Venturi Injector Selection Guide for Fertigation
Calculate the required suction capacity, pressure differential, injection time and connection size before choosing a Venturi injector for an agricultural, greenhouse or landscape irrigation system.
How Does a Venturi Injector Work?
A Venturi injector is a differential-pressure device used to draw fertilizer, acid or another compatible liquid into a pressurized irrigation water stream.
Water enters the injector and accelerates as it passes through the restricted internal throat. The increase in velocity produces a lower-pressure condition at the suction port. When the pressure is sufficiently low, fertilizer solution is drawn through the suction tube and mixed with the irrigation water.
The injector contains no motor-driven metering mechanism. Its suction rate is controlled mainly by the injector model, motive-water flow, inlet pressure, outlet pressure, installation arrangement and suction-side resistance.
Buyers can view available sizes on the Trex irrigation Venturi injector product page or review related components in the irrigation fittings collection .
Two injectors with the same inlet size can have different motive-water requirements and suction capacities. Selection must use the performance data for the actual injector model.
Main Parts of a Venturi Injection Assembly
Injector Body
The body contains the converging inlet, restricted throat, mixing area and outlet connection that create the hydraulic conditions required for suction.
Suction Port
The suction port connects the injector to the fertilizer tank through tubing, a valve, flow-control equipment and suitable non-return protection.
Bypass Valves
In a bypass installation, valves create and adjust the pressure difference that drives water through the injector branch.
Pressure Gauges
Gauges installed at appropriate points help the operator compare inlet and outlet pressure and diagnose a loss of suction.
Suction Flow Control
A compatible valve, rotameter or calibrated control device can be used to adjust and monitor fertilizer solution entering the injector.
Backflow Protection
Check valves, anti-siphon devices and other required protection help prevent irrigation water from entering the fertilizer tank or chemical solution from reaching the water source.
What Information Is Needed to Select a Venturi Injector?
Collect hydraulic and fertilizer-application information before selecting an injector. Missing pressure or flow data is one of the most common causes of incorrect Venturi sizing.
Irrigation Flow Rate
Record the mainline or active-zone flow in litres per hour, cubic metres per hour or gallons per minute.
Mainline flow is important for calculating the final fertilizer concentration, but it is not automatically the same as the flow passing through a bypass injector.
Inlet Pressure
Measure the pressure available at the injector inlet while the irrigation system is operating at its normal design flow.
A static pressure measurement taken when water is not flowing is not sufficient for final selection.
Outlet Pressure
Determine the pressure at the injector outlet or bypass return point. The difference between inlet and outlet pressure drives the injector.
Required Suction Rate
Calculate how many litres or gallons of stock solution must be injected during the available fertigation period.
Fertilizer Properties
Identify the fertilizer or chemical, concentration, temperature, viscosity, density and compatibility with the injector, seals, tubing and valves.
Pipe Connection
Confirm connection diameter and standard, including BSP, NPT, metric threaded or flanged connections where applicable.
- Main irrigation flow rate
- Injector inlet pressure
- Injector outlet pressure
- Required fertilizer solution volume
- Available injection time
- Required suction rate
- Fertilizer or chemical name and concentration
- Pipe size and connection standard
- Bypass or direct-line installation
- Required quantity and destination country
How to Calculate Venturi Injector Requirements
The calculations below establish the required operating point. The result must then be compared with the performance table for the actual injector model.
1. Required Suction Rate
If 120 litres of fertilizer solution must be injected over 60 minutes:
Select an injector that can provide the target suction rate under the actual inlet-pressure and outlet-pressure conditions.
2. Pressure Differential
If inlet pressure is 3.0 bar and outlet pressure is 2.0 bar:
The injector must still leave enough downstream pressure for filters, valves, pipes and irrigation emitters.
3. Differential Percentage
Using an inlet pressure of 3.0 bar and outlet pressure of 2.0 bar:
This percentage is useful for comparing the operating point with model-specific performance data.
Calculate the Solution Injection Ratio
For a suction rate of 120 L/h and an irrigation flow of 20,000 L/h:
This value describes the stock-solution flow relative to irrigation flow. It does not, by itself, determine the final nutrient concentration because the concentration of the fertilizer stock solution must also be known.
Calculate the Required Injection Time
If the tank contains 150 litres and the measured suction rate is 100 L/h:
Allow time for the fertilizer to move through the irrigation network and complete any required post-injection flushing.
Do not assume that every injector will operate correctly at a fixed 20%, 30% or 50% pressure difference. Required pressure differential and suction capacity vary by model and operating condition. Always use the corresponding performance table.
How to Select the Correct Venturi Injector
Define the Fertilizer Volume
Determine how much prepared stock solution must be injected during one irrigation cycle.
Define the Injection Time
Establish the available fertigation period while allowing enough time for irrigation before injection and system flushing afterward.
Calculate the Target Suction Rate
Divide the stock-solution volume by the desired injection time and keep the units consistent.
Measure Operating Pressure
Measure actual inlet and outlet pressure while the irrigation zone is operating—not only static system pressure.
Check the Performance Table
Find a model whose motive-water flow and suction capacity match the required operating point.
Verify Downstream Pressure
Confirm that sufficient pressure remains after the injector for the irrigation system to operate uniformly.
Select the injector by its operating point—not by the largest advertised suction figure. Maximum suction values may be measured under pressure conditions that differ from your irrigation system.
Venturi Injector Selection Checklist
| Selection Factor | What to Confirm | Why It Matters |
|---|---|---|
| Suction capacity | Required litres per hour or gallons per hour | Determines whether the fertilizer tank can be emptied within the available injection time |
| Inlet pressure | Dynamic pressure at the injector inlet | Affects motive-water flow and the ability to generate suction |
| Outlet pressure | Pressure at the injector outlet or bypass return | Determines the pressure differential across the injector |
| Motive-water flow | Water flow passing through the injector | Must be available without disrupting the required irrigation-zone flow |
| Connection size | Nominal diameter and thread standard | Ensures mechanical compatibility with the bypass or mainline |
| Material compatibility | Injector body, tubing, valves and seals | Reduces the risk of chemical attack, leakage or premature component failure |
| Downstream pressure | Minimum pressure required by the irrigation zone | Prevents poor emitter performance and uneven irrigation after pressure is consumed by the injector |
Bypass and Direct-Line Installation
Venturi injectors can be installed in a bypass branch or directly in the main water line. The correct arrangement depends on system flow, available pressure, injector size and the amount of pressure loss the irrigation network can tolerate.
Bypass Installation
In a bypass arrangement, only part of the irrigation water passes through the injector. A restriction or control valve creates a pressure difference between the bypass inlet and return points.
This layout is commonly considered when the mainline flow is greater than the injector’s required motive-water flow.
Direct-Line Installation
In a direct-line arrangement, all water passes through the injector. The injector must therefore be sized for the complete system flow and the irrigation network must be able to tolerate its pressure loss.
For installation and operating details, continue to Fertigation with a Venturi Injector: Setup and Ratios .
Recommended Supporting Components
Pressure Gauges
Use suitable irrigation pressure gauges to measure operating pressure and evaluate the differential across the injector.
Irrigation Flowmeter
An irrigation flowmeter helps confirm system flow and monitor changes while the fertigation bypass is operating.
Irrigation Filter
Review the irrigation filtration guide to reduce contamination entering small irrigation passages and downstream emitters.
UPVC Fittings
Confirm the working pressure and chemical compatibility of UPVC pressure pipes and fittings used in the bypass assembly.
Threaded Connections
Verify BSP, NPT or other connection requirements before selecting UPVC threaded pipe fittings .
Flow-Control Valves
Use compatible valves to isolate the assembly and adjust pressure and water flow without exceeding component ratings.
Fertilizer and chemical injection systems should include the backflow, anti-siphon and non-return protection required by the water source, system design and applicable local regulations. Do not connect a chemical tank to a water supply without appropriate protection.
Venturi Injector Troubleshooting and Common Mistakes
When a Venturi injector does not draw fertilizer, begin by checking pressure, flow direction, suction restrictions and the condition of the injector throat.
No Suction
Possible causes include insufficient pressure differential, incorrect flow direction, closed valves, a blocked suction tube or inadequate motive-water flow.
Low Suction Rate
Check actual inlet and outlet pressure, suction-tube length, tube diameter, fertilizer viscosity, valve restriction and injector deposits.
Unstable Injection
Air leakage, changing irrigation flow, a nearly empty tank or fluctuating pressure can cause the suction rate to vary.
Water Enters the Tank
Reverse flow can occur if pressure develops at the suction port. Shut down the system and inspect check valves, anti-siphon equipment and operating sequence.
Injector Clogging
Undissolved fertilizer, debris or precipitation can block the suction passage. Prepare compatible solutions and keep the suction assembly clean.
Insufficient Zone Pressure
Excessive pressure consumed by the injector can reduce downstream emitter performance. Recheck the hydraulic design or consider a different installation arrangement.
Common Venturi Selection Mistakes
- Selecting only by pipe diameter
- Using static pressure instead of operating pressure
- Ignoring injector outlet pressure
- Using maximum advertised suction as the design rate
- Confusing mainline flow with injector bypass flow
- Ignoring fertilizer concentration and viscosity
- Using long or undersized suction tubing
- Installing without pressure gauges
- Failing to provide backflow protection
- Not allowing time for post-injection flushing
Venturi Injector Selection FAQs
Common questions about pressure differential, suction capacity, injector sizing and fertigation installation.
How do I choose the correct Venturi injector size?
Calculate the required suction rate and measure the injector inlet and outlet pressure under normal operating conditions. Then select a model whose performance table provides the required suction capacity at that operating point. Connection diameter should not be the only selection factor.
How much pressure differential does a Venturi injector need?
There is no universal pressure-differential requirement for every Venturi injector. The required differential varies by model, motive-water flow, outlet pressure and target suction rate. Use the performance data supplied for the actual injector.
How is the required fertilizer suction rate calculated?
Divide the volume of prepared stock solution by the available injection time. For example, injecting 120 litres over one hour requires an average suction rate of 120 litres per hour.
Can a Venturi injector work without electricity?
The Venturi injector itself does not require an electric motor because it uses hydraulic pressure differential to create suction. However, pumps, controllers, solenoid valves or monitoring equipment in the complete system may require electricity.
Why is my Venturi injector not drawing fertilizer?
Common causes include insufficient pressure differential, low motive-water flow, incorrect installation direction, closed valves, an air leak, a blocked suction tube or a clogged injector passage.
Is a bypass installation better than direct-line installation?
A bypass installation is often useful when the irrigation mainline flow is greater than the injector’s required motive-water flow. A direct-line installation may be suitable when the injector can handle the complete system flow and the network can tolerate its pressure loss.
Does a Venturi injector need pressure gauges?
Pressure gauges are strongly recommended because they help verify the operating pressure differential and diagnose low or unstable suction. Gauge range and materials should match the system.
What information is needed for a Venturi injector quotation?
Provide the irrigation flow, inlet and outlet pressure, required suction rate, stock-solution volume, injection time, fertilizer type, pipe connection, installation layout, quantity and destination country.
Need Help Selecting a Venturi Injector?
Send Trex Irrigation your mainline flow, injector inlet pressure, outlet pressure, required suction rate, fertilizer type, pipe connection and order quantity. Our team can help compare suitable injector and bypass configurations.