Page 141 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

SOLID-LIQUID SEPARATION AND INTERCEPTORS

SOLID-LIQUID SEPARATION AND INTERCEPTORS 3.9

Fluid streams may contain iron or steel particles that are small enough to pass through the
finest screens. If this is a problem, a strong magnet capable of lifting several times its own
weight should be suspended in the basket. The magnet should be installed so that all the water
passes over it. This magnet should be encased in an inert material to prevent corrosion.
Baskets, especially when full, are not capable of withstanding the same pressure as the
body. A particular phenomenon called runaway buildup is possible, in which the dirt builds
up and plugs the mesh or perforations, thereby reducing the free area. The pressure in the
strainer increases slowly at first, but faster and faster over time. The water velocity and
pressure inside the basket escalate quickly, which causes the resultant flow to stop or be
reduced to a trickle. This full-line pressure can burst the basket.

Covers
A cover is provided in order to remove and clean the basket. The most common type is
bolted; the bolts must be loosened and removed to provide access to the basket. This type
of cover is the strongest and should be used for high-pressure applications. However,
its removal is time consuming. Another type is the clamping yoke, in which threaded,
tee-shaped handles are used to secure the cover to the body. Often, the cover is attached
to the body with a hinge mechanism, making it very easy to remove. This type of cover
is more expensive than the bolted type.
Another type of strainer is the automatic type, which does not require manual cleaning.
A rotating, circular screen is used as the basket. The water inlet is directed to the inside
of the basket. A rotating backwash inlet inside the basket uses the differential pressure
between the atmosphere and line pressure to produce a localized reverse flow across only a
portion of the basket, thereby allowing continuous cleaning. This type of strainer is appro-
priate for large consumers of water such as raw water inlets from rivers and lakes used for
cooling and process. Automatic strainers are available to 60-in (150-mm) size.

DESIGN CONSIDERATIONS AND SELECTION
CRITERIA

When selecting a strainer, the four main considerations are its physical size, friction loss
through the unit, price, and ease of cleaning. The viscosity and specific gravity of the fluid,
and the degree of perforation of the basket (or size mesh) all influence the pressure drop. In
many cases, the size of the unit is a consideration if it is to fit into an existing space. Generally
accepted practice limits the pressure drop to a maximum of 2 psi (13.8 kPa). Another general
rule is to have a minimum 4:1 open area ratio of the perforations in the basket. In order to
reduce the friction loss, a strainer one or more sizes larger than the pipeline into which it
is installed should be selected. In some cases, the basket may become large and unwieldy
when filled with debris. If this is a possibility, several smaller units in parallel should be
considered. Typical friction loss through a Y-type strainer is given in Fig. 3.3, and for a
basket strainer in Fig. 3.4.
Another consideration is the length of time required between cleanings. Past experience
with the specific application (if available) should give a good idea of the size and amount
of debris expected. Examination of the total suspended solids present in the water analysis
allows calculation of the amount of debris to be expected over a period of time. This, along
with any specific preference, should provide good guidelines for the proposed size of the
unit to be selected.
If continuous operation of the units is critical, duplex strainers should be used. This
allows one to be cleaned while permitting full use of the system. Another method is to
install two strainers in line, the first with larger openings to trap large particles and the

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