Design of Subsurface Drip Irrigation Systems in Humid Areas

The design flow rate calculated for crop water needs must be matched to the manufacturer specified drip line flow rates at the recommended pressure.. This requirement normally leads to “

Design of Subsurface Drip Irrigation Systems in Humid Areas

Garry L Grabow, Assistant Professor

Department of Biological and Agricultural Engineering

North Carolina State University

Campus Box 7625

Raleigh, NC 27695-7625

Garry_Grabow@ncsu.edu

Kerry Harrison, Senior Public Service Associate

Department of Biological & Agricultural Engineering CES

Earl Vories, Agricultural Engineer

USDA-ARS-Cropping Systems and Water Quality Research UnitDelta Center

147 State Hwy T, Box 160

Portageville, MO 63873

VoriesE@Missouri.edu

Heping Zhu, Agricultural Engineer

USDA-ARS-Application Technology Research Unit

1680 Madison Ave

Wooster, OH 44691

Zhu.16@osu.edu

Ahmad Khalilian, Professor

Department of Agricultural and Biological Engineering

TABLE OF CONTENTS

Table of Contents ii

Introduction 1

Design Criteria 2

Water Requirements 2

Soils 4

Management and Operation Considerations 6

Water Quality 8

Water Sources For Subsurface Drip irrigation 12

Surface Water 12

Ground Water 13

Alternative Water Supplies 13

Pumps and Power Sources for Subsurface Drip Irrigation 14

Filtration requirements for Subsurface Drip Irrigation 15

Media Filters 16

Screen Filters 17

Disk Filters 18

Chemical Injection for Subsurface Drip Irrigation 19

Venturi injector 20

Metering Pump 21

General Design Considerations for Chemical Injection Systems 22

Valves for SDI systems 24

Control Valves 25

Vacuum/Air Relief Valves 26

Backflow Prevention 27

Pressure regulating valves 27

Main and Submain Design 28

Slope 31

Lateral Length 31

Field Shape 31

Mainline / Submain Flushing 32

Dripline design 33

Dripline Depth 35

Dripline Spacing 35

Dripline length 35

Dripline Flushing Manifold Design 37

Pipe Size 38

Field Shape 39

Instrumentation and controls 39

Irrigation control 39

Backflushing 43

Chemigation 43

Summary 44

References 44

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1

Subsurface drip irrigation (SDI) is similar to surface drip irrigation, but has drip lines that are buried beneath the surface Although many surface drip systems have lines buried up to a few inches deep, SDI is normally defined as a system that is “permanent”, that is, the drip lines are not taken up every year

Before the design of an SDI system is done, it must be determined that the intended site is suitable for SDI These considerations include factors important to all irrigation systems, but some factors are particularly important to the success of an SDI system These factors include adequate water supply, acceptable water quality, and appropriate topography Another

consideration is management which is important to all drip irrigation systems, and especially important to SDI systems in which drip lines are out of sight

This publication is one in a series of publications that deal particularly with SDI in humid areas These areas, such as the southeastern United States, have particular climate, topography, soils, cropping systems, and water sources that require special consideration when considering and implementing an SDI system The other publications in this series deal with site selection, installation, management, and chemical treatment of water related to SDI systems with particularconsideration given to humid areas

The design of an SDI system is similar to the design of other drip irrigation systems, with additional consideration giving to system flushing and traffic In humid regions, topography and field layout will normally demand extra attention Proper design of a SDI system will ensure uniformity of water application, as well as reduced operation and maintenance cost

DESIGN CRITERIA

The successful design of a subsurface drip irrigation (SDI) system requires that pertinent information be collected and incorporated into the design This information is oftentimes referred to as “design criteria” For an SDI system, these criteria will include information on climate, crops, soils, water quality, and system management and operational considerations

Water Requirements

The SDI system must be sized to deliver the required amount of water to the crop at the time

it needs it The water requirement should be thought of as both a flow rate and a total amount of water The SDI system must be designed to deliver the required flow rate, and the water supply must be adequate to deliver the amount of water required over the growing season The amount

of water required will depend on many things including, climate, crop, and soils

Climate affects the water requirements of an irrigation system by dictating, along with the growth stage of the crop, how much water a crop will need at any time of the growing season Inhumid areas, the water requirement is normally reduced over more arid regions This is due to higher humidity, which reduces the vapor pressure gradient, or driving force, for

evapotranspiration, the sum of the water that passes through a plant (transpiration) and the water that directly evaporates from the soil surface Other factors such as temperature, sunshine and wind influence evapotranspiration With an SDI system, evaporation from irrigation is reduced

to a negligible amount in most cases, since the soil surface is not normally wetted

The crop to be irrigated with an SDI system will also influence its design Different crops use different amounts of water, and also may be grown during different times of the year with different environmental demands The SDI system may be intended to irrigate more than one crop (rotation) in which case the crop with the highest water demand should be satisfied

Small Vegetable Water Use Piedmont

0 0.05 0.1 0.15 0.2 0.25 0.3

March 15 Aug 1

Figure 1 Crop Water Use for Small Vegetables in

the Piedmont Region of North Carolina Note that the peak rate for early planting is about 0.25 in/day

while that for late plating is about 0.18 in/day

The most important aspect of crop water use for SDI design is the “peak” water requirement

or the amount of water that a crop uses during its highest water use period This is because it is during this period that the SDI system must deliver the greatest amount of water The peak use rate that is used to design an irrigation system is normally derived from an average use rate of thepeak use month (NRCS, 1970) Since drip systems are designed to apply small amounts of water frequently, the average peak use rate for the peak week should be used While rain may

be factored in to reduce the irrigation requirement for a season, it should not be factored in when calculating a peak use rate This is because even in humid regions, the probability of receiving appreciable rain in a few-day period with high dependability (80% of the time), is low In addition, moisture in soil storage is normally not considered when designing for peak demand Peak use rates may be calculated using a number of methods (Allen et al., 1997; USDA-SCS, 993) or be obtained through a

local University extension office For

most humid areas, a typical peak use

design rate is 25 inches (6 mm) per

day See Figure 1 for an example of

crop water use rates This depth is

converted to a flow rate gpm (l/m),

which for 25 inches per day is 4.7

gpm per acre irrigated This flow rate

is normally increased by considering

system application efficiency and by factoring in operating times less than 24 hours per day If designing a system to pulse (irrigate multiple times per day), the off-cycle times need to be

included when estimating system daily operating time System inefficiency is derived from multiple sources; manufacturer variation in emitters, pressure variation within the system, and system operation and management The design target for emission uniformity along a lateral is normally 90% This is controlled in the design process by limiting drip line length Considering the other sources of inefficiency, the application efficiency for SDI design purposes can normally

be set at 85% Assuming an application efficiency of 85% and an operating time of 12 hours per day, the system flow rate to satisfy a crop water requirement of 0.25 inches per day is 11 gpm peracre irrigated The total system flow rate is obtained by simply multiplying the per acre flow rate

by the total number of acres to be irrigated by the system Flow rates larger than those arrived at

in the design process may be used, but larger pumps and mainlines will be required, resulting in greater costs

The design flow rate calculated for crop water needs must be matched to the manufacturer specified drip line flow rates at the recommended pressure This requirement normally leads to

“zoning” a field, since the flow rate required to satisfy crop water requirements will likely be lower than the SDI system flow rate if operating the whole field at once (or conversely the application rate of the system exceeds that required for crop needs) For certain situations, such

as small fields, it may be preferable to size the pump and mainline to deliver to the whole field simultaneously If media filters are used in smaller fields, back flushing flow requirements may exceed normal operation flow rates and may dictate pump capacity

such as infiltration rate and hydraulic conductivity These hydraulic characteristics, in turn, play

a part in system performance

Drip lines will need to be more closely spaced in a sandy soil since the lateral spread of water from the drip lines will be less pronounced than in a finer texture soil Of course, crop rotation and cultivation will also dictate drip line spacing, especially for row crops when spacing will usually be a multiple of the row spacing

Slow emitter emission rates may be required on heavy textured soils, such as clay, so that the emission rate does not exceed the hydraulic conductivity of the soil When this happens

“chimneying” may occur in recently installed systems as water takes the path of least resistance

to the surface via void spaces This can also occur in older systems, in which repeated irrigation cycles tend to wash out the fines above the drip line

Although soils in humid areas often lead to a restriction in the depth of the root zone, drip lines will need to be installed deep enough to avoid damage from tillage equipment In general, drip line depths should be shallower in coarser textured soils and deeper in finer textured soils.While design considerations can address many soil-related issues, operation and

management will also insure optimal performance of the SDI system in various soils

Management and Operation Considerations

An essential step in the design of a subsurface drip irrigation (SDI) system is to consider how the use of the area will vary It is not enough to design only for next year’s crop A well-designed system should be in operation for at least ten to fifteen years, so some attempt must be made to plan for the future Important questions that should be asked include:

• Will the same crop be grown each year or will there be a rotation of multiple crops? As previously mentioned, the water requirements and thus the system capacity will vary depending on the crop Other factors such as traffic pattern and types of equipment could also be affected

• Will the entire field be planted to one crop or will it be divided into smaller areas of different crops? The field will most likely be divided into zones and knowledge of the cropping pattern will allow for the most practical layout of the zones, allowing for more efficient management

• Will the different crops in a rotation employ different cropping systems? In humid regions, row crops and field crops are often rotated For example, corn and cotton may both be grown on 30-, 36-, or 38-inch beds without much difference in the production systems However, winter wheat, soybeans or rice will likely have different traffic patterns than the row crops, a fact that should be considered even if they will not use the SDI system for irrigation Another good example is peanuts Even though peanuts may be produced as a row crop, the fact that the fruit is produced underground could affect the placement of the drip tubing In general, row crops will dictate drip line spacing with spacing a multiple of the row spacing If field crops such as winter wheat or soybeans are to be irrigated in

addition to row crops, drip line spacing may be limited to one or two rows, although economic considerations might override efficiency gained from the narrower spacings.

• Is subsoiling a part of the production system? If so, is it necessary because of soil hardpans

or is it done “because everybody else does it”? If it is an essential part of the soil

management plan, then the placement of the drip tubing must be planned around

subsoiling One way to achieve this is to place the drip tubing deep enough to avoid contact with the subsoil plows This is usually not a good alternative since it would mean placing the drip tubing relatively deep in the soil profile and both the placement of the drip tubing and the depth of the subsoil plows would tend to undulate somewhat A better system would probably be to coordinate the horizontal placement of the drip tubing and thesubsoil plows However, for this system to work it is essential to know the location of the drip tubing After several years of normal field operations it is quite easy to lose track of the location of the drip tubing by a few inches, even if the field has been bedded the entire time If the cropping pattern is such that the beds have been periodically destroyed and rebuilt, the location of the beds relative to the drip tubing may have shifted by several inches Just a small error in locating the drip tubing before subsoiling could lead to major repairs of the SDI system

• A final consideration is harder to quantify Is there time and willingness to learn a new system and manage it correctly? With the recent state of the farm economy, many

producers are spread over such a large area that it is all that they can do to keep up with what they already have SDI will most likely be different from anything else the producer

is doing As such, there will be a learning curve associated with it In addition, there will

be system maintenance operations (e.g., acid injection, iron settling, flushing, etc.) not

required or less extensive than for other systems Ignoring the maintenance to save time will most likely lead to a significantly shorter life for the system.

SDI offers opportunities for more efficient use of irrigation water in many situations However, the system must be well planned and maintained to achieve the potential benefits A poorly planned system will not function properly and will require time-consuming repairs

Water Quality

When designing an irrigation system, water quality concerns may include two sources of design criteria, one for the system and one for the crop Water quality criteria for crops normally focus on leaching requirements or application concerns (foliar burning, etc.) In humid regions, where salts do not build up in the root zone, a leaching requirement is not required Also, since SDI systems don’t wet the plant, problems resulting from the contact of irrigation water with the plant are not an issue As a result, water quality criteria for the design of SDI systems in humid areas focuses on irrigation system concerns Emitter clogging is the primary concern with SDI systems as with all drip systems

Groundwater is generally of higher quality than surface water and therefore presents less of

a concern of emitter clogging However, many existing and potential water supply sources for SDI systems in humid areas are from surface water

Water quality will dictate filtration requirements, chemical injection requirements, and management of SDI systems to prevent emitter clogging Causes of emitter clogging in SDI systems may be chemical (precipitates or scale), physical (grit or particulates such as sand and sediment), biological (such as algae or bacteria), and rarely water temperature (affects solubility

of precipitates, or tendency of lime to precipitate) Water temperature can also affect the pressurerating of PVC and polyethylene pipe (ASAE, 1989)

To determine the quality of the water to be used to supply the SDI system, a water sample should be taken Many states have programs that analyze water quality for agricultural use at very reasonable costs Private laboratories may also be used The laboratory provides

instructions on sampling, preserving and transporting water samples The primary constituents that should be analyzed for in water to be used in an SDI system are total suspended solids (TSS), Total dissolved solids (TDS), bicarbonates or hardness, pH, manganese, iron, sulfur and nutrients (Nitrogen and Phosphorus) An example water quality report is shown in Figure 2 Chemical problems can be anticipated by measuring the levels of TDS, bicarbonates or hardness, pH, and iron and manganese High amounts of calcium, magnesium and bicarbonates promote deposition of lime in the SDI system Low pH keeps lime from forming A high pH indicates a higher likelihood of precipitates, such as lime forming Fertigation with certain types

of nitrogen fertilizers can increase pH and enhance the chance of chemical precipitation Lime formation can also be caused by changes in temperature, which changes its solubility Higher water temperatures are more conducive to lime formation than low water temperatures High saltconcentration in water tends to form precipitates in the emitter as it dries between irrigations This hazard is not as great with subsurface drip, since less evaporation occurs from the emitter Also, humid regions do not tend to have high levels of salts in water Other chemical problems occur when manganese and iron precipitate in the form of oxides Well water oftentimes has high levels of manganese and iron, and if the water is subjected to the atmosphere or an aerobic environment, their oxides can form While bacteria indicates a potential biological clogging problem, certain bacteria may also produce iron and manganese oxides also known as iron ochre,which is a combination of the iron oxide precipitate and filamentous algae

Potential physical clogging problems can be diagnosed from TSS levels Suspended solids should be analyzed to determine their composition between inorganic and organic material Biological clogging hazard can be anticipated from bacteria counts and iron levels Bacteriacan form a slime that can clog the system Nitrogen and Phosphorus promote algae growth, so may give an indication of potential biological clogging Some waters contain significant

amounts of nitrogen that can be used to offset fertilizer requirements Plant available nitrogen (PAN) is nitrate and nitrite plus a portion of ammonium and organic nitrogen Table 1 shows the clogging potential of irrigation water in drip irrigation systems

Physical clogging is addressed through filtration systems Surface water supplies normally will have more suspended solids than ground water and will require media filtration and

sometimes a secondary filter Chemical clogging must be addressed in the design process by properly specifying a device for chemical injection Algae and other biological clogging may also be prevented and treated via chemical injection and by painting any above ground PVC pipe

Figure 2 Agronomic Water Quality Report

to reduce light penetration Finally, well-designed flushing manifolds are essential in preventing emitter clogging A good design will also simplify operation and maintenance of filtering and chemical injection equipment Filtration design considerations are discussed in greater detail in asubsequent section.

Table 1 Relative clogging potential of irrigation water for SDI systems (from Pitts, et al., 1990)

Clogging Hazard, based on concentration

WATER SOURCES FOR SUBSURFACE DRIP IRRIGATION

As with any irrigation system, a reliable water supply is critical for SDI Water sources for irrigation include surface water and ground water Both have advantages and disadvantages that may differ with local conditions Water sources should be adequate to supply water during extended dry periods Water for irrigation is needed most when supplies are generally at their lowest level

Depending upon local or state requirements, an agricultural water withdrawal permit may berequired before irrigation system installation

of the formation or aquifer will often limit pumping rates Local topography will determine the feasibility of a pond for irrigation use The required size of an irrigation pond is based on the area to be irrigated For most areas, the general rule of thumb is that one acre-foot of water storage is required for each acre of land to be irrigated This rule assumes no recharge to the pond from ground water or underground springs If the pond is spring fed, storage capacity of the pond can be reduced according to the pond recharge rate

If a pond is not adequate in supplying the needs of an irrigation system, a well may be installed to pump groundwater into the pond to help maintain a suitable pond level The size of the well needed to supplement the pond will depend on the storage capacity of the pond, the

acres irrigated by the irrigation system, and the water demand of the crop to be grown Farm ponds normally require the construction of an embankment to impound water Regulations and permits normally depend on embankment height and will vary by State Wetland regulations may also apply State dam safety officials and a local Army Corps of Engineers office should be contacted concerning regulations and permitting.

Ground Water

The use of ground water for irrigation has increased since the early 1980's In general, ground water is readily available in most areas of the southeastern United States The quantity and quality of ground water is usually better than surface water supplies, although the flow rate

or yield of wells may limit use with larger areas Where ground water is limited, a surface water supply may be a more economical alternative Sometimes groundwater is used to supplement a pond, or in some cases a pond is used primarily as storage for the groundwater This works well when the groundwater yield is limited In this situation, the well may be pumped continually during periods of peak water use

Well drillers are often a good source of information to determine ground water availability and expected well costs As with surface water, permits may be required to use groundwater for irrigation and backflow prevention will be required State officials or well drillers will be able toprovide information on well permitting requirements

Alternative Water Supplies

Most often, the term “alternative” is used to describe either municipal (city or county) water

or wastewater The main factor to be considered with a municipal water supply is cost and availability Costs for municipal water can range into the hundreds of dollars per acre-foot, and flow rates may be limited Local officials should be consulted to determine the cost of water

and the cost of required hook-ups and backflow prevention devices Filtration is still

recommended with municipal water supplies, although the requirements are generally minimal and less expensive than with surface water A single screen filter is normally adequate for municipal water

Wastewater may be a potential water supply, although there are oftentimes restrictions on its use In most cases in humid areas, wastewater is from animal facilities and is treated and stored

in a lagoon Wastewater may also be from industrial operations The source of wastewater will dictate what crops can be grown, and the filtration and chemical injection requirements Most states have strict restrictions on what crops may be irrigated with wastewater Crops that may be irrigated with wastewater are normally limited to those not consumed by humans State health officials should be contacted before considering the use of wastewater for irrigation

Emitter clogging hazard is also higher with wastewater, and therefore filtration requirementsare higher Chemical injection may also be needed to keep precipitates from forming

Consultation with an agronomist is also advisable when investigating the potential use of

wastewater with an SDI system

PUMPS AND POWER SOURCES FOR SUBSURFACE DRIP IRRIGATION

As with most irrigation applications, two general types of pumps are used in SDI:

centrifugal or turbine Centrifugal pumps are more frequently use to pump water from surface supplies such as ponds Turbine pumps (a special type of centrifugal pump) are used to pump water from wells and may be either vertical shaft or submersible Motor sizes are limited by the well size with submersible pumps Some pumps are more efficient than others (in each type category) and are usually available in a wide range of operating efficiencies Pump efficiency is the ratio (expressed in decimal form) of the water horsepower at the discharge of the pump

divided by the power delivered (required) to the input side of the pump Normally, pump cost increases as efficiency increases but savings in operating costs usually compensate for the increased initial cost after a few years of use If possible, the pump should be operated in the midrange of its performance curve A pump efficiency in the range of 75-85 percent is desirable.Smaller centrifugal pumps will likely have lower efficiencies than the desired range.

Power sources for pumps will depend on the availability and accessibility of the energy resource in the local area In most instances, electricity is preferred because of reduced labor requirements Power may not be available where it is required and three- phase power is usually required to operate irrigation pumps greater than 10 horsepower Local power suppliers can provide information on power rates and any demand charges that are often associated with irrigation uses If electricity is not available or desirable alternative sources such as diesel, gasoline, or propane may be used The most common alternate power source is usually gasoline driven engines for small pumps and diesel engines for larger pumps Each has advantages and disadvantages, both management and economic More detailed information on pumping plants and power supplies can be found in Evans et al (1991) and Hanson et al (1997)

FILTRATION REQUIREMENTS FOR SUBSURFACE DRIP IRRIGATION

Filtration is critical in SDI systems Since the emitters are buried, determining the location

of clogged emitters is very difficult For optimum performance, only clean (free of particulate matter) water should be pumped through drip irrigation systems Few, if any, water sources are completely free of particulate matter and many sources (especially surface water) contain high levels The size and type of filter required will depend on the water source and the kinds (if any)

of fertilizer and chemical stock solutions to be injected The pH and the amounts of particulate matter, carbonates and iron will largely determine the filtration system

Filtration systems are placed at the headworks of the SDI system The types of filters used most often are media (sand), disk and screen Media filters are absolutely necessary for any surface water source and especially for wastewater In addition, flowing water usually requires a sand separator to remove sand before it enters the filter system

Media Filters

Media filters are superior for filtering surface water due to their large filter area and

capacity They can be quickly back flushed for cleaning which can be automated At least two media filters are needed, since during the back flushing operation; clean water from one filter is used to remove contaminates from the other filter Media filter sizes vary from 14 inches

(adequate for approximately 2 acres) to 48 inches in diameter Recommended filter capacity ranges from 25 gpm per ft2 of filter cross-sectional area for relatively clean water to 15 gpm per

ft2 for water with a large amount of sediment or particulate matter (Hanson et al, 1997) An illustration of media filters in a back flushing operation is shown in Figure 3 Table 2 shows the

relationship between sand size, equivalent mesh size and particle size removed Generally, it is

recommended to remove particles down to a size ten times smaller than the emitter’s passageway

so that grouping and bridging of particles will not cause clogging (Schwankl et al., 1997; Pitts et al., 1990) The maximum tolerable particle size for an emitter should be provided by the

manufacturer More detail on media filters can be found in (Haman et al., 1994)

Table 2 Relationship between particle size, screen mesh size, and particle size removed

Sand

Number

Equivalent Screen Mesh Size

Particle size removedmicrons

Particle size removed,Inches

Sediment Particle Type

10 inches in diameter Mesh sizes range from 40 to 200 Although some screen filters have automatic back flush capabilities, most must be manually flushed In problem situations, small screen filters are often used on submain lines

Figure 5 Disk filter

Disk Filters

Disk filters are used as secondary filters with surface water systems or as primary filters with well or municipal water sources These filters contain a series of grooved plastic disks that may have an equivalent screen size ranging from 40 – 400 mesh (Hanson, et al., 1997) with 40-

200 mesh the most common Disk filters have the advantage of having more surface area than screen filters and are therefore better suited for higher flow rates and are also easier to clean A disk filter is shown in Figure 5 Some filters have interchangeable screen or disk elements.Maintenance of filtration systems should be considered in the design process Back flushingmay be automated in the case of media filters, and some screen filters’ flushing cycles may be automated by the use of solenoid valves with

controllers

Irrigation dealers as well as Cooperative

Extension Service personnel should be able

to provide additional advice on filtration

systems for SDI systems

Figure 4 Two types of screen filters a.) horizontal steel filter and b.) plastic

spiraling water cleaning filter

CHEMICAL INJECTION FOR SUBSURFACE DRIP IRRIGATION

Chemigation is an inclusive term referring to the application of a chemical into or through

an irrigation system It includes the application of chlorine (chlorination), acids (acidifiction), fertilizers (fertigation), and pesticides Because drip emitters are small, they clog easily Along with filtration, the capability to inject chlorine and acid are important features in an SDI system

In humid regions algae production may be accelerated and therefore chlorination is very

important Other benefits of chemigation are uniform and timely application of fertilizer, reduced soil compaction due to reduced traffic in fields, reduced labor requirements, reduced exposure to chemicals, and reduced environmental contamination

Chlorination is the introduction of chlorine, such as liquid sodium hypochlorite (household bleach), granular calcium hypochlorite (HTH), or chlorine gas into an irrigation system

Chlorination prevents the growth of organic material that can clog emitters It will also oxidize (remove) existing organic material

Acidification is the introduction of an acid, such as phosphoric, sulfuric or hydrochloric (muriatic) into an irrigation system Acid is injected to dissolve chemical precipitates such as lime or to prevent their formation

The design of a chemical injection system involves the selection of the injector, both type and capacity (size) If the injection system is to be used for fertigation, the injection unit should

be sized for this since injection rates for fertilizers are usually much higher than injection rates for chemicals such as liquid chlorine or acid

Two basic types of injection pumps the Venturi injector and the metering pump are

commonly used for injecting fertilizer and other chemicals into drip irrigation systems Field setups for both types should have an adjustable injection rate Any components that will be in

contact with fertilizer, chlorine or acid should be resistant to

corrosion

Venturi injector

As water flows through the tapered Venturi orifice, a

rapid change in velocity occurs This velocity change

creates a reduced pressure (relative vacuum), which draws

the liquid to be injected into the system Since the injection

rate will vary with the pressure differential across the

Venturi, a regulating valve and a flow meter are

recommended for calibrating the system (see Figure 6 The venturi is sized by determining the liquid suction flow (chemical) to motive flow (flow through injector) These vary with injector model (size) and can be found in manufacturers’ tables Normally only a portion of the SDI system flow is routed through the injector This can be accomplished by creating a bypass of the mainline To insure that there is a pressure drop across the bypass, a gate valve or pressure regulator is used (see Fig 7)

Figure 6 Venturi injector

with regulating valve

Figure 7 Venturi with bypass configuration (from Haman