Understanding what technology is available is a first step towards selecting the right pump for metering applications
Reciprocating metering pumps are problem solvers for difficult fluid-handling tasks and applications. Their main function is the metering of a precise fluid volume per stroke in a specified period of time. For this, the accuracy of the metering flow is the critical, value-added element of these pumps. They are used in different industries, such as the chemical, oil-and-gas and pharmaceutical industries for applications with partially specific properties. While operating pressures can range from atmospheric pressure to 3,500 bars, the properties of the fluids can be extreme. Examples are applications with very low or high temperature, very high or low viscosity, thixotropic fluid behavior, shear sensitivity, abrasiveness due to solid contents, toxic or corrosive behavior.
The pumps consist of two main elements: the drive element and the pump head, as shown in Figure 1. The drive element converts the electrical energy into the reciprocating movement of the plunger while the main task of the pump head is the metering of the fluid. For most of the pumps, the stroke length can be adjusted for different flowrates. Due to this, reciprocating pumps have the following three characteristic properties (Figure 2):
- The flowrate is extraordinarily independent of the discharge pressure, in comparison to other types of pumps
- A digital character of the flowrate over time
- A linear dependence of the metering flow on stroke length and stroke frequency
Many different drive designs are available on the market today. Important requirements for these drive elements are as follows :
- Rugged design, capable of sustaining overload
- Adjustable stroke length
- Linearity between stroke adjustment and stroke length
- Harmonic reciprocation motion with continuous plunger viscosity curve over the stroke length
- Constant front dead point, independent of stroke-length setting
The last two requirements are prerequisites for an optimum pump-head operation. A discontinuous viscosity curve can lead to hydraulic pressure shocks. And a constant front dead point is important to reduce the pump head efficiency and metering accuracy at partial stroke length. Figure 3 shows five different principle designs and their properties.
The cam drive concept (Figure 3C) is an interesting solution for small- and mid-size discharge powers. It has a constant front dead point, which is the precondition for small pump-head dead spaces, and linearity between stroke adjustment and stroke length. This guarantees a very precise adjustment. The main disadvantage of the cam drive concept is the shock-like kinematics in partial stroke range. Depending on the adjusted stroke length, the adjustment screw blocks the movement of the plunger abruptly in the suction stroke. This leads to mechanical and hydraulic pressure shocks. With small pumps, these shocks can be acceptable, but for larger discharge powers, cam drives are not optimal.
A good compromise for mid-size to high-hydraulic power is the crank drive with variable eccentricity principle (Figure 3E). As shown in the right-hand side of Figure 3E, a mechanism converts the axial movement of the wedge (1) into a radial movement of the eccentric disk (2) for the adjustment of the stroke length of the crank. Due to the crank drive, the center point of the movement at partial stroke is constant. Thus, the front dead point varies at partial stroke length. This is the only disadvantage of this drive principle.
In recent years, new drive elements with integrated servo drives have been developed. One supplier, for example, has used the existing mechanics of the drive elements . In the automotive industry, servo-driven piston pumps are used to apply adhesive . These meters use roller screw drives for the conversion into a reciprocating movement. This is possible because the stroke frequency is very low with one stroke per minute. Both solutions can generate the partial stroke by controlling the movement of the servo drive. They allow individual suction and discharge stroke characteristics for high-viscosity fluids, easy data communication with other systems and easy integration into process control systems. It is to be expected that more drive elements with servo drives will be required in the future.
Pump heads can be classified as either plunger- or diaphragm-type pump heads.
Plunger pump heads. Plunger pump heads are comparable economic solutions for metering applications when leak-free solutions are not required. They are suitable for extreme operating conditions like very high or low temperature or the highest pressures.
The sealing concept is very important for this type of pump head. All efforts to improve the operational response of seals cannot overcome the fact that leakage flows are unavoidable in dynamically operating seals. Often, small flows are even functionally necessary for lubrication. Most of the sealing solutions have been proven for years. Standard sealing solutions are single- or double-tensioned gland packing for pressure up to 500 bars and temperature up to 400°C. For excluding contamination or infection entering from the outside, for example, in the pharmaceutical industry, sterilizing flushing of the seals (with wet steam, for example) creates safely operating sterile interfaces, as shown in Figure 4. Especially for suspensions, the sealant can be protected by forced flushing of a front-side sealing gap into the working chamber. Various configurations have been applied successfully for this purpose .
Diaphragm pump heads. The main advantage of diaphragm metering pumps against plunger pumps is the hermetically sealed tightness of the pump head. Instead of a dynamic sealant, the pump head is equipped with a diaphragm that serves as a flexible, static seal. Therefore, these pump heads are compatible with difficult-to-handle fluids, such as toxic, flammable and corrosive fluids, as well as slurries, thixotropic, high-viscous or non-lubricating fluids. In terms of the hydraulic power, it makes sense to distinguish between mechanically and hydraulically coupled diaphragms.
For low-pressure applications and smaller flowrates, pump heads with mechanically coupled diaphragms have found a broad market. At these pumps the diaphragm is directly assembled to the plunger, as shown in Figure 5. The diaphragm has to seal and to convey fluid against the operating pressure. This is the reason for the relatively small allowed operating pressure (up to about 20 bars) compared with hydraulically coupled diaphragm pump heads. In many cases, the pump head is combined with cam drives as drive elements. Qualified designs should have at least the following properties:
- A diaphragm lifetime of at least 5,000 operating hours, depending on the operating pressure and the diaphragm deflection
- Diaphragm material with high diffusion strength and chemical stability. Many suppliers use polytetrafluoroethylene (PTFE) or PTFE-coated rubber for this
- Multi-layer diaphragm with integrated diaphragm-monitoring system to indicate diaphragm failure and guarantee tightness — also after diaphragm rupture
If the diaphragm is hydraulically coupled, an operating pressure of up to 800 bars is possible with a PTFE diaphragm, and up to 1,200 bars is possible with a stainless-steel diaphragm. These pump heads are used in a broad range of 0.1 mL/h to about 60 m3/h.
The principle design of a hydraulically coupled diaphragm pump is shown in Figure 6. The diaphragm separates the pressure chamber into an operating chamber and a hydraulic chamber. Therefore, the pressure in both chambers is identical. The diaphragm has the following two functions:
- To transmit the plunger displacement of the hydraulic fluid onto the metering fluid — under normal operating conditions, the pressure on both sides of the diaphragm is nearly identical
- To seal the operating fluid against the environment at the diaphragm clamping area
Due to the oil leakage at the plunger seal, a replenishing valve is needed. It opens when the diaphragm contacts the perforated plate. The hydraulic chamber can also be equipped with a safety valve to prevent overpressure (not shown in Figure 6). Figure 7 shows the operating pressure as a function of time for a diaphragm pump, where one can see the interaction of the valves. In the diagram, the suction and discharge strokes, the opening time of the suction and discharge valves, the compression and de-compression phases and the opening time of the replenishing valve are clearly recognizable.
As mentioned before, the diaphragm material is usually PTFE for pressures up to 800 bars and stainless steel for pressures up to 1,200 bars. State-of-the-art for PTFE material is a double-layer diaphragm with integrated monitoring system to indicate diaphragm rupture and to prevent leakage. The lifetime of a PTFE diaphragm can exceed 10,000 h operating time. Standard material for wetted parts, such as pumps heads and valves, is stainless steel, for example 316Ti (1.4571). For specific process requirements, these parts can be made of special materials, such as stainless steel 316/316L (1.4401/1.4404), Hastelloy or Duplex steel and polyvinyl chloride (PVC).
When the suction line pressure is below ambient pressure during standstill, the start procedure of a metering pump can be critical for the diaphragm. Due to very low leakage at the suction valve and at the piston seal, the diaphragm can move over time to the front dead point. If the piston has remained in the rear position, the diaphragm would be overstretched or damaged at the first stroke after pump start. In order to prevent this, a specific starting procedure is necessary. Pumps have to be started with zero stroke length and slowly increase the stroke length afterwards. Figure 8 shows a modern, patented diaphragm-pump head that prevents this situation from happening. An installed spring tears the diaphragm to the rear position during standstill. Easy and safe starting, even under extreme conditions, is the result. Furthermore, the spring ensures a stable diaphragm displacement and a significantly lower minimum-required suction flange pressure at the pump entry in many applications.
The fluid valves and the diaphragm are the major wear parts of the pump head. In order to reduce lifecycle cost and to increase the pump-head reliability, monitoring systems help to recognize developing failures at an early stage for predictive maintenance. While the double-layer PTFE diaphragms are already monitored, the technology to indicate and to predict valve failures is known [5–7]. The basis for this monitoring system is a pressure sensor to detect the operating pressure as a function of time, as shown in Figure 7, and an accelerometer for acceleration measurement. The accelerometer can be assembled at the front of the pump head, while the pressure transducer can be installed in the hydraulic chamber. At commissioning, a fingerprint of the signals has to be learned by the system. Figure 9 shows the pressure versus time diagram in normal (left) and faulty operation. The replenishing valve in the hydraulic chamber is open for a longer period of time in case of leakages of the hydraulic chamber (due to defective piston rings or hydraulic valves). If the suction valve is defective, the compression starts later and lasts longer, whereas the decompression takes place earlier. This behavior is shown in Figure 10. The reason for this is fluid that is forced back into the suction line if the suction valve does not close properly although the pump produces the operating pressure required. In case of a defective discharge valve, the compression would be faster and decompression slower. The existing pressure in the discharge line supports the compression and maintains the pressure for a longer time period during the decompression. In addition, the acceleration signal indicates clearly the defective suction valve, as shown in Figure 11. This shows that a valve failure can be detected by two independent and redundant signals.
According to estimates by the German Association of the Chemical-Pharmaceutical Industry (VCI), 9% of the investment costs of a process plant in Germany are accounted for by redundant pump systems . At the pump side, the technology for monitoring valves seems to be clarified. But we have a lack of field experience. This could change in the course of the digitalization strategy of the chemical and oil-and-gas industries. Companies, such as BASF, have declared that they want to use data to better forecast maintenance requirements of their plants and reduce unexpected shutdowns . And Shell clearly describes the advantages of predictive maintenance: “The objective is an increase in reliability of the overall asset or part of the asset” . In connection with the discussion of the internet of things (IoT), it is to be expected that the demand for condition monitoring and predictive maintenance systems will continue to increase for metering pumps as well. And that is a very promising development, because only intensive field experience ensures enough confidence in these systems.
1. Fritsch, H., “Metering Pumps (Principles, Designs, Applications),” Verlag Moderne Industrie, Landsberg/Lech, Germany, 1990.
2. Sauter, M.: Innovative Technologien zur Steigerung der Betriebssicherheit von Membranpumpen, Praktiker Konferenz, April 16–18, 2007, Graz, Austria.
3. Vetter, G. (ed.): A Survey of Leak-free Centrifugal and Positive Displacement Pumps, in: “Leak-free Pumps and Compressors,” Oxford, Elsevier Advanced Technology, 1995.
4. Kohlhase, N. and Stritzelberger, M., High Pressure Design of Engineering Pumps, Chemical Engineering Transactions, Vol. 2, 2002.
5. Kohlhase, N., Recording the Heartbeat of Diaphragm Pumps – Monitoring Systems Help to Reduce Costs, Fruit Processing, June 2001.
6. Kohlhase, N. and Sauter, M., Störungsfrüherkennung für Prozess-Membranpumpen, Industriepumpen und – Kompressoren April 2005.
7. Martinez, F, Phillipin, M. Blanding, J, and Schlücker, E.: Dynamic Monitoring for Early Failure Diagnosis and Modern Techniques for Design of Positive Displacement Pumping Systems, Proceedings of 17th Pump Users Symposium, Turbomachinery Laboratory, 2000.
8. Hensel, R., Trend: Condition Monitoring Systeme für Pumpen, www.Fluid.de.
9. www.basf.com/global/en/who-we-are/digitalization.html, July 2019.
11. Milton Roy Metering Pump Technology, Bulletin 210, Revision July 2008.
13. Lewa ecoflow – The Custom Made Metering Pump, D1-160_en, January 2019.
Nils Kohlhase is professor for product development and systematic design at the University of Applied Science in the Hanseatic City of Lübeck (Technische Hochschule Lübeck, Mönkhofer Weg 239, D-23562 Lübeck, Germany; Phone: +49-451-300-5780; Email: email@example.com). He has more than 21 years of experience in the chemical, oil-and-gas, consumer and automotive industries. During this time, Kohlhase was the technical manager at LEWA GmbH, director R&D Europe at Trelleborg Sealing Solutions, manager R&D at Leica Camera AG and global manager R&D at SCA Schucker GmbH & Co KG. He is an expert in systematic product development and modular system development. Kohlhase holds a doctoral degree (Diplom-Ingenieur) in mechanical engineering from the Technical University of Braunschweig, Germany.
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