WO2011104419A1

WO2011104419A1 - Performance monitoring of a pump-valve system

Info

Publication number: WO2011104419A1
Application number: PCT/FI2010/050146
Authority: WO
Inventors: Mats Friman; Jouni Pyötsiä; Ismo NIEMELÄ
Original assignee: Metso Automation Oy
Priority date: 2010-02-26
Filing date: 2010-02-26
Publication date: 2011-09-01

Abstract

In a method for diagnosing a pump-valve system, a valve opening of a control valve and a pressure difference (Δp) over the control valve during normal operation of the control valve is measured. Then, based the measured valve opening data and the measured pressure data and a stored inherent valve flow coefficient (Cv) characteristic curve of the control valve, an actual pressure difference over the control valve as a function of a flow rate (Q_flow) through the control valve, as well as an actual maximum flow rate (Q_max) through the control valve are determined. Finally, based on the pressure difference (Δp) and the actual maximum flow rate (C_max), it is possible to determine a potential reduction (Δp_r) achievable in the pressure difference, if a fully open flow rate of the control valve is resized to a value (Q_fully _{open new}) close to the actual maximum flow rate and a preceding pump is resized accordingly. The potential reduction of the pressure difference (Δp_r) can be reported to a user via a user interface, such as graphical user interface, web-based user interface or voice user interface, or by means of any messaging or data communication mechanism.

Description

PERFORMANCE MONITORING OF A PUMP-VALVE SYSTEM

FIELD OF THE INVENTION

The present invention relates generally to online diagnostics of a pump-valve system. BACKGROUND OF THE INVENTION

A control valve is generally used for a continuous control of a liquid or gas flow in various pipelines and processes. In a processing industry, such as pulp and paper, oil refining, petrochemical and chemical industries, different kinds of control valves installed in a plant's pipe system control material flows in the process. A material flow may contain any fluid material, such as fluids, liquors, liquids, gases and steam.

Successful control valve sizing and selection depend on knowing the operating conditions in the system in which the control valve is to be installed. It is known that distinct information on operating conditions very sel- dom exists. The more assumptions one has to make on flow conditions, the less accurate the control valve sizing is going to be. A common problem is an oversized valve. This means that the valve operates with openings that are too low, within a very narrow opening range and with high installed gain. A high installed gain means that even small changes in the control signal, and respec- tively in valve travel, effect relatively large changes in flow. To control such a loop accurately is very difficult.

Under operating conditions a control valve is part of a process pipeline. A process pipeline often includes a number of pump-valve systems wherein a pump produces a fluid pressure to provide a fluid flow in the pipe- line, and the fluid flow is controlled by a throttling control valve located somewhere in the pipeline after the pump. The pumps are driven by electrical energy. Such throttling flow control means that the "extra" pressure energy produced by the pump is wasted in the throttling control valve. Therefore, it is desired to size the pump-valve system as optimally as possible in order to avoid wasting the pumping energy and to operate the control valve more optimally. A large industrial process may contain hundreds of pump-valve systems, and the amount of wasted pumping energy may be very large. For example, an estimation has been presented that the annual total amount of wasted pumping energy in the process industry in Finland may be even 500 MW which would cor- respond to the energy production capacity of one nuclear power plant. Thus significant savings in energy and cost could be obtained by a correct sizing of the pump-valve systems. Moreover, the wrongly sized pump-valve system results in reduced control performance and control accuracy of the valve.

A process analysis during planning enables a plant design engineer to select a pump based on expected flows in a pipeline. However, actual flows in the process are impossible to predict exactly, and the actual flows typically differ from the estimated designed flows. This may result in a wrongly sized pump. Moreover, when a plant is designed, a pump dimension is often exaggerated in order to secure the operation of the plant. It is also possible to make offline analysis of the pump-valve system. However, such analysis focuses on the identification of pump and system curves and neglects variations of system curve. The analyzed operation period is typically short, while the period should be significantly long (months or years) in order to be representative because the variations in a system curve are sometimes slow, due to fouling, clogging, etc. The high number of pump-valve systems in a plant also makes it labour- some to analyse and recognise inappropriately operating pump-valve systems.

EP0962847 discloses a method and equipment for controlling a pipe network comprising piping, an inverter-controlled pump and at least two control valves. The valve position and the flow through the valve are moni- tored, and the rotational speed of the pump and the position of each valve are adjusted on the basis to the position and flow data received. The rotational speed of the pump is adjusted to be as low as possible but, at the same time, sufficient for maintaining the gain of the valves. The positions of the valves are adjusted to be as open as possible, the valve opening being, however, for the major part of the time, not more than a certain predetermined portion of the opening of the totally open position. As a result, the energy consumption required for the pumping is decreased and the control accuracy of the valves is increased.

This prior art approach is only applicable to few pump-systems in a plant but fails to provide an universal way to reduce the waste of pumping energy in the plant containing a high number of pump-valve systems.

SUMMARY OF THE INVENTION

An object of the present invention is to improve online diagnostics of control valves. This object of the invention is achieved by a method, a valve positioner, systems and a computer program according to the independent claims. Embodiments of the invention are disclosed in the dependent claims.

An aspect of the invention is a method for diagnosing a pump-valve system, comprising

storing an inherent valve flow coefficient (Cv) characteristic curve of a control valve in a memory,

measuring a valve opening of the control valve and a pressure difference over the control valve during normal operation of the control valve, determining, based the measured valve opening data and the measured pressure data and the stored inherent Cv-curve, an actual pressure difference over the control valve as a function of a flow rate through the control valve, as well as an actual maximum flow rate through the control valve,

determining, based on the pressure difference and the actual maximum flow rate, a potential reduction achievable in the pressure difference, if a fully open flow rate of the control valve is resized to a value close to the actual maximum flow rate and a preceding pump is resized accordingly.

Another aspect of the invention is a method for diagnosing a pump- valve system, comprising

storing an inherent valve flow coefficient (Cv) characteristic curve of the a control valve in a memory,

measuring a valve opening of a control valve and a flow rate through the control valve during normal operation of the control valve,

determining, based the measured valve opening data and the measured flow rate data and the stored inherent Cv-curve, an pressure differ- ence over the control valve as a function of a flow rate through the control valve, as well as an actual maximum flow rate through the control valve,

determining, based on the pressure difference and the actual maximum flow rate, a potential reduction achievable in the online pressure difference, if a fully open flow rate of the control valve is resized to a value close to the actual maximum flow rate and a preceding pump is resized accordingly.

According to an embodiment of the invention the resized fully open flow rate of the control equals to the actual maximum flow rate plus a predetermined safety marginal.

According to an embodiment of the invention the method comprises reporting the potential reduction of the online pressure difference via an user interface, such as graphical user interface, web-based user interface or voice user interface, or by email, short message service, multimedia message service, or by another messaging or data communication mechanism.

According to an embodiment of the invention the method comprises further calculating and reporting, via a graphical user interface, a potential sav- ing in pumping energy and/or pumping cost with the potential reduction in the online pressure difference.

According to an embodiment of the invention the method comprises further calculating and reporting, via a graphical user interface, a recommended resizing of the pump to achieve the potential reduction in the online pressure difference, such as a recommended resizing of a pump propeller.

According to an embodiment the method comprises determining an installed flow characteristic of the control valve based on the stored inherent Cv-curve, the online-measured valve opening and the online-measured pressure difference or the online-measured flow rate.

According to an embodiment the method comprises

determining a new installed flow characteristic of the control valve with the resized fully open flow rate and the reduced online pressure difference,

determining the gain of the control valve based on the new installed flow characteristic,

checking whether the gain meets predetermined constrains.

According to an embodiment the method comprises

sorting the values of the determined potential reduction in the online pressure reduction values, and

providing, via a graphical user interface, a graph showing the sorted values the determined potential reduction in function of the number and/or the normalized number of the measurements giving each respective value of the determined potential reduction.

According to an embodiment the method comprises

defining a plurality of operating points indexed by two of the following parameters: a measured valve opening of the control valve; a measured pressure difference over the control valve; a measured flow through the control; a determined pressure difference over the control valve; a determined flow through the control valve; a determined potential pressure difference reduction over the control valve; and

storing in a memory for each of said plurality of operating points a count of measurement hits to the specific operating point and preferably one or more of the remaining parameters.

An aspect of the invention is a valve positioner for operating a control valve, the valve positioner comprising means for implementing steps of any one of the method aspects recited above.

An aspect of the invention is an automation system for controlling a process, the automation system comprising means for implementing steps of any one of the method aspects recited above.

An aspect of the invention is a valve management system compris- ing means for implementing steps of any one of the method aspects recited above.

An aspect of the invention is a system comprising a plurality of pump-valve systems installed in a process, an automation system for controlling the process, and a valve management system, the system further compris- ing means for implementing steps of any one of the method aspects recited above.

An aspect of the invention is a computer program comprising program code means adapted to perform steps of any one of the method aspects recited above when the program is run on a computer or a processor.

By merits of embodiments of the present invention the entire lifecy- cle of pump-valve system (i.e. several years) can be analyzed based on true observed values collected during the entire life-cycle. The analysis and data collection is effective with modest memory requirements. There may be no need to store or retrieve old trend information. Continuous monitoring of pump- ing performance and/or pumping energy can be provided. Analysis of the energy savings potential can be automated. Continuous suggestions of saving potential can be made. Saving potential vs. pumping performance can be shown in one graphical plot.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in greater detail by means of embodiments shown as examples and with reference to the attached drawings, in which

Figure 1 shows a schematic block diagram of an exemplary process automation system and a field device management system;

Figure 2 illustrates an exemplary pump-valve system wherein em- bodiments of the present invention may be applied;

Figure 3 illustrates an exemplary control valve having inlet and outlet pressure sensors integrated into the body of valve;

Figure 4 illustrates examples of pressure curves depicting the measured inlet and outlet pressures of a valve as a function of the actual flow rate;

Figure 5 illustrates an example of a pressure curve depicting the measured pressure drop over a valve as a function of the actual flow rate;

Figure 6 illustrates an example of an installed flow characteristics curve;

Figure 7 illustrates examples of pressure curves depicting the measured inlet and outlet pressures as well as a reduced inlet pressure as a function of the actual flow rate;

Figure 8 illustrates examples of pressure curves depicting the measured or calculated pressure difference and a reduced pressure difference as a function of a calculated or measured flow rate;

Figure 9 illustrates an example of a new installed flow characterics curve;

Figure 10 illustrates an example of a gain of the valve as a function of the flow rate through the valve;

Figure 1 1 illustrates another exemplary pump-valve system wherein embodiments of the present invention may be applied;

Figure 12 illustrates an example of providing discrete operating points indexed by the flow rate and the valve opening h;

Figure 13 illustrates an example of a graph resulting from sorting of calculated pressure reduction values and plotting the sorted values in function of the number n of hits giving each value;

Figure 14 illustrates an example of a graph resulting from sorting of calculated pressure reduction values and plotting the sorted values in function of the normalized number n of hits giving each value; and

Figure 15 illustrates another example of a graph resulting from sorting of calculated pressure reduction values and plotting the sorted values in function of a normalized number hits, and calculated monetary savings (in thousand euros per year) as a function of the sorted pressure reduction values.

EXEMPLARY EMBODIMENTS OF THE INVENTION The present invention can be applied in diagnosis of any pump- valve system comprising a combination of a pump and at least one control valve in a process pipeline.

Figure 1 shows a schematic block diagram of an exemplary process automation system and a field device management system wherein the principles of the invention may be applied to a pump-valve system. The control system block 5 generally represents any and all control room com- puter(s)/programs and process control computer(s)/programs as well as databases in the automation system. There are various architectures for a control system. For example, the control system may be a Direct Digital Control (DDC) system or Distributed Control System (DCS), both well known in the art.

In the example of Figure 1 , only one control valve is shown, but an automation system may, however, include any number of field devices, such as control valves, often hundreds of them. There are various alternative ways to arrange the interconnection between the control system and field devices, such as control valves, in a plant area. In Figure 1 , the field/process bus 3 generally represents any such interconnection. Traditionally, field devices have been connected to the control system by two-wire twisted pair loops, each device being connected to the control system by a single twisted pair providing a 4 to 20 mA analog input signal. More recently, new solutions, such as Highway Addressable Remote Transducer (HART) protocol, that allow the transmission of digital data together with the conventional 4 to 20 mA analog signal in the twisted pair loop have been used in the control systems. The HART protocol is described in greater detail for example in the publication HART Field Commu- nication Protocol: An Introduction for Users and Manufacturers, HART Communication Foundation, 1995. The HART protocol has also been developed into an industrial standard. Examples of other fieldbuses include Fieldbus and Profibus. However, it is to be understood that the type or implementation of the field/process bus 3 is not relevant to the present invention. The field/process bus 3 may be based on any one of the alternatives described above, or on any combination of the same, or on any other implementation.

The field devices may be managed using a field device management and diagnostics system 4. The management and diagnostics system 4 may further be connected to a local area network LAN of the factory, which allows it to communicate with the control room programs, for example. Alternatively, the field device management and diagnostics system 4 or similar func- tionality may be integrated into the control system 5, e.g. into control room or process control computers. The management and diagnostics system 4 may be connected to the field devices (e.g. valve positioner 2) over the field/process bus 3, as described above. For example, each field device may have a dedicated fieldbus connecting it to a HART multiplexer, which is in turn connected to the management and diagnostics system 4. The management and diagnostics system 4 may comprise a computer workstation provided an appropriate management and diagnostics program. Example of a management and diagnostics system is a computer provided with Neles FieldCare software from Metso Automation Inc. Neles FieldCare is an universal FDT/DTM (Field Device Tool / Device Type Manager) -based software. One of the features of Neles FieldCare is on-line condition monitoring which enables to collect on-line data from field devices and provides tools for predictive maintenance planning.

An exemplary pump-valve system wherein embodiments of the pre- sent invention may be applied is illustrated in Figure 2. Such pump-valve system may be controlled and managed by the automation system illustrated in Figure 1 , or any other type of control system, or it may even be a standalone system. In the illustrated example, a pump 23 is provided to pump a flow of material to a process pipeline 25, and a control valve 21 is connected to a process pipeline 25 after (downstream from) the pump 23 to control the material flow of a substance in the process pipeline 25. The material flow may contain any fluid material, such as fluids, liquors, liquids, gases and steam. The control valve 21 is usually connected with an actuator, which turns the closing element of the valve to a desired position between fully open and fully closed positions. The actuator may be a pneumatic cylinder-piston device, for example. The actuator, for its part, is usually controlled by a valve positioner 22, sometimes referred to as a valve controller, which controls the position of the closing element of the control valve 21 and thus the material flow in the process according to a control signal from a controller in an automation system.

According to an aspect of the invention, a pressure difference Δρ across the control valve in the pipeline, and an opening h of the control valve is measured and stored online, i.e. while the control valve is normally operating in the pipeline. In other words, an inlet pressure p1 , provided by the pump 23 in the pipeline 5 upstream from the control valve 21 , is measured by a suitable pressure sensor 24, and the outlet pressure p2 of the control valve 21 to the downstream pipeline 25 is measured by another suitable pressure sensor 26. The pressure sensors 24 and 26 may be separate from the control valve 21 , i.e. they may be attached to the pipeline 5 in a location close to or remote from the control valve. Preferably the pressure sensors 24 and 26 may be integrated into the control valve assembly and arranged to provide the pressure information to the valve positioner 22, e.g. over a wired or wireless connection. Figure 3 illustrates an exemplary control valve 21 having the pressure sensors 24 and 26 integrated into the body of valve at the inlet and the outlet, respectively. The pressure sensors 24 and 26 may be provided with wireless communication means, e.g. radio transmitters, for transmitting the pressure data p1 and p2 to a receiver at a valve positioner 22A. A pneumatic actuator 22B operates the control valve under control of the valve positioner 22A. An example of a suitable control valve 21 wherein pressure sensors could be integrated is Neles RotaryGlobe control valve, Series ZX, from Metso Automation Inc. An example of a suitable valve positioner 22A is Intelligent Valve Controller Neles ND9000 from Metso Automation Inc. An example of a suitable actuator 22B is a Pneumatic double-diaghram actuator, Series E, from Metso Automation Inc.

The pressure difference (pressure drop) Δρ over the control valve may be obtained from the measured upstream and downstream pressures as follows: Δρ = p1 - p2.

In exemplary embodiments of the invention, the valve positioner 22, e.g. a microprocessor of the valve positioner, is arranged to sample the pressures p1 and p2, and the valve position h at a predetermined sampling frequency during the online operation of the control valve, and to store the sample values in an internal memory of the valve controller, and/or to transfer (ac- tively or on demand) the sample values to the management and diagnostics system 4, or to the automation system, over the field/process bus 3. For each type of a control valve there typically is an exact inherent flow characteristic determined by laboratory testing. Valve inherent flow characteristic is defined so that the pressure differential across the valve (Δρ) is kept constant. As the differential pressure (Δρ) is constant, the flow rate (Q) through the valve is proportional to the valve flow coefficient (Cv), as expressed in the simplified equation:

Q = N - Cv - V Ap (1 ) where

N = constant

Q = flow rate through the valve Cv = valve flow coefficient

Δρ = pressure differential across the valve.

Because the valve flow coefficient (Cv) reflects the effective flow cross-section of the valve, the valve inherent flow characteristic shows how the effective flow cross-section changes as a function of relative travel or position h of the valve. Such inherent flow characteristic is often presented in form of a Cv characteristic curve which depicts the Cv value as a function of the opening of the valve. According to embodiments of the invention, an inherent Cv curve may preferably be stored, e.g. in a tabular form, in a memory in the valve posi- tioner, the automation system or the diagnostics system.

The inherent flow characteristic is the shape of a flow curve through the valve with a constant pressure drop across the valve. However, when process piping is attached to the valve, the piping pressure loss which varies as a function of flow rate will cause also the valve pressure drop to vary as a function of flow rate, even if the pressures at the source and receiver were constant. In other words, under operating conditions the differential pressure across a valve is seldom constant in the valve travel range because dynamic pressure losses in the flow cause the valve inlet pressure to fall and the outlet pressure to rise as the flow rate increases. For an installed valve, the depend- ence of the flow rate q on the position h, i.e. the shape of the installed flow characteristic curve, is therefore a function of the process pipeline and of the inherent flow characteristic of the valve.

According to an aspect of the invention, the measured valve opening data and the measured pressure data may be utilized to determine the in- stalled flow characteristic curve of the valve for analysis of the pump-valve system. First a characteristic curve depicting the inlet and outlet pressures p1 and p2, or the pressure difference Δρ, as a function of the flow rate q through the control valve may be defined. To that end, for each entry of the measured inlet and outlet pressures, or of the pressure difference, respective value for the flow through the control valve may be determined. First, using a stored inherent Cv characteristic curve, the required C value for the respective measured opening h can be determined. Then the respective flow rate Q may be determined for each valve opening h based on the determined Cv value and the measured pressure difference Δρ=ρ1 -ρ2 using the flow equation (2), for exam- pie. Q(h) = N ^■ Cv(h)^■ V Ap(h) (2) wherein N = constant, Q(h) = flow rate through the valve at opening h, Cv(h) = valve flow coefficient at opening h, and Ap(h) = measured pressure differential at opening h.

As a result, pressure curves depicting the measured inlet and outlet pressures p1 and p2 as a function of the determined flow rate may be provided as shown in Figure 4. Alternatively or in addition to, a pressure curve depicting the pressure difference Δρ as a function of the determined flow rate may be provided as shown in Figure 5.

Further, the installed flow characteristic curve and the actual maximum flow rate Cw through the valve may be determined using the inherent Cv curve and the measured opening h of the valve. The accuracy of the installed flow characteristic curve will improve with increasing number of meas- urements. An example of an installed flow characteristics curve is shown in Figure 6. It can be seen that the actual maximum valve opening h_{max va}ive reguired by the actual maximum flow rate is small as compared with the designed maximum opening of the valve. This indicates that the control valve is oversized, which offers a possibility of optimizing the pump-valve system.

Referring again to Figures 4 and 5, the flow Q_fun_{y op}en refers to the maximum designed flow rate provided, when the installed valve is fully open. The Qmax refers to the actually observed maximum flow rate in the pipeline required by the process. These example curves indicate that the control valve is oversized, which offers a possibility of optimizing the pump-valve system. More specifically, there is a possibility of determining how much the inlet pressure p1 of the control valve can be decreased, when the control valve is fully open. The decreased inlet pressure p1 means that the pumping height of the preceding pump can be decreased accordingly, resulting in pumping energy savings.

Figure 7 illustrates pressure curves depicting the measured inlet and outlet pressures p1 and p2, as well as a reduced inlet pressure p1 new as a function of the determined flow rate. The new inlet pressure curve p1 _new may be determined by first defining a new maximum designed flow rate Qf_Un_y open new for a fully open valve. The Qf_Un_{y 0P}en new is preferably larger than the maximum flow rate Cw required by the process, with a safety margin SM required in each specific application, e.g. 10-15 %. After having the Qf_Un_{y 0P}en new, a pressure difference Δρ over the control valve, when fully open with the new Q_fun_y open new, may be calculated using the inherent Cv curve and and equation (3), for example

Ap(h) = [Q(h)/Cv(h)]² (3) wherein h = valve opening, Q(h) = flow rate at valve opening h, and Cv(h) = Cv-value at valve opening h. In this case, h=100%, Cv(h)=Cv(100%), and Q= Qf_Un_y open new- The new inlet pressure p1 _new at the flow rate Qf_Un_y open new can then obtained by adding the pressure difference Δρ to the measured outlet pressure p2 at the flow rate Qf_Un_{y 0P}en new, i.e. p1 _new = p2 + Δρ. Similarly, new inlet pressure p1 _new at any flow rate Q can be obtained such that the new inlet pressure curve p1 _new is formed as shown in Figure 7. The potential saving (reduction) in the inlet pressure is the difference Δρ_Γ between the original inlet pressure curve p1 and the new inlet pressure curve p1 _new- The pressure reduc- tion may also be calculated with equation (4)

Ap_r(h) = Ap(h)[1 -(Cv(h)/Cv(100%))²] (4) wherein h = valve opening, Q(h) = flow rate at valve opening h, and Cv(h) = Cv-value at valve opening h.

Figure 8 illustrates a pressure curves depicting the measured or calculated pressure difference Δρ_ν3ΐνβ originaii and a reduced pressure difference Δρ_ν3ΐνβ new as a function of a calculated or measured flow rate. The new pressure difference curve Ap_vai_Ve new may be determined by first defining a new maximum designed flow rate Qf_Un_{y 0P}en new for a fully open valve. The Qf_Un_{y 0P}en new is preferably larger than the maximum flow rate Q_max required by the process, with a safety margin SM required in each specific application, e.g. 10-15 %. After having the Qf_Un_{y 0P}en new, a pressure difference Ap_vaive new over the control valve, when fully open with the new Q_fun_{y op}en new, is calculated using the inherent Cv curve and equation (3), for example. The potential pressure reduction Δρ_Γ (saving) in the inlet pressure is the difference between the original pressure difference Ap_vai_Ve original and the new pressure difference Ap_vai_Ve new- Similarly, the pressure reduction Δρ_Γ can be calculated at any flow rate Q. Alternatively, the Δρ_Γ may be calculated with equation (4), as described above.

In all cases, the inlet pressure saving potential Δρ_Γ is proportional to the corresponding saving potential in the pumping height of the preceding pump, and further to the potential saving in the pumping energy. Based on the above measurements, it is possible to calculate an average flow rate through the valve over a period of time. Thus it is possible to estimate the possible saving in the pumping energy, when the pump provides the reduced inlet pressure pl new or the reduced pressure difference Ap_vaive new instead of the original inlet pressure p1 or the original pressure difference Δρ. Finally, when the price of the pumping energy is known, a corresponding monetary saving in comparison with the original pump-valve system can be calculated.

According to embodiments of the invention, the performance and control accuracy of the control valve with the new inlet pressure and settings may be checked, before making corresponding changes in the pump-valve system. To that end, a new installed flow characteristic of the valve and the gain of the valve within the control range may be calculated. An example of a new installed flow characteristic curve is shown in Figure 9. The min and max indicate the valve openings h at a minimum flow rate and at the maximum flow rate (C ) required by the process, respectively. An example of a gain of the valve as a function of the flow rate through the valve is shown in Figure 10. The min and max indicate the minimum flow rate and the maximum flow rate (Qmax) required by the process, respectively. The gain of the valve with the new installed flow characteristic may be determined using equation (5), for example

G = dQ/dh (5) wherein G = gain, dQ= a change of flow rate Q, and dh= a change of valve opening. Moreover, the gain G may be defined in relation to the maximum flow rate C required by the process rather than in relation to the flow rate Qf_Un_y open new of a fully open valve. Following design constrains may be applied to the gain G within the control range in order to ensure a sufficiently good performance. The ratio of the maximum gain G_max and the minimum gain G_min shall preferably be less than 2, i.e. (G_max/G_mi_n) < 2. Further, the gain shall preferably be equal to or higher than 0.5, i.e. G > 0,5. In the case these design constrained are employed, they may restrict the maximum opening of the valve. On the other hand, the control error of the valve increases also when the gain G of the valve is high. For example, if the gain G within the control range ex- ceeds 3, it means that the error in the flow rate is three times greater that the position error of the valve. Thus, if the position error was e.g. 0,3 %, the error in the flow rate would be 0,9%. Therefore, according to an embodiment of the invention, it is preferable that the maximum gain Gmax always is equal to or less than 2 or 3, i.e. Gmax < 2...3.

Thus, according to embodiments of the invention, the new installed flow characteristic of the valve may be checked against the above-described constrains. Typical problem may be that the gain G becomes too small at large openings of valve, and that the ratio of the maximum gain G_max and the minimum gain G_min exceeds 2. In that case the installed flow characteristic is corrected to remove the problem. If the absolute value of the maximum gain Gmax is exceeds 2...3, the safety marginal may be increased and it may be examined whether it is possible to reduce the gain.

The potential savings may be put into practice by, in addition to defining the new installed flow characteristics for the desired Δρ_Γ available, replacing the original pump with a new smaller pump, or more preferably, replac- ing or processing the impeller of the original pump to provide the required lower inlet pressure p1 or the pressure difference Ap_vaive new- This measure will preferably be made during a maintenance shutdown of the process.

Figure 1 1 illustrates another exemplary pump-valve system wherein embodiments of the present invention may be applied. In this system, the opening h of the control valve is measured as in the exemplary system of Figure 2. However, a pressure difference Δρ across the control valve in the pipeline is not measured, but a flow rate Q is obtained by the flow measurement on the material flow at a suitable point of the pipeline 25. This measured flow Q may be obtained from the flow indicator 30, for instance. The flow indicator 30 is preferably a flow indicator that already exists in the process, or it may be installed in the process for the purpose of the invention. The flow indicator 30 is preferably located after the valve 21 , but it may also be placed at a suitable point of the flow process before the valve 21 , as shown in Figure 1 1 . The pressure drop Δρ across the valve can then be calculated by using the C_v curve of the valve, the measured valve opening h and the measured flow Q. As the inlet and outlet pressures p1 and p2 are unknown, a pressure difference curve as a function of the measured flow rate Q can be provided, e.g. in a similar manner as in Figure 5. Then the analysis may proceed as described in connection with the above exemplary embodiments.

The data gathering and analysis is generally represented with a function block 27 in Figures 2 and 1 1 . The data gathering and analysis func- tionality 27 may be located at the valve positioner 22, at the management and diagnostics system 4, or at the automation system, or in a desired manner be distributed among these, the communication being performed over the field/process bus 3. The block 28 generally represents the functionality of stor- ing of the Cv curve, e.g. in a tabular form or in form of a mathematical equation. Also the block 28 may be located at the valve positioner 22, at the management and diagnostics system 4, or at the automation system.

The data gathering and/or analysis may generate a huge number of stored measurement results and/or calculation results and/or analysis results which must be stored somewhere. For example, a pair of measured Q and h are stored every second results in 3600 stored pair in an hour. In order to reduce the amount of data to be stored, a statistical method may be used for processing the information to be stored. Thereby, the entire life-cycle of the pump-valve system can be stored with limited memory capacity. This is espe- daily significant issue, when the data is primarily stored in the valve positioner. Let us examine an example wherein a flow distribution is presented as a function a valve opening distribution. The flow rate range (e.g. Q_min-Qfuii_y open) and the valve opening range (e.g. 0% - 100%) is subdivided into a number subranges of equal size. The crossconnections of the subranges establish op- erating points. In other words, a number of discrete operating points indexed by the flow rate Q and the valve opening h. This is schematically illustrated in Figure 12. In the illustrated example only five subranges are shown for both the flow rate Q and the valve opening h, resulting in 5 x 5 = 25 operating points (Q,h), but normally there are much higher number of subranges. However, any number of subranges and operating points may be defined depending on the solution. The numbers of Q and h subranges may also be different. Number of hits (n), i.e. number of matching measurements, is included in each operating point. In other words, a pair of Q and h measured at a specific moment increments the count n in the matching operating point (Q,h). Thus, the number of data entries and the required memory capacity do not increase with the time. Moreover, a pressure data, such as a measured or calculated pressure difference Δρ, a pressure reduction Δρ_Γ, etc. may be included in each operating point. In an embodiment of the invention, the pressure reduction Δρ_Γ is calculated and updated at intervals longer than the measuring intervals (e.g. at in- tervals ranging from one hour to days or weeks), and analyzed and reported. In an embodiment of the invention, the pressure reduction Δρ,- is calculated and updated only on demand.

Figure 13 is a simplified illustration of an example case wherein the calculated pressure reduction values Δρ_Γ are sorted, and then the sorted Δρ_Γ plotted as function the number n of hits giving each value of Δρ_Γ, and the resulting graph may be displayed to a user,. The same value of Δρ_Γ may be obtained in more than one operating point, meaning that the hit number of the value of Δρ_Γ may be a sum of hits n in more the one operating point.

Figure 13 is a simplified illustration of an example case wherein the calculated pressure reduction values Δρ_Γ are sorted, and then the sorted Δρ_Γ values are plotted as a function of a normalized number of hits giving each value of Δρ_Γ, and the resulting graph may be displayed to a user. The normalized number of hits may be obtained, for example by an equation 1 - (n)/sum(n), wherein n is number of hits for the specific value of Δρ_Γ, and sum(n) is the total number of all hits. The sorted graphs, like those illustrated in Figures 13 and 14 gives information which portion of process flows is covered with each particular pressure reduction values Δρ_Γ. For example, Δρ_Γ= 0,2 applies to 90% of flow situations. This information enables to select a Δρ_Γ which is used for sizing the new pump impeller, for example.

Embodiments of the invention may further automatically generate reports for impact of impeller replacements on pumping performance, suggestions for magnitudes of pump impeller replacements, calculation of energy and monetary savings of suggested pump impeller replacement.

Figure 15 illustrates another example of a graph obtained by sorting the Δρ_Γ values and plotting the sorted Δρ_Γ values as a function of a normalized number of hits (1 - (n)/sum(n)) calculated after the Δρ_Γ sorting. Figure 15 shows also the calculated monetary savings (in thousand euros per year) as a function of the Δρ_Γ.

The techniques described herein may be implemented by various means. For example, these techniques may be implemented in hardware (one or more devices), firmware (one or more devices), software (one or more modules), or combinations thereof. For a firmware or software, implementation can be through modules (e.g., procedures, functions, and so on) that perform the functions described herein. The software codes may be stored in any suitable, processor/computer-readable data storage medium(s) or memory unit(s) and executed by one or more processors/computers. The data storage medium or the memory unit may be implemented within the processor/computer or external to the processor/computer, in which case it can be communicatively coupled to the processor/computer via various means as is known in the art. Additionally, components of systems described herein may be rearranged and/or complimented by additional components in order to facilitate achieving the various aspects, goals, advantages, etc., described with regard thereto, and are not limited to the precise configurations set forth in a given figure, as will be appreciated by one skilled in the art.

The description and the related figures are only intended to illustrate the principles of the present invention by means of examples. Various alternative embodiments, variations and changes are obvious to a person skilled in the art on the basis of this description. The present invention is not intended to be limited to the examples described herein but the invention may vary within the scope and spirit of the appended claims.

Claims

1 . A method for diagnosing a pump-valve system, comprising storing an inherent valve flow coefficient (Cv) characteristic curve of a control valve in a memory,

2. A method for diagnosing a pump-valve system, comprising storing an inherent valve flow coefficient (Cv) characteristic curve of the a control valve in a memory,

determining, based the measured valve opening data and the measured flow rate data and the stored inherent Cv-curve, an pressure difference over the control valve as a function of a flow rate through the control valve, as well as an actual maximum flow rate through the control valve,

determining, based on the pressure difference and the actual maxi- mum flow rate, a potential reduction achievable in the online pressure difference, if a fully open flow rate of the control valve is resized to a value close to the actual maximum flow rate and a preceding pump is resized accordingly.

3. A method according to claim 1 or 2, comprising

the resized fully open flow rate of the control equals to the actual maximum flow rate plus a predetermined safety marginal.

4. A method according to claim 1 , 2 or 3, comprising

reporting the potential reduction of the online pressure difference via an user interface, such as graphical user interface, web-based user interface or voice user interface, or by email, short message service, multimedia mes- sage service, or by another messaging or data communication mechanism.

5. A method according to any one of claims 1 -4, comprising further calculating and reporting, via a graphical user interface, a potential saving in pumping energy and/or pumping cost with the potential reduction in the online pressure difference.

6. A method according to any one of claims 1 -5, comprising further calculating and reporting, via a graphical user interface, a recommended resizing of the pump to achieve the potential reduction in the online pressure difference, such as a recommend resizing of a pump propeller.

7. A method according to any one of claims 1 -6, comprising determining an installed flow characteristic of the control valve based on the stored inherent Cv-curve, the online-measured valve opening and the online-measured pressure difference or the online-measured flow rate.

8. A method according to any one of claims 1 -7, comprising determining a new installed flow characteristic of the control valve with the resized fully open flow rate and the reduced online pressure difference,

checking whether the gain meets predetermined constrains.

9. A method according to any one of claims 1 -8, comprising sorting the values of the determined potential reduction in the online pressure reduction values, and

10. A method according to any one of claims 1 -9, comprising defining a plurality of operating points indexed by two of the following parameters: a measured valve opening of the control valve; a measured pressure difference over the control valve; a measured flow through the control; a determined pressure difference over the control valve; a determined flow through the control valve; a determined potential pressure difference reduction over the control valve; and

1 1 . A valve positioner for operating a control valve, the valve positioner comprising means for implementing a method according to any one of claims 1 -10.

12. An automation system for controlling a process, the automation system comprising means for implementing a method according to any one of claims 1 -10.

13. A valve management system comprising means for implementing a method according to any one of claims 1 -10.

14. A system comprising a plurality of pump-valve systems installed in a process, an automation system for controlling the process, and a valve management system, the system further comprising means for implementing a method according to any one of claims 1 -10.

15. A computer program comprising program code means adapted to perform a method according to any one of claims 1 -10 when the program is run on a computer or a processor.