Flat-Bottom Basket Strainer for constrained layouts

Flat-Bottom Basket Strainer

Basket strainer is the preferred filtration option to handle high volumes of fluid or to provide large filtration area. Typically, a fabricated basket strainer’s body is constructed using a pipe and a dish (bottom cap). Depending on the diameter of the pipe, the cap constitutes to 25-30% of the total length of the lower body. The lower body is from the center flange to the bottom of the dish cap. In layouts where space is an issue, we can use the same sized basket strainer however the dish is replaced with a plate during fabrication. Such basket strainers are called a Flat-Bottom basket strainer.

They take 20% less vertical space when compared to regular dish bottom basket strainers. The drain, which is usually located at the bottom of the dish in a regular basket strainer, is located at the side for a Flat-Bottom basket strainer. This drain location further reduces the vertical space required to install a Flat-Bottom basket strainer. Pressure drop in a Flat-Bottom basket strainer is the same as a dish bottom type. The filtration area (open area percentage) is also not affected.

So if you are having vertical space issues in your layout and prefer only a basket strainer, try our fully customized Flat-Bottom basket strainer

What factors affect pressure drop across a strainer in clean condition?

DP Curve

One of the major factors considered during selection of a strainer is the pressure drop (DP) caused in the pipeline. Pressure drop is inevitable as the strainer obstructs the smooth flow of the fluid. Turbulence is caused as the fluid hits the filtration area of the strainer. This turbulence causes the pressure to decrease at the outlet of the strainer when compared to the inlet. This decrease in pressure is termed as “Pressure Drop” (PD) or “Drop in Pressure” (DP).

So what are the main factors that affect pressure drop in a strainer at 0% clogged condition? In general, two factors, the flow rate of the fluid and the size of the strainer itself. Theoretically, it is simpler to reduce the pressure drop across a strainer by lowering the flow rate of the fluid. Lower flow rate means slower output of the fluid. In most cases this is not ideal for operating team as this may impact the production or other process functions.

Increasing the strainer size can also lower pressure drop across a strainer. Let’s assume a DN150 (6”) line currently has a DP of 0.6 bar caused due to a 6” basket strainer. This DP can be lowered to 0.2 bar-0.3 bar by changing the strainer size to 8”. A bigger strainer means lesser turbulence thereby decreasing the pressure drop across a strainer. Other factors such as space restriction and use of reducer connections need to be considered before upgrading the strainer size.

One of the common myth is DP can be reduced by increasing the hole size of the mesh or the filter. Increasing the mesh hole size has a bigger impact on the Open Area and little or no significant impact on the DP. So the next time you are faced with a high DP across a strainer in clean or 0% clogged condition, consider reducing the flow rate or increasing the size of the strainer.

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Wedge Wire Screen for Seawater Filtration

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Often a lot of time and importance is given to the selection of the strainer type. It is obvious that the selection of strainer type has significant commercial impact. At times, in tricky seawater intake lines, regular round hole perforation with woven mesh will not work. Well the filtration will happen, however the maintenance team will have a nightmare with an increased length of maintenance duration. What is the difference? Well, a lot is to do with the fibrous nature of particles in seawater.

When seawater is filtered, there are very high chances of fibrous weeds or other particles getting tangled in the round holed perforation. The fibrous particles are stubborn in nature and needs heavy scrubbing by the maintenance team. This scrubbing will eventually rupture the screen. The perforation and mesh are not designed for such heavy scrubbing. In such cases, the best option is to use a solid wedge wire screen. The Wedge wire screen is constructed using solid wires. They are also reinforced and have a very high burst pressure. This robust construction can withstand heavy scrubbing by the maintenance team. The construction is such that there are no circular holes. The holes are between two solid wires and runs across the length of the screen. This style of construction makes it very hard for the fibrous particles to get entangled. Hence for a seawater application or any application with fibrous particles, the best option is to use a solid wedge wire screen.

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Why is mesh lined outside for temporary strainer in gas application?

Temporary Strainers

Any piping engineer will tell you that for a temporary strainer fitted in a gas application, the mesh needs to be lined on the outer side of the perforation and the flow of the gas must be from out to in (not the usual in to out). Have you ever wondered why the flow must be from out to in and the mesh on the outer side of the perforation? It is all got to do with cleaning of the temporary strainer during maintenance.

We all know that in a screen with mesh and perforation, filtration happens at the mesh. The perforation is only there to provide mechanical support to the woven mesh. As a thumb rule, the first point of contact for the fluid must be the mesh. In a gas application, the mesh is lined on the outer side of the perforation and the cone is positioned such that the flow hits the pointed side and flows from out to in. As a result all the unwanted particles, will get settled on the outer side of the cone. During maintenance, the temporary strainer is removed from the pipeline and using an air gun, a blast of air sprayed from in to out. As a result, all the unwanted particles that were trapped on the outer side of the cone are blown away. Now let’s assume that the mesh is lined on the inside and the flow is from in to out. In this case if the air gun is used to blow the air, the unwanted particles will keep getting stuck within the strainer and will not have anywhere to go.

Thus, in order for effective cleaning during maintenance, in a gas pipeline, a temporary strainer must have the mesh on the outer side of the perforation and the flow of the gas must be from out to in.

Fixing Open-Area glitch in Tee Strainers

Tee Strainer

Tee Type strainers (T-strainers) are notorious for their low open area as compared to a Basket strainer or a Y-strainer.   The reason for this low open area is the screen construction style in a tee strainer. The screen is similar to a cylinder cut in to half along its length. In a basket strainer, the screen is more like a complete cylinder. Due to this screen structure, the open area or the filtration area in Tee strainer is significantly lower than a Basket or Y strainer.

So what do we do if the design team opts for a tee strainer over a basket strainer due to budget constraints and the process team still calls for an open area that is similar to a basket strainer?  I suggest two paths. Option one is to add an extension pipe between the neck of the equal tee and the cover flange. The length of the extension pipe can be increased or decreased based on the open area requirement. By adding and increasing the length of the extension pipe, we can increase the length of the screen that sits inside the equal tee. This increase in length of the screen will increase the open area or filtration area of the tee. Personally, I am not a fan of this approach, as this option will alter the outer dimension of the strainer housing. This can adversely affect the structural layout of the pipes especially in space-restricted services in an offshore platform or an FPSO.

The second option is to change the structure of the screen. Instead of having a simple half cylinder, we can construct the screen with pleated or zigzag screens. The number of pleats can be varied to increase the open area. This option does not alter the outer dimension of the strainer housing. Fabricating a pleated design screen needs a lot of expertise, however I still prefer this option. So there you go, open area for a tee strainer can be increased using any of the above two options.