TECHNICAL FIELD

The present invention relates ^* to a method and to a device for measuring the flow rate of a fluid, preferably of a lance of a burner of a gas-turbine plant.

BACKGROUND ART

Some types of gas-turbine plants comprise a combustion chamber provided with one or more burners, each of which comprises at least one primary gas lance and one secondary gas-oil (diesel oil) lance. Gas-oil lances, however, are frequently affected by machining defects that determine variations, which may even be marked variations, in the flow rate of fluid that can be nebulized in a combustion chamber. Gas-oil lances, in fact, have to operate at given pressures to be able to obtain an optimal nebulization of the gas-oil for combustion, and consequently, the presence of machining defects jeopardizes the flow rate of nebulized gas-oil.

It is consequently necessary to test each lance, before installing it in the respective burner, for verifying what is the effective flow rate of fluid that can be nebulized by the lance in the conditions of pressure that will present during use in a combustion chamber. Devices are known for measuring the flow rate of fluid that use an obstacle, which is set within the pipe in which the fluid evolves and causes a known loss of head. The flow rate is calculated on the basis of the measurement of the difference of pressure between a point upstream of the obstacle and a point downstream of the obstacle. This type of device is precise when the ratio between the maximum flow rate and the minimum one is relatively low, for example around approximately 30, and for fluids with stationary behaviour. However, these devices are not very precise when the fluid is two-phase or else contains a gas, for example air, as in the case of the gas-oil used in gas-turbine plants.

Moreover known air devices for measuring the flow rate of fluid of an electromechanical type, which use, for example, a turbine set within the pipe in which the fluid evolves. The turbine reacts to the motion issuing an electrical signal indicating the flow rate. These devices present two main limits: they are not very precise at low flow rates, and effect the measurement of the flow rate at the centre of the fluid vein, neglecting the effects of the limit layer. Finally, also these devices can be used only with fluids in stationary and not two-phase conditions, as in the case of the gas-oil used in gas-turbine plants.

There moreover exist devices for measuring the flow rate of fluids of an electronic type. The most widespread are ultrasound and Coriolis-effeet devices. Said devices, however, are not very precise at low flow rates and are cannot be used for two-phase fluids and fluids with inclusions of gas, as in the case of the gas-oil used in gas-turbine plants.

DISCLOSURE OF INVENTION

An aim of the present invention is to provide a device for measuring the flow rate of a fluid that will be free from the drawbacks of the known art highlighted herein; in particular, an aim of the invention is to provide a device that will be able to carry out a measurement of the flow rate that is reliable and correct for two-phase fluids and fluids with inclusions of gas, as in the case of the gas-oil used in gas- turbine plants .

In accordance with the above purposes, the present invention relates to a device for measuring the flow rate of a fluid, preferably of a lance of a burner of a gas-turbine plant, comprising:

a lance to be tested, through which the fluid flows in use and which comprises a main body and a terminal nozzle;

a fluid tank;

a first weight-measuring assembly for measuring the weight of fluid that traverses the nozzle of the lance; and computing means for calculating, on the basis of the weight measured, the flow rate of fluid that traverses the nozzle of the lance.

Yet a further aim of the invention is to provide a method for measuring the flow rate of a fluid that will be free from the drawbacks of the known art.

In accordance with said purposes, the present invention relates to a method for measuring the flow rate of a fluid, preferably of a lance of a burner of a gas-turbine plant, comprising the steps of:

measuring the weight of the fluid that traverses a terminal nozzle of a lance to be tested; and

calculating, on the basis of the weight measured, the flow rate of fluid that traverses the nozzle of the lance.

BRIEF DESCRIPTION OF THE DRAWINGS

Further characteristics and advantages of the present invention will emerge clearly from the ensuing description of a non-limiting example of embodiment thereof, with reference to the figures of the annexed drawings, wherein:

Figure 1 is a schematic representation of the device for measuring the flow rate of fluid according to the present invention;

Figure 2 is a schematic view of a first detail of the device of Figure 1; and

Figure 3 is a schematic view of a second detail of the device of Figure 1 .

BEST MODE FOR CARRYING OUT THE INVENTION

In Figure 1, designated by the reference number 1 is a device for measuring a flow rate of a fluid. In particular, here and in what follows a device for measuring the flow rate of fluid of a lance 2 of a burner of a gas-turbine plant (not represented in attached figures) will be described and illustrated.

The device 1 comprises a lance 2 to be tested, which is provided with an elongated main body 3 and with a terminal nozzle 4, a fluid tank 6, a filter 7, a pumping group 8, a delivery line 10, which connects the pumping group 8 to the main body 3 of the lance 2, a return line 11, which connects the terminal nozzle 4 of the lance 2 to the tank 6, a by-pass line 12, which connects an opening 9 of the main body 3 set upstream of the nozzle 4 to the tank 6, a cooling circuit 13, and a control unit 14.

Along the delivery line 10, the device 1 comprises a pressure detector 15, which is set downstream of the pumping group 8, a temperature detector 16 for detecting the temperature of the fluid, and a pressure detector 17, which is substantially set at the inlet of the main body 3 of the lance 2.

Along the return line 11, the device comprises a casing 19 that contains the terminal nozzle 4 of the lance 2, a fluid conveyor 20 fluidically connected to the casing 19, and a weight-measuring assembly 21.

Along the by-pass line 12, the device 1 comprises a by-pass valve 23 and a weight-measuring assembly 24. In particular, in the example described and illustrated herein, the fluid used in the device is gas-oil (i.e. diesel oil) .

In a variation not illustrated, gas-oil mixed with water is used.

In detail, the pumping group 8 comprises at least one volumetric pump (not illustrated) designed to guarantee a constant flow rate and a given pressure of the fluid at outlet.

In the non-limiting solution described herein, the pumping group 8 comprises three screw pumps set in parallel, which are able to pump fluid at pressures of up to 70 bar, sufficient to test gas-oil lances for gas-turbine plants. In particular, each screw pump is driven by an electric motor, which is controlled by an inverter for adjusting the speed of rotation of the pumps with a precision of IHz and for adjusting the flow rate from a minimum of 20 1/min to a maximum of 60 1/min approximately.

In a variation, the pumping group 8 comprises three screw pumps set in parallel, each of which is preceded by a booster- type pump for reducing onset of cavitation phenomena in the respective screw pump.

In a further variation the pumping group 8 comprises one or more gear pumps, which are able to pump fluid at pressures of up to 140 bar approximately, which are the values necessary for testing gas-oil lances to be used with gas-oil/water mixtures. In particular, each gear pump is driven- by an electric motor, which is controlled by an inverter for adjusting the speed of rotation of the pumps with a precision of 1 Hz and for adjusting the flow rate from a minimum of 20 1/min to a maximum of 90 1/min approximately.

The pressure detectors 15 and 17 and the temperature detector 16 are configured for sending the data detected to the control unit 14, which, as will be seen in detail hereinafter, will use said data to guarantee the conditions of safety and pressure necessary for proper conduction of the test of the lance 2.

In use, the cooling circuit 13 keeps the temperature of the fluid preferably around 30-40 ⁰ C and comprises filtering means and at least one heat exchanger (represented schematically in Figure 1 by a single box), generally a water heat exchanger.

The casing 19 set along the return line 11 is made of transparent material, preferably polycarbonate, in such a way that the nebulization cone of the lance 2 is visible.

The fluid conveyor 20 collects the fluid coming from the casing 19 and conveys it into the weight-measuring assembly 21. With reference to Figure 2, the weight-measuring assembly 21 comprises two collection vessels 26 set in parallel, two weight sensors 27, connected to the respective collection vessels 26, two inlet valves 28, set respectively upstream of each collection vessel 26, and two outlet valves 29, set downstream of each respective collection vessel 26.

In particular, the weight sensors 27 are load cells from which the respective collection vessels 26 hang. Each load cell 27 converts the force that it undergoes into an electrical signal P _L and sends it to the control unit 14.

In particular, the signal of each load cell 27 is acquired at preset time intervals Δt, which can be regulated as a function of the weighing requirements, between approximately 5 μs and 1 s . In the non-limiting example described and illustrated here the time interval Δt is of approximately 200 ms . The inlet valves 28 and outlet valves 29 are controlled by the control unit 14 and are, preferably, butterfly valves with closing times shorter than approximately 500 ms, in particular of approximately 100 ms .

With reference to Figure 1, the by-pass valve 23 is preferably a globe type valve, opening of which is regulated by the control unit 14.

The weight-measuring assembly 24 is substantially identical to the weight-measuring assembly 21 and sends to the control unit 14 the values of weight p _B p of the fluid that flows along the by-pass line 12, detected by the weight sensors 27.

With reference to Figure 3, the control unit 14 comprises a weight-assembly-control module 30, a pressure-control module 31, a temperature-control module 32, and a computing module 34 (represented in Figure 3 with a dashed line) .

The weight-assembly-control module 30 regulates opening and closing of the inlet valves 28 of the weight-measuring assemblies 21 and 24 through the signals Sn and Si ₂ , and regulates opening and closing of the outlet valves 29 of the weight-measuring assemblies 21 and 24 through the signals S _1n and Su2•

In particular, the weight-assembly-control module 30 governs the inlet valves 28 in such a way that only one inlet valve 28 out of two is always open. In addition, the weight-assembly- control module 30 governs the outlet valves 29 in such a way that each outlet valve 29 is in an opposite state with respect to that of the respective inlet valve 28. In other words, if the inlet valve 28 is open, the respective outlet valve 29 is closed, and vice versa. In this way, the continuity of the measurement of the weight is guaranteed, in so far as once one of the two collection vessels 26 is filled, the other collection vessel 26 is used, whilst the first is emptied out directly into the tank 6.

The pressure-control module 31 receives at inlet the values of pressure Pi and P ₂ detected respectively by the pressure detectors 15 and 17 and is configured for regulating the pressure generated by the pumping group 8, through a signal Sp ₀ MP/ and opening of the by-pass valve 23, through a signal S _B p/ in such a way that the pressure inside the lance 2 in a region corresponding to the nozzle 4 is at least 50 bar higher than the external pressure in the area close to the nozzle 4. Said value corresponds to the condition of pressure for obtaining a nebulization of the gas-oil that is optimal for combustion.

The temperature-control module 32 receives the value of temperature T _F of the fluid detected by the temperature detector 16 and is configured for activating an alarm signal in the case where the temperature T _F of the fluid exceeds a given threshold, preferably of approximately 70° C.

The computing module 34 comprises a filtering module 36 for filtering the values of weight p _B p and p _L coming from the weight sensors 27 so as to eliminate anomalous weight measurements, and a flowrate-computing module 37, which is configured for calculating the flow rate of fluid Q _L that traverses the lance 2 on the basis of the values of weight p _L received by the weight-measuring assembly 23 and for computing the flow rate of fluid Q _B p that traverses the by-pass line 12 on the basis of the values of weight p _B p received from the weight-measuring assembly 24. In particular, the flowrate-computing module 37 calculates the flow rates Q _L and Q _BP on the basis of the values of weight detected by the weight sensors 27 according to the following formulas :

_ p _L (t + At)-p _L (t) _p _BP (t + At)-p _BP (t)

Q ^L ~ At ^QBP ~ At where :

p _L (t) is the weight detected at the t-th instant by the weight sensor 27 of the weight-measuring assembly 21;

P _L (t+Δt) is the weight detected after an interval Δt with respect to the t-th instant by the weight sensor 27 of the weight-measuring assembly 21;

p _B p(t) is the weight detected at the t-th instant by the weight sensor 27 of the weight-measuring assembly 24;

P _BP (t+Δt) is the weight detected after an interval Δt with respect to the t-th instant by the weight sensor 27 of the weight-measuring assembly 24; and

Δt is the interval that elapses between one acquisition of weight and another. The flowrate-computing module 37 is moreover configured for -calculating the mean value Q _M of the flow rate over variable lengths of times .

Basically, the flow rate Q _L that traverses the nozzle 4 of the lance 2 and the flow rate Q _B p that flows in the by-pass circuit

12 are necessary for characterizing the lance 2 and establishing the flow rate that has to be supplied to the lance 2 when the lance 2 is mounted in the burner of the combustion chamber. The gas-oil-supply system of the gas- turbine plant has in fact to supply to the lance 2 a total flow rate Q _TOT =Q _L +Q _BP to obtain an optimal nebulization.

Finally, it is evident that modifications and variations may be made to the device and to the method described herein, without departing from the scope of the annexed claims.