Keeping an eye on fluid contamination
Published: 29 April, 2015
With reference to guidance provided in the book ‘Hydraulic Fluids – A Practical Guide’, Chris Buxton, CEO of the British Fluid Power Association, looks at the topic of fluid contamination in hydraulic systems such as those used on offshore oil and gas platforms, and how a diligent contamination monitoring strategy can help to keep this equipment running safely and efficiently while keeping downtime to a minimum.
Hydraulic systems play a critical role within a host of plant and equipment found on offshore oil and gas platforms. This includes everything from blowout preventers, cranes and thruster and ballast controls, to riser tensioning systems and motion compensators. It is therefore important to ensure that the hydraulic fluid is kept in tip-top condition, because fluid impurity, or a degraded level of fluid integrity, can result in a host of onerous equipment issues. These include wear of parts and components, impaired operation and even plant failure if the problem is left unchecked. Therefore, proven and reliable filters should be installed to minimise fluid contamination. However, it is important that this is also backed-up by a well-regimented condition monitoring strategy. With this in mind, and for the purpose of this article, I would like to draw particular attention to the need for a regular hydraulic fluid contamination control routine as part of a wider condition monitoring regime.
Sampling the fluid
The process of monitoring the condition of hydraulic fluid involves removing samples from the system in order to undertake contamination analysis. However, it is worth pointing out that this needs to be done carefully in order to avoid contaminating the sample; which could then influence the properties of the fluid being analysed. As explained in the book titled ‘Hydraulic Fluids – A Practical Guide’, by Allan Barber, Nigel Battersby and David Phillips, consideration should therefore be given to the following aspects:
• Obtaining a representative sample.
• Using the correct equipment.
• Using the correct sampling procedure.
• Recording details of the sample.
Obtaining a representative sample
As the book explains, it is essential that any sample of fluid taken for analysis is representative of the fluid in the system. For this reason, samples should not be taken from places in the system where only limited or intermittent flow occurs.
The publication goes on to point out that a good example can be obtained using a specially designed sampling valve that has been correctly positioned in the hydraulic circuit. This will normally be in the form of a ball valve or push-on connector fitted to one of the flow lines. The pressure line is the preferred location to take a representative sample. To avoid difficulties with high pressures and flows, a small capillary tube should be firmly connected to the valve. The sampling point should be downstream of an elbow or other fitting to ensure turbulent flow (see ISO 4021) and hence good mixing conditions.
If obtaining a full-blown line sample is not possible, it may be acceptable to take a sample from the active portion of the reservoir. Samples should not be taken from the reservoir drain because the drain plug is normally located at the lowest point of the system to allow water and debris to settle out to be drawn off. Therefore, it is preferable to use a weighted tube to obtain a sample at a predetermined level while fluid is flowing through the reservoir. Wherever possible, the end of the sample tube should be located in an area of mixing, away from any components and baffles and approximately halfway down from the fluid surface (see ISO 3170).
Correct equipment and procedures
We have determined that the aim of sampling is to obtain a representative quantity of the fluid in the system. In order to do this without introducing extraneous materials, there should be careful preparation for collecting samples. In this respect, ‘Hydraulic Fluids – A Practical Guide’ provides the following guidance:
• If particle counting is to be carried out, only sample bottles that have been thoroughly cleaned and dried (e.g., as described in BS 5540 part 3) and validated in accordance with ISO 3722, should be used.
• The sampling valve should conform to ISO 4021 and be clean and dry before extracting the sample.
• A suitable volume of fluid should be used to flush the valve and the sampling port before the sample is taken. If the sample is for particle count analysis, a flush of between 1 to 2 litres is recommended.
• The sample bottle should be filled to about 70% of its capacity to allow adequate shaking before analysis.
• The valve should be opened and closed but not otherwise moved during the sampling procedure to avoid generating additional particulate.
If particle counting is to be carried out, it may be advantageous to draw two separate samples; the first for general analysis and the second for particle count. Once the samples are obtained, they should be labelled correctly, giving as much as possible of the required information (see ‘Recording sample details’ below).
In some cases, systems can be fitted with on-line particle counters or monitors that give direct readouts. This avoids the need to collect samples for a particle count analysis and the problems of extraneous particles entering the sample. However, care must be exercised when performing this type of analysis because errors can be generated. For example, the process of connecting the instrument to the valve generates particles. These must be flushed out before recording the data. Also, instruments using light interruption principles (automatic particle counters) are generally affected by optical interfaces in the fluid, such as air bubbles, water droplets in the oil and contamination by tramp liquids.
Recording sample details
If full use is to be made of the information generated on the sample, it is essential that details – such as fluid name, equipment identification, date, time, machine hours, hours since last fluid change and/or top-up and position of the sampling point – are recorded.
It is also very important to note any environmental factors that may have influenced the behaviour of the fluid. An appreciation of the operation and location of the system can assist in interpreting the results of a used fluid analysis.
‘Hydraulic Fluids – A Practical Guide’ states that the above information, together with the analytical data, can be the basis of an effective predictive maintenance programme, and will allow the chemist or engineer to plot historical data and carry out trend analysis on the parameters measured. The information can also be used to estimate the remaining life of the fluid and, if necessary, to schedule planned maintenance.
The mission-critical nature of plant and equipment used in such sectors as offshore oil and gas means hydraulic fluid effectiveness cannot be left to chance. A regular hydraulic fluid contamination control strategy as part of a wider condition monitoring regime is therefore a non-negotiable.
For more guidance on hydraulic fluids, including information about effective fluid analysis, the Coxmoor Publishing Company book titled ‘Hydraulic Fluids – A Practical Guide’, by Allan Barber, Nigel Battersby and David Phillips, and published in association with the BFPA, is a handy source of information. To obtain a copy please email the BFPA quoting ODEE as your source: enquiries@bfpa.co.uk
