pump dosing
Dosing pump is one of the reciprocating positive displacement pumps. Dosing pump has the capability of suction from negative height (self-priming) and injection of material in pressurized lines. This type of pump is used to inject a chemical substance at a certain pressure, in a certain amount and of course with the ability to adjust to a process such as a cooling tower, boiler, chlorination, reactor, etc.
The structure of a dosing pump consists of a driving force section and an injection section or pump head (liquid end). Each of these sections has types and components that are discussed below:
First part: driving force
Dosing pump is available in three models: solenoid, motorized and pneumatic.
Most manufacturers use solenoid drive power to provide smaller capacities, and in other cases, use electric motor drive power. Pneumatic propulsion is produced in small and very limited models due to lack of market demand.
pump Solenoid Driven dosing :
The structure of a solenoid dosing pump consists of a magnet or solenoid, in which the metal core embedded in it moves and moves a few millimeters due to the momentary electrification of the solenoid. This movement changes the position of the diaphragm or plunger and injects the substance. As soon as the power is cut off, the spring installed behind the metal core returns its position to its previous position and thus the material is sucked. The amount of dosing pump capacity is directly related to this number of strokes and the user can adjust the dosing pump capacity by changing this number.
These pumps are produced in wall mounted models and in higher technologies in tabletop models. Wall-mounted dosing pumps are able to control the amount of chemical injection only by changing the number of strokes. But desktop models have the possibility to change the volume of each stroke and are superior to wall models in this respect.
Solenoid dosing pumps can easily add automatic control options such as control signal inputs and alarm outputs by using the electronic board that is embedded in it.
pump Motor Driven Dosing :
The structure of the dosing pump is a motor consisting of an electric motor and a gearbox, the rotary movement of the motor is converted into a reciprocating movement by the gearbox, and a shaft directly drives the diaphragm dosing pump or plunger dosing pump. brought
In some models, this power transfer is done indirectly by a hydraulic part to the diaphragm (hydraulic diaphragm dosing pump).
In most of the motorized dosing pumps, like the solenoid models, the injection of the substance is done with the driving force, but the suction of the chemical substance is done by a spring installed inside the gearbox (spring return). The suction action in some motorized dosing pumps is performed directly by the motor. These two models have advantages and limitations compared to each other, which are mentioned below.
Advantages of dosing solenoid pumps compared to dosing motor pumps:
- Solenoid pumps have lower initial price, weight, dimensions, energy consumption, current cost and depreciation than motorized dosing pumps.
- Dosing repairs of solenoid pumps are faster and easier than motorized models.
- Control options include signal inputs and automation facilities in dosing solenoid pumps are more accessible and cheaper than motorized models.
- The protection standard of dosing solenoid pumps (IP) is higher than motorized models. Solenoid dosing pumps have IP65 or higher, and motor dosing pumps have IP55 in standard mode, which can be upgraded for a fee.
The advantages of dosing motor pumps compared to dosing solenoid pumps:
- In most cases, the physical resistance and dosage strength of motorized pumps is higher than that of solenoid models.
- It is possible to design and manufacture motorized dosing pumps for special conditions, including high-risk environments or for toxic or flammable substances, as well as tropical regions. But there are many limitations in solenoid dosing pumps due to the presence of an electronic board.
- It is possible to design and manufacture motorized dosing pumps based on global standards such as API675 and special conditions, but it is not possible for solenoid dosing pumps.
- It is more common and possible to use motorized dosing pumps in open environments that are exposed to environmental conditions and sunlight. Dosing solenoid pumps with plastic body do not have proper resistance in the open environment, especially against sunlight.
- In special circumstances, it is possible to couple two or more motorized dosing pumps and use an electric motor. This option is not possible in dosing solenoid pumps.
Comparison of two suction modes in motorized dosing pumps:
In motorized dosing pumps, depending on the design of the gearbox, it is possible to suck the material with spring return or with engine power. In the models where the suction is done with electromotor power, the dosing pump has more suction power, but it has an important limitation that if the suction line is blocked, it will cause serious and heavy damage to the peripheral equipment and the dosing pump itself, and this It is a limitation that return spring models do not have. But on the other hand, their suction power is limited to the power of the spring in the gearbox.
Second part: pump head (liquid end)
pump Mechanical diaphragm dosing
This series of dosing pumps includes the most widely used type of dosing pump. Almost all dosing solenoid pumps and part of dosing motor pumps are of this type. In this model, a metal or plastic shaft is directly connected to the gearbox or solenoid, and on the other hand, it is directly connected to the diaphragm.
Compared to other types of dosing pumps that will be discussed below, this model has advantages such as a more reasonable initial price and easier repairs, but some of its limitations are also significant. Among other things, this model has lower working pressure and accuracy. The maximum working pressure of these dosing pumps is up to 25 bar and its accuracy is 2%.
The important point in setting the minimum and maximum capacity (Max. and Min. capacity) is to pay attention to the allowed range for setting the pump. No dosing pump has the ability to accurately inject the chemical in the range of 0 to 100%. Minimum and maximum allowable range (turndown ratio) is defined for all dosing pumps. This amount in dosing mechanical diaphragm pumps is 3 to 10 or in other words 30 to 100 percent. These pumps are mostly adjustable in the range of less than 30%, but they are not accurate enough.
pump Hydraulic diaphragm dosing
This series of dosing pumps is also known as piston-diaphragm. In this model, the power of the gearbox is transferred to the hydraulic part by a piston, and the movement of the diaphragm is done by hydraulic oil pressure. Dosing hydraulic diaphragm pumps are usually equipped with an internal relief valve, which protects the pump components against increasing line pressure. The working pressure and accuracy of this model is higher than the dosing of mechanical diaphragm pumps.
The maximum working pressure of these dosing pumps with polymer diaphragm is about 400 bar and its accuracy in different models is from 0.5 to 1%.
The minimum and maximum allowable range (turndown ration) defined for this dosing pump is 1 to 10 and in some models 2 to 10.
The technical advantages of this model have led to the use of this model in most sensitive projects, including the petrochemical, gas and oil industries. These dosing pumps can be made according to API 675 standard.
pump plunger dosing
Due to the possibility of using metal alloys in all its parts, this series of dosing pumps can provide higher pressures than other models. In this model, a plunger is directly connected to the gearbox by a shaft, and its back and forth movement in a cylinder causes the injection of the chemical substance. To prevent wear and friction between the plunger and the cylinder, the plunger is restrained by a number of polymer packings, and the design is such that the chemical substance is responsible for cooling the packing. Therefore, plunger dosing pumps are leaky unlike diaphragm models. The permissible amount of leakage is about 1 to 2 drops per minute.
Existence of leaks in plunger dosing pumps are an important and limiting factor in injecting flammable or toxic substances. Actually, we are not allowed to use plunger dosing pumps to inject these substances.
There are differences between plunger and piston pumps, which is why the word piston is not used in the text above. Basically, if the term dosing piston pump is mistakenly seen somewhere, it means the same plunger. Because dosing pump manufacturers usually do not produce piston pumps.
The maximum working pressure of these dosing pumps is about 3000 bar and its accuracy is 0.5%.
The minimum and maximum allowable range (turndown ratio) defined for this dosing pump is 1 to 10.
The technical advantages have made this model to be used in sensitive projects such as the petrochemical, gas and oil industries (of course, for non-toxic and non-flammable materials). These dosing pumps can be made according to API 675 standard.