GB2105467A - A device for controlling the level of liquid - Google Patents

Abstract

The liquid level in a vessel (3) e.g. a condensate collecting tanks controlled by a valve (6) in the outlet conduit (61) operated by a hydraulic servodrive (5). The control means include a sensing tank (2) communicating with a fluidic sensing unit (1) which communicates, via a control pipe (51), with the hydraulic servodrive (5). The pressure applied by the unit (1) varies according to the liquid level in a sensing tank (2). The tank (2) is connected to an annular space (41) communicating with the throat of a venturi nozzle (4) in the outlet conduit (61). Whereby the level in the sensing tank is dependant both on the liquid level in tank (3) and the flow rate from that tank. If the level in tank (3) increases, the difference in levels between the tanks DELTA H falls causing a reduction in flow through pipe (22). The level in tank (2) rises causing the pressure from unit (1) to valve (5) to drop and valve (6) to open. <IMAGE>

Description

SPECIFICATION Device for controlling the level of liquid with a correction feedback obtained by throughflow The invention relates to a device for controlling the level of liquid with a correction feedback obtained by throughflow, which is especially advantageous for controlling the level of condensate in a heat-regeneration system of a steam turbine.

Known designs of arrangements for controlling the level of liquid and its off-take from the controlled system may be distinguished either according to the physical principle, which is used in individual embodiments for converting the information about the liquid level of the controlled system to a suitable inlet signal for the associated members of the control loop, or according to the method of further treatment of the signal generated by a level sensing unit in the associated control circuit.

One of the oldest types of a control loop for controlling a level is e.g. a float type arrangement.

In these systems the information about a liquid level is mostly converted into a force by utilizing the buoyancy of a suitable body immersed in a liquid, the level of which is sensed. In the simplest arrangements, where no large force is required, it is possible that the lifting force of the float sensor may control directly the lift of the control valve.

In modern technical practice mostly an electromechanical control loop is used for controlling a level. In such a loop information about the liquid level is converted into an electric signal by means of a suitable sensing element, the output of which is treated and amplified by an electronic controller, connected to an electromechanical servodrive, the output quantity of which, mostly a lift, changes the position of the throttling element of a control valve. Another, relatively very simple method of converting information about the liquid level into an output hydraulic signal is represented by a fluidic-sensing unit of liquid levels, the principle of which resides in a disruption of a turbulent flow of a liquid, which flows out of an emitting nozzle.The nozzle is situated perpendicularly to the sensed level, in a liquid layer above a sensing nozzle, which is situated coaxially to the said emitting nozzle.The degree of disintegration of a turbulent flow of a liquid is approximately proportional to the thickness of the liquid layer above the sensing nozzle and determines the level of the output pressure signal of the fluidic sensing unit, the output of which is mostly sufficient for an accurate control of a simple hydraulic servodrive. It changes the lift of the throttling element of a control valve.

All the mentioned designs of control loops of a liquid level control have some drawbacks. The main common drawback of most hitherto known designs is the fact that information concerning the condition of the controlled system, which enters into the control circuit proper, is limited to a signal provided by the respective sensing element and corresponding to the liquid level.

This condition of the said designs results in the system of a closed control loop in only one feedback, and its stationary and especially dynamic behaviour is dependent both upon operating conditions and regimes of the controlled system, and upon the time stability of parameters of members of the control circuit. A direct dependence between the input of the control circuit and the speed of its dynamic response, the need for relatively high shifting forces and the elimination of the hysteresis of action members of control loops result in another drawback of such arrangements. A fluidic-hydraulic loop, has another drawback, viz. that it is not possible to change the desired value of the liquid level in this loop within a sufficiently large range by a simple external action.

The said drawbacks may be considerably diminished by a device for controlling the level of liquid with a correction feedback according to the invention, the principle of which resides in the fact that it comprises a collecting tank to which a connecting piping provided with a control valve is connected, a sensing tank into which a sensing unit for liquid level is built in being connected to an action member controlling the control valve.

The aim of the invention is to generally increase the stability of the level control circuit, to make possible an external adjustment of the desired level of liquid in a tank, the level of which is controlled, to reduce the effect of hysteresis caused by friction in an action member of the control loop and of hysteresis of the throttle valve, to decrease the time needed for recontrolling, to decrease the needed input of the loop including a simple adaptation when redesigning control circuits, and to obviate the necessity to adapt the design of pressure tanks if the mentioned redesign is carried out.

The invention provides a device comprising a connecting piping which connects the collecting tank to the inlet of the control valve. In the said connecting piping a Venturi nozzle is built in. The Venturi nozzle is connected to a sensing tank at the point of minimum passage cross-section by means of a piping of a correction feedback. An outlet of the compensation piping is situated in the said sensing tank. In the sensing tank a level sensing unit is situated, which makes the inlet into the control circuit, the outlet of which is connected to a control valve.

In order that the invention may be clearly understood and readily carried into effect, a preferred embodiment thereof is, by way of example, hereinafter more fully described and illustrated in the accompanying diagrammatic drawing showing a device according to the invention, where a fluidic-hydraulic loop for controlling the level of condensate in a regeneration heater of a steam turbine is used.

In the drawing a regeneration heater 3 is connected by means of a connecting pipe 61 to the inlet of the control 6, to the outlet of which is connected a waste pipe 62 which opens outside the controlled system into another stage of a heat-regeneration cascade of a steam turbine, which is not shown in the drawing.

A Venturi nozzle 4 is built into the vertical branch of the connecting pipe 61 such that it communicates at its constriction (smallest crosssection) by means of radial passages 410 with an annular filling space 41. Into the space 41 opens the outlet of a pipe 22 of a correction feedback, which connects the space 41 , through a valve 220 of the desired level value, to the inner space of a sensing tank 2. The inlet of the pipe 22 is situated in the sensing tank 2 below a horizontal partition 200. A fluidic sensing unit 1 is connected to the sensing tank 2 such that a bottom 1 20 of a connecting passage forms a continuation of the horizontal partition 200. A compensating pipe 21 with a built-in throttle valve 210 opens into the inner space of the sensing tank 2.The compensating pipe 21 interconnects the upper part of the sensing tank 2 and the inner space of the regeneration heater 3 at the level of the lower part of a tube bundle 30.

The fluidic sensing unit 1 has two coaxial nozzles arranged perpendicularly with respect to the bottom 120 of the connecting passage, an emitting nozzle 11, situated above, is connected by its upper end to a source of pressure condensate (not shown) and forms, together with a coaxially situated shielding jacket 12, and a sensing nozzle 13, situated below, an interaction space of the fluidic sensing unit 1. The sensing nozzle 13 opens in its lower part through a perforated passage into the inner space of a separator 14, which is connected by means of a two-phase orifice plate 1 5 to the space under the bottom 120 of the connecting passage. The separator 14 is connected in its lower part to the upper cylindrical space 512 of a hydraulic servodrive 5 by means of a control pipe 51.The upper cylindrical space 512 is separated from the lower cylindrical space 511 by means of a movable piston 501 resting on a compression spring 502 and fixed to a piston rod 503. The piston rod 503 is fixed to the control valve 6.

The function of the devices according to the invention resides in the utilization of a hydraulic correction feedback, obtained by the through-flow of liquid from the controlled system through the Venturi nozzle such that this feedback complements the main feedback from the liquid level in the controlled system.

In a steady state the condensate drops from the heat-exchange surface, represented by a tube bundle 30, into the controlled system in the lower part of the jacket of the regeneration heater 3. A constant level of liquid in this system is ensured by discharging the same quantity of condensate through the connecting pipe 61 with the control valve 6, and then through the waste pipe 62.

When the condensate passes through the Venturi nozzle 4, the static pressure decreases at the point of its minimum throughflow cross-section in accordance with the Bernoulli equation. The obtained pressure deviation is transferred through the radial passages 410 into the filling space 41 of the Venturi nozzle 4, into which simultaneously flows the condensate delivered through the pipe 22 of the correction feedback from the sensing tank 2.The supply of the condensate through the pipe 22 of the correction feedback corresponds to the pressure deviation at the level of the Venturi nozzle 4, and to the hydraulic resistance given by the setting of the valve 220 by a level difference H, and also by a pressure difference PoPSN' where P0 is the pressure in the regeneration heater 3 and PSN is the pressure in the sensing tank.As to time, the constant level and pressure in the space of the sensing tank 2 are determined in a general case by a steady state of the condensing process in the sensing tank 2 and in the fluidic sensing unit 1, into which enters saturated steam from the space of the regeneration heater 3 through the compensating pipe 21 and through the throttle valve 210, as well as by a constant delivery of condensate from the emitting nozzle 11 of the fluidic sensing unit 1.

As the level of the disintegration of the turbulent flow of the condensate, flowing from the emitting nozzle 11, and consequently the hydraulic outlet from the system of the fluidic sensing unit 1 and separator 14, depends upon the condensate level in the sensing tank 2, the level control loop is closed and simultaneously the main feedback and the correction hydraulic feedback are established. The separator 14 forms an inlet into the hydraulic servodrive 5, the outlet of which is formed by the lifting of the control valve 6.

A dynamic function of such a closed control loop may be demonstrated by a response of this loop to a sudden change of flow into the controlled system. If the inflow change is positive, the liquid level in the regeneration heater 3 starts rising, and the level difference H between the regeneration heater 3 and sensing tank 2 drops. A drop of the level difference H to a lower value causes reduction of the flow through the pipe 22 of the correction feedback, and in this way a decrease in the condensate discharge from the sensing tank 2 and a rise of the liquid level in this tank. This rise of the liquid level is transferred from the sensing tank 2 into the fluidic sensing unit 1, where it results in an increase of the condensate layer about the sensing nozzle 13. In this way an increased disintegration of a turbulent flow leaving the emitting nozzle 11 is brought about as well as a corresponding decrease of a pressure outlet from the system of the fluidic sensing unit 1 and separator 14. From there the pressure drop is transmitted through the control pipe 51 into the upper cylindrical space 512 of the hydraulic servodrive 5.

As the system of the hydraulic servodrive 5 and control valve 6 comprises mechanically movable parts and therefore elements with friction resistance, the pressure decrease in the upper cylindrical space 512 does not cause an immediate change of the stroke of the mutually interconnected system of the piston 501, piston rod 503 and control valve 6. The stroke of the described system is changed if the resultant of the force caused by the compressed spring 502 and the force corresponding to the difference between the pressure in the lower cylindrical space 511 and the pressure in the upper cylindrical space 512 of the hydraulic servodrive overcomes the friction force corresponding to passive resistances.In this case a sudden change in the lift of the control valve 6, a decrease of its hydraulic resistance, and, with a delay corresponding to the dynamics of the connecting pipe 61, an increase of the flow through this pipe takes place. A relatively very quick change in the flow through the connecting pipe 61 causes a drop of the pressure level, sensed by the Venturi nozzle 4, and, with a delay corresponding to the dynamics of the pipe 22 of the correction feedback, the sensing tank 2 is emptied and simultaneously, much more slowly, the regeneration heater 3 is emptied as well.

A drop of the liquid level in the sensing tank 2 and in the fluidic sensing unit 1 increases, by the already described mechanism, the level of the outlet pressure signal from the system of the fluidic sensing unit 1 and separator 14 and also the pressure in the upper cylindrical space 512.

The adaptation of the lift of the servodrive 5 controls the change of position of the control valve 6 and the discharge of condensate from the controlled system.

By the arrangement of a hydraulic correction feedback in a system of a closed fluidic-hydraulic control loop, may be obtained a considerable increase in the stability of the control process, a reduction of time needed for recontrolling and a decrease of the hydraulic input. The adjustment change of the hydraulic resistance of the valve of the desired liquid level enables the desired liquid level to be changed in an easy way and within a large range even if the device is in operation.

Although the invention is illustrated and described with reference to one preferred embodiment thereof, it is to be expressly understood that it is in no way limited to the disclosure of such a preferred embodiment, but is capable of numerous modifications within the scope of the appended claims.

Claims (5)

Claims

1. A device for controlling the level of liquid in a collecting tank to the outlet of which is connected an outlet conduit provided with a control valve for controlling the flow of said liquid therethrough, the valve being controlled by a hydraulic servodrive, the device comprising a Venturi nozzle, arranged in the outlet conduit between the collecting tank and the control valve and defining between itself and the walls of the outlet conduit an annular space communicating with the constriction of the Venturi nozzle, and a sensing tank the upper part of which communicates, via a compensating pipe controlled by a throttle valve, with the collecting tank, and the lower part of which communicates, via a correction feedback pipe controlled by a valve, with the annular space, the sensing tank communicating with a fluidic sensing unit which communicates, via a control pipe, with the hydraulic servodrive which is thereby controlled.

2. A device according to Claim 1 wherein the sensing tank communicates with the fluidic sensing unit via a connecting passage, and the sensing.tank is sub-divided by a substantially horizontal partition, the connecting passage having a bottom forming continuation of the said partition.

3. A device according to Claim 2 wherein the fluidic sensing unit comprises an emitting nozzle and a sensing nozzle which is coaxial with the former and opens at its outlet into a separator.

4. A device according to any one of Claims 1 to 3 wherein the hydraulic servodrive includes a cylinder containing a piston, one side of the piston being exposed to the pressure of the liquid in the control pipe and the other being spring biassed and connected to the controlled valve in the outlet conduit.

5. A device for controlling the level of liquid in a collecting tank, constructed, arranged and adapted to operate substantially as herein described with reference to, and as shown in, the accompanying drawing.