Technical and Economic Analysis of Constant Speed ​​Pump and Speed ​​Control Pump

1. Fixed speed pump and speed pump working principle and characteristics

When the pump installed in a certain pipeline work, the actual state of work depends not only on the performance curve of the pump itself, but also depends on the characteristics of the entire pipeline. The intersection of these two curves determines the operating conditions of the pump in the piping system. In the same piping system characteristics of the system, for different pumps, the operating point is different. Constant speed pumps and speed pump work point there is a big difference, so that the flow, lift, different efficiency.

Constant speed feed water pump in the water supply system is generally centrifugal, its working principle is the use of centrifugal force generated by the rotation of the fluid to obtain energy. It is driven by a certain power motor to a certain speed of rotation, the fluid to be a certain pressure and kinetic energy. Speed ​​feed pump (YT62 type fluid coupling, for example) has a special piece - hydraulic coupling. It is installed in the prime mover (motor, etc.) and the feed pump between the rotation of the liquid kinetic energy to transmit power hydraulic transmission. Its working principle is: there is a pump impeller at the end of the drive shaft, the driven shaft on the opposite end of the drive shaft with a turbine, impeller and turbine to maintain a certain gap between the two form a cavity, said For the round circle. When the prime mover drives the drive shaft to rotate, the working fluid gains energy in the pump wheel and is thrown from the pump wheel at a certain speed toward the turbine to transfer the energy to the turbine, so as to drive the driven shaft to rotate and realize the power transmission .

Circulation of working oil in the circular circle The circulation can only be maintained if there is a difference in centrifugal force between the impeller and the turbine at different speeds (slip). Under rated operating conditions, slip ≤ 3%, as long as the change in the oil circulation in the circle, that is, to change the oil filling, you can adjust the speed and power transmission driven shaft to achieve stepless speed regulation.

2 constant speed pump, speed pump technical and economic comparison

Hebei Thermal Power Co., Ltd. eight technical transformation project for the two 200MW double-pumping heat units and four 410t / h circulating fluidized bed boiler, a furnace for each 2 units for a unit, plan each unit set the nameplate output 440t / h, lift 1400m 2 speed pump and a fixed speed pump. The following comparison when the system constant speed pump (or speed governor pump) run alone, 2 constant speed pump (or 2 speed governor pump) run side by side, a fixed speed pump and a speed governor pump running side by side.

2.1 constant speed pump (or speed pump) run alone

Methods to change the pump performance curve are variable speed regulation, bucket adjustment and cavitation regulation. The main methods to change the pipeline characteristic curve are outlet throttling regulation and inlet throttling regulation. The most common regulation method is outlet throttling regulation, which is a way to adjust by adjusting the valve opening degree. General fixed-speed pump for the export side of throttling regulation, variable speed pump for variable speed regulation.

Boiler start-up time is longer, and the boiler load to bring 410t / h the entire process of pressing the press side is gradually reached, that is to say if in the process of using speed control pump can only change the speed on To meet the start-up requirements of the furnace. It can be seen from the characteristic curve of the governor pump that the corresponding lift at the flow rate of 120 ~ 450t / h can be raised from 800m (or lower) to 1400m according to the step-up temperature and pressure of the furnace without adjusting the feedwater regulating valve . In this case, the constant speed pump is used, and the water supply control valve needs to be throttled for a long time to satisfy the required flow rate, thus consuming a higher energy head.

Flow from 120 ~ 450t / h, the corresponding lift of 1680 ~ 1430m, excess head can be consumed by the water control valve, see equation (1):

Where N --- motor power consumption, kW;

g --- acceleration of gravity, m / s2;

Q --- mass flow rate, kg / s;

H --- head, m;

η --- pump efficiency,%.

It can be seen that under certain conditions of other parameters, the power N is proportional to the head H. As the pump speed can be achieved by changing the speed of the furnace line to start the request, so the energy saving effect is significant. Such as 120t / h, speed pump than fixed speed pump half the power.

2.2 pump parallel operation

The purpose of the pump parallel is to increase the flow when the pressure head is the same, and then draw the intersection C of the common pipeline characteristic curve b and the pump parallel characteristic curve, that is, the working point when working in parallel, the flow rate is QC and the lift is HC. Parallel work is characterized by: head equal to each other, the total flow for each pump (same performance pump) and the flow of delivery, that QC = 2QA. The project uses two furnace 1 mechanism, when the first two boilers ignition load needs to start the second set of water pump. The following two sets of constant speed pumps in parallel and 1 set 1 and 2 to adjust the situation to make a comparison.

2.2.12 constant speed pump in the water supply system in parallel operation

Constant speed pump is running in parallel by changing the pipeline characteristic curve that is the way to adjust the boiler feed water control valve opening to achieve the purpose of regulating the flow. This work point is with the flow changes, head also changes. The general design point is to design the flow, lift and efficiency of the feed pump under the condition that the feedwater control valve is fully open, which is the economic working point. Although the method of throttling adjustment is simple and convenient, the excess lift generated by the feed water Regulating valve consumed, resulting in energy loss, lower load economy even worse. In particular, Hebei Thermal Power Co., Ltd. for the heating unit, the thermal load by the industrial heat users and heating heat load is very large, the main water flow often changes, long-term by adjusting the opening of the water regulating valve to regulate the flow, will inevitably lead to more than Pure coagulation unit greater economic waste, but also to the water supply valve scouring damage.

When the system is operating at rated load (2Q0), the head is the rated head H0 of the feed pump, the flow is the sum of the rated flow of 2 feed pumps 2Q0, the tube resistance curve at this time is b, that is, the feed water control valve is fully open ; When the system is running at 0.75 rated load (1.5Q0), because the piping characteristic curve becomes b ', that is, the water supply control valve is throttled, so that the working point of the parallel pump changes from A to A ', This time the flow rate of 1.5Q0, head is H1, corresponding to a single pump operating point from B to B', the flow rate of 0.75Q0, head is H1, that is, the lift from the original H0, an increase of H1- H0.

2.2.21 speed control pump and a constant speed pump in parallel operation

If each unit is 2 fixed speed pumps, then add a speed pump. Normal operation through the speed pump to adjust the unit required water flow, you can overcome the water supply pipe pressure is too high defects. This mode of operation requires that the outlet pressure of the governor pump be maintained at the rated outlet pressure of the constant speed pump and the flow rate varies between 30% and 100%, which requires simultaneous adjustment of the feed pump characteristic (by adjusting the feed pump speed) and Tube resistance characteristic curve (by adjusting the water supply valve opening). So that the working point falls on the straight line for the head of H.

Parallel operation, running at full load conditions at point A, the flow rate of 2Q0, head pressure H0, constant speed pump and speed control pump are running at point B. When the load is reduced to 1.5Q0, the governor pump speed is reduced, the pump characteristic curve becomes a2, so that the feed pump total lift curve to a2 '. At the same time change the water supply valve opening, the tube resistance characteristic curve to b '. At this point the operating point A0, constant speed pump is still running at point B (that is, the rated operating point of the pump), the speed control pump is running at B0, the flow rate is 0.5Q0, the pressure head is still H0, so At a flow rate of 1.5Q0, the energy consumption of the H1-H0 head difference is saved compared with the pure constant speed pump system mentioned above.

If at this time does not adjust the tube resistance characteristic curve, the original tube resistance characteristic curve can be extended down and the original single constant speed pump curve intersect, you will find the constant speed pump load is too high, the speed pump can only take less Load, which will lead to constant speed pump overload and speed pump load is too small and fever, vibration and even vaporization of the poor, so when there is a speed pump should be taken at the same time adjust the feed pump characteristic curve and the tube resistance characteristic curve method To adjust the flow of purpose.

2.2.32 speed control pump in parallel operation

When the speed regulating pump is operated in parallel, the speed of the speed regulating pump is changed to make the head and the flow meet the requirements of the boiler, so that when the two constant speed pumps are connected in parallel, the pump can be operated at a lower flow rate (below the rated flow rate) The waste caused; also overcome when one boiler with a full load of another boiler ignition or with a small load, a set of 1 parallel run to make constant speed pump overload, the pump speed is too small pump damage . Ensure the economy and safety of the unit.

3. Conclusion

In summary, the speed pump, fixed speed pump system in parallel than the whole system for the speed control pump to save the initial investment, and all for the fixed-speed pump system to reduce the water consumption unit, especially as a heating unit, In Hebei Thermoelectric Co., Ltd. Eight technical transformation project water supply system to set two speed pump technical economy is better, a fixed speed pump as a backup is more reasonable.

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