Why is my screen filter on bypass?

An issue regularly brought to our attention is a screen filter that went into bypass soon after installation - meaning, it stopped working. 

First let’s clarify what is meant by “bypass mode”: 

Automatic screen filtration uses a pressure differential sensor to detect whether a backwash cycle has successfully cleaned the screen. If yes, the filter returns to normal filter mode. If no, it attempts another backwash cycle. Typically, backwash cycles in New York City water conditions occur 6 - 12 times daily. 

If the cleaning mechanism cannot successfully backwash the screen, then the system will try again. After some number of failures to clean the screen, the filter will go into bypass mode, automatically or manually. 

The principal cause of a series of unsuccessful flush cycles is inadequate suction force to pull collected particles from the screen. And the most common reason for a filter having insufficient suction power is misapplication of the underlying technology – using a hydraulic-type filter instead of a fully electrically powered one for fine (10 or 25 micron) filtration.

Even for trained mechanical engineers, the difference is often not apparent.  Especially when information provided by the factory or vendor obscures the distinction.

To explain why hydraulic-type screen filters can quickly end up on bypass requires some attention to detail:

New York City water has a high level of total suspended solids (TSS).   More than 99.9 % of these particles are smaller than 15 microns.  Since particles are soft and clump together when they are collected against a screen, filtration is not "pass/fail" based on a screen's micron rating.  A 25 micron screen filter is therefore generally very good at reducing overall TSS in NYC water conditions.  A 10 micron screen filter is extraordinary, reducing not only TSS but also Turbidity, actually improving the very transparency of water. In NYC water conditions, filtration coarser than 25 micron - 50, 80 or 100 etc. - offers little reduction of the TSS count, due to the prevailing size of particles.

The hard part of fine screen filtration is not the filtering itself.  It is the scanning of dirt off the screen and restoring it to virtually 100% clean.  This is true for 25 micron and even more so if 10 micron.  The ability of a screen automatically to restore itself to clean status is what enables automatic screen filtration to operate reliably day after day.

The single most important factor in achieving effective screen cleaning is a high pressure differential between the two sides of the screen, which in turn generates a high velocity of water passing through the screen and into the nozzles of the suction scanners that “vacuum” the screen.  It is this velocity that dislodges the contaminants from the screen.  The greater the velocity, the greater the cleaning force. 

In order to capture this high velocity, the suction scanner nozzles must come into direct contact with the screen, as seen below:

Suction power weakens exponentially as the distance between scanner nozzle and screen increases.  Without actual contact, cross currents and turbulence - especially in a water environment - interfere with suction power.  With even a few mm distance from the screen, part of the dirt load, instead of being captured, will either reenter the filtration chamber or remain lodged in the screen.

(Think of vacuuming your floor.  How effective would it be if you hovered over it instead of vacuuming directly on it?) 

Note: As pore size decreases (fineness of filtration increases), the force required to capture collected particles during cleaning cycles increases.  So while the above principle is true at 25 micron, it is all the more critical at 10 micron.

In the video clip below from STF Filtros (manufacturer of the Omicron line), you will see operation of a suction scanner in physical contact with a screen: 

Note the nylon bristle nozzle slowly suctioning off dirt from the screen as it methodically brushes across.  This creates resistance, which must be overcome by an energy source greater than the available water pressure inside the filter:  Electric power.

There is no alternate reality.  A separate energy source apart from the hydraulic power of water moving in the filter - electricity – must independently control the cleaning mechanism.  

Note: For coarse filtration, say 80 micron and greater, incoming hydraulic pressure can be adequate for both filtration and cleaning.  But not at the fine degree of 10 or even 25 micron. 

There are any number of low-cost screen filter brands that rely on hydraulic technology (first developed more than 40 years ago by Filtomat, with patents now expired).  With claims that ignore principles of mechanical physics, the crucial difference between a hydraulic-type and a fully electrically powered suction scanner are not apparent.  Viewing video clips issued by manufacturers of hydraulic-type filters, the two technologies appear to do the same thing.  Depictions of scanner operation show dirt particles magically leaving the screen and obediently entering the scanner nozzles. 

Here is an example of such a video clip from a manufacturer of hydraulic screen filters that shows a simple sequence:  The suction scanner spins rapidly, controlled only by the available hydraulic power within the system.  Without requiring any substantial velocity of water, without nozzles in physical contact with the screen... it all just appears to work perfectly.

Understanding why it really doesn't work that way requires a bit of digging into some basic principles of physics:

With a hydraulically driven motor, the sole source of energy is the available water pressure in the system.  Every unit volume of water under pressure conveys a given measure of energy.  In a closed system where there is no additional input of energy, the existing energy source, hydraulic power alone, must be allocated to all functions.  That is simply not enough to generate the velocity of water required to clean a fine screen.  Additionally, the rotation speed of the scanner is not independently controlled.  Higher incoming pressure causes it to spin more rapidly.  But cleaning a fine screen requires slow, deliberately calibrated, rotation speed and torque independent of water pressure.

Thus in order to compensate for the diminished energy/volume ratio per unit of water, the hydraulic-type filter requires more water per second for its cleaning cycle. To accommodate that higher volume, the suction nozzles must be larger.  This further reduces the velocity into the nozzles, further diminishing the effectiveness of cleaning.  (Think of sliding your finger across a garden hose:  Reducing the aperture increases the velocity, and vice versa).

There are accordingly two reasons for reduced water velocity:

1. The limitations on power in a hydraulic system as noted above.

2. The distance between the scanner nozzles and the screen.

It requires suspending the laws of mechanical physics to achieve adequate cleaning of a fine screen in NYC water conditions with heavy loads of fine particles (high TSS), if relying on a hydraulic-type filter. 

So why don't the manufacturers of hydraulic-type filters provide scanners with nozzles that actually contact the screen? 

Because there is no external power (electrical) to overcome the resistance caused by the friction of nozzles moving against the screen.  If there were any physical contact between the nozzles and the screen, the scanner system would not operate.

Note there are also hybrid designs - hydraulic-type filters that add electric power to the scanner but still maintain spinning suction scanners at a remove from the screen.  These are somewhat more effective than purely hydraulic filters, but often end up on bypass nonetheless, since without actual contact the system lacks sufficient suction power to remove captured particles. 

It took years before the consequences of trying to use a hydraulic-type screen filter in New York City water conditions came to be widely understood within the community of engineers and owners who encountered these operational failures, long after specifying decisions were made.  Today, it seems to us that most mechanical engineers tasked with approving screen filtration have a deeper understanding of the technologies in play.

However, decision-makers often associate success or failure with brand names, not with the underlying technologies.  That is a mistake.  While it is true some factories make better products than others, the most vital determination an engineer or other decision maker can make is recognizing the underlying technology behind the brand.  In the case of screen filtration, that means understanding the critical differences between hydraulically powered and electrically powered filtration.  

More about minimum pressure requirements and the choice between 25 and 10 micron: