causing the particles to bond without liquefying.

Q: What are the main advantages of using sintered metal filters?

A: Sintered metal filters offer high durability, excellent temperature and corrosion resistance, and the ability to withstand harsh environments, making them suitable for challenging industrial applications.

Q: In what industries are sintered metal filters commonly used?

A: Sintered metal filters are widely used across various industries, including pharmaceuticals, food and beverage, chemical processing, and aerospace, for their efficiency in removing particulates from gases and liquids.

 

Porous sintered bronze elements FAQ, applications beyond filtration plus references

Read Part 1 to find out more about sintered porous bronze filter elements and their use in small plastic inline fuel filters >>

Read Part 2 to find out more details about how to make sintered porous bronze filter elements and how they work >>

An FAQ section about sintering, porous sintered bronze fuel filter elements and depth filters

How does depth filtration work?
Fine particles suspended in a fluid passing through a depth filter may impact or run into the filter material and stick or adhere to it. This particle impact and capture is called adsorption.

Depth filters have complex, meandering and irregular interconnecting pores (tortuosity). This tortuosity increases the likelihood that depth filters can capture suspended particles in one or more ways (particle capture mechanisms).

Particle capture mechanisms are also called principles of filtration, mechanisms of filtration and mechanisms of particle capture.

The ways depth filters capture fine particles are (see the illustration)

• Diffusion interception (top)
• Direct interception (middle)
• Inertial impaction or interception (bottom)

Diffusion interception
Tiny particles suspended in liquids are bounced around by the molecules of the liquid. This random bouncing (Brownian Motion) increases the odds that particles bounce out of the fluid stream and bump into the filter material where they may stick.

Direct interception
Depth filter materials have tiny, winding, interconnecting passages or pores (tortuosity). Many of these pores or passages taper or interconnect with smaller passages.

Fluid can easily flow through narrower passages, but particles that are too large cannot pass through. When this happens, filter material directly captures these particles.

Inertial impaction
As the liquid passes through filter material (filter media), the flow follows the path with the lowest resistance to flow. Particles carried along in the fluid have mass and velocity. These combine to give moving particles momentum.

Momentum is the tendency for a moving object to keep moving in a straight line. Unless another force interacts with it, the particle does not change direction. Inertia is this resistance to changing direction.

The winding passages (tortuosity) inside depth filters cause liquids passing through it to change direction repeatedly. When this happens, the momentum and inertia of particles in the fluid cause them to try to keep going on in a straight line. This straight-line motion causes them to strike the filter material where they likely stick.

The technical aspects of depth filtration can be quite tricky. If you are interested in learning more about the mechanisms of particle capture by depth filters, check out this downloadable PDF reference resource:

Principles of Filtration from the Pall Corporation >>

What are filter elements?
Filter elements are the components inside a filter that do the actual filtering. Plastic inline fuel filters direct fuel flow through the filter element, capturing debris and particle contaminants.

Fuel filter elements for small inline filters tend to have a slightly conical or drum shape. These element shapes maximize the element surface areas and flow rate through the filters.

Flow direction matters for small inline filters, usually indicated by flow direction arrows molded into the filter body.

Filter elements are added to the fuel filter during their assembly because the elements arrive as separate parts. This interchangeability allows for variations in filter element material and design. Filter element design variations provide more options for micron rating and flow rate.

Types of fuel filter elements typically used in small plastic inline filters

• Metal mesh screen
• Porous sintered bronze
• Polymer treated cellulose (paper)
• Polymer (plastic) filament mesh screen

How do you clean a sintered bronze filter?A combination of ultrasonic cleaning and appropriate solvents can clean porous sintered bronze filter elements. This approach works best if the solvents flow in reverse through the filter (backflushing or backwashing).

Cleaning the sintered metal fuel elements inside small plastic inline fuel filters is not practical because

• Potentially damaging effects of solvents
• The use of ultrasonic welding to assemble plastic fuel filter bodies

Backwashing can extend the life of a sintered bronze fuel filter but, over time, fine particles build up and cause the filter to become plugged. Even before flow stops, restricted flow can make a filter unusable. The relatively low cost of plastic inline fuel filters means replacing them as needed is usually the best option.

Find out more about cleaning porous sintered filters at the CTG Technical Blog >>

What is a sintered filter?
Manufacturers use a powder metallurgy process to produce porous metal parts like sintered filter elements. In this process, metal powders are sintered or heated to a temperature that causes the powder particles to bond together.

Controlled sintering leaves interconnecting, winding passages or pores throughout the filter body.

The pores allow air or liquid to flow through the filter. They capture particles at the pore openings on the surface as well as down inside the pore passages.

How are sintered filters made?
For porous bronze filter elements, the manufacturing process used is called pressureless sintering or gravity sintering.

Pressureless sintering begins with pre-alloyed, spherical bronze powders poured into appropriately shaped dies or molds.

Metal powders pour freely, much like a liquid. Vibrating powder-filled dies ensure that metal powders fill the dies but do not leave any voids.

Dies filled with bronze powder are moved to a controlled-atmosphere, multi-stage furnace. The furnace heats the dies to the bronze's sintering temperature. Sintering temperatures are always lower than the melting point of the alloy and its primary metal.

Sintering heat causes the metal to diffuse between the powder particles where they touch. This diffusing metal creates narrow metal necks where the powder particles bond together to make mechanically strong parts.

For filter elements, applying sintering heat just long enough achieves the desired filter porosity.

The sintering temperature and die material are also chosen so that the bronze powder does not bond with or stick to the die surfaces. The dies are then allowed to cool.

After cooling, the final step is to remove the new filter elements from the dies.

Plastic fuel filter assembly combines the porous sintered filter element and filter's plastic body components. Ultrasonically welding the body components together holds the filter element tightly inside the finished fuel filter.

Precision and good manufacturing quality control ensure both leakproof welds and proper filter element placement.

Learn more about plastic fuel filter bodies at the ISM blog >>

What is sintering?
Sintering is a powder metallurgy process where metal powders are fused to create mechanically strong parts. The heat used to sinter a metal powder is less than the metal's melting point. But this heat is still high enough to cause the metal to flow together and bond powder particles together where they touch.

Sintering requires controlled-atmosphere, multi-stage furnaces.

What is the sintering process?
Sintering begins with filling a part die or mold with metal powder.

Are you interested in learning more about Custom Porous Metal Heterosexual Filter Supplier? Contact us today to secure an expert consultation!

Pressing or compacting the powder into the die is the next step when making solid parts and porous self-lubricating bearings. The pressure causes the metal powder particles to stick together. While not strong, these "green" parts hold their shape after being released from the die.

For fuel filter elements, the sintering process begins a little differently.

Similar to making solid parts, the process for filter elements begins with pouring metal powder into dies. Vibrating the dies removes any voids or empty spaces. The metal powder remains loose in the dies until sintering.

After being shaped in dies, "green" metal parts or powder-filled dies are exposed to a precise sintering heat. Sintering requires controlled-atmosphere, multi-stage furnaces.

This heat is enough to cause the metal to diffuse between the particles (diffusion bonding). This diffusing metal creates narrow necks of metal that bond the particles together. Additional sintering causes these necks of metal to grow.

This "Sinter neck formation" micrograph (photograph taken through a microscope) shows the metal necks that form between spherical sintered metal powder particles.

Sinter neck formation

Enough additional sintering can produce solid parts. Additional heating causes more and more metal to diffuse between the metal particles. Space between particles shrinks to the point where the parts become almost completely solid (densification).

For porous parts, the right amount of diffusion bonding leaves a pattern of holes on the surface. The holes or pores on the surface connect to a complex winding matrix of interconnecting passages throughout the finished part.

Carefully controlled sintering time and temperature create the required porosity of finished filter elements.

Producing porous metal parts requires careful control of these variables:

• Alloys used
• Particle shape
• Sintering time
• Sintering temperature
• Sintering atmosphere

Metal powders used for making sintered porous fuel filter elements also need to be

• Prealloyed tin bronze
• Highly spherical in shape
• Very uniform in size (narrow particle size distribution)

Carefully controlled sintering creates fuel filter elements with just the right amount of particle fusion. The result is a highly porous metal with controlled pore size and good depth filtration.

Learn more about sintering and sintering theory in this slide deck from Peter M. Derlet at the Paul Sherrer Institut >>

What is particle size distribution?
Particle-size distribution (PSD) is a way of describing the relative amounts of different-sized particles contained in batches of powder or other granular material. PSD provides particle analysis as a relative percentage or fraction of the total batch of material.

Usually, this information comes in a chart that graphically illustrates both percentages and particle sizes. Special measurement tools and techniques provide the data for these charts.

Sorting batches of powder for very consistent particle distribution is possible using sieving and other techniques.

The particle size data generated from analyzing these powder batches create bell-curved particle-size distribution graphs (see the illustration). The more carefully the powder is sieved and sorted, the narrower the bell curve is.

Manufacturers use highly spherical bronze powders with narrow particle size distributions to make sintered filter elements. The ready availability of uniform bronze powders makes it economical to produce sintered filter elements with specific porosity and micron ratings.

These special bronze powders also make it economical to manufacture sintered bronze filter elements small enough for small plastic inline fuel filters.

Shimadzu, an analytical and measuring instruments company, has a great series of articles on particle size distribution >>

What is diffusion bonding?
Diffusion bonding, also called solid-state diffusion bonding, is a metal bonding process that resembles welding. When exposed to sintering heat, metal diffuses between powder particles where they touch.

The heat of sintering dramatically speeds up a tendency for the metal in each particle to migrate towards and intermix with metal from other particles.

Metal blending and fusion bind the particles together by causing metal "necks" to form and grow where the particles touch. These necks link the metal powder particles together.

Over time, exposure to the sintering temperature causes metal necks to grow larger and become stronger. The formerly loose particles become solidly locked together to make a mechanically strong part.

Find out more about diffusion bonding at Wikipedia >>

What is the difference between sintering and melting?
Pressure sintering, solid-state sintering and pressureless sintering (gravity sintering) all use temperatures below the metal's melting point. This temperature is called the sintering temperature. Even though it is less than the metal's melting point, it is high enough to cause the metal to quickly diffuse between each particle (mass transport). This metal diffusion bonds the particles together (diffusion bonding).

At the metal's sintering temperature, heat bonds the powder particles together, creating mechanically strong parts.

Solid parts and densification
Pressure sintering and solid-state sintering techniques produce solid parts starting with metal powders.

These powder metallurgy processes use a combination of heat and pressure to cause the metal to diffuse between the metal powder particles where they touch. This bonds the particles together. The parts are then kept at the sintering temperature long enough (dwell time) to eliminate the empty spaces between the particles (densification or consolidation).

Pressureless sintering and porosity
Pressureless sintering is the specific type of sintering used to make porous fuel filter elements. Filter elements produced using pressureless sintering have a uniform porosity that runs throughout the metal. Pressureless sintering is a relatively economical and useful way to make metal filter elements.

All sintered parts require special furnaces
All sintering requires controlled-atmosphere, multi-stage furnaces. The controlled atmosphere protects metals from exposure to oxygen-rich air. Exposure to oxygen during sintering creates metal oxide films that inhibit particle fusion.

Controlling the atmosphere or gases inside sintering furnaces also provides better, more uniform heat transfer inside the different furnace stages.

What is the sintering temperature?
In powder metallurgy, sintering is the heat treatment used to fuse metal powder particles to make solid parts. Sintering temperatures are less than the melting point of both the alloy and its primary metal.

Sintering temperatures are lower than melting temperatures because sintering does not use melting to bond the particles together. Instead, the heat of sintering is enough to cause the metal to diffuse between the powder particles. Diffusing metal causes interconnecting metal necks to form, grow and bond the particles together where they make contact.

Porous sintered bronze filter elements used in small plastic inline filters are tin bronze, usually about 89-91% copper and 9-11% tin.

• 449 ºF (232 ºC) tin melting point
• ºF ( ºC) copper melting point
• - ºF (740-780 ºC) tin-bronze sintering temperature
• 1,570 °F (854 °C) tin-bronze melting point (C/SAE 620 high-tin bronze)

Find out more about sintering temperatures and the difference between it and molten metal casting at Marlin Steel Wire Products >>

Why is sintering done?
Sintered parts made using suitable materials and sintering techniques can be mechanically strong while still being filled with tiny winding, interconnecting passages. Parts produced this way can be used for filtering, sound dampening, sparging, flame and spark arresting, self-lubricating bearings and more.

Sintering causes metal "necks" to form and grow between the tiny bronze spheres. Metal diffuses or flows together at the particle contact points to create solid metal bonds.

The controlled sintering of porous sintered parts provides parts with metal necks that are narrower than the sphere diameters. The result is tortuous or sinuous winding passages that run through the depth of the porous metal walls.

Because porous bronze filter elements start as highly spherical particles, sintering produces filter elements with controlled porosity and specific micron ratings.

For both porous and solid parts, sintering has advantages:

• Material waste is very low
• Lower secondary machining costs
• Economical for short production runs
• Energy efficient because no melting of the raw material
• Even relatively complex parts have high production rates
• Very good control of finished part properties like hardness

 

What are powder metallurgy's disadvantages?
Making porous sintered filter elements requires the use of powder metallurgy techniques. They have distinct advantages over other manufacturing techniques for making solid parts.

It is important to note the disadvantages of powder metallurgy processes:

• Maximum part sizes may be limited
• Not suitable for highly complex shapes
• Not as strong as parts produced by forging
• Requires specifying just the correct metal powder
• Metal powders require significant time and effort to produce
• Expensive process for low value and low melting point materials
• Not possible to form some more complex features that only machining can create

Horizon Technology has a helpful article outlining some of the significant advantages and disadvantages of using powder metallurgy instead of machining to produce metal parts >>

What is pressureless sintering?
Sintering is a powder metallurgy manufacturing technique. Pressureless sintering (gravity sintering) is just one of many types of sintering.

Steps in the pressureless sintering process

• Pour metal powders into dies or molds
• Vibrate the dies to make sure the powder leaves no voids
• Place the dies in a furnace for controlled exposure to the metal's sintering temperature

Sintering temperatures vary by alloy and are lower than the melting point of the primary metal in the alloy.

During sintering, the metal powder does not melt. The temperature is high enough to cause narrow, metal-to-metal bonds or necks to form where the metal powder particles touch.

The result is a fully hardened, mechanically strong part that has excellent porosity. Porosity is the intricate pattern of winding, interconnecting passages throughout the walls of the finished part.

Pressureless sintering is especially useful for producing filter elements, but the process requires careful control of

• The technical specifications of the metal powder used
• The sintering temperature the powder-filled dies are exposed to
• The length of time the furnace holds dies at the sintering temperature

A careful, precise manufacturing process provides filter elements with good porosity, which makes them very efficient.

Efficiency means porous sintered bronze filter elements are very good at capturing small particles without creating too much restriction or reduced flow through the filter.

What's unique about the bronze powders used to make fuel filter elements?
It is necessary to control both the filter walls' porosity and the filter element's micron rating when making porous sintered bronze filters. This kind of control requires bronze powder with specific qualities.

Bronze powder requirements for making filter elements

• Prealloyed bronze
• Deoxidized bronze alloying
• Controlled spherical particle size
• Highly regular, consistent, spherical shape

Deoxidizing gives these bronze metal powders a light gloss. Because of this, better-quality sintered bronze filter elements tend to be shiny.

How are spherical bronze powders made?
Gas atomization is the process most widely used to produce highly spherical metal powders.

It involves pouring the molten bronze through inert gas jets. The pressurized gas coming out of the jets breaks the molten metal into fine metal droplets. These molten droplets cool down and harden as they fall.

Metal powders made by gas-atomization tend to have a perfectly spherical shape and are very clean.

What about ethanol, methanol and E85 gasohol materials compatibility?
In general, material compatibility with methanol is more challenging than gasoline-ethanol blends, including E85 flex-fuel. Unless used as a racing fuel, most fuel filter exposure to methanol comes from fuel additives. Additives for treating water contamination or for preventing fuel line freezing sometimes contain methanol as an ingredient. Any filter that is methanol compatible is also E85 compatible.

Find out more about gasohol (gasoline-ethanol) blends and outdoor power equipment from OPEI, the Outdoor Power Equipment Institute >>

Low-quality materials and fuel compatibility
Low-quality paper filter element filters may not hold up well when exposed to gasohol (E10, E15 and E85). Lower-quality filters frequently use adhesives to attach the filter paper to the paper filter element's cap or end. These adhesives usually are not compatible with ethanol. The adhesive bonds weaken over time, and adhesive failure allows fuel flow to bypass the filter paper altogether.

Filter body plastics also matter when it comes to compatibility with ethanol, methanol and gasohol blends. High-quality OE and OEM aftermarket fuel filters have plastic bodies made with materials with very good resistance to ethanol and gasohol blends and limited exposure to methanol.

A good maintenance rule of thumb for plastic inline fuel filters is to use high-quality OE replacement and OEM aftermarket plastic inline fuel filters made by reputable manufacturers like ITW Fastex Filtration and ITW Powertrain Components.

For both Powersports and Outdoor Power Equipment (OPE) applications, recommended best practice is to replace fuel filters at least once every year.

Find out more about ANSI-OPEI B71.10- compliant inline plastic fuel filters and shut-off valves from ITW Fastex Filtration and ITW Powertrain Components >>

What are the effects of ethanol on gasoline?
Ethanol or ethyl alcohol is the alcohol added to gasoline to make gasohol blends like E10, E15 and E85. Ethanol absorbs water more quickly than straight gasoline does. When a small engine with only a partially full tank of a gasoline-ethanol blended fuel sits around for a while, moisture can build up in the fuel system and may separate.

Find out more about standard ethanol fuel mixtures at Wikipedia >>

Ethanol, gasohol and absorbed water
If water does settle out, it settles out at the bottom of the tank. This settling happens because water is heavier than both gasoline and ethanol. This settling can also occur in fuel lines and carburetor bowls.

Fuel containing water can create problems for plastic inline fuel filters that use paper filter elements if they are not compatible with gasohol. Water absorption can cause low-quality filter paper to swell and lose strength.

The paper filter elements used in low-quality filters are also more likely to be degraded by exposure to ethanol. This ethanol also causes low-quality filter papers to lose strength.

Filter elements like porous sintered bronze are not affected by water or ethanol in the fuel.

Make sure that the plastic used for the filter housing is ethanol compatible also. Most lower quality plastic inline fuel filters use plastics that are less able to stand up to prolonged exposure to gasohol.

Low-quality filters are also more likely to be assembled using adhesives that may not withstand gasohol.

High-quality OEM fuel filter manufacturers use filter elements, body materials and assembly techniques not degraded by water and ethanol.

Find out more about materials and chemical compatibility with the ISM Chemical Compatibility Chart  >>

Besides fuel and oil filtration, what other applications use porous sintered bronze elements?
Porous sintered bronze is mechanically strong and made with very controlled levels of porosity. These features make porous bronze parts and elements useful for a variety of applications:

• Breathers
• Liquid aeration
• Exhaust mufflers
• Condensation traps
• Flame suppressors
• Powder fluidization
• Pressurized fluid flow control
• Snubbers or pressure snubbers

Breathers
Breathers filter exhaust gases or incoming air. In this way, they serve as oil reservoir vents, air exhaust vents, air intakes and vacuum pressure equalization ports. Breathers can help reduce the exhaust of tiny, suspended oil droplets, although they are not as efficient as dedicated oil mist filters.

Get more information about the BV series breather vents in the ISM catalog >>

Liquid aeration
Pressurized gas passed through porous sintered bronze elements comes out as fine bubbles when suspended in liquids. The gas in these tiny bubbles is absorbed or dissolved into the liquid. Used this way, porous bronze elements can gasify or aerate liquids (sparging).

Find out more about the use of sparging (chemistry) or gas flushing (metallurgy) at Wikipedia >>

Exhaust mufflers
Mufflers or exhaust mufflers dissipate the velocity and dampen the noise level of compressed air coming out of pneumatic valve exhaust ports, air cylinders, air tools and similar applications.

 

MS series speed control muffler cross-section
showing the porous sintered bronze element

Get more information about the MS series speed control mufflers in the ISM catalog >>

Condensation traps
The porosity of sintered porous bronze allows it to retain water vapor by causing water to condense inside the porous bronze. In this way, porous sintered bronze elements are used both for drying exhaust and for protecting devices from corrosion caused by moisture entering the system.

Flame suppressors
Flame suppressors or arrestors prevent a flame from entering or leaving equipment while also providing precise gas flow control.

Porous sintered bronze elements have both high thermal conductivity and porosity. These features allow the flow of combustible gasses while preventing ignition. The porous bronze element absorbs and dissipates the heat from a flame front or combustion wave and stops combustion.

Powder fluidization
Sintered bronze can also fluidize fine powder solids. Fluidization is provided by passing pressurized gas through porous bronze elements.

Fluidization makes it easier to manipulate powder solids by causing them to behave more like a liquid. It does this by increasing the solids-to-air ratio. The increased solids-to-air ratio reduces the density of the gas and powder mix. This change in density allows the particles to become suspended and flow more easily.

Pressurized fluid flow control
The porosity of sintered bronze elements can provide a very precise, constant and controlled pressure drop for pressurized fluid processes.

Snubbers or pressure snubbers
Snubbers protect gauges and pressure instruments from damaging shocks caused by sudden pressure surges and fluctuations in fluid or gas flows. As a beneficial side effect, they also help ensure gauge accuracy by smoothing out pressure transients (spikes).

SNUB series pressure snubber

Get more information about the SNUB series pressure snubbers in the ISM catalog >>

 

Some additional reference resources about porous sintered bronze filters, powder metallurgy and sintering

Advances in Filtration Technology Using Sintered Metal Filters at the Mott website >>

Characteristics and Properties of Copper and Copper Alloy P/M Materials at the Copper Development Association website >>

How Modern Powder Metallurgy Stacks Up Today: A Comparison of Conventional Powder Metallurgy Versus Competing Methods at the Horizon Technology >>

Metal Powder Atomisation Methods for Modern Manufacturing by John J. Dunkley via the Johnson Matthey Technology Review >>

Porous Metal Design Guidebook PDF from SpinTek >>

Sintering Theory by Peter M. Derlet at the Paul Scherrer Institut. It is a lecture slide deck PDF  >>

About this three-part series on sintered porous bronze fuel filters

Sintered Porous Bronze Fuel Filters - Part 1: Benefits of porous sintered bronze filter elements and how they are made  >>
An overview of porous bronze filter elements, why they are so effective in small plastic inline fuel filters and what it takes to produce high-quality sintered porous bronze fuel filters.

Sintered Porous Bronze Fuel Filters - Part 2: About depth filtration and the porous filter elements used in fuel filters >>
More detail about bronze, porous sintered bronze filter elements as depth filtration, how to make them, and their installation and use.

Sintered Porous Bronze Fuel Filters - Part 3: Porous sintered bronze elements FAQ, applications beyond filtration plus references >>
An FAQ about sintering, sintered porous bronze fuel filter elements and depth filters. It also includes an overview of other applications that use porous bronze filter elements, plus a list of references.

 

Selected ITW Fastex Filtration Visu-Filter fuel filters

ITW Fastex Filtration and ITW Powertrain Components Visu-Filter fuel filters are compliant with ANSI/OPEI B71.10-.

The ISM catalog includes

• Product specifications
• Photos and 3D CAD images
• Links to product pages in our e-commerce catalog

Check out our complete lineup of miniature fittings and components >>

Keep in mind we carry thousands of related items like valves, connectors, adapters, gauges and so on.

Get the catalog

 

 

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