Centrifugal pumps used at thermal power plants, according to their purpose, nature of work, type of pumped liquid and parameters, form a group of energy pumps.
The designs of energy pumps are very diverse. These are low, medium and high pressure centrifugal pumps; single-stage pumps with single- and double-suction, single- and multi-stage pumps for clean water and radioactive media, pumps for liquid metal coolant, etc.
However, depending on their purpose, they have a number of common characteristics, according to which energy pumps are divided into feed pumps, condensate pumps, network pumps, etc.
Let’s consider the design features of nutritional energy pumps.
Feed pumps as part of the pumping unit are designed to supply chemically purified feed water to the boiler. According to the functions they perform in the thermal circuit of a modern power plant, they belong to the main power equipment.
The design of feed pumps is largely determined by the steam parameters and the scheme of their inclusion in the system. Feed water temperatures are 105-165 °C, so feed pumps are called “hot” pumps in contrast to “cold” general purpose pumps, and this factor must be taken into account when designing. Feed pumps, as a rule, are connected to the system in parallel, so they must use special flow parts that ensure reliable operation under these conditions.
Basic requirements for the design of feed pumps:
– high reliability and efficiency;
– ensuring free temperature expansion of the stator and rotor elements without disturbing their mutual alignment and the alignment of the pump with the drive;
– external tightness and absence of internal flows at the joints of the rotor and stator parts;
– long service life (15,000-30,000 hours depending on the type of pump);
– protection against reverse flow of feed water from a common pressure pipeline and, at low flows, from unacceptable overheating of water to a temperature close to steam formation (presence of a check valve and recirculation line);
– dynamic stability over the entire range of operating modes;
– a stable, continuously decreasing shape of the pressure characteristic in the flow range from 20-30% Qnom to Qnom with a slope of at least 25% to ensure stable operation of pumps when connected in parallel;
– ease of assembly, disassembly, maintenance and high maintainability of pumps under operating conditions.
Electric and turbo drives are used to drive feed pumps.
In the domestic and foreign energy industries, the turbo drive has received predominant use for powerful feed pumps (N>8 thousand kW), providing a number of advantages during operation. Such pumps are mainly used in power units with a capacity of 300 MW and above.
In power units up to 200 MW, electrically driven feed pumps are prevalent.
The parameters of electric feed pumps are regulated by GOST 22337-77. Depending on the parameters, they have the same type and largely unified design.
Feed pumps are designed to supply water to drum and direct-flow stationary steam boilers with steam pressures of 3.9 (40), 9.8 (100), 13.7 (140) and 25 MPa (255 kgf/cm2). Steam pressure has a significant influence on the design of the feed pump.
In domestic pump construction, at pressures up to 14.7 MPa (150 kgf/cm2), a single-casing (sectional) design is used, and above that, a double-casing design of pumps is used. An exception is the PE 250-180 pump, made in a single-casing design.
For steam boilers with a steam pressure of 3.9 MPa, electric pumps of the types PE 65-45, PE 65-53, PE 100-53, PE 150-53, PE 150-63 have become predominant.
For steam boilers with steam pressure of 9.8 and 13.7 MPa – pumps PE 150-145, PE 270-150 and PE 250-180.
For steam boilers with a steam pressure of 13.7 MPa – pumps PE 380-185/200,
PE 500-180, PE 580-185/200, PE 720-185, PE 780-185-210 and PE 900-185.
Let’s consider a single-casing (sectional) version using the PE 100-53 feed pump as an example, and a double-casing version using the PE 500-180 feed pump as an example.
- Feed pump PE 100-53 belongs to a series of pumps of the same type designed to supply feed water to stationary steam boilers with a steam pressure of 3.9 MPa (40 kgf/cm2).
Figure 1
The feed pump (Figure 1) is a centrifugal, horizontal, multi-stage, single-casing sectional type.
The basic parts of the pump are: inlet and discharge covers. Between them there is a set of unified sections. One section includes a guide vane, which performs the functions of supplying fluid to and removing it from the impeller, and the section housing. In places where the impellers are sealed in the sections and guide vanes, replaceable sealing rings are installed.
The package of sections, together with the covers and the hydraulic foot body, are centered among themselves on cylindrical grooves and tightened with pins, forming the pump body. The tightness of the joints is ensured by the “metal” contact of the sealing belts of the sections, covers, hydraulic foot housing and the installation of rubber sealing rings.
The inlet and discharge pipes are made in the inlet and discharge covers, respectively, and are directed vertically upward. The design of the pipes is flanged.
The supporting surfaces of the legs of the suction and discharge covers are located in a horizontal plane passing through the axis of the pump (to reduce bending forces and misalignment of the unit when the pump heats up). To ensure thermal expansion of the housing along the axis of the pump, special keys are provided in the lower part of the covers.
End seal housings with brackets for attaching bearing housings are attached to the hydraulic foot housing and the inlet cover.
The pump rotor consists of a shaft, a set of impellers, protective jackets, a discharge disk, oil deflectors and fasteners. All impellers, except the first stage, have the same flow path. The impellers on the shaft are secured against rotation using keys. The unloading disk is fixed on the shaft in the axial direction through a protective jacket with a round nut. A thermal gap is provided between the end of the disk bushing and the impeller of the last stage. The assembled pump rotor is dynamically balanced.
The axial force of the rotor is perceived by an automatically acting balancing device – an unloading disk (hydro-heel). To monitor wear of the relief disk, a visual axial displacement indicator is attached to the rear bearing housing. The end seals of the pump are stuffing box type. Bushings are installed in the seal housings, forming cavities for the flow of cooling water in order to ensure the temperature regime in the stuffing box. It is possible to install mechanical seals. The pump rotor is supported by plain bearings with ring (crankcase) lubrication using lubricating rings. To maintain the temperature of the liners in a given range, cooling water is supplied to the bearing supports of the pump. The temperature of the sliding bearing shells is controlled using resistance thermal converters. Alignment of the pump rotor in the stator is ensured by moving the bearing housings using adjusting screws, after which the bearing housings are pinned. The pump rests on the racks of a common foundation slab with four legs located in a horizontal plane. The pump housing is covered with a protective and decorative casing, under which thermal insulation is placed at the pump operation site.
An asynchronous motor with a squirrel-cage rotor is used as a drive.
The pump and motor are connected to each other using an elastic pin-sleeve coupling. The coupling is closed by a guard.
The pump symbol means: PE – electric feed pump;
100 – flow rate in m3/h; 53 – discharge pressure in kgf/cm2.
- Feed pump PE 500-180 is designed to supply feed water to stationary steam boilers with a steam pressure of 13.7 MPa (140 kgf/cm2).
Figure 2
The feed pump (Figure 2) is a centrifugal, horizontal, double-casing pump with a sectional internal casing.
The basic part of the pump is a forged outer casing, which is a cylinder with welded support legs, inlet and discharge pipes.
The inlet and pressure pipes are directed vertically upward, the connection to the pipelines is: inlet – flanged, pressure – welded.
To ensure directed thermal expansion of the pump, the supporting surfaces of the feet of the outer casing are located in a horizontal plane passing through the axis of the pump. The fixed position of the pump axis, ensuring the absence of misalignment during thermal expansion of the housing, is achieved through transverse keys installed in the front pair of legs and longitudinal keys located in the lower part of the housing. The sealing joints of the outer casing have erosion-resistant surfacing. To control the temperature difference in the upper and lower parts of the outer casing, places are provided for installing sensors – resistance thermal converters.
The outer casing is closed at the ends with inlet and discharge covers. The front end seal assembly is located in the welded-cast inlet cover, and a bearing bracket is also attached to it. The discharge cover contains a hydraulic axial force relief unit, and the rear end seal unit and bearing bracket are also attached to it.
The internal pump casing is a separate assembly unit and consists of a set of sections connected to each other by pins and a rotor. The sections are equipped with guide devices, which are secured against rotation with special screws. In places where the impellers are sealed in the sections and guide vanes, replaceable sealing rings are installed. The joints of the sections are sealed using metal contact. To additionally guarantee sealing at the joints, the installation of rings made of heat-resistant rubber is provided.
High pressure joints between the outer and inner casings, the outer casing and the pressure cover are sealed with toothed and flat metal gaskets.
From the third stage of the pump, water is provided for the technological needs of the consumer with a supply of up to 10% of the nominal supply with a pressure of 5.4 MPa (55 kgf/cm2) in nominal mode.
The pump rotor is a separate assembly unit and consists of a shaft, a set of impellers, a discharge disk, sealing parts and fasteners. All impellers, except the first stage, have the same flow path. The impellers on the shaft are secured against rotation using keys. The unloading disk is fixed on the shaft in the axial direction with a nut. A thermal gap is provided between the end of the unloading disk and the impeller of the last stage.
The assembled pump rotor is dynamically balanced.
The axial force of the rotor is perceived by an automatically acting balancing device – an unloading disk (hydro-heel).
The end seals of the pump are mechanical type. In order to ensure temperature conditions, cooling condensate from an external source is supplied to the mechanical seal chamber.
The pump rotor is supported by plain bearings with Babbitt liners and forced lubrication. Forced lubrication and cooling of the bearings of the pump, electric motor and gear coupling is ensured by the use of an oil unit. The oil installation includes: an oil tank, an oil cooler with a double oil filter, two oil pumps, control and shut-off valves. Oil installation pipelines are made during installation of the unit.
To limit axial movements of the rotor towards the pressure cover, a rotor stop with a visual indicator of the axial shift is installed on the rear bearing.
The temperature of sliding bearing shells and mechanical seals is monitored using resistance thermal converters.
Alignment of the pump rotor in the stator is ensured by moving the bearing housings using adjusting screws, after which the bearing housings are pinned.
The pump is installed on an individual foundation frame.
A system of auxiliary pipelines is provided within the pump.
The pump housing is covered with a protective and decorative casing, under which thermal insulation is placed at the pump operation site.
An asynchronous electric motor is used to drive the pumps.
The pump and electric motor are connected to each other using a gear coupling. It is possible to use an elastic plate coupling to increase the reliability and durability of the pump unit, as well as reduce noise and vibration.
The pump symbol means: PE – electric feed pump;
500 – flow rate in m3/h; 180 – discharge pressure in kgf/cm2.
The double-hull design compared to the single-hull design has the following advantages:
– better maintainability (disassembling the pump without disconnecting the pipelines, which allows you to quickly replace the internal casing with a backup rotor);
– better tightness (only one external high-pressure joint);
– better noise characteristics.
Basic advantages of single-casing sectional pumps:
– possibility of starting from a cold state without preheating;
– less metal consumption for the manufacture of the pump compared to double-casing pumps, and therefore lower cost.
