How do dosing pumps work?
Depending on the brand and model, a dosing pump functions in a variety of different methods. All these methods involve taking a measured amount of a chemical and then injecting that product into a pipe or similar vessel. There are a couple of major parts to a dosing pump setup:
- The chemical tank or container. The product that is being dosed,
- The foot valve. This is a one way valve that is attached to a suction line. It is placed into the drum of product and allows the pump to remain primed. It should have a weight on it so it remains in the bottom of the drum of product and sometimes it has a float switch attached to it so the pumps has an alarm activate if the product runs out.
- The pump itself. This can vary in size and materials but is generally a variety of chemical resistant plastic (PVC, PE or similar), rubbers or stainless steel. It has a suction line attached to the inlet and the dosing line attached to the suction. The mechanics of the pump can vary (see below).
- The dosing line this is generally a fairly rigid PVC or PE tube or a reinforced hose. Occasionally in steam, hot water or super high pressure applications the line can be stainless steel. This can have a variety of bleed, pressure relief, air release valves included into it but generally it is just a line.
- The injector. At the point at which the product is injected into the product, there is an injector point. This is a one way valve so that when the dosing pump pushes an amount of product into the line it can overcome the pressure in the delivery pipe and allow the product out into the flow. Once a shot of product is released or the pump stops, the one way valve stops the liquid in the delivery line from going up the dosing line and damaging the pump. The injector also has a spout so that the product is delivered into the middle of the flow rather than the side wall. Over time certain products especially acids and oxidisers like chlorine or peroxide can corrode the walls of a pipe if released right at the edge of the stream. Releasing the product into the middle of the stream also creates a vortex which allows the product to mix properly too which is beneficial to ensure a proper reaction takes place.
- Control system. Occasionally there is a control system installed to ensure the dosing pump is accurate and turns on and off at particular times. This can be as simple as a timer or flow switch right through to a full SCADA or similar central control system with sensors for pH, chlorine and similar and variable rate control to raise and lower the level being dosed. It may also be integrated into a more complex operations system.
The injector assembly for a Grundfos dosing pump
There are 4 different types of dosing pumps, they vary in their action and pumping mechanism and are suited to various different applications, pressures and chemicals. They are:
- Diaphragm type constant injection. Where there is a pump chamber that is filled and emptied by a piston and diaphragm and valves on the inlet and outlet. When the chamber is filled by drawing in the piston, then depending on the amount being dosed (generally the % of the maximum flow rate) then the dosed volume is injected out at a certain speed. These pumps are very accurate and deliver a near constant flow rate of product generally in the 6-250L/hr range. With correct control, they can also deliver variable dosing rates. An example of this is the Grundfos DDE and DDA range.
- Diaphragm type pulse injection. This is where there is a diaphragm mechanism again but instead of a slow and constant flow rate, the pump is controlled by a solenoid coil. This coil sucks in and injects the chemical in pulses with the time gap between pulses providing the control of flow rate. This makes this type of pump much less accurate as the product is delivered in pulses with a time gap between injections. They are generally very simple both mechanically and electrically and are also very cheap. Having said that you get what you pay for and they are only able to deliver properly close to 100% of the pump duty or If the water being treated is then pumped into a tank or similar and then mixed properly. With the much lower price of proper “constant” dosing pumps these days, this sort of technology is outdated.
- Lobe type pumps. This type of pumps allows a certain volume through a set of meshing gear type impellors. The volume in between these impellors is possibly not as accurate as the proper diaphragm pumps and they also have a wearing surface so they are only really suitable when the product is high viscosity and self-lubricating so wear is minimised. They are also difficult to set up accurately for lower flow rates.
- Peristaltic pumps. Peristaltic or lobe pumps are a specialised and very accurate method of dosing. The mechanism works by having a flexible tube which the product has to pass through. This tube is bent in a semi-circle and a small roller on a mechanical arm moves over the outside of the tube. This action captures a “portion “ of the product in the tube and pushes it along and into the dosing tube and the main stream. These pumps are used widely in the medical industry as they are easily sterilised and the pump mechanism can be quickly changed out of it is contaminated or damaged. The downside to these units are that they wear out quickly and as they use a flexible rubber tube for the pump action, they can’t handle pumping into a high pressure stream as they are only good for the burst pressure of the flexible tube.