The present invention relates to a method and an apparatus for determining a power and energy saving, preferably achieved by installation of rotation speed regulation/speed control in rotodynamic machines. A typical example of an application comprises an industrial process plant, which for example comprises a piping system through which a gas, liquid, suspension, or the like is conveyed. The conveyed medium is forwarded and processed by means of rotodynamic machines comprising pumps, fans, compressor, agitators, etc., which in their turn are driven by electric motors, usually of an asynchronous type.
For a given such machine, provided with rotation speed control, the method determines the actual power consumption at a certain point in time and, for comparison, a corresponding simultaneous power consumption for the same machine in the same piping system, when it has no rotation speed control installed but would then have been regulated by an imaginary/fictitious regulation of another type.
The instantaneous power saving becomes the difference between the second and the first-mentioned power consumption. At equal and constant power saving over time, the energy saving is obtained as power saving multiplied by time. For the general case, the energy saving is obtained as the integral over time of the power sawing.
A large number of portable measuring equipments, so called measuring kits, for temporary determinations of volume flow, pressure and powers, are available on the market. These are primarily aimed at determining the efficiency of the machine or, in more general terms, its efficiency for the type of regulation being used during the measurement. The efficiency of the machine itself can be determined directly by a thermodynamic method, i.e. by determining the temperature increase of the medium during its passage through the machine. In certain cases, wireless transfer of certain signals, for example electrical current measured in a remotely located switchgear, is utilized. However, these measuring equipments only provide information about the required power of the machine, without any comparison being made with another technique or with another type of regulation, as is the case with the method according to the invention.
In the patent publication U.S. Pat. No. 4,208,171, there is disclosed a method for determining volume flow for a rotation speed-regulated pump, when rotation speed and certain defined pressure heads are known, so-called indirect flow measurement. This is based on characteristics in the so-called Q-H diagram, compare with
The document EP1548925 A1 describes a method for evaluating the energy consumption in an electric AC motor with associated control circuit, and a comparison is made between operation with and without rotation speed regulation. The energy consumption without speed regulation is calculated in a very rough way for the comparison, which results in considerable errors in the evaluation result
The power and energy saving achieved with a rotation speed regulation compared to another type of regulation has previously been rather unreliable and difficult to judge in size because of variations in, among other things, the facility, operation, etc., which are described further below. The unreliability has meant that concrete information on the economic benefit from introducing a rotation speed control for a given machine has not been possible to determine. One purpose of the invention is to enable a determination of power and/or energy saving with such accuracy that it can form a basis for financial settlements. These can, for instance, relate to an executed installation of a rotation speed control bringing about a certain saving relative to another type of regulation, possibly being used before this installation. This saving can, in its turn, form the basis for a settlement of the financing of the rotation speed control installation, for example by means of profit sharing between a plant owner, on the one hand, who has a machine (pump etc. and an electric motor) in operation in his/her plant, and a supplier of the rotation speed control equipment, on the other hand. In certain cases, the supplier can be replaced by a third party, for example an electricity supplier or a bank, acting as a financier.
The method according to the invention is of particularly great importance in, for example the process industry, where initial investments in its core business are prioritized, whereas margin investments, for example energy saving by rotation speed control normally will not be realized. However, the profitability is so high that other types of financing, for example with profit sharing as exemplified above, are interesting.
One advantage of the invention lies in the fact that, by means of an instantaneous measurement and evaluation, a considerably greater accuracy and actuality is achieved compared to calculations based on a theoretical/ideal condition prevailing, for example, during the projecting, or compared to simple estimates performed at a certain point in time. Since the realisation of a plant, a piping system may, for example, have been changed with regard to, for example, pipe diameter, piping length, included fittings such as bends, valves, as well as piping quality in the form of welds, hanging gaskets, etc. Furthermore, also the internal condition of a pipe with deposits, corrosion damages and, for example, bright polishing of its inner surface, has an influence. At a prevailing temperature, the instantaneous characteristics of a medium (a liquid/a gas) such as density, viscosity and composition, for example consistency of paper pulp suspensions, have an influence. In liquid lines partially filled with gas or air may additional flow resistances of an unknown magnitude arise. In long pipelines, intended for fluid transportation of different liquids in the form of “plugs” with, for example, different liquid density, both the generation of pressure by a pump and the pressure loss in a pipeline will depend on the instantaneous position of the liquid plugs. The corresponding of course applies to the required power. In conventional measurements, wear in machines results in more unreliability. For conventional methods, another complication is added when a worn-out machine is reconditioned. The continuous adaptation to the actual situation, according to the invention, with associated measurements/evaluations, is of an extremely great importance during the periods a financial settlement can last for, which usually means several years.
By means of the invention, a good accuracy is obtained because only the main parameters of influence are studied and utilized for a saving determination. According to the invention, in a preferred embodiment, these are constituted of shaft rotation speed, instantaneous power at this rotation speed, the basic power curve of the machine as a function of the volume flow, and the nature and magnitude of internal losses in prime mover and rotation speed control. By means of the invention, the need for extensive measurements of volume flow, pressure heads, powers, and the characteristics of the medium are eliminated. Further aspects of and advantages with the invention are evident from the following description in connection with exemplifying embodiments according to the invention.
In the following, the invention will be described in greater detail with reference to the accompanying figures of embodiments according to the invention, in which
In many process systems, a typical system curve,
Rotodynamic machines have their instantaneous required power measured on their drive shaft almost exactly proportional to the third power of the machine rotation speed according to the so-called affinity laws. They apply to each individual performance point (Q, H and P), when a flow condition in the machine has equal facing angles for this point. The laws have a term n for a relative shaft rotation speed, wherein n=1 is a full rotation speed:
The pressure head H is common in pumps. The corresponding applies to fans, if H is replaced by a pressure increase, and for compressors, by an enthalpy increase. In certain cases, it may be convenient to replace the volume flow with the mass flow constituting the product of volume flow and the density of the medium.
Deviations from the affinity law for power amounting up to one or a few percent of the power at full rotation speed are constituted of certain machine elements included in the machine, such as shaft bearings and shaft seals, the required powers of which, as a rule, are linearly dependent on the rotation speed. Accordingly, at a known rotation speed, an instantaneous required power can be determined relative to a relevant power at full rotation speed, “a full speed power”.
The invention is, inter alia, characterized in that, in a first stage, shaft rotation speed and required power at this rotation speed are measured for a rotodynamic machine having an already installed rotation speed control, and that, in a second stage, a power saving is calculated compared to the same machine provided with a fictitious, but conceivable, other type of regulation for the same system, when the machine operates at a constant rotation speed/“full rotation speed”,
As an application, in an upper part
In an imaginary throttle regulation, i.e. by throttling/changing pressure with a regulating valve connected in series with the machine, the volume flow is regulated to, for example, a value Q1 at a constant speed n=1 with the required power P2. During rotation speed regulation at the speed n with the same volume flow Q1, a required power P1, which is measurable on the machine shaft, is obtained. The instantaneous power saving ΔP as counted on the machine shaft amounts to P2-P1. The power P0 is determined from the affinity laws (or the same corrected for sealing and bearing friction), from the measured power P1.
P0=P1/n3 and Q1=n·Q0 [1]
In principle, in order to determine a correction P0-P2, a representative reference point Qref, Pref on the power curve at full rotation speed has to be known, as well as the factor for the actual slope kp of the power curve. A simple proportioning yields:
P0=[Q0/Qref·(1−kp)+kp]·Pref and [2a]
P2=[Q1/Qref·(1−kp)+kp]·Pref [2b]
A combination of the equations 1 and 2 will now yield the power P2
P2=n·(P1/n3−kp·Pref)+kp·Pref [3]
Since Pref can be chosen freely in this case, kp·Pref in
The saving related to the machine shaft is then obtained as:
ΔP=P2−P1 [4]
It is appreciated that similar relationships can be postulated for an imaginary valve regulation with a regulating valve connected in parallel with a machine, a so-called shunt regulation.
With the same desired volume flow Q1 and with an imaginary so-called overflow regulation, where the liquid which is not needed can flow over a brim and then be directed away or back to a suction source, slightly more complicated relationships are obtained. The volume flow at full rotation speed n=1 becomes Q0′ and the required power on the pump shaft P0′. Equation 2 is applied to the values Q0, P0, on the one hand, and to Q0′, P0′, on the other hand. After elimination of Qref, this yields:
Q0/Q0′=(P1−kp·Pref)/(P0′−kp·Pref) [5]
For the unknown volume flow Q0′, the previous relationship for indirect measurement of volume flow is used, with the term SQRT[. . . ] for the square root:
Q1=Q0·n=SQRT [(n2−kH)/(1−kH)]·Q0′ [6]
If Q0/Q0′ is eliminated between equations 5 and 6, the sought-after power P0′ is obtained:
P0′=n·(P1/n3−kp·Pref)·SQRT [(1−kH)/(n2−kH)]+kp·Pref [7]
The power saving will then become: ΔP′=P0′−P1.
For a fully unregulated volume flow, as is usually the case when circulating a medium, relationships analogous to overflow regulation apply. Powers are obtained from equations 6 and 7, if kH is set equal to zero.
For the regulation of a machine driven at a constant rotation speed n=1 with on/off regulation (intermittent operation) it applies that, when the machine is in operation, it delivers a volume flow Q0′ and has a required power P0′,
P0″=Q1/Q0′·P0′ [8a]
ΔP″=P0″−P1 [8b]
With Q0/Q0′ from Equation 6, and P0′ from Equation 7, the energy saving is determined in equation 8b. In
In those cases where the slope factor kp of the power curve can be regarded as small or 0, i.e. P2=P0=Pref, or where the level and pressure difference in a system is 0 (Hstat=0 and hence kH=0,
P0=P1/n3 and ΔP=P0−P1 and P0′=P1/n3 ΔP=P0′−P1 resp. [9]
For a refined calculation of the influence of the shaft rotation speed, the bearing and seal friction's relative share of the power at full rotation speed is set to ΔL. The term n3 in equation 1, 3, 7 and 9 should then be replaced with (1−ΔL)·n3+ΔL·n, where ΔL is usually of the magnitude 0.01-0.03 with the largest values for smaller machines.
Equation 2a contains the 3 practically estimated but, from a mathematical point of view, unknown quantities Q0/Qref, Pref and kp, where kp certainly can be estimated rather well from experience values. Provided that an operating condition of a machine varies somewhat over time when volume flow and rotation speed are concerned, the unknown values can gradually be refined through a moving adaptive method in that measured values for speed n and power P0(=P1/n3) are saved for at least 3 different measurement points and that an equation system based thereon is solved. Furthermore, the product kp·Pref in Equation 3 can be determined directly at the flow Q1 and Q0 equal to zero. If the straight line has been replaced with a polynomial, as mentioned previously, coefficients in the polynomial can also be refined with a moving adaptive method.
In the general case, the adaptive method is advantageously based on selecting Q0′ and P0′, respectively, as Qref, Pref. The coefficient kp will then assume a different value kp′. Equation 2 will then become:
P0/P0′=Q0/Q0′·(1−kp′)+kp′ [10]
For Q0/Q0′, the method for indirect measurement according to equation 6, which is inserted into equation 10, is used:
P0/P0′=1/n·SQRT[(1−n2)/(1−kH)]·(1−kp′)+kp′ [11]
In this equation, for one operating point, there are known values from measurement for n and P0=P1/n3. The quantities P0′, kH and kp′ are to be considered as unknown. If kH à priori is considered to be constant, 3 pairs of measurement values (n, P1) for 3 different operating points are sufficient. If Hstat (
In case of large variations in the density of the medium and kH deviating from zero, such as, for example, for thermal updraught in a chimney (chimney draught) enhanced by a flue gas fan, an additional refinement is obtained if the medium temperature is measured and the reference power Pref, P0′ is adjusted accordingly in a way known per se.
The rotation speed of the machine shaft can be measured with several known methods, such as with a tachometer generator, with optical or inductive methods with indication from a black/white disc or from a grooved metal plate, respectively. For rotation speed regulation with a frequency changer, the rotation speed follows the frequency with a small deviation depending of the slip of the electric motor. When the frequency/voltage ratio (type F1) from the frequency changer is kept constant, the slip is proportional to the torque requirement of the machine and enables a corresponding correction of rotation speed. For machines with a so-called quadratic torque, to which rotodynamic machines on the whole belong, the voltage is sometimes lowered more than the frequency (type F2), so that the percentage slip becomes constant at all rotation speeds. Deviations from this are determined by the output power from the electric motor relative to its power rating, and by the slope of the machine's power curve,
A rotodynamic machine 1 and a rotation speed control 3, 3′ can be differently arranged with respect to an electric motor 2, 2′,
The measurement of the machine shaft power between the machine 1 and the motor 2, or between the machine 1 and the rotation speed control 3′, can take place directly by measuring torque in the shaft. The power is then obtained as torque times angular speed. The electric motor's input power, measured immediately before the motor 2, is measured indirectly, which power after reducing the power losses of the motor and the rotation speed control yields the shaft power. The input power to an electric motor 2, 2′ can be measured by the so-called 2 watt meter method or by measuring electrical current. The current measurement is very simple to perform, since only a current transformer, designed as a simple wire coiled around one of the connecting leads/phases of the electric motor, is needed. For power calculation (active power) a power factor (cosφ) must be known. This is specified in electric motor catalogues. The reactive power=the electric motor's power rating·tanφ is fairly constant at different active power outputs, since it depends on the magnetization of the electric motor. This relationship can advantageously be utilized for the power determination.
The same principle can be applied to an indirect measurement before a rotation speed control 3, for example designed as a frequency changer.
Power losses dP2, dP2′, dP3 and dP3′ are calculated in a way known per se and are used, on the one hand, for correction of (indirectly) measured power, if this is not done on the machine shaft and, on the other hand, for determining the required powers from electricity supply systems for the powers which have been determined, i.e. measured and/or calculated, to be valid for the machine shaft. For power, the affinity laws apply strictly to the machine shaft, while the costs are determined solely by the power from the grid 4, 4′. When some or all of the losses in the components (2, 2′, 3, 3′) are small, these two powers can replace each other without practical complications, in all conceivable and reasonable combinations.
The invention is further characterized in that measurements are repeated at least once for each studied period which forms the basis for a financial settlement. Advantageously, time intervals are based on statistical methods with, for example, about 30 measurements for each period. In the general case, measurement takes place with time intervals adapted to the nature of an actual process. Practically, these can vary from parts of a second in, for example, a rapid combustion process, to parts of a day in a sedimentation process. Advantageously, in the normal case, time intervals of the order of 10 seconds to 1 hour are chosen.
An apparatus according to the invention is characterized in input units for receiving measurable values during operation for the machine rotation speed and for directly or indirectly measurable power. Furthermore, there are input units for selection of an imaginary/fictitious type of other regulation, selection of parameters for the shape of the power curve, kp and Pref, and for the hydraulic system parameter kH, as well as for general information describing the specific characteristics of the machine, motor and rotation speed control. If the power curve is expressed by a non-rectilinear curve, coefficients describing it will be added to the input unit. The apparatus further has a timing unit indicating running time, a calculating unit, and a storing unit for the data from the input units, for time, and for calculated results. The results can be displayed directly in an output unit, alternatively be transferred by wire or wirelessly to a receiver. The units are interconnected in a way known per se, and are driven by some form of auxiliary power. Advantageously, a data processor programmed for the above-mentioned technique and based on electronic circuits is used.
Measurement must of course take place during operation, whereas calculations can take place both during operation as well as later. The calculation does not have to take place in the same local device (computer) but can, after transfer of time and measured values to a central computer, take place therein. The power for an imaginary/fictitious regulation method will then be valid for the same point in time as the measurement, in spite of the fact that the calculation does not take place simultaneously.
In its broadest sense, the invention can be varied within the stated principles. Accordingly, measurement/determination of power can take place indirectly with a somewhat lower accuracy from Q-H-curves, upper part of
Number | Date | Country | Kind |
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0801871-5 | Aug 2008 | SE | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/SE2009/050977 | 8/28/2009 | WO | 00 | 3/4/2011 |