New painting processes using robots require a short tube between the paint pressure regulator and the consumer resp. spray head, and are switched frequently and with sharp edges. To avoid or solve rare and hard problems associated with paint pressure regulators one should know about the following.
In contrast to air pressure regulators, paint pressure regulators must be easy to rinse for frequently changing workwear materials (paint change). Complicated paint pressure regulators with separate feedback channel (bypass regulators) are not good flushable. Therefore, paint pressure regulators are realized as main-stream regulators. However, these can only feed-back and regulate if they are actively flown through AND, if pressure builds up under their membrane! Otherwise they are functioning as throddle. These kind of regulators are also called flow-through regulators.
Inexperienced employees sometimes make the mistake of "setting" mechanically adjustable paint pressure regulators without active flow, even with the valve closed at the consumer or spray head. The regulator and the pressure gauge react as expected when increasing the pressure, but usually not on reducing the pressure (paint pressure regulator can not blow off). Only when painting, the "inexplicable" result of a setting without active flow manifests itself. Pneumatically controllable paint pressure regulators are due to the superimposed electronic regulation less problematic than the mechanically adjustable paint pressure regulators. APSON strongly recommends training.
The simple design of lacquer pressure regulators, hereafter referred to simply as (varnish or paint) regulators, is all too easy deceiving about their true functioning. Regulators from APSON are robust and mature. They are especially suitable for painting applications in the automotive industry. Proper choosing and application of regulators as well as avoidance resp. solving of eventual problems requires some knowledge about regulated sections (streches) and regulators, especially in the case of mechanically adjustable regulators. For stable regulation, the regulation strech is to be planned in such a way, that it should not be able to form more than two effective energy storages.
Elements of a regulated section of a varnishing application are hoses, atomizer, switching blocks and valves. A: Hoses that are not layed out as spools, and in which working material is flowing not to quick, are inductive energy storages with low energy content. The effects of such storages on a strech, is regularily negligible. B: Hoses and riser tubes with gas-bubbles under pressure, and spaces with movable membrane (diaphragm) both sides of which are loaded with forces, are capacitive energy storages. Effects of this kind on streches are not negligible. The count of these relevant and effective storages determine the real behaviour of a regulated strech and the effort to realize a good regulation.
The leap response of a lacquer pressure regulator shows in which kind the regulated magnitude reacts to a sudden change in the pressure conditions at its inlet and/or at its outlet – because of the near 100% feedback. The flow and pressure of a liquid in a regulated strech without gas-bubbles propagate without delay. The regulated magnitude thus changes proportionally (P) and without delay with the control magnitude.
Unproblematic are streches with linear behavior and with only one storage – the lacquer space under the diaphragm in the regulator. A storage reacts continuously, but the regulated magnitude follows the control magnitude slightly delayed in time (T). The strech then has P and T characteristic. It behaves like a P-T1 strech (T1 = 1 storage) and requires a matching lacquer pressure regulator to operate stably. Applications with multiple effective energy storages are more likely to cause chaotic oscillations or other problems. Furthermore, streches with non-linear behavior have at each operating point an other dynamic behavior of the controlled magnitude.
Regulators differ fundamentally from control devices in that regulators are back-coupled (feedback) from the output to the input. Extensive literature can be found on the Web. A regulation will be determined mainly by the PID parameters of the regulator. Parameters are by-values for influencing the kind and manner of a regulation. The proportional by-value P stands for the static strength of the feedback. The integral by-value I and the differential by-value D, both determine the dynamic behaviour of a regulator in cases of changing conditions at the inlet and at the outlet!
Electronic regulators, for example, have manifest feedback through dedicated modules resp. channels. Normally, all three parameters are separately adjustable. Lacquer pressure regulators are PI regulators (they have no D component). Furthermore, they must be rinsable and have therefore no feed back through dedicated channels. They are main-current regulators and feed back directly through the working material flow. They are very normal adjustable, but the "parameters" are final (fixed by the physical structure and geometry). Especially in case of mechanically adjustable regulators PRM-xxx, their working range should cover the required working range of the application.
Fig. 1: Lacquer Regulators – Structures, Function-Schemes, Diagramms
There are two kinds of regulators from APSON. The pneumatically controlled regulators PRP-xxx are controlled by compressed air P. The mechanically adjustable regulators PRM-xxx are adjusted by adjusting nut N and compression spring F. These can be equipped with a pressure gauge at the outlet. For each regulator kind there are several types. Lacquer pressure regulators open whenever the force across the membrane M is greater than the force under it. Then the membrane pushes by thrust pin S on the ball K, and opens the throttle between inlet and outlet. But, lacquer pressure regulators start only then to regulate, when under the membrane – while working material flows through – pressure builds up! Only then are these regulators meaningful adjustable resp. readable. Because lacquer pressure regulators feed back directly through the material stream, these have close to 100% feedback and react strong to switching operations and pressure swings at the output!
The "parameters" of the regulator types from APSON stem from long-term experience and are fixed in the type design (this is physically not bypassable)! The regulator types are designed so, that their respective working range is as large as possible. For choosing of an optimal mechanically adjustable type PRM-xxx, APSON recommands real simulations with switching valves, under conditions of production. The choosing of an optimal pneumatically controllable PRP-xxx type is less critical, because this type of regulator will be integrated in a super-ordinate regulation system (see Fig. 2).
Important: A regulator type should be chosen so, that the working range of the regulator "covers" as far as possible the required working range of the application. All valves for rinsing should be equipped with backstroke valves. For "simultaneous" switching of several valves, their inertia must be considered. Per each regulator one should provide only one consumer. With more consumers, all consumer strands must have the same impedance. They may be commuted only pair-wise. Maybe a training by APSON makes sense. The pressure at the input should be as smooth as possible, and be not much larger than the required pressure at the output. At the O-ring in the throttle, then less pressure is to be taken down. Then the O-ring suffers less cavitation and lasts longer. The lifespan of the membrane depends strongly on how much it is stressed. Therefore, it should be checked pro-actively at least every 6 months and exchanged if excessive bulging occurs.
Fig. 2: Typical application with PRP-xxx and PRM-xxx
The "parameters" of pneumatically controllable regulators PRP-xxx are quasi changeable – by means of a superimposed electronic regulation system. This then behaves similar to an electronic regulator. It has a total feedback per sensor S (for pressure or flow-through rate), from the end of the regulation section. A system of this kind requires additionally an electronic regulator (PC or PLC), an electro/pneumatic proportional valve (also a regulator!), and the sensor. The PC measures and computes the regulation magnitude y(t) and controls the regulator, as superimposed regulation loop, via the proportional valve. Such systems are intended for applications in the lacquer realm. They are more elaborate, and less sensitive to the effective disturbance d(t) on their regulation section. For demanding applications with precise dosing, after the PRP-xxx regulator, should be provided a gear pump DP with stepper motor SM. The regulated section for lacquer then ends at the inlet of the gear pump. Then the regulator works as pressure reducer and as pump protection. For the concrete case, eventually some components, for example the sensor, will be eliminated, and other components will be added. For questions, we are happy to help.
The "parameters" of mechanically adjustable regulators PRM-xxx are not changeable. Therefore this regulator types are to be specially chosen, see further above. Their behaviour depend heavily on the dynamic behavior of their outgoing drive path (hoses, valves and atomizer). This regulator types are forseen for less critical applications in the solvent realm. In case of problems, the outlet of a regulator is to be decoupled, for example with a hose with adequate internal diameter. If the pointer hand at the gauge of a regulator is fluttering heavy, and this disturbs, then a damped pressure gauge is recommended. For solving of tenacious problems, pressure gauges may be mounted close to the switching block SB. Thus, the true pressure values which affect the atomizer get observable and the behavior of the application becomes easier to understand.
For special applications, APSON, with sufficient time, can develop, test and deliver an optimal regulator type.