Optimizing Injector Quill Performance

8/11/2019 Optimizing Injector Quill Performance

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WHITE PAPER:

OPTIMIZING QUILL AND INJECTOR

PERFORMANCE IN REFINERY OPERATIONS

FIGURE 2.Quill with Slot

There are dozens of operations in reneries where quills and injectors are used. Before we discuss usage andperformance optimization, let’s clarify the difference between a quill and an injector. The terms are oftenused interchangeably even though the devices are quite different.

• An injector has one or more spray nozzles on a pipeand delivers a specic volume of uid at a speciedpressure drop. See Figure 1. The nozzle(s) convert

uid into a predictable drop size spectrum and providespecic spray characteristics. The use of spray nozzlesallows more control over the distribution of the injectedliquid into the receiving process uid.

• A quill is a pipe with slots or holes. See Figure 2. Theinjected uid ow is uninhibited. The receiving processstream breaks up and mixes the injected uid.

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FIGURE 1.Injector with Spray Nozzle

INJECTORS AND QUILLS: WHAT’S THE DIFFERENCE?




WHITE PAPER: OPTIMIZING QUILL AND INJECTOR PERFORMANCE IN REFINERY OPERATIONS

Should you use a quill or an injector? The answer is somewhat dependent on your operation but, in general,you should use an injector unless you don’t need any control over spray characteristics such as ow rate, dropsize or spray pattern.

The most common uses for quills and injectors are ingas conditioning, chemical injection and water wash.(See below for a complete list.) Here are a few examples that show the performance advantages of injectors overquills.

If you are cooling a gas, it is critical to know drop sizesince it is proportional to how fast the cooling will beachieved and when drop evaporation will occur. Ifdroplets are too large, you may not achieve the desired

cooling effect. This can result in excess uid in the pipeor duct, cause maintenance problems and damage down-stream equipment.

Chemical injection requires a greater surface area ofinjected liquid. Greater surface area requires better gas- to-liquid (or liquid-to-liquid) interaction to ensure efcientheat or mass transfer.

A third example is water wash. The wash water, viasurface area contact, must have sufcient interactionwith the vapor stream in order to dilute the corrosivesand dissolve any water-soluble materials present. Aninjector with a spray nozzle provides more efcientmixing and better drop size breakup than a quill andcan provide more control over the process – somethinga quill cannot achieve.

Injectors cost more than quills. However, given the

long service life requirements of injectors and quills –up to 10 years – the cost differential between the two isinsignicant should a problem occur due to impreciseow. The cost of unscheduled downtime, damage todownstream equipment or incomplete cooling, wash-ing or chemical reactions will greatly exceed the pricedifferential between a quill and an injector.

A. Defoaming InjectorsB. Distillation Column InjectorsC. FCCU Overhead Water Wash

InjectorsD. FCCU Feed InjectorsE. Steam Quench Injectors

F. Slurry Backush InjectorsG. Packed Tower HeadersH. Heat Exchanger InjectorsI. Additive InjectorsJ. Mix Temperature Control (MTC)

Injectors

K. Torch Oil InjectorsL. Regenerator Bypass InjectorsM. Catalyst Reformer Gas Cooling

InjectorsN. Chloride InjectorsO. Fractionator Water Wash

Injectors

P. Coker Off-Gas Cooling InjectorsQ. Flue Gas Desulfurization

InjectorsR. SNCR NOx Control InjectorsS. SCR NOx Control InjectorsT. Desuperheating Injectors




WHAT YOU NEED TO KNOW ABOUT INJECTOR SPR AY NOZZLES

Spray nozzles fall into two categories: hydraulic or air atomizing. A single liquid ows through a hydraulicnozzle. A liquid and a gas ow through air atomizing nozzles. Air atomizing nozzles are often called dual uidnozzles.

Spray nozzles are designed to produce a spray pattern: full cone, hollow cone or at spray.

FULL CONE SPRAYS are formed by swirling the uidinside the nozzle by a stationary vane. There is a distribution of small to larger size droplets throughout the pattern.Among hydraulic spray nozzles, full cone nozzles produce the largest droplets for a given ow rate and pressure.The maximum free passage in full cone nozzles is a bitlimited due to the vane design, so it is important to keep inmind that clogging could be a problem when using liquidswith suspended particulates. If there are particulates in the uid, consider using full cone spray nozzles featuringa maximum free passage design.

Typical uses: overhead water wash, defoaming, torch oilinjectors and vacuum tower spray distributors.

HOLLOW CONE SPRAYS are formed by injecting a streamof uid tangentially into a swirling chamber. The swirl-ing action allows a uniform lm of liquid to dischargefrom the nozzle forming a ring of uid. Hollow cone spraynozzles produce smaller droplets and a tighter spectrumof droplets than full cone or at spray nozzles. This means the droplets are relatively uniform in size throughout thespray. Hollow cone spray nozzles have a large free pas-sage so the risk of clogging is minimal.

Typical uses: ue gas cooling and urea injection for SNCRNOx control, desuperheating, quenching and water wash.





FLAT SPRAY NOZZLES use an elliptical orice to createa at fan spray pattern. The exact shape of the oricedetermines the spray angle which can range from asolid stream to a 120° spray angle. Drop size is medium –

smaller than full cone sprays and larger than hollow conesprays.

Typical uses: steam quench, desuperheating, torch oilinjectors and catalyst reforming cooling.

AIR ATOMIZING NOZZLES are available in a wide rangeof spray patterns – hollow cone, full cone and at spray.Although available as internal or external mix nozzles,injectors in renery operations are generally equippedwith internal mix nozzles. Internal mix dual uid nozzlesproduce the smallest droplets. The size of the droplets isdetermined by the mass and pressures of the atomizinggas which is usually compressed air, nitrogen or super-heated steam.

The liquid and gas streams in dual uid nozzles are typi-cally kept separate until the two uids are brought togeth-

er just behind the discharge orice. This enables mixingefciency to be maximized and the smallest possible drop to be produced. If the two uids are mixed earlier, coales-cence and drag would increase drop size.

Air atomizing spray nozzles designed for operation at lowow rates – 2 to 5 gallons per minute (8 to 19 liters perminute) – can be sensitive to operational pressures. Forexample, if the atomizing gas pressure is too high, it can

choke or shut off the liquid ow due to high back pressurein the mixing region. When a high volume of liquid needs to be atomized – like 25 to 50 gallons per minute (95 to 189liters per minute) – large quantities of gas are required toachieve small droplets. A multi-stage air atomizing nozzleis typically used. These specialty spray nozzles are moreenergy efcient and produce small droplets over a widerange of operating pressures.

Typical uses: gas cooling, fresh feed injectors to FCCunits, emergency quench and urea injection for NOx.

One more selection consideration is the environmentwhere the nozzle will spray. For nozzles to atomize, theyneed to spray into vapor – which is air or any other gasinside a duct system. Atomization does not occur whenliquids are sprayed into liquids. When liquids are sprayedinto liquids, the nozzle is acting more like a dispenser.Spray nozzles with multiple orices may prove advanta-

geous. Breaking the ow into multiple injection points canincrease the mixing of the injected liquid with the processstream if it is also liquid.





DETERMINING SPRAY DIRECTION

There are two ways to spray: co-current or counter-cur-rent. Each approach has advantages and disadvantages.Table 1 below summarizes the pros and cons of eachapproach.

There is no right or wrong answer on spray direction. Thebest direction is the one that will deliver the performanceyou need.

A common rule of thumb is to center the injector in theprocess pipe and spray co-currently. However, thisdoesn’t always produce the best results. See Table 2.This example illustrates how the type of spray nozzle usedand the orientation of it can affect performance.

When the rst and second spray nozzles in the chartbelow are compared, the impact of using a properly sizedspray nozzle becomes clear. Wall wetting is decreased, the amount of water evaporated is increased and dropletsize is smaller at the end of the pipe.

The third spray nozzle in the chart is the same as the sec-ond nozzle but it is spraying counter-current to the vaporstream. There is a slight increase in water contact with the wall as the spray plume opens up. However, a greateramount of water evaporation occurs and droplet size iseven smaller at the end of the pipe.

Which is the better spray direction? In this case, theapplication requirements will determine if 2% watercontact with the wall is acceptable to achieve greaterevaporation and cooling.

CO-CURRENTSPRAYING

COUNTER-CURRESPRAYING

More exibility in where the injector is placed in the pipe 3

Injector must be placed in the center of the pipe 3

Bearding (build-up) on the nozzles is minimized because the nozzle is spraying in the same directionas the process stream

3

Bearding on the injector can occur if there is a high amount of particulate in the process stream. Thebuild-up can increase stress on the injector

3

Impingement on pipe walls possible if injector is not placed in center of process stream 3

Larger droplets created by fallback of sprayed droplets coalescing with newly sprayed droplets 3

Longer residence time of spray by opening up the spray pattern; shearing of the droplets can result insmaller droplets

3

Faster reaction time required for full evaporation of the injected uid 3

NOZZLE TYPE SPRAYDIRECTION

INJECTOR MEANDROPLET DIA.

DV0.5 (MICRONS)

% CONTACT WITHPIPE WALL

OUTLETTEMPERATURE

% WATEREVAPORATED

OUTLET MEANDROPLET DIA

DV0.5 (MICRONS

Hollow cone / large droplets Co-current 1115μ 85% 480°F (249°C) 4% 428μ

Hollow cone / small droplets Co-current 95μ 0.40% 349°F (176°C) 58% 133μ

Hollow cone / small droplets Counter-current 95μ 2.0% 283°F (139°C) 78% 67μ

Initial Process Stream Temperature = 540°F (282°C). Pipe Diameter = 36 inch (914 mm).

All measurements downstream from injector are at 15 feet (4.6 m) – 5 pipe diameters


The placement of an injector in a pipe can signicantly impact performan

TABLE 1: SPRAY DIRECTION PROS AND CONS

TABLE 2: EFFECTS OF NOZZLE TYPE AND ORIENTATION ON PERFORMANCE




INJECTOR DESIGN CONSID ERATIONS

In addition to evaluating the factors that affect sprayperformance, injector design and construction must becarefully planned and validated. Operating conditions andservice life will dictate design, materials of constructionand the level of validation required. Pressure, temperature, corrosion and erosion typically determine processcode requirements. Material options include stainlesssteel, HASTELLOY®, INCONEL® and titanium. Other spe-cial alloys and special coatings can be used in extremeenvironments.

The operating environment typically determines the levelof testing and construction validation required. Testing anddocumentation generally include many of the following:

• Weld quali cations/procedures and weld maps

• Positive Material Identi cation (PMI)

• Non-Destructive Examination (NDE)

– Radiographic Examination (RT)

– Liquid Penetrant Examination (PT)

– Visual Testing (VT)

– Ultrasonic Examination (UT)

– Material Traceability (MTR)

– Hydrostatic Testing (LT)– Ferrite testing of welds

Using computer modeling to validate injector design andperformance is generally recommended. Relying on theo-retical calculations alone can be risky. Computer model-ing is used for two purposes. The rst, Fluid StructureInteraction (FSI), is to evaluate the mechanical stresseson the injector and verify structural integrity. These mod-els reveal if the materials of construction can withstand thermal stresses, high loads, high pressures and vibrationcaused by turbulence in addition to resisting corrosionand erosion.

For example, the velocity of the process stream cancause vortex shedding and premature injector failure. Thelength of the injector, the size of the pipe used for injec-

tor construction and the type of welds are all importantconsiderations. Equally important are the types of forceson the injector in the process stream. A failure inside theprocess pipe is dangerous and it is more costly than usingcomputer modeling to validate the system design before itis nalized and installed.

The second way modeling is used is to validate injectorperformance with Computational Fluid Dynamics (CFD)models. These models predict the interaction of theinjected uids with other uids or a vapor and determine the heat transfer, mass transfer, chemical reactions andother ow related phenomena that will occur when theuids interact under specic conditions. Unforeseen in- teractions can be identied and design modications canbe made before injector construction begins.

Using computer modeling enables injectordesign and performance to be validatedprior to construction and prevents costly andpotentially dangerous design problems.


FSI Model CFD Model




MINIMIZING MAINTENANCE DOWNTIME

Injectors may need to be removed for inspection, rou- tine maintenance or insertion in another location. Safelyretracting the injector so an isolation valve can be closedprior to unbolting the injector from the process pipe is amajor advantage.

There are two types of retractable injectors. One is re-moved manually and the other features a handwheel andscrew mechanism that provides a mechanical assist. Themechanical retractable version provides an additionalsafety level since the injector cannot be retracted anyfaster than the handwheel operator allows. With the useof retractable injectors, the primary injector can be ser-viced safely ofine and redundant injectors can be used

to prevent process disruption.

Using retractable injectors can also help dramaticallyreduce downtime. One renery in the midwestern U.S. isusing mechanical retractable injectors to spray liquid nitrogen into the hydrocracker reactor to cool the catalystprior to replacement. The injectors are inserted and thecooling process begins while the plant is on line. Previ-ously, a partial shutdown of the renery was required be-fore the liquid nitrogen cooling process could begin. Theshorter turnaround resulting from the use of the retract-able injectors saves the re nery an estimated $2,000,000by reducing downtime by two days.


SUMMARY

The guidelines outlined here provide a good starting point when a new or replacement injector is required. However, thespeci cs of your operation will determine what’s necessary to optimize performance. You may nd the following checklisthelpful as you move forward with your next injector project.

q Don’t automatically replace like withlike. Just because performanceseems to be OK doesn’t mean itcouldn’t be better. You may beable to reduce wetting, decreasemaintenance time, extend servicelife and more by revisiting thecurrent design.

q Be meticulous about the detailsand eliminate open issues. Usemodeling to uncover potentialproblems and ensure performance.Fixing problems after an injectoris installed will cost considerablymore than the investment inmodeling.

q Consult with experts even if you arecondent in your specications/design. They have specializedequipment, modeling programsand experience that can helpvalidate your design. They may alsorecommend design enhancementsbased on advancements in spray technology.

Mechanical Retractable Injector



ABOUT THE AUTHORS

Dan Vidusek has 30 years of spray nozzle design experiencewith Spraying Systems Co. He has extensive application ex-perience in a wide variety of industries and now specializes inchemical injection, water wash and gas conditioning applica- tions. Mr. Vidusek has presented papers at many technicalconferences and is currently working with NACE Internationalon updating its publications on injector use in reneries.

Chuck Munro has more than 17 years of experience withSpraying Systems Co. in spray nozzle and injector applicationassistance and troubleshooting. His areas of focus includewater wash, emergency quench and torch oil injection in thepetrochemical and chemical industries. Mr. Munro is a mem-ber of several industrial committees and has been involved inmany different injection optimization studies.

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White Paper No. 109 Printed in the U.S.A. ©Spraying Systems Co. 2012

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HASTELLOY® is a registered trademark of Haynes International, Inc.INCONEL® is a registered trademark of Special Metals Corporation group of companies.


Optimizing Injector Quill Performance

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