the present invention will be described in detail below with reference to the drawings . however , it should be known for one person skilled in the art that the present invention is not limited by the listed embodiments , and any technical solution that accords with the spirit of the invention will fall within the protection scope of the invention . during the test and experiment of liquid nitrogen pump equipments used for nitrogen foam fracturing , the discharge pressure and displacement of the liquid nitrogen pump equipment should be respectively tested so as to give the relationships between the displacement and the pressure , between the input power and output pressure and between the pressure and the loading time . it can be seen that the discharge pressure of the liquid nitrogen pump equipment is key parameter that should be controlled during the overall test and experiment of liquid nitrogen pump equipments . in order to complete the overall test and experiment of liquid nitrogen pump equipment , it is required to set damping to the gas discharged from such equipment , that is to say , to carry out pressure loading . because the gas has large compression ratio , it is very difficult to directly and securely load the gas with large displacement . therefore , the loading experiment apparatus for liquid nitrogen equipment in accordance with the present invention is particularly installed a hydraulic pump truck so that the gas discharged from the liquid nitrogen pump equipment is mixed with water in the hydraulic damping apparatus so as to form a high pressure liquid - gas mixture , and then the pressure adjustment of the gas discharged from the liquid nitrogen pump equipment is indirectly achieved by adjusting the flow rate of the high pressure liquid - gas mixture . thus , the pressure loading of the gas discharged from the liquid nitrogen pump equipment may be indirectly achieved by converting the liquid pressure loading into gas pressure loading . in the loading test and experiment apparatus for liquid nitrogen pump equipments in accordance with the present invention , the hydraulic damping apparatus adjusts the output flow rate of the water discharged from the hydraulic pump equipment therein , while the pressure is adjusted by a pressure adjusting unit . the pressure adjusting unit mainly comprises a pneumatic cut - off valve component and a motor - driven pressure regulating valve , wherein the pneumatic cut - off valve component is used for rough adjustment of the discharge pressure , while the component is used for fine adjustment of the output pressure . meanwhile , nitrogen gas discharged from the liquid nitrogen pump equipment is mixed with liquid discharged from the hydraulic fracturing equipment by using a specialized manifold . thus , the nitrogen gas displacement of the liquid nitrogen equipment may be controlled by the power transmitting system and also the flow distributing system of the equipment per se . the present invention will be described in detail by reference with the examples below . fig1 is a structural schematic drawing of the loading test apparatus for liquid nitrogen pump equipments in accordance with the present invention . the loading test apparatus for liquid nitrogen pump equipments in accordance with the present invention mainly comprises three major parts as follows . 1 . liquid nitrogen pump equipment test unit 100 , which further comprises the following elements that connects with each other from upstream to downstream of liquid nitrogen , liquid nitrogen tank 1 , which is a normal pressure vessel for storing liquid nitrogen and stores liquid nitrogen that is to be used in the loading test of the liquid nitrogen pump equipment . liquid nitrogen pump equipment 2 , which is just the liquid nitrogen pump equipment that is to be tested for its performance in the invention , used for oil - gas field or coal bed methane nitrogen foam fracturing and arranged on the outlet pipeline of liquid nitrogen tank 1 so as to pump liquid nitrogen from the liquid nitrogen tank 1 ; the role of the liquid nitrogen pump equipment 2 is to pump the liquid nitrogen tank at low pressure and low temperature by the high pressure pump so as to form liquid nitrogen at high pressure and low temperature , and then the latter is heated by the specialized liquid nitrogen evaporator equipped on the liquid nitrogen pump equipment 2 , so that the liquid nitrogen at high pressure and low temperature is converted into gaseous nitrogen at high pressure and normal temperature . in one embodiment , the gas displacement of the nitrogen foam fracturing pump equipment used for oil - gas field exploitation or coal bed methane exploitation in accordance with the invention is in the range of 90 to 600 sm 3 / min , which is divided into four grades of 35 mpa , 70 mpa , 105 mpa and 140 mpa according to the discharge pressure grade . it is required for the experiment apparatus to accomplish the full - loading tests on the liquid nitrogen apparatus wherein the nominal output is in the range of 90 to 600 sm 3 / min and the nominal pressure is one of 35 mpa , 70mpa , 105mpa and 140mpa . check valve 3 arranged downstream from the liquid nitrogen pump equipment 2 , which is used to prevent nitrogen gas flowing back . pressure sensor 4 , which collects pressure data of the discharged nitrogen gas and transports it to the control system . 2 . hydraulic damping apparatus unit 200 , which further comprises the following elements that connects with each other from upstream to downstream of water , water tank 5 , which is a normal vessel used for storing water . flow sensor 6 arranged on the outlet pipeline of the water tank 5 , which collects the outlet flow rate of the water tank 5 and transports it to the control system . hydraulic pump truck 7 , which may be a hydraulic fracturing pump truck in practical experiment , is a high power and high pressure hydraulic pump equipment . its displacement should satisfy the highest displacement requirement based on the lowest foam quality requirement , and its highest pressure should be equal to or higher than the highest discharge pressure of the liquid nitrogen equipment . for example , in order to accomplish the test on a liquid nitrogen equipment with a normal gas displacement of 330 sm 3 / min and a normal discharge pressure of 105 mpa , it is required that the mixed foam quality is controlled within 20 % and the hydraulic fracturing pump truck has a liquid normal displacement equal to or greater than 1 . 9 m 3 / min and a discharge pressure equal to or greater than 105 mpa so as to ensure the security during the test and experiment . accordingly , it is required to provide a hydraulic fracturing pump truck of 1500 type ( i . e . its normal discharge power is 1500 kw ) having a normal discharge pressure of 105 mpa , such as a ylc1500 - 105 fracturing truck ( sy / t 5211 - 2009 ). also , the process may be accomplished by using two fracturing trucks having a discharge pressure of 105 mpa and a normal displacement in range of 1 . 0 - 1 . 9 m 3 / min selecting the form of pump trucks is because a equipment with the form of pump trucks will be easy to be placed and installed during the field test . meanwhile , the high power and high pressure hydraulic pumps owned by oilfield enterprises are also commonly in vehicle - mounted form . the hydraulic pump truck may be replaced by pump set equipments provided that the pump set equipments composed of water pumps and power systems not only satisfies the requirement of technical parameters but also is appropriate to be placed and installed in work field . check valve 8 arranged downstream from the hydraulic pump truck 7 , which is used to prevent water flowing back . pressure sensor 9 , which collects pressure data of water and transports it to the control system . manifold system 10 , which is composed of a high pressure water valve , a high pressure gas valve , a check valve that resists high pressure and is appropriate for transporting liquid - gas mixture and also pipelines , allows the high pressure water described above to mix with high pressure and normal temperature nitrogen gas . that is , nitrogen gas at normal temperature and high pressure from the liquid nitrogen pump equipment test unit 100 is mixed with water from hydraulic damping equipment unit 200 , during which high pressure nitrogen gas is mixed into the discharged high pressure water so as to form a high pressure liquid - gas mixture . the pressure of such high pressure liquid - gas mixture is the damping that the liquid nitrogen pump needs to overcome for discharging gas . 3 . pressure adjusting unit 300 is arranged downstream from the hydraulic damping apparatus unit 200 , and comprises a plurality of adjusting unit 310 and also subsequent water recycling equipments . the pressure adjusting unit 300 pressurizes the water discharged from the hydraulic pump truck 7 by adjusting the flow rate of the high pressure liquid - gas mixture discharged from the hydraulic damping apparatus unit 200 . in the pressure adjusting unit 300 , each adjusting unit 310 further comprises the following parts that connect in sequence : liquid - gas mixture output manifold 11 arranged downstream from the manifold system 10 , the role of which is to distribute the mixed high pressure liquid - gas mixture into each pressure adjusting mechanism . pneumatic cut - off valve 12 , which roughly adjusts the discharge pressure of the hydraulic pump truck by using a pneumatic motor as the adjusting mechanism for adjusting the pressure . motor - driven pressure regulating valve 13 , the role of which is to adjust pressure as the mechanism for implementing adjustment by using a servo motor and optionally to carry out feedback control so that the discharge pressure of the hydraulic pump truck is roughly adjusted . in a practical process , around 90 % of the pressure needed to be adjust to can be obtained by using the pneumatic cut - off valve 12 , and the remaining 10 % of the pressure adjustment can be achieved by using the motor - driven pressure regulating valve 13 . for example , when the liquid nitrogen pump equipment to be tested has a normal gas displacement of 330 sm 3 / min and a normal discharge pressure of 105 mpa , a ylc1500 - 105 fracturing truck is provided as the fracturing truck and the discharge pressure is required to be adjusted to 105 mpa , then firstly the discharge pressure may be controlled in range of 92 to 96 mpa using the pneumatic cut - off valve 12 , and subsequently within 105 mpa using the motor - driven pressure regulating valve 13 . throttling hush pipe 14 , which applies pressure loading to the high pressure water discharged from the hydraulic pump truck by utilizing the throttling principle and limits noise by a silencer . the throttling hush pipe is composed by a diffusion section of the throttler and a hush pipe , and the throttling hush pipe is covered by noise reduction and insulation material . the role of the diffusion section is to gradually decrease the flow rate of the high speed liquid - gas mixture , and much noise will be generated along with the decreasing pressure during this process . the noise generated in the experiment and test process is limited by using the noise reduction and insulation material that covers the throttling hush pipe . one or more adjusting units 310 , which are connected with each other in parallel , can be used in the present invention . in addition , the quantities of the pneumatic cut - off valves 12 , the motor - driven pressure regulating valves 13 and also the throttling hush pipes 14 to be used in the test and experiment process may also be determined according to the liquid and gas displacement in each of the adjusting units 310 . in order to ensure that the adjusting unit installation is highly dependable per se , it is required to control the highest flow rate of the water after adjustment . in one embodiment , the highest flow rate of the water is limited within 200 m / s in the apparatus in accordance with the present invention . in this case , if to be accomplished is the test on a liquid nitrogen pump equipment with a normal gas displacement of 330 sm 3 / min , the fracturing truck provided is a ylc1500 - 105 fracturing truck and the liquid displacement is required to achieve 1 . 9 m 3 / min , it will be needed to use two pneumatic cut - off valves 12 and two motor - driven pressure regulating valves 13 , while the quantity of the adjusting unit 310 is one . the outlet of the throttling hush pipe 14 is connected to subsequent water recycling equipments , which comprise in sequence a gathering apparatus 15 , a liquid - gas separator 16 , connecting pipeline 17 , a cooling system 18 and also a recycling manifold 19 , wherein the recycling manifold 19 is connected to the water tank 5 of the hydraulic damping apparatus unit 200 ; the high pressure liquid - gas mixture flows through a plurality of adjusting units 310 and then through the throttling hush pipe 14 so as to be converted into low pressure liquid - gas mixture , and a great deal of heat will be generated along with this process . the role of the gathering apparatus 15 is to collect water and nitrogen gas and transports them into the liquid - gas separator 16 , while the role of the liquid - gas separator 16 is to separate the mixture of water and nitrogen gas formed during the test . the separated nitrogen gas is discharged under security condition , whereas the separated water flows into the water tank 5 through the connecting pipeline 17 , the cooling system 18 and the recycling manifold 19 so as to be recycled . in one embodiment of the invention , the cooling system 18 comprises a cooling column , a plate - type heat exchanger , a recycling pump , valves and pipelines . because energy generated in the test is absorbed by water , which acts as the experiment medium , resulting in temperature rising , the cooling system 18 is used to cool the water recycled in the test and decrease its temperature and plays a role in stabilizing the liquid flow . in other embodiments , other known technology may also be employed for the cooling system 18 . because the controlling to the nitrogen gas displacement of liquid nitrogen equipments is achieved by using the power transmitting system and the flow distributing system of the liquid nitrogen pump equipment test unit 100 and the hydraulic damping apparatus unit 200 per se , no additional power is needed and thus the equipment cost as well as energy can be saved . the above description is made in terms of the structure of the loading test and experiment apparatus for liquid nitrogen pump equipment in accordance with the invention . the test and experiment process will be described in detail below . firstly , the hydraulic pump truck 7 is started and the water discharged from the hydraulic pump truck 7 is pressurized . the water pressure is measured by the pressure sensor 9 . simultaneously , the pneumatic cut - off valve 12 in the pressure adjusting unit 300 is opened for rough adjustment of the water pressure , and the discharge pressure may be preliminary obtained by regulating the pneumatic cut - off valve 12 . the motor - driven pressure regulating valve 13 is used for fine adjustment of the water pressure , and the discharge pressure of the hydraulic pump truck may be preciously adjusted by regulating the motor - driven pressure regulating valve 13 . secondly , the liquid nitrogen pump equipment 2 is started . the gaseous nitrogen discharged from the liquid nitrogen pump equipment is mixed with the liquid water discharged from the hydraulic pump truck 7 and inputted to the mixture output manifold 11 through the manifold system 10 . because the pressure loading has been carried out for the water liquid discharged from the hydraulic damping apparatus unit 200 in advance , the liquid inputted to the manifold system 10 by the hydraulic pump truck is high pressure water liquid . when flowing to the manifold system 10 , the gas discharged from the liquid nitrogen pump equipment has to overcome the pressure already existing therein , that is to say , the pressure of the water discharged from the hydraulic damping apparatus unit 200 is equal to the damping that the liquid nitrogen pump equipment 2 needs to overcome for discharging gas . thus , the pressure loading of the nitrogen gas discharged from the liquid nitrogen pump equipment 2 is realized by using the output liquid pressure loading system of the hydraulic fracturing apparatus . during practical test and experiment , the flow rates of the liquid - gas mixtures discharged from the hydraulic pump truck 7 and the liquid nitrogen equipment 2 are adjusted by using the pneumatic cut - off valve 12 and the motor - driven pressure regulating valve 13 , so as to adjust the pressure of nitrogen gas discharged from the liquid nitrogen pump equipment 2 in real time and thus to verify the accordance of the practical discharge pressure of the liquid nitrogen pump equipment 2 with design indices . thereafter , the liquid - gas mixtures discharged from the hydraulic pump truck 7 and liquid nitrogen pump equipment 2 are gathered to the gathering apparatus 15 and then entering into the liquid - gas separator 16 . after the separation of the liquid - gas separator 16 , the liquid continues to be involved in the recycle , whereas the gas is discharged into suitable atmosphere . for the security of the test and experiment , the quantity of the hydraulic pump trucks 7 required to be provided in the test and experiment is determined according to the gas displacement of the liquid nitrogen pump equipment and the highest gas content permitted in the liquid - gas mixture . in one embodiment of the invention , for example , to be accomplished is the test of a liquid nitrogen pump equipment having a normal gas displacement of 330 sm 3 / min and a normal discharge pressure of 105 mpa , in order to ensure security during the test and experiment , it is required to control the mixed foam quality within 20 %, and the hydraulic fracturing pump truck is required to have a liquid normal displacement equal to or greater than 1 . 9 m 3 / min and a discharge pressure equal to or greater than 105 mpa . accordingly , it is required to provide one hydraulic fracturing pump truck of 1500 type ( i . e . the normal discharge power is 1500 kw ) having a normal discharge pressure of 105 mpa . also , two fracturing trucks having a discharge pressure of 105 mpa and normal displacement in range of 1 . 0 to 1 . 9 m 3 / min may be used , such as ylc850 - 105 ( the normal discharge power is 850 kw ) or ylc1000 - 105 ( the normal discharge power is 1000 kw ), so as to accomplish the test . the overall test and experiment apparatus for liquid nitrogen pump equipments is equipped with data collecting and processing system , which may be linked to the flow rate monitoring system on the liquid nitrogen equipment and will not be described in detail since it is not key point of the present invention . the output parameters of the displacement of the liquid nitrogen pump equipment and the power of the power system may both be collected in the data collecting system of the liquid nitrogen pump equipment per se . the power output parameter of the power system of the hydraulic pump truck may also be collected in the data collecting system of the hydraulic pump truck per se . adjusting to the experiment condition and also the measuring , storing , collecting and outputting of the parameters needed to check out are achieved by the data collecting and processing system provided by the test and experiment system . the flow rate and pressure are measured under every gear of rotation speeds , and the flow rate - pressure curve ( see fig2 ), the input power - discharge pressure curve ( see fig3 ) and the loading pressure - loading time curve for the liquid nitrogen pump equipment ( see fig4 ) are drawn . the test and experiment system has the functions of automatic control and protection , adjustment of the outlet pressure ( pump head ), display of test condition , abnormal alarm and error protection etc . the present test and experiment apparatus may also be used for the discharge flow rate test of high pressure gas equipments , such as liquid carbon dioxide pump trucks , and also the pressure loading , testing and experimenting of the discharged gas thereof . the test and experiment process described above has the following characteristics : ( 1 ) the loading of the discharge pressure of the hydraulic fracturing equipment is carried out by using the hydraulic loading apparatus . specialized manifold is used to mix nitrogen gas discharged from the liquid nitrogen pump equipment with liquid discharged from the hydraulic fracturing equipment . on one side , the experiment and test risk caused by the characteristic of high compression ratio of the gas is reduced by decreasing the gas to liquid ratio of the mixed gas - liquid flow discharged from the entire test system . on the other side , the pressure loading of the nitrogen gas discharged from the liquid nitrogen pump apparatus is carried out by using the discharged liquid pressure loading system of the hydraulic fracturing equipment . by means of transforming from liquid pressure to gas pressure , both the technical difficulty and the security risk caused by applying pressure loading to gas are avoided , and thus security is ensured in tests in accordance with the present invention . ( 2 ) the discharge flow rate and pressure of the hydraulic fracturing equipment are adjusted by using the pressure adjusting system , which is composed by the pneumatic cut - off valve component and the motor - driven pressure regulating valve as main parts , wherein the motor - driven pressure regulating valve is used for fine adjustment of the discharge pressure and its operation is easy but accurate . ( 3 ) during the overall test and experiment of the liquid nitrogen pump equipment for nitrogen foam fracturing , tests at every discharge pressure and every displacement can be accomplished of the liquid nitrogen pump equipment , so as to obtain the relationships between the displacement and the pressure , between the input power and the discharge pressure and between the loading pressure and the loading time , and then output results of the test and experiment . it is clearly seen from fig2 to 4 that the object of the test and experiment has been achieved by the loading test and experiment apparatus for liquid nitrogen pump equipment in accordance with the invention . the present invention has the following advantages comparing to the prior art . ( 1 ) the loading of the discharge pressure of the hydraulic fracturing equipment is carried out by using the hydraulic loading apparatus . specialized manifold is used to mix nitrogen gas discharged from the liquid nitrogen pump equipment with liquid discharged from the hydraulic fracturing equipment . on one side , the experiment and test risk caused by the characteristic of high compression ratio of the gas is reduced by decreasing the gas to liquid ratio of the mixed gas - liquid flow discharged from the entire test system . on the other side , the pressure loading of the nitrogen gas discharged from the liquid nitrogen pump apparatus is carried out by using the discharged liquid pressure loading system of the hydraulic fracturing equipment . by means of transforming from liquid pressure to gas pressure , both the technical difficulty and the security risk caused by applying pressure loading to gas are avoided , and thus security is ensured in tests in accordance with the present invention . ( 2 ) the discharge flow rate and pressure of the hydraulic fracturing equipment are adjusted by using the pressure adjusting system , which is composed by the pneumatic cut - off valve component and the motor - driven pressure regulating valve as main parts , wherein the motor - driven pressure regulating valve is used for fine adjustment of the discharge pressure and its operation is easy but accurate . ( 3 ) the nitrogen gas displacement of the liquid nitrogen equipment is controlled by the power transmitting system and the flow distributing system of the equipment per se . no additional power is needed , and thus the equipment cost as well as energy can be saved .