In separators based on the utilization of a difference in specific gravity, the gravity and unequal specific gravities of various substances are utilized to separate these substances. For example, in a tank in which it is desired to separate solids from water, most of the solid matter is deposited on the bottom of the separator, wherein there is an aqueous layer above the solids. In a corresponding manner, also other substances can be separated from each other, as long as these substances have unequal specific weights. It is naturally possible to use also different filters. Furthermore, separators based on the utilization of a difference in specific weight do not comprise movable parts, wherein they are very reliable and have a long maintenance cycle.

The requirements for oil separators are set in the standard prEN 858-1.

Furthermore, a sufficiently long delay must be selected for the separation volume in oil separators. The following table presents, as examples, a selection of oil separators of different sizes and their minimum volumes according to the standard prEN 858-1. The sludge volume must be at least (L x NS)/fd (I), in which NS is the nominal size, fd is the oil density coefficient, and L is 100, if the nominal size is greater than 10 and sewage is collected from a location in which the expected sludge content is low. L is 200, if sewage is collected from a location in which the expected sludge content is average. L is 300, if sewage is collected from a location in which the expected sludge content is high. In this case, however, the smallest sludge volume is 600 1. The nominal size (NS) is obtained, in the case of rain water, by multiplying the rain water discharge (Qr) with the oil density coefficient (fd), and in the case of sewage, by multiplying the design discharge (Qs) with two and the oil density coefficient. The suitable oil separator

is the closest NS value greater than the calculated nominal size. The minimum oil volume is obtained by multiplying the NS value by the figure 15, and the minimum separation volume is obtained by multiplying the NS value by the figure 180. Vsludae (I) Voil (I) Vseparation (I) Vtotal (I) NS 3/600 600 45 540 1185 NS 3/900 900 45 540 1485 NS 6/600 600 90 1080 1770 NS 6/1200 1200 90 1080 2370 NS 6/1800 1800 90 1080 2970 NS 10/1000 1000 150 1800 2950 NS 10/2000 2000 150 1800 3950 NS 10/3000 3000 150 1800 4950 NS 15/1500 1500 225 2700 4425 NS 15/3000 3000 225 2700 5925 NS 15/4500 4500 225 2700 7425 NS 20/2000 2000 300 3600 5900 NS 20/4000 4000 300 3600 7900 For grease separators, all the volume requirements are set in the standard prEN 1825-1. The sludge volume must be at least 100 x NS value (I), the grease storage volume must be at least 40 x NS value (I), and the separation volume must be at least 240 x NS value (I). The grease storage volume is included in the separation volume. The volume requirement also includes the supply and discharge areas.

Furthermore, the minimum surface area requirement for the grease separation part is 0. 25 x NS value (m2), if the nominal size is determined by verifying the dimensions of the separator. The grease separator shall have a total fall of at least 70 mm, of which 50 mm in the sludge space and 20 mm in the grease separation space. The nominal size can be calculated according to the following formula.

NS = Qs x ft x fd x fr QS = maximum sewage discharge (I/s) ft = sewage temperature coefficient

fd = grease density coefficient fr = inpedient factor The suitable grease separator has the closest NS value greater than than the calculated nominal size. The following table presents, as an example, a selection of grease separators of different sizes and their minimum volumes according to the standard prEN 1825-1. Vsludge (I) Vgrease (I) Vseparation Vinput&output Vtotal (I) Agrease NS 2 200 80 480 24 704 0, 5 NS 4 400 160 960 24 1384 1, 0 NS 7 700 280 1680 51 2431 1, 75 NS 10 1000 400 2400 51 3451 2, 5 NS 15 1500 600 5600 80 5180 3, 75 NS 20 2000 800 4800 80 6880 5, 0 Plastic composites with their various alternatives have seized the market of e. g. underground separator tanks. Plastic tanks are easy to handle, mechanically and chemically resistant, and quickly mountable. For this reason, the aim is normally to make separator tanks preferably of plastic. However, to manufacture a body for a plastic tank, a mould is needed, wherein a separate mould must be provided for each tank body of different size.

It is an aim of the invention to reduce the number of moulds for the manufacture of bodies for oil and grease separator tanks with a plastic structure.

This aim can be achieved by making bodies for oil and grease separator tanks in only a few sizes, wherein the required oil and grease separators of different sizes and complying with the standards (prEN 858-1, prEN 1825-1) are formed of a combination of one or more tanks.

Thus, each different tank size is not manufactured separately. That is, only a few different tank sizes are preferably used to produce, for example, the selections presented above.

More precisely, the method according to the invention is characterized in that the total tank volume required for the use is formed by tank parts connected in series in the direction of the separation flow, connected to each other by ducts.

Considerable advantages are achieved by the present invention. As all the different sizes can be advantageously formed of only a few tanks of different sizes, the costs of manufacturing and storage are considerably reduced. This is due to the fact that considerably fewer moulds are needed for the manufacture of the tanks, wherein only a few moulds and a few tanks of different sizes need to be kept in storage, their combinations being sufficient to manufacture the oil and grease separators of the selections presented above. As a result, considerable savings in the manufacturing and storage costs are achieved by the firm.

In the following, the invention will be described in more detail with reference to the appended drawings, in which Fig. 1 shows an oil separator consisting of one tank according to an advantageous embodiment of the invention in a principle cross-sectional view, Fig. 2 shows another oil separator consisting of two tanks according to an advantageous embodiment of the invention in a principle cross-sectional view, Fig. 3 shows a grease separator consisting of one tank according to an advantageous embodiment of the invention in a principle cross-sectional view, and Fig. 4 shows another grease separator consisting of two tanks according to an advantageous embodiment of the invention in a principle cross-sectional view.

The general idea of the invention is that advantageously two or more different body sizes 1 a, 1 b (Fig. 2) for a tank are first selected in such a way that different combinations of one or two such tanks 1 (Fig. 1) are sufficient to cover the desired total volumes. The ratio between the volumes of these two tanks of different sizes is preferably substantially 1/n (n = number of tanks of different sizes), to achieve a gradation which is as even as possible between the different total volumes. To be able to form all the combinations formed by these tanks, the units formed of two or more different tanks must be connected in series with each other in the direction of the separation flow.

In the following, two substantially spherical tank bodies 1 a, 1 b of different sizes will be used as an example. The smaller body 1 b has a volume of about 2. 2 m3 and the larger body 1a has a size of about 4. 3 m3. According to the need, the tanks can be equipped with the required ducts 6, filters 7 and partition walls 8. It is naturally obvious that the invention is not limited solely to these sizes, but the body of at least one of the tanks can also have a size different from the examples.

Furthermore, it is obvious that the number of sizes of bodies for different containes can also be different, and also the shape of the body can be different from the substantially spherical shape. For example, the body of the tank can be substantially cylindrical or cubical. The size of ducts to be used is selected according to the nominal flow into the separation volume. In connection with the following examples, the selected duct size is given as the nominal size (DN xxx).

In the case of an oil separator 2a (Fig. 1), the next step is to calculate the nominal size (NS) for a suitable tank. On the basis of this, a tank body 1 a, 1 b (Fig. 2) or a combination of them, with a suitable size, is to be selected. The suitable total volume is greater than ( (Ufd+15+180) x NS value) (I), in which L equals to 100, 200 or 300, depending on the expected quantity of the sludge 3. If the suitable total volume is achieved with one tank 1 (Fig. 1), the tank body is equipped with suitable ducts 6 and a filter 7 in such a way that also all the other volume requirements set for the oil separator are met.

If the suitable total volume is achieved with two tanks 1 a, 1 b (Fig. 2), the oil separator 2a can be advantageously placed, in the direction of the discharge flow, on or close to the bottom of the latter tank, because the filter 7 will then not take the volume of the sludge 3. Furthermore, in the flow direction of the separation flow, the first tank is selected on the basis of the separation volume required for the sludge 3, wherein the second or latter tank is selected according to the separation volume required for the oil 5 and the required oil volume.

In the case of a grease separator 2b (Fig. 3), the next step is to calculate the nominal size (NS) for a suitable tank. On the basis of this, a tank body 1a, 1b or a combination of them, with a suitable size, is to be selected. The suitable total volume is greater than (100+40+240) x NS value (I). If the suitable total volume is achieved with one tank 1, the tank body is equipped with suitable ducts 6 and a partition wall 8 in such a way that all the other volume requirements set for the grease separator are also met. In other words, the separation volume for the sludge 3 must be at least 100 x NS value (I), the storage volume for the grease 9 must be at least 40 x NS value (I), and the grease separation volume must be at least 240 x NS value (I). The grease separator shall have a total fall of at least 70 mm, of which 50 mm in the sludge space and 20 mm in the grease separation space. Furthermore, it should be noticed that the surface area 10 of the grease separation part is at least 0, 25 x NSvalue (m2), or the grease separator must pass a standard flow test, in which the grease separator must not discharge more than 25 mg/I of the test oil.

If a suitable total volume is achieved with two tanks 1a, 1b (Fig. 4), no separate partition wall 8 will be needed, because a partition wall is functionally formed by the duct 6a used as an interface between these two tanks and the walls of the tanks. Furthermore, in the flow direction of the separation flow, the first tank is selected on the basis of the separation volume required for the solids 3, wherein the second or latter tank is selected according to the required separation volume for the grease 9 and the required surface area 10.

From one small tank 1 b, it is possible to manufacture, for example, the following oil separators 2a (Fig. 1) and grease separators 2b (Fig. 3) which meet the different requirements and were presented in the selections used as examples.

Oil separators : NS 3/600, ducts DN 110 NS 3/900, ducts DN 110 NS 6/600, ducts DN 160 Grease separators : NS 2, ducts DN 110 NS 4, ducts DN 110 The inner diameter 13 of the smaller tank 1b is 1750 mm, and the height of the discharge duct 6c is 1200 mm. The difference in height between the discharge duct 6c and the supply duct 6b is 80 mm (55 mm + 25 mm). As a result, the effective volume of the sphere is 2150 I. The following table gives the volumes obtained for the tank and the delay for the oil separator. The delay is calculated by the maximum discharge in the separation volume for oil 5. The oil storage volume is calculated with a thickness of 150 mm for the oil layer by reducing the volumes of the filter 7 and the supply duct 6b and the discharge duct 6c. The separation volume is calculated by deducting from the effective volume the sludge volume, the oil storage volume and the filter volume. The sludge volume is reduced, if the filter is mounted in the sludge space. If the filter diameter 12 is e. g. 530mm and the volume possibly taken by the duct is not considered, the filter takes 22 1 /10 cm of the sludge volume, that is, with the values of the following table, the filter can be lowered into the sludge space by 26 cm with a sludge volume of 600 1, and by 40 cm with a sludge volume of 900 1, still complying with the volume requirements of the sludge space. In the oil separator NS 6/600, the selected minimum delay of 180 s is not quite achieved, but the achieved delay of 175 s can still be considered sufficient. hsludge Vsludge (I) VOll (I) VSeparation Delay (s) (mm) (I) NS 3/600 550 657 275 1055 352 NS 3/900 700 988 275 760 253 NS 6/600 550 657 275 1050 175 = Delay in oil separation part For the grease separator 2b (Fig. 3), the volumes given in the following table are achieved for the smaller tank 1 b. The location of the partition wall 8 is specified to be 300 mm from the centre line 11 of the sphere towards the supply duct 6b. The thickness of the grease layer 9 is 150 mm, and the grease storage volume is included in the volume of the separation part.

Vsludge (I) Vseparation (I) Vgrease (I) Agrease (m2) NS 2 220 1920 290 2,0 NS 4 425 1715 290 2,0 From one large tank 1a, it is possible to manufacture, for example, the following oil separators 2a (Fig. 1) and grease separators 2b (Fig. 3) which comply with the different requirements and were presented in the selections used as examples.

Oil separators : NS 6/1200, ducts DN 160 NS 6/1800, ducts DN 160 NS 10/1000, ducts DN 160 NS 10/2000, ducts DN 160 Grease separators : NS 7, ducts DN 160 NS 10, ducts DN 160 The inner diameter 13 of the larger tank 1 a is 2140 mm, and the height of the discharge duct 6c is 1610 mm. Consequently, the effective volume of the sphere is 4340 I. The following table gives the volumes obtained for the tank and the delay for the oil separator. The values of

the table are calculated as for the smaller sphere above. If the diameter 12 of the filter 7 is e. g. 530mm and the volume possibly taken by the duct is not considered, the filter takes 22 1/10 cm of the sludge volume, that is, with the values of the following table, the filter can be lowered into the sludge space by 40 cm with a sludge volume of 12001, by 72cm with a sludge volume of 18001, by 60cm with a sludge volume of 1000 1, and by 62 cm with a sludge volume of 2000 1, still complying with the volume requirements of the sludge space. In the oil separator NS 10/2000, the selected minimum delay of 180 s is not quite achieved, but the achieved delay of 163 s can still be considered sufficient. hsludge Vsludge (I) VOjl (I) Vseparation Delay (s) (mm) I NS 6/1200 700 1288 390 2468 411 NS 6/1800 900 1959 390 1796 299 NS 10/1000 650 1133 390 2621 262 NS 10/2000 950 2136 390 1630 163 '= Delay in oil separation part For the grease separator 2b, the volumes given in the following table are achieved for the larger tank 1a. The location of the partition wall 8 is determined in this case to be 410 mm from the centre line 11 of the sphere towards the duct. The thickness of the grease layer 9 is 150 mm, and the grease storage volume is included in the volume of the separation part. Vsludge (I) Vseparation (I) Vgrease (I) Agrease (m2) NS 7 710 3690 333 2,07 NS 10 1040 3360 333 2,07 From the large tank 1 a and the small tank 1 b, it is possible to manufacture, for example, the following oil separators 2a and grease separators 2b which comply with the different requirements and were presented in the selections used as examples.

Oil separator : NS 10/3000, ducts DN 160 In this case, the first tank is the larger tank 1a, in which sludge 3 is separated from sewage 4. Thus, sewage which contains water and oil 5 is led to the second tank. That is, oil is separated from sewage in this second or smaller tank lb. On the bottom of this second tank, there is no sludge space, wherein the filter 7 can be mounted on or close to the bottom of the tank. In the oili separation space, or the second sphere, the delay is 196 s. Below, no more tables will be presented, because the calculations are performed on the basis of the principles and examples given above.

From the small tank 1b and the large tank 1a, it is possible to make, for example, the following oil separators 2a and grease separators 2b which comply with the different requirements and were presented in the selections used as examples.

Oil separators : NS 15/1500, ducts DN 200 NS 20/2000, ducts DN 200 Grease separators : NS 15, ducts DN 200 From two large tanks 1a, it is possible to make, for example, the following oil separators 2a and grease separators 2b which comply with the different requirements and were presented in the selections used as examples.

Oil separators : NS 15/3000, ducts DN 200 NS 15/4500, ducts DN 200 NS 20/4000, ducts DN 200 The present invention is not limited solely to the above-presented embodiments, but it can be modified within the scope of the appended claims.