Flue gas waste heat effective utilization system for gas-steam combined cycle generator sets

Abstract

The present invention provides a system for effectively utilizing waste heat of flue gas of a gas-steam combined cycle power generation unit, comprising: a water supply main line; a return water main line; a waste heat exchange device, the waste heat exchange device comprising a waste heat exchange module, a water inlet branch and a water outlet branch; a return bypass across the waste heat exchange device; a starting heater, the starting heater is arranged at the water outlet end of the water supply main line; an auxiliary steam main pipe, the steam outlet end of the auxiliary steam main pipe is connected to the starting heater; a natural gas heating device, the natural gas heating device comprises a heater, a water inlet branch pipe, a water outlet branch pipe, a heating flow channel, a natural gas input pipe, a natural gas output pipe and a natural gas heat absorption flow channel; an expansion water tank, the expansion water tank is connected to the water supply main line or the return water main line; a circulating water pump, the circulating water pump is arranged in the return water main line. The present invention can effectively utilize the waste heat of flue gas of the waste heat boiler, increase the natural gas temperature and improve the operating efficiency of the generator set.

Description

Technical Field

[0001] The invention relates to the technical field of boiler waste heat recovery, and in particular to a system for effectively utilizing flue gas waste heat of a gas-steam combined cycle generator set. Background Art

[0002] The existing natural gas pressure regulating station is used by multiple gas-steam combined cycle power generation units. The natural gas pressure regulating station area is equipped with two natural gas water bath furnaces, which are used to heat the natural gas so that the temperature of the natural gas at the outlet of the pressure regulating station meets the inlet temperature requirement of the gas turbine front module (about 35°C). According to the actual operation data in recent years, the two water bath furnaces have a total annual gas consumption of about 600,000 cubic meters, which is high in gas consumption and operation cost. In addition, if you want to further increase the natural gas temperature to improve the power generation efficiency of the gas-steam combined cycle power generation unit, the amount of gas consumed will increase further, and the comprehensive power generation efficiency will decrease instead.

[0003] The exhaust gas temperature of the waste heat boiler of the existing gas-steam combined cycle power generation unit is about 90°C, which has a certain amount of waste heat energy. If it is directly discharged into the atmosphere, there will be a certain degree of energy loss. Summary of the invention

[0004] In view of the shortcomings of the prior art mentioned above, the technical problem to be solved by the present invention is to provide a system for effectively utilizing the flue gas waste heat of a gas-steam combined cycle power generation unit, which can effectively utilize the flue gas waste heat of the waste heat boiler, replace the existing natural gas water bath furnace, heat the natural gas through circulating water, improve the explosion-proof safety level of the gas-steam combined cycle power generation unit, effectively reduce the fuel consumption of the power plant, reduce the emission of flue gas pollutants, and increase the natural gas temperature to improve the operating efficiency of the generator set.

[0005] In order to solve the above technical problems, the present invention provides a system for effectively utilizing waste heat from flue gas of a gas-steam combined cycle power generation unit, comprising:

[0006] Water supply main pipeline;

[0007] Backwater main line;

[0008] A waste heat exchange device, the waste heat exchange device comprises a waste heat exchange module, a water inlet branch and a water outlet branch. The waste heat exchange module is arranged at the tail flue of the waste heat boiler. A heat exchange flow channel is formed in the waste heat exchange module. The two ends of the water inlet branch are respectively connected to the heat exchange flow channel and the return water main line, and the two ends of the water outlet branch are respectively connected to the heat exchange flow channel and the water delivery main line;

[0009] A return bypass spanning the waste heat exchange device, with both ends of the return bypass being connected to the water supply main line and the return water main line respectively;

[0010] A start-up heater is provided at the outlet end of the water delivery main pipeline;

[0011] An auxiliary steam main pipe, the steam outlet end of the auxiliary steam main pipe is connected to the start-up heater;

[0012] A natural gas heating device, the natural gas heating device comprises a heater, a water inlet branch pipe, a water outlet branch pipe, a heating flow channel, a natural gas input pipe, a natural gas output pipe and a natural gas heat absorption flow channel, the heating flow channel and the natural gas heat absorption flow channel are both formed in the heater and are mutually matched in position, the two ends of the water inlet branch pipe are respectively connected to the heating flow channel and the start-up heater, the two ends of the water outlet branch pipe are respectively connected to the heating flow channel and the return water main line, the natural gas input pipe is connected to the gas inlet end of the natural gas heat absorption flow channel, and the natural gas output pipe is connected to the gas outlet end of the natural gas heat absorption flow channel;

[0013] An expansion water tank, which is connected to a water supply main line or a water return main line;

[0014] Circulating water pump: The circulating water pump is installed in the return water main line.

[0015] Preferably, a water outlet gate valve and a water outlet check valve are sequentially provided on the water outlet branch along the water flow direction.

[0016] Preferably, a water inlet gate valve, a water inlet regulating valve and a water inlet check valve are sequentially provided on the water inlet branch along the water flow direction.

[0017] Preferably, the flue gas waste heat effective utilization system further comprises a wastewater discharge pipe, one end of which is connected to the startup heater, and the other end of which extends to the wastewater pool.

[0018] Preferably, there are multiple waste heat exchange devices, and all waste heat exchange devices are arranged in parallel with each other.

[0019] Preferably, a metering unit is provided on the natural gas input pipe.

[0020] Preferably, a pressure regulating unit is provided on the natural gas output pipe.

[0021] Preferably, a first return gate valve, a filter, a return check valve and a second return gate valve are sequentially provided along the water flow direction on the return water main line; the first return gate valve and the filter are both located upstream of the circulating water pump, and the return check valve and the second return gate valve are both located downstream of the circulating water pump.

[0022] As described above, the system for effectively utilizing waste heat of flue gas of a gas-steam combined cycle power generation unit of the present invention has the following beneficial effects: In the present invention, before the first power generation unit is started, since the temperature of the flue gas discharged by the waste heat boiler is not high enough, a startup heater is used to heat the circulating water, and the circulating water that has absorbed heat then flows into the natural gas heating device to transfer the heat to the natural gas in the natural gas heat absorption flow channel, so that the temperature of the natural gas in the natural gas output pipe reaches the requirement. When the above-mentioned startup heater is in the heating mode, the water inlet branch and the water outlet branch of the waste heat heat exchange device are in a non-circulating state, the reflux bypass is in a circulating state, and the auxiliary steam main pipe is in a steam supply state to pass steam into the above-mentioned startup heater, thereby heating the circulating water to a preset temperature, thereby heating the natural gas to a preset temperature. At the same time, the circulating water pump is in a running state. When the exhaust gas temperature of one waste heat boiler meets the temperature requirement for heating the circulating water, the water inlet branch and the water outlet branch of the waste heat heat exchange device are in a circulating state, and the reflux bypass is in a non-circulating state. After the outlet water temperature of the outlet branch reaches the preset value, the startup heater is disabled and the tail flue gas heating mode of the waste heat heat exchange device is switched. At this time, the heating degree of the natural gas heating device is controlled by adjusting the frequency of the above-mentioned circulating water pump and the flow of the water inlet branch and the water outlet branch. When multiple generator sets are operating normally, whenever the exhaust temperature of a waste heat boiler meets the heating requirements, the corresponding waste heat exchange device is immediately turned on, thereby entering the heating mode in which multiple tail flue gases jointly heat the natural gas. When one of the waste heat boilers or one of the waste heat exchange devices fails, the corresponding water inlet branch and water outlet branch are automatically closed. If only one waste heat exchange device is in operation, it will automatically interlock and put into use the startup heater before exiting operation, and switch to the heating mode of the startup heater. If the temperature of the natural gas in the natural gas heating device does not reach the preset value in the tail flue gas heating mode, the startup heater is automatically put into use to ensure that the temperature of the natural gas meets the safe operation requirements of the gas turbine. Therefore, the flue gas waste heat effective utilization system of the above-mentioned gas-steam combined cycle power generation unit can effectively utilize the flue gas waste heat of the waste heat boiler, replace the existing natural gas water bath furnace, heat the natural gas through circulating water, improve the explosion-proof safety level of the gas-steam combined cycle power generation unit, effectively reduce the fuel consumption of the power plant, reduce the emission of flue gas pollutants, and further increase the natural gas temperature to improve the operating efficiency of the generator set. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 Shown is a schematic diagram of a system for effectively utilizing waste heat from flue gas of a gas-steam combined cycle power generation unit according to the present invention;

[0024] Component number description

[0025] 1 Water supply main line

[0026] 2 Return main line

[0027] 21 First return valve

[0028] 22 Filters

[0029] 23 Backflow check valve

[0030] 24 Second return valve

[0031] 25 Backflow bypass

[0032] 3 Waste heat exchange device

[0033] 31 Waste heat exchange module

[0034] 32 Water inlet branch

[0035] 321 Inlet Gate Valve

[0036] 322 Water Inlet Regulating Valve

[0037] 323 Inlet Check Valve

[0038] 33 Water outlet branch

[0039] 331 Outlet Gate Valve

[0040] 332 Water outlet check valve

[0041] 34 Heat exchange channel

[0042] 4 Start the heater

[0043] 5 Auxiliary steam main pipe

[0044] 6 Natural gas heating device

[0045] 61 Heater

[0046] 62 Inlet branch pipe

[0047] 63 outlet branch pipe

[0048] 64 Natural gas inlet pipe

[0049] 641 Measuring unit

[0050] 65 Natural gas export pipe

[0051] 651 Voltage Regulator Unit

[0052] 66 Natural gas heat absorption flow channel

[0053] 7 Expansion tank

[0054] 8 Circulation water pump

[0055] 9 Wastewater discharge pipe

[0056] 91 Wastewater pool DETAILED DESCRIPTION

[0057] The following is a description of the implementation of the present invention by means of specific embodiments. Those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification.

[0058] It should be noted that the structures, proportions, sizes, etc. illustrated in the drawings of this specification are only used to match the contents disclosed in the specification for people familiar with this technology to understand and read, and are not used to limit the limiting conditions for the implementation of the present invention, so they have no substantial technical significance. Any modification of the structure, change of the proportion relationship or adjustment of the size, without affecting the effects and purposes that can be achieved by the present invention, should still fall within the scope of the technical content disclosed by the present invention. At the same time, the terms such as "upper", "lower", "left", "right", "middle" and "one" quoted in this specification are only for the convenience of description, and are not used to limit the scope of the implementation of the present invention. The change or adjustment of their relative relationship should also be regarded as the scope of the implementation of the present invention without substantially changing the technical content.

[0059] Considering that the quality requirements for natural gas heating sources are relatively low, and the exhaust temperature of the waste heat boiler of the generator set is about 88°C, the waste heat of the flue gas can be recovered as a heat source to replace the existing natural gas water bath furnace.

[0060] Based on this, Figure 1 As shown, the present invention provides a system for effectively utilizing waste heat from flue gas of a gas-steam combined cycle power generation unit, comprising:

[0061] Water supply main line 1;

[0062] Backwater main line 2;

[0063] The waste heat exchange device 3 includes a waste heat exchange module 31, a water inlet manifold 32 and a water outlet manifold 33. The waste heat exchange module 31 is arranged at the tail flue of the waste heat boiler. A heat exchange channel 34 is formed in the waste heat exchange module 31. The two ends of the water inlet manifold 32 are respectively connected to the heat exchange channel 34 and the return water main line 2, and the two ends of the water outlet manifold 33 are respectively connected to the heat exchange channel 34 and the water delivery main line 1;

[0064] A reflux bypass 25 spanning the waste heat exchange device 3, with both ends of the reflux bypass 25 being connected to the water supply main line 1 and the water return main line 2 respectively;

[0065] Starting the heater 4, which is arranged at the water outlet end of the water supply main pipeline 1;

[0066] An auxiliary steam main pipe 5, the steam outlet end of the auxiliary steam main pipe 5 is connected to the start-up heater 4;

[0067] The natural gas heating device 6 comprises a heater 61, a water inlet branch pipe 62, a water outlet branch pipe 63, a heating flow channel 67, a natural gas input pipe 64, a natural gas output pipe 65 and a natural gas heat absorption flow channel 66. The heating flow channel 67 and the natural gas heat absorption flow channel 66 are both formed in the heater 61 and are mutually matched in position. The two ends of the water inlet branch pipe 62 are respectively connected to the heating flow channel 67 and the start-up heater 4, the two ends of the water outlet branch pipe 63 are respectively connected to the heating flow channel 67 and the return water main line 2, the natural gas input pipe 64 is connected to the air inlet end of the natural gas heat absorption flow channel 66, and the natural gas output pipe 65 is connected to the air outlet end of the natural gas heat absorption flow channel 66;

[0068] An expansion water tank 7, the expansion water tank 7 is connected to the water supply main line 1 or the water return main line 2;

[0069] The circulating water pump 8 is arranged in the return water main line 2.

[0070] In the present invention, the main water delivery line 1 and the main return water line 2 are both used to transport circulating water. The number of the waste heat exchange device 3 can be one or more, and all the waste heat exchange devices 3 are arranged in parallel with each other, and a one-to-one correspondence is formed between the waste heat exchange device 3 and the waste heat boiler. Before the first generator set is started, since the temperature of the flue gas discharged by the waste heat boiler is not high enough, the startup heater 4 is used to heat the circulating water, and the circulating water that has absorbed heat then flows into the natural gas heating device 6, and transfers the heat to the natural gas in the natural gas heat absorption channel 66, so that the temperature of the natural gas in the natural gas output pipe 65 reaches the required level. When the above-mentioned startup heater 4 is in the heating mode, the water inlet branch 32 and the water outlet branch 33 of the waste heat exchange device 3 are in a non-circulating state, the return bypass 25 is in a circulating state, and the auxiliary steam main pipe 5 is in a steam supply state to pass steam into the above-mentioned startup heater 4, thereby heating the circulating water to a preset temperature, thereby heating the natural gas to a preset temperature. At the same time, the circulating water pump 8 is in operation.

[0071] When the exhaust gas temperature of one waste heat boiler meets the temperature requirement for heating circulating water, the water inlet branch 32 and the water outlet branch 33 of the waste heat heat exchange device 3 are in a flowing state, and the reflux bypass 25 is in a non-circulating state. When the outlet water temperature of the water outlet branch 33 reaches the preset value, the start heater 4 is disabled and the tail flue gas heating mode of the waste heat heat exchange device 3 is switched. At this time, the heating degree of the natural gas heating device 6 is controlled by adjusting the frequency of the above-mentioned circulating water pump 8 and the flow rates of the water inlet branch 32 and the water outlet branch 33. When multiple generator sets are operating normally, whenever the exhaust gas temperature of one waste heat boiler meets the heating requirement, the corresponding waste heat heat exchange device 3 is immediately turned on, thereby entering a heating mode in which multiple tail flue gases jointly heat the natural gas.

[0072] When one of the waste heat boilers or one of the waste heat exchange devices 3 fails, the corresponding water inlet branch 32 and water outlet branch 33 are automatically closed. If only one waste heat exchange device 3 is in operation, the startup heater 4 is automatically interlocked and put into use before the operation is stopped, and the heating mode of the startup heater 4 is switched.

[0073] If the temperature of the outlet natural gas of the natural gas heating device 6 does not reach the preset value in the tail flue gas heating mode, the heater 4 is automatically started to ensure that the temperature of the natural gas meets the safe operation requirements of the gas turbine.

[0074] The above-mentioned circulating water pump 8 can be used in pairs and spare. When one of the circulating water pumps 8 trips, the other circulating water pump 8 is automatically started; the natural gas heating device 6 can also be used in pairs and spare. When one of the natural gas heating devices 6 fails or leaks, the other natural gas heating device 6 is automatically turned on.

[0075] Therefore, the flue gas waste heat effective utilization system of the above-mentioned gas-steam combined cycle power generation unit can effectively utilize the flue gas waste heat of the waste heat boiler, replace the existing natural gas water bath furnace, heat the natural gas through circulating water, improve the explosion-proof safety level of the gas-steam combined cycle power generation unit, effectively reduce the fuel consumption of the power plant, reduce the emission of flue gas pollutants, and further increase the natural gas temperature to improve the operating efficiency of the generator set.

[0076] Specifically, the flue gas waste heat effective utilization system of the present invention can increase the temperature of natural gas by 20°C on the basis of 35°C, which can correspondingly improve the operating efficiency of the gas turbine, and can improve the efficiency of the gas turbine by about 0.0832%. The power generation units of the whole plant can save 490,000 cubic meters of natural gas per year, or even more. In addition to the gas consumption of the original water bath furnace (calculated at 600,000 cubic meters / year), 1.09 million cubic meters of natural gas can be saved throughout the year, and sulfur dioxide emissions can be reduced by about 1.74 tons, nitrogen oxide emissions can be reduced by about 7.19 tons, and smoke dust emissions can be reduced by about 0.72 tons throughout the year. After the implementation of the flue gas waste heat effective utilization system, the safety risks brought by the use of water bath furnaces in the pressure regulating station area can be eliminated, and the explosion-proof safety level of the gas-steam combined cycle power generation unit can be improved; in addition, the utilization efficiency of energy can be improved, and it has good economic benefits. It can also reduce the emission of pollutants, be low-carbon and environmentally friendly, comply with national industrial policies, and have good social benefits.

[0077] The startup heater 4 can be a steam-water shell and tube heat exchanger, and the auxiliary steam in the auxiliary steam main pipe 5 is used for heating steam, and the startup heater 4 is drained. Considering that when a single generator set is running, the waste heat exchange device 3 corresponding to the waste heat boiler fails, in order to ensure that the above flue gas waste heat effective utilization system can operate continuously, the startup heater 4 is designed to heat the natural gas to at least 30°C when a generator set is running at full load. Considering that the role of the generator set in the power grid is to carry peak load and extreme operating conditions, the number of waste heat exchange devices 3 is designed according to the natural gas flow rate when four generator sets are running at full load, to ensure the safe and stable operation of the four generator sets.

[0078] In order to avoid the reverse flow of circulating water in the water outlet branch 33 and to adjust the flow state of the water outlet branch 33, a water outlet gate valve 331 and a water outlet check valve 332 are sequentially provided on the water outlet branch 33 along the water flow direction.

[0079] In order to avoid the reverse flow of circulating water in the water inlet branch 32 and adjust the flow state of the water inlet branch 32, the water inlet gate valve 321, the water inlet regulating valve 322 and the water inlet check valve 323 are sequentially provided on the water inlet branch 32 along the water flow direction.

[0080] In order to recycle the waste water in the startup heater 4 , the flue gas waste heat effective utilization system further includes a waste water discharge pipe 9 , one end of which is connected to the startup heater 4 , and the other end of the waste water discharge pipe 9 extends to a waste water pool 91 .

[0081] In order to detect the flow rate of natural gas, a metering unit 641 is provided on the natural gas input pipe 64 .

[0082] In order to detect the pressure of natural gas, a pressure regulating unit 651 is provided on the natural gas output pipe 65 .

[0083] In order to filter and regulate the circulating water in the above-mentioned return water main line 2, the above-mentioned return water main line 2 is provided with a first return gate valve 21, a filter 22, a return check valve 23 and a second return gate valve 24 in sequence along the water flow direction. The first return gate valve 21 and the filter 22 are both located upstream of the circulating water pump 8, and the return check valve 23 and the second return gate valve 24 are both located downstream of the circulating water pump 8.

[0084] In summary, the flue gas waste heat effective utilization system of the gas-steam combined cycle power generation unit of the present invention can effectively utilize the flue gas waste heat of the waste heat boiler, replace the existing natural gas water bath furnace, heat the natural gas through circulating water, improve the explosion-proof safety level of the gas-steam combined cycle power generation unit, effectively reduce the fuel consumption of the power plant, reduce the emission of flue gas pollutants, and increase the natural gas temperature to improve the operating efficiency of the generator set. Therefore, the present invention effectively overcomes the various shortcomings of the prior art and has a high industrial utilization value.

[0085] The above embodiments are merely illustrative of the principles and effects of the present invention, and are not intended to limit the present invention. Anyone familiar with the art may modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by a person of ordinary skill in the art without departing from the spirit and technical concept disclosed by the present invention shall still be covered by the claims of the present invention.

Claims

1. A system for effectively utilizing waste heat from flue gas of a gas-steam combined cycle power generation unit, characterized in that: include: A main water supply line (1) for conveying heat medium circulating water; A return water main line (2) for conveying heat medium circulating water; A waste heat exchange device (3), the waste heat exchange device (3) comprising a waste heat exchange module (31), a water inlet branch (32) and a water outlet branch (33), the waste heat exchange module (31) being arranged at the tail flue of the waste heat boiler, a heat exchange flow channel (34) being formed in the waste heat exchange module (31), the two ends of the water inlet branch (32) being respectively connected to the heat exchange flow channel (34) and the return water main line (2), and the two ends of the water outlet branch (33) being respectively connected to the heat exchange flow channel (34) and the water supply main line (1); a water inlet gate valve (321), a water inlet regulating valve (322) and a water inlet check valve (323) are sequentially arranged on the water inlet branch (32) along the water flow direction; and a water outlet gate valve (331) and a water outlet check valve (332) are sequentially arranged on the water outlet branch (33) along the water flow direction; A return bypass (25) spanning the waste heat exchange device (3), wherein both ends of the return bypass (25) are respectively connected to the water supply main line (1) and the water return main line (2); A starting heater (4) is provided at the water outlet end of the main water delivery pipeline (1); An auxiliary steam main pipe (5), wherein a steam outlet end of the auxiliary steam main pipe (5) is connected to a start-up heater (4); A natural gas heating device (6), the natural gas heating device (6) comprising a heater (61), a water inlet branch pipe (62), a water outlet branch pipe (63), a heating flow channel (67), a natural gas input pipe (64), a natural gas output pipe (65) and a natural gas heat absorption flow channel (66), the heating flow channel (67) and the natural gas heat absorption flow channel (66) are both formed in the heater (61) and are mutually positioned, the two ends of the water inlet branch pipe (62) are respectively connected to the heating flow channel (67) and the start-up heater (4), the two ends of the water outlet branch pipe (63) are respectively connected to the heating flow channel (67) and the return water main pipe (2), the natural gas input pipe (64) is connected to the gas inlet end of the natural gas heat absorption flow channel (66), and the natural gas output pipe (65) is connected to the gas outlet end of the natural gas heat absorption flow channel (66); An expansion water tank (7), the expansion water tank (7) being connected to the water supply main line (1) or the water return main line (2); A circulating water pump (8), the circulating water pump (8) being arranged in the return water main line (2); When the startup heater (4) is in the heating mode, the water inlet branch (32) and the water outlet branch (33) of the waste heat exchange device are in a non-circulating state, the reflux bypass (25) is in a circulating state, and the auxiliary steam main pipe (5) is in a steam supply state to pass steam into the startup heater (4), thereby heating the circulating water to a preset temperature, thereby heating the natural gas to a preset temperature; at the same time, the circulating water pump (8) is in an operating state; when the exhaust gas temperature of a waste heat boiler meets the temperature requirement for heating the circulating water, the water inlet branch (32) and the water outlet branch (33) of the waste heat exchange device are in a circulating state, and the reflux bypass (25) is in a non-circulating state; when the outlet water temperature of the outlet branch (33) reaches a preset value, the startup heater (4) is stopped and the tail flue gas heating mode of the waste heat exchange device (3) is switched.

2. The system for effectively utilizing waste heat from flue gas of a gas-steam combined cycle power generation unit according to claim 1, characterized in that: The flue gas waste heat effective utilization system further comprises a wastewater discharge pipe (9), one end of the wastewater discharge pipe (9) is connected to the start-up heater (4), and the other end of the wastewater discharge pipe (9) extends to a wastewater pool (91).

3. The system for effectively utilizing waste heat from flue gas of a gas-steam combined cycle power generation unit according to claim 1, characterized in that: The number of the waste heat exchange devices (3) is plural, and all the waste heat exchange devices (3) are arranged in parallel with each other.

4. The system for effectively utilizing waste heat from flue gas of a gas-steam combined cycle power generation unit according to claim 1, characterized in that: The natural gas input pipe (64) is provided with a metering unit (641).

5. The system for effectively utilizing waste heat from flue gas of a gas-steam combined cycle power generation unit according to claim 1, characterized in that: The natural gas output pipe (65) is provided with a pressure regulating unit (651).

6. The system for effectively utilizing waste heat from flue gas of a gas-steam combined cycle power generation unit according to claim 1, characterized in that: A first return gate valve (21), a filter (22), a return check valve (23) and a second return gate valve (24) are sequentially arranged on the return water main line (2) along the water flow direction; the first return gate valve (21) and the filter (22) are both located upstream of the circulating water pump (8), and the return check valve (23) and the second return gate valve (24) are both located downstream of the circulating water pump (8).

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