Pumps & Systems, July and August 2007
A review of traditional metering pump designs, along with a look at some of the new technologies that are being introduced.
The metering pump is a positive displacement chemical dosing device with the ability to vary capacity manually or automatically as process conditions require. It features a high level of repetitive accuracy and is capable of pumping a wide range of chemicals, including acids, bases, corrosives or viscous liquids and slurries.
Basic Components
The pumping action is developed by a reciprocating piston which is either in direct contact with the process fluid, or is shielded from the fluid by a diaphragm. Diaphragms are actuated by hydraulic fluid between the piston and the diaphragm. Metering pumps are generally used in applications where one or more of the following conditions exist:
- Low flow rates in ml/hr or gph are required
- High system pressure exists
- High accuracy feed rate is demanded
- Dosing is controlled by computer, microprocessor, DCS, PLC, or flow proportioning
- Corrosive, hazardous, or high temperature fluids are handled
- Viscous fluids or slurries need to be pumped
The metering pump is usually driven by an AC constant speed motor. Variable speed, pneumatic, and hydraulic drivers are also utilized. The liquid end design and materials of construction are determined by the service conditions and the nature of the fluid to be handled. Temperature, flow rate, fluid viscosity, corrosiveness and other factors are considered.
The drive mechanism translates the rotary motion of the driver into reciprocating movement. Industrial duty pumps will submerge this portion of the pump in an oil bath to assure reliability during continuous operation. Pump flow rate is adjustable by varying stroke length, effective stroke length, or stroking speed. Most metering pumps are supplied with a micrometer screw adjustment. The micrometer can also be replaced by an electronic or pneumatic actuator to adjust pump flow rate in response to process signal.
Metering Pump Characteristics
The pumping action is developed by a reciprocating piston. This reciprocating motion develops a flow easily represented by a sine wave. Actual flow rate is determined by the following formula:
Flow rate = Displacement x Cycles per unit of time
The metering pump flow vs. stroke characteristic curve is linear. It is not, however, necessarily proportional, in that 50 percent stroke setting may not equal 50 percent flow. This is due to the fact that the calibration line may not pass through 0 on both axes simultaneously.
By measuring flow at two stroke settings, plotting both points and drawing a straight line through them, other flow rates vs. stroke can be accurately predicted. The steady state accuracy of a correctly installed industrial grade metering pump is generally +/- 1.0 percent or better. Although a metering pump can generally be adjusted to pump at any flow rate between 0 and its maximum capacity, its accuracy is measured over a range determined by the pump's turndown ratio.
Most metering pumps have a turndown ratio of 10:1, which means that the pump is within its accuracy rating anywhere between 10 percent and 100 percent of capacity. New generation metering pumps have been introduced that feature higher accuracy, and a greater turndown ratio of 100:1. Some of these new designs will accurately dose anywhere between 1 percent and 100 percent of capacity.
The maximum capacity of a metering pump is determined by gear ratio, piston diameter, and motor RPM.
Liquid End Designs
The liquid end, which is referred to as the wetted part of the pump, is selected to meet the specific service conditions of the application. Required flow and pressure ratings are considered, as well as the physical and chemical properties of the liquid. The liquid end's ability to protect the environment is also a major consideration when dealing with toxic or hazardous chemicals.
All liquid ends have several features in common. First, the liquid is drawn into the wetted end by the rearward motion of a piston, and expelled by the forward motion. To achieve this, the metering pump is supplied with check valves at the suction and discharge connection points. The check valves contain and release the chemical based on system conditions and gravity.
During the suction portion of the stroke, the motion of the piston lifts the suction ball check from its seat, allowing liquid into the pump. At the same time, the piston's motion and system back pressure hold the upper check valve (discharge) closed. This is then reversed during the discharge stroke.
Check valves are available in several different designs and configurations. There are single or double ball configurations and poppet style check valves. Selecting which type to use can be determined by the manufacturer based on capacity required of the specific pump.
For example, slurries or liquids with large fibers or particles can cause a single ball to leak if particles are trapped between the ball and seat. Therefore, a double ball check offers more stability and accuracy. On the other hand, since each check valve causes some resistance in the flow path even when open, viscous fluids are better handled with a single ball suction check valve.
Packed Plunger
The packed plunger style liquid end is the only liquid end in which the piston is in direct contact with the process fluid. This direct contact offers a number of advantages, including: high suction and discharge pressure capabilities; high temperature resistance, and lowest NPSH requirements.
The reciprocating piston requires packing to seal the wetted parts from the atmosphere. This simple design is effective, but places limitations on the use of packed plunger pumps in certain applications. Because a small amount of controlled leakage past the packing must be expected, this style liquid end should not be used with hazardous or toxic chemicals.
Additionally, the friction between the piston and the packing results in wear that increases leakage. Periodic packing adjustment is necessary to maintain volumetric efficiency. To avoid problems associated with leakage, consider a diaphragm style liquid end. The packed plunger can handle pressures up to 15,000-psi, and temperatures to 600-deg F (with special modifications).
Disc Diaphragm
Certain disc diaphragm liquid ends use a teflon diaphragm to act as a barrier between the piston and the process fluid. The piston's pumping motion is applied to hydraulic fluid which causes the diaphragm to flex back and forth as the piston reciprocates.
The hydraulically actuated diaphragm operates with equal pressure between the hydraulic and process fluids. This eliminates diaphragm stress, since the pressure is essentially equal on both sides at all times. Two contour plates encase the diaphragm to contain its travel.
The hydraulic and process fluids pass through carefully engineered holes in the contour plates in order to come into contact with the diaphragm. Relief and refill valves control the volume of hydraulic fluid. An automatic air bleed valve continuously purges air from the hydraulic fluid.
The diaphragm style pump is sealed, making it an excellent choice for hazardous, toxic, or corrosive chemicals. For extra protection, double diaphragm and leak detection modifications are available, although they are considered redundant since this design is extremely durable.
Because the process fluid must pass through relatively small holes in the contour plate, the disc diaphragm liquid end is not the best choice for slurries. With a few exceptions, disc diaphragms are usually not the best choice when pumping viscous fluids. The disc diaphragm is capable of handling fluids where the required injection pressure is 3500-psi or greater and the fluid temperature exceeds 250-deg F.
Mechanically Actuated Diaphragm Design
A mechanically-actuated diaphragm pump represents the best balance between low pump cost and high quality performance. Because it has zero diaphragm leakage, it makes a great pump for critical and otherwise expensive chemicals or where environmental issues are involved.
The mechanically-actuated series is an excellent choice where slurries and abrasive chemicals are required up to the pump's maximum flow and pressure ranges. They are also well tolerant of high viscosity liquids, providing an economical solution for a variety of difficult applications.
Mechanically-actuated pumps operate with a plunger directly attached to the diaphragm. This attachment generally takes place from a bolt and clamp being placed through the plunger and through the diaphragm. The direct attachment of the piston to the diaphragm connects the pump's drive and motor to the liquid end. The motion of the pump drive moves the plunger back and forth, thereby causing suction from the supply tank and pumping the fluid of choice through the attached conveyance infrastructure.
This type of pump generally finds pressure peaks at 175-psi, but is only limited to flow as a matter of wetted end volume. Maximum life of the pump can be achieved by replacing the diaphragm at the recommended service interval. Leak detection can be easily found from the air-filled chamber residing generally at atmospheric pressure on the drive side of the liquid end.
As with any chemical where gas binding can be a problem, it is recommended that a degassing valve be used to release off-gases from the agitation or pressure changes experienced by a liquid having off-gas characteristics. Some of these liquids that can generate off-gases as a result of pressure losses are NaOCl, H2O2, and some specialty chemicals.
Mechanically-actuated pumps work well in these applications, providing 10:1 turndown as a standard. The addition of VFD technology and remote stroke control will bring the turndown as high as 100:1. Mechanically-actuated diaphragm pumps are easily maintained and provide years of service for little effort.
Metallic Diaphragm Liquid End and Critical Head Service
Metallic diaphragm metering pumps are ideal for use in critical, high pressure applications such as oil and gas platforms and specialty industrial applications. They are especially useful where temperatures and pressures of both the environment and the process chemical can be variable or otherwise difficult. These pumps are known for their longevity and durability in many difficult applications.
Metallic diaphragm metering pumps are hydraulically-actuated in the same manner and style as a standard hydraulically-actuated drive liquid end. However, the teflon or other usual diaphragm material is replaced with a special metal alloy particular to the application to produce higher pressures than more traditional materials. The metal design of the diaphragm also manages difficult chemicals such as abrasives, slurries and other special requirement compounds easier and more efficiently than its more standard version.
Many oil and gas offshore drilling platforms require metallic diaphragms because of their high reliability and longevity.
Advanced Liquid End Technology: High Performance Diaphragm
A high performance diaphragm (HPD) liquid end operation is similar to a disc diaphragm in that it is hydraulically actuated and utilizes the same shape and diaphragm. It is similar to a tubular diaphragm in the respect that the process fluid has a "straight through" path through the liquid end. Its low NPSH requirements are similar to that of a packed plunger liquid end.
The primary advantages of a HPD are the unique design features that separate it from traditional design.
A hydraulically actuated diaphragm liquid end design requires a refill system to compensate for hydraulic fluid that bleeds past the piston or through an air bleed valve during normal operation. Hydraulic fluid is also expelled from the chamber through the internal relief valve when the system experiences excess pressure, and therefore must also be replenished. A HPD features a mechanically actuated refill system (MARS) that offers a number of advantages over traditional refill systems. To understand the advantages of a MARS, traditional refill systems must first be explored.
Traditional Designs
Traditional designs use a system that refills the chamber when a vacuum is created by the inability of the diaphragm to move beyond the hydraulic contour plate. It also refills when the suction is momentarily or permanently starved by accidental valve closure, insufficient NPSH, or other similar occurrences. When this happens, the hydraulic fluid chamber is overfilled because a vacuum has been created even though the diaphragm has not been able to travel rearward.
To avoid diaphragm rupture due to overfilled hydraulic oil, a process side contour plate stops the diaphragm's forward travel, and forces the hydraulic relief valve to open, thus expelling the excess fluid. The contour plate is a concave (actually, concavo-convex) disc that supports the diaphragm and limits its travel. The plate has a series of holes bored through it to permit the fluid to come into contact with the diaphragm. The pattern and size of these holes requires careful engineering to maintain the contour plate strength required to withstand the force of the diaphragm experienced at operating pressure.
The hydraulic contour plate does not cause any problems in pump operation since the hydraulic fluid passes easily through the contour plate holes. However, a process contour plate, required by traditional disc diaphragm liquid ends, places limitations on the types of process fluids the pump can handle (such as slurries) since the process fluid must also pass through contour plate holes. The process contour plate also creates a pressure loss which raises the NPSH requirement of the liquid end.
MARS Design
A MARS eliminates the need for a process contour plate by assuring that the hydraulic fluid can only be refilled when the diaphragm has traveled all the way back to the hydraulic contour plate. The diaphragm presses against the MARS valve, which only then permits a poppet valve to open from the vacuum created by insufficient hydraulic fluid. Hydraulic overfill is therefore impossible.
With the process contour plate gone, the straight through path of the process liquid makes a HPD a perfect choice for slurries and viscous materials. It also lowers the NPSH requirements of the pump, since pressure loss through a process contour plate is eliminated.
A MARS also simplifies HPD start-up. Unlike other hydraulic liquid ends, the refill valve does not need adjustment. Additionally, since a HPD hydraulic fluid cannot be overfilled, there is no need to perform delicate procedures to synchronize hydraulic fluid balances (a difficult task required for tubular and other double diaphragm liquid ends). With a HPD, just fill the reservoirs and turn it on.
HPD Preshaped Composite Diaphragm
A HPD features a preshaped PTFE/elastomer composite disc diaphragm. On the process side, the chemical resistance of PTFE is utilized. On the hydraulic side, the elastomer imparts favorable elastic and mechanical factors.
A composite diaphragm eliminates the inherent problems of pure PTFE diaphragms. PTFE tends to cold flow when compressed between two metal parts (such as those required to seal the hydraulic side from the process side). A HPD composite diaphragm features an integral O-ring seal around the perimeter of the diaphragm, which provides a better seal between hydraulic and process fluids than conventional diaphragm materials. A HPD is capable of handling pressures up to 3025-psi and temperatures up to 300-deg F (with special modifications).
Metering Pump Drive Mechanisms
Ideally, drive mechanisms feature gears that are submerged in an oil bath to assure long life. Capacity can be adjusted while the pump is running or stopped, ± 1.0 percent accuracy over a 10:1 turndown ratio.
Hydraulic Bypass
Some designs feature a hydraulic bypass mechanism that uses a piston with a constant stroke length that pumps hydraulic fluid, thus transferring the pumping motion to a diaphragm. Therefore, this type of drive can only mate with a hydraulically actuated diaphragm liquid end.
Capacity is varied by changing the location of a hydraulic bypass port over the piston's path of travel. If the port is positioned at 50 percent of the piston's stroke length, hydraulic fluid will be relieved through the port during the first half of the piston's stroke, and pumped against the diaphragm during the remaining half.
This type of drive is often called hydraulic lost motion, because a portion of the piston's travel does not transmit pumping energy when the capacity adjustment is less than 100 percent. Hydraulic bypass style pumps can develop reciprocating piston motion by way of a worm gear set and eccentric.
In these type of pumps, a piston pumps hydraulic fluid, which either forces the diaphragm to flex or is relieved through the bypass port. A control valve positions the port based on a desired capacity setting. A disc diaphragm liquid end can feature simplex or duplex liquid ends, with maximum capacities ranging between 0.43-gph and 85-gph (170-gph Duplex), and maximum pressures up to 1800-psi.
In some hydraulic bypass style pumps, capacity can be varied by positioning a stroke adjust sleeve over bypass ports bored through the hollow piston. When operating at 100 percent the ports are covered, which traps hydraulic fluid in the hydraulic pumping chamber. Once trapped, the piston's pumping action forces the hydraulic fluid to flex the diaphragm.
A cup valve attached to the diaphragm closes all hydraulic paths to the diaphragm when it reaches the full forward position. This eliminates a process contour plate, as well as excessive hydraulic pressure on the diaphragm, since any excess hydraulic fluid in the hydraulic pumping chamber cannot reach the diaphragm and is forced through the internal relief valve to the fluid reservoir.
A hydraulic bypass style pump can be an excellent choice for a mid-range metering pump capacity at low pressure. Its design is more economical than high pressure pumps in the same capacity range, without sacrificing ruggedness and accuracy. The "straight through" process fluid path allows this metering pump to be applied to many of the same services as the HPD (high performance diaphragm) liquid end.
Polar Crank
Some metering pumps use a polar crank, an advanced and reliable variable stroke length drive available in high pressure/high flow industrial duty metering pumps.
In the polar crank drive, a high speed worm gear reduces the RPM supplied by the motor, and provides the lower RPM to a rotating crank. A connecting rod with spherical bearings on each end links the crank to the crosshead and piston assembly. The worm gear and crank assembly pivots in an arc about the worm shaft center to change stroke length. The piston stroke length is determined by the angle of the assembly.
For example, when the pump is at zero stroke, the worm/crank assembly is in a vertical position. The crank then rotates in a vertical plane, and one end of the connecting rod revolves with it. The crosshead and the piston remain stationary because no reciprocating action is produced. When the pump is adjusted full stroke (or maximum capacity), the rotating crank is moved to its maximum angle from the vertical axis.
At the top of the rotation cycle, the connecting rod is pushed forward, moving the crosshead and piston to the full forward position at the end of the discharge stroke. As the crank continues to rotate, the angle of the crank causes the connecting rod to pull the crosshead and piston until it reaches the full rearward position, at which point the connecting rod has reached the bottom of the rotation cycle.
Regardless of the stroke length setting, the top of the rotation cycle always forces the crosshead and the piston to the full forward position at the end of each discharge stroke. This assures complete scavenging of the liquid end during each stroke cycle. The angle of the polar crank can be adjusted in infinite increments between zero and maximum stroke for extremely accurate controlled volume pump settings.
A polar crank drive can feature maximum capacity ranges between 0.033-gph (125-mL/hr) and 2510-gph depending on frame size, stroking speed, and plunger diameter. Discharge pressures are rated up to 7500-psi and up to eight pumps can be multiplexed and driven by one motor. Polar cranks can include HPD, packed plunger, disc diaphragm, or tubular diaphragm liquid ends.
To achieve a high thrust capacity and extend component life, some polar crank drives feature a pressurized lubrication system. This positive oil pressure lubrication ensures long bearing life and permits the pump to operate at very high suction and discharge pressures.
As the crosshead moves forward during the discharge stroke, oil from the reservoir is drawn up through a ball check into a cavity in the crosshead. During the suction (rearward) stroke the lubricant is trapped. It is then forced through the crosshead, into the crosshead connecting rod bearing, through the hollow connecting rod, and finally to the crank connecting rod bearing.
By forcing the oil through this path, every moving part is lubricated during every complete cycle of the pump. To reduce the wear of moving parts and extend oil life, a magnetic strainer cleans the oil before it enters the pressurized system.
Advanced Drive Technology
A new innovation in metering pumps combines advanced gear reduction design with electronic variable speed drives to achieve twice the accuracy over a turndown ratio (100:1) ten times greater than traditional designs, with ± 0.5 percent steady state accuracy.
The most significant difference from traditional systems is the close relationship between the driver and drive mechanism and how they enhance overall pump performance. The operating characteristics of this technology are the result of a unique constant stroke length drive mechanism that depends on a special electronic variable speed drive to vary pump flow rate.
Traditional designs utilize worm gear sets to convert motor rotation to reciprocating motion through an eccentric or similar mechanism. Worm gears are the best choice when the drive mechanism is required to incorporate a variable stroke length adjustment. They operate well at high rotary speeds, due to an oil shield that develops between the gear surfaces. Unfortunately, they lose that shield at lower speeds, which causes wear and raises motor torque requirements. This limits worm gears to a 10:1 turndown ratio.
This new technology utilizes a special helical gear set. Since there is no need for stroke length adjustment, the gear set is very simple. Helical gears are known for operating with low noise and low friction. Combined with a basic scotch yoke mechanism to develop reciprocating motion, this gear set runs quietly and efficiently with few moving parts. All moving parts are submerged in oil to ensure long life.
Figure 1 indicates the greatest advantage of the special gear design. The flat torque curve (vs. the worm gear set) allows the technology to operate at or below 1 percent of speed without placing extra demand on the motor, thus permitting a 100:1 turndown ratio. The simple drive mechanism also allows the product to be easily duplexed within the same housing by adding a second piston and liquid end opposite the primary liquid end. The capacity can therefore be doubled economically and efficiently.
Conventional AC and DC variable speed drives are limited to turndown ratios between 5:1 and 30:1. This is insufficient to take advantage of the turndown capabilities of this gear design. New technology in brushless DC motors and controllers has created advanced drives capable of strong operation at low speeds. The variable speed drive can deliver rated torque at less than 1 percent (100:1 turndown) of maximum rated RPM while maintaining steady state speed control at better than ± 0.1 percent. It works well with the drive mechanism.
The flexibility of a 100:1 turndown ratio permits these metering pumps to be applied where a wide range of dosage rates are required. It can also provide built-in growth potential by pumping efficiently in systems requiring a fraction of the pump's capacity for the short term during startup or early phases of an expanding project. These pumps can provide all this without compromising accuracy or drive power.
This new technology can respond instantly to changes in dosage rate. Its constant stroke length does not upset the balance within the liquid end hydraulic system; that is, hydraulic fluid volume remains constant. When the hydraulic balance is disturbed, as in variable stroke length designs, the full result of dosage changes can take minutes or hours. The instant response can provide smoother operation in closed loop or automated systems, and also assure proper dosage at all time in systems requiring very close dosage tolerance.
The ± 0.5 percent steady state accuracy over the full turndown ratio is a result of the constant stroke length and the precise speed control of the drive. This level of accuracy provides maximum chemical economy while assuring stable automatic operation and optimum process quality.
Metering Pump System Components
Proper metering pump system operation depends upon the selection of appropriate system components suited for the application requirements.
Safety Relief Valves
Most piping systems require the use of an external safety valve to protect the piping from over-pressure. Diaphragm pumps feature internal safety valves to protect the pump, but external safety valves are still recommended. Safety relief valves should match the operating pressures of the pumps being used. Typical valve materials include specialty steel, 316SS, alloy 20, and PVC.
Back Pressure Valves
To prevent unmetered liquid from free-flowing through the pump, metering pump systems require a greater pressure in the discharge line than the suction or inlet line. When the process does not supply a minimum of 25-psi above the suction pressure, a back pressure valve is required. Typical valve materials include specialty steel, 316SS, alloy 20 and PVC.
Pulsation Dampeners
The metering pump's reciprocating motion provides a pulsating discharge flow. Applications requiring a steady flow can eliminate over 90 percent of the pulsations with a pulsation dampener. Typical dampeners are available for pressures to 1000-psi. Sizing is based on cubic inch/stroke displacement of the specific pump.
Calibration Columns
Metering pumps should be factory tested. Once installed, pump calibration should be periodically determined to verify proper operation, especially after the performance of any maintenance. Calibration columns can provide an inexpensive means of assuring pumping accuracy.
Mixers
Accurate dosing requires proper mixing of the solution being pumped. Typical mixers are direct drive, high speed units designed for mixing medium and low viscosity fluids and dispersion of light solids.
Tank Chemical Feed Systems
Tanks are constructed of steel, stainless steel, and polyethylene, and are often available with the pumps and mixers mounted, plumbed, and ready for installation.
Strainers/Sludge Traps
A metering pump's check valves should be protected from particles and debris by installing a strainer in the suction line. When pumping concentrated sulfuric acid, a sludge trap is required to trap sludge particles while providing easy cleaning or flushing. Foot valves and strainers can be used for applications that pump fluid from replaceable drums. "Y" type strainers are sometimes used for inline protection in systems.
Chemical Dosing Systems
A typical chemical dosing pre-engineered system provides manual control and all accessories to allow for proper operation. Other systems can pace the dosing from a single input or use a line of instruments to provide a full closed-loop solution.
Instruments
Water quality meters can be used as standalone products or in conjunction with a chemical dosing system. Water quality, including pH, ORP, chlorine residuals, DO (dissolved oxygen), and other necessary water quality measurements, should be measured.
Laurel Bloch is the marketing manager for Milton Roy, 201 Ivyland Road, Ivyland, PA 18974-0577, 215-441-0800, Fax: 215-441-8620, www.miltonroy.com.