Selecting and installing filter nozzles

A filter nozzle is a small part that decides whether a filter works. Too fine a slot and it blocks; too coarse and it passes the media. The rest of the specification is about the plate it sits in.

Also searched as: filter nozzle selection · filter nozzle slot size · abs filter nozzle installation · filter nozzle flow rate · nozzle plate filter media retention

Send the filter design Specification sheet (PDF)

Slot size is set by the media, not by the flow

The slot has one job in normal operation: retain the filter media. Start from the media grain size you are actually using, then apply the rule your specification names - the same rule appears in different wording across markets, and it is the number that decides the nozzle, not the flow capacity.

Two failure modes come from getting this wrong, and they look completely different on site:

Too coarse	Media passes into the underdrain. The filter loses its bed, the nozzles look fine, and the problem is first noticed as media in the clearwell
Too fine	The nozzle blocks during backwash or with coagulant carry-over. Head loss rises, backwash distribution becomes uneven, and the bed eventually breaks through in one area

Flow per nozzle, and the count

The count follows from the plate area and the spacing you choose; the resulting flow per nozzle then has to sit inside what the nozzle can pass at the designed head loss, in both filtration and backwash. Backwash is usually the governing case, because the flow is higher and the head loss budget is tighter.

Use the site calculator rather than a spreadsheet you inherited: filter nozzle count calculator. It takes the plate area and the spacing and returns the count, and the same page links the published flow figures for the nozzle range.

The most common procurement error is ordering nozzles against a plate area that later changes. Confirm the final plate dimensions before the order - the count is cheap to change on paper and expensive to change after casting.

Plate type decides the installation sequence

Cast-in-place concrete plate	Nozzles are set in the formwork or cast with inserts, so the installation tolerance is a formwork tolerance. Get the nozzle positions right before the concrete
Precast / block plate	Positions are set by the block, and the joint between blocks is where leakage and media bypass appear
Stainless or ABS plate	Threaded or clipped nozzles, with the plate flatness and the gasket doing the sealing work

Our ABS nozzles and fittings are supplied with the plate arrangement agreed against the drawing, and the pipe range in the same material covers the underdrain connections: ABS filter nozzles · concrete filter plates · cast-in-place filter plates · ABS pipe and fittings · guide: nozzle and plate guide.

Frequently asked

How do I choose the slot size?

From the media grain size, using the rule named in your specification, not from the flow rate. A slot that is too coarse loses the bed; one that is too fine blinds during backwash. Confirm the grain size actually being supplied before fixing the slot.

How many nozzles does my filter need?

Plate area divided by the spacing you choose, which then has to give a flow per nozzle inside the published capacity at the design head loss in both filtration and backwash. The site calculator does the arithmetic; the published nozzle data sets the limit.

Can nozzles be replaced without removing the media?

Usually not - the bed and often the support gravel have to come out to reach the plate. That is why the plate and nozzle decision should be made once, with the final dimensions confirmed, rather than revised during construction.

ABS or stainless steel nozzles?

ABS is the standard for municipal gravity filters because it is corrosion-resistant, light and inexpensive. Stainless appears where temperature, abrasion or a specification requirement rules ABS out. Both are supplied against the plate drawing.