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Sanitary Strainers: Perforated or Wedgewire Filter Inserts- Which Should I use?

November 6, 2014
basket-strainer

Sani-Matic Basket Strainer

Since we re-launched this blog, we’ve hit on most of the types of sanitary process equipment we deal with at Holland. One category of equipment we’ve overlooked, however, is sanitary strainers. Today’s post will take a look at sanitary perforated sheet and wedgewire strainers inserts and some of the advantages and disadvantages of each.

To begin, the most common type of sanitary filter insert used in the industry today is a perforated sheet type strainer with a wire mesh overscreen. Perforated inserts will commonly use 14 gauge sheet metal perforated with 60 degree staggered 1/8” or ¼” holes. The 60 degree stagger pattern is preferred because of its superior strength and large open area ratio, increasing filter capacity. Wire mesh linings are often used with the perforated sheets for finer filtration and take advantage of the reinforcement provided by this thick material perforated sheet material. Mesh screens are generally available in both stainless and cloth bag versions. The perforated type of insert is economical.

A common alternative to perforate filter inserts with mesh over screens are wedgewire inserts. Wedgewire inserts have a unique set of advantages that make them ideal for the high purity processing industry. A wedgewire insert is made by welding specially shaped wire to support rods, thereby creating a continuous slot. The wedged shaped profile of the wire allows for fine filtration. With hole sizes as small as 0.005”, filtration as fine as 100 mesh is possible with a wedgewire insert.

The two primary types of wedgewire insert construction are reverse formed and inverted wrap construction. Reversed formed wedgewire inserts feature external support rods which provide a smooth, unobstructed screen surface on the inside of the strainer. The alternative to reverse formed inserts are inverted wrap inserts. Utilizing internal support rods, inverted wrap elements are used as an alternative when the desired basket diameter is too small. The durable, welded construction, allows for a wedgewire insert to outperform and outlast perforated or woven mesh inserts in high pressure applications.

Wedgewire inserts have a few other advantages perforated inserts other than ease of use. The continuous hole spacing means that total percent open area is much higher in a wedgewire insert than with an equivalent woven mesh screen. This increases filter capacity and reduces pressure drop. This higher percentage of open surface area is also going to mean that these inserts are much more cleanable than traditional perforated tubes with mesh inserts. This is huge in a sanitary application where we need to validate or prove that we’re cleaning something.

One other very important advantage wedgewire inserts have over mesh type screens is increased product safety. Mesh screens can fatigue over time.  This can result in individual wire pieces of the stainless steel screen breaking off and migrating downstream into the product.  Using wedgewire inserts eliminates this risk.

Overall, we try to steer our customers towards wedgewire inserts. While they are priced at a premium to perforated inserts, the overall performance of wedgewire decreases total cost of ownership and makes these our “go to” in challenging filtration applications. In processes where we need to minimize pressure drop by maximizing percent open area and assure that we are going to be able to clean the system, the wedgewire is our go to. That’s not to say perforated and wire mesh inserts don’t have their place. For simple applications, such as pump protection, an inline perforated strainer will work just fine. If you have any additional questions about which insert type is right for you, contact a Holland Sales Engineer today.

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