GB2222789A - Paint spraying systems - Google Patents

Abstract

A spray painting system comprises a paint reservoir 16, a feed line 12, which feeds a plurality of spray guns 10, and a return paint line 13 from the spray guns. Reservoir pumping means (which may be a pump 14 or reservoir pressurizing means) feeds paint from the reservoir into the feedline 12. A feedline pump 15 is connected either in the feedline 12 as shown or in the return line 13. This maintains high pressure in the feedline and intermediate pressure in the return line, and paint circulation past the paint guns, without the need for any non-return or back pressure valves. A link 20 may be provided to provide a path for a cleaning pig. The feed line may be formed into a loop with a pump (Fig. 2). The return line may be extended to provide a loop path for a cleaning pig (Fig. 3). <IMAGE>

Description

Paint Sprayirz; Systems The present invention relates to paint spraying systems, and more particularly to the paint circulation paths in such systems.

The spray gun of a standard basic commercial spray painting system is such that it is desirable for there to be a continuous flow of paint through the gun, with only a part of the paint actually being sprayed out by the spray head of the gun, and the rest being returned to the paint reservoir. Such systems frequently also include a plurality of spray guns. Such a system therefore normally comprises a paint reservoir, a pump feeding paint from the reservoir into a feed line which feeds a plurality of spray guns, and a return paint line from the spray guns to the reservoir.

With certain types of paint, it is desirable for the flow rate of the paint through the system (and in particular, through the feed line) to be held at a fairly high value. A development of the standard basic system has therefore been devised in which the feed line is extended past the inlet side of the spray guns to form a by-pass line, which returns a substantial part of the paint flowing in the feed line directly to the reservoir. To maintain the pressure in the feed line at an adequate value, this by-pass line includes a back pressure regulator valve.Such a valve consists esstrltidllL of a ball held against on aperture by a spring or similar device, so that the pressure at the aperture is hrld at the value required to open the valve against the force of the spring.

(This type of valve also acts as a non-return valve.) This development is well known.

However, the passage of certain types of paint through a back pressure regulator valve can cause damage to the paint. This can happen, for example, with m tallic paints (which contain small flakes of metallic material held in suspension). Hence in the standard systems, metallic paint eventually deteriorates, causing shading and discolouration, and in due course has to be discarded arid a fresh batch of paint used.

h further divt:opmrrt has recently been proposed in OB 2 182 867 (Grdco Thz). In this systems, the bypass line feeds a second pump which in turn fees the feed line (which is on the high pressure side of the main (supply) pump).

Thus the bypass line and part of the feed line form a closed loop including the second pump (which operates as a circulating pump). This arrangement reduces paint degradation, since the paint no longer has to pass through a back pressure regulator valve. It also reduces the power consumption of the system, since the paint passing through the bypass line has to have its pressure raised by only a small amount to return it to the pressure in the feed line, with only the fresh paint drawn from the reservoir having to have' its pressure raised by the full value of the feed line pressure.

In all the systems described so far, however, there is a substantial problem which has not yet been discussed. It is normally desirable to maintain the pressure in the return line at somewhat above half the pressure in the feed line; typical pressures are around 800 kPa (120 psi5 in the feed line and 275 kPa (40 psi) in the return line. The pressure in the feed line is obviously generated by the pump or pumps. It is possible to achieve the desired pressure in the return line by means of a suitable constriction or other resistance therein.

However, this is inconvenient for various reasons; for example, a constriction is difficult to adjust and is sensitive to variations in the flow rate (and hence to how many of the spray guns are operative) and the consistency of the paint.

In the standard system, this problem is normally overcome by providing a back pressure regulator valve in the return line. Thus in the standard basic system there is one such valve, and if a bypass line is provided, there are two such valves, one in the bypass line and the other in the return line. This arrangement has, however, the disadvantage that the passage of the paint through the valve can cause paint degradation.

The system proposed by Graco lacks any specific means for controlling or maintaining the pressure in the return line. In fact, in that system the return line is not fed back into the reservoir. Instead, it is fed to a T junction in the feed line on the low pressure side of the main pump, and a simple non-return valve is included in that line between the T junction and the paint reservoir.

It will however be clear that that modification of the basic system has little effect on the pressure in the return line. (In fact, the pressure at the T junction must be slightly negative, because the supply pump has to draw paint through the non-return valve, and there must therefore be a slight pressure drop across that valve.) The insertion of a regulating valve in the return line (or between the T junction and the reservoir) in the Graco system would solve the problem of maintaining and controlling the pressure in that line. However, it is obvious that that would reintroduce the problem of paint degradation.

The main object of the present invention is to provide a system in which the pressure in the return line is effectively .controlled and regulated without the need for the paint to pass through any regulating valves. A further object is to provide a system in which cleaning by means of a cleaning pig is facilitated.

Accordingly the invention provides a spray painting system comprising a paint reservoir, a feed line which feeds a plurality of spray guns, reservoir pumping means feeding paint from the reservoir into the feedline, and a return paint line from the spray guns, the return line being connected to the outlet side of the reservoir pumping means and a feedline pump being connected between the return paint line and the feed line.

Preferably, the system also includes a bypass line which is connected back to the feed line to form a closed loop, and a loop pump included in the closed loop. Preferably the loop pump is connected in the loop downstream of the spray guns and upstream of the entry point of fresh paint to the loop.

A link line may be included which either links the feed and return lines or links the ends of the return line (via a pump in the latter case) to facilitate cleaning of the lines by a pig passed through the lines. In the latter case, the mode of operation of the system may be modified so as to normally maintain circulation in the return line.

The reservoir pumping means comprises either a pump or reservoir pressurizing means.

Various embodiments of the invention will now be described, by way of example, with reference to the drawings, in which: Figs. 1 to 3 are schematic views of the embodiments.

Fig. 1 shows a spray painting system having no bypass line. A plurality of spray guns 10 having spray heads 11 are connected between a feed line 12 and a return line 13. The feed line 12 is fed from a paint reservoir 16 through two pumps 14 and 15 connected in series, and the return line 13 is connected to the junction between the two pumps 14 and 15.

The pump 15 is required to operate with a high flow rate, to maintain a substantial flow round the loop formed by the feed line 12, through the spray guns 10, and back along the return line 13. The pump 14 operates with a somewhat lower flow rate, having to supply the amount of paint used by the spray guns 10 by being sprayed out of the heads 11. Pump 14 has to maintain this replacement flow against the pressure in the return line, which, as mentioned above, is typically .in the region of 275 kPa. The feed line has to be maintained at a pressure in the region of 800 kPa, so the pump 15 has to generate only the difference between these two pressures.It is evident that the only substantial energy losses from the system are those necessarily resulting from the operation of the spray guns 10, i.e. the energy loss of the paint sprayed out by the stray heads 11 and the energy losses of the paint passing through the spray guns 10 from the high pressure feed line 12 to the somewhat lower pressure return line 13.

A valve 17 may be provided in parallel with the pump 14 to permit draining of the system back into the reservoir 16. Obviously further valves can be added at other suitable points for the same purpose. Further such valves and regulating valves (not shown) can also be added to enable the system to revert to conventional operation should either of the pumps fail.

This arrangement could be modified by connecting the pump 15 in the return line 13 rather than in the feed line 12. That would impose a somewhat lower flow rate loading on the pump 15, and a somewhat higher pressure loading on the pump 14.

Fig. 2 shows a spray painting system having a return line. This system is obtained from the system of Fig. 1 by adding a bypass line 18 onto the spray gun end of the feed line 12, inserting a third pump 19 between the pumps 15 and the feed line 12, and connecting the bypass line 18 to the junction of pumps 15 and 19. It will be evident that pumps 14 and 15 operate substantially as before, while pump 19 has to maintain a relatively high flow rate at a very low pressure difference.

This arrangement could be modified in the same way as the Fig. 1 arrangement, and also by connecting the pump 19 in the bypass line 18 rather than in the feed line 12.

In all these. paint spraying systems, it is desirable or necessary for the various lines to be easily cleanable. One convenient way of cleaning a line is to pass a cleaning device (commonly termed a pig) through it, forcing it through the line by pressure behind it. This is facilitated if it can be done by using the paint normally in the system. In the Fig. 2 system, the feed line 12 is part of a closed lQop including also the bypass line 18 and the pump 19, and thus a pig can be circulated round that loop without requiring any modification of the system. However, the return line 13 is not part of a closed loop; and in the Fig. 1 system, no line is part of a closed loop.

To facilitate such pigging in the Fig. 1 system, a link line 20 can be added between the free ends of the feed line 12 and the return line 13, as shown, with valves 21 and 22 which are normally closed to isolate these two ends from each other. When the valves 21 and 22-are opened, the lines 12 and 13 and the pump 15 form a closed loop round which a cleaning pig can be passed.

To facilitate pigging in the Fig. 2 system, the system can be modified as shown in Fig. 3 by the addition of a link line 23 including a pump 24 and joining the free end of the return line 13 to the junction between pumps 14 and 15, as shown. Two valves 25 and 26 normally isolate the link line 23 and pump 24, and are opened to convert the return line 13 into part of a closed loop (completed by the link line 23 and the pump 24), for cleaning by a pig.

The system of Fig. 3 can be operated in another mode, by leaving the valves 25 and 26 normally open and operating the pump 24 continuously. This mode of operation permits the flow rate of the paint in the return pipe 13 to be maintained at a level which prevents stagnation of paint in that line regardless of the amount of paint being used by the spray guns 10 (which affects the amount flowing past the guns and into the return pipe 13). If this mode of operation is adopted, the valves 25 and 26 may be omitted.

These systems can be further modified by removing the pump 14 and replacing it with means for pressurizing the reservoir 16 as an alternative means of forcing paint from the reservoir 17 into the system at the pressure of the return line 13. Also, the details of the layout of the pipework can obviously be changed.

It will be evident that all these arrangements maintain the pressure in the return line high without requiring the use of regulating valves. Further, there is no paint return from the return line into the reservoir. Hence any paint which deteriorates in the course of its circulation round the various lines will be maintained in the various lines, and hence used relatively rapidly. This is preferable to it being returned to the reservoir, since although that would dilute the degraded paint, the degree of degradation could slowly build up throughout the whole system.

Claims (9)

C Iims

1 A spray painting system comprising a paint reservoir, a feed line which feeds a plurality of spray guns, reservoir pumping means feeding paint from the reservoir into the feedline, and a return paint line from the spray guns, the return line being connected to the outlet side of the reservoir pumping means and a feedline pump being connected between the return paint line and the feed line.

2 A spray painting system according to claim 1 wherein the reservoir pumping means is connected to the return paint line side of the feedline pump.

3 A paint spraying system according to either previous claim including a bypass line which is connected back to the feed line to form a closed loop, and a loop pump included in the closed loop.

4 A paint spraying system according to claim 3 wherein the loop pump is connected in the loop downstream of the spray guns and upstream of the entry point of fresh paint to the loop.

5 A paint spraying system according to any previous claim including a link line which links the feed and return lines to facilitate cleaning of the lines by a pig passed through the lines.

6 A paint spraying system according to any previous claim including a link line which links the ends of the return line via a pump to facilitate cleaning of the lines by a pig passed through the lines.

7 A paint spraying system according to claim 6 wherein the mode of operatio; of the system is such as to normally maintain circulation in the return line.

8 A paint spraying system according to any previous claim, wherein the reservoir pumping means comprises a pump.

9 A paint spraying system according to any one of claims 1 to 6, wherein the reservoir pumping means comprises reservoir pressurizing means.

1G A paint spraying system substantially as herein described and illustrated.