Boiler feed pump
The boiler feed pump is also called the feed pump (see reactor pump) and is designed as a multi-stage radial flow pump. (See also Multistage Pump.)
They are used to feed steam generators (such as boilers or nuclear reactors) a certain amount of feed water corresponding to the amount of steam discharged. Today, all boiler feed pumps are centrifugal pumps.
The design of the boiler feed water pump in terms of power input, material, pump type and drive largely depends on the development of power station technology. The trend for fossil fuel power plants is for larger and larger power plants (>1000 MW in 2011). This resulted in a boiler feed pump with a driving power of 30-50 MW.
Until 1950, the average pressure of the pump outlet cross section (the discharge pressure of the feed pump) was in the 200 bar area. By 1955, the average exhaust pressure had risen to 400 bar. In 1950, the mass flow rate was about 350 tons/hour, while today's flow rate is 3,200 tons/hour (up to 4,000 tons/hour in some cases). The boiler feed pump operates at a fluid temperature of 160 to 210ºC. Under special circumstances, the temperature of the treated fluid may be even higher.
The mass flow rate of the feedwater pump used in a 1600 MW nuclear power plant is as high as 4000 tons/hour, and the discharge pressure of the feedwater pump is 70 to 100 bar.
Until around 1950, boiler feed pumps were made of non-alloy steel. Since then, they have been made of steel with a chromium content of 13-14%. This material change must be made by introducing new chemical feed water components. The development of high-strength, corrosion-resistant and corrosion-resistant martensitic chromium steel with good seizure resistance and the continuous development of all pump components (bearings, shaft seals, pump hydraulic systems, etc.) have paved the way for the feed of today’s boilers It is a pump of 4500 to 6000 rpm.
The mass flow of centrifugal pumps increases rapidly with the increase of unit output in the power station. Today, the full-load feedwater pumps used in traditional 800 to 1100 MW power plant units consist of four to six stages with a stage pressure of up to 80 bar. The feedwater pump of the 1600 MW nuclear power plant is of single-stage type.
In conventional power stations exceeding 500 MW, the full-load feedwater pumps are increasingly driven by steam turbines. In most cases, the speed of the condensing turbine used is 5000 to 6000 rpm.
Electric motors usually drive part-load feedwater pumps in fossil fuel and nuclear power plants. The speed control of the electric feed water pump can be achieved through a fluid coupling (such as a variable-speed turbine coupling) or through an electric closed-loop control system through a thyristor (the maximum drive power rating in 2011 is about 18 MW).
Currently, four options for installing boiler feed pump drives are commonly used.
Low-speed booster pumps are usually driven by the free shaft end of the turbine through a speed reduction gear, or directly driven by the free end of an electric motor.
Single-suction or double-suction booster pumps are used to generate the NPSHR required by the system for the downstream connected high-speed boiler feed water pump.
For conventional power stations, the boiler feed pump is designed as:
Multi-stage barrel extraction pump
Ring section pump
Boiler feed water pump: barrel pumping type with water outlet
Boiler feed water pump: ring section model with perforated table
The difference between these two types is only in the structure of the pressure holding shell, which will affect the manufacturing cost and ease of installation. There is no difference in operational reliability and durability under abnormal operating conditions. The size of the rotating part and the runner can be designed to be the same.
The following describes two aspects of choosing between annular cross-section and barrel extraction pumps:
The lower the mass flow and the higher the pressure, the higher the material and manufacturing cost of the barrel pump. This does not apply to annular section pumps.
When repairing pumps installed in the system, barrel-type extraction pumps have some advantages over annular section pumps. If the rotor must be replaced, the syringe (see pump housing) can remain installed in the pipeline.
If there is no complete backup pump, or the replacement of the pump is very time-consuming, this is important for the availability of power plant equipment.
For nuclear power plants, single-stage feedwater pumps with double inlet impellers (see double-suction pumps) and double volutes are usually used. See figure 6 boiler feed water pump
Boiler feed pump: cast iron double suction reactor feed pump
Casting pressure-holding housing parts are increasingly being replaced by forged parts. For example, this feed pump can be designed to have a flow rate of about 4200 m3/h and a head of about 700 m at a speed of 5300 rpm.
For boiling water reactors, the head of the reactor feed pump is about 800 m, and for pressurized water reactors, the head of the feed pump is about 600 m. The flow rate is approximately twice that of a similar boiler feedwater pump in a fossil fuel power plant.
For boiler feed water pumps, two factors related to the shell wall thickness must be considered: pressure load and different temperature conditions that need to be endured. These two conditions can be met by using high-strength ferrite casing material. The material can keep the wall thickness thin enough to avoid any overload caused by temperature fluctuations, but it must also have sufficient thickness to ensure the necessary resistance Internal pressure safety.
Barrel pumps and barrel pump casings are usually made of non-alloy or low-alloy ductile steel. Surfacing is used for all surfaces in contact with feed water to cover them with corrosion- and erosion-resistant materials.
In order to weld the pump into the pipeline, if the material of the nozzle to be connected comes from a different material group, an adapter must be provided.
The barrel cover on the discharge side (including discharge pressure) is fixed by a larger torque-free stud. The seal is provided by the profile joint, which is only pressurized by the main pressure (up to a few hundred bars) without any external force.
Ring section pump
The casing of the annular section pump is preferably made of forged chromium or carbon steel plated with austenitic (iron solid solution) material.
The sealing elements between the shells of each stage (see stage) are sealed by metal-to-metal contact, and each shell is axially clamped by the tie bolts between the suction and exhaust shells (see pump housing) Close together.
The thermal shocks that cause various thermal expansions mainly cause additional loads on the sealing surfaces of the tie rods and the platform housing.
The common feature of barrel suction pumps and annular section pumps is that the greater the wall thickness, the greater the thermal stress caused by thermal shock, which in turn will shorten the service life of the pump. It is often necessary to provide spray water at a pressure between the suction and discharge pressure of the pump. This problem is solved by pumping water from one of the pump stages of the barrel pump and the annular section pump.
A stage of excavating the boiler feed pump
In the case of a ring-shaped section pump, it is easy to divide the flow under medium pressure through the discharge nozzle in one of the stage housings. See Figure 5 Ring Section Pump
For the barrel extraction pump, the inside of the barrel is divided into three pressure zones, so part of the flow under the required intermediate pressure can be directly led to the outside. See Figure 4 Barrel Pump
The sealing function is realized by a special-shaped joint between the exhaust and tapping pressure, and a metal-to-metal joint between tapping and inlet pressure. See figure 7 boiler feed water pump
Especially for special-shaped joints, it can meet the requirements of any temperature shock and make the sealing surface have a large degree of relative movement.
The pump shaft of the boiler feed pump has a small static deflection, because the spacing between the bearings is as small as possible, the diameter of the shaft is relatively large, and the impeller usually shrinks to the shaft (to achieve high performance). The pump shaft is generally insensitive to vibration and runs smoothly during normal operation (see Smooth Operation) without any radial contact. The diameter of the hub at the rear of the impeller is increased, and the geometry of the impeller inlet is designed to be a minimum diameter to reduce the residual axial force that must be absorbed by the balancing device (see axial thrust).
The rotor of a single-stage reactor feed pump is even harder than that of a boiler feed pump, and its static deflection is smaller than that of a multi-stage boiler feed pump.
Axial thrust balance
Some impellers of boiler feed pumps used in conventional power plants are arranged on the impellers to cause axial thrust. See figure.
The magnitude of the axial thrust depends on the position of the operating point on the characteristic curve, the speed and the amount of wear of the internal gap (see Controlled Gap Seal). If abnormal operating conditions occur, for example, additional interference forces may be generated. Cavitation.
On larger boiler feed water pumps, the axial force at the pump rotor is balanced by a hydraulic balancing device through which the processed fluid flows. Balancing devices are usually combined with oil-lubricated thrust bearings (see Plain Bearings). Since the balancing device absorbs more than 90% of the axial thrust, relatively small thrust bearings can be used. The balance device may include a balance disk with a balance disk seat, or a balance drum or double drum with a corresponding throttle bush.
The axial thrust generated in the reactor feed pump with double inlet impeller (see double suction pump) is hydraulically balanced. The remaining thrust is absorbed by the oil-lubricated thrust bearing. See figure 6 boiler feed water pump
Balance the radial force on the pump rotor
The radial force comes from the weight of the rotor, mechanical imbalance or hydraulic radial thrust. The radial force is balanced by two oil-lubricated radial bearings and a throttle gap, and the treated fluid flows along the axial direction along the throttle gap. This throttle gap is located at the impeller neck on the inlet side of the impeller, or in the case of a multi-stage boiler feed pump used in a conventional power station, at the outlet side of the impeller (interstage bushing) and the balance drum. If the rotor is in an eccentric position, a re-centering reaction force will be generated in these gaps, which depends largely on the pressure difference and the gap geometry (LOMAKIN effect).
When the feedwater in the gap is not a pure liquid phase due to abnormal operating conditions, the LOMAKIN effect will be severely reduced (see cavitation).
The hydrostatic effect of the gap is more helpful than the mechanical stiffness to reduce the deflection of the shaft. The system is designed in such a way that the operating speed is always kept far away from the critical speed of the rotor, which can additionally absorb the hydraulic excitation force (especially in low flow operation).
An additional diffuser or double volute can reduce radial thrust.
Common shaft seals for boiler feed water pumps are mechanical seals, floating ring seals and labyrinth seals. Nowadays, packing seals are less common. (See also shaft seal).
Warm up and keep warm
Transient or low-flow operating conditions will put additional load on the boiler feed pump. This leads to additional stress and strain, as well as to deformation of the component, which has various effects on its function.
Nowadays, almost all boiler feed water pumps must be able to cope with cold start (high temperature shock load) and semi-warm start without causing any damage. During these startup procedures, hot feed water suddenly flows into the cold pump, which causes the internal components to heat up faster than the pressure boundary. Depending on the starting frequency and the gradient curve of pressure and temperature (duty cycle), this may shorten the service life of the pump.
On machines with particularly thick walls, heat will spread more slowly in the thicker parts, increasing internal stress.
Generally, contact between the rotor and the stator cannot be ruled out because the narrow gap serves as a controlled gap seal. This applies to the impeller neck on the inlet side of the impeller, the discharge side gap between the impeller, diffuser and interstage bushing, and balance devices with multiple throttle gaps (depending on design).
For example, critical operating conditions such as steam bubbles cannot be completely avoided in the inlet pipeline. The brief contact between the stator and the rotor can cause high unbalanced forces in the narrow gap. Therefore, the material pair must not only be resistant to corrosion and erosion, but also must be particularly resistant to wear (with good seizure resistance). Die-cast chrome steel and special gap geometry have proven successful.
Under operating conditions with very low or zero flow, such as the rotating gear mode of a boiler feed pump driven by a turbine, a temperature layer will form in the processed fluid, which may cause the rotor to deform, and also after a slight delay. Will cause deformation of non-rotating parts. Once the gap is closed, the rotor will experience a significantly higher friction torque, which will overload the rotating gear and cause the pump to stop. In this case, the temperature on the rotor will no longer be equal, which will further aggravate the deformation of the rotor.
This may cause the pump to stop for several hours. Usually, the only remedy is to let the machine cool down to reduce or eliminate the critical temperature layer and deformation.
Several measures can be taken to optimize the thermal performance of the pump:
Avoid large temperature differences in and on the pump
The cold zone (shaft seal zone) is thermally isolated from the zone through which the hot fluid passes (hydraulic system and balance device) with the help of the insulation chamber system; heat seal is provided to prevent convective flow and a special thermowell.
Isolate the outside of the pump.
It is usually supplied by throttling pressure, and the pump is preheated or insulated by forced flow through the machine.
Temporarily or permanently interrupt the cooling water supply in the mechanical seal (secondary circuit) area.
Limit critical operating conditions (ΔT) (top/bottom of the barrel shell) and/or operating parameters of the ΔT between the shell and the feed water.
Reduce the impact of large temperature differences
Use the rotating gear to put the pump in standby mode.
Use synchronous rotating devices (to minimize or prevent actual downtime).
Drain water from critical heat dissipation areas.
Choose good thermal characteristics when choosing a shaft seal
Install non-contact seals (floating ring seals).
The above measures are usually used for barrel pumps (barrel extraction pumps) because their external dimensions, wall thickness, driving force (turbine with rotating gears) and working mode are considered to be more critical than annular section pumps. If possible, these measures are always automated to ensure the availability of the pump set.
Minimum flow valve
The minimum flow valve (automatic circulation valve) can ensure the minimum flow, thereby preventing the material in the pump from vaporizing or causing low flow cavitation due to an impermissible temperature rise, and damage during low flow operation.