System pressure for indoor irrigation applications is often either overthought or undervalued. When designing irrigation systems knowing the available flow and pressure is critical. One of the most common grower responses we get when asked these questions is, “I have 80 PSI. We have tons of water.” Although this may be true, and pressure directly impacts flow rates, understanding how these two are different is extremely important.
Flowrate is the actual volume of water available from your source. This could be from an onsite well, city or district supplied, or other sources. Most commonly referred to in gallons per minute (GPM). In many indoor growing applications, source water is treated and stored or stored directly into holding tanks.
Pressure is the force or strength used to move water through pipes or other paths created by a change in altitude or pumping and is primarily shown in pounds per square in (PSI). In the case where the source water is stored, the system designer determines the flow and pressure available to the irrigation system.
One of the significant reasons for non-uniformity in flow rates is pressure differences. For example, a 20% change in pressure will result in a 10% flow variation. With many indoor irrigation systems turning to drip emitters as the standard, the required system pressure must be precise. But can there be too much pressure regulation? Most definitely.
Here is a common scenario. Stored RO water is pumped using a variable frequency drive with an on-demand pressure pump set to a constant pressure, 40 PSI.
The water goes through a filter, flow restrictor, hydraulic injection, and pipes out to the zone. Once at the zone or room, “whip kits” connect the piping to the benches or racks. These whip kits often consist of a manual control valve, electric solenoid valve, check valve, screen or disc filter, inline pressure regulator, and quick-connect fittings for hose connections to the table or racking system. From there, we have a manifold with poly tubing and pressure compensating emitters. What’s the issue here? The redundant pressure regulation.
Most pressure compensating emitters have an operating range between 14 – 58 PSI. This means that they flow a consistent volume of water per emitter within that pressure range, with fluctuations only coming from manufacturing coefficients and any potential plugging issues. Within a properly designed irrigation system, the need for another pressure regulator at the “whip kit” is un-needed.
The inline regulator does not allow for proper flushing and can be a point at which the distribution uniformity of the system is affected. The emitter itself is the regulating device and is paired with a VFD-controlled pumping system, and no additional regulation is needed.
Knowing the relationship between flow and pressure is critical for proper irrigation system design. Often overlooked and overdesigned.