Through a glass darkly - we throw some light on the darker side of pumps and dosing

Is the surface finishing industry getting the safest, most efficient and most cost-effective results from its use of metering pumps and dosing technology? Perhaps not, argues a technical team from Gee & Company - a UK industry leader in chemical dosing technology - who suggest a new look at best practice.

As far as dosing equipment is concerned, few industries better demonstrate a simple truism: improve your heath and safety environment and performance benefits will naturally follow. It's an argument that works equally in reverse. Get your pumps and dosing systems correctly planned, specified and maintained - and the working environment will become healthier and safer.

How often do you see a metering pump balanced on top of a carboy squirting a corrosive chemical down an unprotected line, which is then bouncing around by the edge of a bath - with an operator stood only a metre or so away? Remember, the chemical that is coming out of the line is under pressure and could quite happily squirt several metres in any direction.

System installation apart, is the pump the correct one for the application? In other words, is it a high pressure pump where only a low pressure pump would be acceptable? Is it an air-driven pump where it should be an electric motor-driven pump - or vice versa? Does the pump have the right type of wet end for the chemical being used? Also, are the materials of pump construction compatible with the chemicals being dosed? If they are not, this can lead to dramatic system failures.

A further consideration when selecting pumps and dosing systems is the chemical that as being dosed. Has it got a high viscosity? Does it have a high SG, which will make suction lift difficult? Does it 'gas', like sodium hypochlorite or hydrogen peroxide, which will gas-lock the head of the pump and mean that it will cease to deliver chemical? Does it precipitate or crystallise, which can lead to blockages of dosing lines and then develop into dramatic failure of the dosing system?

Although the metering pump may be running and is assumed to be dosing, but is it necessarily delivering the required quantity? Poor validation can have costly results, causing downtime and production losses through rejected finished material or process.

Dire thoughts, perhaps, but every one of these potential pitfalls can be overcome by consideration of the many factors, influencing the pump specification, design of the dosing system and through-life maintenance.

Why use dosing pumps at all?

One of the advantages of using metering pumps for the control of addition chemical is that the make-up chemical is added proportionately to its consumption rate. The alternative is to add it manually - jugged or bucketed or otherwise - into the process solution at a frequency that depends on consumption rate. But it is obvious that the controlled addition of chemical through the day gives a much more consistent process operation.

Adding chemical slowly in this way is a proven way to lower operating cost. This is because the system can be set run at the optimum rate all the time - instead of alternating between the maximum and minimum - with the inevitable high level of contingency maximum as the process solution falls off in quality.

Pumps for every purpose

There is no shortage of choice when it comes to identifying the right pump for the right job - whether it be for delivering either small quantities of chemical at high rates of accuracy or larger quantities with lower accuracies.

For the large displacements, the choice includes double diaphragm, air-operated pumps and progressive cavity pumps. Look at the smaller end of the metering market and there is a whole array of pump types available.

For chemical metering pumps, there are perastoutics (?spelling), plunger, and diaphragm pumps. Within the diaphragm options and according to need, there are hydraulically actuated diaphragm, mechanical diaphragm, spring return and positive return mechanical diaphragm types.

Pumps need motors and, once again, there is no shortage of choice. There are air motors and electric motors - the latter available in single phase or three phase, where three phase can be used with an inverter drive to change the speed. Finally, there is the solenoid-driven option.

Surface/metal finishing

The sectors within the surface and metals finishing industries that benefit from progressive chemical addition extend from general engineering right through to microelectronics production and also to specialist applications within the aerospace and nuclear industries. Control accuracies increase as the level of quality control in the industry rises.

Users include surface conversion coating coating markets, where the surface is modified by phosphating onto steel or chromating onto aluminium. Also included are electrolysis concealing solutions - such as those used in electroplating operations - for which smaller additions of chemical are made, in direct ratio to the electrical load.

Another of the numerous applications is for barrel deburring mechanical cleaning operations that require the addition of specialist chemical on a regular basis.

Other sectors include the more specialist areas of chemical processing, such as the manufacture of printed circuit boards and micro-electronics. Here, addition rates are controlled and there are potentially more chemicals being added and with a much higher threshold of quality and control.

For the production of printed circuit boards, systems can include the relatively complex, consuming as many as fifty different types of chemicals at varying rates. Contrast this with a simple electro-plating operation, which may have just one or two brightener or addition chemicals to be added, proportional to production consumption.

For Gee & Company, one of the more sophisticated storage and dosing systems was designed and installed for a large printed circuit board production facility at GE, handling about forty different chemicals. For reasons of efficiency, safety and greater quality, it was decided that the chemicals would be stored on one floor, with the technical production equipment located on the floor above.

Within the preparation area, storage drums of various forms, shapes and sizes are used, including powders that are made up into liquids. They are then dosed by precision metering pumps in closed pipes to the process equipment on the production floor. The whole system is controlled by PLCs, so that the volumes of chemicals being added is carefully validated.

Dosing validation

In general, one of the big problems of running a chemical process line is that of trusting the operator to add the chemical correctly. If there is a process problem, the ability to validate the addition within the chemical dosing system is clearly important.

To do this, self-calibrating pumps, flow-checkers and load cells can be used. All the information they generate can be fed back to the microprocessor, which will be interrogated if there is a problem. This data will also be used to validate the costs of running the chemical processes, so that accurate costs of chemical additions are maintained.

Dosing validation and process control and data is also greatly influenced by the selection of pump and ancillary equipment. Health and safety is very much driven by appropriate specification and installation of the chemical dosing system as a whole. As we shall see, the two go hand in hand.

Pump selection and system installation

Due to the pulsating nature of metering pumps, the related suction pipe work needs to be oversized, especially for the addition of a viscous material or one with a high SG. If working on suction lift, a foot valve and strainer should always be installed: the strainer to stop any debris from entering the pump head and the foot valve to maintain liquid in that suction leg.

For gaseous or very high viscosity materials (high SG), it is generally better if the system operates with flooded suction: in other words, with the metering pump at low level being fed from a static storage tank. Where the chemical has a likelihood of gassing, the use of degassing heads on the metering pumps is highly recommended. Some of the pumps on the market try to handle this problem by auto venting from the head itself, while other pumps like the Signal S200 KKS are specifically engineered to overcome the problem.

The next consideration is that a chemical metering pump is quite often capable of generating in excess of 10 bar back pressure and many pumps are installed with just a single dosing line off to the point of application. Quite often there are isolation valves installed in these lines, of a type that are commonly used for general pump installations but which, for metering pumps, are not recommended.

It is important that a pressure relief valve is installed on the pump discharge line, which can either spill back to the suction side of the pump or, alternatively, back to the chemical container. The benefit so gained is critical. It means that, if for any reason the pump becomes dead-headed, the result will neither rupture the pipe work nor blow the head off the metering pump.

A pressure-sustaining valve should always be incorporated on the line, which will allow the line to stay full and will give a constant back pressure for the pump to work against, which greatly increases its accuracy. It also ensures that the correct amount of material is delivered on every stroke, whether the pump is located one metre or 100 metres away from the addition point.

Turning to the pump itself, the selection of the correct materials of construction for both the wet end and the suction dosing lines - plus the chemical storage tank - is extremely important. If incorrectly selected, it can lead to dramatic failures, with either the material dissolving or becoming embrittled. The wet end of a chemical metering pump comprises of many components, including check valves, check seats, the diaphragm and the actual body carrying the valves and the material head itself.

A wide variety of materials of construction are available, but generally materials like PVDF are acceptable for most products or, if more robust construction is sought, stainless steel can be utilised. It should be pointed out, however, that stainless steel is not inert to all products and if, for example, sodium hypochlorite is being dosed, it will lead to failure of the steel.

In many plants, the sight of the chemical metering pumps just sat on the top of a tank is far from uncommon. This, we believe, is not acceptable under current health and safety regulations. For this reason, Gee strongly recommends that all metering pumps and associated discharge and suction valve work should be housed within a chemically inert enclosure, with a clear PVC or similar lift-off access screen.

Enclosing the equipment behind a screen in this way allows the operators to view the pump and make sure that it is operating correctly and safely before the screen is lifted off for maintenance work or calibration. A further advantage of enclosing pumps behind lockable screens is that it stops unauthorised people from tampering with them, which can result in loss of chemical, loss of production and unwanted down time.

Location of dosing pumps within an enclosure means that a built-in drip-tray can be incorporated at the base of the unit. This is not only good for housekeeping because it contains any leakages or spills that may occur during maintenance. But, by installing level detection, any problem can be detected and the system either can either be shut down or drawn to the attention of the maintenance department.

Self-calibrating pumps and other improvements

Pump calibration has been with us for a long time. However, with today's technology, it is no longer always necessary to have to recalibrate pumps, as there are models available that will automatically self-calibrate. This means, for example, that if the pump is set at 5 litres per hour and there is variation of the back pressure against which it is dosing, then the pump will automatically increase or slow down its delivery rate to maintain the correct addition levels.

Such a key advance in combining metering pumps with automatic flow checking means that it is also important to utilise pumps with built-in electronics, rather than the external PLCs that greatly add to the cost of the project. The flow checker employed on the Signal series 100 and 200 pumps, for example, automatically feeds back into the electronics of the pump. In effect, therefore, the pump itself converts into a dosing system, rather than being just a stand-alone pump. Such features are not generally available on other types of metering pumps.

There may be a view that metering pumps are all much of a muchness and that anyone can supply them. However, it is recommended that they are sourced from reputable manufacturers where there is not only a vast array of choice available, but also an appropriate level of technical advice and support.

For example, for dosing a high viscosity material, it is probably necessary to have to utilise steel check valves with the possibility of spring loading to assist them. A similar argument applies to the selection of peripheries because, again, there are various types of pressure loading, pressure relief and chemical dosing lines.

Where gaseous materials are being added, it is no use deploying a pump with an inferior de-gassing head. It is better to specify one that is purpose-designed fully to engineer the problem out in the first place. One of the approaches, for example, is to use a combined-headed pump like the Signal 200 KKS model. This pump utilises a diaphragm to re-circulate the gaseous material against no back pressure into the container, hence carrying the gas back to the container and filling the chamber of the plunger pump which then actually meters the product.

Many of the chemicals used in the metal finishing industries do have adverse effects on some of the materials used on dosing lines. Although this may not immediately become apparent, in time the tubing loses its plasticity and then becomes brittle - a formula for potential failure.

Gee and Company does have the advantage that have developed from thirty years' operation across a wide variety of industries. These range from the stringent requirements of drinking water to the fast production needs of the paper industry and right through to the safety-critical needs of the petrochemical market. There is technology transfer of all this related intelligence and experience to its application in the metal finishing industries.

Maintenance matters

So many of the dosing systems that we see around the industry are installed in good faith but not maintained and fail on quite a regular basis. Not only is it important to design and install system appropriately, but also to ensure that the correct service and support back-up is maintained. The use of a manufacturer's service contract to support this type of equipment not only to ensure its correct operation and function in this way, but also makes sure that the systems are calibrated, recalibrated and checked.

This is demonstrated at Jaguar Cars in West Bromwich, for which Gee designed and installed a dosing system for the addition of highlighter. Jaguar's service records should show that Gee has maintained and supported the system every six months under a contract that has run now for five or six years. This indicates that to have a correctly serviced installation maintains the system operation for many years and therefore enhances its reliability.

Vital as such a maintenance programme is, the health and safety issues raised by selecting the right pumps and dosing equipment that are fully compatible with the chemicals being added are equally important. Key to this is the separate containment of dosing equipment within a self-contained and lockable enclosure.

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