As a consequence of economic constraining and increasingly stringent environmental and occupational health and safety regulations, it becomes imperative that applications requiring the dispensing and metering of expensive and hazardous chemicals be accomplished with the utmost accuracy, care and control.
When excess chemical is dispensed into the service, addition of another chemical is required to offset this action, resulting in increased expense. Conversely, if enough chemical is not dispensed into the process, the batch may be unsatisfactory for use and discarded and the processing repeated at further expense. Environmental and safety concerns are also important considerations.
It is thus important to plan the entire installation from the day tank or other liquid source up to the injection point and determine which accessories will be included. The following is a general outline covering typical metering pump applications, and the accessory items that have been developed to help enhance the performance of your metering pump, for the optimum control of chemical being used.
A – Chemical Tank
The chemical tanks is ether used to store chemicals or blend chemicals. It is generally a good idea to have the suction above the floor of the tank so that if there are any solids that have settled out they are not drawn into the suction line.
C – Suction Strainer
A Strainer on the suction feed line is another item generally overlooked and can contribute to the proper operation, life and accuracy of the components downstream.
Small bits of debris often find their way into supply tanks and will foul the function of the check assemblies, imbed themselves in the diaphragm or score the piston and cause premature failure of the pump. If the pump allows this debris further downstream, you may see failure of the back pressure valve or pulsation dampener or even more typically, the injection valve will get fouled and fail.
The strainer should always be installed, periodically checked and cleaned. This will give a large pay back by ensuring the uninterrupted service and longer life of vital and more expensive components downstream.
E – Calibration Cylinder
Typically, metering pumps are used without proper calibration. The manufacturer provides a performance curve detailing the general discharge capacities of the pump. These curves are normally derived under controlled conditions, using water as the testing fluid. Given the large variety of chemicals available, with varying viscosities and specific gravities, and the wide differences in suction conditions on the pump and discharge piping, it is only reasonable that each application needs to be calibrated individually.
Calibration must be repeated from time to time due to wear and vibration in the system. Calibration cylinders are available in St.St, PVC, polypropylene construction. The cylinder tubes are manufactured from glass or PVC. They provide an excellent way to periodically check the performance and accuracy of your metering pump.
G – Pulsation Damper Air Release Valve
This is required to refill the damper. H – Discharge Pipe Drain Valve Tee off the discharge of the pump at the lowest point back to tank or install an inline pressure relief valve that has the ability of venting back to the feed tank or calibration cylinder.
J – Dosing Pump Suction Isolating Valve
This valve is used to isolate the pump when maintenance is performed to protect the mechanic from the chemical.
L – Metering Pump
Whether diaphragm or piston style, these pumps generally incorporate check valves as the mechanical source to isolate the flow of the chemical, at each stroke of the diaphragm or piston. The response time of the check valve assemblies, enabling them to reset at the end and beginning of each stroke is essential to the performance and continuous accuracy of the metering pump.
N – Discharge Piping
Specify piping suitable for the discharge pressure. Discharge pipe size is not as critical as the suction pipe size. Matching the pipe size to the discharge connection size is usually sufficient. When installing a metering pump system, it is always imperative that you install as many unions in the line as possible to allow for ease of maintenance and repair of the various components used.
Typically, this can be easily accomplished with the use of true union valves, which also serve the purpose of isolating valves. When handling very hazardous chemicals and indeed any chemicals, always remember to design your piping system with a way to drain off the chemical and vent any built up pressure before service begins. This will prevent most of the spillage and reduce potential hazard in the workplace.
P – Discharge Isolating Valve
As with the suction isolating valve this valve is used to isolate the discharge piping from spilling back chemical when maintenance is required.
R – Discharge Pressure Gauge
In order to check if the metering pump is operating correctly, it is essential to install a pressure gauge on the discharge line. The pressure gauge shows the actual working pressure of the metering pump. This value must not exceed the maximum allowable pressure of the pump.
T – Flowmeter
Dosing pumps installed correctly are very accurate, however in some cases the flow can be monitored using a flowmeter. When using a flowmeter a pulsation damper is generally required to ensure a smooth measurable flow.
B – Suction Isolating Valve
Provide valves on both suction and discharge for ease of maintenance. Select large port, quick opening valves. A ball valve has a generous opening and is easily stroked from full closed to full open position. A needle valve would not be an acceptable suction valve as the port design would cause a restriction. It is also required in conjunction with the calibration cylinder to isolate the chemical tank.
D – Calibration Cylinder Isolation Valve
When running a calibration this valve is opened and the suction isolating valve is closed. When in normal operation this valve is closed and the suction valve is open.
F – Suction Pulsation Damper
A bottle type pulsation damper may be required in some installations where the suction piping is long or has many bends, this is because metering pumps ‘push’ against pressures but they do not ‘pull’ very well. The damper will reduce the effects of acceleration pressure losses, thus assisting the pump to perform correctly.
I – Suction Piping
Limit the suction to 1.5 m in a suction lift application if possible. A foot valve must be used in a top mount installation. Typically, combination foot valve strainers are used.
Limit the length of a flooded suction to 2.5m or seek application assistance from PMPS. Use an adequately sized line. Minimize bends, elbows or other restrictions.
The single, safest rule of thumb for selecting suction pipe size is to use one size larger than pump suction connection. Piping may be the same size as suction connection for slow speed pumps used with low viscosity chemicals. As a practical matter, do not use hard piping smaller than ½”. For low pressure, low temperature, low flow applications that use plastic tubing, 3/8” is a practical minimum size.
K – Suction Piping Drain Valve
Is used to drain the suction piping when maintenance is required.
M – Stroke Length Actuator
When remote or automatic control of the pump is required the dosing pumps can be fitted with a stroke length actuator, this can either be pneumatic or electrically actuated. The control signal could be 4….20mAmp, 0..10V, pulse, or profibus.
O – Pressure Relief Valve
When using motor driven pumps or solenoid pumps capable of higher pressures than you line is designed for (i.e. 150 psig), an inline pressure relief valve must be installed to protect the line from over-pressure and possible splitting which could cause uncontrolled discharge of hazardous chemical into the area.
Always install the pressure relief valve in the line closest to the discharge of the pump and ensure that there are no isolation valves or components capable of closing the discharge line off prior to the relief valve. Pressure relief valves are available in a wide range of materials and sizes with adjustable pressure settings.
Q – Pulsation Damper
Pulsation is another problem that is typical with most metering pumps and in some cases cannot be tolerated by the application. Pulsation dampeners are available in a variety of sizes and materials for such situations and help remove a high degree of pulsing and surging in the line.
They only work if installed properly, in the right order in the line, and with the right pressure bladder to offset the incoming pulse. It is important that the directions supplied by the manufacturer regarding recommended set pressure and location of the dampener are adhered to.
S – Pressure Keeping Valve
Many metering pump applications dispense to atmospheric conditions or into a process with less than 1.5 bar pressure and more typically, into a process with erratic system pressure. These applications require back pressure to ensure a constant pressure for the discharge check assembly on the metering pump to work properly. It is crucial that Back Pressure Valves be installed in the discharge piping of the pump to ensure a constant pressure for the discharge check assembly to work under.
This allows for the repeatability of a constant fluid discharge per stroke, and accuracy desired. Additionally, back pressure valves automatically provide anti-siphon protection and are available in a variety of sizes and materials.
U – Injection Valve / Quill
A check valve, normally spring loaded is used for the purpose of isolating your discharge chemical line from your process line. This valve should be utilized and installed at the point of injection and have an isolation valve in close proximity. There are all types of injection valves available to accommodate the various requirements of injection needed, dependent on the process pipe line size and volume.
Injection quills ensure that the chemical is fully dispersed into the centre of the process line and provide for a more homogenous mix in the pipeline. A built-in check prevents back siphoning.
Properly designed, installed and maintained metering pump applications should provide dependable and accurate service with minimal downtime. In order to minimize maintenance and system problems, and optimize the efficient use of chemicals, a modest investment in the suggested accessories is recommended. If you have any questions regarding application installation or applications that are a problem and/or continuous expense, please contact our office for some free and friendly consultation.