J. Pošta, R. Zewdie

Czech University of Agriculture, Faculty of Engineering, Prague, Czech Republic

The replacement of crude oil products by other materials is currently motivated above all by the search to acquire materials with better ecological character and utilize a part of The Agricultural Fund for non-agricultural objects. One of the obstacles to wide usage and application of biodegradable hydraulic liquids are currently the undefined and unclarified questions concerning its utility and effectiveness.

The use of hydraulic equipment in tractors, mobile technology, agricultural and construction machines, etc. presents a serious threat to the environment because of oil losses in the fields which lead to the contamination of water and soil (Pošta, 1995; Chrást et al., 1995). In the same way thousands of tons of fossil oil products enter the soil and water annually. This leads, together with construction solutions, to look for an alternative method of reduction of harmful effect, acceptable biodegradable hydraulic liquid that does not, in case of leakage, cause large damage and protects the environment.

Along with the use of a biodegradable hydraulic liquid in the hydraulic system of a machine together with the presently used classical mineral oil as a complex it is possible to predict (Zewdie, 1997; Hartweg et al., 1989):

1. the different acquisition costs
1. different technical life of both types of hydraulic oils
1. possibly different life range of hydraulic elements in hydraulic systems
1. different degrees of contamination of the environment
1. the use of surplus agricultural land for technical aspects, ecology, employment, etc.

The objective of this contribution is to adapt the theoretical and methodical analysis of a given problem concerning total techno-economical effectivity.

To reach the established objective of the work it is necessary to:

1. elaborate and compile the theoretical part of the problem
1. elaborate all methodologies including essential method for realization of the experiment
1. execute the verifying experiment
1. apply the result of the experiment to the methods required to analyze the effectivity of Biodegradable Hydraulic Oil.

The renewal (replacement) of liquids in a hydraulic system could be understood as an optimization normative for a renewal, extension or a reduction of the interval in which the exploited liquid has an impact on the total effectiveness. The contribution analyzes this problem from techno-economical point of view which includes all influences connected with the discharged liquids, due to different wear out intensity of elements of hydraulic aggregates and with respect to ecological requirements.

THEORETICAL PRINCIPLES OF SOLUTION

Professional literature substantiated that the effectivity of technical object is determined by the sum of acquisition (production) and operational costs to its lifetime per unit (number of km, work done, production etc.). It means, we are talking about the so called average unit price of a given technical object (Pošta, 1993; Havlíèek et al., 1989).

If the condition of technical object is evaluated by the change of a selected diagnostically signal S, as the indicator of the effectivity of the object the renewal time is determined by a hidden value S_o of a given signal, then the ratio of the sum of acquisition and operational cost to its lifetime is minimal.

The operational costs are supposed to increase with the increasing volume of the work performed (i.e. with increasing time of operation), due to failure mechanisms. If the result in form of the average unit costs of renewal and operation of the equipment can be identified with sufficient accuracy with the similar value of the selection set, then the selected diagnostic signal can be stated, (Havlíèek et al., 1989)

(1)

where:

As every discrete value S_i of the diagnostic signal has the corresponding variable operational times for the respective element t_i(S_i), from which it is possible to calculate the mean value t(S_i), to determine the function t(S), Fig. 1, and after a statistical evaluation, the equation (1) can be transformed to a general form of the effective function of renewal

(2)

where:

As the unit renewal costs u_o(t) = N_o t ^-1 are a permanently decreasing function of the mean operational time and the unit operational costs are an increasing function, the summary function u(t) has a local minimum u(t_O), whose coordinates are dependent on the items, that form the function (1) or (2), and obtaining them is the general purpose of the whole optimization solution.

The methods of the determination of the standard for the liquid renewal in a hydraulic system should therefore be as follows:

1. A perspective diagnostic signal Q is chosen for the evaluation of the state of the considered liquid and a real range of this signal is estimated, where the experiment will be held. It is obvious, that the experiment and the whole optimization solution can be carried out for several different signals and then the signal with the most obvious results can be recommended for practical application.
1. For every chosen discrete value Q_i of the Q signal, for which the experiment is carried out, the selection set of hydraulic systems is monitored during operation, for which the liquid replacement occurs at Q_i value. The number of these sets is the same as the number of the selected discrete values of Q_i.
1. For every replacement of the liquid at the technical state of Qi the individual operational time - the individual replacement interval t_vi(Q_i) - is recorded and for the same state Q_i also the mean replacement interval t_vi(Q_i) is calculated; it can be expected, that with increasing values of Q_i also the values of t_v(Q_i) will increase and that there is a possibility to describe this fact by the t_vi(Q) relation.
1. Each discrete value of Q_i has a corresponding level of the technical state of the observed liquid and of the observed hydraulic system. It can be expected, that with increasing values of Q_i the physical and technical characteristics of the liquid will deteriorate, so each subsequent value of Q_i will correspond higher instantaneous operational costs.
1. These experimental data are transformed into the form of the functional relations of the technical life of the investigated element of the system. And the average unit costs of the mean interval of the operational time up to the liquid replacement, t_v, which enables to determine the optimal mean interval of replacement of the filling of the hydraulic liquid t_v, and also the optimal value of the diagnostic signal S_o for the renewal of the liquid charge belonging to this interval. This principle is illustrated in Fig. 1.

Fig. 1 The principle of the assignation of the optimal value of the diagnostic signal of preventive replacement

To obtain the necessary source materials for the verification of the possibility of the application of the above mentioned theoretical constructions, the laboratory test of the hydraulic system was carried out with biodegradable liquid of Czech production. Observed were the changes of selected parameters characteristic for the technical state of the hydraulic system as well as for the liquid:

• total content of contaminants
• viscosity
• water content
• regime of wear out
• changes in acidity number

The analyses were carried out repeatedly, the gained results were statistically evaluated.

The testing equipment consisted of two basic units, that were formed by a high-pressure hydraulic aggregate and a loading equipment. The scheme is given in Fig. 2.

Fig. 2 Scheme of laboratory apparatus

The testing equipment was adjusted for automatic operation. Operational times, the pressures at measurement points, the number of cycles, and the temperature of hydraulic liquid were continuously monitored.

The first stage of the experiment was carried out for 400 hours (120 000 loading cycles). The control samples of the liquids were extracted every 50 hours of operation. During the experiment the temperature in the primary circuit was maintained on 54 ± 3 ^oC. The loading pressure was set on 14 Mpa.

A number of particular data were obtained during the experiment. An example can be given by the data relating to the parameter “total content of contaminants”, see Table 1.

Table I Experimental ascertained and rated values

It is apparent during this experiment that the minimum sum function was not accomplished. Behalf the chance attestation applicability of a designed methodology have been accordingly experimental data replaced by a theoretic function. The sum function has been expressed in common conformation

(3)

where:

For the assignation of arguments and indexes of the correlation of the theoretic sum function was applied the standard method of correlation and regression analyses. It is apparent, that designed theoretic functions are very well described by experimental data. By the extrapolation of a theoretic function these were assigned optimal values for recovery of hydraulic oil. It is once again an exigency to admonish, that those values need additional experimental acknowledgment.

Table II Parameters and correlation indexes of theoretic functions

Under equation (3) there were rated theoretic values of a sum function behalf diagnostic signal S₁ = content of aggregate contamination and S₂ = operating time. Values of u(S)_T of theoretic and values u(S)_E of experimental function are in a table III and pictures 3 and 4. Appropriate values of diagnostics signals (@  S₁) and (@  S₂) are assigned by a linear interpolation.

Table III The values of the function u(S₁), u(S₂)

Besides the characteristics of the biodegradable liquids at the moment of their production, there is very important there are also changes of these characteristics during the operation. The knowledge of these relations is necessary for the control of the optimal replacement of liquids.

The performed experiments have proved the possibility of the monitoring and the usage of the selected parameters of the technical states of the hydraulic system with a biologically degradable liquid. The selected parameters and the method of their determination are therefore suitable for this purpose.

We can recommend the following procedure to the users who want to achieve an optimal control of renewal of biodegradable hydraulic liquids:

• continuous monitoring and recording of the operational time for the liquid
• with drawing of control liquid samples and performing analyses for the total content of impurities, viscosity and acidity number at determined intervals of operational times
• determining whether the optimal time for renewal has been reached by substituting the obtained values into the theoretical equations. If yes, carrying out the replacement of the liquid.

1. HARTWEG, A., KEILEN, K.: The environmental benevolence of biological oils. Scottish Forestry, 43, 1989 (4)
1. HAVLÍÈEK, J. e.a.: Provozní spolehlivost strojù. (Operational reliability of machines.) SZN, Prague, 2nd edition, 1989, 616 p., ISBN 80-209-0029-2
1. CHRÁST, V., ŠÈERBEJOVÁ, M., FILÍPEK, J.: Hodnocení úèinnosti odmašovadel. Sborník vìdecko-pedagogické konference "Jakost, spolehlivost, materiály a technologie". Prague, 1995, p. 33-36, ISBN 80-213-0230-5
1. POŠTA, J.: Provoz a údržba strojù z ekologického pohledu. (Operation and machine maintenance from ecological point of view.) In: Proceedings of the International Conference “Actual questions of current agriculture and forestry”. ÚZPI, Prague, 1995, p. 69-70.
1. POŠTA, J.: Zabezpeèování spolehlivosti strojù v provozu II. (Reliability assurance of machines operation.). Czech society for Quality, Prague, 1993, 144 p.
1. ZEWDIE, R.: Vliv biologicky odbouratelných kapalin na životnost prvkù hydraulických soustav a celková efektivnost jejich užití. (The impact of biodegradable hydraulic oil on hydraulic elements and its durability and total effectiveness in hydraulic system.) TF, CZU, Prague, 1997, 131 p.

Doc. ing. Josef Pošta, CSc.

Dr. ing. Retta Zewdie

technická fakulta

Èeská zemìdìlská univerzita v Praze

Kamýcká 129

165 21 Praha 6 - Suchdol

Tel. 02/24383266

E-mail: POSTA@TF.CZU.CZ

Při použití biologicky odbouratelných kapalin v hydraulických soustavách strojů lze při komplexním pohledu, při porovnání s dosud užívanými minerálními kapalinami, očekávat (Zewdie, 1997; Hartweg et al., 1989):

• rozdílné pořizovací náklady na hydraulické kapaliny,
• rozdílný technický život biologicky odbouratelných a minerálních kapalin,
• možné rozdíly v životnosti strojních prvků hydraulických systémů,
• rozdíly ve stupni poškozování životního prostředí,
• možnost nepotravinářského využívání zemědělské půdy se všemi doprovodnými jevy, tj. uchování kulturní krajiny, pracovní příležitosti aj., a současně také úsporu a uchování zásob ropy, včetně vedlejších efektů z toho plynoucích.

V odborných publikacích je dostatečně zdůvodněno, že vhodným integrálním ukazatelem efektivnosti užití technického objektu je suma pořizovacích (výrobních) a provozních nákladů, připadajících po vyčerpání technického života objektu na jednotku jeho doby provozu. Jedná se ukazatel “průměrné jednotkové náklady”, (Pošta, 1993, Havlíček et al., 1989;). Toto kriterium bylo použito i v této práci.

• celkový obsah nečistot,
• změna viskozity,
• obsah vody,
• režim opotřebení
• změna kyselosti kapaliny.

The compensation of oil products in different suitable materials is motivated first of all by the effort to discover materials with better ecological properties, to lessen the dependence on producers of oil and cultivate agricultural land for non food purposes.

The use of the hydraulic appliances in mobile machinery, especially with tractors, utility vehicles, agricultural and construction machines, etc. conceals an extraordinary risk in the pollution of water and soil from leakage of hydraulic liquids in the field.

During the use of biodegradable liquids in the hydraulic systems of machines it is possible at complex view when making comparison with current mineral liquids, expect (Zewdie, 1997; Hartweg et al., 1989):

• different actual costs of hydraulic liquids
• different technical life of biodegradable and mineral liquids
• possible differences in the life-time of the machine elements of hydraulic systems
• differences in the level of the damage in environmental life
• possibility of the non food use of agricultural land with all concomitant phenomenon’s, e.g. the conservation of cultural landscape, work opportunities, etc. and simultaneously also the cuts and conservation of oil reserves including side effects.

Theoretical and methodical process of the problem of overall technical-economical effectiveness in the use of biodegradable hydraulic liquids was the primary object of the work. To reach a conclusion it was necessary:

• to carry out a theoretical analysis of effects influencing overall effectiveness of an appliance using biodegradable hydraulic liquid
• to propose methodics in the determination of overall effectiveness in the use of these liquids
• to verify experimentally the accuracy and realness in the proposal and the way of discovering the entry data needed for the proposed methodics.

In professional publications it is sufficiently stressed that a suitable integral indicator of the effectiveness of the use of technical object is the sum of entry (production) and operational costs resulting after the exhaustion of the technical duration time of the object in the unit at the time of operation. It is the question of the indicator “average unit expenses” (Pošta, 1993; Havlíèek et al., 1989). This criterion was also used in this work.

For the determination of the normative for the exchange (restoration) biodegradable liquid in the hydraulic system the following method was proposed:

• It is chosen a promising diagnostic signal for the evaluation of the technical state of the liquid and it is conjectured the extent of the real values of this signal.
• It is followed the dependence of the value of the selected signal on the time of the process of liquid for every element of selected set.
• For every change of the liquid it is recorded the individual value of the diagnostic signal and its corresponding value at the time of the process.
• These experimentally reached data are statistically processed and are replaced by theoretical dependence which make it easier to obtain the optimal values of diagnostic signals and corresponding time of operation.

It was proposed and produced a laboratory experimental appliance which made it possible to observe and register all required data. The appliance is schematically demonstrated in picture 2.

During this experiment verification was followed by these diagnostic signals:

• overall content of impurities
• change of viscosity
• content of water
• regime of wear out
• change of acidity of liquid

There were obtained experimental data which were processed according to suggested methodics. The results in relation to the most suitable diagnostic signal are presented in tables I up III and graphically demonstrated in picture 3.

As the most suitable and the most accurate diagnostic signal appeared in the conditions of the experiment “overall content of impurities”. For this signal there were processed all figured out data, stated theoretical dependence on the time of the operation and determined normative for the change of the liquid. There was confirmed the suitability and reality for the use of the proposed methodics for optimal change of hydraulic liquid and for the evaluation of overall effectiveness and the use of biodegradable hydraulic liquids.

To those using hydraulic appliances with biodegradable hydraulic liquid is now possible to recommend the following procedure of the optimal regulation in the change of the liquid:

1. Record continuously the times during the operation of liquid (appliance).
2. During a time span of 100 operational hours to measure the samples of liquid and to determine the overall content of impurities, viscosity and number of acidity.
3. By the placement of discovered values to relations derived in this work to determine if the optimal moment of a change in liquid was reached. If yes, change must take place.
4. This process to apply to all diagnostics signals and after a certain time of this process to select a signal which will be for the concrete conditions the best. The change in liquid will be further programmed by this signal.

biodegradable hydraulic liquid; viscosity; content of impurities; regime of wear out; diagnostic signal