Comparison of Fabricated and Cast Strainers

Which do you need - a cast or fabricated basket strainer design?

Pipeline strainers are often considered a commodity product because the primary design criteria are considered “universal”:

PRIMARY DESIGN CRITERIA
1. material of construction
2. connection type
3. connection size
4. style simplex, duplex or automatic
5. element retention

 

This article explains the differences of cast and custom fabricated strainer designs, complementing the following articles:

1. How to Compare and Select Cast Simplex Strainers, Avoid the pitfalls related to relying on only size and cost

2. How to Select a Pipeline Strainer, Three design criteria for proper strainer selection

3. Effects of Fluid Velocity and Differential Pressure on Pipeline Strainers, Considerations for proper strainer sizing
   

 

Simply put, most customers use fabricated strainers to maximize filtration efficiency for their specific application. Sometimes the liquid characteristics or engineering specifications necessitate using a custom fabricated design, but you can benefit from using a custom fabricated strainer design even when a cast design would work.

 

Open Area Ratio
Cast strainer designs have a fixed open area ratio (OAR); this is the ratio of the open area of the filter element to the open area of the inlet nozzle. As this ratio increases the frequency of element cleaning decreases. The OAR is mostly dictated by the strainer design, with Y and T styles having low ratios (1:1 or 2:1) and basket strainers of 5:1 or better OARs.

Although it is not a guarantee, most fabricated strainers have a higher OAR than cast strainers and that is because cast strainers are designed to minimize cost. It is possible to cast strainers with thinner wall thicknesses and overall smaller dimensions compared to their fabricated versions which are physically larger due to the required welding procedures.

OAR Trade-Off
You might enjoy an initial cost savings and quicker delivery purchasing a cast strainer at the expense of more frequent basket cleanings. Labor hours related to strainer maintenance are often overlooked during the procurement process, but it should not because pipeline strainers last decades and those labor hours add-up over the years. Its not only labor hours, personnel exposure to the process liquid and spillage contribute to overhead costs also.

Applications Requiring Fabricated Strainers
Some applications require features which cannot be provided in cast designs, for example allowing for corrosion. Due to the manufacturing technique for cast strainers it is not possible to customize the thickness of the body or nozzles. Such strainers have varying wall thicknesses with the design meant to withstand a certain pressure and temperature combination and cannot be altered from the tooling used at the foundry.

Fabricated strainers can be made from thicker materials, their connections can be customized by size, type and location and they are available in exotic alloys that cast versions are not.

Examples of Customized Fabricated Strainers
One of the most common customizations differentiating fabricated strainers from cast strainers is attaching smaller sized connections to match the pipeline it will be installed into; this is done to improve the OAR.

Liquid can be transferred at high velocities and therefore smaller pipeline sizes are used to reduce cost and space but this can be problematic for a pipeline strainer. Once the inlet velocity exceeds about 10 FPS there is an increased likelihood that the internal strainer basket is damaged. Strainer baskets are typically sealed against the strainer body through the downwards force the cover applies to the arched spring-like basket handle. At high velocities the baskets begin to spin, damaging both the basket and strainer body.

In such situations the strainer body can be oversized, so perhaps you are essentially installing a 6" strainer into a 4" pipeline. A 6" fabricated strainer can have 4" connections, doubling the OAR from 7:1 to 14:1 and reducing the velocity by almost 50%. If a cast 6" strainer was used, a separate transition spool would need to be fabricated for the inlet and outlet, increasing the overall cost and number of potential leakage points.

Mesh lined baskets reduce the OAR of a given size strainer significantly unless offset by using a larger size basket commensurate with a larger sized
strainer body.

Code Stamps, NDT and NDE
Only fabricated strainers can be provided with an ASME code stamp due to the specific manufacturing requirements related to code stamping. Applications requiring magnetic particle testing, ultrasonic weld/wall thickness tests and non-standard welding procedures (or even a requirement of minimum welder qualifications) are indications that a fabricated strainer is required. These types of requirements are often industry specific with the oil and gas industry being notorious for ensuring stringent manufacturing and quality control procedures are used throughout the manufacturing process.

Alternate Nozzle Locations
The basic pipeline strainer design has inlet and outlet nozzles 180° apart at the same elevation, often referred to as "inline". This is the most common nozzle orientation, however there are applications which benefit from nozzles having the inlet and outlet on the same side (referred to as "loop" configuration) and even 45° or 90° offset. Most of the time these special nozzle orientations are driven by limited space as might be characteristic of portable systems located on trucks and ships.

Advantages of Cast Strainers
If your flow rate (velocity) and OAR can be satisfied with a cast strainer then you benefit from lower costs and quicker lead times. Cast strainers are mass produced and require less labor; the labor that is used is often low-cost labor because most cast strainers originate from foundries in China and India. These countries do not have the environmental and labor protection laws that manufacturers in the USA are subject to, so it's just not possible to cast strainers in the USA at a competitive price.

As previously mentioned, the amount of material used for a cast product is often less due to sophisticated tooling used by the foundry, a big factor contributing to lower cost. So even after you account for the ocean freight cost to North America, cast strainers almost always cost less than a locally fabricated strainer.

Yoke Style Access Covers

Most cast strainers have quick-opening strainer basket chamber covers which require only a few turns of a T-Bolt to access the strainer basket. Fabricated designs typically have a fully bolted cover design with an option for a hinged cover and swing-bolts which add significantly to the cost of the strainer due to the extra labor.

Quick Delivery
The mass production nature of cast strainers results in physical inventory and lead times reduced to whatever is required to fabricate the strainer basket (typically within 1 week). Fabricated strainers must go through a rigorous process of drawing generation/approval, acquiring raw materials, cutting/bending and welding procedures in addition to various non-destructive testing (NDT), resulting in at least 4 to 6 weeks for shipment after receipt of an order. If there are supply chain interruptions or a backlog of orders that lead time can easily extend to 12+ weeks.

 

Made In the USA
There are increasing requirements for government agencies and their vendors to purchase products of which 55% of their value originates from the USA. It is difficult to supply a cast strainer for such applications because the strainer body itself is often cast overseas. One way around this (prior to the January 2021 update), was to re-source the chamber cover hardware (bolts/nuts), fabricating the basket, re-testing (hydrostatic test) the body and re-painting the strainer so it can be considered to have enough USA value to satisfy the Buy American Act requirements. These added parts and procedures increased the cost and lead time, reducing the biggest advantages of cast strainers. Now that they Buy American Act has been updatcASTed, 95% of the iron and steel content must be of US origin to satisfy the Buy American Act. An exception can be made if the foreign cast strainer is 20-30% less expensive than the USA version.

Let us leverage our strainer sizing and selection experience from the previous 20 years to assist you with your next strainer application. We approach your project from the perspective of solving a problem or satisfying specific goals; explaining and quoting multiple options to ensure you are making an informed decision. Our website at https://fdpp.com has many articles, PDF files and automated sizing tools enabling comparison of several brands of strainers; our assistance is only a phone call or email away and has no cost or obligation to you.