Page 140 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

SOLID-LIQUID SEPARATION AND INTERCEPTORS

3.8 CHAPTER THREE

A bronze body is preferred for brackish, saline, and seawater service. It is also often
used for potable water services in smaller sizes. Its cost is about double that of cast iron.
Carbon steel bodies are used where moderately high temperature and pressure conditions
are encountered, and where resistance to high thermal and mechanical shock is required.
Carbon steel components are the materials of choice where fire hazards exist, such as in the
petroleum and petrochemical industries. Their cost is equal to bronze. For high pressures
and temperatures over 1000°F, chromemoly steel is usually specified for bodies.
Stainless steel is the preferred body, basket, and screen material for the pharmaceutical,
food-processing, and chemical industries because of its resistance to corrosion and con-
tamination and ease of cleaning. Stainless steel costs about 4 times as much as cast iron.
Plastic strainer bodies are available in all of the materials used for pipelines. Baskets and
screens of all metallic materials are also available.

Basket and Screen Construction and Materials

The actual collection and retaining of the debris, dirt, and other particles in all strainers is
done by the basket or screen that is placed inside the body of the strainer. The size of the
openings through a screen is referred to as mesh, and the size of the openings through a
basket is referred to as perforations.
The term mesh describes a screen that uses a woven wire cloth manufactured from the
material chosen for the intended service. The most common material for all applications is
stainless steel. Mesh screens are generally available in standard sizes from 20 to 200 mesh,
with a variable wire size used depending on the mesh. The mesh size does not indicate the
particle size retention, since the size of the mesh opening is determined by the diameter and
number of wires per inch. For example, a 100 mesh means 100 vertical and 100 horizontal
strands of wire per inch. For critical applications, a screen should be selected on particle
retention capability, not mesh size.
Standard perforated screens are manufactured from a light or heavy gauge sheet metal
available in standard sizes generally ranging in diameter from 1/32 to 1/4 in. The baskets
should be assembled by either welding or brazing. The strongest are of welded construc-
tion. Solder is much weaker and more easily broken. If a basket strainer is required for fine
straining, it is common practice to add a wire mesh liner inside the perforated bucket since
the screen alone is too weak to provide the necessary mechanical strength.
The material to be used depends both on the intended water service and the body that
the strainer is installed into. The most commonly used basket materials are brass and
stainless steel. Cast iron strainer bodies commonly use baskets of brass and stainless
steel, depending on the intended service. Bronze bodies usually require Monel metal
baskets because of the severe service required. Stainless steel baskets are used with
stainless steel bodies.
The baskets are made from sheet metal with a wide variety of diameter perforations.
A Y strainer is generally furnished with 1/16-in perforations in sizes up to 4 in, 1/8-in
perforations in larger sizes for liquid service, and 3/64-in perforations for steam service.
If finer straining is required, a wire mesh screen fitted inside a basket is used. In this case,
generally accepted practice limits the perforations in the basket to 50 percent of the wall
area in order not to lose strength. In addition, this combination usually provides the best
ratio of maximum flow rate with adequate strength. The mesh and basket should be an
integral unit, with the mesh fastened to the basket both at the top and the bottom to prevent
any debris from bypassing the unit.
By generally accepted practice, the open area perforation ratio should be about 4:1 to
avoid excessive pressure drop through the unit. A smaller ratio will require frequent cleaning.
Additional strainer basket area can be obtained by using a pleated basket. If finer filtration is
required after the strainer has been in service, a mesh liner can be added inside the basket. If
the size particle to be removed is known, the perforations should be slightly smaller.

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