Point Source on- line Emitters
On-line, single outlet, point- source emitters, (once available in only a few non-pressure compensating types) are now produced by many manufacturers, and are available in a wide variety of designs and flow rates. The most common type installed in commercial installations are “pressure compensating” (PC) emitters, which are specified by the majority of designers mainly due to the fact that the prescribed flow rate will remain relatively constant regardless of changes in dynamic pressure within the system. These hydraulic conditions may be present on drip systems with long runs or significant changes in elevation. PC emitters are also often selected due to their self-cleaning capability.
In most situations PC emitters are the best choice, but not always. PC emitters design usually incorporates a diaphragm which fluctuates in response to incoming pressure, thereby providing more uniform flow than a non PC emitter. Because of this design, PC emitters usually have a minimum operating pressure requirement of 8-12 psi. If the inlet pressure never reaches the minimum level, the emitter may enter the “flush mode” where the diaphragm is opened to allow increased flow. Non-PC emitters generally have a lower minimum pressure requirement.
So in certain situations, non-PC emitters may actually be the better choice. These time-tested, economical, on-line emitters are usually produced in a .5, 1, 2, or 4 gallon per hour pre-set models. Most utilize a “turbulent flow” design, (no diaphragm) where the water is controlled and accelerated by forcing it through a labyrinth of small passageways. They can be utilized on relatively level systems with little elevation change (20 feet or less) and still achieve excellent uniformity. Although these durable emitters lack some of the added features of PC emitters (like built in check valve), they are more economical and may hold up better over time, especially when used on systems supplied with non-potable, or re-claimed water.
Non PC emitters of one form or another are also a good choice for gravity flow systems which frequently operate at pressures less than 10 psi.