Cleaning the screen without stopping the process — designed in at the start, not retrofitted after the first shutdown.
A single-chamber strainer has to be isolated to be cleaned. On a batch process or a line with spare capacity that is a manageable interruption. On continuous duty it is not, and the strainer becomes the reason the plant stops.
Two arrangements solve it. A duplex strainer places two chambers in parallel with a changeover valve, so flow is diverted to one side while the other is isolated, drained, opened and cleaned. An automatic self-cleaning strainer keeps a single chamber but cleans the screen in place under flow, discharging debris through a purge connection.
A single-chamber strainer on continuous duty will eventually force a shutdown to clean — usually when the dirt load is highest and the process can least afford it.
A duplex body, its changeover valve and its access clearance take considerably more space than a single chamber. Adding them after the piping is built is expensive and sometimes not possible.
The changeover itself has to be considered — transferring flow between chambers should not expose the process to a pressure transient or admit air into a liquid line.
A duplex arrangement is sized so that each chamber alone carries the full design flow within the allowable pressure drop, because that is the operating condition during cleaning. Each chamber needs its own drain and vent so it can be depressurised and drained before the cover is opened, and the changeover valve is selected for tight isolation of the offline chamber.
An automatic self-cleaning strainer suits continuous duty with a steady or high solids load, cleaning on a differential pressure set point, on a timer, or both. It removes the manual cleaning task entirely, at the cost of a powered actuator, a control interface and a purge line that has to be routed and disposed of.
Six governing inputs, one evaluation, four engineered outputs. The defining question is what the process can tolerate during cleaning, and that answer drives the rest.
Each duplex chamber is sized for full design flow on its own, because single-chamber operation is the condition during cleaning — not an upset case.
| Requirement | Possible Solution |
|---|---|
| Manual cleaning acceptable, flow must not stop | Duplex Strainer |
| No manual intervention available or practical | Automatic Strainer |
| High or variable solids loading in continuous duty | Automatic Strainer |
| Remote or unattended installation | Automatic Strainer with monitoring |
Duplex suits continuous duty where manual cleaning is available but downtime is not. Automatic suits continuous duty where manual intervention itself is impractical, and requires power, a control interface and a purge route.
For continuous-duty selection we also need to know what the process can tolerate during cleaning and what utilities are available at the location.
Duplex bodies are supplied cast or fabricated depending on size and pressure class, in carbon steel, low-temperature carbon steel, stainless steel, duplex, super duplex and nickel alloys. Baskets, changeover valve internals and sealing elements are selected for the same service conditions as the body.
Allow clearance above or beside each chamber to withdraw the basket, and specify davit arms or hinged covers on larger sizes where manual handling is impractical. Every chamber needs a drain and a vent. On an automatic strainer, the purge discharge has to be routed, collected and disposed of in a way the site can actually accommodate — plan it with the piping, not afterwards.
A duplex strainer has two chambers and a changeover valve; flow is diverted to one while the other is cleaned by hand. An automatic self-cleaning strainer has one chamber and cleans the screen in place under flow, discharging debris through a purge valve. Duplex removes the downtime; automatic removes the manual task as well.
Each chamber is sized to carry the full design flow within the allowable pressure drop on its own, because single-chamber operation is exactly what happens during cleaning. Sizing each chamber for half the flow would exceed the pressure drop limit whenever the strainer is cleaned.
A properly selected changeover valve transfers flow between chambers without shutting the line. The transfer still needs consideration — venting and filling the standby chamber before changeover avoids admitting air into a liquid system.
Rarely without significant rework. The body, changeover valve and access clearance need considerably more space than a single chamber, so it is far cheaper to specify it during piping design than to retrofit it.
A power supply for the actuator, a control input — differential pressure switch, timer, or both — and a routed purge discharge with somewhere for the backwash effluent to go. These are site requirements, not options.
Send flow rate, line size, fluid, solids loading, allowable pressure drop and what the process can tolerate during cleaning — plus the utilities available if automatic cleaning is being considered.
Submit RequirementUnsure whether duplex or automatic is justified? Describe the duty, the solids loading and the manning arrangement and we will work through the comparison.
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