Ever more engineering consultants and power plant owners are opting for concrete volute pumps as cooling water pumps for their installations, writes Elisa Wielinger
Concrete volute pumps are vertical volute pumps whose casing is made of concrete instead of steel. Concrete volutes are a well-known technology which has been applied in the hydro turbine business for more than a century.
The first part of this article compares conventionally-installed pump types for cooling water with concrete volute pumps to demonstrate the advantages of the latter. The article then focuses on the design parameters, technology and construction procedure of concrete volute pumps, while also citing some references of pumps supplied by Andritz.
Concrete volute pumps (CVP), vertical volute pumps with steel casing (VVP) and vertical line shaft pumps (VLSP) are used as cooling water pumps for power plants. These three pump types can operate in different ranges as stated in the table on p28.
For discharges below 5 m3/s, the VLSP is the best choice economically, because it is still a standard product and therefore offers price advantages over the other stated pump types.
In the following comparisons, only the design range in which all pump types can operate (H up to 40 metres and Q up to 20 m3/s) and where it is feasible to operate them (Q> 5 m3/s) is considered.
There are three main cost impacts which have to be looked at: acquisition costs, operating costs and maintenance costs.
It is not possible to state an exact price difference between the pump types as there are many factors influencing the price, such as the pump size, the materials used, etc.
For cooling water pumps with a discharge higher than 5 m3/s, the acquisition costs of a CVP will be lower than for a VVP or a VLSP, as the material costs of a concrete casing are lower than those of a steel casing.
The material costs strongly depend on the medium transported: if sea water is the medium and super duplex steel is required as material, a vertical line shaft pump, for instance, can cost up to twice as much as a concrete volute pump. If mild steel can be used for the medium in question, the price difference will be smaller.
The same is true for the comparison between the concrete volute pump and the steel volute pump, but the difference will not be as high, as the steel volute pump has a more compact design compared to the vertical line shaft pump and therefore fewer steel parts are required.
A major cost factor on the civil engineering side is the additional construction depth which is required for a vertical line shaft pump. The inlet chamber of volute pumps is designed for less submergence, which cannot be achieved for the typical design of the VLSP.
When it comes to operating costs, none of the pump types can be favoured, as they all have approximately the same efficiency.
Concerning the maintenance costs, the CVP and the VVP are advantageous in that they comprise fewer parts due to their compact design. The concrete casing of the CVP is resistant to corrosion and erosion and is more or less maintenance-free.
The pull-out unit of the CVP and the VVP ensures easy inspection and maintenance. The VLSP can also be supplied as a pull-out pump type, but it makes the construction of the pump more complex. The discharge elbow, for example – a component that is subject to considerable strain – has to be specially designed for pull-out, resulting in higher costs.
It can be concluded from the above line of argument that, from the techno-economic point of view, concrete volute pumps are the best solution when large amounts of water are to be delivered. The concrete casing has a long service life, is easy to maintain, is resistant to corrosion and erosion, and its material and construction costs are low.
Concrete volute technology and construction procedure
Based on its long experience and on the high demands customary in the hydro turbine business, Andritz Hydro has adapted its technology to concrete volute pumps and has developed a concreting system, which can be described in the following four steps:
1. Placement of formwork and pouring of primary concrete: A wooden formwork for the inlet elbow and the volute is put in place and the so-called primary concrete is poured. The requirements for pouring conform to standard concrete manufacturing methods.The primary concrete requirements are as follows:
• Self compacting concrete (SCC) 1 cm;
• Minimum concrete cover 4.6 cm;
• Surface reinforcement with approximate diameter of 1.4 cm at 10 cm spaces;
• Concrete dimensions >80 cm;
• No special load-bearing reinforcement required due to low internal pressure.
The formwork is removed after the concrete has dried and can be reused several times for other pumps, if required. Civil engineering contractors can be provided with instructions for the reinforcement and concreting. Homogeneous concrete without major joints at the volute surface is obtained due to the utilization of the formwork.
2. Installation of embedded parts: Pump parts such as the pit liner, mounting device, support ring and cutwater are positioned and anchored at the primary concrete before being embedded in the secondary pour. Experienced supervisors assist throughout the construction and commissioning.
3. Pouring of secondary concrete: The secondary concrete is poured. The division into primary and secondary concrete leads to usual tolerance limits for the primary concrete and to exact positioning of embedded parts in the secondary concrete.
4. Installation of pull-out pump unit: The pull-out unit including the impeller, shaft, shaft sealing, bearings and covers is installed.
Wooden formwork for the inlet elbow and volute
Elisa Wielinger is a sales engineer at Andritz Engineered Pumps.. www.andritz.com