By Annette van Dorp, Freelance Journalist
In critical fields of application, such as the pumping of boiling liquids encountered in power plants or in the chemical and petrochemical industries, sustainable solutions are required. Damage to pump systems usually occurs due to the selection of the wrong pumps which can result in cavitation. Cavitation-critical applications include applications that deal with hazardous substances such as liquid gases, hydrocarbons, or condensates. The pumping of condensates in particular, is a demanding task in the field of power plant engineering.
Special Centrifugal Pumps of the V-AN Type
Self-regulating pumps based on the V-AN process, offer an innovative manner of working and provide a sustainable solution to issues like cavitation. Unlike conventional centrifugal pumps, these pumps operate without any suction. They are safe to run dry and never encounter cavitation. The NPSH value can therefore be ignored during the planning of the plant.
V-AN-type vertical pumps are intrinsically safe, and function in the following manner; pressure equalization is established between the inlet on the impeller and the gas phase of the pumped liquid in the inlet tank upstream of the pump. The basic requirement to accomplish this is the vertical construction of the pump. Furthermore, the hydrodynamic seal behind the impeller plays an important role. Since hydrodynamic sealing systems are wear-free and work without extra sealing fluids, they have low life cycle costs and can be operated for several years with low maintenance requirements and high annual pumping times.
“It runs when it runs dry,” stated Frank Bungartz, the third-generation Managing Director of the company of the same name. “It is one of the outstanding features of the special centrifugal pumps of the V-AN process.”
The design, which is constantly being further developed by the manufacturer, provides a primary and a secondary seal for the special V-AN-type centrifugal pumps. For the primary seal, back vanes (radial fins) are arranged on the rear of a standard centrifugal pump impeller. As the speed of the pump increases, the centrifugal forces move the liquid to be conveyed outward.
What is Cavitation?
Cavitation occurs when liquid turns to vapor at low pressure and bubbles are formed. This phenomenon occurs because there is not enough pressure at the suction end of the pump, or insufficient Net Positive Suction Head (NPSH) available. As the liquid passes from the suction side of the impeller to the delivery side, the bubbles implode. What usually announces itself by crackling noises has a destructive effect; it results in the interruption to the pump discharge flow, defective components, and can lead to pump failure and a need to replace the application.
This prevents the medium from coming into contact with the shaft seal when the pump is operated. A sealing system is installed as a secondary seal, consisting – depending on the application – of a stuffing box, a mechanical seal, a lip seal, or a magnetic coupling. This secondary seal either functions only as a standstill seal, or acts as a point of contact with the gas phase of the pumped material during pump operation.
Using this process, a balance is then established between the pressure at the impeller inlet and the gas equilibrium line. The special centrifugal pump that uses the V-AN process only pumps as much volume as is pushed into it. Similar to a siphon, no more liquid will follow.
The Intrinsic Safety* V-AN type centrifugal pumps are typically used for pumping mixtures of boiling, solid-laden and/or toxic liquids. They can be used in pits, closed containers (such as slop tanks) or vacuum filters, centrifuges, distillation columns, evaporation plants, or wastewater pits without any problems, see Figure 3.
*Intrinsic safety is a technical property of a device or system which, based on specific design principles, ensures that no unsafe condition will occur even in the event of a malfunction.
Many pumps use the medium to be pumped to sufficiently lubricate the seal and cool the seal used in a pumping system. Dry-running safety is the ability of a pump to operate permanently without any medium. This is achieved by decoupling the sealing unit from the pumped liquid.
Pumping a Condensate from a Water-Steam Circuit
In a power plant process, for example, boiling hot condensates are discharged from the water-steam circuit. The condensate is then led into an evaporation tank (flash tank with a high degree of turbulence), and is pumped into a neutralization tank, after corresponding cooling as part of the subsequent process. Conventional pumps also require a condensate collection tank in order to store a condensate which is almost vapor-free. For conventional pumps, additional level control would be required to ensure that a sufficient NPSH value is present across the level of the liquid (NPSHA> NPSHR+0.5m). The required construction height could only be achieved by building a pit. Through the use of the self-regulating special centrifugal pump according to the V-AN process, it is possible to achieve savings in terms of the construction height. Pressure equalization to the impeller inlet prevents cavitation and the vapor that passes through to the pump does not cause any damage. Even with entrained gas or vapor bubbles, no stalling occurs. Neither regulation nor dry-running protection is required for trouble-free pumping.
Energy from the Deep
Geothermal energy is a clean, virtually emission-free, renewable energy source that is generated from geothermal reservoirs with hot water. Heated by magma, miles below the earth’s surface, the vapor and brine is used to drive turbines for the generation of electricity. In the last few decades, companies have built more than 150 geothermal power plants worldwide. The pump technology described in relation to the self-regulating and intrinsically safe pumps of the V-AN process has been in use for almost 20 years in many of these plants. Although the pumps used for this process started off ‘small’, with an installation size of 20 – 30 m3/h, pumps reaching up to 100 – 150 m3/h in dimension, are now much more common.
With conventional pump systems the infrastructure costs to reach areas that are not critical for cavitation are high, due to the expenses for cooling the condensate or frames, pits, etc. Condensate pumps are frequently run at a low rotational speed and with an over-sized design in order to avoid cavitation as far as possible. Among other things, this is at the expense of high energy consumption. The benefit of non-suction pumps for the geothermal energy is therefore clear.
Unlike conventional condensate return systems, no large feed tank is necessary.
Profitability Right from the Start
When the correct pump is selected for an application during the planning phase of a sustainable plant, considerable investments can be saved. “The special design and technical features of the vertical self-regulating centrifugal pumps based on the V-AN process make it possible to dispense with expensive construction measures such as pits or to significantly reduce the construction height,” explained Dr.
Michael Schwanse. As a member of the manufacturer’s management team, he is aware of numerous examples
from his long-standing practical experience.
Favorable operating costs (for example, due to the economical monitoring expenses) and low maintenance costs pay off; as do the long service lives. Whether for the emptying of residues from tanks, for example, from tanker trucks, or for the pumping and collection of condensates, pumps based on the V-AN process have been in use worldwide for many years, successfully and economically, both in the construction of new plants and in the retrofitting of existing plants.