SPECIAL SECTION | Flow & Filtration
The Temporary Strainer Problem
Why premature removal during commissioning wrecks equipment.
Pratik Desai | Titan Flow Control
| IMAGE 1: Types of temporary strainers (Images courtesy of Titan Flow Control, Inc.)
Three weeks after startup, a maintenance crew opened a water booster pump, expecting to find it in good condition. Instead, they found it was damaged. The impeller vanes had deep radial grooves, the wear rings were worn beyond tolerance and both seal faces were pitted, not from cavitation but from hard particulate impact.
The pump operated smoothly on the first day, but by week three, the discharge pressure started to drop, causing the vibration alarms to go off. After some investigation, the crew discovered the issue was the result of a decision made on day five of commissioning, when someone removed the temporary strainer from the suction line.
The differential pressure (DP) across the basket strainer remained low and steady for two days. A quick visual inspection revealed almost nothing in the mesh, and the line looked clean. Consequently, the temporary strainer was removed, and the system continued to run without it.
The line was not clean. Weld slag from the new piping remained in dead legs and low points throughout the system. Gasket shreds and Teflon tape fragments were lodged at branch connections. During steady-state flow, this debris stayed in place. However, every time the system cycled (startup, shutdown, load change), velocity transients mobilized small amounts at a time, repeatedly feeding directly into an unprotected pump.
A temporary strainer worth a few hundred dollars, pulled five days too early, led to a $40,000 pump being sent to the shop for a rebuild. This is a common and preventable equipment failure during commissioning.
Why Does This Keep Happening?
The root cause is usually procedural rather than technical. Temporary strainers are pulled too early because surrounding systems—such as schedules, criteria or decision-making processes—are missing or vague.
Schedule pressure is the primary driver. Commissioning timelines are shortened, and everyone is working toward a mechanical completion date set months or years in advance, before anyone knows the actual field conditions. Removing the temporary strainer becomes a punch-list item that must be checked off to turn the line item green. Instead of making sure the system is clean, the focus is on when the strainer can be pulled out.
Differential pressure readings can be misinterpreted. A clean differential pressure reading across the strainer does not necessarily indicate the entire piping system is clean. It simply means the strainer basket is not loaded at that moment. Debris movement in the new piping is intermittent rather than continuous. Material tends to settle in dead legs, at low points and around tees and branch connections. It only moves when flow conditions change during startups, shutdowns, thermal cycles or sudden load shifts. A DP gauge reading steady state will not show what occurs during transient conditions.
Visual inspections can create a false sense of confidence. Someone may pull the basket, see little and declare it clean, but fine particles like sub-200-micron weld scale, silica or coating fragments pass through. Holding a strainer basket up to the light will not reveal particles that could damage a mechanical seal with a 5-micron face gap.
Most commissioning procedures lack written removal criteria. They state “remove temporary strainers” as a line item. They do not specify when, based on what evidence or who has the authority to decide. Without defined criteria, the decision automatically falls to whoever is closest to the strainer when someone asks, “Are we done with this?”
The situation in water treatment is similar. A municipal plant activates a new raw water intake line. The contractor removes the temporary strainer after 48 hours because the basket appears clean. Two weeks later, the plant starts to lose control of a downstream series of butterfly valves. When they remove the valves for inspection, they find sand and zebra mussel shell fragments packed around the edges of the disc. The strainer was there to protect those valves, but it was removed before the line had been flushed long enough to clear out the sediment that had been sitting in the pipe since installation.
What Happens When Strainers Come Out Too Early?
The damage to equipment follows a predictable pattern. Whatever is immediately downstream of the strainer takes the hit: the first pump, the first control valve, the first heat exchanger.
Centrifugal pumps are the most common components to suffer damage. Weld slag and mill scale are tough, abrasive and irregularly shaped. When they reach the impeller, they gouge the vanes and erode the wear ring clearances. As these clearances widen, internal recirculation increases, efficiency decreases and the pump begins to run rough. Seal faces become pitted due to particulate passing through the seal flush. Once the faces are damaged, leakage occurs. In severe cases, vibration caused by impeller imbalance can also damage the bearings, requiring a rebuild of the entire wet end.
Control valves sustain damage at the trim. Particulates can cause scratches on the plug and seat surfaces, and as these surfaces become rough, the valve cannot shut off properly, which leads to seat leakage. In order to compensate, the operator may drive the actuator harder, accelerating wear on the packing and stem. In a throttling service, scratches can alter the flow coefficient (Cv) and cause unpredictable cavitation patterns. A valve designed and sized for clean service now behaves differently, and the control loop begins hunting.
Heat exchangers foul. Particulates often build up on tube surfaces, particularly near the inlet tube sheet, where the velocity decreases and flow direction changes. Heat transfer rates decrease gradually at first, then become more noticeable as fouling thickens. In severe cases, the tubes can become completely blocked, creating dead zones where stagnant fluid accelerates corrosion. This issue is particularly severe in water and wastewater systems. Stagnant areas foster conditions for microbiologically induced corrosion (MIC).
Instrumentation gets unreliable. Pressure transmitter impulse lines clog, flow sensors collect debris and sampling ports plug. Calibration drift occurs when process conditions no longer match calibration, leading to distrust in readings. Operators rely on gut feel rather than data.
On a gas processing platform during the commissioning of a glycol dehydration unit, the temporary strainers were removed after the initial flush cycle. The line appeared clean. Over the following weeks, weld slag from the new piping gradually entered the lean glycol circulation pump. The slag eroded the faces of the mechanical seal, and the seal began leaking glycol. That leak caused a unit shutdown due to a fire hazard in the regeneration section. The replacement seal had a six-week lead time. The cost of the temporary strainer taken out to save a few pounds per square inch (psi) of pressure drop? Possibly $300. The cost of the shutdown, the seal replacement and the lost production? In the millions.
| IMAGE 2: Illustration of a typical temporary cone type strainer installation
The 5-Point Removal Checklist
Wait to remove the temporary strainer until all five conditions are met.
- Minimum runtime threshold exceeded Count operating cycles, not clock hours. Debris collects in dead legs, branch connections, low points and anywhere the pipe diameter or direction changes. At steady state, it stays there. What moves it is the change in conditions: the velocity spike when a system starts up, the pressure drop when it shuts down and the expansion and contraction of the pipe as the temperature swings between ambient and operating. A system that has been holding steady for 72 hours has only proven it can run. It has not been proven that the pipe is clean. A system that has been through three startup/shutdown sequences and three temperature swings has at least forced those conditions to occur. As a rule of thumb, it should be either three complete thermal cycles or 72 hours of continuous operation at the designed flow rate, whichever comes last.
- Three consecutive clean inspections Pull and inspect the strainer basket regularly. Every 24 hours of operation is a good starting point. Check for any new debris buildup. Weigh what is found. Take a photo of the basket each time. Removal requires three consecutive inspections showing no new material in the basket. Not “almost nothing.” Nothing. If debris is still being collected on the third inspection, reset the count and keep going.
- DP is stable at baseline The differential pressure across the strainer should be at or near the clean basket baseline and remain there—not just for a single snapshot reading, but for a sustained period that includes at least one startup or load change. Debris shifts during transients, so checking DP at steady state only shows the calmest part of the cycle and does not reflect the full cycle.
- Downstream equipment undergoes inspection and cleaning Before removing the strainer, inspect the first piece of rotating or close-clearance equipment downstream. Remove the pump’s seal flush filter. Check the valve trim if the valve has been in service. Examine the instrument impulse lines. If there are signs of particulate contact on downstream equipment while the strainer remains in place, it indicates the strainer is functioning. This does not mean removal is safe; it suggests that removing it would be premature.
- Approval is signed off by operations and engineering Removing the strainer is an engineering decision, not a convenience call made by whoever happens to be on shift. Document sign-off should be required from both the commissioning engineer and the operations lead before any temporary strainer is removed. This establishes accountability by requiring someone to review the inspection logs, the DP trend and the condition of downstream equipment before signing off. It prevents the “somebody said it was okay” failure mode that causes most of these issues.
Integrating the Checklist Into a Commissioning Plan
The removal checklist only works if it is included in the procedure before commissioning begins. If strainer removal criteria are only in someone’s head or in an email thread, they will not survive the pressure of a condensed startup schedule.
Write the five-point criteria directly into the commissioning procedure as a hold point. Strainer removal does not proceed until the hold point is cleared. Treat it the same way a hydrostatic test hold or an alignment check would be treated, giving it the same procedural weight. Assign a specific person, by name and job title, who will be responsible for strainer inspections and authorized to approve removal.
Brief the contractors at the start of precommissioning. Temporary strainer management should have its own agenda item. Contractors need to understand that strainer removal requires documented criteria, not a field judgment call, and that no strainer is removed without a signed-off checklist.
The solution is simple: Use the checklist. Leaving a strainer in for an extra week causes a minor drop in pressure. Pulling it a week early can destroy a $40,000 pump and shut down a plant. Wait until the data shows the system is ready.
Pratik Desai is a marketing engineer at Titan Flow Control, Inc. and has been with the company for nearly two decades, contributing across marketing, engineering, operations management, product management and product development. He may be reached at pdesai@titanfci.com. For more information, visit titanfci.com.
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