Abstract:
The instant invention provides a gas depressurization station, comprising a preheater for preheating a gas, an expander having an output shaft for expanding and depressurizing the gas, and an air compressor coupled to the output shaft for creating high pressure air for use in controlling the depressurization process. The invention is also directed to a method for safely depressurizing a gas, comprising: passing the gas through a preheater and a gas expander which drives an output shaft; driving an air compressor attached to said output shaft to produce compressed air; and controlling the pressure of said compressed air by means of control throttling means; whereby unwanted variations in the speed of rotation of said output shaft may be safely controlled by said control throttling means.

Description:
[0001]    This application claims priority from U.S. provisional application No. 60/275,094 filed Mar. 12, 2001. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The invention is applicable in the area of natural gas depressurization wherein there is enhancement to the power that may be generated by preheating the gas. This enables each depressurization gate station to produce a substantial quantum of power, with a heat rate which is 50-35% better then the existing best gas turbine power plant.  
         DESCRIPTION OF THE PRIOR ART  
         [0003]    Depressurization of natural gas is conventionally done by means of a throttling valve. The depressurization action is an isenthalpic process, known as a Joule-Thompson expansion. Because of relationships between gas properties and inlet and outlet pressures, the J-T expansion can result in either a temperature increase or decrease. The controlling parameter is known as the Joule Thompson temperature inversion curve and it can be plotted for any gas.  
           [0004]    If energy is extracted from the gas, then the process would be known as a reversible, adiabatic expansion, if the expanding gas is kept in complete thermal isolation. Due to a reduction in enthalpy of the gas, there is substantial cooling taking place. An earlier provisional patent application looks at the possibility to acquire heat from the atmosphere, by expanding in three stages and communicating with the atmosphere in between stages.  
           [0005]    A further important aspect of natural gas depressurization with power generation is that safety of the system must be maintained at all times. In the case of a throttling valve, the element (piston) of the valve which provides the orifice through which the pressure step down takes place, moves up and down in response to pressure signals in the downstream distribution circuit as such. All changes are accommodated.  
           [0006]    The element of risk arises especially if the valve or in the case of depressurization with power generation, the expander device allows excessive flow through onto the downstream side, without any demand for such flow. This can happen if the element in the throttling valve sees restricted movement or if there is a grid trip, in the case of an expander coupled to a grid feeding alternator.  
           [0007]    In the latter case above, where power is being fed to the grid, this ensures that expander and alternator run at grid frequency and all changes in mechanical power at the expander are converted to changes in electrical power, without any change of speed. This is a property of synchronous alternators connected to a grid which is much larger than the generator output. However, if the grid is tripped or disconnected from the alternator, this condition no longer applies. The mechanical power flow then has nowhere to go and the result is an immediate speeding up of the expander and alternator. In the case of a volume based expander, speeding up will mean more volumes being passed through per second, resulting in possible overpressurization of the downstream circuit, with no regard to actual downstream demand.  
           [0008]    Since downstream structures and end users are designed for gas pressures within a narrow range, the overpressurization will result in a substantial safety hazard. Whilst an electrical system to provide a dummy load would be switched in every time the grid trips, enabling the speed to remain at approximately the value at the time of the trip, there is concern that an electrical failure in this dummy load system could still put the system at risk. Further, the simple mechanical failsafe system which is part of the throttle valve is not matched in terms of durability and reliability, in an electrical dummy load activation system.  
           [0009]    Williams U.S. Pat. No. 6,155,051 looks at running a compressor with a natural gas expander, to provide the necessary heat to preheat the gas. The following points are noteworthy, concerning this patent: (1) The &#39;051 patent uses only a portion of the gas to provide power by gas expansion; (2) The &#39;051 patent only purpose in compressing air is to provide heat of expansion to the incoming natural gas; and (3) another purpose in preheating the natural gas is to reduce Joule-Thompson cooling which may occur in the throttling valve process. No further use is made of the compressed air.  
           [0010]    The present invention provides a mechanical failsafe system equal in simplicity and durability to the failsafe operating mode of the throttle valve, for use together with and to back up the dummy load type failsafe system.  
           [0011]    In natural gas depressurization, adding a quantum of preheat to the gas prior to entry to the expander will enable the gas to expand without any cooling of the gas below atmospheric temperature. Typically, for gas incoming at 1000 psig and outgoing at 150 psig, increasing the incoming temperature to 115° Celsius will result in a temperature after expander of around 0° C. However, a portion of the natural gas will have to be consumed for this purpose.  
           [0012]    Because the gas is already compressed at entry to expander, the heat rate for the process is up to 50% less than for a typical Gas Turbine Combined Cycle power plant with an overall thermal efficiency of 55%. In all such conventional plants, compression power is fed back from the expander output, which is not required in this case.  
           [0013]    The present invention is directed to a system to maximize the power output from each such depressurization location, without loosing the benefit of a good heat rate. Further, direct heating of natural gas to a temperature of 115° C. with a gas flame at a temperature of 1,750° C. will result in a substantial loss of available energy or exergy. The invention therefore embodies a method whereby the exergy loss could be minimized. Further, the invention embodies a system which uses a property of one of the mechanical devices to provide a failsafe, mechanical means of preventing overpressure protection. Further, the invention incorporates a mathematical formulation concerning establishing the optimum plant sizing for the enhancement of power output.  
         BACKGROUND TO THE INVENTION  
         [0014]    Natural gas is compressed to high pressure for transmission from wells to consumers, typically the pressures in the main interstate pipelines are of the order of 1000-3000 psig. On the other hand, natural gas is distributed to consumers at much lower pressures. Typically, for domestic consumers, the pressure may 20-40 psig and for commercial consumers, 100-150 psig.  
           [0015]    The pressure in the interstate high pressure transmission pipeline is reduced to the distribution pressure in a “gate station”, by means of a pressure reducing throttling valve. Several types of throttling valve are to be found, the main types being Direct Operated Valves and Pilot Operated Valves.  
           [0016]    The change of pressure between the high pressure transmission pipe and the low pressure distribution side constitutes a loss of potential energy. If the gas is placed within a suitably conFigured expansion system such energy may be usefully utilized to produce shaft work, which may be converted to electric power.  
           [0017]    However, there are crucial differences between the expansion and pressure drop in a throttling valve and that in a power generating device. Most crucially, the former is theoretically an isenthalpic or constant enthalpy expansion, typically known as a Joule-Thompson expansion. In such an expansion, depending on the conditions, the outgoing temperature may be lower or higher than the incoming temperature. The governing parameter is known as the Joule Thompson inversion temperature curve In particular instances where there is significant temperature drop, heating is required to bring the gas temperature back to a reasonable level  
           [0018]    In the case of a direct gas expansion with power generation, where the gas expands in isolation, the theoretical description is isentropic or adiabatic, reversible expansion. External work or energy is extracted from the gas stream, this means the outlet enthalpy of the gas is lower than the inlet enthalpy. Invariably, this means the outlet temperature is lower than at inlet. Because of the substantial pressure difference between the incoming main and distribution main, if a direct expansion is carried out, the result is a very low gas temperature exiting the expander, see FIG. 1.  
           [0019]    Such low temperatures are unacceptable because a substantial number of the components in the gas will liquefy or solidify. Therefore a method is required to capture the useful energy in the pressure difference between transmission mains and distribution mains, without substantial temperature decrease. A simple method suggested has been to preheat the incoming gas by burning a portion of the natural gas.  
           [0020]    Preheating of the gas to around 115° C. will enable the following conditions to be achieved:  
           [0021]    Typical Case  
                                                       Natural gas flow   10 Kgs/sec           Power generation   2340 kW           Heat input   1900 kW           Heat rate   2950 kJ/kWh           Final gas temperature   0° C.                      
 
           [0022]    The average heat rate in the case of a Gas Turbine Combined Cycle with a thermal efficiency of 55% overall would be approximately 6545 BTU/kWh. Because the compression power need not be subtracted in the present case, the heat rate is only 50% of the GTCC case quoted, indicating a low fuel consumption compared with other cycles where compression power is fed back from the output.  
           [0023]    If most of the gas is diverted through an expander, coupled to an alternator, then if there is a trip in the grid, the expander would overspeed, resulting in overpressurization of the downstream circuit During grid connected operation, because the grid is large compared with each individual alternator, the frequency and hence the speed of rotation remains constant. This condition does not apply when the alternator is disconnected from the grid. The back EMF and hence the reverse torque velocity vector which opposes the input torque to the alternator (from the expander) collapses. The input torque then has a tendency to accelerate the rotating masses until one of several mechanisms is able to absorb the incoming power.  
           [0024]    In the case of the expander, under conditions where the alternator is disconnected, the pressure difference causes the machine to speed up until (a) the pressure difference is lessened due to over pressurization of the downstream circuit and (b) the mechanical power is dissipated through fluid friction within the expander.  
           [0025]    Under these conditions the expander reaches an equilibrium speed, typically in the case of Francis type water turbines in hydro applications, this is 2.2 times the synchronous speed. In hydro applications, discharging more water after the grid is disconnected does not matter. This is certainly not the case in natural gas depressurization. Any tendency to overspeed, followed by excessive pressurization of the downstream circuit, is completely unacceptable.  
           [0026]    The present invention therefore embodies unique means to mechanically control and absorb the output on grid disconnection, without imposing any load during grid connected operation, or by imposing a load which leads to further benefit.  
           [0027]    The benefit realized by imposition of a mechanical device is enhanced, in this invention, by further use of the output from the mechanical safety device. The invention therefore embodies the integration of both a safety device and a power generation device, for substantial added benefit.  
         SUMMARY OF THE INVENTION  
         [0028]    The present invention is directed to a gas depressurization station, comprising a preheater for preheating a gas, an expander having an output shaft for expanding and depressurizing the gas, and an air compressor coupled to the output shaft for creating high pressure air for use in controlling the depressurization process.  
           [0029]    The instant invention also provides a gas depressurization station, comprising a preheater for preheating a gas, an expander having an output shaft for expanding and depressurizing the gas, and a fail-safe safety system comprising: an air compressor coupled to the expander output shaft; and control throttle means to throttle the air output from said air compressor; whereby unwanted variations in the rotation speed of said shaft may be controlled by said control throttle means.  
           [0030]    Furthermore, the invention provides a method for safely depressurizing a gas, comprising: passing the gas through a pre-heater and a gas expander which drives an output shaft; driving an air compressor attached to said output shaft to produce compressed air; and controlling the pressure of said compressed air by means of control throttling means; whereby unwanted variations in the speed of rotation of said output shaft may be safely controlled by said control throttling means. 
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0031]    [0031]FIG. 1 describes graphically the relationship between final temperature and final pressure in direct expansion of the gas.  
         [0032]    [0032]FIG. 2 illustrates a brief schematic of the invention.  
         [0033]    [0033]FIG. 3 shows the safety system schematic of the invention.  
         [0034]    [0034]FIG. 4 describes an alternative embodiment of the safety system.  
         [0035]    [0035]FIG. 5 illustrates the thermodynamic cycle diagram of the invention.  
         [0036]    [0036]FIG. 6 describes in detail the schematic of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0037]    The present invention provides the following novel developments in the technology of gas transmission and safety handling:  
         [0038]    A system to depressurize natural gas wherein the power generated by a natural gas expander is used to compress air, as the first step in a combined cycle.  
         [0039]    Air compressed through natural gas expansion is preheated and expanded in an air expander, to generate power.  
         [0040]    The combination of a first natural gas expander, followed by an air compressor, followed by an air heater, followed by a hot air expander, wherein extra nett output is produced over and above the power available in natural gas expansion only.  
         [0041]    A system where heat is provided by natural gas combustion to achieve a high temperature in an air stream thereby utilizing a substantial quantum of the availability in a high temperature flame.  
         [0042]    A system which recovers heat after expansion in an air expander and preheats the air stream from the compressor in an exhaust gas recuperator, prior to entry to a natural gas fired heater.  
         [0043]    A system which first provides heat of compression of air to the natural gas stream, then acquires heat from the recuperation of hot exhaust air from an air expander, then is heated to a high temperature by natural gas combustion and is then expanded to produce useful work.  
         [0044]    A system in which the flow and pressure of air from an air compressor driven by a natural gas expander may be independently controlled, provided the total energy change (increase)in the air stream is equal to the energy change (decrease)from the natural gas expansion process.  
         [0045]    A system in which there is a critical value of air flow from an air compressor driven by a natural gas expander wherein the heat required for preheating natural gas exactly equals the sensible heat available in the compressed air flow, resulting in a system where in the ideal case there is no heat rejected from the total system  
         [0046]    A system in which for air flow above the critical value, there is heat rejected to atmosphere and for air flow below the critical value, combustion of natural gas directly for preheating the natural gas, is required.  
         [0047]    A power generation system in which the heat rate is 35-50% better than a conventional gas turbine combined cycle because of the availability of a compressed fluid medium, which may expanded.  
         [0048]    A power generation system which recovers hitherto wasted energy whilst further optimising the potential at each gate station site.  
         [0049]    A power generation system consisting of a first natural gas expander to which preheated natural gas is supplied, followed by a compressor which is mechanically driven by the first expander, followed by an alternator to which the compressor is mechanically connected by a shaft, followed by a second air expander to which the compressor is mechanically connected by a shaft, in parallel the air compressed is used to preheat the natural gas, it is then heated in an exhaust gas recuperator, then further heated to a high temperature in a natural gas heater, then expanded in an air expander.  
         [0050]    A mechanical, shaft power circuit consisting of a first natural gas expander, followed by a second, air compressor, followed by a third, alternator or synchronous generator, followed by a fourth unit, which is actually a second expander, all on a common shaft.  
         [0051]    An air circuit consisting of a first compressor, followed by a heat exchanger to provide heat to natural gas, followed by a recuperator to heat incoming air, followed by a gas firing heat exchanger to further increase output, followed by an air expander wherein the enthalpy change in the air is converted to shaft power.  
         [0052]    A natural gas circuit consisting of a heat exchanger to carry out air preheat, followed by a natural gas expander where the change in enthalpy is converted to shaft power.  
         [0053]    An integration of the mechanical power, air and natural gas circuits to provide a system of safe natural gas depressurization with generation of additional shaft power and with zero heat rejection to atmosphere, under certain conditions, resulting in high overall efficiency and low heat rate.  
         [0054]    A safety system in which the air compressor itself is used to control the speed of the natural gas expander and hence the air thoughflow, by virtue of using a throttled flow a compressed air to absorb the power generated in the natural gas expander.  
         [0055]    A safety system in which a simple gravity driven system is used to shutoff air flow to the heating and recuperation system.  
         [0056]    A system in which a spring operated pressure reducing valve of the very same nature as the conventional throttling valve is used to throttle air flow from the compressor, thereby absorbing power generated by the natural gas expander, in a controlled manner.  
         [0057]    A system in which the rotating natural gas expander and air compressor closely coupled to it act as a mechanical, fail safe, safety device, to eliminate any possibility to over-pressurization in the downstream circuit, by means of. a throttle valve which controls the air stream from the compressor.  
         [0058]    A safety system in which on grid failure, the solenoid holding off valve colsure is denergised, allowing main flow circuit valve to close, thereby pressurising the air compressor, which causes the parallel throttle valve circuit to open, followed by control of the air flow by this throttle valve, which leads to effective control of the rotational speed of the compressor and natural gas expander  
         [0059]    The invention consists of a natural gas preheater, a natural gas expander, an air compressor and air expander, as shown in FIG. 2. The air compressor also acts as the mechanical failsafe safety element which is an essential requirement for safe operation.  
         [0060]    The natural gas expander and air compressor are integrally connected through a shaft coupling. The fail safe safety system as embodied in the invention involves shutting down the main air flow valve within 10 milliseconds of grid failure and allowing the compressor safety valve to open, see FIG. 3.  
         [0061]    The safety system consists of a pressure reduction valve connected in a parallel path to the compressor high pressure side, and a fail safe main air pathway shutoff valve. On grid failure, the air pathway to air heater and expander is closed off and all of the compressor air flow is diverted to the pressure relief valve indicated. The pressure relief valve then acts in an exactly analogous manner to the original gate station PRV and would, in fact be the same type of valve. It controls the pressure development within the compressor, thereby controlling compressor power input and hence power absorbed from NG expander.  
         [0062]    This present invention also embodies means by which gas pressure may be controlled in a simple way, in an analogous manner to the original gate station pressure control system and employing a standard pressure reduction valve, The gas flow volume passing through the expander is now under complete control by these means. Only air passes through the compressor and it&#39;s PRV circuit.  
         [0063]    The invention embodies means whereby natural gas pressure can be controlled as the gas is passed through an expander, by employing an air compressor directly coupled to the expander shaft and throttling the air output from the air compressor.  
         [0064]    Furthermore, the invention embodies means to usefully utilize the excess power available in always rotating the compressor, by using compressor output in a thermodynamic cycle.  
         [0065]    Referring to FIG. 2, the natural gas first passes through a preheater, then through a natural gas expander which enables the enthalpy change to be converted to shaft power. The natural gas is then discharged directly to the distribution network at the correct lower pressure. The expander is mechanically connected to the air compressor, which also acts as the primary element in the safety device, see FIG. 3. In normal operation where the unit is connected to the grid, the compressed air proceeds to the natural gas air preheater, where it&#39;s temperature drops substantially. Thereafter, it proceeds to the recuperator and then main air heater, whereby through combustion of natural gas, the temperature is raised to between 900-1100° C. The heated air is then expanded in the air expander and the resulting shaft work is supplied to the alternator.  
         [0066]    The alternator receives shaft work from both the NG expander and compressor and from the air expander, through shaft projections at either end. The shaft work supplied to the alternator from the NG expander is nett of the work supplied to the compressor. After air expansion, the air still has sensible heat remaining within it, see cycle diagram, FIG. 5. This sensible heat is given up to the incoming cold compressed air in the recuperator.  
         [0067]    The invention also embodies means whereby in the case of the air cycle, there is no heat rejected to atmosphere, see cycle diagram, FIG. 5. The closed air cycle, as such, does reject heat, from the compression step to the natural gas, therefore there is no contravention of the Second Law of Thermodynamics. In the particular circumstances pertaining, the natural gas heating and expansion is an open cycle process, hence it is not governed by the Second Law of Thermodynamics. The incoming preheat is converted into shaft work and the gas input and output enthalpy remain the same, with slight variation due to the change in pressure, in the case of a real gas.  
         [0068]    The invention therefore embodies means in certain configurations whereby with the integration of a closed cycle air cycle and an open cycle natural gas expansion process, no heat is rejected to the atmosphere. In cases where the airflow is large in comparison with the natural gas flow, there is more heat in the compression process than required for natural gas expansion and in such cases a quantum of heat would be rejected to atmosphere. In other cases, correspondingly, where the air flow is small in comparison with natural gas flow, there is insufficient heat in the compressed air—in such cases additional heat has to be provided, by burning natural gas.  
         [0069]    The invention further embodies means to enhance the power output from a natural gas depressurization station, by connecting the natural gas expansion process to a closed Brayton type air cycle, with recuperation. The sensible heat in the air after compression will be transferred to the natural gas thereby enabling very efficient recuperation. In the typical gas turbine case, recuperation can only take place between the outgoing air temperature and the compressed air temperature, hence the heat of compression is effectively lost, unlike in this case.  
         [0070]    With reference to a usual Brayton cycle, which can be represented as an adaibatic compression, followed by a constant pressure heat addtion, followed by an adiabatic expansion, followed by a constant pressure heat rejection, 100% recuperation is never possible because of the elevated temperature of the gas or air, after compressor  
         [0071]    The invention is also directed to a natural gas depressurization system and a fail safe safety system in an integrated device, with substantial synergies arising from the dual use of one and the same item., namely, the air compressor  
         [0072]    The natural gas expansion system consists of a first preheater, HEATEX I in the main flow diagram, FIG. 6, overleaf Natural gas enters at I at ambient condition and main pipeline pressure, shown here as 1000 PSIG. It is then preheated by the sensible heat in compressed air entering at 9 and leaving at 10. The natural gas acquires a temperature well in excess of the ambient typically between 100-250° Celsius, a temperature of 115° C. is indicated in the example.  
         [0073]    The natural gas at slightly less than pipeline pressure and with an elevated temperature is introduced to the first expander, EXP 1, where the gas is depressurized from 1000 psig to 150 psig in the example, the incoming and outgoing pressures could be at various levels in other examples and cases. In the course of the depressurization, the enthalpy change is converted into shaft power. At 5, the expander is connected up with a standard air compressor. The compressed air flow may be determined independently of the natural gas flow and constitutes a first independent variable. The temperature and pressure of air exiting compressor is 192° C. and 74 psig respectively, in this example. In the example shown the flow of air is I 1 Kg/second. The compression pressure may also be set quite independently of the natural gas system pressure variations and constitutes a second independent variable. In the example, the pressure ratio is 5:1, giving an outlet temperature and pressure from the compressor of 192° C. and 75 psig. Many other combinations of pressure, temperature and flow may be adopted.  
         [0074]    After the expansion of natural gas to the requisite lower pressure has taken place, a small portion of the gas flow, typically 2-5%, is diverted for heating purposes. This is shown as an offiake line at point 4 in FIG. 6. The hot air from the compression process is now diverted to the first heat exchanger, HEATEX1, where after transferring sensible heat to the natural gas flow, the colder compressed air is sent out at 10 and at point 11, enters a recuperator, RECUP 1, which is also a heat exchanger. RECUP 1 has the task of transferring sensible heat from the exhaust air flow leaving the air expander, to the incoming cold compressed gas In RECUP1, the temperature of the incoming compressed air is increased to an approach value of around 10-30° C.  
         [0075]    At this point in the description, the thermodynamic pressure-volume diagram given in FIG. 5 will further illustrate the concept. The natural gas entry, preheating and expansion are represented by the thick line A-B-C. The heavy dotted line is the envelope demarcating the ambient temperature. The natural gas heat pick up in process A-B is exactly equal to the compressed air heat release in the section E-F in the air cycle diagram.  
         [0076]    After the compressed air has transferred sensible heat to the natural gas, it is reheated in the recuperator, the relevant process line is F-E′ in FIG. 5. The heat pickup in section F-E′ is equal to or less than the heat given up in section H-C, which represents the process on the other side of the recuperator, je where the exhaust air transfers heat and is cooled to near atmospheric temperature. It is apparent that in a theoretical or ideal diagram, since heat rejected by the air cycle, represented by H-C, is fully absorbed by the natural gas, after which a conversion to shaft power takes place, there appears to be no reject heat, leading to a contravention of the Second Law of Thermodynamics. In fact, the Second Law cannot be applied overall because the natural gas forms part of an open cycle—the Second Law is only applicable to closed cycles. If the analysis is extended to the original natural gas compressor station, thereby reconstituting a closed cycle for the natural gas, then there is reject heat in the form of sensible heat loss from the compressed gas, after the compressor station  
         [0077]    Heat addition in HEATEX2 is represented in FIG. 5 by the section E′-G and expansion in EXP2, by the section G-H. Within the closed air cycle, the section H-C constitutes heat rejection, which, conventionally may be recuperated or transferred to the incoming stream if the temperature at E is less than the temperature at H. In this case the quantum of recuperation is the difference in temperature between the temperature at H and temperature at E, the remaining sensible heat (le TE-30) must be rejected to atmosphere. In the other case, je where the temperature at H is less than at E, all of the sensible heat represented by the section H-C has to be rejected to atmosphere.  
         [0078]    Returning to the flow diagram, FIG. 6, after heat addition in HEATEX2, the air enters the second expander, EXP2, where the pressure reduces to just above atmosphere, with attendant production of shaft power. By virtue of the shaft interconnection with the alternator, ALT1, this shaft power is converted to electrical energy. In the example given, FIG. 6, a net 4630 kW is shown as output, with consumption of 5280 kW in terms of heat provided by the natural gas. This constitutes an overall system thermal efficiency of 82%, in the example given. The thermal efficiency in this system is therefore much higher than any comparable heat driven device hitherto developed. There are two main reasons for this:  
         [0079]    1) The supply of a compressed gas, which is depressurised through means embodied in this invention and  
         [0080]    2) Eliminating all discharges of sensible heat to the atmosphere (in the ideal case)  
         [0081]    The measure of efficiency given is not strictly correct because it does not account for the compression energy supplied at the gas compressor station. At the locality of the gate station, where the device embodied in this invention is implemented, the efficiency is correct because the compression energy is supplied without any mechanical or financial penalty.  
         [0082]    The major independent variables enabling the output of EXP2 to be varied independently of the natural gas flow are, the air pressure and flow at COMP1 By means of varying these parameters, a range of power outputs may be achieved. However, a condition arises where for any natural gas flow, for any air compression pressure, a particular flow gives rise to the condition of no natural gas consumption for heat input to the natural gas preheat process and no rejected heat from the (ideal) system. This condition is called the NGRH condition (or No Gas input or Rejected Heat condition).  
         [0083]    If the flow is below NGRH, for a given air compression pressure, then an extra heat input to the natural gas flow, by burning natural gas, is required. If the flow is above NGRH, then some heat is rejected to the atmosphere. Mathematically, this is represented by:  
         Qa&lt;Q NGRH , RH=0, IH1&gt;0  
         Qa&gt;Q NGRH , RH&gt;0, IH1=0  
         Qa=Q NGRH , RH=0, IH1=0  
         [0084]    Here Qa is the air flow, Q NGRH  is the crtical flow , RH is the rejected heat and IH1 is the natural gas combustion heat input to the preheat process, in HEATEX 1.  
         [0085]    The invention is further directed to an overpressurization Safety System. Further to the discussion of the invention in terms of it&#39;s operating parameters, devices, modules and functions, the invention embodies a safety device integral with the functioning and operation of the air compressor, See FIG. 3.  
         [0086]    When the grid is disconnected, the collapse of the back EMF in the alternator leads to a collapse of the force which maintains the speed constant. This leads to the RPM increasing in the mechanical system, leading in turn to overpressurization of the downstream circuit. The invention embodies a pressure control system as given in FIG. 3, whereby, in addition to the compressor air offtake which leads to the heat exchanger, at K, an additional air release circuit on the high pressure side is provided, the connecting pipe is indicated at G.  
         [0087]    In the event of the grid being disconnected, the electrically activated solenoid F, which holds up lever D when the grid is on, is deactivated, resulting in the lever D descending under the influence of gravity acting on weight E. This causes rotation of the lower section of lever D, around the fulcrum indicated at C, thereby moving lever B which is attached to a piston within passage A. This piston is pressure balanced in that the compressor pressure acts on both sides, the passage to the rear (towards B) for pressure balancing is not shown. Further, the piston attached to lever B has a clearance fit in the passage A, facilitating eay movement.  
         [0088]    Movement of piston attached to B shuts off the passage at K thereby interrupting the flow of compressed air to the heat exchanger HEATEX 1. There is pressure build up in the compressor. The other circuit on the high pressure side now comes into operation. Valve piston J on valve seat H lifts up at a predetermined pressure, releasing compressed air to atmosphere. The valve P is not only a conventional safety release valve found as standard on any compressor, but is of the same type and with the same or similar operating characteristic as the throttling valve in the natural gas circuit. It has the function of modulating the discharge pressure of the compressor, such that any tendency to overspeed is controlled.  
         [0089]    By means of a device to change the compression of the valve spring, the set point of the valve may be changed to cater for any changes in overall conditions. When the system is operating with valve P in action, the energy released by the expansion of natural gas in EXP 1 is dissipated by means of the expansion of compressed air to atmosphere through valve P, which acts as a throttling valve. In the circumstances indicated, there is interruption of the flow to 1-IEATEX1, but in many cases, it may be desirable to keep flow to HEATEX1 going without shutoff Under these circumstances, both sections A-K-E and G-P-L would be connected to the air flow pipe after HEATEXI, as shown in FIG. 4.  
         [0090]    The system indicated constitutes a mechanical fail safe system to prevent overpressurization of the downstream circuit, on grid failure it acts under the influence of gravity and built in forces to control the rate of rotation of the expander and hence the gas throughput. The system may be used continuously, instead of the conventional throttle valve, to control the depressurization process.  
         [0091]    Thus, the present invention is well adapted to carry out the objects and attain the ends and advantages mentioned as well as those which are inherent therein. While numerous changes may be made by those skilled in the art, such changes are encompassed within the spirit of this invention as defined by the appended claims.