Bulk oil filtration in heavy industry

While oil producers filter their products at many stages to ensure that products enter and leave refineries clean, it's a simple fact of life that further handling, such as packaging into oil drums or transporting by road or rail tanker, will likely cause further contamination.

It's not unusual to expect oils with cleanliness levels to ISO standards to arrive at a user site with a cleanliness level of ISO 22/21/18. (This three-digit ISO code denotes the number of particles larger than 4, 6 and 14 microns you would find in a sample of 100 millilitres of fluid). However, for clean new oils and fuel it is recommended that the following target cleanliness values are achieved prior to installation:

• Heavy-duty gear oils ISO 18/17/13
• Engine, transmission, turbine and hydraulic oils ISO 16/15/12
• Diesel fuels, high-pressure hydraulic servo-valve systems ISO 14/13/11

According to Donaldson Australasia, the most effective place to install bulk filtration is as close as possible to the final point of decanting the product. It makes most sense to filter out the majority of contamination using large, easily serviced filters at the bulk dispensing points rather than trying to filter oil or fuel on expensive mobile or industrial equipment, potentially delaying production.

The benefits of filtration are probably best explained by looking at the consequences of not filtering oils and fuel before use. Consider the following example of a gear pump circulating dirty oil with a cleanliness level of approximately ISO 22/21/18 at 264 L/min versus a similar pump circulating oil cleaned to ISO 16/14/11.

The typical life of the pump circulating the dirty oil could be two years or less and it would have pumped over 4 tonnes of contaminant, whereas in the example of the pump with the clean oil, it would have pumped approximately 23 kg of dirt and would have a life expectancy in excess of 14 years.

Designing a filtration system

When designing bulk filtration systems, the value proposition to keep in mind is that the filtration is there to provide the lowest cost of ownership via clean oil or fuel.

Usually, the most difficult decisions are where to place filtration and what level of filtration efficiency to install to achieve desired cleanliness levels. These decisions are most often dictated by existing infrastructure. To try to answer some of these questions there are other practical issues to consider.

Single pass filtration?

Theoretically it is possible to clean oil or fuel on a single pass through a filter; however, fluid viscosity, flow rates and serviceability will dictate whether this is viable.

In general, single pass filtration is not a viable proposition for bulk lube and fuel applications for the following reasons:

• Fine filters have relatively low dirt-holding capacity in comparison to coarse filters.
• Fine filters have high differential operating pressures in comparison to coarse filters. This is particularly critical when dealing with heavy lubricants or working in cold climates.
• In order to use fine filtration in a single pass the filter needs to be sized sufficiently large enough to handle both the flow rate and have adequate dirt holding. This invariably makes it an expensive design with high maintenance requirements.

Bulk delivery

Filtering fuels and lubricants coming off bulk delivery vehicles is beneficial, but there are some practical challenges, such as how to avoid cross contamination of fluids and how to minimise turnaround times for delivery vehicles. Unloading a vehicle at more than 400 L/min pumping a high viscosity engine oil or gear lube can be a taxing problem.

Even if an oil supplier delivers a relatively clean product, the exercise often comes to nothing simply because the customer's storage facilities are simply not able to maintain cleanliness. Poorly stored drums, open storage tanks, inadequate air breathers and poor handling practices will ensure that all prior filtration is meaningless.

Ideally, oil drums should be stored on their sides with the bungholes submerged. Moisture will migrate through the thread of a bung, especially if the drum has standing water resting on top. Dust and water will certainly ingress any drum left open to atmosphere.

Bulk storage

A bulk storage tank without a proper breather filter will ensure that achieving an ISO cleanliness level better than 18/17/13 is virtually impossible. Typically, a liquid filter rated at 10 microns absolute will have an efficiency of around 2 to 3 microns when used as a breather; this is more than adequate for most bulk storage applications. (Breathers need to be sized correctly and maintained to avoid collapsing large storage tanks.)

(Note: On very large storage tanks there are strict regulations about restricting airflows. Installing any kind of air breather may be unacceptable.)

Storage tank design can significantly impact cleanliness levels, particularly with regard to the removal of water and the build-up of sludge and bacteria.

Mounting bulk storage tanks with a slight gradient will ensure that free water will collect in the smallest possible area, which can be detected and removed if a clear pipe with a drain is installed.

Installing the inlet to the tank with a diffuser set below the surface of the oil helps prevent stirring up any sediment settled. It also helps reduce aeration, which supports the growth of bacteria.

As mentioned earlier, existing infrastructure usually dictates the overall design criteria for a bulk lube or fuel system

Basic filtration design

While each bulk filtration project will be site specific, there are some basic design criteria to consider.

Ensure that any inlet filtration for a bulk storage tank is sized sufficiently large to handle both the flow and the contaminant load. A single filter may be able to handle a large flow rate, but can it handle large quantities of contaminant without requiring frequent servicing and without having to follow difficult maintenance procedures?

Ideally, the system should be designed such that the oil or fuel is circulated as many times as possible through the filters prior to use. Single pass filtration is expensive and not always effective.

Designing suitable storage vessels and preventing contaminant ingress before and after delivery is money well spent; reservoir items such as breathers, inspection hatches and dispensing points properly designed and installed will significantly improve overall system cleanliness.

Finally, there is little purpose in installing filtration unless it can be serviced and its performance monitored. Installing filters which are too small will soon lead to dissatisfaction by maintenance personnel and lubricant suppliers alike. It's not uncommon to find undersized filter assemblies either bypassed or operating without filter cartridges. Unlike most closed circuit hydraulic systems, bulk filtration has to continually filter out new contaminants. Along with efficiency, dirt-holding capacity and flow rate issues are primary design criteria.

Soft contaminants

While the problems associated with filtering out small, hard particles of minerals or metals is fairly well known and understood, a less obvious problem for many filter applications is the short life caused by the presence of soft contaminants existing and resulting from the fluid itself.

These filter contaminants may be by-products of oil oxidation or degradation in the form of varnish particles. They may be the result of mixing different fluids or they may even be oil additives. In some instances, chemical reactions or breakdown of the media may cause short life. Problems like these and resulting short filter life are so ubiquitous that many standard filterability tests have been developed to test the ability of an oil or fuel to be filtered. Examples of this type of test include:

• ASTM D 6824-03 Standard Test Method for Determining Filterability of Aviation Turbine Fuel
• ASTM D 6426-02a Standard Test Method for Determining Filterability of Distillate Fuel Oils
• ISO 13357-1:2002 Petroleum products - Determination of the filterability of lubricating oils - Part 1:Procedure for oils in the presence of water
• ISO13357-2:1998 Petroleum products - Determination of the filterability of lubricating oils - Part 2:Procedure for dry oils
• CETOP RP 124H RP 124H Filterability of Hydraulic and Lubricating Oils

From an application point of view, the last thing that anyone would want to happen is that short filter life should result in the non-use of filters. When filter housings are left empty, this simply passes the problems down to the end use. Therefore, it is important to evaluate used filters to determine the cause of the short element life. Often, the problem can be identified and fixed with minor changes in the system.

Example:

A hydraulic filter with 16 hours of service became completely plugged and was submitted for test, along with a sample of new bulk oil and used oil that was taken from the 16-hour filter. The filter was submitted to ultrasonic vibration in petroleum ether to remove the contaminant. This solution was filtered through a 1.0 µm Nuclepore. SEM was used to view the membrane.

The results show the membrane was completely closed off with a resin or varnish-like material. The base size of the soft, organic resin-like particulate appeared to be from sub 0.1 µm to 0.5 µm in diameter. Some of the varnish formed fibrous looking tendrils within the resinous blobs.

This user's application was the filtration of bulk oil into other smaller containers. Their filter lasted less than 3785 L before plugging, a much lower capacity than expected. A section of contaminated media was cleaned with petroleum ether and the solids collected on a Nuclepore membrane.

The solids were a light tan-coloured, thick material that could be peeled off the membrane. The contaminant did not look like particulate or metal fragments or even an additive that might be in the oil. Thermal gravimetric analysis (TGA) and differential scanning calorimeter (DSC) analyses were performed to try to determine the nature of the contaminant.

The results indicated the material was a polymer material, probably polyethylene. SEM images were taken of the contaminant and all of the images showed the same plastic-like particulate plugging the media. Also, EDS was used to identify the minerals that remained after the sample was heated to 750°C using the TGA. The elements in the TGA residue were: calcium, silicon, phosphorus, zinc aluminium, sulfur, potassium and titanium. Calcium, silicon, phosphorus and zinc are possible oil additive components.

Several filters were returned for analysis because of short life on a hydraulic 'run-in' station. The filters were heavily filmed with a gum-like material. It almost totally fused the pores and blinded the media. The contaminant that was removed from the media and collected on a membrane was SEM imaged. The image showed thick soft, gummy material plugging the filter.

Premature filter failure is invariably caused by contamination; however, as the necessity for finer filtration grows, the products of contamination are often not those solids that the filter was designed to remove, but rather resins, gums and polymers.