Filter

But we appreciate that for people who are new to the business, filtration can be a complicated topic, and it can be frustrating to find simple answers to your questions about duplex strainers. To help simplify matters for both our customers and for people in the field who are looking for more information, we've written this quick guide to what you need to know about duplex strainers, starting with the most basic question: what is a duplex strainer?

Duplex strainers are filters – A filter is any device which removes impurities from a substance that passes through it. One type of filter is a “strainer,” which physically blocks unwanted substances from moving through a pipeline system, and one type of strainer is a duplex strainer, which can be used to remove contaminants or to control the size of particles.

Operation is straightforward – Although there are many types of duplex strainer, the general principle is straightforward. There are two baskets, and each performs the function of an ordinary simplex strainer: removing impurities, particles, or debris from the substance passing through a pipeline system. Valves connect the baskets to each other and to the pipeline system, and operating these valves controls the flow to one or both of the baskets.

Maintenance isn't difficult – In most applications, both baskets will be open for normal operation, but the pipeline system can function adequately with only one. While one basket is closed, the other can be cleaned, repaired, or maintained. This may be done manually, but many duplex strainers may even perform these functions automatically.

Duplex strainers operate continuously – The key distinction between duplex strainers and other types of strainer is that cleaning duplex strainers does not require the pipeline system to be stopped. They are essential for continuous flow processes because the process is not interrupted by maintenance of the duplex strainer.

Duplex strainers have many uses including a wide range of applications, mostly industrial. As an integrated part of a pipeline system. They can be used with water, oil, fuel, or other liquids. They are used in the processing of chemicals, food, fuel, pharmaceuticals, minerals and mining, paper, water, minerals, sewage and waste management, and a variety of other areas.

Duplex strainers are diverse – Given the tremendous number of industries that rely on strainers, it's no surprise that there are as many different ones as there are applications. Some are primarily intended to remove unwanted or dangerous particles from a substance that will reach consumers directly, while others simply remove particles that are hazardous to the operation of the pipeline system itself. Even though they may function in the same way, they may have very different appearances based on their materials, conditions, and safe usage limits.

Duplex strainers are part of our lives – Throughout the average day, you may encounter many things that rely on them. A glass of juice with breakfast? That juice was probably squeezed through a duplex strainer. Reading the morning newspaper? The pulp for the paper and the ink for the presses went through one. Taking a shower? Wastewater passes through one in your local sewage system. Drive to work? The fuel you use probably passed through many on its journey from crude oil to your car. They are everywhere!

Sediment filters work through a process called mechanical filtration. Mechanical filtration physically blocks unwanted particulate matter from infiltrating your water supply. You can think of mechanical filters as a screen door. You want the refreshing breeze to flow through your home, but you do not want bugs or leaves blowing in with it. In this example, the screen door acts as a mechanical filter. Sediment filters allow water to flow into your home while capturing the dirt and sand the water is carrying. Sediment filters are the net that catches the particulate matter traveling in your water.

Some sediment filters use expansive surface areas to catch large amounts of debris. Other sediment filters use a depth gradient to filter out suspended particles. These force water through thick walls of filter media that become increasingly tight as the water nears the core, filtering out smaller and smaller particulate matter along the way.

Where are sediment filters used?
Sediment filters are used across a wide array of applications. Restaurants and coffee shops use sediment prefilters to ensure the quality of their food and beverages. Whole house filtration systems employ sediment filtration to eliminate particulate matter from entering your faucets and showers and to protect the lifespan of other filters. Your pool filter cartridges are a form of sediment filter, blocking dirt and clay from muddying your pristine swimming water. Any instance in which clean water is imperative, you will likely find some form of sediment filter.

What does a sediment filter remove?
Sediment filters remove visible particulate matter, and any particles of dirt, sand, dust, and debris that can be caught by its micron-rated capacity. In addition to particulates, they can also remove cloudiness, also known as turbidity, from water. This causes water to turn yellow, orange, or brown.

Sediment filters do not remove chemicals, heavy metals, bacteria, or dissolved particulate matter. Likewise, they do not improve the taste or smell of water. They are primarily a defensive and preservative filtration method. Sediment filters are most effective when serving as prefilters for other filtration systems. This is why sediment filtration very often works in conjunction with other filtration methods, like reverse osmosis or ultraviolet purification.

Basket strainers are important pieces of equipment used in a wide array of filter applications. It is a closed pressure vessel with a cleanable screen element that filters particulate out of the process liquid to protect valves, pumps, and machines downstream. These strainers are designed to be installed horizontally and to provide quick access to the strainer basket for cleaning. The two most common styles of basket strainers are simplex (single basket) and duplex (dual basket).

Simplex Strainers
With the simplex design, baskets can be removed for cleaning only when the process is shut down and thus are used for "batch" applications or processes which can be interrupted for basket cleaning. Usually, shut-down time is minimal due to the quick-opening basket chamber cover closures.

Once the flow has stopped and the strainer chamber isolated, a vent valve is used to relieve the chamber pressure and to facilitate quicker draining with the drain valve or plug at the chamber bottom.

Once the basket has been cleaned of particulate, it is placed back in the strainer and the application process may continue. Its common practice to use a spare basket to quickly replace the dirty basket and then the dirty basket can be cleaned and set aside for the next cleaning cycle.

Simplex strainers are available in both cast and custom fabricated designs to match the pipeline pressure class and material.

Duplex Style Strainers
The duplex strainer design is for continuous duty applications which cannot tolerate an interruption in flow for basket cleaning; i.e. the system flow cannot be interrupted.

A duplex strainer is essentially two simplex strainers connected with a three-way valve used to divert flow from the dirty basket chamber to the clean basket chamber.

The style of valve used to divert the flow differs with cast and custom fabricated designs. Cast designs require only a lever to divert the flow because there's one three-way valve being operated. The flow remains continuous because as one side of the valve closes, the other side opens.

The custom fabricated designs often use four butterfly valves; sometimes they can be linked to a single operator, however more often they need to be individually opened and closed, thus the order they are manipulated becomes important. You begin by opening the inlet valve to the clean chamber, followed by opening the outlet valve for the clean chamber, then closing the outlet valve for the dirty chamber and finally closing off the inlet valve to the dirty chamber.

Once the dirty chamber is isolated, the access and change-out procedure is the same as it is for a simplex strainer.
The choice of a cast or custom fabricated design is typically dictated by the design criteria for the application; castings are suggested wherever applicable to reduce cost and lead time.

Sizing Guidelines
There are four general steps to take when sizing for a strainer:
1) Ensure that the pipeline flow velocity and clean differential pressure falls within the standard design range of the strainer.
2) The strainer should be compatible with the process fluid and satisfy both design pressure and temperature requirements.
3) Consider the quantity, type, and nature of debris to be removed.
4) Choose the proper screen perforation/mesh for your application, if you choose something unnecessarily restrictive it will increase the clean differential pressure and thus require more frequent cleaning.

Cv Value
Basket strainers typically have an open area ratio (OAR) of 6:1 and sometimes as high as 8:1. This is the ratio of net free area of the screen to pipe area. The area is calculated with a clean basket and as the basket begins to clog, the ratio will drop. As material collects on the basket surface, the available amount of open area is reduced thereby resulting in reduced flow through the strainer and/or exhibiting a higher differential pressure.

Most strainer elements are perforated and for finer retentions a second layer of wire mesh cloth is welded to the basket interior.

The finer the retention, the less open area the straining element has because there is a certain amount of structural material required for a sturdy strainer basket design. Strainer baskets are generally designed to withstand up to 20 PSI differential pressure, however a properly sized strainer will begin with <=2 PSI differential pressure and be cleaned once the differential pressure increases by 5 PSI because at this point nearly all the basket surface area is covered and the differential pressure will begin to increase exponentially. The "trick" is to allow sufficient time to divert flow before the differential pressure increases sufficiently to damage the strainer basket.

In summary, it is important to know if an application is batch or continuous before deciding between a simplex and duplex strainer design.