By Helmut Voss, Sergej Gerz and M. Ayatollahzadeh
There is a wide range of highly sophisticated dosing tasks in water technology and process engineering which place high demands on dosing pumps in terms of precision, reliability and, in particular, monitoring the dosing process. Examples of these complex applications include dosing antiscalants and antifouling agents in reverse osmosis ( RO ) processes and in the growth sector of diaphragm filtration. Further critical examples include dosing various biocides and defoaming agents in paper production and dosing cleaning agents and disinfectants in the case of clean-in-place (CIP) applications for the food and beverage industry. Reliable dosing of concentrated acids and lyes has proved to be a difficult task in the detoxification and neutralization processes of industrial process and waste water treatment. The new generation of digital dosing pumps has raised the bar in terms of precision and functional reliability when it comes to these complex applications.
Industry continues to use conventional systems for dosing and process monitoring despite the fact that these no longer satisfy the high demands placed on them. Newly developed digital flow monitors give the user precise control of dosing behavior. Sophisticated diagnosis systems detect errors in the dosing head and report malfunctions reliably and without delay, even at the lowest dosing rate settings in the ml/h range.
The following gives an overview of the reasons for dosing errors and the conventional solution available on the market and explains the basic principles behind the new flow monitors for dosing monitoring.
Reasons for dosing errors
The following are the most common causes of faults when dosing with diaphragm pumps:
- Air and/or gas bubbles in the dosing head
- Cavitation bubbles in the dosing head
- Leakage in the suction or pressure valve
- Inadmissibly high operating pressure or system pressure fluctuations
These operating states can be detected and evaluated using the indicator diagram for the dosing process as will be explained in greater detail below.
Conventional solutions for monitoring volumetric flow
Up until now, the most commonly used solutions for dosing monitoring in the case of diaphragm dosing pumps have operated according to the ‘floater principle’. This method involves moving a float upwards in a pipe during the pressure stroke. The float falls again during the suction stroke of the pump. This movement of the float is recorded visually, magnetically or inductively. Depending on the manufacturer, adaptation to the dosing rate of the pump is achieved by moving the switch and/or adding a bypass. This is precisely where the weaknesses in this system lie. These settings have to be adapted to the relevant operating conditions of the pump and when changing the stroke length and/or the stroke frequency. This method of dosing monitoring can often not be used in the case of large variations in stroke frequency or when dosing viscous liquids.
Operating principles of the new flow monitors
The indicator diagram displays the pressure profile across the suction and pressure strokes of the piston or diaphragm.
Indicator diagram for error detection
When the indicator diagram described above has a properly functioning diaphragm pump as a reference stored in the microprocessor, even minor faults can be detected by means of changes in the current indicator diagram.
Figure 2 shows two frequently occurring faults in the dosing process through comparison with the error-free pressure profile (curve 1). Should there be air and/or gas bubbles in the dosing chamber (curve 2), pressure builds up more slowly due to the much greater elasticity. This means that it takes longer to reach the system pressure and that the dosing volume of each stroke is significantly smaller.
In the case of large errors on the suction side of the pump, extensive cavitation can occur as shown in Figure 2 (curve 3). The reasons for this can be overly small cross-sections, excessive levels of suction or excessive viscosity. Vapor pressure dominates the entire suction stroke and only dissipates late in the pressure stroke. This scenario also results in a smaller dosing volume per stroke due to the delay in reaching the system pressure.
Figure 3 shows two further frequently occurring faults in the dosing process through comparison with the error-free pressure profile (curve 1). If leakage occurs in the suction valve (curve 2), pressure builds up more slowly due to the leakage in the suction valve and the pressure falls before the outer dead center is reached as soon as the leakage rate is larger than the current flow rate of the pump.
Should leakage occur in the pressure valve (curve 3 in Figure 3), a rise in pressure occurs before the end of the suction stroke when the leakage rate in the pressure valve is larger than the current suction flow of the pump. Moreover, the pressure falls more slowly at the start of the suction stroke as a result of post-flow in the pressure valve.
This indicator diagram is also suitable for detecting further faults. For example, if the set system pressure is exceeded, this is identified and evaluated.
A new product by one manufacturer has a dosing pump that also incorporates a pressure sensor. This solution uses the existing microprocessor for both motor control and processing the measurement values.
As can be seen from Figure 4, the microprocessor continuously records the pressure in the dosing chamber and, using the motor control, the motor position which can also be used to determine the diaphragm position on the basis of the drive kinematics. This means that the microprocessor can generate the indicator diagram on a continuous basis. Evaluation algorithms have been developed for this solution to enable detection and evaluation of the dosing errors described in greater detail above.
The basic parameter setting for monitoring volumetric flow has been selected to cover a wide range of standard applications. Depending on the error in question, a fall in the dosing rate of only 30 percent can be reliably detected.
Moreover, it is now possible to read out representative points on the indicator diagram from the memory and use them for the purpose of diagnosing errors.
The processor also takes an average of the values to continuously calculate the feed pressure in the dosing chamber. This value is representative of the current system pressure and can be queried at any time at the press of a button.
Outlook: basis and potential for further development
Continuous monitoring of diaphragm dosing pumps by recording and automatically evaluating the current indicator diagrams opens up many opportunities for operational monitoring:
- As a status parameter, the pressure enables a process to be established which is largely independent of the pump size.
- The indicator diagram is largely independent of speed, viscosity and temperature. This establishes the same system for detecting errors across the pump’s entire volumetric flow range (in some models) without defining a working point.
- Recording the current system pressure enables users to largely balance out the pressure-dependent fluctuations in the flow rate of the pump by adapting the speed.
- There is still significant potential in terms of detecting and quantifying errors by expanding the quantity of data recorded and the microprocessor performance, by fine-tuning the evaluation algorithms and by drawing on other experiences with this process.
Mehdi Ayatollahzadeh, Dipl. Ing, is Product Manager, Dosing Pumps for Grundfos Alldos at the manufacturing facility in Pfinztal, Germany and has over 25 years service with the company. He can be reached via email to Tosh@ALLDOS.com
About the product
Grundfos Alldos manufactures Digital Dosing Pumps with Plug & Play Solutions.
About the company
Grundfos Alldos is an innovative supplier of high-quality products and services in the environmental technology and water treatment sectors. They develop, manufacture and market dosing pumps, disinfection systems, dosing systems as well as the associated measuring and control technology for these sectors. Using their knowledge of the markets and processes, the firm possesses the ability to adapt to customer requirements. Grundfos Alldos is a member of the Grundfos Group.