Difference between revisions of "Backflush Filters"

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Backflush Filters (view source)

Revision as of 05:17, 16 September 2012

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[[File:Backflush_filters_03.jpg|thumb|right|Backflush]]
[[File:Backflush_filters_03.jpg|thumb|right|Backflush]]
[[File:Backflush_filters_04.jpg|thumb|right|Backflush Model]]
[[File:Backflush_filters_04.jpg|thumb|right|Backflush Model]]
 
This filter is used as pressure filter in the main flow and in a hydraulic system has the function of filtering and removing the contaminants of a hydraulic fluid. These contaminants mainly occur in the form of metallic abrasions, lacquer, rubbed off seal parts, dust, and scales. When no backflush filters are installed these dirt particles are taken to all points of a hydraulic system and can thus impair the functional reliability of the system. The backflush filter consists of the filter housing, the filter elements, and the control components.


Most drinking water systems use filters to collect, catch, or gather particles from an incoming flow. When the filter’s pores become clogged, they need to be cleaned. One of the best ways to clean a drinking water system’s filter is to backflush it, meaning reversing the flow and increasing the velocity at which water passes back through the filter. This, in effect, blasts the clogged particles off of the filter.
==Features==
Although every filter is unique, the principles of backflushing are similar for all of them. One key ingredient to a good filter backflush is using clean water, usually out of the clear well, first storage tank, or distribution system. This Tech Brief examines the most common filters: conventional and direct filtration. These filters use either pressure or rapid rate gravity processes.
 
*Used in industry
*Continuous filtration supports rational production processes
*Large filter surface area due to 2-cylinder filter elements
*Low backflush flow rates and optimal cleaning of the filter element improve filtration efficiency
*Perfect synthesis of ecology and economy
*Mature engineering and robust design
*Compact design
*Filter ratings from 25 to 1000 μm absolute, other ratings on request
*Easy to service




==When to backflush?==
==Operating principle==
    
    
The cleanliness or cloudiness (turbidity) of the water coming out of the filter just before it goes into the clear well is the best way to determine when to backflush. A good rule of thumb is 0.1 nephelometric turbidity units (NTU) on each individual filter’s effluent. This may sound a little extreme when the combined filter effluent (CFE) for conventional and direct filtration systems is 0.3 NTU. But 0.1 NTU gives the operator time to react to any problems within the treatment system. Head loss on the filter also indicates the need to backflush. Head loss is usually measured with a negative pressure gauge. As the filter gets clogged, more negative pressure is created. The pressure usually starts near zero pounds per square inch (psi) or approximately one foot of head loss on the clean filter. Then the pressure will increase in a linear fashion in the negative direction to approximately -2.5 to -4 psi on rapid-rate, gravity filters and some pressure filters or about six to 10 feet of head loss.
-The filter housing contains two filter elements with pleated wire cloth cylinders through which the medium flows and contaminants are trapped on the inside of the element.
The more clogged the filter, the greater the head loss.  


This calculation may be different depending on the filter type and make. Some small plants will just have a clear tube with water indicating pressure differences. The water level in the tube rises as the pressure difference increases. For every one psi measure, there is about 2.31 feet in a column of water. For every one foot in a column of water, there is 0.434 psi of pressure difference. The clear tube can be marked with the pressure difference in negative psi, or it could have a single mark, indicating it is time to backflush.
-When a defined differential pressure is reached or after a settable time interval, the fully automatic backflush process starts.


A couple of other indicators really only work when a water system has consistent raw water turbidity. These indicators are gallons filtered or run time. When the raw water is consistent, operators usally can tell when to backflush based on the pump’s run time.
-When the backflush start time is reached, first of all one of the flush valves opens and the gear motor starts to turn the external filter element. Thereby the whole filter surface bypasses the cleaning nozzle.


==How long to backflush?==
-The process medium that has already been filtered flows at high speed in the opposite direction through the vertical slot, which is located directly on the filter element. The trapped contaminants are discharged from the system via the flush pipe.
 
 
Backflush until the water runs clear (provided there is enough clean water to do unlimited backflushing). Most systems don’t have unlimited amounts of clean water, which is why it is important not to get the filter too dirty or overextend the filter run time. Some small systems don’t want to use a lot of water while backflushing because they may have limited water set aside in the clear well, or the system could be dealing with drought and must conserve as much water as possible.
-The flush valve closes when the filter element has been turned approximately 400° and the second flush valve opens in order to clean the internal filter element similarly. Afterwards the gear motor is switched off and the flush valve closes again. The backflush process is completed in only a few seconds.
However, systems should not think of it as wasting water because the filters must be clean to ensure good water quality for their customers. There are several techniques that can be used to enhance or speed up the backflushing process:
 
-Since the element is turned, only the part covered by the cleaning nozzle is actually cleaned; the remainder can continue to be used for filtration ® operation is not interrupted.
 
 
==Steps of Backflushing==
The necessary steps are described in the following:
 
-The filter comprises a bowl with a cover and a gear motor.
 
-The bowl contains a vent port, a drain port and a filter element.
 
-The filter must be filled and vented before it is put into service. It must not be operated with the full pump flow when empty.
 
-Switch on the filter controller and start a flushing process with the hand release. If the viscosity of the medium is very sensitive to temperature, the filter controller should not be switched on until the filter reaches its normal service temperature.
 
-The filter controller must be switched off if the plant is not in service.
 
-In order for the backflushing process to be efficient, there must be operating overpressure during the flushing process on the outlet side of the filter.
 
-Backflushing starts automatically after a defined time or when the maximum differential pressure is reached. If the differential pressure exceeds 3 bar, the filter must be removed from service or changed over to bypass. Then dismantle the filter and clean the wire cloth cylinder (refer to "Cleaning").
 
-When a flushing process is tripped, the gear motor is switched on and a flush valve for the flushing medium outlet opens. The medium flows from the clean side through the filter element and into the internal nozzle as the element is turned by the gear motor.
 
-After turning approximately one time the first flush valve closes and the second one opens. As the filter element is still turned by the gear motor, the second filter element is cleaned.


*Add a surface wash system. This system is a series of water jets that can be fixed or a revolving apparatus and comes on at the beginning of the backflush cycle and usually ends before the middle of the cycle. There also can be another series of water jets in the media bed itself (subsurface wash) that come on when the filter bed is fully fluidized (full backflush). For a surface wash to work adequately, at least 45 psi is needed as well as a properly installed vacuum breaker. The flow for fixed nozzles should be two gallons per minute per square foot and 0.5 gallons per minute per square foot for rotating system.  
-The flushing medium flows through the wire cloth at high speed, so that the contaminants trapped in the filter are detached and discharged via the flushing outlet and the flush pipe connected to it.


*Add an air scour system. This system is a series of small air pipes with diffusers in the under-drain or above the under-drains that blow uncontaminated air to help break-up mud balls. (The process is similar to when kids blow in the straw in their drinks.).
-The filter controller is programmed so that the flush valves close again and the gear motor is switched off after approximately 2¼ turns of the filter elements.


*Always spray the side of the filter walls down with clean water using a potable water hose during the backflush on a rapid-rate gravity filter.  
-To clean the filter, switch off the filter controller, dismantle the gear motor, loosen the cover fixing screws and remove the cover.


*Increase or vary the backflush rate, but not too much since the support gravel could become displaced and creating a big problem.
-The complete filter element can now be lifted vertically out of the filter. To clean the filter element manually, spray it with steam, compressed air or water from the outside towards the inside.


-Pre-treat the element with a suitable solvent if the dirt cannot be removed easily. It may be necessary to dismantle the pleated wire cloth cylinder.


==After Backflush==
-The service of the filter has to be done biannual. The operation of the filter controller, flush valves, and the gear motor has to be checked. All gaskets, O-rings and bearing bushes also have to be checked for wear or damage and replaced as necessary. The pleated wire cloth cylinders have to be checked for damage to the wire cloth and the seals as well.
 
Several techniques can be employed to avoid the dreaded turbidity spike, which is the first slug of water that comes through the filter after the backflush cycle once the filter is operational again. This turbidity spike can occur anywhere from a few minutes to 40 or more minutes after returning to filter operation mode. The turbidity can vary from a negligible NTU to as high as 1.5 NTU or more. If the turbidity on the individual filter gets any higher, there may be problems with the filter or the filter backflush.
One easy way to avoid the turbidity spike is to let the filter “ripen.” Ripening the filter means that the operator allows the filter to sit for a while after the backflush cycle. Some small plants can even perform the backflush at the end of the day, before the filter is shut down and allow the filter to sit overnight. Filter ripening takes from 30 minutes to 24 hours. In most cases, however, the longer the filter sits, the better. Filter ripening doesn’t cost anything and requires no special piping, but many systems don’t have the luxury of this time.




==Related Topics==
==Related Topic==
*[[Automatic Backflush Filter]]
*[[Backflush pool filter]]




==Video==
==Video==
<youtube>kYXtRO0-ePI</youtube>
<youtube>kYXtRO0-ePI</youtube>
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