Expert

Summary.

Power units currently operate in a wide range of load variability, which translates into a wide range of pump performance adjustments, especially feed and condensate pumps. For this reason, it is important to choose the right regulation method, which should, above all, ensure high energy efficiency. However, the adjustment method also affects the vibration level. The nominal rotational speed of the pumps is selected at the design stage so that it is far from the critical speeds. However, during regulation by changing the rotational speed, there is a risk of resonance. Moreover, if the pump operates with a system with flat characteristics, which is typical for feed and condensate pumps, reducing the efficiency while maintaining the discharge pressure causes the pump to leave the optimal operating range. This causes internal recirculation flows, which are an additional source of forcing vibrations at frequencies other than rotational and blade frequencies.
The paper discusses the above phenomena and methods of limiting the increase in the vibration level.

1. Introduction.

An increased level of vibration of the pump unit shortens its service life, and if the vibrations are transmitted through the pipelines to adjacent elements, it also increases the risk of their damage. Therefore, the dynamic condition of pump units should be monitored, and if an increased level of vibration is found, the causes of this phenomenon should be determined and removed.

Mechanical sources of vibration excitations of pump units are widely known, such as misalignment of the pump and motor or unbalance of rotating elements. They give symptoms in the form of vibrations at the rotational frequency or its multiples and are relatively easy to diagnose and eliminate.

There are also hydraulic forces originating from recirculation flows and turbulence that appear in the pump when operating with a capacity different from the nominal one. This phenomenon becomes particularly important when power units operate within a significant range of load changes, reaching 50-100% of maximum power. Lowering the block parameters translates into a change in pump efficiency, which leads to their operation outside the optimal range.

When choosing a method for regulating pump parameters, one should take into account not only the minimization of energy consumption but also the reduction of the vibration level. Both aspects are interconnected, because vibrations forced by internal turbulence in the pump constitute a mechanism for the dissipation of hydraulic energy.

2. Danger of resonance when regulating by changing the rotational speed.

A dangerous increase in vibration levels occurs if the pump operates at a rotational speed close to the critical speed. To avoid this, pumps are usually designed so that the nominal speed is far from the critical speed. Most often, the first critical speed is at least 20% higher than the nominal speed. In the case of pumps with high nominal speed (above 3000 rpm), sometimes the first critical speed is below the nominal speed. These are the so-called "supercritical pumps". In their case, during start-up, the pump goes through resonance, and then after reaching the nominal speed, the vibrations stabilize.

When operating at a constant nominal speed, there is no risk of resonance resulting from operation near the critical speed. However, this risk cannot be ruled out in the case of speed control. First of all, in the case of "supercritical pumps", resonance may occur as a result of hitting the first critical speed. However, this danger also occurs for pumps for which the first critical speed is above the nominal one. Resonance may occur not only when the pump is operated at a speed close to the critical one, but also at a speed close to half of it. If, as is most often the case, the first critical speed is in the range of 1.2 - 1.4 times the nominal speed, then reducing the speed to 0.6 - 0.7 times the nominal speed as a result of regulation will lead to resonance. In practice, such a phenomenon usually does not pose a threat to feed pumps or condensate pumps in power units, because they operate in systems with flat characteristics (i.e. the discharge pressure changes relatively little with changes in efficiency), and this causes the range of rotational speed changes to be narrow, reaching down no further than 80% of the nominal speed. The phenomenon of resonance caused by the operating speed overlapping half of the critical speed is highly probable or even inevitable for deeply controlled pumps, where the change in rotational speed reaches down to 50% of the nominal speed. It is particularly dangerous for long vertical pumps, which are naturally more susceptible to vibrations.

If the pump, due to the wide range of parameter adjustments, is to operate with a drive with a frequency varying in the range of several dozen percent, encountering one of the resonance frequencies is basically inevitable. When selecting the pump, however, it should be ensured that the drive frequency at which the pump will most often operate is not close to the resonance frequency. Unfortunately, in practice this requirement is often ignored. Taking advantage of the fact that the variable speed drive makes it possible to easily change the pump parameters, the required analyzes are omitted, hoping that the parameters will be "adjusted on the inverter". If the pump achieves the most common parameters at the resonant frequency of the drive, it can be detuned by changing the impeller diameter. After reducing the impeller, the pump requires a higher rotational speed to achieve specific parameters, which allows it to move away from the resonance frequency.

3. Vibrations caused by hydraulic forces.

In addition to mechanical forces related to rotation, pumps also have forces arising from liquid flow. Their source is turbulence that occurs when the pump capacity differs from the nominal one. The blade angles in the flow system are designed so that, at nominal capacity, the flow to the impeller or guide blades is shock-free. However, when the pump operates with a capacity different from the nominal one, the direction of liquid inflow does not match the blade angle and vortices appear in the flow. (fig.1) When there is a significant difference between the current and nominal efficiency, recirculation flows appear in the inlet and outlet areas of the pump in the form of larger-scale vortices, which are a source of low-frequency vibrations. Collectively, these turbulences cause vibrations of various frequencies (noise), generally below the blade frequency.

Another factor forcing flexural vibrations of the pump shaft is the so-called axial thrust resulting from uneven pressure distribution around the impeller in scroll pumps, which occurs at capacities significantly different from the nominal ones.

Fig.1. Swirl at the blade inlet.

It is generally accepted that the permissible operating range of the pump (fig.2) is from 0.8 to 1.1 of the nominal capacity. In this range, the vibration level is the lowest, while at lower or higher capacities the vibrations increase as a result of the hydraulic excitations described above.

Fig. 2 Dependence of the vibration level on performance.

With extensive throttling or bleed control, the pump often operates outside the recommended characteristic range and with increased vibration levels.
Using regulation by changing the rotational speed does not always prevent this phenomenon. For each rotational speed, the pump has a characteristic curve with a similar permissible operating range. As a result, on the collective chart for various rotational speeds, the permissible operating range in which vibrations are at a low level looks as follows: fig. 3.

Fig. 3. Permissible operating range for variable speed.

If the regulation takes place in a system with flat characteristics, the change in efficiency takes place with a small change in pressure (lifting height), as shown by the arrow in the figure. fig. 3. In this case, the pump goes out of the recommended operating range, which results in an increase in vibrations. This situation occurs for feed and condensate pumps that operate in systems with flat characteristics, and with the current method of operating power units, their efficiency is often limited to 50% of the nominal level. This phenomenon is unavoidable for 100% pumps, which enter an unfavorable operating range when capacity is reduced. This can be avoided by using a larger number of pumps operating in parallel (e.g. 2 x 50%, 3 x 33%). In a 2 x 50% system, if there is a significant drop in efficiency, one pump can be turned off and the remaining pump operates in a favorable range. Therefore, in blocks intended for operation in a wide range of power regulation, it is more advantageous than 100% feed and condensate pumps to use a larger number of pumps operating in parallel, both in terms of energy and vibration levels.

In the case of pumps operating in parallel, if speed control is used, it should be applied to all pumps. A system used for cost-saving reasons, in which only one of the pumps operating in parallel has an adjustable rotational speed, is unfavorable because the pump, when the rotational speed is significantly reduced, begins to operate at too low efficiency and in an unfavorable range, which leads to a significant increase in vibrations.

4. Pumps regulated by changing the angle of the impeller blades.

In pumps in which parameters are regulated by changing the angle of the rotor blades, there are technological problems with proper balancing of the rotor, which may lead to an increase in vibrations at the rotational frequency. Rotors made using casting technology have shape errors resulting from shrinkage during the solidification of the metal in the mold and the resulting unbalance. A commonly used dynamic balancing method involves subtracting part of the material (e.g. by milling) at a fixed location. However, this treatment is only effective with a specific position of the shoulder blades. If the angle of their position is then changed, the balancing in the previous position is no longer effective. To avoid this phenomenon, blades with very limited shape errors should be used, which is difficult to obtain for castings. The blades should therefore be mechanically processed.

Increased vibration levels are also caused by increased clearances in the blade adjustment mechanism, which may occur after a certain period of operation.

5. Summary.

• The method of parameter regulation used affects not only the energy consumption but also the level of vibration of the pumps, and thus their service life.

• With a wide speed control range, there is a risk of resonance. This threat should be analyzed at the pump selection stage.

• Regulation by changing the rotational speed in systems with flat characteristics (as for feed pumps and condensate pumps) leads to the pump leaving the recommended operating range. For this reason, it is advantageous to divide the required capacity into several pumps operating in parallel.

Dr. Eng. Grzegorz Pakula