Multipass Method (Particle Counting and Filtration Testing)

The multipass method (also called the multipass filter test or ISO multipass filtration test) is a standardized laboratory test procedure for evaluating the filtration efficiency and contaminant capacity of hydraulic and lubricating oil filters — in which a controlled quantity of test contaminant (fine silica particles) is continuously injected into a flowing oil stream upstream of the test filter, with particle count measurements taken simultaneously upstream and downstream of the filter at multiple particle size thresholds, and the filtration ratio (Beta ratio, β) calculated as the ratio of upstream to downstream particle counts to express the filter's capture efficiency as a function of particle size.

Key Takeaways

The Beta ratio (filtration ratio) at a given particle size x — written βx — is defined as the upstream particle count per unit volume divided by the downstream particle count per unit volume for particles larger than x micrometers: βx = (upstream count per mL) / (downstream count per mL); a β10 = 200 means that for every 200 particles larger than 10 micrometers upstream of the filter, only 1 passes through — corresponding to a capture efficiency of 199/200 = 99.5%; β values of 100 to 1,000 are typical for high-efficiency filters used in hydraulic systems and oilfield fluid processing equipment.
The multipass method was standardized as ISO 16889 (Hydraulic Fluid Power — Filters — Multi-Pass Method for Evaluating Filtration Performance of a Filter Element), adopted internationally by fluid power industry associations, and is the basis for filter performance ratings used by all major filtration suppliers (Parker Hannifin, Pall, Donaldson, Eaton Filtration) in hydraulic and lubrication oil system filter specifications for oilfield equipment including hydraulic power units (HPUs), mud pump stroke counters, draw-works and crane hydraulics, and BOP accumulator systems on drilling rigs.
In the petroleum production and processing context, the multipass method is applied to evaluate filters used in crude oil dehydration trains, gas dehydration glycol systems, amine gas treating units, injection water filtration skids, and hydraulic control systems — critical filtration points where inadequate filter performance allows contaminant particles to damage pumps, plug flow control valves, contaminate glycol or amine solvents, or cause injection well injectivity decline by delivering particulate-laden water into the formation.
Filter media used in oilfield applications includes: glass fiber depth filter media (high dirt-holding capacity, moderate pressure drop, used in large-volume filtration applications like injection water); pleated cellulose or synthetic fiber surface filter elements (lower dirt-holding capacity, sharper cutoff particle size, used in hydraulic control systems requiring tight cleanliness specifications); and coalescers (glass fiber or fiberglass media with hydrophobic or hydrophilic treatment that separates emulsified water from oil or vice versa by coalescing small droplets into large droplets that separate by gravity).
ISO cleanliness codes (ISO 4406) provide a standardized language for expressing the particle contamination level of oil samples taken from hydraulic and lubrication systems — reported as three numbers (e.g., ISO 18/16/13) representing the particle counts per mL in three size ranges (larger than 4 μm, larger than 6 μm, and larger than 14 μm respectively), with each number representing a range of particle counts on a binary scale; equipment manufacturers specify required cleanliness codes for hydraulic system oil, and the multipass test beta ratios of installed filters determine whether the system can achieve the required cleanliness levels under normal operating contamination ingress conditions.

Fast Facts

The multipass test method was developed in the late 1960s and early 1970s by the fluid power industry to replace earlier, less reproducible single-pass filter rating methods that gave misleading comparisons between filters. Before the multipass method became standard, filter efficiency was often expressed as the "nominal rating" (the particle size at which the filter was claimed to remove some percentage of particles) — a loosely defined and inconsistently applied measure that allowed manufacturers to claim efficiency at particle sizes their filters did not reliably capture. The ISO 16889 multipass standard, first published in 1999, established a reproducible, third-party testable methodology that allows direct performance comparison between filters from different manufacturers and provides the Beta ratio data needed for hydraulic system filtration design. Oil sampling and particle counting services using automated particle counters (HIAC Royco, Beckman Coulter particle counters) are offered by oil analysis laboratories as part of oil condition monitoring programs for critical oilfield equipment.

What Is the Multipass Method?

Filtration efficiency — how completely a filter removes particles of various sizes from a fluid stream — is not a fixed property of the filter medium alone. It depends on flow rate, differential pressure, the nature and concentration of contaminant particles, and whether the filter is clean (new) or loaded (partially plugged with previously captured particles). A filter test that measures efficiency under only one set of conditions can be misleading if the operating conditions change, and a filter that performs well at low contamination concentration may perform very differently when heavily loaded.

The multipass method addresses this by continuously injecting a standardized test contaminant (ISO fine test dust, a well-characterized silica powder with a known particle size distribution) into the oil stream upstream of the test filter throughout the test, while simultaneously measuring particle counts both upstream and downstream of the filter. Because fresh contaminant is continuously added, the filter progressively loads with captured particles during the test, and the Beta ratio measurements capture the efficiency across the entire service life of the filter element — from clean to fully loaded (end-of-life, indicated by reaching the maximum allowable pressure differential).

In oilfield applications, the multipass test Beta ratios printed in filter element data sheets are used by equipment designers and maintenance engineers to select filters that will maintain the required oil cleanliness level (expressed as an ISO 4406 code) for the specific equipment. A hydraulic power unit feeding a subsea BOP accumulator system, for example, may require ISO 14/12/9 cleanliness — achieving this in a system with normal particle ingress requires filter elements with β6 greater than 200 or β3 greater than 1,000, and the multipass test data verifies whether a given filter element achieves these performance thresholds.

Multipass Method in Oilfield Hydraulic and Fluid Systems

Hydraulic control systems on drilling rigs — including the BOP accumulator system, draw-works hydraulic brake, top drive hydraulic motor, and iron roughneck hydraulic actuators — use mineral oil hydraulic fluid that must be maintained within tight cleanliness specifications to prevent valve spool wear, servo valve contamination, and pump cavitation damage. The filter elements protecting these hydraulic systems are specified by their multipass test Beta ratios, typically β10(c) greater than 200 or β6(c) greater than 75 for high-pressure servo valve systems. The "(c)" designation refers to the revised ISO 16889 contaminant (ISO medium test dust) versus the earlier standard. Filter replacement intervals are based on differential pressure indicator readings (change filter when pressure drop exceeds a threshold indicating heavy loading) rather than time-based intervals, because loading rate depends on contamination ingress that varies with rig environment and operating conditions.

Injection water filtration for seawater injection or produced water reinjection systems uses multimedia filters (sand, anthracite, garnet dual media, or membrane cartridge filters) whose performance is characterized by a multipass-equivalent efficiency test. The required filtration specification for injection water is typically driven by the injectivity requirements of the injection well formation — tight carbonate reservoirs may require 2-micron absolute filtration to prevent pore throat plugging that reduces injectivity, while coarser sandstone injection zones may tolerate 10-micron filtration. The Beta ratio at the design particle size cutoff determines whether the filter system meets the injection water quality specification throughout the full service cycle from clean to backwash.

Gas dehydration glycol filtration uses particle and coalescer filters to remove liquid hydrocarbons, iron sulfide scale particles, and formation fines from the glycol regeneration loop in TEG (triethylene glycol) and DEG dehydration units. Contaminated glycol accelerates corrosion of heat exchanger tubes and reboiler surfaces and causes foaming that reduces the glycol contact efficiency in the absorber column. Multipass test data is used to specify the coalescing and particulate filtration stages in the glycol filtration package, ensuring that the glycol cleanliness meets the operational requirements for the specific gas processing duty at acceptable filter replacement frequency and operating cost.

Multipass Method Across International Jurisdictions

Canada (AER / WCSB): WCSB drilling operations and oil sands processing facilities use hydraulic fluid filtration specified to ISO 16889 multipass test standards for all critical hydraulic control systems. Oil sands mining operations (Suncor, Canadian Natural Resources Horizon, Imperial Oil Kearl) use extensive hydraulic fluid cleanliness monitoring programs for the hydraulic systems of haul trucks, shovels, and hydraulic excavators operating in the abrasive oil sands environment — the high silica content of oil sands material is a severe contamination source that necessitates aggressive filtration to achieve the required ISO cleanliness codes for high-pressure hydraulic systems in this equipment. WCSB produced water injection schemes at Pembina, Cardium, and other waterfloods use filtration systems designed to the multipass standard to protect injector well injectivity.

United States (API / BSEE): BSEE requirements for offshore drilling equipment specify that BOP accumulator systems and related hydraulic control systems meet defined reliability and performance standards — the filtration performance of hydraulic fluid filters used in these systems is characterized by ISO 16889 multipass test data. API Specification 16A (Specification for Drill-Through Equipment) and API Spec 16D (Specification for Control Systems for Drilling Well Control Equipment) reference fluid cleanliness requirements for BOP control fluid that are achieved through filter elements with defined multipass performance. Onshore hydraulic fracturing equipment (pumping units, blending trucks) also uses hydraulic systems requiring multipass-rated filtration to maintain reliability during the high-utilization fracturing operations typical of unconventional well completions in the Permian Basin, Marcellus, and Eagle Ford.

Norway (Sodir / NORSOK): NORSOK I-005 (Machinery — Condition Monitoring) and NORSOK R-003 (Safe Use of Lifting Equipment) reference hydraulic fluid cleanliness requirements for offshore machinery and cranes that are achieved through ISO 16889-rated filter systems. Equinor and other NCS operators maintain centralized oil analysis programs that include particle counting according to ISO 4406 to monitor hydraulic system cleanliness against the target codes specified for each equipment type, with filter replacement triggered when particle counts exceed target values rather than on fixed time intervals. The harsh North Sea offshore environment — with high humidity, vibration, and temperature cycling — places additional stress on hydraulic systems and increases the importance of rigorous filtration to maintain system reliability on NCS platforms.

Middle East (Saudi Aramco): Saudi Aramco's gas processing plants (Ras Tanura, Abqaiq, Shaybah NGL facilities), pipeline facilities, and offshore platforms use ISO 16889-rated filtration throughout their hydraulic control and lubrication oil systems, with Aramco's central inspection and maintenance organization managing oil analysis sampling programs to verify that system cleanliness codes are maintained within Aramco Engineering Standards specifications. Aramco's filtration specifications for injection water at Arab Formation seawater injection schemes (SWIP — Sea Water Injection Project) target 2-micron beta-rated filtration to protect Arab D carbonate reservoir injectivity, with filter performance verified by periodic multipass-equivalent efficiency testing of the membrane filter elements used in the final polishing filtration stage before injection.