Auto Dosing Equipment

In life in general if something is under control it performs well. This is no different with metal pretreatment process solutions, as the better they are controlled the more consistent results.

In the past when the majority of cleaners and Light Weight Iron Phosphates were powder chemicals automatic dosing was the exception rather than the rule. However, in recent years the trend has been to liquid chemical concentrates which has resulted in the increased use of automatic dosing equipment.

Speaking from personal experience having introduced automatic dosing equipment into many companies we have not had one single complaint. In fact most of our customers consider it to be the best thing since the proverbial sliced bread.

Automatic dosing can be effected in many ways from a simple dosing pump to a fully integrated computerised system. These typical applications are described as follows.

TIMED DOSING

As the name implies this is done by using dosing pumps which doses chemical on a predetermined time scale. The solution strengths are regularly checked and the timers adjusted as required.

ADJUSTABLE DOSING

This is done by installing an adjustable dosing pump. The dosing pump is set to administer chemical at a set rate. When the solution strength is checked, this rate can be altered by adjusting the speed control on the pump to cater for any change.

PROPORTIONAL DOSING

On solutions such as rust inhibitors which do not show a significant pH or conductivity change a pump can be fitted to the water feed supply which will add chemical at a predetermined ratio.

On the above installations there is the regular use of manual input to make the adjustments and solution strength variations. However, this is better than chemical checks alone and manual additions as small regular chemicals additions are always better than one large shock addition.

Moving on from this are control systems that are fully automated which require the use of conductivity or pH sensors to be fitted in the process solution. Process solutions on pretreatment plants are normally controlled using conductivity; however, in some instances pH control is required.

CONDUCTIVITY CONTROL

This method of control measures the electrical resistance of the process solution and is known as or electrodeless conductivity.

Conductivity ControlThe system operates by immersing the dip cell in the process solution, which in turn sends a signal to the electrodeless conductivity controller. When the solution strength falls below the predetermined set point the dosing pump is activated and supplies chemical until the process is back up to strength. The control is very accurate and a digital display of the strength is shown on the controller which operates around a narrow range above and below the set point, known as the hysteresis.

When used for rinse control, the units are set on a lower scale and when the set point is reached, this activates a solenoid valve which supplies the fresh water.

Electrodeless conductivity control is well practiced and proven within the pretreatment industry proving to be extremely reliable in service. The instrumentation is sound and the dip cells very robust, which after the initial calibration require little maintenance apart from regular cleaning of the contact area.

The units can be supplied as stand alone or integrated in a fully controlled computer programme.

pH CONTROL

This method is used on certain installations where the process solution does not shown a conductivity change or where a visual display of the pH is required on a rinse overflowing to foul sewer. Historically it has been used on effluent treatment plants which rely on close pH control to drop out heavy metals and ensure the discharges to foul sewer are in the desired range.

PH ControlThis method operates in the same manner as conductivity control, whereby, the dip cell is immersed in the process solution to monitor the solution strength. This obviously reads the pH of the bath and activates the pump or solenoid valve in the case of rinse control when the set point is reached.

This method although reliable is not as robust in use as conductivity control. The dip cells are usually glass and are very delicate. The dip cells require calibrating on a regular basis and have a limited life.

So in summary conductivity control is the preferred choice and pH control should only be used when the application demands its use.

With all chemical solutions regular titration testing should be done regardless of the use of automatic dosing equipment. However, there is a case for a reduced checking frequency.
Automatic control and dosing facilitates supply in larger containers, eliminates handling, reduces chemical and water costs and above all gives accurate solution control with consistent results. The overall merits of installing automatic dosing equipment can be summarised as follows:-

  1. Allows supply in larger containers such as 200 litre drums and 1000 litre IBC'S with reduction in chemical prices for bulk supply.
  2. Eliminates the disposal costs of 25 litre polypropylene drums thus improving the carbon foot print.
  3. Eliminates the lifting of 25 litre drums and inherent health and safety implications.
  4. Reduces the handling of hazardous chemicals.
  5. Supplies in larger containers reduce ordering and invoicing frequencies.
  6. Gives a content accurate solution strength control.
  7. Provides a digital display of the solution strength.
  8. Accurate control reduces chemical costs.
  9. Rinse water control gives cost savings.
  10. Reduces the number of chemical control checks.

At B C Metal Finishing we have been at the forefront of automatic control for over the last twenty years during which time we have gained valuable experience on the many installations we have supplied. We have the advantage of being a Pretreatment Chemical Supplier thus understanding the needs of all our customers. We can still offer simple dosing pumps to the state of the art conductivity and pH controllers. As previously stated we do not have one dissatisfied customer to whom we have introduced automatic dosing equipment.


Back