Use of Centrifugal Pumps with Difficult Pumping Media
Frank Bungartz
Paul Bungartz Gmbh
& CO. KG
Düsseldorf, Germany

When pumps are used in the field of chemistry, reliability and operational safety are of the utmost priority. The reason for this is that when aggressive, corrosive or toxic chemicals and mixtures or waste waters are being pumped, this must be carried out without causing any damage. Even if additional difficulties such as temperatures, solid matter in the medium or small quantities with an irregular feed make the pumping process more difficult, it is essential that the plants, environment and pump are not negatively affected.

Examples of problematic substances are ammonium nitrate, nitro phosphate or tetrachloride. The requirements for liquids, gaseous, laden with solid matter or pumped directly from a vacuum, are just as high as for the use of pumps in molten salt, for example. The latter is one of the extremely hot applications. The usual standard centrifugal pumps, which are the most frequently used pumps with a share of approximately 80 – 90% of applications, are usually unable to cope with these complex problems. In contrast, they do not represent an insurmountable obstacle to sturdy special centrifugal pumps. Special pumps made by Bungartz are completely reliable for media containing solids, for example, as they work without enclosed pumping chambers and do not require any valves. They work with a hydraulics system and an impeller, as well as housing with a seal and bearing unit.

The main decisive factors when selecting the pump are an exact description of the parameters, such as the place of use, the conditions and – in particular – the media to be pumped. The more precise the details, the better the preconditions for the selection of the important pump hydraulics and the shaft seal. As a rule, the pump manufacturer is responsible for this design. At the same time, the pump or pump system must fulfill the task inexpensively and with a low level of maintenance. The sealing technology is an essential component for the optimum use and a long service life of the pumps. For this reason, the selection and combination of the seal and bearing unit is highly important.

Sealing problem and sealing system

The sealing system seals off the rotating shaft against the surrounding environment. Here too, the type of application and the medium to be pumped play an important role. The potential hazards associated with this medium are the determining factor for the selection, depending on the national law and regulations (explosion protection, TA Luft (Technical Instructions on Air Quality Control) etc.). Furthermore, the type of seal has a large influence on the performance and the life cycle costs of the pump. Sealing systems such as a stuffing box, single or double mechanical seal, magnetic coupling or even a canned motor are available for selection.

In the case of the canned motor pump, the pump and motor form one unit. In contrast to the double mechanical seal, which is sealed using a separate medium, this does not operate independently of the pumping medium. The medium flows around the containment can and slide bearings. Magnetic coupling and canned motor pumps can only pump gas-laden or magnetizable media and solids with the addition of measures such as external flushing. Within the context of a comparison of the life cycle costs, the high energy losses due to induced eddy currents (in the containment can/can) and viscosity effects must also be taken into account.

If centrifugal pumps are used with a magnetic coupling, the motor shaft is fitted with an external magnetic rotor in the so-called block design. It transfers the magnetic forces through a containment can to the internal magnet rotor. In the case of conventional magnetic coupling pumps with slide bearings lubricated by the pumping medium, the containment can, which hermetically seals the medium from the surrounding environment, and the slide bearings must not run dry; they will react directly with malfunctions if there are solids in the pumping medium. In order to prevent this, it is necessary to ensure the dry-running safety of the pump for many types of media. This guarantees that the pumps are not damaged even during phases in which little or no fluid is pumped. However, in the case of conventional pumps, it is only possible to provide dry-running safety by means of complex and costly additional measures.

Definition of dry-running safety

Dry-running safety is the ability of a pump to handle difficult operating situations without damage or causing a dangerous situation. The difficult situations include pumping processes in which there is only a little or even no pumping medium inside the pump casing, or where gas components occur in the pumping medium.

Hydrodynamic seal

The advantage of the hydrodynamic shaft seal is its friction-free mode of operation. The hydrodynamic seal was invented and patented by Paul Bungartz in the 1930s. It has been continually developed by Bungartz up to today. The fundamental motive for the need for such sealing physics was the existing problem of ensuring that the transition between the pump casing and rotating shaft was permanently tight; i.e. the task is to pump the medium away from the critical shaft gap transition and in this way to protect the following shaft seal system. In centrifugal shaft seals, ribs are arranged concentrically on the rear of an ordinary centrifugal pump impeller with the shaft stub. The effect of these ribs is that as the rotational speed of the pump increases, the fluid is carried along by the centrifugal forces – i.e. it is induced to rotate and therefore to be conveyed towards the outside.

Figure 1: MPCH Dry Run
Energy saving centrifugal pump for difficult and complex pumping tasks

The back vanes on the rear of the impeller reinforce this effect, so that the rotational speed of the rotating ring of liquid can reach up to 90% of the pump speed. The rotating ring of liquid reduces the pressure that is applied to the external diameter in the direction of the shaft to the ambient pressure. If the design is correct, no liquid is then found at the shaft penetration while the pump is operating. A friction shaft seal is then superfluous. However, there are problems when the system is at a standstill: the seal is ineffective. A standstill seal is required.

One example of the combination of the hydrodynamic seal with a centrifugal system has been created for a horizontal pump. As soon as the pump is started and the hydrodynamic seal takes pressure off the shaft penetration, the shaft is moved axially in the direction of the suction side of the pump. As a result, the conical stuffing box packing used here is separated radially from the shaft. The standstill seal is completely friction-free during pump operation. In combination with stuffing boxes, pumps with these sealing systems are ideally suited for use in the fertilizer and paper industry for pumping non-toxic products containing solids.

The hydrodynamic sealing technology is also referred to as the basis of the Bungartz “sealing philosophy”. With the so-called secondary seals – which in the past took the form of stuffing box packings, but today are mostly double mechanical seals – they ensure leak-tightness when the system is at a standstill. Almost all pumps of the manufacturer combine the hydrodynamic shaft seal as a primary seal with a downstream secondary seal capable of dry running.

The advantages are clearly seen in practice: The pumping medium is kept away completely from the actual seal during operation and also during a standstill in the case of very aggressive applications which place high demands on seals. This permanent dry-running safety results in an extremely long service life. Because the hydrodynamic shaft seal operates without contact and without any wearing parts, no maintenance whatsoever is necessary. Since they are operated for several years with little maintenance and high annual running times, they also offer the advantage of low life cycle costs. Pumps of this type are particularly suitable for continuous operation in the chemical and primary industry.

Horizontal centrifugal pump MPCH Dry Run with permanently dry-running magnetic coupling

Bungartz is the only manufacturer to also build magnetically coupled pumps with a hydrodynamic shaft seal - that is, with dryrunning containment cans. “It doesn’t matter what is pumped at the front in the pump head, the secondary seal and the magnetic coupling are not affected during operation. In many cases we work with sealing gas blanketing,” explains Michael Wolf, who represents Bungartz as a Consulting Sales Engineer in the Asian region. Thanks to the unique pump characteristics, the company – which is known for its problem solutions - also offers the basis for the optimal selection of magnetic coupling pumps. The horizontal special pump MPCHDryRun has proven its worth in the pumping of hazardous liquids and / or solid-laden, gas-containing media.

The special technical features begin with the magnetic coupling, which is permanently dry-running and depressurized. A ceramic containment can located between the inner and outer magnet rotor is depressurized and enables eddy current-free magnetic field transmission without product contact and therefore dry operation.

The bearing mount consists of a shaft with grease-lubricated roller bearings which have a minimum service life of 32,000 hours. The life-long lubricated roller bearings are extremely low-maintenance. These increase the efficiency compared to conventional magnet pumps with plain bearings. Safety advantages are also provided by the hydraulic relief on the impeller. As a result, and through the use of compensating bores, the shaft gap pressure drops to the inlet pressure. A labyrinth seal between the impeller and shaft bearing
separates off any solids. These cannot enter the bearing area. It is possible for the impeller, for example, to be equipped with enclosed vanes so that it is tailor-made for every application. Between the back of the impeller and the shaft bearing there is the product-free labyrinth area through which the sealing gas flows. This sealing system and the rest of the structural design ensure a high temperature gradient and therefore a lower bearing temperature. A special lip seal ring upstream of the bearing is activated if the nitrogen supply fails. The ceramic containment can the actual hermetic seal, is thus subject only to low pressures, and operates without product contact. Since the bearing and magnetic coupling are not in contact with the pumping medium, this magnetic-coupled pump can be operated more or less independently of the liquid. "It also runs when it runs dry," is the core statement on the outstanding feature of this special centrifugal pump.

The triple seal arrangement guarantees very high operational safety levels, even if the flow of the medium is interrupted. The design impresses even in the case of faulty operation, since it is based on an intrinsic safety method which is incorporated into the system of the pump: Dry-running lip sealing rings automatically secure the system.

The use of roller bearings and a ceramic containment can also have a positive effect on the energy balance of the MPCH Dry Run. The eddy current-free magnetic field transmission results in savings in terms of energy costs (for example: for a pump running time of 8,000 operating hours, the annual savings in Germany are up to € 8,000). In addition to the positive energy balance, the operating reliability, a maintenance interval of three years and a very long service life pay for themselves over the long term.

Figure 2: V-AN
Self-regulating centrifugal pumps reduce plant costs
and can be used as problem-solvers
in a wide range of applications.

Conclusion: For the pumping of hazardous liquids and/or solid-laden, gas-containing media, the permanently dry-running magnetic coupling pumps of the MPCH Dry Run type have proved their value all over the world.

Intelligent Interaction

If the properties of the permanently dry-running magnetic coupling pumps are combined with the vertical centrifugal pumps of the V-AN range, further problems with pumping applications can be solved. V-AN centrifugal pumps made by Bungartz have become wellknown problem-solvers. With the V-AN pump, the special pump manufacturer Bungartz provides a reliable, tried and tested solution: The V-AN regulates itself in order to adapt to the incoming flow rate. Advantage: Due to the pressure reduction without cavitation up to the boiling point, the emptying of liquid gases is also unproblematic. All pumps in the V-AN range are dry-running and operationally safe, self-ventilating and have a low NPSH value (<0.1 m).
They are suitable for use with fluctuating inflow volumes, media at their boiling point or gaseous media. They are often used for tanker and tank wagon emptying. One important factor in pump selection in this area is the residue-free draining of containers and tanks. The pumps in the V-AN range ensure complete discharging without additional pressure. When discharging is carried out from above, this can even be used with heavy media up to a density of 2.4 kg/dm³. The V-AN range is also frequently used with gaseous or boiling media. They are ideal for separation processes with fluctuating inflow volumes because the pumps adapt themselves independently to changing volume flows.

It is possible to dispense with the otherwise commonly used additional equipment which is very prone to failure, such as min/max circuitry, speed regulation by means of a frequency converter or volumetric flow measurement. A further criterion when selecting pumps is to save plant costs by reducing the overall height. This is where the pumps of the V-AN range also offer advantages; considerable investment cost savings are possible if the pumps are incorporated in good time into the planning phase (no pits, scaffolding, buffer tanks, etc.). They are in demand all over the world for the residual emptying of all types of containers, for the pumping and collection of condensates on vacuum filters, centrifuges, distillation columns, evaporation plants or on slop containers.

A complete pumping system consisting of the V-AN self-regulating centrifugal pump, a pump stand and feed vessel can be used to save space for tanker emptying. It is connected directly to the power supply and the pipes. As all Bungartz pumps, the system can be used entirely without any control unit, regulation or additional monitoring. The V-AN range of pumps has no problem with reliably pumping gases, acids or alkalis – from sulfuric acid, through sodium hydroxide solution to substances such as acrylamide, propylene, isopropanol, ammonia or special items such as maleic anhydride – down to the very last drop.

Figure 3: M-UMOR
Large circulating pump (900 m3/h) with semi-axial impeller for pumping ammonium nitrate

V-AN centrifugal pump versions
Fields of application (pumping media) with corresponding shaft seals
  • liquid gases and other TA Luft media, permitted for solids
  • double gas-lubricated mechanical seal with low gas sealing pressure
  • liquid gases and other TA Luft media, permitted for solids, suitable for TA Luft
  • double liquid-lubricated mechanical seal with low sealing pressure
  • for non-toxic liquids under atmospheric pressure, permitted for solids
  • dry-running, self-lubricating stuffing box
  • tried and tested problem-solving pump as a robust alternative to the gas-lubricated DGLRD, suitable for TA Luft with restrictions,
  • now with gas-sealed shaft seal and new lip seals
  • heavy media, e.g. sulphuric acid, oleum, zinc tetrachloride, generally solid-free media suitable for TA Luft
  • liquid-lubricated magnetic couplings
  • liquid gases as well as other TA Luft media, permitted for solids, suitable for TA Luft
  • dry-running magnetic coupling – supplied externally with sealing gas
Legendary for a good reason
These legendary types M-MOR, M-UMOR slurry pumps are suitable for the pumping of solid-containing, corrosive liquids, especially for suspensions – including acid, fertilizer and sand mixtures. Legendary not only because they can pump corrosive, abrasive, sticky, gaseous, crystallizing or even gelling media without any problems. Pumps from the MOS / UMOS and MOR / UMOR range are highly appreciated worldwide due to their sturdy design and used in a wide variety of plants, e.g. for the pumping of substances such as ammonia, ammonium nitrate, urea solutions or phosphoric acid. Legendary also because they can also be found where other pumps have already given up. Recently, one of these pumps, which had been in use since 1948, was removed from service in a fully functional condition. It has now been replaced by a new model. The horizontal centrifugal pumps operate either with a dry-running magnetic coupling or with the hydrodynamic shaft seal described, where the back vanes seal the shaft gap on the open impeller (star wheel). A centrifugal controller integrated into the bearing block moves the rotor in the direction of the suction connection during start-up. The gap between the conical shaft sleeve and the stuffing box packing prevents the signs of wear which are otherwise found there. When the pump is switched off, the rotor is pulled back into the packing space by a spring force. Here, a stuffing box seals the shaft gap as a standstill seal.

The pump speed can be adjusted without problems by means of the impeller back vanes and an additional seal expeller – without impairing the hydrodynamic sealing effect. Further significant advantages result from the innovative use of a wear-resistant material based on silicon carbide, which is used in the area of the impeller and the housing. Additionally, the silicon carbide ceramic can be incorporated selectively where direct wear occurs.

The large circulating pumps (1,200 m3/h) of the M-UMOR type, which have just been put into operation, are completely reliable thanks to the contactless hydrodynamic shaft seal with seal expeller and centrifugal forcecontrolled standstill seal. Operating reliability and generous maintenance intervals add up to massive cost advantages with all of the other special features