Sizing and Selecting the Proper Metering Pump

Metering pumps have long set the standard in chemical-injection applications, but only if the operator selects a model that addresses all of the many operational variables. This article will explain the different metering-pump technologies and show how the user should select the proper size and style.

By Tom O’Donnell

A typical process application can consume large amounts of chemicals during daily operations. While the volumes may be large, though, they must also be very precise, with exact amounts delivered according to strict injection schedules.

The pressures - from very high to very low - that the chemicals are injected at must also be governed precisely. This demand for precise flow rates at predetermined schedules and pressures requires the use of a very specific pumping technology, one that can ensure successful, reliable adherence to the injection schedule and fluid volumes.

Metering pumps have - whether mechanically, hydraulically or electronically actuated - risen to the fore as a technology of first choice for the injection of chemicals in processing and agricultural applications. Yet within the realm of metering pumps there are many different styles and modes of operation.

Know The Variables

When determining which metering pump is the best option for a chemical-injection application there are a number of variables that must be taken into consideration, including:

• Flow Rate. Metering pumps should never be oversized, meaning that determining the exact flow rate that is required for the application is of paramount importance. With that in mind, a metering pump should be sized so that the maximum expected flow rate is 85% to 90% of the pump’s capacity, which will leave room for additional capacity, if needed.
• Materials of Construction. Metering pumps are available in a variety of materials, most commonly 316 stainless steel, C-20 stainless steel, PVC and Kynar® PVDF. When selecting a metering pump’s materials of construction, the corrosion, erosion and solvent action of the chemical must be taken into consideration. The effects of erosion must also be considered when the chemical takes the form of an abrasive slurry.
• Chemical Makeup. Standard metering pumps are typically able to handle clear liquids with viscosities ranging from water-like to 1,500 cPs. Chemicals with viscosities that approach 5,000 cPs or have light suspensions will require special liquid ends. Those with viscosities up to 20,000 cPs or that contain up to 10% solids will require special diaphragms, while ones that automatically vent accumulated gas will need their own variety of liquid ends.
• Driver. Drivers, which can be powered by electricity, water, gas, air or the sun, must be selected according to the utilities that are available. Driver selection must also take into account any environmental hazards that may be found in the operating area, with the operator realizing that pumps used in remote locations may not be able to be inspected as often as those in controlled environments.
• Environment. Determine if the pump will be operating indoors or outdoors. If used outdoors, the pump must be sheltered from direct sunlight. Any pumps that will be used in freezing temperatures can only pump fluids that will not freeze at that temperature.
• Method of Control. The operator must know how the pump will be used, either manual continuous operation, on/off operation of operation that is governed by a process signal.

When all is said and done, method of control may be the most important variable when choosing a metering pump, so let’s take a closer look. Many styles of metering pumps allow their flow rate to be adjusted manually through the use of a micrometer dial. Adjusting this dial changes the pump’s stroke length and allows the pump to be operated anywhere between 10% and 100% of its rated flow capacity.

Metering pumps with micrometer dials may also feature a variable speed drive that allows adjustment of the pump’s stroke speed. Using the two in unison can allow additional adjustability or turndown capabilities over the range of the drive, depending on the pump’s stroking speed.

For example, a pump operating at 75 strokes per minute (spm) would produce a 5:1 turndown ratio on speed with the variable speed drive and a 10:1 turndown ratio on stroke length via the micrometer dial.

Rising up: although metering pumps have become the tech of choice for chemical injection, there remains many choices

Flow rates can also be controlled automatically by a process signal and through the implementation of electric or pneumatic positioners that can adjust the stroke length, which will deliver a full 10:1 turndown ratio. In this method of control, the number of doses will remain constant with the size of each dose reduced, resulting in doses that are uniformly distributed in a constantly flowing line.

Metering pumps that use a variable speed drive will deliver a turndown ratio that is determined by dividing the pump’s stroke speed by its minimum operating speed. A variable speed drive will enable the pump to inject a dose of the same size on each stroke, but since the stroke speeds will change, the doses themselves will be less frequent.

Additionally, it is not practical to use a variable speed drive on motor-driven pumps that normally operate at speeds less than 100 to 150 spm since slowing the motor causes each stroke to take longer to complete from start to finish. However, electronic metering pumps, which are pulsed by a solenoid, can operate at less than one spm because the timing of each stroke from start to finish is uniform at every stroking speed.

Planning an Installation

So now that all of the variables have been identified and reviewed, it’s time to design the pumping system, keeping in mind any location or environmental concerns that may be present. For best results, the system’s design should originate from the liquid source or feed tank and work its way to the injection point while keeping in mind that metering pumps will be able to “push” against great pressures but will struggle to “pull” over longer distances.

This means that suction lift should be limited to no more than four feet (1.2 meters) and that a foot valve should be used in top-mount installations. Flooded suction is always preferred in a metering-pump installation as it makes the pump easier to prime, but the length of the flooded suction should be limited to six or seven feet (1.8 to 2.1 meters). And, as with all pump installations, adequately sized lines should be used and piping runs should have minimal bends, elbows or other design characteristics that may restrict or limit flow.

Components Considerations

• Suction Piping. The traditional rule of thumb is to use suction piping that is one size larger than the pump’s suction connection, though it is acceptable to use piping that is the same size as the suction connection if the metering pump will be operating at a slow speed when transferring low-viscosity chemicals. Generally, do not use hard piping that is smaller than 1/2” in diameter or that is smaller than 3/8” in diameter for low-flow applications that use plastic tubing.
• Discharge Piping. The size of discharge piping is not as critical as that of suction piping, but the piping must be suitable for the discharge pressure. Typically, matching the pipe size to the discharge connection should be sufficient to ensure proper and reliable operation.
• Flanges/Unions/Compression Fittings. At least one of these must be installed at the pump’s suction and discharge ports in order to more easily facilitate maintenance procedures
• Isolation Valves. Large-port, quick-opening isolation valves should be placed at both the suction and discharge ends of the installation as a way to ease maintenance operations. Ball valves are usually the best choice for this operation, while needle valves should not be used because their design will cause the creation of a flow restriction.
• Calibration Column. Because metering pumps very often feature pulsed flow at low volumes a draw-down calibration column is the most accurate and convenient method to measure pump performance, provided that the liquid in the column draws down smoothly and stops precisely at the end of each suction stroke. A tall, thin column should be used to ensure ease of reading and reporting accuracy.
  Calibration columns can also be helpful in determining if any wear has occurred or dirt has accumulated in the pump’s check valves; if the liquid in the column “bounces,” that may indicate that the valves are worn or dirty.
• Relief Valve. Though the metering pump may be constructed with an internal relief valve, it is recommended that an external relief valve also be installed. The external relief valve should be set at 50 psi (3.5 bar) or 10% above the maximum operating pressure, whichever is greater. Any chemical that flows through the relief valve is piped back to the feed tank. Using transparent tubing for the relief valve will allow the operator to observe any returned fluid and identify any impurities.
• Back-Pressure Valve. This component is only necessary when the installation does not produce adequate back pressure and the pump does not contain a built-in back-pressure device. Back-pressure valves are also required when a system has a low-pressure injection point that is hydraulically lower than the feed tank.

Conclusion

In process or agricultural applications where precise, reliable, efficient injection of chemicals is critical to optimise performance, metering pumps have been proven over the years to deliver the required method of operation.

However, knowing that requiring a metering pump is not nearly enough to ensure that the best pump will be chosen for the job. A long list of important variables must be considered, along with an array of pump components that will help optimise performance, if deployed appropriately. Successful chemical injection via metering pump will only be achieved if all variables are taken into consideration and system installation adheres to suggested protocols.

Tom O’Donnell is director of business development for Neptune Chemical Pump Company and PSG.