Sizing correctly for fine filtration

OMICRON TWIN 21300-10(H) 316L with 42,600 cm² screen area

Reliable screen filtration requires the correct alignment of these values:

  • flow rate
  • micron degree
  • incoming pressure
  • screen area

Here is the rule for predicted water conditions at a given flow rate:

The finer the filtration degree you want, the more screen area you need, and the higher the minimum inlet pressure required.

So we wonder why it is that the distributor of the screen filter line we used to represent -- until the point that we lost confidence in its capacity to provide 10 micron filtration in New York City's challenging water conditions -- appears not to understand this ironclad rule of mechanical physics.

After all, the screen is the screen, whether in their filter or an Omicron.

Could it be because the largest filter offered by this distributor is about the same size as one of our smallest?  Are they just making claims based on the best they have to work with?

Whatever the reason, we are seeing consistently undersized specifications proposed by this distributor.  Below are screen sizes the distributor originally offered as viable solutions which, after evaluation by engineers or owners, were rejected in favor of much larger Omicron filters.

Note: As indicated below, the distributor often proposed a secondary stage by Triple Clear, which is another story.  In every case that was also rejected by the engineer or owner.

Incoming Max Flow / Pressure

Their proposal

Revised to Omicron

Project 1
200 gpm / 55 psi

9000 cm² screen /
25 micron (+TC)

21,300 cm² screen / 10 micron  (55 psi with flush line pump is sufficient to support 10 micron.)

Their cm² per gpm: 45
Our cm² per gpm: 106

Project 2
400 gpm / 42 psi

9000 cm² screen /
10 micron (+TC)

33,000 cm² screen / 10 micron located downstream of house pumps to leverage high pressure

Their cm² per gpm: 23
Our cm² per gpm: 82

Project 3
600 gpm / 30 psi

4500 cm² screen /
10 micron

33,000 cm² screen / 10 micron located downstream of house pumps to leverage high pressure

Their cm² per gpm: 7.5
Our cm² per gpm: 55

Project 4
660 gpm / 45 psi

4500 cm² screen /
20 micron (+TC)

42,600 cm² screen / 10 micron with booster pump to increase inlet pressure to 80 psi.

Their cm² per gpm: 6.8
Our cm² per gpm: 65

Project 5
700 gpm / 35 psi

12,000 cm² screen /
10 micron (+TC)

42,600 cm² screen / 10 micron located downstream of house pumps to leverage high pressure

Their cm² per gpm: 17
Our cm² per gpm: 61

What jumps out at us is not only the consistent undersizing, but the overall inconsistency of approach.  On one project they propose 9000 cm² @25 micron for 200 gpm (45 cm² per gpm), on another they offer 4500 cm² @10 micron for 600 gpm (yes, only 7.5 cm² per gpm).

But we presume that in every instance the distributor assured decision makers their solutions would be reliable.  Our 20+ years of direct field experience -- including with the very product line they are now promoting -- lead us to disagree. 

The reason is this:  New York City water has a far higher level of Total Suspended Solids (TSS) / Turbidity (NTU) than any major city in the US.  While the term "city" water quality is generally understood to imply a Turbidity (NTU) value not exceeding 1.0, in New York City NTU is routinely 1.2 -- or higher.  That is, in fact, an enormous difference. 

When we size filtration for a NYC application, we are guided by the empirical reality of presenting water conditions.  Not what a piece of sales literature might suggest is good enough.  There simply is no substitute for the judgment that derives from direct experience.