Centres of expertise; DERA
Centres of expertise DERA
Keywords DERA,Fuels, Lubricants
The Defence and Research Agency (DERA) Fuels and Lubricants Centre
The Fuels and Lubricants Centre was opened in 1995 and is situated in purpose built facilities at Pyestock near Farnborough. It brings together the fuels and lubricants related activities of the three Ministry of Defence laboratories, previously well known within the petroleum fuels and lubricants industry, of Woolwich, Cobham and Harefield to join the existing Petroleum, Chemistry and Technology Division of Naval Engineering already at Pyestock.
The Fuels and Lubricants Centre is 64km (40 miles) east of London and is served by the M3 motorway junction 4a (20 miles from junction 12 on the M25).
The Centre is also accessible by rail with a direct 40 minute service from London Paddington to Farnborough every 30 minutes.
The Centre, in addition to its primary duty of providing a service to the MOD, is also able to offer the benefit of its facilities to prospective clients such as non-UK military clients and commercial organisations. The defence industry has always bred technologies which have crossed over into the non-defence area and technologies which have changed the face of military combat have also changed the face of the commercial world. DERA, having had over 40 years experience of fuels and lubricants used in a wide variety of applications in the air, at sea and on land, is therefore able to provide testing and evaluation services, an investigative and analytical resource, development and consultancy and to form partnerships with industry to exploit the technology within DERA. The staff are also able to work with client organisations to develop and enhance their own in-house technology utilising the skills and expertise developed over many years.
The Fuels and Lubricants Centre is able to carry out the majority of the laboratory based standard test methods associated with a very broad range of fuels, lubricants, greases and associated products in addition to some standard, and some now less standard,rig and engine tests. The Centre also has a number of specialised items of equipment which are not readily available in other laboratories.
The analytical equipment available encompasses the majority of the instrumentation in common usage together with a number of specialised items and techniques which are not yet widely available. However probably the most significant feature of the analytical resource is the knowledge and expertise of the staff who, as a result of the equipment evaluation role required by the military customer base, possess an extremely broad and in depth knowledge of the majority of analytical techniques and instrumentation available.
Another key ability of the Centre is the condition monitoring capabilities of both lubricants and, perhaps more importantly, of equipment and machinery. Expertise exists in off-line condition monitoring with Inductively Coupled Plasma (ICP) spectrometers, a sophisticated data management system and related supporting instrumentation such as scanning electron microscopes and X ray techniques, including the recently developed X-ray microprobe instrumentation. The Centre is also actively involved in on-line condition monitoring including particle detection, vibrational analysis and equipment performance monitoring.
The Fuels and Lubricants Centre can be a powerful resource when used to investigate problems and failures, and the in-depth knowledge and expertise of the staff can provide an authoritative, objective and independent assessment of the design, operation or servicing of a wide range of industrial plant and machinery and of the lubricants and greases used within them.
A further point of interest is the ability of the tribology projects team to undertake detailed tribological studies using a range of test rigs and equipment suitable for a broad range of loads and lubrication regimes supported by a detailed knowledge of the materials and surfaces in contact with lubricants and fuels. The arduous conditions in many items of military equipment have enabled the staff to develop an extensive knowledge and understanding of the many facets of lubrication and lubricating products under a wide range of loads and conditions.
Staff within the Fuels and Lubricants Centre would be pleased to discuss the facilities and resources available. A principal contact within the Centre is N. Hopkinson, DERA Fuels and Lubricants Centre, Building 442, DRA Pyestock, Farnborough, Hampshire GU14 OLS. Telephone: + 44 (0) l 252 374326; Fax: + 44 (0) l 252 512136; E-mail: nahopkinson@dera.gov.uk
Specific testing facilities
The Centre has a number of dedicated test facilities of which two are highlighted.
Grease testing
The Centre is particularly well equipped to evaluate the properties and performance of greases and is able to undertake a wide range of specialist test methods.
The Centre has an extensive range of analytical equipment and techniques available enabling detailed and comprehensive grease characterisation, analysis and testing to be undertaken, and to provide authoritative advice and consultancy on a wide range of grease and related matters.
In addition to the standard tests, the Centre also has a number of specialised items of laboratory equipment and test rigs which are not readily available in other laboratories or facilities which are of direct relevance to grease manufacturing and grease marketing organisations.
Through a long involvement with the grease and lubricant industries, the staff in the Centre have developed considerable expertise relating to the use and application of greases and,importantly, on failure analysis. The ability to examine failed bearings and related grease lubricated components to determine the mode and cause of failure is of considerable value to both suppliers and users alike.
Members of the Fuels and Lubricants Centre are active participants in the European Lubricating Grease Institute (ELGI) and in the parent organisation, the National Lubricating Grease Institute (NLGI).
Engine and rig test facilities
The engine test facility is able to undertake evaluations of the performance of fuels and lubricants using dedicated, fully equipped engine test cells. A number of standard petroleum industry engine test methods, developed by the Institute of Petroleum(IP) and the Co-ordinating European Council (CEC), are available. These tests simulate many typical lubrication conditions and the Lubrication Engineering Facility is recognised as fully qualified to run and assess test sequences. This facility also develops and runs specialised test methods to investigate specific lubrication conditions or to assess more complex engine designs than is possible with standard test methods. The Mechanical Rig Test Laboratory has a number of specialised mechanical test rigs used to simulate specific lubrication regimes and operating conditions. Many are well known established test methods developed by the IP and the American Society of Testing Materials (ASTM). There are, in addition, a number of specially developed test rigs which have been custom-built to provide state of the art capabilities for the investigation of lubricant and material interactions and behaviour often under extremely arduous conditions.
Specialised areas of investigation
DERA Fuels and Lubricants Centre has developed particular areas of expertise in a number of specialised areas related to its primary function, some examples of which are described in more detail as follows.
Hot liquid process simulator
The Hot Liquid Process Simulator (HLPS) is a research tool designed to simulate a variety of heat transfer situations commonly encountered in a wide variety of industries. The equipment is versatile and is able to simulate the conditions encountered across a wide range of applications where liquids are in contact with heated metal surfaces. Such conditions may be found in conventional heat transfer systems where a fluid is heated and used to transmit that heat from the calorifier to the working area, in a heat exchanger application where the working fluid is used to remove heat from process fluids or in situations where the lubricating fluid is in contact with hot surfaces such as in aviation gas turbines or steam calendar cylinders found in paper-making plants.
In all such severe applications where a fluid, normally petroleum or petrochemical based, is in contact with hot surfaces problems of thermal and oxidative degradation of the fluid can occur.
There are a number of methods available to enable the heat transfer process and the fluids intended for use in heat transfer situations to be studied in the laboratory. Some of the more common include a variety of coking tests, thermal and oxidative stability tests, plate type heat exchanger rigs and tests using heated wires. However, the method that most closely mimics the conditions of the heat transfer process and permits the evaluation of the test fluid by the precise measurement and control of key parameters is one using a process simulator which enables the following parameters to be studied:
temperature and pressure regulation;
local heat flux determination;
measurement of surface and bulk fluid temperatures;
evaluation of various tube metallurgies;
testing of small fluid samples;
assessment of a variety of fluid degradation mechanisms.
The test fluid is pumped over a tube which can be heated by electrical resistance means to temperatures of up to 600°C with a carefully controlled thermal gradient being maintained along the length of the tube. The reservoir, pump and delivery pipework can all be heated to permit high viscosity products to be evaluated. The heater tube can be obtained in a variety of metallurgies, 316 stainless steel or aluminium alloy JFTOT specification being two of the more usual materials.
A typical test duration for an aviation turbine lubricant is 20 hours and the performance of fluids assessed by the degree of discoloration/lacquering formed on the heater tubes or by weight of the deposit formed. In the latter case a typical "bad" result for such a lubricant would be a few milligrams. It is not normally necessary to increase the severity of the test to the point where significantly heavier deposits are formed (see Figure 1).
In addition to aviation turbine lubricants, the HLPS has been successfully applied to the evaluation of the oxidative stability and deposit forming tendency of crankcase lubricants,air compressor lubricants, and, naturally, heat transfer fluids.
The compression ignition tendency of "fire resistant" hydraulic fluids
In many hydraulic systems,particularly those in hazardous environments such as underground mining, steel mills or the oil and petrochemical industries, it is common practice to use fire resistant hydraulic fluids instead of the more common mineral oil based fluids. There are a number of different types of fire resistant hydraulic fluids but they generally fall into two main groups; those containing water and those based on fluids less flammable than mineral oil. However, it is the water glycol based hydraulic fluids that are in widest use as such fluids typically have many performance characteristics that are very similar to those of mineral oil based hydraulic fluids and demonstrate excellent fire resistance.
Figure 1 The HLPS tubes show the results obtained when different synthetic lubricants were evaluated
In 1983, however, there was a serious explosion in one of the riser tensioner legs on the Chris Chenery offshore rig in the North Sea. After investigation it was believed that the explosion could be attributed to the ignition of the water glycol based hydraulic fluid in the riser tensioner following rapid compression.
It was apparent that following the explosion on the offshore rig, consideration would have to be given to the possible hazards that could arise when hydraulic fluids are subjected to rapid compression particularly in an air-fluid environment. For such applications the existing test methods did not provide any information about the behaviour of hydraulic fluids in such circumstances.
The modified cetane engine test
Various test methods,based on modifications to a standard cetane test engine (ASTM D613)traditionally used to determine the ignition quality of diesel fuels, were proposed. The most prominent were the American MIL-H-22072B and the French E 48619 methods used for water glycol based hydraulic fluids intended for military application. However neither test method was particularly successful and the Fuels and Lubricants Centre developed a more sophisticated test method which,while also using a modified cetane test engine, did produce a reliable and repeatable assessment of the tendency of a hydraulic fluid to ignite under conditions of rapid compression.
Samples of the test fluids are injected into a motored Cetane engine as the compression ratio is gradually reduced from 50:1 down to 20:1. Pressure transducers fitted to the cylinder head of the engine enable the compression of the air-fluid mixture to be followed and ignition, which causes an additional increase in pressure, to be observed. The test terminates when the fluids cease to ignite as the compression ratio is reduced.
Figure 2 Water glycol fluids, limit of ignition
Figure 2 shows the results obtained from four commercially available water glycol based hydraulic fluids and clearly demonstrates the very different behaviour of the fluids when subjected to compression ignition testing in the modified cetane engine test rig. It is interesting to note that Fluid 1, although classified as a fire resistant hydraulic fluid, is igniting at a compression ratio of only 25:1 which is very close to the compression ratio used for commercial diesel engines. Such a fluid would be extremely hazardous under rapid compression conditions.
The elastohydrodynamic point contact test facility
The ability of a fluid to form and maintain a lubricating film between heavily loaded contacts under elasto hydrodynamic lubricating (EHL) conditions is determined by the"pressure-viscosity coefficient" of the lubricant. The parameter describes the ability of a fluid to increase its effective viscosity in an EHL contact such as that between a ball and the track in a spherical bearing, between a cam and a cam follower or between the meshing teeth in gear pairs. While it is impossible to measure directly the pressure-viscosity coefficient of a lubricant, the analysis performed on the test facility enables the effects to be accurately studied and comparisons between the performance of different lubricants to be assessed.
The elastohydrodynamic rig
In the equipment, the lubricant film thickness in a rolling contact between a superfinished steel ball with a polished glass (or sapphire) disc can be measured over a range of rolling and sliding velocities.
Film thicknesses in the range of 0.1-1.0µm are measured using an "optical interferometry" technique over rolling speeds of 0.25 to 4.5m/s. Using a glass disc a maximum contact pressures in the region of 0.7GPa can be used, although pressure of up to 1.2GPa may be achieved if a sapphire disc is used. Such conditions are typical of those in rotating shaft and gear transmission systems.
An interferogram of a typical contact produces a colour fringe image such that each of the colours seen in the contact can be related to a calibrated separation of the two surfaces, i.e. the film thickness. By running a series of tests under a range of rolling speeds the relationship between film thickness and speed can be established (Figure 3).
The performance of different fluids can be rated by comparing the graphical plots of the film thickness against speed determinations (under the same temperature and load conditions), as shown in Figure 2, enabling an accurate estimation of the relative performances of fluids to be made. Fluids A, B and C are of similar viscosity at ambient pressure.
Figure 3 Relationship between film thickness and rolling speed of contact
The EHL test facility is particularly relevant for the development of new and improved lubricants or other functional fluids, for assessment of fluids or new applications or for situations where new designs impose significantly increased operating conditions.
Summary
Military requirements for fuels and lubricants differ in a number of ways to those designed for civilian use. They must be capable of withstanding extreme conditions of climate and ambient temperature, be stable over long periods without forming deposits which could cause filter blockage, and be suitable both for older and state-of-the-art vehicles. In order to ensure that the specialist requirements of military were met, the MOD has for many years carried out investigational and development programmes at a number of centres throughout the UK. The modern Fuels and Lubricants Centre at Pyestock was created from three long-serving MOD establish-ments, and is now a Centre of Excellence in the Mechanical Sciences Sector with an 80-strong scientific workforce. The Centre itself does not produce fuels or lubricants but works closely with oil companies to produce suitable specifications. The Centre interprets MOD's requirements for industry,adjudicates the manufacturers, tests the products against the specification and monitors in-service performance. Despite the Centre's strong links with industry, its principal customer is the MOD. Only 4 per cent of last year's income was from industry with the remainder roughly divided between the Services(75 per cent) and the Procurement Executive (25 per cent). Civil work is,nevertheless, on the increase, and the Centre is actively seeking to further increase its activity as an independent provider of expertise and services in the non-military fuels and lubricants area.
David Margaroni
