TECHNICAL FIELD OF THE INVENTION

The present invention relates to a system for heating the gas inside systems for reducing the gas pressure in a main gas pipeline.

The present invention also concerns a method for heating the gas inside gas pressure reducing systems in a main gas pipeline.

DESCRIPTION OF THE STATE OF THE ART

It is common knowledge that distribution networks are used to transfer gas between remote geographical regions, and particularly to distribute fuel gas from a source to where it is supplied to the various industrial or civil users.

Such networks comprise a plurality of pipelines through which the gas being distributed flows.

According to the known state of the art, the gas is delivered from a source into a gas main by means of which it is transported at a pre-set pressure, which typically varies in the range of 12 to 74 bars.

Along this distribution line, there are units for regulating and measuring the gas flow, also called first-stage stations, for the purpose of providing the physical connection between the gas main and the line reaching the users. A primary purpose of these stations is to achieve a decompression of the gas, reducing it from the above-mentioned delivery pressure to a lower pressure for the line delivering the gas to the user, i.e. to pressure values typically in the range of 1.5 to 5 bars, and up to 12 bars in certain applications.

This reduction is achieved by means of special pressure regulators.

It is also well known that said stations provide for a phase in which the natural gas coming from the main is heated so that its temperature downstream of the pressure reducer does not drop to values below 0°C due to the effect of expansion. Natural gas, in fact, contains a very small percentage of water, which would freeze and could pose problems for the instrumentation and the pressure regulator at the station. The temperature of the gas leaving said stations is therefore maintained preferably at an optimal value above zero, and advantageously at around 6°C.

During the heating phase, a certain amount of thermal energy is consequently transferred to the natural gas coming from the main. This heating phase typically takes place through suitable systems with water/gas tube bundle heat exchangers fed with water that has been preheated with methane gas boilers.

The water/gas tube bundle heat exchangers release energy in the form of heat to the natural gas in transit.

The water preheating circuit preferably consists of one or more boilers for the production of hot water, preferably relying on thermopile power systems, plus a series of gate valves for intercepting the flow of water, one or more open expansion tanks, one or more circulators located on the delivery side of the hot water circuit, two or more water/gas heat exchangers, and all the necessary connection piping. The natural gas is preheated, as mentioned earlier, before its pressure is reduced.

Downstream of the pressure regulators that reduce the pressure of the gas, the temperature of the gas is measured by means of a thermostat on the pipeline. This pipeline thermostat controls the temperature of the outgoing gas and governs the switching on/off of the boilers, and the switching on/off of the circulators. The preheating temperature for the hot water circuit is set manually by means of the thermostats installed on the boilers. The switching on/off of the boilers and/or circulators is consequently governed by means of the pipeline thermostat. The temperature of the water in the preheating circuit comes to be at a pre-set value that is adjusted manually by an operator taking action on the thermostats on the boilers. The water temperature value is therefore set at said fixed value irrespective of the actual demand for energy to heat the gas.

The heating system for gas pressure reducing units of the known type has several drawbacks, however.

A first drawback lies in that, although the system is controlled by a pipeline thermostat, it constantly generates increases and decreases in the temperature of the gas leaving the unit. This temperature departs all the more from the optimal value of 6°C, the more the temperature of the water coming from the boilers differs from (i.e. it exceeds) the optimal minimum value sufficient to heat the gas.

In fact, the operators setting the thermostats on the boilers rely on their own experience and normally exaggerate when setting the temperature for the hot water on the boilers vis-a-vis the optimal minimum temperature required. This in order to avoid risking any problems in particularly demanding operating conditions, as in winter, for instance. This gives rise to a considerable waste of energy for the pressure reducing unit. Another drawback of the known art relates to the measurement of the temperature of outgoing gas. In the hours of lower gas consumption, and especially during the night in winter, when the gas flow rates are minimal, the pipeline thermostat measures the ambient temperature instead of the temperature of the gas in transit, since the lack of consumption means there is no flow of gas, which remains practically at a standstill inside the pipeline. Due to the effect of the above-described conditions, the pipeline thermostat pointlessly enables the water to be heated in the boilers, with a further wastage of energy for the unit. The object of the present invention is to overcome at least some of the drawbacks of the known art.

In particular, it is the object of the invention to provide a system for heating the gas in gas pressure reduction systems that has a better energy performance than the known systems.

It is another object of the invention to provide a gas heating system for gas pressure reducing systems that guarantees a temperature of the outgoing gas that is more stable than in the known systems.

A further object of the invention is to optimise the thermal energy used in the heating systems of first-stage gas receiving and pressure reducing stations.

SUMMARY OF THE PRESENT INVENTION

The present invention is based on the general consideration that the object is to provide a system for heating the gas inside a gas pressure reducing system in which the thermal power needed to heat the gas is adjusted on the basis of the instantaneous flow rate of the gas measured after the reduction of its pressure. According to a first embodiment, the subject of the present invention is a gas heating system according to claim 1, i.e. a gas heating system applicable to a gas in transit between an inlet duct and an outlet duct, wherein the pressure of said gas inside said inlet duct is higher than the pressure of said gas inside said outlet duct, and comprising:

- thermal power generation means for heating said gas between said inlet duct and said outlet duct;

- means for controlling said thermal power generation means,

said system comprising means for measuring the flow rate of said gas in said outlet duct that are connected to said means for controlling said thermal power generation means.

According to a preferred embodiment of the invention, the system comprises means for measuring the temperature of the gas in the outlet duct that are connected to the means for controlling the thermal power generation means. The thermal power generation means preferably comprise means for heating a heating fluid, and means for exchanging thermal energy between the heating fluid and the gas.

The control means advantageously comprise means for controlling the temperature of the heating fluid.

The thermal power generation means preferably comprise a line for delivering the fluid from the heating means to the exchanger means, and a line for returning the fluid from the exchanger means to the heating means.

According to a preferred embodiment, the system comprises means for measuring the temperature of the fluid in said delivery line.

According to another preferred embodiment, the system comprises means for measuring the temperature of the fluid in the return line.

The heating means preferably comprise a boiler for the production of hot water. The exchanger means preferably comprise a tube bundle heat exchanger.

The means for measuring the gas flow rate advantageously comprise a gas meter operatively connected to a volume corrector suitable for producing the value of the gas flow rate as an output.

The control means preferably comprise a PLC unit.

According to a second aspect, the subject of the present invention is a system according to claim 7, i.e. a system for reducing the pressure of a gas between an inlet duct and an outlet duct, comprising pressure reducing means between said inlet duct and said outlet duct, the system comprising a heating system as described above.

According to a third aspect, the subject of the present invention is a method according to claim 8, i.e. a method for heating a gas in transit between an inlet duct and an outlet duct, wherein the pressure of said gas in the inlet duct is higher than the pressure of said gas in the outlet duct, comprising a phase for heating said gas between said inlet duct and said outlet duct, wherein said phase for heating said gas is controlled on the basis of the flow rate of said gas through said outlet duct.

It is preferable for the gas heating phase to be controlled on the basis of the temperature of the gas inside the outlet duct.

According to a preferred embodiment of the invention, the gas heating phase comprises a phase for heating a fluid and a phase for exchanging the thermal energy of the fluid with the gas.

The temperature of the fluid in the heating phase is advantageously controlled on the basis of the gas flow rate in the outlet duct.

It is preferable for the temperature of the fluid in the fluid heating phase to be controlled on the basis of the temperature of the gas in the outlet duct.

It is even more preferable for the temperature of the fluid in the fluid heating phase to be controlled on the basis of the difference between the temperature of the gas in the outlet duct and a pre-selected reference temperature.

The thermal energy used in the gas heating phase is calculated preferably in relation to the efficiency of the heating system.

This efficiency value is preferably an estimated value.

The method advantageously comprises phases for recalculating the estimated efficiency value in relation to a real efficiency value for the heating system.

BRIEF DESCRIPTION OF THE FIGURES

Further advantages, objects and characteristics, as well as further embodiments, of the present invention are described in the claims and will be clarified below by means of a description in which reference is also made to the attached drawings. In said drawings, corresponding or equivalent characteristics and/or component parts of the present invention are identified by the same reference numbers. In particular, the drawings contain the following:

- figure 1 is a functional diagram of a system according to a preferred embodiment of the invention;

- figure 2 shows a system according to a preferred embodiment of the invention; - figure 3 shows a simplified flowchart of the operations involved in a method according to a preferred embodiment of the invention, implemented in the system shown in figure 2;

- figure 4 shows details of several operations in the flowchart in figure 3;

- figure 5 shows details of several operations in the flowchart in figure 3;

- figure 6 shows a variant of the system in figure 2.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

Although the present invention is described below with reference to embodiments shown in the drawings, the present invention is not limited to the embodiments described below and illustrated in the drawings. The embodiments described and illustrated merely clarify certain aspects of the present invention, the object of which is stated in the claims.

The system according to the invention as described below is particularly suitable for reducing the pressure of the gas in a network for distributing gas to industrial and/or civil users.

This is on the understanding, however, that the inventive concept can be extended to any type of system in which a gas pressure reduction is required.

Figure 1 is the functional diagram for a system 1 for reducing the pressure of a gas according to a preferred embodiment of the invention.

Figure 2 shows a system 1 for reducing the pressure of a gas consistently with the content of the diagram in figure 1.

The system 1 is used to reduce the pressure of natural gas coming from a gas main 2 and being delivered to an outlet duct 3.

The system 1 substantially consists of a pressure regulating and measuring unit, or first-stage pressure reducing station, wherein the gas main 2 is the primary line for carrying the natural gas and the outlet duct 3 is the line for delivering the gas to the users.

The gas main 2 and the outlet duct 3 are intercepted by respective opening/closing valves 2a and 3a.

The gas main 2 is the primary line for carrying the natural gas, which travels at a pre-set pressure Pi,gas that is typically in the range of 12 to 74 bar, and at a preset temperature Ti,gas that is typically in the range between 5°C and 20°C, with mean values around 5°C. The outlet duct 3 is the line for delivering the gas to the users at a pressure Pu,gas typically in the range between 1.5 and 5 bar (and as high as 12 bar in certain applications), and at a pre-set temperature Tu,gas that may typically vary between 5°C and 15°C. It is preferably, as we shall see below, for the temperature Tu,gas of the gas at the outlet to be kept at values as close as possible to 6°C.

Between the gas main 2 and the outlet duct 3 there are gas heating means 4 and gas pressure reducing means 5. The gas heating means 4 are located upstream of the gas pressure reducing means 5.

The gas pressure reducing means 5 preferably comprise piloted pressure regulators.

The heating means 4 (which are easier to see in figure 2) preferably comprise a water/gas tube bundle heat exchanger 6 connected to a boiler 7 for the production of hot water. Upstream of the exchanger 6, there is a filter 13 designed to remove any impurities contained in the natural gas to protect the regulating and measuring instruments.

The boiler 7 is preferably of the free-standing type with a thermopile power system.

The boiler 7 is preferably complete with a thermostat 7a for controlling the temperature of the heated water, which is connected in series with the digital thermostat typically installed on the boiler 7.

Between the boiler 7 and the exchanger 6, there is a delivery line 8 that carries heated water from the boiler 7 to the exchanger 6, and a return line 9 that carries the cooled water from the exchanger 6 to the boiler 7.

There is an expansion tank 12 connected to the delivery line 8 that is designed to compensate for variations in the volume of water in the heating circuit and to maintain the appropriate quantity of heating water. In a variant of the invention, the expansion tank could be connected to the return line 9 instead of the delivery line 8.

Circulators 30 are also located along the delivery line 8 (there are two of these in the embodiment illustrated) to enable a forced recirculation of the water in the heating circuit, since the expansion tank is open, in order to reduce the system's hysteresis times.

The boiler 7 is advantageously of the type fuelled with methane gas, in which case a suitable gas supply line 10 connects the boiler 7 to the outlet duct 3 in the system 1.

The heating means 4, and particularly the boiler 7, are appropriately connected to control logic 14.

The control logic 14 sends the thermostat 7a of the boiler 7 an indication of the temperature T1,H20 needed for the water that is to flow to the exchanger 6 through the delivery line 8.

A first temperature detector 15 measures the temperature of the water T1,H20 in the delivery line 8 and sends this value to the control logic 14.

A second temperature detector 16 measures the temperature of the water T2,H2O in the return line 9 and sends this value to the control logic 14.

A gas temperature detector 17 is associated with the outlet duct 3 and measures the temperature Tu,gas of the gas downstream of the gas pressure reducing means 5. The gas temperature detector 17 is preferably in the form of a temperature sensor. The temperature Tu,gas measurement is sent to the control logic 14.

Means 18 for measuring the instantaneous flow rate of the gas Qu,gas are associated with the outlet duct 3 and used to record the instantaneous flow rate of the gas downstream of the pressure reducing means 5.

This instantaneous gas flow rate Qu,gas measurement is sent to the control logic 14.

In particular, the means 18 for measuring the instantaneous gas flow rate Qu,gas comprise a gas meter 19 that sends an impulsive signal to a volume corrector 20 suited to produce the instantaneous flow rate Qu,gas as an output in the form of standard cubic metres per second, i.e. with a defined unit of time.

In variants of the invention, other types of measuring device could be used, such as means for measuring mass, or a Venturi meter, to provide as output values that indicate the instantaneous flow rate of the gas Qu,gas.

The control logic 14 preferably consists of a programmable logic controller (PLC) unit of known type, i.e. a unit capable, among other things, of recording different signals coming from respective sensors/detectors and processing their values to generate suitable control signals. In the embodiment described herein, the output from the control logic 14 is a control signal for the thermostat 7a on the boiler 7 that establishes the temperature Tl,H2O to which the heated water must be heated. This command overrides the digital thermostat on the boiler 7, which is installed in series with the analogical thermostat 7a.

Along the various pipelines in the system 1, there is also a series of on/off valves, suitably positioned to allow for action to be taken for servicing purposes or to enable the flows in the water heating circuit to be controlled.

At suitable points along the various pipelines, there are also devices for recording the relative pressure of the natural gas in a known manner, consisting for instance of Bourdon spring pressure gauges, and possibly also pressure transducers suitably wired with electric connections.

According to the invention, the instantaneous reading by the means 18 for measuring the gas flow rate Qu,gas, combined with the gas temperature measurement Tu,gas in the outlet duct 3, enables an appropriate control of the heating means 4 so as to optimise the energy needed to heat the gas, as we shall see more clearly below from the description of how the system 1 operates.

For the description of how the system 1 in figure 2 operates, reference is made below to the diagrams shown in figures from 3 to 5.

Figure 3 schematically shows the operating principle behind the system 1 in a methane gas distribution network. More in particular, it illustrates the operations performed by the control logic 14.

In a first processing unit 100, the value of the instantaneous flow rate Qu,gas of the gas at the outlet and the value of the return temperature of the water T2,H2O are processed to provide as output a first value of the temperature at which the heated water V l,H2O has to be delivered.

The calculations performed in the processing unit 100, and operatively implemented in the control logic unit 14, are better illustrated in figure 4.

The rated values for the system 1 are used for this calculation, which are:

- the incoming pressure Pi,gas of the natural gas, expressed in bar;

- the outgoing pressure Pu,gas of the natural gas, expressed in bar;

- the incoming temperature Ti,gas of the gas inside the gas main 2.

These three values - Pi,gas, Pu,gas and Ti,gas - are used to calculate, with the aid of the enthalpy diagram for methane, the thermal energy that has to be transferred to the natural gas arriving in the system 1 in order for said gas to emerge downstream, after its pressure has been reduced, at a temperature Tu,gas of 6°C. From the enthalpy diagram for methane we thus obtain a value for the enthalpy change Ah, expressed in J/kg.

Using the value for the instantaneous flow rate Qu,gas of the outgoing gas, as measured by the measuring means 18 and expressed in standard cubic metres per second (sm3/s), the value of the thermal power CI, expressed in Watts, that has to be transferred to the incoming gas is calculated (unit 101) as follows:

Cl= Qu,gas*pCH4*Ah;

where pCH4 is the volumetric mass of the methane, expressed in kg/m3, and Ah is the previously-calculated enthalpy change.

The next processing step (unit 102) takes into account the efficiency El of the heating system in the system 1 in order to calculate the effective thermal power C2 that the boiler 7 has to provide.

The result will consequently be the thermal power C2 (at the outlet from the unit 102) that has to be used to heat the water in order to guarantee that the temperature of the outgoing gas, after the pressure reduction, will be 6°C, i.e. C2= C1/ E1

The value of the heating system's efficiency El is an estimate made by the operator, based on their know-how and experience in the field, and may depend on various factors, such as the insulation of the circuit where the water transits, the state of efficiency of the boiler, and so on. In a variant of the method, as we shall see in more detail later on, this efficiency value can be recalculated from time to time by the control logic 14, based on its processing of the values identified in the system 1. The calculated efficiency will thus tend towards the real and effective value of the efficiency of the heating system in the system 1. The previously-calculated value of the thermal power C2 (at the outlet from unit

102) is subsequently processed (unit 103) to provide a temperature value ΔΤ (at the outlet from unit 103). This temperature ΔΤ represents the temperature difference between the temperature T2,H2O in the heated water return line and the estimated temperature T' l,H2O of the heated water in the delivery line, i.e. ΔΤ = T' l ,H2O - T2,H20 = C2 / (mH2O*CpH2O)

where:

- T' 1,H20 is the estimated temperature of the heated water produced by the boiler 7 and carried to the exchanger 6 through the delivery line 8;

- T2,H2O is the temperature of the water in the return line 9 from the exchanger 6 to the boiler 7;

- mH2O is the mass flow rate, expressed in kg/s, of the water pumped by the boilers;

- CpH20 is the specific heat at a constant pressure of the water, expressed in J/(kg*°k).

The temperature of the water in the return line T2,H20 is advantageously measured by the second temperature detector 16.

The water delivery temperature T' 1,H20 is then calculated by adding the value of the previously-calculated temperature difference ΔΤ (at the outlet from unit

103) to the temperature T2,H2O measured in the water return line.

The temperature T' 1,H20 calculated in this way (at the outlet from the unit 100) represents a first estimate of the temperature T' l,H2O at which the heated water must flow from the boiler 7.

This temperature T' l,H2O is calculated with no control being performed on the effective temperature Tu,gas on the gas in the outlet duct 3.

Said control is performed in the next processing unit 150, as shown in figure 3.

The previously-calculated first temperature value T' l,H2O is processed and corrected, taking into account the temperature of the gas Tu,gas actually measured at the outlet duct 3, and the desirable reference temperature for the outgoing gas Tu,gas_rif. The final temperature value Tl,H2O at which the heated water has to flow from the boiler 7 is given at the outlet from said processing unit 150. This value is transmitted by the control logic 14 to the thermostat 7a on the boiler 7.

The temperature of the outgoing gas Tu.gas is advantageously recorded by the gas temperature detector 17 associated with the outlet duct 3.

The desirable reference temperature value for the outgoing gas Tu,gas_rif is advantageously set at 6°C.

The processing unit 150, shown in detail in figure 5, is basically a feedback system for controlling the final temperature Tl,H2O at which the heated water has to flow from the boiler 7 as a function of the temperature of the outgoing gas Tu,gas. Said control system may be of the proportional-integral-derivative type or based on any combination thereof, depending on the type and the dynamic conditions of the system.

An example of the possible feedback methods implemented in the processing unit 150 is given below.

The calculations performed in the processing unit 150 are implemented operatively in the control logic unit 14.

The deviation S between the value of the desirable reference temperature for the outgoing gas Tu,gas_rif (preferably set at 6°C) and the temperature value measured in the outgoing gas Tu,gas is calculated first, i.e.

S = Tu,gas_rif - Tu,gas

The value of this deviation S is used (in unit 151) to calculate a coefficient of error er (as output from unit 151), i.e.

er = S*( Qu,gas* Cpgas/QH2O*CpH2O);

where:

- Cpgas is the specific heat at a constant pressure of the gas in standard conditions, expressed in J/(kg*°k);

- QH2O is the flow rate of the water from the boiler 7 to the exchanger 6, expressed in kg/s;

- CpH20 is the specific heat under constant pressure of the water, expressed in J/(kg*°k).

This coefficient of error er represents the value to be added to the previously- calculated first temperature value T' 1,H20 (output from unit 100), i.e. Tl,H2O = T' l,H2O + er.

The temperature value T1,H20 thus represents the final temperature at which the heated water must flow from the boiler 7.

It is clear from the above description that the thermal power needed to guarantee an optimal outgoing gas temperature can be calculated starting from the instantaneous flow rate measured for the gas in transit through the outlet duct. The system consequently enables the amount of heat needed to heat the gas to be estimated using the instantaneous flow rate of the gas as a control parameter, whereas conventional systems rely on the temperature of the outgoing gas alone. This enables the inertia of the system to be limited, thereby increasing the efficiency of the system for regulating the temperature of the outgoing gas.

The system also enables an appropriate adjustment of the temperature of the water heated by the boiler to provide the effective thermal energy required by the system, with a consequent improvement in the system's energy efficiency by comparison with the known systems, in which the temperature of the heated water is substantially fixed.

In another embodiment of the invention, a self-teaching function is included in the system 1 with a view to optimising its performance.

As mentioned previously, this is achieved by means of a more accurate calculation of the value of the efficiency El of the heating system in the system 1 , using this parameter to calculate the thermal power C2 (unit 102, in figure 4). One way to calculate the value of the efficiency E' l to use in the above- mentioned formulas is explained below.

At regular intervals, the mean value erm of the coefficient of error er, the mean value Clm of the thermal power CI, and the mean value C2m of the thermal power C2 are calculated.

The values of er and C2m can be used to calculate a corrected value for ATc, i.e. ATc = C2m / ( mH2O * CpH2O ) + erm.

Then the thermal power C3 can be calculated, which corresponds to the real thermal power that the boiler 7 needs to provide, i.e.

C3 = ATc * mH2O * CpH2O

Finally, the new efficiency value E' 1 is calculated, i.e.

E' l = Clm / C3.

In subsequent calculations, this new efficiency value E' 1 for the heating system in the system 1 can be used instead of the efficiency value El estimated in advance. In successive calculations the efficiency ΕΊ will gradually tend towards the real value of the efficiency of the system 1.

Figure 6 shows a variant of the system 1 ' according to the invention that differs from the one shown in figure 1 in that it includes two boilers 7, 7', instead of one, and two parallel lines for the passage of the gas, fitted with respective gas heating means 4, 4' and gas pressure reducing means 5, 5'.

The two boilers 7, 7' are suitably connected in parallel and are connected to the same delivery line 8 and to the same return line 9.

A thermostat 7a is associated with both the boilers 7 and 7', and connected to the control logic 14.

Providing two boilers 7, 7' guarantees the continuity of the supply of heat even in the event of one of the two boilers being switched off or needing servicing 7, 7'. Providing two parallel pipelines for the passage of the gas, with respective gas heating means 4, 4' and gas pressure reducing means 5, 5', guarantees the continuity of the supply and of the heating of the natural gas, even in the event of one of the two boilers 7, 7' being switched off or needing servicing.

As far as the operating principle for this system 1 ' is concerned, the same applies as described above with reference to the preferred embodiment of the invention. The gas heating system described in the previous embodiments preferably comprises a boiler for heating the heated water and a water/gas tube bundle heat exchanger. In other embodiments, the heating system may be different, however, e.g. a system with a heat pump and a tube bundle exchanger, or boilers with plate exchangers, or even heat pumps combined with plate exchangers, etc. Here again in these variants, the thermal power released to the gas before its expansion will be advantageously controlled on the basis of the gas flow rate identified after its pressure has been reduced.

It has thus been demonstrated herein that the present invention enables the previously-stated objects to be achieved. In particular, it enables the development of a gas pressure reducing system with a better energy performance than the known state of the art.

While the present invention has been described with reference to the particular embodiments illustrated in the figures, this is on the understanding that the present invention is not limited to the particular embodiments illustrated and described therein. Further variants of said embodiments come within the scope of the present invention, as stated in the claims.