Quick answer: preliminary FRP vessel diameter comes from design service flow divided by the selected filtration rate, then converted from area to diameter. Check media-bed depth, freeboard, backwash flow, pressure, and control-valve flow limits together. This worksheet establishes a design basis; it does not replace final media, valve, and vessel datasheets.
Start with one clear duty flow: average, peak, or the capacity required while another unit is unavailable. PT Watermart Perkasa can match the result to FRP vessels, filter media, and automatic control valves once raw-water data, treated-water target, and installation conditions are available.
Inputs required before sizing
Do not begin with a vessel size that happens to be in stock. Record water quality, hydraulic duty, and physical limits so neither filtration nor backwash rate is chosen from pipe size alone.
| Symbol | Input | Unit | How to set it |
|---|---|---|---|
| Qs | Design service flow | m³/h | Peak flow the operating filter must actually deliver |
| Vs | Design filtration rate | m³/m²/h | Media guidance and influent quality; not one universal value |
| Hb | Media-bed depth | m | Media datasheet, treatment duty, and underdrain arrangement |
| Fbw | Backwash rate | m³/m²/h | Media datasheet at design water temperature for required bed expansion |
| Qbw | Available backwash flow | m³/h | Pump curve, actual pressure, piping, and concurrent demand |
| Hf | Available freeboard | m | Shell height above media after allowing for backwash expansion |
| Pv | Design pressure and temperature | bar; °C | Maximum system conditions and vessel, valve, and fitting ratings |
Also obtain turbidity, iron/manganese, organic matter, particle size, pH, operating hours, and treated-water target. These inputs determine pretreatment need, likely bed fouling rate, and whether one vessel provides sufficient availability.
FRP vessel diameter and filter-area worksheet
Select Vs from media documentation and influent conditions, then calculate using m³/h and m² consistently.
- Calculate minimum filter area.
A = Qs ÷ Vs - Calculate initial hydraulic diameter.
D = √(4 × A ÷ π) - Select an available vessel diameter equal to or above D. Do not round down, because actual filtration rate rises.
- Calculate actual area from the selected diameter.
Aactual = π × D² ÷ 4 - Calculate actual filtration rate.
Vs,actual = Qs ÷ Aactual - Check bed height and freeboard. Internal vessel height must accommodate the underdrain, Hb, Hf, and connection allowance; freeboard must remain adequate when the bed expands during backwash.
| Check | Formula or evidence | Pass condition |
|---|---|---|
| Service area | Aactual = π × D² ÷ 4 | Actual area is not below minimum area |
| Actual filtration rate | Qs ÷ Aactual | Does not exceed media and influent limit |
| Media volume | Aactual × Hb | Matches media arrangement and vessel volume |
| Expansion space | Shell height less underdrain and Hb | Meets media-datasheet expansion allowance |
| Pressure/temperature | Vessel and valve datasheets | Every component rating exceeds design condition |
Backwash and control-valve worksheet
Do not size backwash from service rate. Use the media manufacturer’s Fbw at design water temperature, then calculate the required flow for the selected vessel.
- Calculate required backwash flow.
Qbw,required = Aactual × Fbw - Compare it with available backwash flow. Pump and pipework must deliver
Qbw,requiredat the pressure remaining at the vessel, not merely at an unloaded pump-flow point. - Check valve limits. The control-valve datasheet must cover service flow, backwash flow, port size, pressure drop, operating pressure, and cycle sequence for the configuration.
- Check drain and water source. The drain must receive backwash flow; the tank or source must supply one cycle without disrupting the service that remains online.
| Item | Value to enter | Decision |
|---|---|---|
| Selected vessel diameter | m | |
| Actual area, Aactual | m² | |
| Media backwash rate, Fbw | m³/m²/h | |
| Required backwash flow | Aactual × Fbw m³/h | |
| Pump flow available at duty point | m³/h | Confirm against pump curve |
| Control-valve backwash limit | m³/h | Refer to selected model datasheet |
| Drain capacity | m³/h | Must not restrict required Qbw |
Worked preliminary example: 10 m³/h filter
This example shows the calculation order; it does not recommend a media rate. Suppose service demand is 10 m³/h and selected media/process information sets an initial rate of 10 m³/m²/h. Minimum area is 10 ÷ 10 = 1.00 m²; hydraulic diameter is √(4 × 1.00 ÷ π) = 1.13 m.
If the available vessel diameter is 1.20 m, actual area is π × 1.20² ÷ 4 = 1.13 m² and actual service rate is 10 ÷ 1.13 = 8.84 m³/m²/h. With a 0.80 m media bed, media volume is about 1.13 × 0.80 = 0.90 m³. Do not infer backwash rate from this example: enter Fbw from the media datasheet, multiply it by 1.13 m², then check pump, valve, drain, and freeboard.
Pre-purchase vessel and control-valve checklist
- Raw-water analysis and treated-water target establish the filter duty and pretreatment need.
- Service flow accounts for operating hours, peak flow, and capacity while one unit is unavailable.
- Filtration and backwash rates come from media documentation or approved design at relevant water temperature.
- Vessel diameter is rounded up from hydraulic diameter and actual rate is recalculated.
- Internal height allows for underdrain, full media bed, and freeboard during backwash expansion.
- Pump, pipework, valve, and drain can handle backwash flow at the actual duty point.
- Pressure, temperature, ports, connections, and internal distributor suit the final arrangement.
- Valve sequence—service, backwash, slow rinse, and fast rinse where used—matches the process and available water.
- Layout provides access for media loading, valve replacement, sampling, and maintenance.
Handoff to water-filter components
Send the completed worksheet, water analysis, PFD or layout, available pressure, and pump curve to PT Watermart Perkasa. These data support a checkable selection of FRP vessels for water filtration, filter media, control valves, and supporting components.
Where filtration is RO pretreatment, also use the RO membrane and pressure-vessel sizing guide. Procurement must still use final model numbers and datasheets for every component.
Sources
- Watermart FRP vessel page, for available vessel sizes and product configurations.
- Watermart filter-media page, for connecting the media duty to required design inputs.
- Watermart control-valve page, for selecting a model from verified flow and cycle data.
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