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12.44 CHAPTER TWELVE

the smallest amount of open area and are highly prone to plugging. One common system,
which works well, consists of a lateral pipe with drilled orifices pointed down toward the
bottom of the vessel with a layer of coarse plastic mesh protector wrapped over the lat-
eral and a layer of plastic or stainless screen of the desired size covering the mesh pro-
tector. Another common system is similar to the first except that the drilled orifices are
covered with a continuous wedge wire sleeve. These two lateral systems are among the
most widely used and are considered by some to be superior to strainer designs. Strain-
ers are reasonably good collectors, but they have too much open area to be inherently
good distributors. They do not create enough pressure drop to create equal distribution at
the low flows used during regeneration. A second problem is that to obtain reasonable
open area in the strainer itself, it is generally higher than 1 l& to 2 in. high. The extra
height creates an area of poor distribution toward the bottom of the strainer. Various man-
ufacturers have tried to overcome this problem by the addition of check valves within
the strainers, putting shrouds over the strainers, or using a large number of very small
strainers, with some success.
Large systems with large amounts of freeboard generally benefit from a separate chem-
ical distributor. There are two advantages. The first one is that the chemical distributor
can be designed for good distribution at the regenerant flow rate. The second advantage
is that by locating the chemical distributor close to the resin bed, the volume of liquid in
the freeboard area does not have to be displaced during the regeneration and rinse steps.
This reduces the rinse requirement. Some systems, such as mixed beds and certain types
of countercurrently regenerated exchangers, have liquid collector distributors that are
buried within the resin bed. These distributor/collectors generally are designed in a sim-
ilar fashion to the underdrain. There is significant hydraulic force against these distribu-
tors during the service cycle and potentially during the regeneration cycle, and if they are
not very firmly braced, they are prone to breakage.

Piping Design
Smaller systems, and in particular the single home type softeners, generally have multi-
port valving systems. Multiport valves are low in cost and are fairly reliable up to about
2 in. in size. They are widely available. Their main disadvantage is that they have a fixed
cycle. Although several options are available in some designs, they are generally consid-
ered not suitable for most countercurrently regenerated systems. Larger systems almost
always employ a valve nest and a piping tree to deliver and collect the various flows.
Many piping system materials are available; however, PVC is most commonly used
for smaller systems, and either plastic-lined steel or stainless steel is most commonly used
for larger systems. The most important feature of the valves used is that they be capable
of tight shutoff over many cycles. The diaphragm-type valve has proved itself to be a very
good choice for this purpose, albeit it has higher pressure drop and costs more than other
types of valves. Two other valve-type choices are commonly used. Ball valves and, in
larger systems, butterfly valves are frequently used.

Chemical Storage and Dilution
Some small systems simply employ open-top tanks to mix and store the chemical solu-
tion at the required concentration prior to regeneration. Larger systems generally use some
type of bulk chemical storage and dilution stations. In these systems, the concentrated
chemical is pumped usually to a mixing T and blended with dilution water prior to en-
tering the exchange tank. The smaller systems with dilution tanks generally require that