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CHOOSING TANK AND VESSEL WASHING SPRAY NOZZLES

 
 Clean-in-Place Guidelines for Consumer Packaged Goods Manufacturers.pdf

 

With the broad selection of tank cleaning devices available today, the thought of manually cleaning a tank, mixer, duct or any other vessel that has been used to hold or convey a liquid or dry product should be a thing of the past.  Unfortunately, reports surface too often of injuries and even asphyxiation deaths from workers entering tanks for cleaning. What makes these especially disturbing is the fact that commonly available, inexpensive products are available that make these situations preventable.

Automated tank washing processes can be adapted to virtually any equipment configuration, large or small, simple or complex, easy or difficult. With at least a dozen manufacturers of such devices, there is no shortage of equipment choices. The most difficult question in many situations is simply, "What will work best for my application?" For purposes of this presentation, we will limit our scope to evaluating the individual tank washing devices, both static and rotating, as are commonly available from a large group of manufacturers, and how they are used. Full CIP systems and how they are configured are beyond what we can cover in this type of venue. We will consider various types of cleaning situations, requirements of different tank sizes and configurations and the spectrum of tank washers available. (Also for purposes of this presentation, the word "nozzle" means a device to spray, not a port or opening on a tank. I will use the term "spud" for the latter.)Let's begin the process by describing cleaning functions in a way that we can use throughout the discussion. For convenience's sake, here are three "classes" of cleaning which present different challenges. The term "soil" describes any material that we want to clean from the affected surfaces.

Class I

Here the soil is simply a liquid residue or light powder that does not adhere aggressively to the surface. Cleaning is simply a matter of rinsing the surfaces with a solution that washes away the soil and possibly disinfects the interior. Spray impact does not contribute significantly to the process. An example of this would be a tank used to hold milk.

Class II

A slightly more difficult situation where the soil does not rinse away so easily, yet it can be dissolved by the cleaning liquid. Spray impact helps move the process along but over time and with enough liquid, the surfaces will ultimately come clean. An example of this would be a tank that held molasses or white glue. It doesn't wash off easily, but water will soften it and wash it away.

Class III

The most difficult situation where the soil does not dissolve in the cleaning liquid. The process depends on the impact of the spray and washing action of the liquid to breakup and carry the soil away. An example of this would be a tank that held a powder that does not dissolve in water. Any caking on the walls needs to be blasted off and washed away with enough force to keep the soil moving.Of course, there are infinite variations and shades between these three designations based on the application. None the less, these should help as you evaluate your own specific needs. The sizes and configurations of tank washers make them particularly suited to cover specific portions of the spectrum. Choosing an inadequate design will cause obvious problems, but making a choice that is overkill can be costly in purchase price and ongoing liquid consumption.

Major Design Groups

While there are dozens of tank washer designs from various manufacturers, for the most part they can be separated into three general classifications:

1. Static-Single or cluster nozzles designed to cover large areas without rotating.
2. Free spinning reaction heads-Rotating heads powered by the reaction force of tangentially directed sprays.
3. Mechanically driven-Units driven by liquid pressure or external power using gear drives for controlled pattern rotation.

There are a variety of designs within each class, each with its own advantages and drawbacks. No single design is appropriate for every application. Advantages balance downsides and there are performance tradeoffs at every turn. Your best position is to understand each of these and make the effective choice for your specific needs.

Here are the primary considerations for the major design classes. There are certainly exceptions to every generalization, but this information applies in the widest range of cases.

Static Tank Washers


The simplest and generally least expensive tank washers are nozzle heads that do not rotate, but provide wide coverage through the use of a cluster of individual orifices or slots. They come in a variety of sizes and shapes, but like most designs, they can be grouped into sub-classifications.

Spray Balls static-cip-spray-nozzle-tank-washing


These use a hollow shell, shaped like a ball or small pumpkin, with a large group of small holes that form many small solid stream sprays when the head is pressurized. In some cases there can be slots around the "equator" that send out a fan of spray in combination with the small holes. 

Best Applications


Class I and possibly light Class II cleaning with smaller tanks, usually smaller than 10' in diameter. The design adapts well to specialized and directional coverage requirements given their ability to concentrate spray wherever it's needed. They can mount in any position since they have no rotating parts.

Drawbacks

 

The tank surface where the spray actually hits receives the impact but this is a very small portion of the inside surface. The balance depends on wash-down action. Small orifices are clog prone, which can form voids in the spray. Additionally, spray balls frequently become the "system strainer' due to their ability to trap debris. Make sure you install them where they can be cleaned out easily or you must have a very clean liquid supply. Otherwise they gradually fill with any junk carried by the liquid which drastically hurts their effectiveness.


Cluster Full Cones   cluster-spray-nozzle-tank-washing-cleaning-cip


Here a group of full cone nozzles are mounted on a central hub, which acts as a common liquid supply. The sprays overlap so there is a cloud of droplets, which provides very complete coverage. Flow rates range from less than 10 gpm up to several hundred gpm, so the size range is huge.

Best Applications

 

Class I and possibly light Class II cleaning with smaller tanks, usually smaller than 8-10' in diameter. In most cases the smaller the tank, the better, as impact falls off very quickly as the spray travels from the orifice. With larger tanks, the impact is minimal, but the coverage is very complete. If the application demands a gentle spray, this would be the first choice.

Drawbacks

 

When making a selection, there are a number of considerations that apply to the whole variety. In general, the main points are as follows:

Best Applications

 

All classes of cleaning, especially I and II. Class III is also certainly possible if the impact requirement is not huge.

 
Free Spinning Reaction Heads    tank-vessel-cip-washing-spray-nozzle 
 

This segment of the spectrum is the largest and most varied. Reaction powered rotating tank washers are available from at least a dozen manufacturers. They can range from tiny heads less than I" in diameter to huge units spraying hundreds of gallons per minute.

Given the variety of designs, free spinning heads can handle everything from a bucket to a tank 30' to 40' in diameter. However, spray impact falls off quickly as distance increases.

Drawbacks

 


When looking at a specific design and matching it to an application, think through the particular features with the following points as your guide:

Flow Rate


It is difficult to give universal guidelines for how much liquid it takes to wash a tank since conditions vary so widely Suggestions range from .2 to .5 gallons per minute per square foot of internal tank surface. If there are few obstructions so the spray can propagate easily, and the tank is not too large, the lower end is probably adequate. For more difficult installations or aggressive soil, a higher flow rate may be required. Higher flow units normally produce larger droplets, which carry over greater distances and hit with more impact.

Spray Distance



Material of Construction

 

In this product category, almost all designs are either made from stainless steel or plastic, primarily PTFE (Teflon using DuPont's name) but a few others like PVDF and polypropylene are also available.


All-stainless steel units are very durable and can be subjected to extreme temperature ranges. The main drawback is ensuring that you have an appropriate grade of stainless if your environment is particularly corrosive. Units in exotic materials are normally very expensive.


All-plastic units are getting to be more common. They are normally inexpensive, especially those made from injection molded PVDF. All-PTFE units are an excellent choice for the most corrosive applications. While inexpensive, they are disposable since they cannot be reconditioned.


Stainless steel with plastic bearing inserts are a good compromise. They can be disassembled easily and the inexpensive plastic inserts replaced as they wear.


Nozzle Design


Tank washers have an amazing variety of nozzle and orifice designs:


Drawbacks


There are many, however with the selection of products available you can choose a design that allows you to minimize the effects for your situation. Look through the list of specific points and you will get an idea of what to watch for. Some apply to all designs and are unavoidable.

Gear driven units wash using a specific pattern, which eventually covers the entire interior of the tank. While this is very thorough, these units have to run for a minimum length of time to complete the task. With each revolution they typically advance 5" to 10" so they need 40 to 80 or more turns to make a complete circle.

True scrubbing may require more than one circle. Shortening the time will leave areas that are not hit directly by the spray. If a light wash is all that's required, an incomplete cycle might be enough.

Water quality becomes a greater concern where there is a more sophisticated mechanism. Debris or abrasive products in the liquid can stall or cause premature wear on the moving parts. Manufacturers go to great lengths to minimize the effect of this, but filtration is still recommended. Some models are more tolerant than others are, but it is difficult to get objective information if you don't have direct experience.

Most units give 360' coverage. There are very few units that only spray one direction.

When looking at a specific design and matching it to an application, think through the particular features with the following points as your guide:

Nozzle count

 

Units are available with 1 to 6 solid stream nozzles, with 2 and 4 the most common. The total flow of the unit is divided among all the nozzles. Adding nozzles gives more coverage with each rotation so the interior is swept completely in less time.  Fewer nozzles concentrate the cleaning power and increase spray distance for a given flow rate. The streams are heavier and have higher impact when not divided among too many orifices.

Operating Pressure


Gear driven units can run at anything from normal water line pressure to 20,000 psi. Obviously, no single design covers that whole range. You need to look at your needs and consider how far you must go to get the job done.

Each design will specify an operating range. The low end is the amount to make it move and the high end the maximum speed the mechanism is designed to withstand. With too much pressure the unit may rotate too fast which will make it wear out too quickly.

While higher pressure cleans better, it takes a lot of horsepower to provide an adequate volume of water at that level. A small high pressure head that sprays 20 gpm at 10,000 psi has to have a 200 hp pump behind it. Even 35 gpm at 1,000 psi takes 30 hp. Make sure you have the pumping capacity before you consider entering the pressure stratosphere.

"Lance" designs

 

Some manufacturers offer versions called lances where the main drive mechanism stays outside the tank and only the actual washing head extends inside on a pipe. When size is critical, a lance can keep the diameter to a minimum to allow insertion through an opening too small for a self-contained unit.

The overall lance length limits the insertion distance. If there is a large variety of tank sizes, this can become a draw back.

For explosive atmospheres or high purity installations such as pharmaceuticals, many users like to keep as much of the mechanism as possible outside the tank.

If you need to operate at a wide range of pressures, a lance can give independent control of the rotation speed regardless of the liquid pressure.

The power source on a lance can be the spray liquid, or an external drive mechanism using an electric, hydraulic or compressed air motor to provide the rotation force. This can provide flexibility for the installation, but requires an additional connection to the unit.

Lubrication


Some units have sections of the mechanism sealed and packed in oil or grease. This protects the more complex gear trains and minimizes wear. While this was more typical in years past, there are some units currently available that still use this approach or it is available as an option. The main drawback is the potential for contamination if the washing medium breaks through the seal and forces the grease out into the process. On the upside, a sealed unit can be a way to cope with poor quality water since the water flows through less of the mechanism. With the development of more sophisticated materials, most tank washers are now are lubricated only by the washing medium. Well designed units direct the flow through the internal passages to keep them flushed with fresh supplies of liquid so no sediment or debris accumulates.

Serviceability


As a piece of mechanical equipment with moving parts, a tank washer will eventually require maintenance. With most you have the option of returning them to the manufacturer or doing the work yourself. Since the mechanisms vary in complexity between manufacturers, this is something to keep in mind. You can always ask to see the service manuals they supply before you buy.

Hybrid Designs


There are a few tank washers that resist classification into one of these three groups. Since they are usually unique to one manufacturer, it is difficult to go into much depth without giving a sales pitch. If you carefully study the literature you receive from various manufacturers, you may find models with attributes that fall between the groups. Given the number of units on the market, there is little reason to believe that you can't get exactly what you want within practical limits. The key point is to shop around and discuss the matter with people you trust.

General Considerations and Practices


Regardless of the tank washer design you choose there are some considerations that are universal. These suggestions are exactly that: suggestions. Nothing is absolute. Nothing works every time for every application. Think through these approaches and decide what sounds best for your specific installations.

Location and Mounting

tank-vessel-washing-cip-spray-nozzle-location-and-mounting
In most cases, tank washers work best when mounted one-third of the way down from the top of the tank-vessel-washing-cip-spray-nozzle-spray-pattern

tank. This uses gravity to assist in the cleaning process as the cleaning solution washes down the walls.

Depending on the diameter, more than one may be needed. If there are internal obstructions such as agitators or mixing  paddles, more than one will be required to make sure there are no shadows where the spray is blocked. If possible, it helps to run the agitator during the cleaning cycle.

If the tank is very tall such as a silo, it will help to have more than one tank washer operating at different levels.

If the tank is horizontal and very long but must be washed from only one location, a horizontal lance can help the spray reach into the ends.

Most washing takes place by inserting the tank washer through a spud or other access point. For portable washing applications, simply attach the tank washer to the end of a pipe and feed it with a hose. Just be careful not to bang the head on the spud or tank wall.

For permanent installations, it is best to attach the tank washer to a pipe with a connection or flange that mates with the spud. Make sure that the material and washer design you use is appropriate for contact with the product should that be the case between washing cycles.

More sophisticated units offer retractable mechanisms that can insert and withdraw the lance for cleaning and operating cycles. While generally expensive, this offers the ultimate in automated solutions.

If contamination is a concern, there are units designed specifically for critical installations such as dairy, food or pharmaceutical use.

Most "sanitary" tank washers are the static spray ball variety or smaller free spinning units. More complex mechanisms offer too many potential sources of contamination for these applications.

True "sanitary" tank washers need to conform to extensive design and mounting requirements to ensure that they do not retain either product or washing solutions after cleaning and operating cycles.

For the most complete analysis on the topic, 3-A offers a new standard (just adopted officially in November 1998) for tank washer designs. Ask a potential nozzle supplier if their unit is certified to 3-A Standard 78-00 for tank washers. This is the most sophisticated standard currently available.

For this application there are also specific piping and mounting requirements which are addressed in the 3-A standard.

The Cleaning Process


Each cleaning situation is different. Washing a tank can be as simple as a water rinse or as complex as high pressure blasting with solvents that must be collected and incinerated. If you are not sure how to approach a new application, here is one method that has many applications:

Water pre-rinse, using your lowest quality water. This wets down the interior and removes any loose soil.

First washing rinse, using a hot alkaline cleaning solution such as 1% sodium hydroxide.

Water rinse to wash out the alkaline cleaner. This can be re-used next time for the pre-rinse cycle.

Second washing rinse using an acid solution to neutralize any alkaline residue and make sure no scale is left behind from hard water. Shocking the soil from one end of the pH scale to the other also helps break it down.

Final rinse with clean water that can contain any specific agents you want to leave in the tank prior to the next operating cycle.

There are any number of chemical companies that can help you select the best cleaning agents for your specific needs and equipment.

Consider this carefully, making sure you are not creating hazardous wastes from the cleaning process that will require special treatment.

You ultimately have to decide what is appropriate for your situation based on your specific needs.

Portable Cleaning vs. CIP


Once you have studied your applications and chosen the appropriate tank washing nozzles, you now have the choice of either moving the tank washer from tank to tank or creating a CIP (clean in place) installation by mounting it in one place where it will be turned on for each cleaning cycle.

 

Portable tank washing approaches work the best in the following situations:

 

Some plants have facilitated portable washing with carts that can be wheeled or driven around carrying the necessary chemicals, pumps and maybe even water. Something like this coupled with a selection of tank washing nozzles can be a practical way of moving around to cover all your applications with minimal manpower.

On the other hand, the broad selection of tank washing nozzles and their generally economical pricing makes permanent installation more and more attractive. The thought of turning a valve or pushing a button from a console to wash one or a group of tanks certainly has its appeal. Such CIP systems can be configured to wash an entire production line. A CIP system can be as simple as a mounted tank washer connected to a water supply pipe. On the other end of the spectrum, it can be highly sophisticated with automated controls to ensure that product is never contaminated with cleaning solution. Use the same analysis to select a tank washing nozzle for CIP use that you would for any application.

There are many engineering companies that can help you design a system with whatever level of complexity is required for your needs.

Cleaning Validation


The move from manual cleaning to a tank washing nozzle requires either faith that it is working or some way to determine that your equipment is clean enough that it will not contaminate product.

Some products are easier to contaminate than others. A bin that holds black plastic pellets for an injection molding machine can be cleaned two or three times a year with little chance of contamination other than a pencil or hardhat falling in. On the other hand, if that same bin is having the black pellets washed out to be replaced with white, a few that stay behind and get carried into the white pellets can spoil many critical parts.

While that is an extreme example, the same could apply to a tank that holds two colors of paint or two different pharmaceutical products. Contamination in such a case could spoil product or even be deadly.

The question is how to validate the cleaning process to ensure that the tank washer you have so carefully selected is doing the job.

This is particularly critical if you are using an automated process where there will be no inspection of the washing operation on a day to day basis. To make things more complicated, if you are using a tank washer that rotates, there has to be a way to verify that it is indeed rotating since that is critical to the cleaning process.

Verifying that soil has been removed can be accomplished in a number of ways depending on the degree of tolerable contamination. Two critical examples include dairy and pharmaceutical applications. A common dairy method is to take bacterial cultures using swabs following very defined procedures. This indicates whether disinfection has been sufficient to reach accepted levels. In pharmaceutical applications, tanks can be coated with proteins other substances that fluoresce under UV light. Inspection using this method shows any weakness to the washing process.

If your validation process also demands verification that the unit is turning, the sophisticated method is to insert a pressure sensor into the wall of the tank that can measure the changes in pressure on its surface as the spray passes by. There should be regular undulations in the measurement at the same frequency of the nozzle's rotation. A cruder method would be to listen through the wall of the tank using a stethoscope or similar device for the same pulsation's.

These examples are at the high end of the spectrum. In less critical applications, you can verify cleaning and mechanical performance through regular inspection. The problem is that any loss of vigilance in your inspection regimen could result in a contaminated batch of product. Tank washers, like most things in life, are not foolproof or infallibly reliable.

Conclusion


While not the silver bullet for every situation, tank washing nozzles can fill an important role for cleaning all manner of product handling equipment. When used properly through careful analysis and engineering, they can perform even the most critical operations easily and economically. In the process, your maintenance personnel stay outside the confined spaces, away from potentially harmful chemical contact.