What is a dosing pump and how does it work?

What is a dosing pump and how does it work?

What is a dosing pump?

A dosing pump is a small, positive displacement pump. It is designed to pump a very precise flow rate of a chemical or substance into either a water, steam or gas flow. A dosing pump will deliver this precise flow rate of chemical or other product by a number of different methods but it generally involves drawing a measured amount into a chamber and then injecting this volume of chemical into the pipe or tank being dosed. Dosing pumps are used in a variety of applications from agriculture, industry, manufacturing to medicine.

A dosing pump is generally quite small and is powered by either a small electric motor or air actuator. They are controlled either by an external control system or more commonly an internal pump controller that can alter the flow rate, the on/off function and also things like alarms and warnings for run dry, degassing and low product levels.

How do dosing pumps work?

Dosing pump process diagramDepending 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:

  1. The chemical tank or container. The product that is being dosed,
  2. 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.
  3. 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).
  4. 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.
  5. 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.
  6. 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.

 

A suction spear and foot valve for a dosing pump.

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:

  1. 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.
  2. 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.
  3. 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.
  4. 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.

A peristaltic pump

Once a dosing pump is primed, this means all the air is pushed out of the lines right up to the injector, it is then set for its required dose rate or set to work from its input signal. If its working from an input signal, then it may need to be calibrated to dose and alter its dose according to a measurement. Once operating, then the pump works at its required flow rate until a stop signal pauses the action or similar.

 

 

If there is a product level problem or similar sometimes an alarm activates to warn the operator, if product runs out then the system may also shut down or provide a higher alarm to avoid untreated water or liquid causing a problem further up the stream .

As a function, a dosing pump is designed to be reliable so once it is properly set up, it should look after itself and not require large amounts of input.

What do you use dosing pumps for?

Dosing pumps have a large range of applications across a number of industries. This ranges from water treatment, agriculture, industrial, manufacturing, medical, food processing and mining. Generally dosing pumps are set up to inject a product into a water or fluid stream to cause a chemical or physical reaction. This may be as simple as adding an acid or caustic chemical to water to get the pH into a desired range or adding chlorine to kill bacteria. They may be also used for other products like flocculants to make a solid settle out of a liquid or alter its properties.

Dosing may also be used to dose chemicals into a stream to make products. This can be used for things like manufacturing like getting a regular consistency for a glue or additive in fibreboard making to ensuring a brine for meat manufacture is a consistant quality. Dosing pumps can also be used for high pressure and high temperature applications like dosing a boiler feed with corrosion or scale inhibitor or a catalyst in a smelter feed.

How do you set up dosing pumps?

As the components of a dosing system are very important, how they are set up is also critical to it all working well. The first thing to ensure is that all components are compatible with both the product being dosed and the feed stream. Things like high corrosive, high pressure and high temperature need specialised parts for them so that things don’t burst, melt or corrode.

Once everything is compatible, a good location for the dosing point should be selected. The thing to note are that it should be located for the product to mix well and not effect any other pipe parts, ie if it is placed before a valve or water meter, then these parts could be prematurely corroded not read properly or have a build-up of scale or sludge.

Once the dosing point is selected, then the pump needs to be located properly. This needs to be located next to power or compressed air to make it work, generally next to or below the dosing point to prevent siphoning and protected from water, dust, sun or heat damage. Even though a dosing pump can handle very corrosive products etc. it is generally not very weather resistant and their casing has a fairly low IP rating so they sometimes need to be protected in a cabinet or housing to maximise working life.

Another factor that needs to be considered is the product container. This depends on the product itself, how much is being dosed and how much needs to be stored at the pump. If the pump is hard to access or going to be left for long periods of time, then a large amount of product needs to be stored. This may be possible with a bulk tank or an IBC container as long as there is lifting or bulk fluid handling equipment on site. If not the service interval may have to be shorter and the container smaller but more handle able. Another method to consider especially if the product has a limited shelf life if to have on the spot make up. That is bringing in the base chemical or ingredient in as a solid and then making it up into a consistent solution with water and dosing this. There are large capital outlays for the equipment for this method but for high throughput situations and if the cost savings are there then it can be justified. An example of this is polymer manufacture for water treatment or salt to hypochlorite manufacture.

As some rather dangerous chemicals are used in the dosing process another large factor is safety for operators, end users and surrounding environment. The main thing to look at for this is the product MSDS. This should have been consulted to assist in seeing the compatibility with components. With the MSDS info and manufacturer data, you should be able to dertermine:

  1. Corrosive nature of products and toxicity.
  2. Required PPE for operators and the surrounding environment.
  3. If you need bunding or similar storage protection to prevent spillage or other accidents to a minimum.
  4. If control systems need to be specialised to ensure that under or overdosing is minimised if this is critical for performance.

If all this is considered then the system should be efficient and safe. One of the big keys with dosing is that if you cut corners then things won’t work how they should properly and cost, safety and output will be compromised.

In this article we have discussed dosing pump systems, their components, how the pumps work and how they are set up properly. If you would like more information or would like to discuss a dosing pump install or supply then please call our staff today on (08) 9721 3577 or send us an email.

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