1. Introduction

The vibration level of pump units is an important parameter for predicting their service life. It is also an important criterion for assessing the quality of the pump. For the above reasons, among others, the level of vibrations should be taken into account as one of the factors influencing the selection of the pump unit. If the pump unit is purchased based on a technical requirements specification, the permissible vibration level should be specified in this document. Whether such a situation occurs depends largely on the industry for which the pump unit is intended and the technical standards applicable there. It can be said that habits and traditions play a significant role in this respect. In some industries, almost no attention is paid to the issue of vibration levels. The opposite situation occurs, for example, in the energy and petrochemical industries, where significant importance is attached to vibrations. Most specifications of technical requirements for power pumps specify requirements regarding the level of vibrations, which should be assessed positively. However, there are situations when the ordering party arbitrarily imposes excessive requirements.

This text is devoted to discussing the vibration requirements that should be formulated at the stage of selecting and purchasing pump units.

2. Parameters used to assess vibrations of pump units.

Various vibration parameters can be measured, but the root mean square value of the vibration velocity expressed in mm/s is commonly used as the basis for assessing the dynamic state of the machine. It is a physical quantity related to the kinetic energy of vibrations. In accordance with the PN-ISO 10816-1 standard, measurements are typically made on non-rotating bearing housings, in two directions mutually perpendicular and perpendicular to the shaft.

In the vast majority of cases, the vibration speed is a sufficient criterion for the dynamic assessment of the pump unit at the stage of selection and acceptance tests.

Other parameters such as: vibration displacement, vibration acceleration, relative vibrations of shafts towards non-rotating elements, spectral analysis are a valuable diagnostic tool, but their inclusion in acceptance tests seems unnecessary, except in special cases, as they increase the cost of testing.

3. Vibrations as a physical phenomenon

Pump vibrations, from the point of view of physics, are a mechanism for converting energy from the drive engine into the kinetic energy of the oscillatory movement of pump elements, which is then dissipated as heat due to internal damping in the material. In a pump, this is a side effect, unintentional and unfavorable, because the energy from the drive should be transferred to the liquid. There is no room in this text for a broad description of the vibration theory, which has extensive literature, e.g. [1, 2]. In order to justify the conclusions to be drawn from this article, we will only mention here that the level of vibrations depends on the intensity of excitations, i.e. the mechanisms by which vibration energy is transferred, and on the stiffness, natural frequency and damping capacity of the pump structure and its foundation.

Factors forcing vibrations can be divided into two basic groups. The first of them are mechanical forces. These include factors such as unbalance of the rotating unit, misalignment of the pump and motor, interaction of the impeller blades with the spiral tongue or guide vanes. The manufacturer of the pump unit has full control over the factors from this group, whose responsibilities include the design and construction of the pump so that these forces are at an appropriately low intensity level. It should be noted that full elimination of forces of this nature is not possible, because, for example, the influence of the centrifugal force acting on the rotating unit cannot be completely avoided, because even with perfect balance, some centrifugal force will occur as a result of the deflection of the shaft, which never has an infinitely large stiffness. Similarly, when rotors with a finite number of blades are used, their interaction with non-rotating elements cannot be fully eliminated.

The second group are hydraulic forces. This includes vibrations generated by cavitation and as a result of the pump operating away from optimal performance. In the latter case, vortices (recirculation flows) are created in the flow system, absorbing energy from the drive and transferring it to generate vibrations. While in the case of mechanically forced vibrations their frequency is specific (usually rotational or vane), in the case of hydraulic forced vibrations the vibrations have the nature of noise without a clearly dominant frequency. Vibrations generated by recirculation flows usually occur in a frequency band clearly lower than the rotational one. Avoiding recirculation flows and the hydraulic forcing they cause is virtually impossible when operating far from nominal parameters. For this reason, the manufacturer (designer) of the pump may be required to maintain a certain level of vibration in the performance range close to the nominal one. However, when the pump operates outside this range, some increase in the vibration level is inevitable. This statement is confirmed in the literature. As an illustration of it in Fig. 1 taken from [3, 4] the vibration speed level increases as performance moves away from optimal.

Fig. 1. Increase in the vibration level for various types of pumps when operating with a performance different from the optimal one.

In terms of the pump structure's ability to dampen vibrations, the manufacturer (designer) is responsible for ensuring that the pump structure has stiffness and vibration damping capacity appropriate to the expected excitations. In particular, the pump structure cannot have natural frequencies corresponding to the expected excitation frequencies (and therefore primarily the rotational and blade frequencies), and the critical rotational speeds (i.e. those at which the shaft deflection arrow increases due to centrifugal force) must lie far from rotational speed. (usually they should differ by at least 20%). However, the pump manufacturer does not have full influence on the structure of the pump unit's workstation, which is often designed by a separate designer. There are known situations when the pipeline system exhibited natural frequencies close to the pump's rotational frequency. There are also known cases of vibrations being generated by hydraulic forces generated on valves. The classic method of installing the pump unit on a solid concrete foundation usually ensured adequate vibration damping ability. However, in the case of economical installation of pump units, e.g. on steel structures, the stiffness of such support may turn out to be insufficient, as a result of which the level of vibrations of pump units that worked stably on concrete foundations increases excessively with this method of installation.

Adjusting the pump parameters by changing the rotational speed over a wide range creates a probability, bordering on certainty, that one of the natural frequencies of the pump unit structure will be encountered, which leads to resonance.

It follows that if the manufacturer of the pump unit is to be responsible for the level of vibrations at the workstation, he should at least be able to provide an opinion on the design of the workstation so as not to be liable for design errors resulting in an increase in vibrations. Acceptance tests in accordance with EN-ISO 9906 standard should take place at the manufacturer's factory station because there are optimal conditions for measuring hydraulic and energy parameters. This statement, however, does not apply to vibration measurement, because factory test stands are provisional and often do not have adequate support stiffness. IN [3, 4] it is stated that the expected vibration level at a properly designed workplace is up to 2 mm/s lower than at a factory test stand. The conclusion is that if the required vibration level is confirmed during tests at the factory workstation and an increase is observed at the target workplace, it should be assumed that the target workplace is defectively designed.

4. Standards regarding vibration levels

In the current legal system, the use of standards is not mandatory. However, since these are documents based on good technical practice, it is advisable to use them to agree the requirements between the manufacturer and the user.
Data on permissible vibration levels can be found in several standards. Containing general technical requirements standard [5] PN-EN ISO 5199 divides pumps into machines with rigid and flexible support and provides permissible vibration levels for pump axis height h below and above 225 mm. The pump is considered to be a rigidly supported pump if the lowest natural frequency is at least 25% higher than the rotation frequency.

According to the author, this approach is questionable because it sanctions an increase in the vibration level for the so-called pumps with flexible support, i.e. those for which the critical speeds are close to the rotational speed, which is not a safe design practice. Moreover, making the permissible vibration level dependent only on the height of the axle, without taking into account other factors, seems to be too far-reaching a simplification.

For the above reasons, it is recommended to use the standards discussed below.

Standard [7] API 610 is commonly used in the petrochemical industry. According to the author, due to the fact that it is developed at a high technical level, it also deserves to be taken into account in other industries. This standard for horizontal pumps with a rotational speed of up to 3600 rpm and a power of up to 300 kW sets the permissible vibration level of 3.0 mm/s. For higher rotational speeds and power per stage, it allows for an increase in the vibration level (according to the graph depending on power and rotational speed), but not more than 4.5 mm/s. For vertical pumps the limit is set at 5.0 mm/s. What is important API 610 standard states that the limits given above should be met in the area of ​​optimal pump operation (i.e. close to the nominal capacity), and in the remaining part of the permissible operating area there may be an increase in vibration by 30%.

The standard is entirely devoted to the issue of vibration standard [5] PN-ISO 10816. It should be treated as the primary source of guidelines regarding vibration levels. It should be noted that this standard consists of seven parts, and the most important part is for the criteria of acceptance tests part of PN-ISO 10816-1 containing general guidelines and part of PN-ISO 10816-7, containing specific criteria for pumps that are less stringent than the general criteria in parts 1 valid for other rotating machines. It should be assumed that the establishment of less stringent criteria for pumps was intended by the authors of the standard to take into account the fact that pumps are inevitably characterized by the above-mentioned hydraulic forces, which do not occur in other machines.

In Part 7 of ISO 10816, pumps are divided into two categories:

Category I – pumps for which a high level of reliability and safety is required (e.g. pumps for hazardous media, for critical applications, etc.)

Category II – pumps for general and less critical applications

The standard provides separate vibration limits for the above categories. In addition, the limits set for each category are divided into four zones:

Zone A – vibrations for pumps immediately after commissioning

Zone B – vibrations acceptable in long-term operation without time limits

Zone C – vibrations considered unacceptable in long-term operation, but do not require switching off the pump yet. Pumps can operate in this zone for some time, but entering it requires immediate planning of remedial measures (e.g. renovation).

Zone D – unacceptable vibrations which may result in pump failure. Entering this zone should result in turning off the machine.

For the above-mentioned categories and zones, the standard provides the following vibration value limits:

The above limits are set for pumps with impellers with 3 blades and more, which reflects the fact that higher vibration levels should be expected (and accepted) for two- or single-blade impellers.

ISO 10816-7 standard in chapter 3.4 further states that vibration limits should be maintained within the optimal capacity range (generally 70% to 120% of the capacity at which the highest efficiency occurs), while outside this range vibration levels may increase due to increased hydraulic excitations. The standard contains a chart illustrating this, consistent with an analogous chart in the API 610 standard.

5. Realistic requirements

As stated above ISO 10816 provides for a rational and flexible approach allowing the manufacturer and user to agree on appropriate vibration limits for individual cases based on engineering considerations. The intention of the authors of the standard is to take into account the fact that in certain physical conditions there are objective, physical reasons for an increase in the level of vibrations. Attempting to keep vibration limits at the lowest level in these conditions would require the use of excessively heavy, stiff pump structures, which is not technically and economically justified.

In practice, there are cases where the authors of technical requirements specifications for pumps impose vibration requirements that go further than ISO 10816 often taking advantage of the contracting authority's dominant position over the bidder. The desire of users to achieve the lowest possible vibration levels is understandable, but this cannot be done without understanding the objective technical premises that are the basis of the engineering approach contained in ISO 10816 standard.

Typical examples of excessive tightening of requirements included in tender specifications include the following:

1. Requiring vibration limits z parts 10816-1 regarding other rotating machinery instead of the limits z parts 10816-7 developed especially for pumps. This does not take into account the fact that pumps have special types of excitations (e.g. hydraulic) that are not present in other rotating machines, as a result of which pump vibrations are characterized by a higher level.

2. Treating all pumps as critical from category I, which is also inconsistent with the philosophy of the standard, which assumes rational adaptation of requirements to the situation.

3. Requirement to maintain vibration in zone A for a multi-year warranty period. Zone A applies to pumps in perfect technical condition immediately after startup. During operation, this condition gradually deteriorates, e.g. due to wear of the rotors, resulting in deterioration of the balance of the rotating unit, or due to increasing bearing clearances. These facts were taken into account in the standard by assuming that they should remain in operation at zone B. Therefore, the ordering party has the right to require that during acceptance tests the limits be obtained zone A but should take into account that during operation the vibrations may increase to the permissible values zone B.

4. Requirement to maintain vibration limits within a specific zone throughout the entire pump parameter control range. In some pumping systems, the required capacity range is well beyond the range considered optimal (70% to 120% Q_n). Both ISO 10816 standard, and 610 API based on rational engineering premises, they state that outside this range, an increase in the level of vibrations should be accepted, in 610 API specified at 30% of the recommended values ​​in the optimal performance range. It is therefore technically unreasonable to expect that a pump operating, for example, at 25% of its nominal capacity will not show an increase in vibration levels.

5. Requiring the supplier of the pump unit to take responsibility for the level of vibrations at the workplace, the design of which the supplier had no influence on.

In the long run, setting excessive requirements as above, inconsistent with rational assumptions ISO 10816 standards, may force pump manufacturers to design structures that are excessively stiff, heavy and expensive, which would not be economically justified.

6. Summary and conclusions

• Determining the requirements for the vibration level of pump units in the technical requirements specifications constituting the basis for the purchase is a recommended practice, leading to the purchase of pumps of appropriate quality and durability.
• The following are recommended as appropriate standards defining the method of measurement and acceptance criteria Normy. ISO 10816 and API 610.
• The optimal practice is to use the flexible approach from the ISO 10816 standard, which allows for rational adaptation of requirements to the situation. Examples of imposing stricter requirements, such as those discussed at the end of section 5, are not consistent with sound engineering practice contained in the standard.
• If the supplier of the pumping unit is required to guarantee a specific level of vibrations at the workstation, he or she should have the right to at least provide an opinion on the design of the workstation in order to indicate any design errors.
• If acceptance measurements take place at a factory test stand, the vibration levels found there may be higher than at the target workplace due to the makeshift nature and lower stiffness of the structure at the test stand. Possible exceedance of the permissible vibration level at the factory workstation (by no more than 2 mm/s) should not be a basis for rejecting the pumps, as a drop in the vibration level should be expected at a properly designed target workstation and the final verification should take place there. However, if an increase in the vibration level is found at the target station compared to the factory station, it is presumed that the station is incorrectly designed.

Dr. Eng. Grzegorz Pakula

Literatura:

1. Gryboś R., Machine vibrations, Silesian University of Technology Publishing House, Gliwice, 2009
2. Morel J., Vibrations of machines and diagnostics of their technical condition, Polish edition: Polish Society of Technical Diagnostics, French edition: Directions des estudes et recherches d'electricite de France, 1992
3. Operating rotodynamic pumps away from design conditions, Eurpopump, Elsevier Science Ltd, 2000
4. European Association of Pump Manufacturers, Guide to forecasting the vibrations of centrifugal pumps, Europump publications, 1992

Standards:

5. PN-ISO 10816, parts 1-7. Assessment of machine vibrations based on measurements on non-rotating parts,

6. PN-EN ISO 5199, Technical requirements for centrifugal pumps, Class II

7. API 610 Centrifugal Pumps for the Oil, Petrochemical and Gas Industries, 11th Edition

In recent years, preparations for covering the surface of a flow system have appeared on the market. They are advertised as a method to improve the energy efficiency of pumps and increase their wear resistance. As part of marketing efforts, it is suggested that pump efficiency can be increased by up to several percent.

The fact is that the hydraulic efficiency of the pump depends on the surface roughness of the flow system. Excessive roughness increases the friction losses of the liquid against the walls, which reduces efficiency. In pump technology, casting technology is most often used to produce the essential elements of the flow system. Depending on the type of technology (sand molding, die casting, etc.), the type of material being cast and the care taken to make the casting, a roughness of several to several dozen μm is achieved. The final surface quality of the casting is achieved by cleaning it. The simplest method is sandblasting, but other technologies are also used, such as pumping a corundum suspension through the rotor for several hours. It is obvious that the quality of the casting surface is related to the technology used and therefore affects the price. Covering the casting surface with preparations that improve smoothness can be considered as a method to eliminate the roughness of a casting made using cheap technologies.

In order to assess the possibility of assessing the improvement in pump efficiency as a result of coating its elements with preparations improving smoothness, our company carried out a number of experiments consisting in measuring the efficiency of various types of pumps made using traditional foundry technology, and then repeating the measurements for pumps with coatings offered by various manufacturers. The result was an improvement in efficiency of 1.5 to 4% compared to a sandblasted medium-quality iron casting. It should be noted, for comparison, that painting the casting with ordinary anti-corrosion paint improved the efficiency by approximately 1%. Due to the fact that coating pump elements with preparations improving smoothness is not a cheap procedure, an experiment was also carried out in which individual elements of a single-stage pump with a collecting spiral were successively covered with preparations in order to estimate the impact of coating a given element on the efficiency. The active sides of the blades, the outer rotor discs, the entire rotor and finally the inner surface of the spiral were covered. The total increase in efficiency reached approximately 3%, and this effect increased approximately evenly as subsequent elements were covered. The tested pump was a medium-speed pump with a speed factor of 35. Based on theoretical considerations, it can be concluded that for slow-speed pumps (with lower speed factors), the greatest benefits can be obtained from covering the impeller, and for high-speed pumps from covering the liquid discharge elements (spiral, guide vane). ).

 Another issue that raises doubts in the case of this type of preparations is their susceptibility to falling off the pump surface during operation. This depends, of course, on compliance with the coating technology recommended by the manufacturer, and in particular on the quality of the casting preparation. Experiments were carried out in which the manufacturer entrusted the pump's interior with the preparations to ensure that the treatment was performed in the recommended manner, and then the pump was put into normal operation at the user's place. The pump was later inspected to assess the condition of the coverage. Such inspections were carried out after a period of operation ranging from several hundred to several thousand hours. Each time, larger or smaller losses in the covering were found.

In summary, based on the collected experience, it can be stated that as a result of covering the surface with preparations that increase the smoothness of the surface of a new pump made using typical technology, an improvement in efficiency of 2-4% can be expected. The durability of this type of coatings during operation is not fully satisfactory. Probably, the use of coatings for worn-out pumps with severely degraded surfaces of the flow system may bring higher results in the form of improved efficiency, but the durability of the preparations' adhesion to such damaged surfaces is even more questionable.

Dr. Eng. Grzegorz Pakula      

Minimum power consumption required to pump a liquid of specific gravity γ [N/m³] with efficiency Q [m³/s] at lifting height H [m] is the so-called useful power, which we calculate from the formula:

N_u = γ QH [W].

The power consumption would be equal to the useful power if pumping took place without any losses, with 100% efficiency, that is, if all the energy taken from the power source was transferred to the liquid. In reality, such a situation never occurs because the pump unit consisting of a pump and a motor (usually electric) operates with an efficiency of less than one hundred percent. Electric motor efficiency η_s is the percentage of power taken from the network transferred on the shaft to the pump, and the efficiency of the pump η_p is the percentage of power absorbed on the shaft by the pump from the engine, transferred to the pumped liquid. The efficiency of the pumping unit is the product of the pump efficiency and the engine efficiency, and the actual power consumption for pumping is:

            N = γ QH / ( η_s η_p).

It follows directly from this that in order to reduce energy consumption, pumps and motors with the highest possible efficiency should be used. These efficiencies vary for individual types of pumps and motors, but you should be aware that the possibility of achieving significant energy savings by replacing a properly selected pump or motor with a more modern machine is limited. In recent years, new designs of energy-saving motors have been developed. The increase in efficiency compared to older designs is at the level of 2-3 percent. A similar level of benefits can be obtained by replacing an older pump with a more modern one. In total, by replacing the pump unit with a more modern one, you can achieve an increase in efficiency not exceeding 5%. We are talking here about a comparison of the efficiency of a modern pump unit with a pump unit of an older design but in good technical condition, i.e. possible benefits resulting only from progress in machine construction.

Of course, higher energy savings can be achieved by replacing the worn-out pump unit with a new one, because the efficiency of engines, and especially pumps, decreases due to the deterioration of their technical condition. The decrease in pump efficiency resulting from its poor technical condition may exceed ten percent. However, please remember that properly carried out renovation allows you to restore the initial efficiency of the pump. Basing the economic calculation on the estimation of savings that can be achieved by replacing an old pump with a modern pump is not entirely correct, because the efficiency of the latter can be improved through renovation, which is usually less expensive than purchasing a new pump, especially since the installation of a new pump often requires additional expenditure on changing the foundations and pipeline system. Before deciding to replace the pump with a new one, you should consider renovating the existing pump as an alternative, which, depending on the quality of workmanship and the technical condition of the pump, allows you to improve efficiency by several to even a dozen or so percent.

Even greater energy savings can be achieved by eliminating errors in the selection of pumps. These errors may concern both the pump type and its parameters.

            Examples of using the wrong type of pump include:

a) Using self-priming pumps where it is not necessary. Self-priming pumps enable start-up without the hassle of venting and pouring liquid, but due to their design, they usually have lower efficiency compared to typical pumps. They should therefore not be used where self-priming is not necessary.

b)  The use of sewage pumps with the so-called free-flow impellers resistant to clogging with solid bodies, in places where such a phenomenon is not likely. Free-flow impellers enable pumping sewage containing fibrous bodies or other large solids, but they have reduced efficiency. Their use for pumping pre-treated sewage, when impeller clogging is unlikely, leads to energy losses.

c)  Use of submersible pumps in applications other than deep wells. Submersible pumps powered by wet engines, thanks to their design enabling long-term, reliable operation without maintenance, are perfectly suited for use in wells. However, it should be remembered that wet submersible engines have energy efficiency reduced by several percent compared to traditional dry engines. Therefore, the use of deep-well pumps outside wells (e.g. in the so-called water jackets) increases energy losses.

d) Using several smaller pumps connected in parallel instead of one larger one. The achievable efficiency of a pump generally increases with its efficiency. Therefore, using several smaller pumps instead of one larger one, unless it is necessary, e.g. due to the need for regulation, reduces the pumping efficiency.

e) Use of single-stage pumps with low efficiency. It is not possible to build a high-efficiency pump if its head is too high in relation to its capacity. In such cases, multistage pumps are used to achieve better efficiency, but have a higher price. The use of single-stage pumps in such a case allows for some savings on the purchase cost, but leads to an increase in energy consumption during operation.

You should be aware that using a pump with a high catalog maximum efficiency does not mean that the pump will operate with such efficiency. The efficiency at the operating point depends on the selection of the pump for the pumping system. If this selection is incorrect, i.e. if the pump operates at a capacity significantly different from the nominal one, the actual efficiency may be much lower than the maximum. Practice shows that we often encounter such a situation. This is not always the result of an error in pump selection. Often the selection was correct, but the required parameters changed, for example the pump operates with lower efficiency than expected due to a drop in water consumption. If the pump has parameters (efficiency and lifting height) higher than required, it can be better adapted to the system, and thus improve energy consumption rates, by reducing the impeller diameter or reducing the rotational speed.

The use of an appropriate control method in cooperation with a pump in a pumping system in which the required parameters change provides great opportunities for saving energy consumption. This is a typical situation for water supply networks in which water demand fluctuates significantly during the day. In such a case, the pump is selected for maximum parameters, and in periods when the demand decreases, a certain regulation method should be used. The simplest of them is throttling the pump with a valve in the discharge pipeline. Compared to this method, significant energy savings can generally be achieved by using speed control. The effects of using this method are greater the greater the share of flow losses in the pump lifting height and the greater the range of efficiency fluctuations. Choosing the right parameter adjustment allows you to achieve energy savings of up to several dozen percent.

So far, we have discussed the possibilities of reducing energy consumption by improving pumping efficiency, assuming that the parameters (capacity Q and lifting height H) appearing in the formula we have no influence on power consumption. This assumption is not always correct. For example, the required pump head includes both the geometric head (on which we have little influence) and the amount of flow losses. By reducing losses, we can lower the pump head and, according to the formula given at the beginning, reduce power consumption even with an unchanged level of efficiency. The diameter of the cables has a strong influence on the amount of losses. By eliminating sections of the network with too small a diameter or unnecessary fittings causing losses (elbows, reducers), we can reduce the required pump lifting height. A similar effect can be achieved by using zones of different pressure in the network. If the entire network is powered by one pump, its lifting height results from the need to provide the required pressure at the most distant or highest point of the network. The pump's lifting height can be reduced if it supplies only those parts of the network where lower pressure is required, and increased pressure at specific points is achieved by smaller zone pumping stations. Overall, this leads to lower energy consumption.

When lowering the required pump lifting height by optimizing the pump system, we must remember to adapt the pump to the changed requirements. If we left the pump parameters unchanged, with a reduced lifting height required by the system, the pump would operate with excessive efficiency, and therefore with excessive power consumption. Reducing the pump head can be achieved by reducing the impeller diameter, reducing the head or replacing the pump with another one, which may also prove cost-effective.

Podsumowując:

Pumping energy savings can be achieved in the following ways:

1. Using more modern pumps and engines (effects of several percent)
2. Maintaining the pump unit in proper technical condition through proper maintenance (effects ranging from several to several percent)
3. Eliminating selection errors regarding both the type of pump and its selection to the requirements of the system (effects ranging from several to several dozen percent)
4. By using the appropriate regulation method and optimizing the system (effects of several dozen percent).

The effects that can be achieved depend on the initial state and are higher the more this state deviates from the optimal one.

Dr. Eng. Grzegorz Pakula

1. INTRODUCTION.

The energy consumption for driving the pumps constitutes a significant part of the total energy consumption for the power unit's own needs. For example, in the case of power units with a capacity of 200 MW, the sum of the power consumption of the pumps operating in them reaches 10 MW, i.e. 5% of the unit's power.

Boiler pumps have the highest power consumption. In a typical 200 MW unit solution used in the Polish energy industry, the pumps are installed in a 3 x 50% system, i.e. two pumps working in parallel provide the required efficiency, and the third pump is a reserve. The power consumption of each of the two operating feed pumps is approximately 3 MW. Such significant power consumption is to some extent inevitable because it results from the need to increase the pressure at a specific stage of the thermodynamic cycle constituting the basis for energy generation in a thermal power plant. However, the actual power consumption exceeds the physically necessary minimum due to the fact that the efficiency of the pump unit is less than one hundred percent. Maintaining this efficiency at the highest possible level makes it possible to reduce energy consumption for the power unit's own needs.

2. EFFICIENCY OF PUMPING SYSTEMS.

Feed pumps operating in power plants are generally equipped with devices enabling measurement of basic parameters (efficiency, pressure, motor current consumption), based on the indications of which the efficiency of the pump unit can be estimated. These are industrially accurate measurements that do not provide the full required accuracy, but allow for an assessment of the order of magnitude. On Fig. 1 Shown is an example of the relationship between efficiency and performance of the pump unit, obtained on the basis of measurements, which is directly related to the load of the block. The results shown in the figure can be considered slightly better than average because data from other power plants showing a lower level of efficiency are available.

In order to assess this condition, it can be assumed that a new feed pump of this type should have an efficiency of around 81%, while the engine from the period when the pump set was installed should have an efficiency of around 94%. As a result, the expected efficiency of the pump unit should be 0.81 x 0.94 = 76.1%. As seen on rys.1 the actual efficiency of the team is approximately 10% lower. This difference causes excessive losses, the elimination of which enables the reduction of energy consumption for the unit's own needs. Below, the sources of losses will be discussed and opportunities to reduce them will be indicated.

Fig. 1. Efficiency of the boiler feeding pump in a 200 MW unit depending on efficiency.

3. POSSIBILITIES OF IMPROVING THE EFFICIENCY OF FEED PUMPS.

3.1. Increasing the initial efficiency of the pump.
The simplest option that comes to mind would be to use pumps with efficiency higher than the above-mentioned 81%. This seemingly obvious possibility is, however, unrealistic because for the parameters that occur here, a higher level of efficiency is difficult to achieve. In the monograph [1] you can find charts showing what efficiency can be expected from the pump depending on its efficiency and speed. They show that for pumps with parameters corresponding to pumps feeding a 200 MW boiler, an efficiency level of 81% is expected and good on a global scale. Perhaps the offer of some pump manufacturers could include pumps with slightly higher efficiency, but any differences are lower than the permissible tolerances for the efficiency of a specific model. [3], so there is no certainty that the delivered pump will have an efficiency that fully matches the offer. It follows that changing the pump type is not an effective direction, as it requires significant expenditure on purchasing the pump and adapting the station to it, while the effects are insignificant and uncertain.

3.2. Using a 100% pump instead of two 50% pumps.
You can also consider replacing pumps operating in parallel 2 x 50% with one pump. This solution may bring certain effects at the nominal operating point. From the mentioned charts [1] providing the relationship between the expected efficiency and the efficiency, it follows that doubling the efficiency from 400 to approximately 800 m3/h may result in an increase in efficiency by approximately 1-2%. Therefore, also in this case, the achieved increase in efficiency is insignificant, and the improvement only occurs when operating around the nominal parameters. The use of a 100% pump instead of two 50% pumps causes complications in regulation and increased energy consumption when the unit operates at reduced power, as discussed below.

3.3. Use of an energy-saving motor.
The efficiency of high-power engines at the time when the 200 MW units were built was 94%. Currently, energy-saving engines are available, the efficiency of which with these parameters reaches 97%. It is therefore advisable to consider replacing worn-out engines with new ones as an alternative to their renovation.

3.4. Renovation economy.
One should ask oneself what explains the ten percent difference found (p.2), close to the nominal efficiency, between the efficiency of the pump unit estimated on the basis of measurements and the efficiency expected for the new pump and engine, which should be at the level of 76%.

Firstly, the efficiency of 76% results from multiplying the efficiency of the pump and the engine and does not take into account the efficiency of the torque converter working between them. Even when operating without speed reduction, with minimal slip, the torque converter has an efficiency of 97%. Therefore, the efficiency of the pump unit with a torque converter at the nominal point can be estimated as 0.81 x 0.97 x 0.94 = 73.8%. A greater drop in efficiency when operating at nominal capacity occurs when the pump is selected with excess parameters. In such a case, even when the block is operating under full load, the torque converter's rotational speed is reduced, which causes a significant increase in losses. Excessive pump parameters are therefore harmful from the point of view of energy consumption. The unfavorable impact of the efficiency of the torque converter on the efficiency of the assembly is much greater when operating at reduced parameters, as discussed below.

Even taking into account the loss on the torque converter, there is a significant difference between the expected and measured efficiency, which cannot be explained otherwise than by a reduction in the initial efficiency due to the deterioration of the technical condition of the machines.
Measurements of the parameters of feed pumps operating in power plants show that the current pump efficiencies are often at the level of 70% (compared to 81% for a new pump), and in extreme cases known to the author, there are pumps with a maximum efficiency not exceeding 60%.

This indicates that the differences in the efficiency of new pumps, which, depending on the manufacturer and the quality of a given unit, are most often in the range of 80-82% have a smaller impact on the level of energy consumption of feed pumps. However, the decisive factor is the operational efficiency, which is reduced compared to the initial one due to wear. As mentioned, the efficiency of pumps in operation may be as high as 60-70%.

For the pump operating at the parameters Q = 450 m3/h, H = 1800 m and the specific gravity of water 9400 N/m3, which parameters correspond to the parameters of 50% of the pumps feeding the 200 MW unit operating at full power, the hydraulic power (i.e. the minimum power on the shaft pump required based on the laws of physics) is 2115 kW. If the pump has an efficiency of 81%, the actual power consumption will be 2611 kW, while with an efficiency of 70% it will be 3021 kW. As it follows, due to the deteriorated technical condition, the increase in power consumption due to the deterioration of the technical condition of the feed pump may be at the level of 400 kW, which, when operating for approximately 7000 hours a year, gives a difference in energy consumption of approximately 2800 MWh per pump (and they operate simultaneously two). The cost of this additional energy exceeds the cost of overhauling the pump.

Elimination or at least reduction of these additional losses is possible thanks to an appropriate renovation policy. A several percent drop in efficiency can be explained by natural wear and tear that occurs over several years. However, if the pump has an efficiency of 70% or less, it suggests that it was overhauled in an improper manner, using spare parts of inappropriate quality, which resulted in a reduction in efficiency. The decision to send the feed pump for major overhaul is usually made on the basis of monitoring the so-called movement parameters (vibrations, temperatures, etc.). Taking into account the fact that the cost of renovation is lower than the cost of additional energy consumed as a result of reduced efficiency, the frequency of renovations could be determined on the basis of energy efficiency monitoring, so as to maintain the operational efficiency of the pump close to that of the new pump.

Additionally, it is recommended to apply the following principles in the renovation policy:
a) When choosing a renovation contractor, you should not only take into account the price but also, or rather primarily, the effect in the form of the level of energy efficiency obtained after the renovation,
b) The criterion for accepting the overhaul should be testing the pump parameters at a testing station, allowing for measurement of the efficiency that the pump achieves after the overhaul.

3.5. Parameter adjustment.
As stated above visible on rys.1 the reduction in the efficiency of the pump unit for nominal capacity from the expected level of 74% to approximately 66% can be explained by the deteriorated technical condition of the pump. A similar level of efficiency deterioration occurs over the entire performance range. However, as seen in rys.1 when the efficiency is reduced, there is a further decrease in the efficiency of the unit, even to the level of 45% with the efficiency reduced to half of the nominal one, which is a consequence of the change in parameters compared to the nominal ones, forced by the operation of the unit with reduced power.
Pumps feeding boilers are usually regulated by changing the rotational speed. This control method is usually the best in terms of energy effects, but in this case it does not prevent the increase in losses. As described in [4], when the rotational speed changes, a given point on the pump characteristic shifts in such a way that the efficiency decreases proportionally to the rotational speed and the lifting height proportionally to its square. The efficiency does not change approximately. If the pump characteristics are drawn using this method at different rotational speeds, the points with equal efficiencies lie on parabolas. (fig.2). In theory, these parabolas originate from the origin of the coordinate system, but in practice, below a certain rotational speed, too much reduced compared to the nominal one, this theory ceases to apply because there is a deeper decline in efficiency. For this reason on rys.2 only a fragment of the graph for higher rotational speeds is shown, where the parabolic shape of the constant efficiency line corresponds to reality.

Fig. 2. Cooperation of a pump regulated by changing the rotational speed with a system with flat characteristics.

Regulation by changing the rotational speed is most effective when the highest efficiency parabola coincides with the system characteristics. This situation is possible in circulation systems (e.g. heating) where there is no static lifting height. However, the characteristics of the boiler power supply system are flat. This is due to the fact that a high boiler feed pressure is required, in addition to which losses in the pipelines increase, theoretically, with the square of the efficiency but are of a lower order of magnitude than the feed pressure. An example of the characteristics of the boiler power system, i.e. the relationship between the required pump lifting height and the efficiency, is shown in Fig. 3. This is a characteristic calculated on the basis of the same measurement data as the efficiency of the unit in fig.1. As seen from rys.3 the supply pressure (corresponding to the head at zero capacity) is above 1400 m. As the efficiency increases due to flow losses, the head increases to 1800 m. Individual measurement points show a scatter typical for industrial measurements, but generally they are arranged in accordance with the theory near parabola. (For the sake of clarity, it should be added that the characteristics of this system may change due to changes in its configuration, e.g. water intake for injection).

Fig. 3. Characteristics of the feed pump system determined on the basis of measurement results (data for one of two pumps operating in parallel).

It should be noted here that one of the possibilities of reducing energy consumption to drive pumps is to reduce losses in the pipeline, e.g. by using fittings (valves, filters, etc.) with lower resistance coefficients. This goes without saying, but is beyond the scope of this article focusing on pumps.

As seen from Fig. 3 the characteristics of the pipeline show that the pumps feeding the boiler at full capacity corresponding to the operation of the unit at full power must produce a lifting height of 1800 m, while when operating with the capacity reduced by half, the lifting height drops to 1500 m. As shown illustratively in Fig. 2 when controlled by changing the rotational speed, this causes the pumps to leave the area of ​​optimal efficiency. The location of the optimal efficiency point shifts along the efficiency axis in proportion to the rotational speed. Therefore, for optimal efficiency to occur at half the nominal capacity, the revolutions should be reduced to half. However, with a flat system characteristic, in order to reduce the efficiency to half, it is necessary to reduce the revolutions to a lesser extent and the point of optimal efficiency does not move along the characteristic along with the operating point, but remains at higher efficiency.

In addition to the deterioration of the pump efficiency resulting from its control characteristics, the drive efficiency also deteriorates. With a significant drop in power consumption resulting from a decrease in efficiency, the engine efficiency deteriorates due to its underload. However, this is not a significant decrease and is usually in the range of 1-2%. More serious losses occur in the torque converter. On rys.4 the efficiency of drives with adjustable rotational speed is shown according to [2]. When controlled using a frequency converter, losses are relatively small over a wide range of rotational speeds. However, for a traditional torque converter, the efficiency decreases quickly as the speed decreases.

Fig. 4. Comparison of the efficiency of variable speed drives.

A typical feed pump unit in a 200 MW unit, according to the structure from the 70s, when these units were designed and built, consisted of a multi-stage pump driven by an electric motor through a gear that increases the speed and a hydrokinetic clutch used to adjust the parameters. The technical solutions available at that time imposed specific conditions on the selection of pumps. Adjustment via the torque converter was only possible downwards. In order to compensate for the reduction in parameters due to wear, the pumps were selected with a certain headroom. This was unfavorable from the energy point of view because as a result, the pump units operated with increased slip on the hydrokinetic coupling, which resulted in increased losses in the coupling. However, the issue of regulation was not of fundamental importance, because at that time the units usually operated with a load close to the maximum.

Currently, power units with a capacity of 200 MW operate in a wide range of loads, relatively often at half the nominal power, which makes it necessary to limit the pump efficiency to approximately half. Let us consider, for example, the operation of pumps on parameters recorded in reality as shown below fig 3. These are the measured parameters of one of the two pumps operating in parallel, so the total efficiency supplied by the pumps to the boiler is twice as high as in the chart. As you can see, the parameters are concentrated around two typical operating points: Q = 760 m³/h and H = 1730 m and Q = 400 m³/hi H =1500 m. Na Fig. 5 a typical, exemplary characteristic of a 100% pump selected at point Q = 760 m is shown³/h and H = 1730 m, which parameters the pump achieves at a rotational speed of approx. 4050 rpm. The efficiency at this point is 81%. If, due to the reduced load on the block, there is a need to limit the capacity to 400 m³/h, and the characteristics of the system show that the lifting height at this capacity is 1500 m, then as can be seen from fig. 5, To achieve these parameters, the pump speed should be reduced to approximately 3450 rpm. The efficiency at such an operating point will be 70%. As seen from Fig. 4, with this degree of speed reduction, the efficiency of the drive with a traditional torque converter will be 80%, and the total efficiency of the pump unit will be 0.7 x 0.8 = 56%. Power consumption at Q = 400 m³/h and H = 1500 m will be 2798 kW (assuming a density of 9400 kg/m³ as for typical supply water temperature).

Losses can be reduced by replacing the old-type torque converter regulation with regulation by a frequency converter. As follows from rys.4 the efficiency of the drive with the converter will be approximately 92%, and the efficiency of the pump unit will be 73.6%. However, replacing the torque converter with an inverter requires significant investment outlays.

Fig. 5. Regulation of 100% pump operation.

It should be emphasized that qualitatively we are dealing with the same situation if, instead of a 100% pump as in the above example, a total of two 50% pumps operate. Their combined characteristic, created by summing the efficiency of both pumps at a given lifting height, shown in Fig. 6, is close to the characteristic of the 100% pump and behaves in a similar way when controlled by changing the rotational speed. In this case, to reduce the efficiency from 760 to 400 m³/h, the rotational speed should be reduced from 3490 to 3060 rpm (to 87.6%). Pump efficiency at reduced speed with a capacity of 400 mXNUMX³/h will be 70%. Due to the slightly lower degree of rotational speed reduction (87.6%), the efficiency of the drive with a traditional torque converter will be approximately 83%, and the efficiency of the entire pump unit will be 0.7 x 0.83 = 58.1%. Power consumption at Q = 400 m³/h and H = 1500 m will therefore amount to 2696 kW.

Fig. 6. Regulation of two 50% pumps operating in parallel.

Therefore, if, as is a common practice, when reducing the power of the block, the efficiency is regulated by reducing the speed of both pumps simultaneously, then also in such a case the efficiency of the unit will drop to a similar level as for the 100% pump. However, if we have a 2 x 50% pump system, when the efficiency is reduced to 50%, instead of limiting the efficiency of both pumps at the same time, one of the pumps can be turned off and the required efficiency can be obtained from one pump, which then operates close to its nominal efficiency. This situation is shown in Fig. 7. The pump operating independently will achieve Q = 400 m³/h and H = 1500 m at a rotational speed of approx. 3350 rpm, therefore reduced compared to the speed during parallel operation at point Q = 760 m³/h and H = 1730 m in the ratio 3350/3490 = 0.96.

Fig. 7. Single pump operation 50%.

Pump working individually at point Q = 400 m³/h and H = 1500 m will achieve an efficiency of 81%, and the drive via a traditional torque converter will achieve an efficiency of approximately 89% at a given reduction in rotational speed. As a result, the efficiency of the pump unit will be 0.81 x 0.89 = 72.1%, and the power consumption will be 2173 kW. As you can see, changing the regulation method by turning off one of the two pumps instead of uniformly reducing the efficiency of two pumps operating in parallel allows for lower power consumption with reduced efficiency by over 500 kW. This is a saving that does not require any investments, only changes in the method of operation. For the sake of clarity, it should be noted that frequent shutdown of one of the pumps to some extent shortens its period between overhauls, because each start and stop is an unfavorable condition for the pump. However, this effect does not eliminate the possible energy benefits.

The above example calculations were carried out on selected pump characteristics with typical mileage and for operating points found in a real power plant. For other types of pumps and other operating parameters of the unit, the results may differ quantitatively, but important qualitative conclusions remain valid, which are formulated in the summary below.

The above considerations regarding capacity regulation for clarity were carried out on the characteristics of new pumps. The effect of deterioration of efficiency during operation, mentioned above, is superimposed on phenomena related to regulation.

4. SUMMARY AND CONCLUSIONS.

1. The level of energy consumption by pumps feeding the power unit's boiler is largely influenced by the current energy efficiency of the pumps, which, as measurements show, in many cases differs significantly from the efficiency of new pumps. In order to limit the serious energy losses resulting from this, an appropriate renovation policy is recommended, taking into account the following principles:
a) Pumps should be sent for overhaul not only on the basis of operational criteria but on the basis of energy efficiency monitoring
b) The criterion for selecting a renovation contractor (apart from the price) should primarily be the energy efficiency obtained after the renovation, which should be verified during measurements at a test station after the renovation.

2. For units operating in a wide range of power changes (e.g. down to 50% of the nominal power), significant energy savings when operating with reduced efficiency can be achieved by replacing old type fluid couplings with more modern regulated drives, e.g. frequency converters. However, this requires investment outlays, the payback period of which should be estimated before making a decision.

3. Significant energy savings can be achieved without investment outlays by changing the method of operating pumps operating in parallel 2 x 50%. By turning off one of the pumps instead of limiting the efficiency of two pumps operating in parallel, you can achieve a very significant improvement in efficiency in the range of 50-60% of the maximum efficiency.

4. A 2 x 50% pump system is much more flexible in terms of regulation than one 100% pump in systems with flat characteristics, such as the power supply system for the power unit's boiler. Even though the 100% pump is able to achieve slightly higher efficiency near the maximum capacity, for blocks operating often with power reduced to 50% of the maximum power, it is recommended to use a 2 x 50% pump system. This allows, when one of the pumps is turned off, to operate at a boiler efficiency in the range of 50-60% of the nominal one with an efficiency close to the maximum, which is not possible to achieve with a 100% pump.

5. The old practice of selecting pumps for maximum parameters and regulating only downwards is now not optimal. Currently, there are control methods available that also enable increasing parameters. When using them, it is beneficial to select pumps with the parameters at which they most often operate (not necessarily maximum) because then the highest efficiency is achieved at the most frequently occurring operating point. Selection based on this principle requires analysis for each case, based on data on the expected hourly distribution of the unit's operating parameters.

Dr. Eng. Grzegorz Pakula

Literatura:

1. W. Jędral, Centrifugal Pumps, PWN Scientific Publishing House, Warsaw 2001,
2. W. Misiewicz, A. Misiewicz, Regulated drives in pump systems of heat sources, National Energy Conservation Agency, Warsaw 2008
3. G. Pakuła, Offer efficiency of pumps, Pumps Pumping Stations, No. 2/2012
4. G. Pakuła, Performance regulation of two pumps operating in parallel, Pumps, Pumping Rooms, No. 3/2011