Profilter - Rotating Screens
PROFILTER - the new generation of rotating screens is the result of many years of experience in water treatment, particularly at water intakes for heat and power generating plants. A thorough analysis of technical problems arisign in the process of use and maintenance of existing designs has led to a new design of rotating screen, providing for high quality of water screening. The new technology of the PROFILTER screens enables their use in chemically aggressive environment without negative effects for their life time, which makes their use and maintenance easy and cost effective. The advantages of the PROFILTER screens have been achieved due to original designs of the individual components which made it possible to get rid of important and troublesome drawbacks of the traditional designs based on jointed steel frame sections.
The design and assemble of the new screens make them applicable both for entirely new installations and for modernisation of existing water treatment systems by replacement of the old types of screens. The pertinent cost analysis proves that in the phase of general overhaul it is absolutely cost effective to replace an old type rotating screen with the new PROFILTER.
The principle of operation of the PROFILTER screen is the same as in the traditional rotating screens. The two versions of application: with raw water delivered from the inside or outside the screen drum have also been saved. Fig.1 shows the first way of application as more frequent in use. It is the new designs of the particular parts of the device that makes it entirely new in quality aspects.
The main feature of the PROFILTER screen is the substitution of the jointed steel screen sections (frames), as used in the traditional rotating screen, with a continuous 'endless' screen (1) stretched between two rubber carrier belts (2) on both sides (Fig. 2). The distance between the rubber belts is kept by means of horizontal stiffening beams (3) made of steel sections, fixed on the outside of screen. The strainer assembly of such a form is hung around the toothed drive wheels (4), its low part being immersed in the well. The rubber carrier belts have teeth on their inner sides, pressed in the process of vulcanization, which makes them function as toothed belts in drive transmission from the drive wheels (4) to the strainer assembly. Resulting from its design, the strainer assembly of the PROFILTER screen uses a minimum amount of steel, as compared with the traditional type. The minimum fraction of steel in the screen assembly makes it affordable to use acid resistant steel for manufacture of all the movable components that operate in water environment. In effect, destruction of such components resulting from corrosion has been practically eliminated, which makes the new PROFILTER screen applicable in chemically aggressive environment, e.g. in sea water. Fig.2 At the same time the new design, without joints operating in water, has radically solved the acute problem of quick wear and tear of the movable parts due to friction accompanied by destructive influence of sand and other hard particles of screenings dragged with raw water. As a result, the new type of rotating screen proves excellent durability and many times longer life time in comparison with the traditional designs.
Excellent tightness which does not degrade in the process of long time use is a feature that gives important superiority to the new PROFILTER design. This valuable advantage has been achieved owing to an original way of guidance of the strainer assembly in the well. Details are shown in a horizontal cross section (Fig. 3). During rotation, the strainer assembly runs between outer (7) and inner (8) slide ways. The outer slide ways make abutments for the horizontal beams of the strainer assembly in the case of force by the water volume that may result from screen choking and increased water head (H). Under normal conditions of use such force will practically not exist due to effective seil-cleaning. The inner slide ways are fixed to the walls of the well in such a way as to keep the required tension of the toothed carrier belts and screen. Fine tightness is ensured by means of rubber sealing strips (10) running along the inner slide ways. The sealing strips serve their function in two ways. From one side, they are tight pressed against the moving carrier belts, thus eliminating inevitable cracks between the belts and the ways. From the other side, the sealing strips are pressed against the well walls, to eliminate leakages between the inner slide ways and the walls. This makes the sealing system of the PROFILTER screens very tolerating for the surfaces of the well, while ensuring the required quality of screening process. The effectiveness of sealing by means of rubber strips consists in the use of force of water running through screen, which creates a mechanism of self-sealing, another original solution applied in the new PROFILTER rotating screens.
As it is widely known, the quality of water treatment and the life time of the rotating screen depend mostly upon maintaining the screen clean itself, i.e. on the effectiveness of the removal of screenings sticking to screen. The successively cumulating screenings, especially with the screen stopped, tend to choke the screen slots. With the screen choked, the difference of water levels on both sides of the screen, i.e. water head, increases, which leads to an increase in water force against the screen, increased friction forces, deformation of the screen, and, as a consequence, poorer screening, resulting finally in more frequent failures and shorter life time. In the PROFILTER screen, this problem has been solved by means of effective self-cleaning, which involves a number of design details. From the inner side of the strainer assembly, narrow shelves (6) have been fixed to the stiffening beams, serving as some sort of baskets to extract screenings from the raw water and lift them up while the strainer rotates. In the screen version shown in Fig.l, raw water is delivered to the interior of the screen and clean water is taken outside. The screenings lifted up by the shelves drop to the gutter (5) when the particular shelf reaches its top position. The residual screenings sticking to the screen are flushed down to the gutter with wide streams of clean water. The design of the drive system has been specially adapted for the self-cleaning function, as well as for the ease in manual removal of screenings from the gutter, which may be sometimes necessary. The gutter is placed inside the drive system (Fig.2) and its cross section is large enough. If it is necessary to manually remove screenings from the gutter there is no problem with applying the proper tool for the purpose, and even a man can easily get inside to make some repairs, without the necessity of disassembling the screen. A new design of drive system has been developed for such a self-cleaning function. The toothed wheels of the drive assembly are joint by a set of beams (9), making a rigid drive drum, instead of a typical, centred shaft. Such a design of the drive system and the gutter inside has helped with application of additional flushing of the gutter. Two examples of gutter flushing are depicted in Fig. 4. In the first version there is free access to the gutter which passes with its full length through the drive drum. This version ensures a variety of ways of delivering water for flushing the gutter. The second version is applicable for such cases when the existing design of drive transmission onto the strainer assembly gives no access to the end of the gutter. Water for flushing of the gutter is thus delivered through the hollow shaft of the drive system. Flushing of the gutter prevents gathering screenings in it and reduces to a minimum man's intervention in screen cleaning, which in practice means only periodical unloading of the screenings heaped beneath the gutter outlet. The elements of the self-cleaning mechanism are put into operation automatically when necessary.
The new complex design of the PROFILTER screens assumes a maximum ease for the personnel in the activity related to the screen operation control. Experience shows that manual control of screen functions, i.e. starting and stopping its rotation, opening and closing the flushing water, is unreliable when based on the visual supervision by the personnel. A sudden 'wave' of screenings flowing in to a stopped strainer can quite often cause damages because the reaction of the personnel to start screen rotation is usually too late. As an alternative for such situations it has been usual practice to keep the screen rotating all the time whenever the whole system of water supply is required to function. And the obvious result of such an alternative is sooner wear an tear and higher energy consumption, that is higher costs of maintenance. This is no longer the case with the new PROFILTER rotating screens which employ automatic control for the optimisation of screen functions adequately to the need. Automatic control is employed for switching on screen rotation and self-cleaning when a wave of screenings flows in and adjusting the speed of screen rotation to the density of screenings: The automatic control unit (Fig. 5) employs a microprocessor controller whose software can be easily adapted to the given conditions of screen use. The automatic control of screen motion is based on the measurement of the water head which varies with screenings density. If the water head exceeds a pre-set maximum threshold as a result of a wave of dense screenings, the screen is automatically put into operation at its nominal rotation rate and continues to perform so until the water head drops below a pre-set minimum level. For continuous removal of screenings that inflow slowly and do not cause a detectable water head, there is a possibility of arranging 'idle' rotation of the screen, just as a self-cleaning measure for the screen only.