Different large industrial processes use oil circulating lubrication systems in which lubricating oil is circulated by a single continuous system to all parts of the process that need lubrication. In an oil circulating lubrication system, the lubricant is al- ways returned from the object of lubrication into an oil tank in a central unit. Since, depending on the process, the oil always gathers some impurities, such as rubbish, water and air bubbles, as it is circulat- ing in the system, it is necessary to purify the oil at some stage during the circulation.

Traditionally, the return line of an oil cir- culating lubrication system is provided with a coarse strainer to separate coarse impurities from the oil flow before they get into the oil tank. The oil tank itself is generally constructed or designed with a view to accomplishing removal of water or air from the oil by the tank itself or separate spaces in conjunc- tion with it. The tank may also be provided with sieve-like partitions to separate impurities.

However, the use of coarse strainers in the return pipes before the oil tank is relatively diffi- cult and inefficient. The strainers need to be cleaned relatively frequently to ensure a continuous and smooth oil flow. Thus, when fine and efficient strain- ers are used, they get clogged in a short time and re- quire frequent maintenance. On the other hand, if clog-proof strainers requiring little maintenance are wanted, then it is necessary to use coarse strainers giving poor purification results. A further drawback are the interruptions in oil flow or oil leaks caused by the cleaning of the strainers. Moreover, prior-art strainer arrangements mix the returning oil flow, mak-

ing it more difficult to separate the water and air contained in the oil.

The object of the present invention is to eliminate the drawbacks described above. A specific object of the present invention is to disclose a new type of prefiltration unit for oil circulating lubri- cation systems which is efficient and reliable, which can be easily installed in both new oil tanks and in tanks already in use in oil circulating lubrication systems and which makes it possible to improve the overall purity of circulating oil, i. e. to reduce the amounts of solid matter, water and air contained in the oil.

As for the features characteristic of the in- vention, reference is made to the claims.

The prefiltration unit of the invention for separating coarse impurities from the oil flow in an oil circulating lubrication system before the oil tank comprises a filter placed in conjunction with the oil tank. According to the invention, the prefiltration unit comprises an oil flow space mounted in an opening in the top of the oil tank, an inlet space connected to the flow space via a flow connection and provided with an air lock, the returning oil being passed into said inlet space via an oil flow inlet pipe, and at least one filter between the flow space and the oil tank for filtering the oil received from the inlet space into the flow space before it flows into the oil tank. Thus, in the prefiltration unit of the inven- tion, an air lock is used to cut off air flow and pre- vent the entry of air and humid gases into the oil pu- rifying areas.

The filter preferably forms a space extending substantially downward from the flow space, so that the oil can freely and smoothly flow from the flow space into said space. The downward extending space forming the filter is preferably located completely

above the oil surface of the oil tank in an operating situation, i. e. when some of the oil in the system is in circulation. Thus, the filter works efficiently while the oil is flowing downward along the filter en- velope and clean oil is passed through the filter en- velope and impurities flow down without turbulence along the interior surface of the filter envelope to the bottom of the filter. Thus, the filter preferably consists of a cylindrical envelope made of a filtering material and provided with a bottom.

The inlet space preferably extends substan- tially downward from the flow space, so that oil re- turning into the inlet space after circulating through the air lock gets into a smooth rising flow, in which, before getting into the flow space, the oil stays long enough to permit the oil to stratify according to the impurities contained in it. In other words, air bub- bles rise to the surface and are largely separated from the oil while heavier impurities remain in lower strata in the flow. In this way, a more efficient fil- ter operation is achieved.

The top of the flow space is preferably pro- vided with a cover which covers the flow space and is provided with a port for an oil flow inlet pipe and a door for filter maintenance and/or filter change. The flow space also preferably comprises an overflow aper- ture through which the oil flow is passed directly into the oil tank if the filters do not pass a suffi- cient flow of return oil through them. The overflow aperture may [pass the oil] through a maintenance opening outside the flow space comprised in the cover.

The flow space and/or its cover are/is pref- erably mounted on the edge of an opening in the top of the oil tank or on a collar attached to or mounted on the edge. As the various parts of the unit are all in- side or below the flow space, the whole unit can be raised and lowered directly in a vertical direction

into the desired opening, so the prefiltration unit can be installed in any new oil tank or in any oil tank already in use, only by making the required open- ing in the top of the oil tank.

As compared with prior art, the prefiltration unit of the invention has significant advantages. The invention can be constructed as a modular cassette, so it can be very easily installed in existing oil tanks as well. The unit can also be lifted off e. g. when the oil tank is to be cleaned. The unit makes the oil flow before the oil tank significantly smoother, which means that air and gas will be more effectively sepa- rated in the oil tank, too. The unit ensures that no oil is allowed to splash or flow out into the environ- ment during use or in different maintenance situa- tions. The prefiltration unit of the invention also allows different kinds of treatment of oils of differ- ent degrees of purity before they are passed back into the oil tank as the unit can be provided with several inlet spaces from which the oil flows can be directed to different filters, and the oil flow already smoothed and partly stratified in the inlet space can be divided in the flow space and passed to different filters.

In the following, the invention will be de- scribed in detail with reference to the attached draw- ing, wherein Fig. 1 presents a diagram representing a prefil- tration unit according to invention, installed in con- junction with an oil tank, Fig. 2 presents a top view of the prefiltration unit in Fig. 1 without a cover, and Fig. 3 presents a top view of the prefiltration unit in Fig. 1 with a cover.

The prefiltration unit of the invention in the oil circulating lubrication system presented in the drawing consists of a module or cassette placed in

an opening 4 made in the top 3 of an oil tank 1. This module is mounted hanging from a collar 14 attached to the edges of the tank top. The module comprises a box- like case 15 provided with a straight and flat bottom, said case extending downward from the collar 14 and forming a flow space 5.

Extending downward from the bottom of the flow space is an inlet space 7 provided with an air lock 6, i. e. with a suitable dividing wall, into which space the oil to be purified is supplied via an oil supply pipe 8 extending into the inlet space. Also ex- tending downward from the bottom of the flow space 5 in this embodiment are four cylindrical filters 2 whose sides and bottom consist of a relatively fine cribrose structure with a sieve opening of the order of e. g. 50 pm.

The flow space 5 is provided with an overflow aperture 12 located at the other edge of the flow space 5, in its upper part, to allow the oil to flow directly down into the oil tank 1 if the filters 2 do not pass enough oil through them.

The flow space 5 is provided with a cover 9 completely covering it. This cover is provided with two ports 10 for oil supply pipes 8. Moreover, the cover 9 is provided with a door 11 placed directly above the filters 2 to allow filter maintenance and inspection as well as filter change. In addition, placed outside the flow space 5 in the cover 9 is a maintenance door 13, which makes it possible to carry out filter maintenance and to check the operation of the filters 2 from outside, also allowing maintenance of the oil tank 1 as a whole. Any oil flowing through the overflow aperture 12 will pass via this same main- tenance opening into the oil tank 1.

The prefiltration unit of the invention works as follows. When the oil to be purified flowing from the supply pipe 8 reaches the inlet space 7, the flow

becomes smooth and steady, and the air contained in it already begins to be separated. The oil flows through the air lock 6, which prevents any humid air in the return piping from getting into the oil tank and into contact with the oil flow to be purified. From the air lock in the inlet space, the oil rises smoothly into the flow space 5, in other words, the oil cannot get directly from the inlet space 7 into the oil tank 1 or even into the filters 2. This ensures that the flow is smooth and free of turbulence and that efficient pre- filtration is achieved.

From the flow space 5, the oil flows into the spaces formed by the filters 2, which are located sub- stantially above the oil surface 16 in the tank.

Coarse impurities are removed from the oil as it passes through the filters into the oil tank, so the oil is ready to be recirculated into the lubrication system. If the system is switched off, then all the oil will return into the tank 1 and the oil surface 17 will rise, covering the filters 2. However, this will not affect the operation or filtering efficiency of the system.

If for some reason the amount oil flowing from the oil circulating lubrication system is larger than normal and especially if the filters 2 are rela- tively dirty and have a poor pass-through performance, then a larger amount of oil may gather in the flow space 5. In this case, the overflow aperture 12 in the upper part of the flow space wall will let the oil flow into the oil tank 1 without purification, ensur- ing that no oil can escape from the system.

In the foregoing, the invention has been de- scribed by way of example with reference to the at- tached drawing while different embodiments of the in- vention are possible within the scope of the inventive idea defined in the claims.