Vessel and valve selector by flow rate
Enter the duty flow and filter media; get the smallest FlowShell FRP vessel from DWT's spec sheets, the service and backwash flows the valve must support, and a recommended DWT-stocked Runxin valve.
Clack quotes 37–49 m/h backwash for fine (0.45–0.55 mm) sand with 20% freeboard minimum, which matches conventional rapid-sand practice; the default here is the bottom of that band. Coarser and denser gradings need MORE upflow to fluidise, not less, and cold water needs less than warm for the same expansion — so confirm the rate against your actual grading and the coldest water the filter will see, then check the valve and the drain can pass it. DWT standard fills allow ~30% freeboard.
Results
| Recommended vessel | FRP1865-4 (18" x 65", 4" top) |
|---|---|
| Manufacturer service-flow guidance | 4–5 m3/h (per the FlowShell spec sheet) |
| Bed velocity at this duty | 24,4 m/h |
| Media fill (settled bed) | ~189 L (77% of vessel volume) ≈ 303 kg |
| Freeboard for backwash | ≥30% of bed depth (bed expands ~20%) |
| Valve must deliver (service) | 4 m3/h |
| Valve and supply must deliver (backwash) | 6,07 m3/h (101 L/min) |
| Recommended Runxin valve (filtration) | F75 / N75 (53510 / 53610) |
FRP1865-4 carries this duty at 24,4 m/h, inside the manufacturer's 4–5 m3/h guidance for the vessel. The F75 / N75 (53510 / 53610) is the smallest DWT-stocked filter valve rated for the governing flow here (6,1 m3/h, set by backwash). Facing the display, the inlet is on the RIGHT — flow runs right to left. This is where the direction flips from the smaller 1" valves. Confirm tank thread and riser size when ordering, and check the whole installation can actually deliver the backwash flow — an undersized valve or feed pump is the most common cause of a failed media filter.
Spec sheet: FlowShell FRP1865-4 technical data sheet (PDF)
Formula used
vessel: smallest FlowShell size where Q ≤ sheet band max AND Q ÷ A ≤ vs,max
v_actual = Q ÷ A · A = π × d² ÷ 4
Qb = A × vb
valve: smallest stocked range rated ≥ max(Q, Qb)
fill = vessel volume × media fill fraction · rest = freeboard
- Q
- duty (service) flow, m3/h
- A
- bed area of the recommended vessel, m2
- vs,max
- upper design service velocity for the media, m/h
- v_actual
- actual service velocity in that vessel, m/h
- Qb
- backwash flow the valve and supply must deliver, m3/h
Vessel flow bands are the manufacturer's service guidance from DWT's published FlowShell spec sheets (DWT-DS-I-001 to -010); the media check keeps bed velocity inside the selected media's design band, which can call for a larger vessel than the generic guidance. Fill and freeboard figures are per-media design values from the manufacturers' data sheets (Clack, Dryden Aqua, Rotocarb, Purolite, DWT calcite sheet) — the settled bed fills the stated share of the vessel and the rest is freeboard so the bed can expand during backwash without washing media to drain. Valve ratings are Runxin catalogue rated flows; filter and softener valves are not interchangeable in service.
Equipment for this duty
Send the water analysis and duty point, and the branch confirms exact selections, pricing and lead times — usually within one working day.
Request a quoteResults are indicative engineering estimates based on standard formulas and typical assumptions. They are not a design service; confirm any design against a full water analysis and the relevant manufacturer’s data.