Examining the myths v logic of minimizing pressure drop when commissioning and managing filtration strainers
Specifying a filtration strainer that minimizes pressure drop and therefore achieves optimum media flow depends on a firm understanding of all the variables.
The specification stage is critical when commissioning a strainer solution. The experience, knowledge and correct procedures in making the correct choice from the wide range of options will determine if the operator achieves the objective or will have to endure the consequences.
Pressure drop and the limitation of simply maximizing open area
When looking at pressure drops for strainer and filtration equipment, it is often mistaken that an increased open area for the strainer screen will lead to better pressure drop performance. This is a misconception that will later lead to higher pressure drop results than those earlier calculated by the manufacturer.
The actuality is that the key to lower pressure drops is to lower the velocity of the service itself and to minimize flow disruption, and while an increased open area may provide certain advantages such as longer run cycles and additional time between cleaning and maintenance, it is often not a major contributing factor to improved pressure drop performance.
Reaching an optimum solution therefore requires an assessment of both the influence and interaction of the following broad categories of service conditions and fluid properties, level of filtration and particle retention, strainer sizing and strainer type.
Understanding resistance co-efficient and the importance of CFD analysis
Where an optimum pressure drop is required then determining a mix of specification variables that will minimize flow disruption will depend on confirmation of the resistance co-efficient. This takes into consideration the volume of the vessel, the design of the screen and its seating arrangement, as well as the perforation size/mesh sizing of the actual screen itself.
The resistance co-efficient is then multiplied against flow conditions. This predicts the pressure drop more accurately against older methods of utilising pressure drop curves on ambient water services.
The analysis is then supported with CFD analysis (Computational Fluid Dynamics).
CFD analysis uses a model of the strainer itself and an active fluid simulation that is rendered to show how the fluid will interact with the strainer and filtration equipment. This provides feedback on the performance of the design.
Most strainer manufacturers should be fully aware of the requirements for CFD modelling and simulation; and where a service within the scope of supply is particularly critical, the specification process should be flagged as requiring CFD simulation to support any pressure drop results initially estimated by the supplier. This is especially relevant where there are discrepancies between vendors on their pressure drop figures.
In even more critical examples where schematics have been provided, a supplier should be able to present a simulation incorporating on-site piping layouts. This will ensure that any bends or additional pipework equipment will not greatly impact downstream equipment.
Correct specification procedures are an important part of the strainer supplier offering
The potential supplier should therefore have the experience to comprehend the required specification procedure from the perspective of specific operational requirements.
While strainers and filters are often considered one of the last items for procurement, it is important to understand their critical nature and the factors that contribute to their performance. Often, they play a vital role in protecting critical high-cost impact equipment such as pumps and compressors.
A failure of a strainer or poor pressure drop performance can be costly for the operator. Downtime due to a failure or increased service frequencies or compromised flow performance will create exponential long-term cost implications for the operation.
