S6A Meter Mount Display
S6A Meter Mount Display
Let’s be honest, we all know that simpler is better. The simplicity of a peristaltic style metering pump makes it a very reliable method for injecting a wide variety of chemicals into water treatment applications. Understanding the variables that result in wear on the pump components, especially wear to the pump tube assembly, can assist the reader in properly specifying the pump for a specific application.
Peristaltic pump technology
The human body uses “peristalsis” action to move food through the digestive tract. The wave-like muscle contractions progressively squeeze the digestive tract, essentially “pushing” the food through. It doesn’t get any simpler than that.
One of the greatest benefits of a peristaltic pump is its functional simplicity. Peristaltic pumps utilize a circular pump “head” and simple rotating roller that is designed to pinch the tubing and gently squeeze the fluid through specially designed tubing, as shown in Figure 1.
Figure 1: Typical peristaltic pump
They can effectively pump both fluids and gasses, eliminating the possibility of siphoning, vapor locking or loss of prime even when operating at very low output rates. Nearly continuous output results in a finer dispersion of chemical in the piping system when compared to pulsating type pumps such as diaphragm pumps. Figure 2 shows the near continuous output of chemical in the flow stream when using a peristaltic pump versus the interrupted chemical dispersion when using a diaphragm pump.
Figure 2: Intermittent vs. continuous flow
Fewer components result in very low maintenance costs when compared to the cost of rebuilding more complex pumps that require a large number wetted components such as metal springs, o-rings, valves, check balls, etc.
Commonly called a squeeze tube pump, the new generation of peristaltic chemical metering pump is quite different from the low-pressure laboratory pumps most people are familiar with seeing in a hospital setting. These industrial workhorses are now capable of pumping aggressive chemicals such as 12% sodium hypochlorite (chlorine), 50% sodium hydroxide, 97% sulfuric acid, and 85% phosphoric acid against system pressures up to 125 psi. Some models include such features as tube failure detection systems, flow verification sensors, and sophisticated control electronics for PLC interface and connection to SCADA systems.
Pump System Components
For analysis purposes, the peristaltic pump assembly can be broken down into five main components; 1. pump tube, 2. pump head and roller, 3. motor, 4. control electronics and 5.motor/electronics housing. Note that in some models, the control electronics (VFD, motor starter, PLC, etc.) are housed in a separate enclosure.
Variables to tubing wear
Many manufactures rate the life of their tubing by the number of effective operating hours before failure. While this rating may be effective for comparing the life of tubes used in the same pump under a specific set of operating parameters (for example; pumping water with a specific pump head type, at 0 psi, at a fixed RPM), there are many variables that will affect the number of hours a given tube will last in an actual application. Care should be taken to specify the pump components and operating parameters to achieve the greatest tube life possible in an application.
- Tubing materials – The tubing material must withstand the chemical being injected, return to its original shape after many thousands of occlusions (compressions), and operate at the required system pressure. Specifying the optimum tubing material is critical for a successful application.
- Chemical resistance – Chemical incompatibility will result in a breakdown of the tubing material properties, often manifested as a change in the stiffness of the material, either softening or hardening. In most cases, chemical resistance problems will be apparent within the first few days of use. However, in some cases, the chemical will attack the tubing material slowly over a long period of time, reducing the life of the tube.
- Dimensions – Larger tube diameters and thinner wall thicknesses will generally result in reduced tube life expectancy.
- Material properties – The physical properties of the tubing material will greatly influence not only its suitability for general use in a peristaltic pump, but also the amount of time the tube will last in a particular application. The peristaltic pump tube must be capable of precisely returning to its original shape many thousands of times after being squeezed by the roller. Many tubing materials lack this memory making them unsatisfactory for peristaltic pump applications. Tubing manufacturers offer a variety of tubing formulations, many of which are suitable for use in peristaltic pumps and many which are not. The end user must be cautious when selecting the tubing material for the application. Most pump suppliers will either offer assistance with the tubing selection or offer pre-assembled “tube assemblies” designed specifically for their peristaltic pumps, greatly reducing the possibility of miss-application.
- System pressure – The pressures acting on the tubing will directly affect the tube’s life. Both the inlet and outlet pressures should be considered and particular attention should be paid to “hidden” variables that can add to the system pressure such as piping system components and fluid viscosity.
- System pressure – The most obvious (and perhaps most influential) variable affecting tube life is the piping system pressure. But often, system components and installation factors that can increase the pressure at the pump tube are overlooked. For example, most manufacturers recommend installing a check valve in the discharge piping directly after the pump tube to prevent the system fluid from flowing back through the pump during routine pump maintenance or pump tube rupture. A spring loaded check valve or back pressure valve will increase the pressure at the pump tube by a value equal to the cracking pressure of the valve. For example, if the system pressure is 50 psi and the back pressure valve is set at 20 psi, the effective pressure at the pump tube is 70 psi. Therefore, valves with high cracking pressures should be avoided.
- Another often overlooked variable that can increase the pressure at the pump tube is the physical distance from the pump to the point where the chemical is injected into the system, especially important to consider when injecting viscous fluids. The pressure at the pump tube will increase as the distance from the injection point increases, the chemical viscosity increases, and the discharge-piping diameter decreases. Imagine trying to drink a thick milkshake through a skinny, 100 foot straw! Small diameter orifices in fittings should also be avoided when pumping viscous chemicals.
- Number of occlusions – The tube life is affected by the number of times the tubing must be pinched (number of occlusions) in order to pump a given amount of chemical. Reducing the number of occlusions will increase the life of the tube. Four variables affect the number of occlusions required to inject a given amount of fluid; the diameter of the tubing, the diameter of the pump head, the number of rollers on the roller assembly (occlusions per revolution), and the motor rpm. Some manufacturers use the total number of occlusions, rather than time, when estimating their tube life expectancy.
- Tubing diameter – A larger diameter tube will inject more chemical per occlusion (trap more chemical between two pinched rollers) than a smaller diameter tube. Therefore, a large tube can output more chemical with less occlusions, resulting in less wear, than a smaller tube.
- Pump Head Diameter – Similar to the tubing diameter, the pump head diameter will affect the amount of chemical per occlusion. Larger diameter pump heads will result in more chemical being pumped per revolution.
- Number of rollers – A given peristaltic pump model may have anywhere from one (offset cam type roller) to six or more individual rollers which squeeze the tube, pinching off the captured fluid and delivering it to the discharge end of the pump tube. Multiple rollers per assembly result in slightly smaller volumes of chemical injection per revolution, less pulsation and a reduced likelihood that an individual roller will wear out resulting in lost pumping capability. However, since tube life is directly proportional to the number of times the tube is pinched per revolution, the cost associated with the higher number of rollers is tube life.
- Motor rpm – Unlike many types of pumps, peristaltic pumps are capable of operating at very low revolutions per minute (rpm) while maintaining very high accuracy, repeatability and priming capability. Therefore, to increase tube life, specify the pump so that the typical operation of the pump is at the lower end of the operating output adjustment range, resulting in the fewest number of occlusions. The maximum possible rpm of a specific pump model will vary from manufacturer to manufacturer with maximum motor rpm of 650 being not uncommon, though at this high rpm, tube life will be greatly diminished. Some pump models have effective turndown ratios of up to 10,000:1 resulting in a minimum effective rpm of 0.01!
- Amount of tubing squeeze – Simply pinching off (occluding) the tube is not enough, the rollers must squeeze the tubing the exact amount required to ensure that the fluid being pumped is effectively trapped in the tubing and delivered to the injection point. Factors such as system pressure, suction lift, fluid viscosity, tube material, and others will affect the amount of squeeze required for a particular application. If the tube is under-squeezed, the fluid can escape or flow backward toward the suction side of the pump tube when the roller rotates in the head. This can occur when the pump is operated against a higher system pressure than recommended. If the tube is squeezed too much, it is being subjected to more force than is necessary and tube life will be diminished. Properly matching the roller design with the type of tube being used will result in the most efficient pump design and longest tube life for a particular application. Figure 3 shows the squeeze action of a peristaltic pump.
Pump Head and Roller Design
The roller diameter, roller materials, type of bearing surfaces, and pump head design can also affect the life of the pump tube as well as the life of the roller assembly. Schematic of a pump head is shown in Figure 4.
Figure 4: Schematic of pump head
Roller diameter – A large diameter roller will pinch off a greater surface area of the tube while rotating, resulting in lower tube life; however, large rollers will rotate fewer revolutions per roller assembly revolution, potentially resulting in longer roller life.
Roller bearings – The roller must rotate on a shaft, therefore the type and design of the bearing surfaces can increase or decrease the life of the roller. The design of the bearing surface can also assist in preventing chemicals and debris (from tubing surface wear) from entering the roller axle area causing drag on the roller.
Roller material – The roller assembly materials of construction should be of sufficient strength to withstand the repeated compressions of the pump tube while offering resistance to the chemicals that may potentially be spilled in the pump head area. The roller assembly must also have the dimensional stability to withstand variations in ambient temperatures and rotational forces without affecting the amount of squeeze on the pump tube.
Pump head – As with the roller assembly, the pump head materials of construction must also withstand any spilled fluid that may enter the head. The diameter of the head will also affect the amount of fluid pumped per revolution, with larger pump heads discharging more chemical per revolution than smaller pump heads.
All of the parameters such as system pressure, number of occlusion, tube chemical resistance, tube squeeze and roller bearing inefficiency impact tube life as shown in Figure 5.
Figure 5: Components affection tube life
Chemical spills – If left alone, the pump tube will eventually fail. Depending on the operating pressure, type of tube, and many other factors, the chemical may leak out slowly or squirt out dramatically. Manufacturers offer a number of different methods for protecting the roller assembly, pump head and area surrounding the pump from chemical spills. Some manufacturers include drain ports to remove chemical, float switches to shut down the pump when a spill occurs and a cup fills, and electronic sensors to shut down the pump when chemical is detected in the pump head area. Some methods are more effective at quickly turning off the pump and reducing the amount of chemical spilled. Based on the effectiveness of the method, the pump head and roller assembly may incur damage resulting in drag on the roller assembly and reduced roller and tube life.
A variety of motors ranging from small, fractional horsepower shaded pole AC gear motors, to large C-frame AC and DC powered gear motors are used with peristaltic pumps. Many peristaltic pump manufacturers include the motor as part of the pump assembly which helps take the guesswork out of specifying the correct motor to use for a given pump assembly. As with any pump, care should be taken to properly specify the motor for the pump and the intended operating environment.
The control electronics must be carefully selected to properly control the motor as well as providing for any remote control and communications capabilities such as analog input motor speed control, analog output pump speed feedback to SCADA, alarm outputs, pump status, etc. As with the motor, many pumps include the control electronics as part of the assembly.
Typically, a peristaltic pump enclosure protects the motor and control electronics from the operating environment while the pump head area of the pump is either unprotected or sealed in its own enclosure separate from the motor and controls. Manufacturers offer a variety of enclosures for the motor and control circuitry ranging from small plastic housings to explosion proof metal enclosures. Many pumps are supplied without any enclosure at all. As with the motor and control electronics, the user should take care to specify the pump system with a proper enclosure that is designed to provide the protection needed for the application environment, as shown in Figure 6.
Figure 6: Fully enclosed peristaltic pump
A typical setup of peristaltic pumps with integral motor and controller provides the necessary chemical feed to the cooling water system is shown in Figure 7.
Figure 7: Peristaltic pumps providing chemical feed
Many variables affect the service life and maintenance requirements of a peristaltic pump. By carefully assessing the application, the user can properly specify the pump and components to minimize service and maintenance requirements and maximize the life of the pump.
Mr. Bill McDowell is a Sales Engineer with Blue-White Industries and has over 29 years with the company. He has held various positions with Blue-White Industries including Project Engineer and Director of Engineering. Additional information can be obtained from Blue-White Industries at, 5300 Business Drive, Huntington Beach, CA 92649. Phone 714-893-8529, Fax 714-894-9492, or email@example.com; www.blue-white.com
A search for technology to increase system accuracy, reduce maintenance costs, and enhance an advanced SCADA system led a Rancho Cucamonga, Calif., utility to replace its diaphragm-type pumps and gas–chlorine injection system with peristaltic pumps.
BY Bill McDowell
Built in 1989, the 60-mgd Lloyd Michael Treatment Plant (LMTP) in Rancho Cucamonga, Calif., treats raw water from the California Aqueduct system to provide drinking water for a multicity service area. In November 2012, the Cucamonga Valley Water District began upgrading the plant to enhance treatment processes and comply with new federal water quality standards. The upgrade is expected to be completed by spring 2014.
As part of the upgrade, the anionic and cationic polymer, ferric chloride 43 percent, sodium hydroxide 50 percent, and gas–chlorine chemical-feed systems will be replaced. Jerry Griffith, plant mechanic, began looking for new technologies to increase each system’s accuracy, reduce maintenance costs, and integrate operations into an advanced supervisory control and data acquisition (SCADA) system.
The plant’s diaphragm pumps and gas– chlorine injection system had a variety of problems that needed to be reduced or eliminated during the upgrade. Challenges included system maintenance, chemical metering accuracy, ease of use, SCADA system requirements, system flexibility for emergency operations, and limited space requirements.
The pulsating diaphragm pumps required frequent adjustments and maintenance. “They always needed cleaning, the oil needed to be replaced frequently, and the stroke length needed to be adjusted manually,” said Griffith.
The pulsating diaphragm pumps were also hard on the piping system, causing occasional leaks. Piping and ancillary components, such as pulsation dampeners, calibration columns, and pressure regulator valves, also required additional maintenance and floor space and made the system more complex. In addition, the diaphragm pumps weren’t providing much information to the SCADA system.
As part of the upgrade, the gas–chlorine injection system will be replaced with a liquid chlorine and ultraviolet (UV) system.
The gas system is expensive to maintain, costing $10,000 per year for scrubber and injector cleaning and maintenance. Using lower-concentration liquid chlorine and UV technology will help the plant maintain lower trihalomethane (THM) levels. The new liquid chlorine system, which uses peristaltic pumps, will be installed in the chemical room. When the gas–chlorine system is removed, the area will be transformed into a much-needed workshop.
CHOOSING THE RIGHT CHEMICAL PUMP
After reviewing and testing various types of pumps on the market, LMTP personnel chose peristaltic-style metering pumps to replace the diaphragm pumps in all applications, including gas–chlorine injection, for several reasons.
Low Maintenance. Although peristaltic pumps require periodic tube changing, such maintenance is predictable and inexpensive. For example, Griffith replaces the pump-tube assembly of the new anionic polymer system every six months, regardless of wear.
Ease of Use. The peristaltic pump is easy to use, and the pump-tube assemblies can be replaced quickly and easily. In addition, the menu-driven software and display allow operators to quickly adjust the pump’s many electronic features.
Higher Accuracy. Even when pumping high-viscosity polymers, the peristaltic pumps are accurate to within about 3 percent over their operating output range and over the life of the tube. The SCADA system can easily set and maintain 1 ppm without requiring operators to make manual adjustments.
SCADA Ready. The peristaltic pumps communicate with the SCADA system better than the diaphragm pumps did. Now more process information is available to the SCADA system, including multiple alarm outputs and output volume data. In addition, the system can react more quickly to commands, such as a quick shutdown of the system. The highly automated plant is monitored and controlled in real time using handheld devices. Rob Hills, water treatment superintendent, can now access and control anything in the plant with his smart phone or personal digital assistant.
Flexibility. The peristaltic pumps are self-contained. The motor and controller are located inside the pump enclosure for portability. The pump’s small size and light weight allow operators to move the pump to a remote location if treatment is required at a different injection point. For example, if a system failure requires a particular section of pipe to be shutdown, the pump can be relocated as required and run manually to prevent plant shutdown. With the San Andreas Fault less than a mile away, LMTP operators are alert to potential damage to piping systems from earthquake activity. They try to maintain as much system flexibility as possible.
Space Requirements. The peristaltic pumps occupy a smaller footprint, further increasing efficiency in the chemical room and reducing maintenance. The entire gas chlorination system will be replaced by peristaltic pumps and relocated to the chemical room with the other systems.
Quiet Operation. The new peristaltic pumps produce significantly less noise in the chemical room. Operators didn’t realize how loud the diaphragm pumps were until they were gone, according to Griffith. Less noise helps reduce the stress of working in the chemical room for extended periods.
Consistency. With the features and capabilities to handle all applications, the peristaltic pumps reduce the complexity and amount of operator training required as well as the number of spare parts necessary for system maintenance.
Customization. Custom designed by LMTP staff, the new anionic and cationic polymer, ferric chloride, and sodium hydroxide chemical systems feature
- a plastic texture-coated flooring grate system over the containment area.
- a below-grate flushing pipe system.
- quick-release polyvinylidene fluoride cam-lock inlet and outlet pump fittings.
Peristaltic pumping technology has simplified maintenance and helped LMTP personnel function more efficiently, maintain and upgrade plant equipment, and comply with federal water quality standards.
Bill McDowell is a sales engineer with
Blue-White Industries (www.blue-white.com),
Huntington Beach, Calif.
Article available on Opflow Magazine’s Website: http://dx.doi.org/10.5991/OPF.2013.39.0020
Maintaining the correct amount of chlorine for effective drinking water system disinfection in a large municipal drinking water system can be challenging. Piping system lengths, variable flow rates and demands, and other factors contribute to the difficulties in maintaining the optimum level of free chlorine throughout the entire system.
One method of increasing the length of time that the chlorine remains effective in the system is to add ammonia. With the addition of ammonia, chloramines are formed resulting in not only a more stable and longer lasting disinfecting residual than free chlorine, but also the additional benefit of a reduction in the amount of initial chlorine injection required and a similar reduction in unpleasant chlorine odor and taste.
Although the mixing of ammonia with chlorine to form chloramines is a safe and effective means to treat drinking water, the addition of ammonia can create a potential hazard if the chlorine is not present. The proper chlorine/ammonia ratio must be maintained to form the chloramines. For this reason, system designers are careful to select the most reliable injection system components possible that also allow for variable flow rate requirements and permit continuous monitoring and remote control by SCADA systems.
Chuck Boone, the Mechanical Maintenance Supervisor at the Irvine Ranch Water District (IRWD) in Irvine, CA, became concerned about the new IRWD reservoir management system (RMS) pilot project when the diaphragm pumps chosen for the chlorine injection task repeatedly failed. Although sensors in the system detected the failure and safely shut down the system, it became obvious that a more reliable chlorine injection pump system was required.
The cause of the diaphragm pump failures was traced to the pumps losing prime due to vapor locking. Vapor locking is caused by gases escaping from the fluid and building up in the pump head preventing the valves from operating correctly. This phenomenon commonly occurs when the pump is sitting idle, such as at night or when the system demands are low. The IRWD maintenance mechanics worked with the diaphragm pump manufacturers to install de-gassing valves and other devices that would permit the pump to automatically expel the built up gasses from the pump head, but these measures were unsuccessful. Looking for a better way to inject chlorine, the IRWD team turned their focus to peristaltic pump technologies.
Commonly called “squeeze tube pumps,” the new generation of peristaltic pump is quite different from the low pressure, non-industrial peristaltic pumps most people are familiar with seeing in a hospital setting. These industrial pumps are now capable of long tube life and output pressures to 125 PSI. Some models also include such features as tube failure detection systems, flow verification sensors, heavy duty weatherproof enclosures and sophisticated electronics for connection to SCADA systems.
Peristaltic pumps use a circular pump “head” and simple rotating roller design to gently squeeze the fluid through a piece of specially designed tubing. With no valves to clog, metal springs to corrode or ball seats to fail they can effectively pump both fluids and gasses, eliminating the possibility of vapor locking and loss of prime. A peristaltic pump’s output is not affected by changes in the system pressure (it therefore does not have a pump output curve) making its output much more consistent than a diaphragm pump.
In a chloramine application, it is critical that the ammonia pump inject at a proportional rate with the chlorine pump and automatically deactivate in the event of a chlorine pump failure. The new generation of variable speed peristaltic pumps meets the requirements for both the chlorine and ammonia pumps in a chloramine application.
Manufacturers of these pumps include many of the features used by large municipal water treatment systems such as scalable 4-20mA (analog) and high speed pulse (digital) input and output signals. These I/Os not only permit the SCADA system complete control of both pumps but they can also provide solutions for external data logging, remote diagnostics and driving multiple pumps and devices from the primary pump.
The scalable analog output signal provided the IRWD team a simple method for proportionally driving the ammonia pump directly off of the chlorine pump.
The pumps offer outputs to 33.3 gph, a 100:1 turndown ratio and continuous feed. With output pressure ratings to 125 psi and its ability to pump gases, they are suited for use in chlorine dosing applications.
|Article Peristaltic Pumps Excel in Chloramine Application|
Diaphragm Metering Pumps
This type of metering pump will require you to be a bit more knowledgeable about the pump valves, as well as proper priming and adjustment characteristics. Once you understand the pump and work within its normal limits, you should be assured of a successful program.
Peristaltic Metering Pumps
Peristaltic metering pumps are a good choice when pumping dirty fluids that may contain trapped gases or particulate matter, into lower pressure systems. Newer peristaltic pump designs are capable of pressures to 124 psi.
There are more tubing options available for modern peristaltic metering pumps, offering more chemical resistance and longer tube life.
Tube failure has been well addressed with Blue-White’s Exclusive, Patented Tube Failure Detection system (U.S. patents 7,001,153 and 7,284,964).
These pumps are initially easier to begin using than diaphragm metering pumps.
Peristaltic & Diaphragm metering pumps –
Diaphragm metering pumps excel at pumping clean, aggressive chemicals into high-pressure systems, and require very little maintenance. A variety of wetted parts materials are available for chemical resistance. However diaphragm pumps can lose their prime, and can be difficult to prime, especially if the fluid is dirty or contains trapped gases.
Peristaltic metering pumps excel at pumping dirty fluids that contain trapped gases or particulate matter into lower pressure systems. Modern peristaltic pump designs are capable of pressures to 124 psi. Peristaltic pumps will require periodic changing of the pump tube.
Research and a good understanding of both the installation requirements, and the pump’s operating parameters and maintenance requirements, are vital to choosing the best pump for your application.
Getting the Job Done
Paddlewheel flowmeters are easy to install and operate, resulting in a very low overall cost of ownership. Relatively low cost paddlewheel style electronic flowmeters are proof that high cost doesn’t always mean better value.
The components of a system must be able to perform the required task – get the job done – and meet the other physical requirements of the application. Excess capability, features, and accuracy are a waste of money. You will get the most value for your money by purchasing system components that meet the demands of the system without being overkill. While expensive, high technology solutions may be available for your application; low cost paddlewheel flowmeters offer high accuracy solutions to many flow system applications – not just displaying the flow rate and the total accumulated flow amount.
A FEW APPLICATIONS
Paddlewheel flowmeters are used to accurately measure and dispense preset volumes of water or other chemicals.
Water Dispensing System
Water dispensing systems commonly use preset cycle timers to dispense water. When the timer is activated, the system begins dispensing water until the preset time cycle times out. These preset (fixed) cycle timers can result in accuracy problems because they do not actually measure the flow rate! Any problem in the system that results in a change in the flow rate will result in an error in the amount of water dispensed. Some examples include worn pump components and changes in the system pressure, either of which can result in changes in the pump output. The cycle timer control cannot compensate for various flow rates because it is not measuring the flow rate.
Paddlewheel sensors actually measure the amount of water dispensed. When the dispensing system is activated, the electronic flow controller starts the pump and opens the correct dispensing valve. The sensor begins to output electrical pulses. These pulses are then counted by the electronic flow controller. Changes in the output flow rate of the pump will not affect the sensor count. When the correct amount of water has been dispensed, the dispensing valve is closed and the pump stopped.
WATER TREATMENT SYSTEMS
Paddlewheel flowmeters can control chemical metering pump outputs.
Chemical metering pumps are used to inject chemicals such as chlorine and acids into water systems. The chemicalmust be injected into the system at the proper rate to achieve the correct water/chemical proportions. Depending on the application, too much or too little chemical can result in series problems. In a system that has changing water flow rates, a fixed feed rate chemical injection metering pump alone is not capable of reacting to changes in the flow rate of the system. Paddlewheel flowmeters can be used to start and stop inexpensive, fixed feed rate metering pumps resulting in the proper amount of chemical injection. When the meter has measured a specified volume of flow that has passed through the system, the chemical pump is turned on for a pre-programmed amount of time. This simple system results in a pump on time (chemical) per flow volume (water) ratio.
Variable speed pumps are used when near continuous injection of chemical is required. These pumps can be controlled directly by the sensor’s output signal. The high speed sine wave signal can be input directly into the pump’s electronic speed control. The pump speed, and therefore the amount of chemical injected, is programmed to react to changes in the frequency output by the flow sensor. A minimum pump speed is programmed for a frequency and a maximum pump speed is programmed for another frequency resulting in a speed:frequency ratio (pump output rate per flow rate).
Paddlewheel flowmeters can verify chemical injection has occurred.
Paddlewheel sensors are capable of measuring chemical metering pump output rates as low as 1 ounce per minute. When installed on the metering pump, the flow sensor can be used to alert the system operator that an error exists in the system. Some metering pumps include electronics that react to the paddlewheel output signal. If the metering pump should fail to inject chemical due to a pump malfunction, clogged fitting, exhausted chemical container, etc., an alarm output is triggered.
FLOW RATE AND RANGE ALARMS
Paddlewheel flowmeters can monitor system flow rate.
FLOW RANGE ALARM
When a system’s flow rate is critical, a paddlewheel flowmeter can be used to alert the system operator if the rate increases or decreases out of a programmed range. The electronic display can be programmed with a high and low rate amount which will trigger an alarm output signal if reached. The alarm can automatically reset or latch. Trigger and release values can be set, with hysteresis, which will eliminate “flickering” that can occur when the flow rate is at the alarm value.
Paddlewheel flowmeters work best with clean fluids. Particles and debris can prevent the paddle from spinning properly.
Install the pipe fitting in a location that includes a proper length of straight pipe before and after the meter. Because the paddle is inserted only a small distance into the flow stream, the flow stream must be a consistent velocity across the entire inside pipe diameter to obtain an accurate reading. The straight length of pipe will allow any swirl patterns in the flow stream to dissipate before contacting the paddlewheel. Swirl patterns can be caused by obstructions such as an elbow, tee, pump, etc. The minimum straight length of pipe required will depend on the type of obstruction before the paddlewheel. The absolute minimum is typically ten times the nominal pipe size before the meter and 5 times after. Thus, a 4” pipe would require a minimum of 40” (10 x 4) of straight pipe before the paddlewheel and 20” (5 x 4) after. Refer to the manufacturers instructions for specific requirements.
Paddlewheel flowmeters may not function properly with high viscosity fluids. High viscosity fluids will tend to produce a laminar type flow profile. In a laminar flow profile, the center of the flowing fluid is moving faster than the outer edge. A turbulent flow profile, where the fluid velocity is the same across the entire pipe diameter, is required for accuracy. The fluid’s Reynolds Number must be greater then 4000 to ensure a fully developed turbulent flow profile. The Reynolds Number is a dimensionless number that combines the effects of viscosity, density, and flow velocity to identify either a turbulent or laminar flow profile.
REYNOLDS NUMBER EQUATION
The pipe must be full of water at all times. When the system starts and stops, any air in the line may lead to an erroneous reading.
Size the meter to work within the published operating range. Although the meter may read at flow rates other than published, the meter may not be accurate at these rates.
Be sure the saddle is properly installed. Saddle installation, pipe size, alignment and adjustment, is critical to an accurate reading.
HOW PADDLEWHEEL FLOWMETERS WORK
Paddlewheel flowmeters consist of three primary components; the pipe fitting, the paddlewheel sensor, and the display/controller. These components can be purchased separately or as a package to meet the particular requirements of the application. The paddlewheel sensor is designed to be inserted into the pipe fitting. Approximately one half of the paddle protrudes into the flow stream. Fluid flowing through the pipe causes the paddlewheel to spin. As the magnets that are imbedded in the paddle spin past the sensor, electrical pulses are produced that are proportional to the rate of flow. The manufacturer publishes the number of output pulses produced, per volume of flow, for each specific pipe fitting. This number is called the K-factor.
PIPE FITTINGS – Various pipe fittings styles are available. Some fitting styles are designed to install directly into the pipeline using various connection methods such as male or female threads, socket weld, socket fusion, and butt fusion joints . These “in-line” fittings are available in a variety of materials such as PVDF, polypropylene, and stainless steel. They are available with and without union connections. Because the manufacturer can control the inside diameter of the fitting, in-line fittings are available in a variety of operating flow ranges to accommodate various applications.
Saddle style fittings are designed to mount directly on an existing pipe. The saddle is installed by simply drilling a hole in the pipe and clamping the saddle onto the pipe. Cutting the pipe and installing special adapters is not necessary. Saddles are available in a variety of materials.
PADDLEWHEEL SENSORS – The Paddlewheel sensor consists of the paddlewheel with its imbedded magnets and the electronic sensor. Manufacturers offer sensors in a variety of materials to meet most applications. Two types of sensor outputs are available, AC coil and Hall Effect.
AC Coil sensors generate an AC sine wave that is proportional to the flow rate. Because they generate their own power, these sensors do not require external input power. The signal range for an AC coil type sensor is limited to approximately 200 feet due to possible noise interference and voltage drop.
Hall Effect type sensors output a digital, current sinking, DC square wave that is proportional to the flow rate. Circuitry that is sensitive to magnetic fields is triggered by the spinning paddle. This circuitry requires external input voltage to operate. The signal range for a Hall Effect type sensor is approximately 1 mile.
DISPLAYS/CONTROLLER – Flow displays and controllers are used to receive the signal from the paddlewheel sensor, convert the signal into an actual flow rate or flow total value, and display the values. The processed signal can now be used to open and close valves, start and stop pumps, indicate high or low flow rate alarms in the system, or transmit 4-20mA and TTL level pulse signals to external equipment such as a PLC, chart recorder, metering pump, etc.
Paddlewheel flow sensors and display meters/controllers offer low cost solutions to a variety of water system applications.
ABOUT THE AUTHOR
Bill McDowell is a Sales Engineer with Blue-White Industries. He has been with Blue-White Industries for 20 years and has also held the position of Project Engineer and Director of Engineering. Bill resides in Garden Grove, California with his wife Jana and their two children Jillian and Sean.
For additional information, contact Blue-White Industries,
5300 Business Drive, Huntington Beach, CA 92649.
Phone 714-893-8529, Fax 714-894-9492,