Hose Cleanliness: Mitigating the Risk of Contamination

How can hose cleanliness be described? The ‘textbook’ answer suggests that hose cleanliness is the analytical and statistical measure of the solid particle contamination present in a sample hose assembly or its individual components. It is typically given in a certain tolerance allowed per size or count.

While this is a very straightforward concept, in practice the ‘amount’ of contamination allowed in supplied hose assemblies are commonly defined by industry standards or internal standards. It is therefore important to understand the different approaches to determining the degree of contamination, the methods to mitigate the risk of contaminates, and how to achieve the required cleanliness levels expected by the customer.

By Azael Resendiz, Individual Contributor

Danger of Containments

All components in a hydraulic system have contamination on some level or another. Bulk hoses, metals, ancillary equipment, and accessories have certain level of contamination on them. The fluid that passes through a hose assembly are also inherently contaminated. At plain sight, these components might not exhibit contamination, but as cleanliness standards are based in small particles, weights, and minute sizing they are often not detectable by plain sight inspection.

Although many individuals believe that ‘new’ means ‘clean’, current requirements in mechanical components often find that ‘new’ does not always meet the minimum cleanliness standards. To ensure that a hose meets these requirements, several end users go to the extent to develop guidelines that define the amount of contamination allowed in the components and fluids utilized in their systems.

Current requirements from the manufacturers of hoses, pumps, valves, and other hydraulic accessories, demand that the hydraulic fluid be free from large particles and contaminants. Depending on the size of passages or surface finishes, long exposure to foreign large particles in the hydraulic oil could be detrimental to the longevity and functionality of the system.

While filters are fitted in hydraulic systems to remove contaminates, and combat impurities, failings in the startup of a system can result in fluid contamination. During start up, the filtration system might not be able to come in contact with all the fluid that will be present in hose assemblies and passages, which could allow large particles in the fluid to find their way to critical hydraulic components. In some cases, the longevity and dependability of the system is therefore directly dependent on the initial start up.

Cleanliness Requirements

It is common for end user to define the required cleanliness of their hose assembly, or component, through their documentation. Most requirements will outline the means by which the contamination is to be measured and the limits that the assembly has to abide to. The most commonly used measurement methods in the industry are: Gravimetric Method, Distribution Analysis, and Maximum Particle.

Gravimetric Method

The Gravimetric Method gathers a sample of contaminates by passing clean fluid through a hose assembly, agitating the assembly, and then filtrating the solvent fluid through fine membranes. The increase in weight of the membrane after the fluid has been passed through the hose assembly will dictate the amount of contamination.

The results are presented in a weight of contaminant per wetter area, usually mg/m2. In the case of a hydraulic hose assembly, the wetter area is the area in which the fluid will make internal contact with the hose assembly during agitation. The industry standard ISO 4405 outlines the sample gathering method with great detail, and it is usually accepted as one of the two preferred method of sample gathering for cleanliness testing in hydraulic hose assemblies.

Distribution Analysis

In Distribution Analysis the end user supplies a three-digit number that is tied directly to a scale number defined by the industry standard ISO 4406. Using a laser particle counter, an analysis is performed on a fluid sample, which determines the number of contaminants in the specified sample. This is then used to determine the scale number. For example, after extracting a fluid sample from an assembly and utilizing a laser particle counter, the sample yield results 1,000 particles of 4 μm per average diameter. The ISO 4406 digit associated with this result will be 17 based on the scale. Therefore, the first of the three required digits result will be a 17, see Table 1. This process be repeated for the next two results which will provide the next two corresponding digits on the scale.

It is important to understand that this scale is directly tied to ISO 4406 and does not necessarily translate to physical phenomena, or relate in any form to the Gravimetric Method.

Maximum Particle

Maximum Particle, as the name suggest, is the maximum size of a particle allowed in a hydraulic hose assembly. Allowable particle sizes are usually dictated by the end user and are directly tied with efforts to mitigate early wear and tear on the assembly. These sizes are in the micron range, usually span from 300 μm and upwards, and have different geometric and material allowances. It is important to note that all of these figures vary per application, end user, performance requirements, and specific material restrictions.

Maximum Particle measurements are fairly straightforward. After gathering a sample by gravimetric extraction, the sample is placed under a microscope and imaging software. After scoping the sample, the largest particle(s) are found and reported. Technological advances also aid in the gathering of maximum particle analysis. Automated microscopes scan the membrane and report back on the largest particle found, along with a possible material type.

Controlling Contamination

Contamination control is a large complex topic and many users find it difficult to find a good starting point. Fortunately, most specifications outlined by the industry, and by end users, are within the reach of the hydraulic hose manufacturing operations. There are three aspects of contamination control that a manufacturing operation needs to pay attention to: environment, process & handling, and mitigation.


The most basic means of contamination control is a clean environment. Successful assembly lines are aware of cross-contamination and actively attempt to mitigate it. While there are of course limitations to the environmental cleanliness of a manufacturing floor, investing time in finding operations that produce a large amount of contaminants and mitigating them, can drastically reduce the amount of contamination that is transferred to hose assembly components or assemblies.

Process & Handling

The next level of contamination control is removing contaminates derived from the process and handling stages of production. Processes such as: hose cutting, presenting the components for assembly, handling components from one station to the next, and storing finished products with connection ends that are open to the environment, are all situations where an assembly can be exposed to contamination. Being aware of these situations allows the manufacturer or supplier to isolate the listed processes in order to protect the components from exposure to as many contaminants.

It is important to have a physical process in place to remove contamination. Common practices include utilizing pressurized air, pressurized foam projectiles, or fluid flushing the assemblies. The level of cleanliness is often also directly proportional to the type of cleaning method that is chosen.


In a small assembly shop, or on-site service, air and foam projectiles is often sufficient. On high volume assembly lines with stringent cleanliness requirement, however, flushing or projectile cleaning (or a combination of both at different stages of the process) and special handing techniques might be required. There are several equipment suppliers that can help with the mitigation of contamination in hose assembly, in order to satisfy a customer’s demand. Alternatively, having a discussion with the quality department in a facility, could aid in the draft of a ‘game plan’ to improve overall cleanliness in the assemblies.

Final Thoughts

Being conscious of the importance of contamination elimination, and having an understanding of how to test for and control contamination in a production environment, is an important part of being a tier one supplier.

The best time to start mitigating contamination is now, whether the business is an independent service hub or a large production line, controlling contamination in hose assemblies will provide added benefits to the product; it will provide the customer with peace of mind, and a good impression of a well-organized operation.


Azael S. Resendiz is a Sr. Design Engineer with 10+ years of experience in the fluid conveyance industry, specifically with hydraulic power transfer applications. His experience is in Hydraulic Hose, Coupling Design, Subsea Umbilicals, Cable Design, Manufacturing Engineering and Engineering Management.

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