By: Chris Pasquali; CEO Factory Direct Pipeline Products, Inc.
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
and isolating the other chamber so that its strainer basket can be removed.
The valves can be standard ball or butterfly type or, for user simplicity, using
a pair of 3-way ball valves.
The Duplex Concept
Although initially it might seem that duplex strainers cost significantly more
than a pair of strainers in piping with valves, if you consider the cost for
the valves, construction of a manifold and the engineering/labor time involved
to achieve “duplexing of simplex strainers”, the engineered duplex strainer
design ends-up being cost effective.
The four basic duplex strainer designs are described by the valve type used to
divert the flow from one chamber to the other.
This represents the latest and most modern incarnation of duplex strainer
design. Made from castings, these are for pipelines up to 4”. The
operation is akin to an ordinary ball valve, so diverting the flow from one
chamber to the other is as simple as manually moving the diverting lever towards
the chamber you want to put into service.
Valve Cartridge Style
Besides the ease of operation, this design provides the most compact footprint
possible. The sealing efficiency is just like ordinary ball valves – you
can expect a completely isolated basket chamber.
It is possible that the valve seats require periodic replacement, especially if
your fluid is particularly abrasive or aggressive to the buna or Viton seals.
This style is used by several duplex strainer manufacturers; it is the standard Titan duplex strainer design and Eaton 53BTX duplex strainer design. The Keckley duplex strainer is also similar.
Prior to the valve cartridge style, the plug diverting style represented the
most compact and modern duplex strainer design. This design is used for 6” and
8” size pipelines because most duplex strainers for smaller pipelines are best
served with the valve cartridge style.
Plug Diverting Style
The plug diverting style also utilizes a casting consisting of two strainer
basket chambers and between them, a diverting valve.
To divert flow from the dirty to the clean chamber, the valve must be lifted off
the valve seat (strainer body) using a jackscrew – just a few turns is all it
takes to ensure the valve can be actuated with the diverting lever. The
valve needs to be re-seated with the screw jack prior to accessing the isolated
One aspect of this design is that it is common for operators to forget to lift
the valve from its seat … it might require a “persuasion bar”, but it is
possible to actuate the valve without operating the screw jack. The
consequence is wear between the valve and seat.
The interface between the valve and seat is metal-to-metal, which might be
advantageous for some applications involving aggressive fluids. Unfortunately,
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 used whenever possible
because it’s a little softer than cast iron, carbon steel or stainless steel
strainer bodies. Bronze and stainless steel strainers with like material
diverting valves have that potential wear issue if the lifting jackscrew is not
The other characteristic of this style is that the isolated chamber is not
“completely isolated”; there is an acceptable leakage rate. The idea is
that accessing and cleaning the strainer basket does not take that long, so some
leakage is okay. The leakage rate increases with wear of the diverting
plug and valve body interface.
This design utilizes specially configured valves and actuators to accomplish
“duplexing of simplex strainers”.
The Eaton duplex strainer model 50 is an example of a diverting plug valve style duplex water strainer. This was formerly known as a Hayward duplex strainer model 50, however they sold this product line to Eaton in 2005. If you call or email our office we can cross reference the different part numbers for spares or replacements.
Butterfly or Ball Valve Diverting
Although it is possible to use ball valves for flow diversion, resilient seated
butterfly valves are compact and are less costly. This style of duplex strainer
is only available with cast iron bodies and therefore most applications involve
water, so complex ball valves or double/triple offset butterfly valves are not
This design does not incorporate a unifying, single diverting device; it
requires manipulation of all four valves in a specific order to divert flow
It is possible to utilize special three-way trunnion ball valves for 2” – 12”
pipelines and link them together to a single operator. After satisfying
your design requirements, we will often suggest ways you might enhance your
system with suggestions like this.
This design provides yet another way of simultaneously diverting flow between
strainer basket chambers. An advantage of the sliding valve design is that
the valve is completely out of the flow path.
Sliding Valve Design
The isolation valves are mounted to screws and slide along rails cast into the
strainer body when the handwheel is rotated. There are two valves and
handwheels which are connected by a chain drive, so operating one handwheel
moves both inlet and outlet valves simultaneously.
The casting used for our design minimizes the pipeline centerline above grade,
which also locates the basket chambers at an ergonomically friendly position.
Furthermore this design uses multiple small baskets (four per chamber) to
further improve ergonomics.
The sliding valve style strainer is used for raw water intake systems and they
are offered in cast iron and cast steel materials for 8” – 24” pipelines.
This is where the secondary design considerations such as the overall footprint,
ergonomics, flow characteristics, particle load, pricing and lead-time all come
We know what were are doing, we have been doing it for a long time. Once
we have the basic design criteria for your application we will select what we
believe to be the best design and provide you with suggestions; if you would
like, we will even quote multiple versions for your comparison.
Chris Pasquali has been trained by Hayward Flow Control and Eaton Filtration,
having provided sales and engineering support since 2001.