The Clack WS1CI control valve includes a useful differential pressure (DP) function that allows a filter to automatically initiate a backwash when the pressure drop across the filtration system becomes too high.
This can be particularly useful on sand filters, multimedia filters, activated carbon filters and other backwashing media filters, where increasing differential pressure can indicate that the filter bed has accumulated contaminants and requires cleaning.
In this guide, we explain how the Clack WS1CI differential pressure function works, how to connect a suitable pressure switch and how to choose an appropriate backwash trigger point.
What Is Differential Pressure Backwashing?
As water passes through a clean filter bed, there will naturally be a small pressure loss between the inlet and outlet.
As suspended solids and other contaminants accumulate within the media, resistance to flow increases. This causes the pressure upstream of the filter to become progressively higher relative to the downstream pressure.
The difference between these two pressures is known as differential pressure, often written as ΔP.
For example:
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Filter inlet pressure: 3.5 bar
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Filter outlet pressure: 3.25 bar
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Differential pressure: 0.25 bar
As the filter becomes loaded, the outlet pressure might fall to 2.9 bar while the inlet remains at 3.5 bar:
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Filter inlet pressure: 3.5 bar
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Filter outlet pressure: 2.9 bar
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Differential pressure: 0.6 bar
A differential pressure switch can detect this increase and signal the WS1CI to initiate a backwash.
Does the Clack WS1CI Measure Differential Pressure?
No. The WS1CI does not contain a pressure sensor.
Instead, the WS1CI circuit board has an external input labelled DP SWITCH. A separate differential pressure switch is connected to this input.
The pressure switch monitors the pressure before and after the filter. When the differential pressure reaches the chosen set point, the switch closes an electrical contact.
The WS1CI detects this contact closure and can then initiate a regeneration/backwash.
Importantly, the Clack DP input requires a dry contact. External voltage should not be applied to the DP SWITCH terminals.
How to Connect a Differential Pressure Switch to a WS1CI
A suitable differential pressure switch will normally have two pressure connections:
HIGH pressure connection
Connect this to a pressure tapping on the inlet/upstream side of the filter.
LOW pressure connection
Connect this to a pressure tapping on the outlet/downstream side of the filter.
The switch should also provide a volt-free/dry electrical contact, typically using COM and NO terminals.
The electrical connection is then:
Differential pressure switch COM + NO → Clack WS1CI DP SWITCH input
When the preset differential pressure is reached, the contact closes and signals the WS1CI.
How Does the WS1CI Prevent False Backwashes?
Water pressure can fluctuate considerably during normal operation.
For example, opening several outlets simultaneously may temporarily increase the pressure drop across the filter. You would not necessarily want a short pressure fluctuation to trigger a complete backwash cycle.
The WS1CI therefore requires the differential pressure switch signal to be present for an accumulated two minutes before acting on it.
This helps prevent temporary flow or pressure fluctuations from unnecessarily triggering a regeneration.
Clack WS1CI DP Programming Options
Within the WS1CI programming there are several options for the differential pressure input.
OFF
The differential pressure input is disabled.
dPon0
A sustained DP switch signal initiates an immediate regeneration/backwash.
For many backwashing media filters, this is the most useful setting because excessive differential pressure indicates that the filter requires cleaning.
dPdEL
A DP switch signal causes a regeneration to be scheduled for the valve's programmed delayed regeneration time.
This may be preferable where backwashing should only take place at a particular time, such as overnight.
HoLd
The external switch signal is used to prevent regeneration while the contact condition is present.
For conventional differential-pressure-controlled filtration, dPon0 or dPdEL will normally be the relevant options.
What Type of Differential Pressure Switch Do I Need?
A wet/wet differential pressure switch should be used for a water filtration application.
Do not confuse this with the inexpensive differential pressure switches commonly used for air and HVAC applications. These may not be designed for water to enter the pressure connections.
The switch should ideally have:
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Wet/wet operation suitable for water
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Adjustable differential pressure set point
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Suitable maximum static/system pressure
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SPDT or suitable NO dry electrical contact
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Pressure range appropriate for the expected filter differential
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Suitable enclosure protection for the installation environment
One example is the Dwyer Series DX Wet/Wet Differential Pressure Switch, which is specifically designed for liquid applications and can be used to initiate filter backwashing as differential pressure increases.
Other equivalent wet/wet differential pressure switches can also be used, provided they offer a suitable dry contact output and appropriate pressure ratings.
Where Should the Pressure Connections Be Installed?
For most installations, the simplest arrangement is to measure differential pressure across the complete filter assembly.
The high-pressure tapping is installed immediately upstream of the filter and the low-pressure tapping immediately downstream.
The arrangement is approximately:
Water Supply → HIGH Pressure Tapping → WS1CI → Filter Vessel → LOW Pressure Tapping → Treated Water
Both pressure tappings are connected to the differential pressure switch.
The switch's electrical contacts are then connected to the DP SWITCH terminals on the WS1CI circuit board.
This allows the system to monitor the increase in pressure loss through the complete filtration system as the media becomes loaded.
What Differential Pressure Should Trigger a Backwash?
There is no single correct setting for every filter.
The appropriate differential pressure depends on factors including:
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Filter media
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Vessel diameter
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Media bed depth
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Service flow rate
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Clean-bed pressure loss
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Available incoming pressure
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Required downstream pressure
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Nature and concentration of the contaminants being removed
For many media filtration applications, a differential pressure somewhere around 0.5–0.7 bar may provide a useful starting point, but the correct value should be established for the individual system.
The best approach is to measure the pressure drop across the filter when it is clean and operating at its normal design flow.
For example:
Clean filter
Inlet pressure: 3.5 bar
Outlet pressure: 3.25 bar
Clean differential pressure: 0.25 bar
If the differential pressure switch were subsequently set to 0.6 bar, the filter would be allowed to accumulate approximately another 0.35 bar of pressure loss before a backwash was initiated.
The actual setting should always take the filter design and available system pressure into consideration.
Why Use Differential Pressure Instead of Outlet Pressure?
It is generally better to measure differential pressure rather than simply fitting a pressure switch to the filter outlet.
Imagine the incoming water pressure normally sits at 4 bar but temporarily falls to 3 bar because of changes elsewhere in the water supply.
A simple outlet pressure switch might interpret this as a blocked filter even though the filter itself is perfectly clean.
A differential pressure switch compares the pressure on both sides of the filter.
It is therefore responding primarily to the increasing resistance through the filtration system rather than simply reacting to changes in incoming water pressure.
Should You Still Programme a Day Override?
In most applications, yes.
Differential pressure provides an excellent way of determining when a heavily loaded filter requires backwashing, but there may also be circumstances where the filter should be backwashed even though the DP threshold has not been reached.
For example, a lightly used filter might operate for an extended period without accumulating enough solids to trigger the pressure switch.
A sensible configuration can therefore combine:
Differential pressure backwashing + maximum day override
The differential pressure switch initiates an earlier backwash when the filter becomes heavily loaded, while the day override ensures that the media is still periodically backwashed during periods of low contaminant loading.
The appropriate maximum interval will depend on the filter media and application.
Differential Pressure Does Not Increase the Filter's Rated Flow
A DP-controlled backwash system can help optimise filter operation, but it should not be used as a way of exceeding the recommended service flow of the media or vessel.
Filter performance still depends on factors such as:
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Filtration velocity
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Media type
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Bed depth
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Contact time
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Vessel diameter
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Contaminant loading
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Backwash expansion and flow rate
Differential pressure control simply provides a better indication of when the filter is becoming hydraulically loaded and requires cleaning.
It does not compensate for an undersized filter.
Typical WS1CI Differential Pressure Setup
A typical automatic media filter installation might therefore be configured as follows:
Control valve: Clack WS1CI
Filter: Sand, multimedia, activated carbon or other backwashing media
DP switch: Adjustable wet/wet differential pressure switch
Pressure sensing: Filter inlet and outlet
Electrical output: Volt-free/dry contact
WS1CI DP mode: dPon0 for immediate backwash, or dPdEL for delayed backwash
DP set point: Determined from clean-bed pressure loss and system requirements
Day override: Retained as a maximum backwash interval
Need Help Setting Up a Clack WS1CI Filter?
GAPS Water Treatment supplies Clack WS1 control valves, pressure vessels, filtration media and components for automatic backwashing water filtration systems.
If you are designing a differential-pressure-controlled filter, the correct vessel size, media selection, service flow and backwash flow should all be considered alongside the DP switch setting.
Contact us with your water analysis, required service flow rate, incoming water pressure and proposed filter size, and we can help determine a suitable filtration configuration.