The main function of a filter separator is to clean the process gas to achieve the desired cleanliness standards.

The typical impurities found in contaminated gas that the filter separator can remove include:

• Liquids such as water and hydrocarbons
• Sand, abrasives and corrosive solids
• Chemicals found in pipelines and plants

Excellent removal of the contaminants will protect other system components downstream from damage and corrosion.

The filter separator removes solid and water contaminants from the gas stream using three different methods.

In the first stage, the incoming contaminated gas impinges the filter elements support, also known as risers.

The impingement drastically reduces the velocity of the gas which helps remove heavier contaminants through impaction, deflection, and action of gravity.

Also, the gas stream then proceeds to element stage stages where the gas passes through coalescing filter elements from outside to inside flow.

Furthermore, the elements restrain particulate contaminants and some liquid droplets.

The gas then enters the final separation stage which contains either a wire mesh mist eliminator, a centrifugal device, vane, or mesh-vane assembly.

Filter separator system

1)      Wire Mesh

Wire mesh eliminators can remove droplets down to 2 microns although most models remove droplets above 8 microns.

A knitted wire mesh eliminator should have varying thickness and density.

The gas stream impinges the wire mesh pads vertically, enabling them to coalesce the liquid droplets.

As the gas rise to the outlet, the liquid drops to the sump.

You should not use wire mesh pads in the process that are likely to form hydrates or foul.

Similarly, avoid using them for more corrosive processes.

Though you can use several materials to construct wire mesh pads, stainless steel is more suitable because it prevents any possible oxidation.

Compared to other types of separating devices, wire mesh eliminators have a very low-pressure drop and are cost-effective.

2)      Vane Mist Eliminators

The vane mist eliminator forces the gas stream through a torturous path.

As the gas stream goes through the device, it hits the vane walls, which remove the droplets through the force of inertial.

You can use several materials such as carbon steel, aluminum, and stainless steel to construct a vane type mist eliminator.

Vane type mist eliminators can easily fit in small size filter separators because they do not require large space.

Consequently, they can achieve similar filtration efficiency to wire mesh separators.

Just like wire mesh separators, you should limit their use in process application which have sticky and fouling impurities.

The main benefits of using vane type mist eliminators are longevity, compact design, and lower pressure drop. Unlike wire mesh separators, vane type mist extractors do not readily plug.

3)      Centrifuge Devices

You should consider the characteristics of your process application to enable you to select the most appropriate design of a centrifugal mist eliminator.

This device has a higher pressure drop and lesser filtration efficiency compared to wire mesh mist extractors and vane mist separators.

They also have a narrow range of operation and some designs are less suitable for applications with a slug.

The main difference between the two designs of filter separators is the orientation. However, both designs of filter separators conform to the same principles of operation.

·         Vertical Filter Separator

Vertical separator

This design is suitable when the liquid to gas ratio is high. The filter separator can have a single or double compartment fitted with two different sumps or a single sump.

You will increase the height of the vessel if you need to increase the ability to hold coalesced liquid.

It has a sensitive liquid level sensor which triggers an alarm to shut it down in case the liquid in the sump surpasses the preset level.

Vertical filter separators occupy lesser floor space and are suitable for systems with limited installation area.

·         Horizontal Filter Separator

Horizontal filter separator

This orientation is suitable for applications that have a large volume of dissolved gas and entrained liquids are present in the process fluid stream.

Horizontal separators can handle process applications with higher flow rates and provide ample space for the release of suspended liquid droplets.

Though they require a larger installation floor space compared to a vertical filter separator, the size can significantly shrink when constructed as a double-barrel vessel.

The lower barrel will provide sufficient space for the collection of removed liquids.

A horizontal filter vessel also provides a lower pressure drop if installed in line compared to a vertical configuration.

You can use a filter separator in a gas that has already gone through a demister.

You can also use the device in a sensitive process for the protection of downstream equipment.

Some of the common application of a filter separator include:

• Gas distribution systems
• Gas storage facilities
• Gas compressors
• Metering stations
• Pressure reduction stations
• Refineries
• Power plants
• Natural gas plant
• Distribution pipelines
• Glycol dehydrators
• Petrochemical plants
• Chemical plans
• Desiccant bed protection

A good filter separator should possess the following characteristics:

• Efficient separation of vapor, water, oil, solid and other impurities from the gas stream
• Conform to respective regional industry and regulatory standards
• Compatible with a wide range of application
• Possess differential pressure gauges and liquid level transmitters
• Easy to open for quick element replacement
• Flexible designs to fit the space restrictions and application conditions
• Guaranteed performance, that is, efficiency and durability
• High-efficiency filter elements with large dirt holding capacity
• Resistant to corrosion

You will benefit from the following advantages of using filter separators:

• Suitable for both offshore and onshore applications
• Can achieve 99.98% contaminants removal efficiency down to 0.3-micron particle size
• Enables fast and easy element replacement
• Easy to maintain
• Premium designs increase production and lower the downtime cost
• Does not require special tools and skills to operate
• Protects system components from damages due to efficient filtration
• Safe to use
• Saves on installation space
• Can operate under harsh conditions because of quality construction materials

A filter separator consists of the following main components:

• Ports: The inlet port allows the contaminated gas to enter the filter vessel and the outlet port allows clean gas to leave the filter.
• Pressure relief valve: It releases excess pressure inside the housing
• Head/Cover: It provides quick access to the filter elements for replacement
• Sump: Collection of removed liquids
• Sump divider plate: Separates the sump for collecting coalesced liquid and separated liquid
• Pipe risers: Supports the coalescing filter elements. They also act as a primary barrier for removal of free, large liquids droplets and large particulate contaminants
• Coalescing filter elements: Traps solid contaminants and coalesce small liquid droplets into large ones which can be removed with gravitational force.
• Mist extraction devices: They remove tiny coalesced liquid which can re-entrain the clean gas.
• Housing: Host all the components of the filter separator.
• Differential pressure gauge: Gives the pressure drop readings across the filter element.
• Liquid level transmitters: Detects the level of fluid within the sump inside the vessel.
• Drains: Removes the coalesced and separated liquid from the sump

There are only two types of filter separators which depend on the design, that is, vertical and horizontal filter separators.

You can also choose from available options on the kind of element cartridges, alloy constructions and coatings, and the type of mist extraction devices.

Depending on the nature of the application, you can choose oil water separator filter or coalescer filter separator.

The size of a filter separator depends on the filtration requirements and design of the device.

Filter separators that remove water will be smaller compared to filter separators that remove light hydrocarbons.

Usually, the coalescing element compartment has a lot of influence on the equipment size compared to the separating section.

You have to determine the size of the filter separator mathematically depending on the device orientation and mist elimination device you are using.

The diameter is very critical, especially for vertical vessels which have a higher stream drag force compared to horizontal vessels.

You should work out the minimum diameter necessary to allow the coalesced and separated water to settle in the sump.

The following factors will also affect the size of a filter separator:

• Fluid velocity
• Drop size
• Level of contamination
• Properties of the liquid contaminants such s surface tension and viscosity

When connecting the filter separator to the piping, you should take note of the following constraints to ensure that piping does affect the efficiency of the equipment.

• Avoid using contractions, pipe expansions, and valves on the pipe diameter attached to inlet nozzle as they will create tiny liquid droplets.Use a fully open gate or ball type feedline valve under circumstances that do not allow for the exclusion of the valve.Also, locate valves that reduce pressure further upstream as practicable in the feed pipe where you cannot avoid them.

• Avoid using bends within the in the pipes connecting the inlet nozzles because they will affect the distribution of gas stream within the filter separator.
• You can use a pipe reducer in the pipeline connected to the outlet ports. However, the location must be twice the diameter of the outlet nozzle from the vessel top.
• The design and layout of the reducer should not have pockets that can encourage the accumulation of liquids and increase slug flow.

You should periodically change the filter elements depending on the change out pressure drop recommended by the OEM.

Usually, the change out pressure drop varies between 7 and 10 psi.

Ensure that you replace the filter element at least once in a year even if it has not reached the change out pressure drop yet.

The period to change out pressure drop varies greatly to the flow rates and level of gas contamination.

The vessel can take up to 25 years when subjected to suitable working conditions and maintenance and therefore will not require periodic replacement.

When selecting a filter separator, you should consider the following key factors:

Filter separator system

·         Gas Stream Velocity

The internal components and configuration of the equipment should not accelerate the velocity of the gas stream once it enters the vessel.

This is important to avoid the shearing of the liquid droplets into tiny drops that can easily re-entrain the clean gas.

·         Pressure Drop

You should select filter separators which have the lowest pressure drop across the filter elements but yet delivers efficient filtration.

For vessels that operate below 500 psi, ensure that you choose filter separators that have a pressure drop of at least 1 psi or lower.

Similarly, the pressure drop should not exceed 2 psi for vessels that operate beyond 500 psi.

You should not choose any filter separator that has more than 0.5 psi pressure drop across the element arrangement.

This pressure drop will multiply four times when the element is 50% plugged with particulate contaminants.

·         Service Life of the Housing

You should carefully examine the suitability of the housing for your process requirements.

Selecting a high-quality vessel constructed using a durable and corrosive resistant material will last longer and give you better service.

Usually, a good vessel can last between 20 and 25 years when used under suitable conditions with proper maintenance.

·         Performance

You should consider the level of desired filtration and the cost of not achieving it.

By so doing, you can factor in the risk of exposing the downstream equipment to a certain level of a re-entrained liquid.

Do the repair, maintenance, and downtime cost worth installing the filter separator you are choosing.

You should make a decision based on a cost-benefit ratio associated with installing a less efficient device.

·         Cost

The ownership cost is one of the most important aspects when selecting the filter separator. You should sum up all the costs listed below to determine if you are within your budget of operation and avoid running into losses.

• The initial purchasing price of the filter vessel.
• Cost of installation, element replacement, and maintenance.
• The cost of energy required to overcome the vessel and element pressure drop
• Operation and downtime costs
• Environmental cost
• Cost of disposing of the elements

·         System Operating Condition

The filter separator you choose must be compatible with the process application flow rate, pressure rating, and temperature range to enable a smooth flow of operations.

Reverse flow filter separator is a self-cleaning separator that optimizes the gas flow and reduces the cost of the element life cycle.

The incoming process gas will first pass through the plenum then proceed to the collection tubes then filter elements.

The gas velocity is significantly reduced in the de-entrainment section through sizing to facilitate the removal of any particulate contaminant remaining.

The particles either drop out or attach to the large liquid droplets and drain to the sump

A quality filter separator should conform to the requirements of the following industry standards and codes:

• ASME Sec U, U2 or VIII
• PED 2014/68/EU
• ASME B16.5
• ASME B16.47
• NACE MR0175
• API 14C
• EN 10204 type 3.1

Filter separator system

You can test the filtration efficiency of a filter separator using several tests.

You can conduct a dust spot efficiency test to determine the percentage retention of particulate contaminants.

In the case of liquid contaminants, you can conduct two tests including:

• Course water coalescing test.
• Emulsified water coalescing test.

You can also do a confirmatory test on the filter housing and elements to check if the filtration efficiency matches the one described by the manufacture.

These tests include:

• Proof and burst pressure test
• Helium leakage test
• Media migration test
• Dirt holding capacity test
• Collapse test

Other examinations that you can do include ultrasonic examination, wet and dry, hydrostatic testing, X-ray test, and stress relieving test.

The material for constructing the medium for a separator will depend on the type of liquid droplets you want to remove

The material for constructing the coalescing element should attract the liquid droplets to ease the amalgamation process.

When constructing a medium for removing water droplets, you should go for hydrophilic material such as fiberglass and stainless steel.

However, when you want to remove light weight hydrocarbon contaminants such as oil, then you should select oleophilic material such as polypropylene and fluoropolymers.

However, the process gas is usually contaminated with both water and oil impurities.

Hence, some of the most suitable and high performing media comprises a mixture of both oleophilic and hydrophilic materials.

Furthermore, such medium comprising of different materials with diverse properties are usually more effective due to synergy compared to the single-material medium.

You can also use nylon, micro-glass, and polyester.

Fiberglass is the most common type of medium because the material is widely available and is cheaper.

Yes, you can clean the filter separator housing, coalescing elements, and the mist collectors.

Because filter elements are the most sensitive, confirm with the product manufacturers on the best techniques of cleaning them.

You can also consider self-cleaning filter separators that use diverse technologies to remove dirt from the outer surface of depth cartridges and vessels periodically.

You should consult the manufacturer when designing the filter separator given that a filter separator design is proprietary.

The following factors will affect the choice of a given design.

·         Characteristics of the Contaminants

The characteristics of the contaminants will determine the choice of material for constructing the filter separator medium and housing.

The choice of a design will depend on the composition of the fluid mixture.

For example, you will use different materials for constructing medium for removing suspended hydrocarbons and water droplets.

·         Gas Velocity

The velocity of process gas as it enters the separation area can be overlooked when designing the filter separator.

If the velocity of the gas exceeds the maximum allowable gas velocity, then the fine droplets will shear back into the mist and re-entrain the filtered gas.

·         Performance and Filtration Efficiency

The general performance of the filter in removing impurities found in gas greatly depends on the design of the filter.

When designing a filter separator, you have to consider the nature and size of impurities you want to remove.

Laboratory tests may reveal that the filter separator can achieve 99.98% removal of water drops and particulate contaminants of a specific micron rating.

However, this efficacy cannot be guaranteed under field conditions because of numerous environmental and other external influences.

You should select a horizontal filter separator with different sump compartments when the process gas is heavily contaminated with liquids.

The selected separator should remove and dump the coalesced liquid and inlet free liquid in separate sumps.

·         Surface Area for Filtration

The surface area for filtration depends on the dirt holding capacity of the coalesce filter cartridges you are using.

You can get this information from the product specifications and filtration capacity graphs that are supplied along with the elements.

If the incoming process gas is highly contaminated, then you should select a filter with double or triple the approximate filtration surface area.

·         Pressure Drop

The filter separator assembly should maintain a very low-pressure drop and achieve the desired level of filtration efficiency.

The pressure drop will vary from one filter separator model to the other.

Normally, a clean filter separator has a pressure drop between 1 to2 psi.

You can schedule regular element replacement or cleaning when the pressure drop exceeds 10 psi.

You should take care not to exceed the pressure drop level beyond which the cartridges will collapse.

In most cases, a maximum pressure drop is 25 psi though it might vary between manufacturers.

·         Pressure and Temperature Rating

Ensure that the filter separator design is capable of withstanding the system pressure and temperature.

The filter separator assembly should be subjected to nondestructive tests such as pressure testing, ultrasonic and radiography tests.

Besides, the design must take into consideration the pressure spikes that are likely to occur in the system application as well as the minimum pressure.

Also, the material of construction must be able to withstand the temperature of the process application.

·         Purpose and Usage

The design should fit into the process application, incorporating all the special needs that the application may require.

You need to know what kind of equipment you will be protecting and the type of contaminant that you want to remove.

You also need to know where the filter separator will be operating.

The knowledge will help you match the design with all the environmental conditions of the location of the installment.

For example, you cannot use a filter that is designed for offshore applications in systems that are submerged under saline water or alkaline lakes.

·         Material of Construction

Selecting the most appropriate material goes a long way to ensure reliability and operation safety.

You need to consider the system operating temperature and pressure, resistance to corrosion, and chemical compatibility.

Also, consider the surface treatment options that are available for the material you select.

·         System Piping and Connection

You should determine and consider pressure drops and velocity limits as a result of fluid transmission through piping.

Placement of inlet and outlet ports should also take accounts of the fluid characteristics to avoid unprecedented problems.

Ensure that you appropriately connect the equipment in a way that will ensure maximum filtration.

·         Maintenance

You should design the system to allow for easy and speedy servicing and maintenance.

Appropriately place the access enclosure to support easy access to all the assemblies.

When installing the filter vessels, you should provide clear information on the casing about the model and appropriate information to avoid possible mix-ups.

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