What Is a Filter? Cartridge vs Bag

A filter is a device that removes solid particles, contaminants, or impurities from a fluid stream (liquid or gas) by passing it through a porous medium. Filters are required in oil and gas facilities to protect equipment, ensure product quality, and maintain process efficiency. The two most common types for liquid service are cartridge filters and bag filters.

Filters differ from strainers in their ability to remove finer particles. Strainers typically handle particles above 40 microns and are designed for coarse protection, while filters remove particles down to sub-micron levels for fine purification.

Cartridge Filter vs. Bag Filter

Feature	Cartridge Filter	Bag Filter
Filtration range	0.1 to 100+ microns	1 to 200+ microns
Flow capacity per element	Lower (small surface area per cartridge)	Higher (large surface area per bag)
Particle removal efficiency	Higher—better for fine filtration	Lower—better for bulk particle removal
Dirt-holding capacity	Lower per element	Higher per bag
Changeout	Multiple cartridges per housing	Single or few bags per housing
Typical cost per change	Higher (more elements required)	Lower (fewer, less expensive elements)
Best application	Fine polishing, low-contamination streams	Pre-filtration, high-solids-loading streams

Filter Types in Oil and Gas

Filter Type	Description	Application
Cartridge filter	Pleated or depth-type elements in a pressure housing	Amine filtration, seal oil, lube oil, chemical injection
Bag filter	Fabric bag in a housing; flow from outside to inside the bag	Cooling water, produced water pre-treatment
Sand filter	Gravity or pressure vessel with sand/gravel media	Produced water treatment, injection water
Coalescing filter	Special media that merges small droplets into larger ones for separation	Gas dehydration, fuel gas conditioning
Activated carbon filter	Carbon bed adsorbs dissolved contaminants	Amine purification, condensate polishing
Self-cleaning filter	Automatic backwash or scraper mechanism	Continuous-duty, high-flow applications

Common Mistake

Specifying a filter based solely on its nominal micron rating can be misleading. Nominal ratings indicate that the filter removes a percentage (often 60-90%) of particles at the stated size, while absolute ratings guarantee 99.9%+ removal. For critical applications such as compressor seal oil or hydraulic systems, always specify an absolute micron rating to ensure adequate protection.

Key Filter Specifications

Parameter	Typical Range
Housing design code	ASME Section VIII (pressure vessels)
Housing material	Carbon steel (standard), stainless steel (corrosive service)
Design pressure	Matched to piping system design pressure
Cartridge materials	Polypropylene, polyester, fiberglass, stainless steel
Flow direction	Outside-in (cartridge), outside-in (bag)
Differential pressure alarm	Typically 1.0-1.5 bar (signals element replacement)
Connection type	Flanged nozzles per pipe class specification

Filters are typically installed downstream of pipeline strainers and upstream of sensitive equipment. Differential pressure instruments monitor filter condition and trigger element changeout.

Read the full guide to oil and gas equipment