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Principles for selecting compressor and pump drive solutions 1. Brief description In many chemical plants, pumps and compressors are used, and their energy consumption is also quite large in a chemical plant. Once these devices are put into operation, these pumps and compressors are required to work continuously for about one to two years. Therefore, the drivers of these pumps and compressors must be very reliable. While ensuring reliable operation and control of the device, they must also require minimum investment and energy consumption. It can be seen that the selection of compressor and pump drive solutions is particularly important. Common compressor and pump drivers are as follows: (1) Electric motor drive (2) Steam turbine drive (3) Diesel engine drive (4) Gas turbine drive compressor and pump Which driver to choose depends on the process of the device and the location, geographical location and surrounding environment of the device. In the absence of special requirements, generally diesel engines and gas turbines are rarely used as drivers. Especially in areas where natural gas, separated gas or petroleum cracked gas are scarce, gas turbines cannot be used as drivers. The following takes the selection of a compressor drive scheme for a certain project as an example to illustrate the scheme comparison and selection principles when selecting motors and steam turbines for compressor and pump drivers. In a certain engineering supporting device, two compressors (one air compressor and one refrigeration compressor) are required for normal production, and the reactor by-produces a certain amount of medium-pressure steam. Therefore, whether the compressor is driven by a motor or a steam turbine, which one is more energy-saving, economical and reasonable? The comparison and optimization of plans are more important. Introduction to the steam system The by-product steam parameters of the reactor are: pressure P1=4.1MPa (G), temperature t1=343°C, and steam volume Q1=54.29t/h. The steam parameters for thermal users are: pressure P2=0.3MPa(G), temperature t2=144℃ and saturated steam volume Q2=44.84t/h. Based on such a set of steam consumption and steam production conditions, the following two compressor driving schemes are available. Now let’s compare these two options. 2. Compressor drive scheme (1) Scheme 1: The motor-driven compressor scheme uses the by-produced steam to drive a back-pressure steam turbine to generate electricity. The back-pressure exhaust steam is supplied to 0.3MPa(G) steam heat users, and the generated electricity is integrated into the 6000V power grid in the factory. The two compressors are driven by two explosion-proof motors respectively, and the power required by the motors is provided by the 6000V power grid in the factory. 1. The generator output electric power is calculated from the water vapor enthalpy entropy diagram (is diagram): under the conditions of P1, t1, the enthalpy of steam is: i1=3075kJ/kg, under the conditions of P2, t2, the enthalpy value is: i2=2735kJ/kg, The adiabatic enthalpy drops to the enthalpy value i=2590kJ/kg, then the effective enthalpy drop is: △i=i1-i2=340kJ/kg, the adiabatic enthalpy drop is: △i=i1-i=485kJ/kg, at this time the internal efficiency of the turbine is: Within eta = △i/△i = 70%. This value is a normal value for back-pressure low-power turbines. The output electrical power is: (assuming that the pipe efficiency is eta pipe 98%, the steam turbine mechanical efficiency eta machine is 97%, the coupling efficiency is eta coupling 99%, and the generator efficiency is eta electricity 98%) N=Δi Q1 η pipe eta machine NX coupling NX electricity / 3.6 = 4728.8kW 2. Motor input power calculation: Since the turbine power N1 and N2 driving the two compressors are 3450kW and 1345kW respectively, the rotation speeds are 7063r/min and 9481r/min (rev/min) respectively. In order to match the motor speed with the compressor speed, a speed increaser must be installed. Assuming that the power of the motor is the same as that of the steam turbine, the electric power required by the motor from the 6000V power grid in the factory is: (Assume that the transmission loss of the power grid in the factory is calculated as 3%, the loss of the speed increaser is calculated as 2%, the motor efficiency is η, and the electric motor is 98%) N'=(N1+N2)/=5147kW 3. The power difference between the two ΔN=N'-N=418.2kW It can be seen that the output electric power of the generator in Scheme 1 cannot meet the input power of the motor. (2) Plan 2: The steam turbine driven compressor plan uses the by-produced steam to drive the back-pressure steam turbine to directly drive the compressor. The turbine back-pressure exhaust steam supplies steam heat users with 0.3MPa(G). 1. Calculation of steam flow entering the two compressor steam turbines a. Calculation of the steam flow rate entering the steam turbine of the air compressor. Since the air compressor is manufactured by a domestic manufacturer, the turbine supporting the compressor also belongs to the domestic manufacturer. According to the data provided by the domestic turbine manufacturer, when N1=3450kW , n=7063r/min, and the steam inlet parameters are at P1 and t1, and the exhaust steam parameters are at P2 (at this time, the adiabatic enthalpy drops to the enthalpy value i = 2590kJ/kg), eta = 80.5%, and the turbine inlet steam volume is calculated according to the following formula: (Assuming that the pipeline efficiency is eta pipe is 98%) m1 = N1 When the steam inlet parameters are at P1 and t1, and the exhaust steam parameters are at P2 (at this time the adiabatic enthalpy drops to the enthalpy value i = 2590kJ/kg), eta = 67%, the correction coefficient of rotation speed to efficiency is A = 0.95, the correction coefficient of superheat to efficiency is B = 1.006, the correction coefficient of back pressure to efficiency is C = 0.985, then the turbine intake steam volume is calculated according to the following formula: (Assume that the pipeline efficiency is 98% for eta pipe) m1=N2×3.6/=16.15t/hc. The total steam consumption of the two turbines is m=m1+m2=48.61t/h. 2. The difference between the turbine steam consumption and the by-produced steam is Δm=m-Q1=-5.68t/h. Therefore, the by-produced steam in option 2 not only meets the requirements of the turbine for steam volume, but also saves money. 3. Comparison and Analysis of Plans (1) Energy consumption The motor-driven compressor plan not only causes energy waste, but also the generator output electric power cannot meet the motor input power requirements (418.2kW difference). With the steam turbine directly driving the compressor plan, the by-product steam not only meets the steam turbine requirements for steam volume, but also has a surplus (5.68t/h). If the excess steam is sent to an off-site thermal power plant for power generation, the additional electrical power that can be generated is calculated by the following formula: N1 = Δi condensation Δm η tube eta machine eta connection NX electricity / 3.6 = 909.5kW In the calculation of the above formula, considering the condenser, the exhaust steam pressure is 0.007MPa (A). At this time, the steam enthalpy value is: i4 = 2450kJ/kg (assuming that the exhaust steam humidity of the condensing steam turbine is considered to be 5%), then the effective enthalpy Decrease: △i condensation = i1-i4 = 625kJ/kg, assuming that the pipe efficiency is eta pipe is 98%, the steam turbine mechanical efficiency is 97%, the coupling efficiency is eta coupling is 99%, and the generator efficiency is eta electricity is 98%) It can be seen that the energy consumption of the steam turbine directly driven compressor solution is lower than that of the motor driven compressor solution, and the power saving value is: N province = N1 + ΔN = 1327.7kW. The above-mentioned saved electric power can generate 1327.7kWh of electricity per hour. Calculated as 8000 operating hours per year, it can generate 10621600 kWh / year. The electricity price per kWh is calculated as 0.18 yuan, which can save more than 1.9 million yuan per year. Its economic value is considerable. The results of the above quantitative analysis show that in this project, the steam turbine driven compressor solution is more energy-saving than the electric motor driven compressor solution. (2) Main equipment purchase costs and land area. Since the electrical explosion-proof level of the location where the compressor of this device is located is classified as dIIAT2, and there is an external steam supply source for startup (supplied by a 3.9MPa (G) steam pipe network), there is no need to install a large explosion-proof motor for startup. Therefore, the steam turbine driven compressor solution has two less large explosion-proof motors, two less speed increasers, one less 6000kW back pressure turbine generator set and two more small power turbines than the electric motor driven compressor solution. In addition, the steam turbine driven compressor solution requires one less power plant room than the electric motor driven compressor solution. The results of the above qualitative analysis show that the steam turbine driven compressor solution saves equipment investment and floor space than the electric motor driven compressor solution. (3) Steam balance results After adopting the steam turbine driven compressor scheme, the by-product is 4.1MPa(G), and the steam at 343°C is still excess 5.68t/h. After the pressure is adjusted by the regulating valve, it is merged into the 3.9MPa(G) pipe network (the 3.9MPa(G) steam pipe network is the startup steam pipe network and is supplied from outside the device). The 0.3 discharged by the back-pressure steam turbine MPa(G), 143℃ steam heating users will have 3.77t/h (derived from m-Q3) left, which will be incorporated into the 0.3MPa(G) steam pipe network for use by other process equipment and public engineering users. Through the above calculation and analysis, the results are listed in Table 1. Plan Comparison Table Table 1 No. Comparison Category Plan 1 Plan 2 Conclusion 1 Energy consumption cannot be self-balanced. There is a saving in steam. Plan 2 is good 2. Major equipment costs are more and less. Plan 2 is good 3. It occupies a larger area and is smaller. Plan 2 is good 4. Steam balance supplies low-pressure steam to the external pipe network and supplies medium-pressure steam to the external pipe network. Plan 2 is good. Based on the analysis of the above engineering examples, it can be concluded that when there is by-product steam or steam source, The steam turbine-driven compressor solution is superior to the electric motor-driven compressor solution in terms of energy saving, investment, and land occupation. Therefore, as far as this project example is concerned, the steam turbine driven compressor solution is the best solution. 4. Conclusion Based on the above engineering examples and similar engineering experience, in the engineering design of compressors and pumps, the selection of drivers is generally carried out according to the following principles: (1) For drives below 500kW, whether for normal operation or backup, electric motors should be regarded as the preferred targets. Under normal circumstances, the relationship between the price of steam turbines and their power changes is not as great as the relationship between the price of motors and their power changes. To choose a driver with a power below 500kW, the cost of a steam turbine is higher than that of a motor. Therefore, whether for normal use or as a backup, the motor should be the first choice. Unless there are special requirements, such as NEMA SM23 and API614 regulations: As an oil pump that supplies oil to the lubrication system, sealing system and regulating oil system of steam turbines, compressors and pumps, there should be a main oil pump and a backup oil pump, and the main and auxiliary oil pumps should use different energy sources for power. Unless otherwise specified, the main oil pump shall be driven by a steam turbine and the backup pump by an electric motor. (2) For drives above 500kW, the steam turbine as the drive should generally be regarded as a backup. If the required drive power exceeds 500kW, the motor will be used as the drive under normal circumstances, and the steam turbine will only be used as a backup. If high and medium pressure steam decompression measures have to be adopted in order to achieve steam balance in the device, the above-mentioned backup steam turbine will operate normally and the motor will operate as a backup. (3) For drives with power above 1000kW, steam turbines should generally be considered commonly used as drives because condensing steam turbines and extraction condensing steam turbines are more expensive than backpressure steam turbines at the same power, and the land area or civil construction investment is larger, and the piping is much more complicated. But the gas consumption is much smaller. Therefore, the selection of condensing and extraction condensing steam turbines is generally decided after technical and economic comparison when the steam balance is allowed. (4) Steam turbine drive has the following outstanding advantages over motor drive: 1. When the motor starts, the instantaneous current is 5 to 10 times higher than the rated current. If the motor power is quite large, the operation of the power supply may be affected. This situation does not exist at all with steam turbine drives. 2. As a motor of electrical equipment, it is easy to cause sparks and thus the possibility of fire is relatively high. This possibility does not exist with steam turbines. Therefore, steam turbines are much safer in terms of safety. 3. The high speed of the steam turbine can adapt to the driving requirements of high-speed compressors and high-pressure pumps. The motor cannot, and equipment such as a speed increaser must be added. (5) Compared with motor drive, the disadvantage of steam turbine drive is that the workload of daily maintenance and parking maintenance is larger than that of motor, and it is not easy to repair. The operating status of parameters such as bearing temperature, shaft vibration, shaft displacement, lubricating oil pressure and oil temperature, and oil tank level must be monitored at all times. The system is complex and occupies a large area. 3. Analysis of the power generation of the turbine: W=α1·(hgr-h extraction)+α2·(hgr-hq) In the formula, α1-extraction steam accounts for the share of the total steam volume ； α2-The share of steam supply in total steam volume ； Assuming that the deaerator and low steam consumption are not considered, α1+α2=1 1. Use tgs=150°C, α2=1-α1 W1=(hgr-hq)-α1•(h pumping-hq) 2. Use tgs=104°C, α1=0, α2=1-α1=1 W2=hgr-h pumping. Therefore, when the extraction steam pressure is not equal to the steam supply pressure, h pumping > hq, W1