Hydraulic filtration: ISO cleanliness codes, selection of filter elements and contamination control

1. Introduction

The reliability of the functioning of hydraulic systems in industrial production directly depends on the purity of the working fluid. Research shows that up to 70-80% of hydraulic component failures such as pumps, valves, hydraulic motors and cylinders are caused by fluid contamination. This leads to premature wear, unscheduled equipment downtime, significant repair and component replacement costs, and reduced overall production productivity. A properly designed and maintained filtration system is a critical element in ensuring the longevity and efficiency of hydraulic equipment. This article provides an in-depth technical overview of hydraulic filtration principles, cleanliness standards, and methods for selecting and implementing effective pollution control solutions.

2. Fundamental principles

2.1. Types and sources of pollution

Contamination of hydraulic fluids is divided into several main types:

• Solid particles: Metal wear particles (steel, bronze, aluminum), dust, fibers, liquid oxidation products. The sizes of these particles vary from micrometers to several hundreds of micrometers. Even particles 5-15 microns in size, invisible to the naked eye, can cause significant abrasive wear on the precision surfaces of hydraulic components.
• Water: Can enter the system from the air through breathing filters, during fluid changes, through leaky seals. Water accelerates the oxidation of the liquid, causes corrosion, reduces lubricity, promotes cavitation and can lead to the destruction of additives.
• Air: Dissolved or free air can cause cavitation, increase fluid compressibility affecting control accuracy, and accelerate oxidation.

Sources of pollution:

• Built-in: Component manufacturing residues, scale, dust after installation of a new system or repair.
• Ingressed: Dust from the environment due to leaky seals of cylinders, rods, breathing filters of tanks, dirt during fluid replacement.
• Generated by: Wear products of moving parts (pumps, valves, cylinders), oxidation products of hydraulic fluid under the influence of temperature.

2.2. Filtering mechanisms

Filter elements work according to different principles:

• Surface filtration: Particles are retained on the surface of the filter material. These are usually thin nets or membranes. Effective for large particles.
• Depth filtration: Particles are trapped in the layer of porous material. This mechanism is used in most hydraulic filters, where the material consists of many randomly arranged fibers forming a labyrinth for the liquid.
• Absorption: Some filters can absorb water or other polar pollutants thanks to special materials.

2.3. Beta coefficient (βx)

Filtering efficiency is quantified by the Beta coefficient (βx), which is determined by the ISO 16889 standard (multipass test). This ratio shows how many times more particles of a certain size (x micrometers) are retained by the filter than pass through it.

Beta coefficient formula:

βx = (Number of particles with a size ≥ x μm before the filter) / (Number of particles with a size ≥ x μm after the filter)

For example, β5 = 200 means that for every 200 particles of 5 µm or larger that enter the filter, only one particle of the same size passes through it. This corresponds to a filtration efficiency of (200-1)/200 * 100% = 99.5% for particles 5 μm and larger. The higher the βx value, the more efficient the filter. For modern high-performance hydraulic systems, the recommended values ​​of βx(c) ≥ 1000 for critical particle sizes.

3. Technical characteristics and standards

3.1. Purity codes ISO 4406

The International Standard ISO 4406:2017 (formerly ISO 4406:1999, which is still widely used) is the main method for classifying the purity of hydraulic fluids. It sets a three-digit code that represents the amount of solids in 1 ml of liquid for three different sizes:

1. First number: number of particles ≥ 4 μm (ISO 4406:2017) or ≥ 2 μm (ISO 4406:1999).
2. Second number: number of particles ≥ 6 μm (ISO 4406:2017) or ≥ 5 μm (ISO 4406:1999).
3. Third number: number of particles ≥ 14 μm (ISO 4406:2017) or ≥ 15 μm (ISO 4406:1999).

Each number represents a "purity class" corresponding to a range of particle counts on a logarithmic scale. For example, the code 18/16/13 means:

• Class 18: 130,000 - 250,000 particles ≥ 4 μm per 1 ml.
• Class 16: 32,000 - 64,000 particles ≥ 6 μm per 1 ml.
• Class 13: 4000 - 8000 particles ≥ 14 μm per 1 ml.

Other standards, such as NAS 1638 and SAE AS4059, are preliminary or specific to certain industries (eg, aviation). Although they can still be found, ISO 4406 is the most common in the industry.

3.2. Characteristics of filter elements

• Nominal and absolute filtration fineness:
  • Nominal: Specifies the size of particles that the filter can retain with a certain efficiency (for example, 90%). This indicator is less accurate.
  • Absolute: Specifies the particle size that the filter can retain almost completely (for example, 98-99%). It is determined by a multiple-pass test ISO 16889 with a specific value of βx (for example, βx ≥ 75 or βx ≥ 200).
• Filter element material:
  • Cellulose (paper): Economical option, but has less dirt capacity and efficiency than synthetic materials. Sensitive to water.
  • Microfiber (synthetics): High efficiency (high βx), significant dirt holding capacity, resistance to water and chemicals. It is widely used for high requirements for cleanliness.
  • Metal mesh: Used for coarse filtration or in systems where element cleaning is required.
  • Water absorbing materials: Special elements for removing free and emulsified water.
• Collapse Pressure: The maximum pressure drop that the filter element can withstand before deformation or collapse. Typically 10 bar, 20 bar or 210 bar for high pressure items. Standard ISO 2941.
• Flow Rate: The maximum flow of liquid (l/min) that can pass through the filter without excessive pressure drop.
• Fluid Compatibility: Filter and seal materials must be compatible with the type of hydraulic fluid (mineral oils, synthetic fluids, hydroglycol HFCs). Standard ISO 2943.

3.3. Compliance with standards

All filters and filter elements supplied by UNITEC-D meet international quality and safety standards, including CE marking and UkrSEPRO certification, which confirms their suitability for use in Ukrainian industry.

In addition to ISO 4406 and ISO 16889, the following are also important:

• ISO 2942: Checking the integrity of the manufacturing of the filter element.
• EN 12792: Hydraulic power fluid - Filters - Terminology.
• DSTU ISO (relevant standards): Ukrainian national standards harmonized with international ones.

4. Guide to selection and calculation

Choosing the right filter and filter element is a multi-factorial process. The following criteria must be taken into account:

4.1. Selection criteria

1. Target fluid purity level (ISO code): Determined by the sensitivity of the most sensitive component in the system. For example, servo valves may require 16/14/11, proportional valves 17/15/12, gear pumps 19/17/14.
2. Hydraulic system type: High precision systems (servo hydraulics) require finer filtration than general purpose systems.
3. Working pressure and flow: Determines the type of filter (pressure, drain), its design and size.
4. Type of hydraulic fluid and its viscosity: Affects the choice of element material and its throughput (pressure drop).
5. Temperature range: Affects the choice of sealing materials and fluid viscosity.
6. The rate of contamination: Determines the required dirt capacity of the filter.

4.2. Types of filters and their location

• Suction filters (Suction Filters): Protect the pump from large particles. They usually have coarse filtration (60-250 microns) and are located inside the tank or on the suction line. It is important to minimize the pressure drop across them to avoid pump cavitation.
• Pressure filters (Pressure Filters): Protect sensitive components located after the pump. They are installed on the high pressure line. Requires a strong housing and elements with a high burst pressure (eg 20 bar or 210 bar). They provide high fineness of filtration (3-10 μm, βx ≥ 200).
• Return Line Filters: Protect the hydraulic tank from contaminants returning from the system. They are installed on the drain line in front of the tank. They usually have a filtration fineness of 10-25 microns (βx ≥ 75). This is the most common type of filter.
• Autonomous filtration units (Off-line / Kidney Loop Filters): Installed in a separate circuit for continuous filtration and polishing of liquid, regardless of the operation of the main system. Can provide very high levels of cleanliness (βx ≥ 1000) and water removal.
• Breather Filters: Protect the tank from dirt and moisture from the surrounding air, which comes in when the liquid level changes. Can be combined with a moisture absorber.

4.3. Filter selection matrix

The table below provides general recommendations for filter selection for various types of hydraulic systems. The actual selection should be based on a detailed analysis of the specific system and the requirements of the component manufacturer.

4.4. Filter size calculation

The size of the filter (its throughput) must be calculated taking into account the maximum fluid flow and the minimum desired pressure drop. A general rule of thumb is to select a filter with a rated capacity that is 20-30% greater than the system's maximum operating flow, especially for drain filters where the flow may be pulsating. This provides a margin for element contamination and prevents premature operation of the bypass valve. For example, for a system with a maximum flow of 100 l/min, it is worth choosing a filter with a throughput of 120-130 l/min.

5. Best Practices for Installation and Commissioning

1. System Flushing: New hydraulic systems, as well as systems after overhaul, must be thoroughly flushed before commissioning. For washing, a flushing pump with a filter with a fineness of filtration 1-2 classes higher than the target operating level is used, until the target ISO code is reached. This allows you to remove built-in impurities.
2. Selecting the filter housing: The housing must have adequate strength for the working pressure and be equipped with a pressure drop indicator. The bypass valve in the filter housing should be set to a pressure that meets the manufacturer's recommendations (eg 3 bar for most drain filters).
3. Installing Breather Filters: Install breather filters on the hydraulic tanks. Respiratory filters with air filtration fineness of 3 microns and moisture absorption properties (with silica gel) are recommended to prevent the ingress of particulate matter and water from the air.
4. Cleanliness when replacing elements: Filter elements must be replaced in the cleanest possible conditions. Use clean tools and gloves. Make sure there is no dust or dirt around the filter housing. Always replace the O-rings.
5. Filling the system: The hydraulic fluid must always be filled into the system through a filtration unit (filter press or filter cart) with the appropriate fineness of filtration, even if the fluid is supplied as