Chemical metering by definition requires an accurate, consistent and repeatable pumping of the chemicals that are being injected. Since accuracy if often measured in percentage of the flow rate it is understandable that meeting that specification becomes harder and harder as the chemical dosing rate gets smaller. It is easy to meet +/-1% on a flow rate of 100 gallons per hour as the range is about 2 gallons. Now the same range shrinks to 0.02 or 1/50 of a gallon if the flow rate required is 1 gallon per hour. At AquFlow we have Series 900 pumps made especially designed and manufactured for low flow applications where the lowest flow rate achievable is 0.14 gallons per hour. Imagine meeting the +/-1% criteria on this pump where the range is 0.0014 gallons. In other words, we need to be within 5 ml in flow for accuracy and repeatability which is essentially a few drops over a period of one hour of pumping. AquFlow Series 900 pumps meet that requirement quite efficiently. However proper application in the field is critical to the success of low flow chemical dosing. We will explore some of the challenges and how to overcome it. These challenges have to do with the low flow rates regardless of the pump type or technology used although we will be mentioning hydraulic actuated diaphragm pump in our example.
Air Pockets in the System: The displacement of the pumping element which is the diaphragm in this case is very small and it is not capable of displacing or pushing out even smallest of air pockets. So when the system is started up it has nothing but air filled in it. That air needs to be displaced by filling up the system with liquid against no back pressure. The piping needs to be designed to allow air to be vented out easily. It means that pipes should have gentle slope upwards and putting a vent at the highest spot to allow air to get out. Any amount of air pocket trapped in the system (on the discharge side) will absorb either partially or completely the discharge stroke varying the chemical pumping rate.
Air in the Pumping Chamber: Any air on the suction side will enter the pumping chamber, sit there expanding and collapsing with each stroke reducing or stopping any chemical from being pumped out. Here again proper piping, venting and degassing valves are very useful in pushing the air out of the system.
Chemicals that are prone to off gassing: Chemicals that evaporate easily will form pockets of gas in the system which will lead to the same problems that we discussed above. Same or similar solutions discussed above need to be applied here as well. Low suction pressure will make this off gassing more probable. Increase the suction pressure by providing a higher NPSHA (Net Positive Suction Head Available).
Piping diameter and length on suction as well as discharge: Chemical in the pipe whether on suction or discharge is a means to an end and not the desired objective which is to inject the chemical into the process where it is needed. When the chemical flow rate coming from the pump is only a few ml/minute it will take a very long time for it to fill up the volume inside the piping. The larger the diameter of the pipe and longer the length of the pipe the greater the volume that needs to be filled up by displacing the equivalent amount of air. There is no reason to use a 1” or even a ½” pipe ID when the flow rate is only a fraction of a gallon per hour. Often it takes several hours under ideal condition for chemical to fill up the pipe volume and even show up at the other end.
Start Up Procedure: Most of these challenges occur at the start up stage when the pump and the system is being primed by purging the air. As a rule the smaller the flow rate the longer you should allow for the purging of the air and priming. The best way to start is by opening up all valves on the discharge side and letting the pump run at its maximum capacity against no back pressure. Having back pressure will either slow down or prevent purging of the air both in the system as well as in the hydraulic drive of the pump itself. Once the air is completely purged you can gradually increase the discharge pressure by closing the back pressure valve to the required pressure. This process of purging the air would need to be repeated anytime air is introduced in the system.
Once these pumps start operating at the desired flow rate and pressure they will continue for a long time and if you are using hydraulically balanced diaphragm pumps like AquFlow Series 900, you do not have to worry about even scheduled maintenance for months and years.