Dosing

Features Characterizing a Good-Quality Dispensing System

1.    Requisites
The following requisites are to be considered a must when planning a purchase and then setting up a good-quality dispensing system:
•    Reliability
•    Speed
•    Precision
Reliability accounts for the first and foremost important characteristic to be met by an automatic  dispensing system since, by choosing to utilize an automatic distribution for chemicals within the plant, therefore eliminating the manual operations of weighing, transporting and feeding chemicals into the machines, it is no longer necessary to employ man-hours previously spent to this purpose. As a natural consequence, down-grading the number of people employed in the production department is quite natural.
While this is certainly an extremely positive side-effect for overall running costs, it must be considered that, should the automatic dispensing system ever stop to work, the global production of the plant would dramatically suffer, since the remaining manpower in the department could not possibly keep up manually with the expected working schedule. This is why it is important to work with something you can definitely count on.
Speed is yet another important quality to be sought after when choosing an automatic dispensing system. In fact, it must not delay or add unexpected waiting time periods to the production schedule, but rather it should only maximize the plant production output.
It is therefore important to consider that both the lay-out and the mechanical characteristics of the automatic dispensing system will play a significant role in obtaining the best possible results.
Precision is very important as well:  an automatic dispensing system will further improve recovery of investment costs if it makes you save on chemicals to be dispensed – normally a big expense in the budget of any dye-house.
By experience we can safely affirm that a good-quality automatic dispensing system accounts for 15% savings on the total purchase of chemicals.  A considerable amount, indeed, that will not be actually recovered if the following conditions are not met.

2.    Analysis of a Dispensing  System: Mono-pipe vs Multi-pipe Systems
The two broad classes of dosing systems present on the market today are the mono-pipe and the multi-pipe systems.
Mono-pipe systems are those which, after a centralized product selection, metering and pumping unit,  develop the distribution network along a central back-bone pipe to which each machine is connected by means of a three-way diverter valve.
Multi-pipe systems are those which, after a centralized product selection, metering and pumping unit, develop the distribution network  through a series of pipes, each of which is dedicated to a single machine.
In a multi-pipe system the increased complexity of the distribution network does not, in fact, bring about any significant advantage to the user. Being in any case limited to one pump, all operations of dosing have to be serially sequenced in exactly the same way as in a mono-pipe system. This implies that no speed advantage can be noticed.
Furthermore, while some manufacturers claim a speed improvement due to separate dosing/water washing actions,  little attention is paid to the fact that using line-pressure rather than pump pressure-head for washing is a much less efficient system. Normally, water-line pressure is around 3 bars while a good dispensing pump can deliver at 9 bars. Thus, water speed in the piping is much higher and the washing efficiency is far better in a mono-pipe system, so less water is transferred to the machines or discharged: less waste and quicker operation.
Other manufacturers claim that air can be used as a displacement medium to move liquids through a pipe: consider blowing air in a pipe containing soap and imagine what happens…
On the other hand,  a mono-pipe system is far less expensive to build: as an example, take a dye-house in which 30 machines are mounted into two different rows in a 20-meter wide and 100-meter long building and where the dispensing system can be placed on either side of the two rows.
To serve all machines a mono-pipe system will only need a  220-meter  long stainless-steel piping of small section, whereas a multi-pipe system needs to be connecting  the dispensing system to each individual machine and the resulting total length of piping to be laid is approximately 1,800 meter-long (just for the horizontal pipes, excluding vertical stretches). Considering that PVC piping has to be supported at least every 3 meters, in the latter case  600 supporting brackets need to be set: quite an expensive operation, indeed.
And these setting –up expenses are totally left to the Customer, not ever being mentioned in the offer price, so beware of your total final costs!
Pozzi has focused on mono-pipe systems since the very beginning of their involvement in dispensing systems over 25 years ago.
The following considerations are about the possible technical strategies to be adopted when designing the lay-out of an efficient automatic mono-pipe dispensing system for liquid chemicals. We shall examine the consequences of such strategies on the performance of the final chemical, following a comparison between POZZI Leopoldo Srl DOSATEX automatic dispensing system and other similar equipments available on the market worldwide.
 
 
2.1    Flow-meters
Inductive magnetic flow-meter (normally used with low-cost dispensing systems)
 
Picture 1:  Example of magnetic flow-meter
Such measuring sensor is normally adopted in low-cost automatic dispensing systems because it has the great advantage of being very inexpensive and rather reliable.
Under the typical working conditions of a dispensing system for liquid chemicals, anyway, it does not turn out to be a good choice because a great number of different chemicals (ranging from acidic, alkaline, ionic tension-active etc.) pass, one after another, through this type of measuring sensors.
In fact, in a mono-piping system the chemical is pushed by water, therefore creating a separating area in which the chemical to be dispensed mixes for a brief time with the pushing water; during the next dispensing operation the water in the piping creates another separating area with the next chemical, and so on.
Separating areas within the magnetic flow-meters will create polarization problems, resulting in hardly correct measurements, the net result being a fairly precise measurement of the chemical and of water, but also an unreliable measurement of the separating areas.
It clearly follows that the closer to each other the separating areas are, the more relevant their   percentage incidence is when measuring the total amount of chemical to be dispensed, therefore lower the precision of the flow-meter.  A direct consequence is that dispensing small quantities will be far less reliable than dispensing large quantities.
Another considerable limitation of magnetic flow-meters lies in the very same technology they utilize: the chemical to be dispensed must be conductive; otherwise it is impossible to measure its speed. The drawback in this case is that you have to restrict the range of chemicals to dispense, so that all of them can be correctly measured.
A further problem is given by the measured unit, i.e. the voltage generated by a conductor traversing orthogonally a magnetic field, since such voltage is extremely small and proportional to the speed of the passing fluid. As a consequence, it is practically impossible to use this kind of flow-meter for both very large and very small quantities to be dispensed, without altering its geometry. In fact, whereas for large quantities of chemical to be dispensed you need piping with a big diameter, you will not be able to use the same big-diameter piping with small quantities in order to obtain the necessary minimum speed (2 m/sec) for a readable signal by the flow-meter since the time for passing through the meter would result lower than the mechanical opening and closing time of the valves necessary to control it.

The Corioli’s Effect-based Mass Flow-Meter Adopted by POZZI DOSATEX
 
Picture 2:  Inside of a Corioli's mass meter
Contrary to an inductive measuring system, a mass flow-meter is not based on the speed of the fluid passing through it, but rather it weights its mass as it goes through.
The great advantage of this ability is self-explanatory: the measurement does not depend on the density of the fluid. In order to better understand this concept, let’s assume that we want to measure caustic soda in the summertime with a 30°C temperature and in the winter with a 10°C temperature. Thermal expansion, which is natural in any fluid, involves in this case an approximate 5% variation in the volume of the chemical to be measured, therefore resulting in a measurement with a 5% error when using an inductive flow-meter.
No such error is possible when correctly using a mass flow-meter, since the weight of the chemical to be dispensed will be the same even if its volume changes due to the different outside temperature.
IT SOUNDS THEREFORE QUITE BIZARRE THAT SOME MANUFACTURERS BOAST UP TO 1% PRECISION FROM THEIR DISPENSING SYSTEMS, WHEN A SIMPLE WEATHER TEMPERATURE CHANGE NATURALLY INVOLVES A 5% ERROR, EVEN ASSUMING THAT THE INSTRUMENT ITSELF IS PERFECT AND DOES NOT INTRODUCE ANY SYSTEM ERROR.
Another considerable advantage of a Corioli’s effect-based flow-meter is that, not depending on the speed of the fluid passing through, it makes it possible to very accurately measure also small quantities of chemical to be dispensed.
Furthermore, its measuring ability does not rely on the conductivity of the measured fluid; therefore, it can be used with all kinds of chemicals to be dispensed.
Another advantage is to be found in the fact that this type of flow-meter can give you a direct reading of the density of the measured fluid and this particular ability turns out to be extremely useful if you need to control the quality of purchased chemicals.
NOTE: It must be remembered that this instrument may be used under many different configurations depending on the adopted controlling software; in fact, when used merely as a replacement for the magnetic flow-meter, it will just perform as such, without adding those other interesting and useful density control characteristics that make the POZZI Corioli’s effect-based flow-meter a better system.
This kind of instrument is extremely reliable. It is normally used in oil and pharmaceutical industry.
Its only defect lies in its price: it costs 4 to 5 times more than a magnetic flow-meter.

2.2 Valves needed for the selection of chemicals

Chemicals connected to a dispensing system are selected for dispensing by valves, which may be quite different from each other, but for the characteristic of being, in most cases, three-way type.
The reason behind this choice is that the third way is normally used as a free passage way that allows for the creation of a collector to transfer the fluid into the other valves, once they are mounted and aligned one after the other. This collector does not cost anything because the very same valves physically create it.
 Picture 3:  Competitors’ plastic dosing-valve assembly
POZZI technical choices for DOSATEX have been altogether different: in favor of reliability and safety, we selected the safest and most reliable valves on the market for our dispensing systems.
A two-way valve is in itself safer and more reliable than a three-way one, since it is much simpler as for construction and ampler seal allocation, resulting in the long term a far better performer as for resistance to wear-and-tear.
Clearly, with two-way valves we had to add a collector as another (normally not foreseen) component of the chemical selection unit.
This brings forth another important choice: as we have seen, in other dispensing systems the three-way valves create the collector, which becomes a very long passage way among a big number of valves individually connected to a different chemical. On the contrary, in POZZI DOSATEX  this collector is split up in several different units, each supporting 10 chemicals at most and each more definitely separated from the others by another valve.
 Picture 4:  POZZI stainless-steel dosing-valve assembly
This kind of architecture allows for a number of barriers considerably higher to avoid the possibility of cross pollution,  since basically there is no opportunity for any two chemicals to mix even in the case of a leaking valve.
Such added safety feature does not belong to any other kind of dispensing system: in fact, in any three-way-valve-based system, should one of the chemical selection valve leak during a dosing operation, the different chemicals would mix. This mixture, in turn, may result in a serious danger for both the dyeing process and, above all, safety, since non-compatible chemicals might even become explosive.
 
 
2.3    Valve Control System
Like any other competitor, POZZI did not invent unbreakable valves. Every mechanical part, in fact, normally undergoes wear-and-tear and ultimately breakage.
A complex dispensing system may have hundreds of valves, distributed over a large served operating area.  Once the average lifetime of a valve has elapsed, this great number of potentially critical elements inevitably brings forth a conspicuous maintenance effort. Maintenance means a lot of wasted time for the entire system, translating into high operational costs.
POZZI solution for this kind of problem is a special sensor connected to every valve, which checks its closing, its opening, and, especially, investigates the way the valve moves. Such modality is, in fact, a good indicator of the present condition of the valve.
Picture 5:  POZZI valve sensors
By continuous monitoring the valve modality of opening and closing it is possible to fairly well estimate its remaining lifetime, avoiding the total stop of the production process. In other words, the DOSATEX software is able to automatically signal which valve in the whole system needs maintenance before it becomes a more serious and urgent a problem. Planned maintenance interventions mean practically zero down-time for the plant, this representing a huge benefit for the overall production efficiency and costs. Obviously, hundreds of sensors distributed on the entire system mean a higher expense, if compared with a Competitor’s system, but the enormous advantage deriving from them is so relevant to make you totally recover the initial investment.

2.4    Pumping Unit

The pump adopted by POZZI DOSATEX system offers several major characteristics: volumetric lobes pumping technology, fluxed mechanical seals, high power: in other words, the best on the market.
 Picture 6:  Lobe pump
A good-quality dispensing system can be considered such depending on its speed serving the various connected machines, that is on how quickly and precisely it can transfer chemicals along the distribution piping.  A low-efficiency or low-power pump, in fact, may very well undermine the production process of the entire plant.
The inverter-controlled lobe pump of POZZI DOSATEX is connected to a 5.5 kW motor and with a nominal transfer capacity of over 100 l/min with a max pressure of 9 Bars it has all the necessary power to push the chemical through the piping.
Meaning to save money, Competitors use pumps with only about 3kW power, which indeed results in a lower initial price for the Customer, but also in longer dispensing time periods that delay the overall dyeing production process of the entire plant.

2.5    Signaling Distribution Network

The electrical interconnection of valves distributed on a vast area may mean a relevant cost, which normally may be underestimated at the time of purchasing a dispensing system, but it will prove considerable when the Customer decides to implement it.
Each valve, in fact, needs to be connected to the central system with electrical signals that control its closing and opening, besides other signals indicating its working modality (open valve, closed valve, opening time, closing time): basically, thousands (mostly remote) connections may cause reliability problems and high costs.
Unlike many Competitors’ dispensing systems, POZZI DOSATEX utilizes a TCP/IP network, by all means identical to a computer network normally used everywhere. A single network cable connects a central point with each and every control subsystem cluster located close to the valves to be checked.
The advantage is obvious:  very few wires to be pulled,  great connection reliability,  highest control ability, low-cost installing.
Once again, the only disadvantage of such distribution network lies in its price, since each control subsystem must come with its own PLC. This initial higher price will be recovered very soon, since the wiring necessary to install it is far simpler and less expensive than with any other system.

2.6    Remote Control of the System

Last, but not least, each PLC subsystem is easily reachable by a simple Internet connection applied to the central control computer, this being an enormous advantage for the dispensing system and, ultimately, for the plant.
POZZI technicians are, in fact, able to connect with the Customer’s dispensing system ANYWHERE they are, guaranteeing the maximum possible assistance any Customer might ever wish for, i.e. they can provide the remote reconfiguration of any instrument connected to the system.
We know by experience that 98% problems of a complex dispensing system can be solved this way, meaning once again a very low expense for the Customer, since no technician is needed to physically reach the plant site. POZZI offers contracts for remote assistance that considerably lower the total cost of ownership of the plant.
 
 
3.    Conclusions
Nowadays, the market certainly offers a wide variety of dispensing systems and with it also the range of prices, depending – as we have just seen – on the utilized components and on the system architecture.
It is obvious that it is not possible to select one dispensing system simply on the basis of its price, but rather it is important to take into account all future, possibly even hidden expenditures that might derive from, first of all, mounting the system (it is important to remember that all necessary piping to connect valves is included in POZZI DOSATEX offer) and, eventually, from the low efficiency due to the frequent down-time periods generated by inadequate technology and/or use of cheaper components.

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