Which Duplex Strainer Design is the "Best"?

Comparing the Four Main Duplex Strainer Designs

One of the most common strainers used in piping systems are duplex strainers. They are designed for applications that cannot tolerate downtime due to strainer basket clogging. Automatic strainers are another option, but due to their complexity they are typically only practical for 8” and larger pipelines.

 

The Duplex Concept

The simplest duplex configuration consists of a pair of strainers in your piping, connected with a valve manifold to direct flow through the desired chamber while isolating the other chamber so its strainer basket can be removed. The valves can be standard ball or butterfly type, or for simplicity, a pair of 3-way ball valves.

 

If you are using four valves (one for each strainer’s inlet and outlet), the operator must operate each valve in a specific order to ensure uninterrupted flow. Using a pair of three-way valves (one on the inlet manifold and one on the outlet manifold) requires manipulating both valves simultaneously. This is where an engineered duplex design becomes advantageous — the strainer bodies and valve actuators are designed to simplify operation.

 

Although it might seem that duplex strainers cost significantly more than a pair of strainers with valves, when you factor in the cost of valves, manifold construction, engineering, and labor, an engineered duplex strainer often ends up being more cost-effective.

 

The four basic duplex strainer designs are categorized by the type of valve used to divert flow from one chamber to the other.

 

1. Valve Cartridge Style

This is the latest and most modern incarnation of duplex strainer design. Made from castings, these are typically used on pipelines up to 4”. Operation is similar to an ordinary ball valve — diverting flow from one chamber to the other is as simple as moving the diverting lever toward the chamber you want to put into service.

 

Besides ease of operation, this design provides the most compact footprint possible. Sealing efficiency is like ordinary ball valves — you can expect a completely isolated basket chamber.

 

This style is used by several manufacturers. It is the standard design for Titan duplex strainers and the Eaton 53BTX. The Keckley duplex strainer is also similar in concept.

 

The valve seats may require periodic replacement, especially if your fluid is abrasive or aggressive to buna or Viton seals.

 

2. Plug Diverting Style

Prior to the valve cartridge style, the plug diverting style represented the most compact and modern duplex strainer design. This design is commonly used for 6” and 8” pipelines (most smaller pipelines are better served by the valve cartridge style).

 

The plug diverting style uses a casting with two strainer basket chambers and a diverting valve between them. To divert flow from the dirty to the clean chamber, the valve must be lifted off the valve seat using a jackscrew — just a few turns are needed to allow the diverting lever to move. The valve must be re-seated before accessing the isolated chamber.

 

One characteristic of this design is that it is possible for operators to forget to lift the valve from its seat. While it may require a persuasion bar, it is possible to actuate the valve without using the jackscrew, which causes wear between the valve and seat.

 

The interface between the valve and seat is metal-to-metal. The only way to replace the valve seat is to replace the entire strainer body. To reduce seat wear, a cast bronze diverting plug is often used because it is slightly softer than cast iron, carbon steel, or stainless steel bodies.

 

Another characteristic is that the isolated chamber is not completely isolated — there is an acceptable leakage rate. The idea is that accessing and cleaning the basket does not take long, so some leakage is acceptable. Leakage increases as the diverting plug and body interface wears.

 

The Eaton duplex strainer Model 50 is an example of this style (formerly the Hayward Model 50 before Eaton acquired that product line in 2005).

 

3. Butterfly or Ball Valve Diverting

This design uses specially configured valves and actuators to accomplish “duplexing of simplex strainers.”

 

Cast versions using this approach are for pipelines 10”–18” and utilize resilient seated butterfly valves to divert flow. A single geared actuator is connected to each of the four valves. The actuator and linkage ensure flow is diverted between chambers without interruption.

 

Fabricated versions are used for higher-pressure applications and when cast iron bodies are not acceptable (2” to 48” pipelines). These typically use four gear-actuated resilient seated butterfly valves, although double/triple offset butterfly valves or ball valves can also be used. For special valve requirements, we can supply the strainer bodies and flanged manifolds and let you procure the valves you prefer.

 

This design does not use a single unifying diverting device — it requires manipulation of all four valves in a specific order. It is possible to use special three-way trunnion ball valves (2”–12”) linked together to a single operator.

 

4. Sliding Valve Design

This design provides another method of simultaneously diverting flow between strainer basket chambers. An advantage is that the valve is completely out of the flow path.

 

The isolation valves are mounted to screws and slide along rails cast into the strainer body when the handwheel is rotated. Two valves and handwheels are connected by a chain drive, so operating one hand wheel moves both inlet and outlet valves simultaneously.

 

The casting used minimizes the pipeline centerline above grade, which also positions the basket chambers at an ergonomically friendly height. This design often uses multiple smaller baskets (four per chamber) to further improve ergonomics.

 

The sliding valve style is commonly used for raw water intake systems and is offered in cast iron and cast steel for 8”–24” pipelines.

 

Which Duplex Strainer Design is Best?

Often the design criteria of your application will dictate a specific design. For example, if you require a 4” Inconel duplex strainer, only the custom fabricated style will work. Likewise, a 2” 316SS application will usually be more economical with a cast valve cartridge design than a custom fabricated one.

 

However, for an 8” water application where cast iron is acceptable, you may have diverting-plug, butterfly valve, and sliding valve designs to choose from. This is where secondary considerations such as overall footprint, ergonomics, flow characteristics, particle load, pricing, and lead time come into play.

 

We have extensive experience with all duplex strainer designs. Once we understand your basic design criteria, we will recommend what we believe is the best design for your application and provide suggestions. If you prefer, we can also quote multiple versions for your comparison.

 

Ready for Help Selecting the Right Duplex Strainer?

Choosing the right duplex strainer design depends on your pipeline size, material requirements, flow characteristics, and maintenance preferences. We can help you evaluate the options and recommend the most suitable and cost-effective solution.

 

Submit Duplex Strainer Inquiry Call 908.362.9981 Now