Precision dosing for alternative reciprocating pumps

In modern industry, optimal control of the process is growing more and more important. Historical techniques for process optimization relied fully on the capabilities of the metering pump; today attention is focused more on measuring equipment. This does not mean there are no specifications for the pump. The pump still needs to be highly reliable and stable. But stability need not be derived from the absolute accuracy of the traditional metering pump, but rather can stem from the linearity and reproducibility of alternative solutions.

Today the most commonly used methods of dosing liquids are:

• Gravity Systems — Achieve low accuracy and relatively low flows, but are inexpensive and require no maintenance.
• Centrifugal Pumps — Mainly used in large flows with limited pressures, where absolute accuracy is less important.
• Rotary PD Pumps — Probably the most prominent alternative to traditional metering pumps. The ability to pump both large and small flows, at pressures up to 17 bar and almost any viscosity, make them attractive for controlled systems. The disadvantages are internal slip and wear.
• Traditional Metering Pumps — Reciprocating pumps suitable for both high and low flows, pressures, and viscosities.

The question here is: How can nontraditional reciprocating pumps offer an alternative solution? Further, how do these alternatives compare with traditional systems? To give a proper answer, we have to know more about the various systems for dosing liquids and the demands placed on the pumps.

Nontraditional pump solutions require a system with feedback or control. To achieve maximum accuracy, the pump performance must be monitored and adjusted where necessary. This can be achieved in various ways, for example:

• Measuring the flow and comparing it against a set point.
• Measuring the system pressure and comparing it against a set point.
• Determining the weight of a partial stream by placing the vessel on load cells and comparing this with a given recipe.
• Observation of the quality of the end product, and manual adjustment during the process.

The above methods are listed in order of accuracy, but all are used in industry today. Other variations are available, but the scope of this article will focus on these techniques. Examples of each method follow, with notes to put them in the context of the users’ requirements. All have been in operation for several years and have established reliability.

Pump dosing requirements

Ultimately the user is interested in the end product. The system should be as simple as possible without making any concessions to performance — extra items increase cost and may require care and maintenance.

In addition, there are other parameters to be considered base on system demand, such as:

• Linearity — The output of the pump should be proportional to the output of the system. Should the system increase output by 10 percent, the speed of the pump will increase by 10 percent, and the flow must rise by 10 percent. Without this linearity, either the response time will be too long, or the system may start to hunt.
• Repeatability — Where there may be many different duty points, the ratio between speed and flow needs to be fixed. The pump must give consistent, repeatable flows when moving from point to point and back again.
• Response — The pump should respond instantly to a change in flow or pressure. If the time between the adjustment and stable output is too long, the quality of the process is compromised, and the product may be spoiled.

In all these aspects, alternative reciprocating pumps have proven performance — using a high-quality reciprocating pump in combination with a good motor and frequency drive results in accuracy similar to conventional metering pump technology.

System control

Unlike traditional metering pumps where flow is controlled by changing the stroke length, alternative reciprocating pumps require speed adjustment. In the past, mechanical speed converters were the only option. They were high in price, responded slowly to changes, and suffered wear on mechanical parts leading to nonlinear response.

These disadvantages rendered this type of equipment less attractive than conventional metering technology. With the development of smaller and more affordable frequency drives, a whole new market developed for speed-controlled systems.

But it is not only the favorable size and price of frequency drives that makes them suitable for use with reciprocating pumps. A modern drive is also advantageous because:

• It adapts both analogue (4-20 mA, 0-10 V) and digital inputs.
• It has built in controllers (PI/ PID).
• It provides a variety of outputs for feedback systems.
• It offers alarm functions to warn of performance issues.

Application reports

Dosing with a flow system

With the development of better, cheaper, and more reliable flowmeter packages, the accuracy of a flow system is often based on the flowmeter rather than the pump. Besides low pulsation, there are essential preconditions to achieve accuracy, such as linearity and reproducibility. Reciprocating pumps are a good fit for meeting these requirements, allowing the system to rely on the readings of the flowmeter.

A variety of flowmeters are available for such applications. The most common types are: mass flow (Coriolis); magnetic flow; positive displacement (e.g., oval, gear, or piston); and vortex. In the example illustrated hereafter, a gear-type meter is used.

Application: Flow control

Soft foam is used in furniture or as sound deadening material. The manufacturing process itself is simple — i.e., mix two liquids in proportion and a chemical reaction will occur with an enormous increase in volume. But incorrect proportions can result in a fire.

In one example, the manufacturer uses three reciprocating pumps to inject MDI (Table 1).

Table 1. Specifications for three reciprocating pumps used in a flow-based control system to inject MDI.

The system consists of two independent working units. The pumps supply the liquid at a pressure of 50 bar through a nozzle at the top of the tank equipped with level control. The tank feeds the pumps. At the end of a long discharge line, the liquid is injected at a pressure of approximately 90 bar into a rotary mixing head.

A flowmeter installed approximately two meters from the mixer gives a digital signal to the process computer, and the pumps are adjusted via the motor controller (Figure 1).

Figure 1. A flowmeter installed approximately two meters from the mixer gives a digital signal to the process computer, and the pumps are adjusted via the motor controller.

To achieve maximum precision, the system starts up with all process parameters in circulation mode. After stability of flow and pressure is established for five minutes, three-way valves switch over and the actual process begins.

Pros:

• Increasing flowmeter reliability improves operation of entire system
• Can be fully automated
• System parameters can be reviewed on location
• Good controllability

Cons:

• High costs associated with high-quality pump, drive, and controller
• Piping and auxiliary equipment have to suit flowmeter
• Extra equipment required to achieve required accuracy (pulsation damper)
• Quality flowmeters more expensive than pumps

Other dosing units working on the flow principle:

• Spraying water and salts in proper ratio on cocoa mass
• Adding green and red dye to fuels
• Dosing additives into discharge of high-pressure systems
• R&D on applying RO systems in chemical industries
• Precise dosing of water and oil for continuous dough production
• Dosing of glycol to stabilize high-temperature cement

Dosing with a pressure system

Although less accurate than control by flowmeter, dosing with pressure is popular due to its simplicity. The theory behind dosing by pressure is based on the following relationship between pressure and flow:

(bar)

In this simple formula, one can see that where there is a fixed opening, an increase in flow will automatically result in increased pressure, and a decrease in flow will result in a drop in pressure.

In the field, the pressure gauge or sensor drives system accuracy. With only a small variation in pressure, pump speed must be adjusted to achieve an acceptable level of accuracy. This is only possible by using a modern pressure sensor in combination with a fast-responding controller and frequency drive.

Application: Pressure control

A manufacturer of proteins is using two pumps to preserve the product (Table 2).

Table 2. Specifications for two pumps used in pressure-based control system to preserve protein product.

In the system, the nozzles are positioned in the filling line to the containers. When the product passes a certain point, the acids are injected.

Based on experience, the operator adjusts the speed to give the pressure that provides optimum results. This is the set point of the process, and from this point the controller takes over, maintaining the pressure. Monitoring both the pressure and pump speed, the operator will intervene as soon as there is an unacceptable deviation. This particular system has been in operation for over five years, providing good accuracy and reliability.

Pros:

• Simplicity gives high reliability
• Cost-effectiveness
• All system parameters can be reviewed onsite

Cons:

• High demands for the quality of pressure sensor/drive and controller
• Monitoring required
• Nozzle wear directly influences flow
• Extra equipment may be required (e.g., pulsation damper)

Other dosing units working on the pressure principle:

• Spraying fats to give a glossy surface to pet food
• Dosing additives into reaction vessels or piping
• Injecting flavorings into sausages

Dosing based on weighing

Control by product weight is common in systems dosing into solids. This is due partly to the high accuracy of the weight-cells. When load is put on the weight-cell the metal is stretched, and the increase in length indicates a value for the weight.

Application: Weight control

Concrete production commonly uses this system. There are high-tech and bulk suppliers who mix in a variety of additives to achieve higher strength, longer pot life, and even color variation.

A large mill is put on weight-cells and the cement, sand, and gravel are fed in by means of a conveyer belt. To optimize the strength of the concrete, an additive is sprayed through a nozzle onto the sand. It is very important that the quantity of this additive is always in correct proportion, so triple control is utilized. The product recipe is stored in a process computer.

The conveyer starts and components are dumped on the belt one by one. When the sand arrives, the basic speed of the pump (Table 3) is directly influenced by the speed of the belt (Figure 2). From that moment on, the weight of the two is exactly defined (i.e., weight of the sand + weight of the liquid). Both the ratios and the absolute weight are standard for filling the mill with the other products.

Table 3. Specifications for pump used in weight-based control system to support concrete mixing.

Figure 2. The conveyer starts and components are dumped on the belt one by one. When the sand arrives, the basic speed of the pump is directly influenced by the speed of the belt.

This particular system has been in operation for over five years, and the customer is very satisfied with the accuracy and reliability.

Pros:

• Simple and reliable
• All system parameters can be overviewed onsite

Cons:

• Requires high-quality pressure sensor/drive and controller
• Monitoring required
• Extra equipment may be required (e.g., pulsation damper)
• High initial cost

Other dosing units working on the weight principle:

• Spraying water and salts in proper ratio on cocoa beans
• Additives dosing in a water line

Dosing by manual control

Although human senses may seem the least accurate means of control, there are very many systems operated in this fashion. People can look, smell, taste (although not in every industry), and thus judge the quality of the product. But is this still within the scope of precision pumping? Even using this method it is not possible to check every product. The operator has to rely on the stability of the system and typically can sample only at random.

Application: Manual control

One company manufactures modern office furniture, where it uses two pumps. The process of making desktops consists of two stages:

• Putting a wooden insert in a mold
• Injecting all open spaces with plastic

The company makes more than one model, one size, and one color desktop, so the production process and pump output need to be modified accordingly (Table 4).

Table 4. Specifications for a pump used in manual control system to support desktop manufacturing.

The liquids pumped are Polyol and MDI, similar to the soft-foam application mentioned earlier, but in different ratios. To achieve the required color of the top layer, Polyol is blended with a pigmented liquid. The unit starts in a circulation loop and when flows are stable the valves are opened, and a controlled amount of Polyol and MDI comes out of the nozzle and starts a chemical reaction (Figure 3). By constantly weighing the Polyol, the MDI content is slowly increased or decreased. The ratio is adjusted by hand as long as required to get the optimum mix. Then the filling of the molds starts, relying on the pump accuracy and stability to give consistency during the production run.

Figure 3. The unit starts in a circulation loop and when flows are stable the valves are opened, and a controlled amount of Polyol and MDI comes out of the nozzle and starts a chemical reaction.

Pros:

• By far the simplest design
• Cost-effective
• Changing products or recipes can be accommodated very quickly
• Very suitable for limited-quantity runs

Cons:

• Can take a long time to stabilize the process
• Quality changes within batch may not be noticed
• Extra equipment required to adjust and control the system
• Accuracy depends on human input

Other dosing units working on the manual principle:

• Spraying water and salts in proper ratio on cocoa beans
• Additives dosing into a waterline

Application advantages

What features make reciprocating pumps excellent alternatives for high-precision pumping?

• Proven accuracy and reproducibility
• Very small size relative to performance
• High speeds do not require gearboxes
• Three- or five-diaphragm designs offer very small pressure and flow variations; standard control equipment can often be used without the use of a pulsation damper
• Competitive pricing
• Broad range of component materials readily available, providing lower cost over pump life
• Easy and quick maintenance; no specific adjustments or repair procedures are necessary
• Fast delivery time

Hans Rietveld has 22 years of experience in the pump industry. For the last 14 years, he has worked for Promotec, a provider of specialty pumping systems based in Hilversum, Netherlands. Promotec primarily serves the oil and gas and chemical industries and is particularly experienced with sealless pump technology. Mr. Rietveld currently serves as technical director for Promotec.