Using Pressure Regulators for High Irrigation Uniformity
Designing a quality irrigation system for your nursery is all about 1) choosing the right emitter type, and 2) controlling the pressures within your system so that all emitters get the right water pressure. If an emitter is supplied with water pressure within its specification range it will deliver the right flow rate and the right pattern. If all emitters experience nearly the same pressure, they will all output the same amount of water and your irrigation, and your crop, will be uniform.
Getting the right pressure to each emitter is much easier said than done. Forcing large amounts of water through a pipe causes pressure losses (“line losses”) which result in higher pressure at the beginning of the run and lower pressure the end. If the pipe is a submain supplying laterals or a lateral supplying spray stakes or sprinklers, plants at the beginning of the run will get more water than those at the end. Pressure losses through elbows, tees and other fittings can cause additional problems. Finally, hills and other elevation changes in your nursery directly affect pressure – emitters at low elevations will experience higher pressures and emitters at the tops of hills will have lower pressures.
Choosing the right diameters for your mainlines, submains and laterals to manage pressure losses is an involved topic which I touch on in my article How to Design a Spot-Spitter System. This current article is on how to select pressure regulators and where to locate them in your system. Proper use of pressure regulators can give you the irrigation uniformity your plants need and, in some cases, can even save you money by allowing you to downsize some of your piping.
What is a Pressure Regulator?
A pressure regulator is an automatic valve that continually adjusts its opening to keep water pressure immediately downstream at a fixed, preset value regardless of upstream pressure (within a range). Pressure at the regulator's input may vary, but the pressure at its output stays constant and at the level you specify.
Due to line losses, elevation changes, pump wear and a host of other factors, you have pressure variations throughout your system. The pressure regulator lets you choose points within your system where the variations are removed and pressure is known and constant. Choosing these points correctly results in consistent, high irrigation performance.
Types of Pressure Regulators
There are three types of pressure regulators that are commonly used in agriculture: diaphragm valves, spring-type pressure regulators and pressure compensating emission devices. Each has their place in irrigation which I summarize below.
Diaphragm Valve: A diaphragm valve is a metal or plastic valve in which water flows through a passageway that is made up of a flexible rubber diaphragm on one side. In a pressure regulating configuration, water pressure upstream of the valve is fed back to the diaphragm through a small pilot valve. This causes the valve opening size to be reduced when pressure downstream of the valve is higher than the preset pressure, and enlarged when it is lower. The result is constant downstream pressure. The “preset pressure” is set by adjusting the pilot valve. The basic mechanism of a diaphragm valve can be fairly expensive, so they are generally more practical in larger sizes. Typical agricultural diaphragm valves range in diameter from 2” to 12”. The pressure setpoint on diaphragm valves is adjustable, which is both an advantage and a disadvantage. The adjustable setpoint allows you to tune downstream pressure to just the level you need. However it’s an adjustment that can be set wrong by field labor, and can drift out of spec with time and use.
- Spring-type Pressure Regulator:A spring-type pressure regulator operates on a similar concept to the diaphragm valve but uses a spring and plunger to vary its opening size instead of a flexible diaphragm. The operating mechanism of a spring-type pressure regulator is simple and can be economical even for small diameters. For this reason, they are popular for diameters ranging from ¾” to 1.25”. Two-inch diameter is a crossover size at which a diaphragm valve or a spring pressure regulator can be practical. The Senninger Pressure Regulator line includes the medium flow PMR series, available in ¾” and 1” diameters, which regulate pressure for flow rates ranging from 2-20 gpm. The Senninger PR series is available in a 1.25” diameter and can regulate pressures for flow rates ranging from 10-32 gpm. Finally, the 2” Senninger PRU series regulates pressures of flows ranging from 20-100 gpm.
- Pressure Compensating Emission Devices:A pressure compensating (“PC”) emission device is drip emitter, sprayer or micro sprinkler with a built-in pressure regulating device that keeps the flow rate of the emitter constant over a range of line pressures. Even when PC emission devices are used, pressure regulators may be required at points within the system to avoid damaging the emitters or tubing, but a PC emission device is like having a miniature pressure regulator at every plant. The cost of a PC emitter is much lower than a conventional pressure regulator, but the overall cost of PC emitters is high since there are a very large number of emitters in an irrigation system. Bringing pressure control right to the plant yields the highest uniformity, but usually at the highest cost. The Spot-Spitter PC Assembly, which makes each Spot-Spitter in a nursery irrigation system pressure compensate, is a great example of a PC emitter.
Where to Locate Pressure Regulators
Since your ultimate goal is to have the right pressure at each emitter, locating pressure regulators as close as possible to the emitters yields better performance. However, this can be at higher cost because it may require a larger number of regulators. For example, consider a nursery container system made up of 16 zones, each zone made up of four laterals and each lateral supplying fifty containers with one 15 GPH spray stake in each. The flow rate required by each lateral is 12.5 GPM and each zone is 50 GPM. Let’s say the water supply capacity is 200 GPM at 60 psi so you can run four zones simultaneously. This will all work if you use ¾” poly laterals, 2” PVC supply manifolds for each zone, and a 4” line connecting the pump to all of the zones.
Your choices are as follows:
- Install one 4” pressure regulator at the pump output
- Install one 2” pressure regulator at the input to each zone (16 regulators total)
- Install one ¾” or 1” regulator at the input to each lateral
- Install one PC Assembly at each container
We’ll explore each of these options below.
Pressure Regulator at the Pump (Option 1):
Option 1 only requires one valve, but a 4” pressure regulating diaphragm valve can cost over $1,000 for a plastic body and upwards of $2,000 for metal. Therefore, the fact that only one valve is required doesn’t necessarily make it the most cost-effective approach. However, depending on the output pressure of the pump a 4” diaphragm valve may still be required to protect downstream components. The maximum input pressure for most spring-type pressure regulators is usually 100 psi or less, and many valves and fittings are rated at 150 psi. Therefore, if the pump outputs 200 psi, pressure reduction at the pump output is required.
Can only one pressure regulator at the output of the pump give you good pressure control throughout your system? The answer is that it depends. There is no way to compensate for pressure variations due to line losses and elevation changes if they are downstream of the regulator. If your system is on relatively flat ground, your pump is located relatively close to all of the irrigation zones (within 50 feet, let’s say) and your pipes have appropriately large diameters, pressure variations downstream of the regulator will be small and performance should be good with only one regulator. On the other hand, if there are long distances between the pump and the plants it supplies, there is the potential for large pressure losses that will negatively affect irrigation uniformity. If this is the case, it’s better to locate your pressure regulation closer to the emitters.
Pressure Regulator at the Zone (Option 2):
In this example there are 16 zones so this approach requires 16 two-inch pressure regulators. The cost of a 2” diaphragm valve ranges from $225 to about $300 depending on construction. The cost of a 2” spring-type pressure regulator is $125 on GrowIrrigation.com (Senninger PRU series). The advantage of the diaphragm valve is that it can also act as an on/off control valve for the zone with little additional cost. If the PRU is used, a 2” control valve must also be purchased for each zone. Depending on which approach you choose, the total cost for pressure regulation at the 16 zones ranges from about $2,000 to about $5,000 assuming an additional 4” regulator is not required at the pump output.
Pressure Regulator at the Lateral (Option 3):
This example system has 64 laterals, so installing a regulator at each row requires 64 pressure regulators. Each lateral requires 12.5 GPM, which is in range of the Senninger PMR Series spring-type pressure regulator. On GrowIrrigation.com a ¾” Senninger PMR regulator is $10.50 in quantity 25+ and a 1” PMR is $12.10. Therefore, the total cost of pressure regulators at the laterals ranges from $672 to $775, with the much more important benefit that pressure control is right at the start of the lateral, resulting in very good uniformity if the lateral is designed well.
As mentioned in the previous section, even if pressure regulators are installed at the laterals a diaphragm-style pressure reducing valve will still be required at the pump output if pump pressure can exceed 100 psi. However even if this is the case, in this example cost/benefit weighs heavily in favor of installing a Senninger PMR regulator at the start of each lateral.
Pressure Regulator at the Emitter - PC Emitter (Option 4):
This example system has 64 laterals with 50 spray stakes each, or a total of 3,200 spray stakes. The cost of a 36” Primerus PC Assembly on GrowIrrigation.com is $40.00 per bundle of 25 or $1.60 each. Placing one at each spray stake is a total cost of $5,120 (which also includes the cost of 36” of spaghetti tubing at each container). The PC Assembly has the additional benefit that it is a non-leak (“CNL”) device, which means all spray stakes turn on at the same time when the zone energizes and they all turn off at the same time when it de-energizes. This is an important benefit in high frequency irrigation of high value crops. If designed well this is the highest-performing solution with near-perfect uniformity, but it is cost prohibitive in many cases. It should be noted that the maximum input pressure to the PC Assembly is 60 psi, and most poly laterals are rated at 60 psi or less, so this solution will additionally require a diaphragm-style pressure reducing valve at the pump output.
Summary of This Example
In this example, installing a spring-type pressure regulator at the input to each lateral is a very cost-effective way to achieve high uniformity. Even if a pressure reducing valve is required at the pump output, the additional cost of the PMR regulators offers a great insurance policy against pressure variations downstream of the pump. However, let’s be clear that this is the result for this particular example. The right solution for you will depend on all of the specifics of your nursery and you will need to go through the same analysis we performed here. Feel free to call GrowIrrigation for help.
Just a few words about overhead sprinklers such as the Senninger Wobbler. Uniformity is as important for overhead sprinklers as it is in microirrigation and pressure losses occur through mainlines, supply laterals and risers. The low cost of spring-type pressure regulators makes them ideal for overhead sprinklers. A pressure regulator installed between the top of each riser and the base of each sprinkler ensures all sprinklers output at the same rate without much added cost.
Where to Go for Help
Placement of pressure regulators is one component of an irrigation system design that has many other considerations. Find additional resources on the GrowIrrigation Blog. You can also contact me directly at JMcDonald@GrowIrrigation.com or (442)279-3152.