FR2522539A1 - PROCESS FOR THE PROTECTION OF A POWDER COATING FACILITY AGAINST EXPLOSION HAZARD - Google Patents

Abstract

A.PROCEDE FOR PROTECTING A POWDER COATING PLANT AGAINST THE RISK OF EXPLOSION. B. PROCESS CHARACTERIZED IN THAT THE BIRTH OF AN ELECTRIC ARC OF INADMISSIALLY HIGH ENERGY BETWEEN, ON THE ONE HAND, THE PART TO BE TURNED IN 18 AND OR ITS SUPPORT DEVICE 19 AND ON THE OTHER HAND, A CONNECTION TO EARTH 22 IS INTERRUPTED BY THE APPLICATION OF A HIGH CONTROL VOLTAGE 24 BETWEEN THE PART TO BE COVERED AND EARTH 25, ANY PRE-EXISTING INSULATIONS BEING THEN BROKEN BY A DISRUPTIVE ELECTRICAL DISRUPTION OF LOWER ENERGY, OR IN THE OTHER IN CASE, THE POWDER COATING INSTALLATION 10 IS SWITCHED OFF WHEN A PREDETERMINED MINIMUM CONTROL CURRENT 28 IS CROSSED DUE TO TOO HIGH CONTACT RESISTANCE. C. THE INVENTION APPLIES IN PARTICULAR TO PLACEMENTS FOR LAYING METAL PARTS.

Description

i -

  "Process for the protection of a coating plant

  by powder against the risk of explosion ".

  The invention starts from a process for the pro-

  tection of a powder coating plant under

  high voltage against the risk of explosion due to

  charging loads for charging electrical parts to be coated, these parts being capable of being connected with

  limited contact resistance, preferably by

  intermediate of an endlessly circulating support device,

with a connection to the ground.

  Such methods known as "earthing control" usually operate in such a way that the electrical resistance between a room to

  coat and the supporting device carrying it, either

  safety with a lower DC voltage, resistance

  contact must not then exceed a value of 1 M Ohm.

  In the case of a higher contact resistance, there is the risk that the capacity constituted by the part

  to be insulated in conjunction with the high voltage

  plicated during powder coating, accumulates enough

  energy for lighting during a discharge, the mixture of powder and air to be sprayed In particular in the case of small pieces of work, it frequently proves impossible to fulfill the conditions for maintaining this

  imposed contact resistance Due to the relative weight

  reduced parts and unavoidable training when

  powder coating of thin insulating films on the

  positively maintained, it is not certain to maintain the contact resistance

  below 1 M Ohm, which results in frequent stops

  quents of the installation. It is moreover known that electric arc discharges with an energy of less than 5 M-W / S do not

  are not dangerous with regard to the risk of explo-

  in such powder coating installations.

  Arcs of this type can not ignite the cloud of powder and are, therefore, safe, if

  although even in the case where the contact resistance

  surée with a lower voltage is above a

  Ohmr, the installation does not have to be obligatorily

rôtée.

  The purpose of the installation is to create a procedure

  protection die escaping these disadvantages and concerns for this purpose, a method characterized in that the birth of an electric arc of an inadmissibly high energy between, on the one hand, the part to be coated and / or its support device and, on the other hand, a connection to ground, is interrupted by the application of a high voltage of

  control between the part to be coated and the mass, isolates

  any pre-existing conditions being broken

  by a disruptive electric discharge of energy more

  in the other case, the installation of

  by powder being switched off at the time of crossing

  from a predetermined minimum control current d C to a

  contact resistance too high.

  The process according to the invention defined above

  above the measurement of the contact resistance between the part to be coated and its support device, the advantage that the hazardous insulation layers on the support device, can be distinguished from

  non-hazardous insulation layers without stopping the installation

  3.- lation Thanks to the proposed provisions, it is certain, in a simple and economical way, that the small residues of insulation between the part to be coated and the device of

  medium, are burned out by an electric arc.

  With a predetermined limited energy When the part to be coated is then subjected to the high voltage applied during the powder coating, no high-energy electric arc can also intervene, so that,

  on the one hand, the risk of explosion is excluded, and that, from

  on the other hand, it is ensured of a continuous operation, although, from the pure point of view of the measurement technique, an inadmissibly high contact resistance could have been observed The installation is stopped only when crossing to the bottom of a minimum current of control

  predetermined, because the crossing down of this cou-

  rant indicates the existence of insulation layers

  inadmissibly important that can not be

  burned with the control high voltage

cated.

  Other features of the invention

  consider other advantageous forms and improvements of the process indicated above. The use as a control high voltage of a DC voltage which is preferably polarized in the same way as

  the continuous high voltage applied to the spray gun

  used, has proved particularly successful thanks to the use of DC voltage in both cases,

  and, in particular, the use of a single pola-

  it is ensured that the electric arc involved in the powder coating plant can not

  to release a higher energy than an inter-

  possibly coming into the control phase In this

  In this case, it is advantageous to use both

  In the case of the control, a negative DC voltage is required because such a voltage, because of known 4-physics processes, is easier to control from the point of view of insulation than positive DC voltage.

  where insulation distances are more important than

  should be maintained In the case of

  usually occurring between the insulated hanging part and the support device, DC control voltages between 1 and 5 KV have been proven, although control voltages are

  higher are eligible without the energy of

  critical light for the powder-air mixture is reached.

  The invention will be explained in more detail with reference to an exemplary embodiment shown on

the single figure attached.

  In this figure, a coating chamber is designated by 10, in which chamber is made a

  powder coating with a spray gun 11.

  Reference numeral 12 denotes the entry of the powder into the pis-

  projection tolet he and the reference 13 the entry of

  the air in this gun Inside the pistol

  jection 11, is disposed a high voltage electrode 14,

  which is applied from a source of

  continuous flow 15, a negative DC voltage of about 100 kV A cloud of coating powder and air escapes to the output 16 of the spray gun 11, the powder particles being then negatively charged

  because of the high negative DC voltage applied.

  These powder particles are designated 17.

  The parts to be coated in room 10 are

  with the reference number 18 and, like the other

  parts of the installation are shown schematically.

  It may be in this case, small parts, such as

  reflectors of headlights or more important parts

  for example, cabinets of refrigerators or similar household appliances The parts 18 to be coated are suspended from support devices 19, which are 5.-

  in turn attached to a transport chain 20 that

  Place in the direction of the indicated arrow Both the parts to be coated 18 that the support devices 19 and the transport chat 20 are made of electrically conductive material, in particular metal The current circuit is closed on a ground connection 22

  via a sliding contact 21.

  Via another contact

  23, a high control voltage, also negative, is applied to the parts to be coated 18 before their entry

  in the coating chamber 10, this control voltage

  the level of which can reach 20 kV but which is preferably below 5 kV being applied between the sliding contact 23 and a ground connection 25 The control high voltage 24 is obtained in a control device 26 which has an internal resistance

  predetermined 27 to limit the current, an organ of

  current 28 and a DC voltage source 29.

  The output of the control device 26 is connected by the

  connecting conductors 30 and 31 to a device

  switch 32 which allows the user to stop

coating.

  The device which has just been described

  As follows: Before the parts to be coated 18 penetrate into the coating chamber 10, a negative control high voltage of, for example, 1.5 kV is applied thereto via the sliding contact 23. maximum current permissible for safety reasons of 10 m A, a contact resistance of 150 k Ohm is established, which is far below

  the maximum permissible contact resistance of 1 Mohm.

  As a lower limit value for the control current

  likely to be detected by the measuring device of

  current 28, it has been fixed in the present example of reali-

6.-

  sation, a value of 5 m A, which corresponds to a resistance

  300 K Ohm when this current value

  control is crossed down, inadequate insulation

  In this case, the switching device 32 switches off the coating device 18 and the carrier 19, or the transport chain 20, is signaled. too low a control current and therefore too much contact resistance

  between the part to be coated 18 and its metallic support

  that too much energy would be accumulated before

  mage of the electric arc during the subsequent application of the continuous coating voltage of for example 100 kV, because of the capacity of the part to be coated, this energy can then be above the energy of ignition of the projected coating powder The capacities measured during the intervention of an insulating layer between the

  parts to be coated 18 and the support device 19,

  at around 20 to 80 p F about this capacity of the applied voltage, an energy is established

  released in the electric arc, up to 1/2 CU 2 Cet-

  the capacity is valid for the various parts which are usually lacquered after the application of high voltage powder coating process. This process operates in a known manner in that the powder-laden particles adhere to the metal part 18 and are, during a subsequent process, melted and cured at temperatures of 1200 C to 1800 C to constitute a

lacquer layer.

  On the basis of a maximum permissible energy of the electric arc of 5 m W, it is established for a capacity of the pieces of work of 20 to 80 p F, according to the dimension of

  these pieces of work, a maximum control voltage ad-

  missible 24 from 10 to 20 kV For additional security

  However, the control voltage is limited to a value of 1 to 5 kV, preferably a value of 1.5 to

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  7.- 3 k V As soon as this control voltage is applied, the

  thin layers existing between rooms 18 and the

  support 19 and possibly also between the

  19 and the transport chain 20 are eliminated.

  by the burners, so that it is not necessary to stop the installation despite the original insulation layer. In the actual coating chamber, higher electric arc energies are not required. can not intervene when applying a high voltage

  also negative, so that the ignition of the mixture can

  air, and hence the risk of explosion, are excluded.

  process according to the invention is insensitive to pollutant

  and ensure that the electrical charging of

  these to be coated 18 in the coating chamber 10, remains below the predetermined value, although in the case of a pure measurement of resistance under DC voltage, an inadmissibly high insulation would have been obtained, insulation which in the case of art control process

  earlier, would have resulted in the shutdown of the

lation.

  In the embodiment which has just been described, a negative DC voltage has been used both for the coating process and for the control process. The use of DC voltage and in the same direction ensures that when subsequent intervention of a DC voltage of the same level, the contact to the ground 22

  is ensured, so that no electric arc of higher energy

  than that which has been predetermined, can not be

  kill Taking into account the isolation conditions of others

  However, we could use instead of

  Negative control voltage 24, also a positive control voltage. It is also possible to

  implement an alternating voltage as a voltage

  24, in which case, however, for reasons of

  In the case of safety sounds, the current passing through the internal resistor 27 must be limited to a value of less than 4 m. This would correspond for a control voltage of again 1.5 kV to an internal resistance of 375 k Ohm. and a contact resistance of not more than 375 k Ohm when the installation is shut down for currents 4 2 m A. 9.-

Claims (4)

R E V E N D I C A T IO N S   1 Method for protecting a plant   high-voltage powder coating against the risk of explosion due to discharges due to electrical loading of the parts to be coated, these parts being capable of being connected with a limited contact resistance, preferably by intermediary   endless circulating support device, with a connection   to the mass, characterized in that the birth of   electrical arc of an inadmissibly high energy between, on the one hand, the part to be coated (18) and / or   its support device (19) and, on the other hand, a connection   ground connection (22) is interrupted by the application   a high control voltage (24) between the part to be coated and the mass (25), possibly isolations   pre-existing are then broken by an electrical discharge.   in the other case, the powder coating plant (10) being switched off when a predetermined minimum control current (28) is crossed out at a resistor contact too high.   2. The process according to claim 1, wherein   characterized in that, depending on the insulation conditions between the part to be coated (18) and the neighboring parts (19, 20) grounded (22), a high control voltage (24) is applied which in the case of the disruption of an existing insulation or a discharge to a grounded part, limits the energy of the electric arc to a value of less than 5 m W.   3. Process according to claim 2, characterized   characterized in that the control high voltage (24) is a DC voltage of less than 20 kV, preferably in the range of 1 to 5 kV,   Process according to any one of