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MINIMISING PRESSURE PULSATIONS
Information from Sulzer Pumps, Finland
The main task of the headbox feed pump in a paper machine approach system is to create a uniform flow and pressure to the headbox. Other requirements are good efficiency, reliable design and a smooth inner surface to avoid fibre hang-up.
Paper quality is strongly influenced by the design of the paper machine approach system and the quality of its components. The headbox feed pump is one component in the paper machine approach system which has an effect on the paper quality. Fig.1 shows a typical paper machine approach system.
Figure 1. PM Approach System
Figure 2. Variation of basis paper weight MD
Pressure pulsations, initiated by the headbox feed pump, might create unacceptable variations of paper basis weight in the machine direction, Fig.2.
As there is no centrifugal pump operating without pulsations, it is necessary to reduce the pulsations to a minimum through design and high quality manufacturing.
This article explains the basic principles of pressure pulsations initiated by a centrifugal pump, and how to minimize them. Other possible sources of pressure pulsations are also mentioned as well as special aspects of design and manufacturing of the headbox feed pump.
2. Pressure pulsations in the paper machine approach system
As a paper machine approach system is complex, the sources of pressure pulsations, Fig.3, can be initiated, reinforced or dampened by the different components installed in a paper machine approach system.
Figure 3. Sources of pressure pulsations in paper machine approach system
As there are several factors which can cause pressure pulsations in a paper machine approach system, it is essential to design, manufacture and install all the components, as well as the whole system, in such a way that the pressure pulsations are as low as possible to ensure a trouble-free start-up of the paper machine and to produce a high and constant quality paper.
3. Basics of pressure pulsations
Frequency, Fig.4, is the number of vibrations in a certain time, eg cycles per second, when the unit Hz (Hertz) is used or cycles per minute (cpm).
Figure 4. Basics of pressure pulsations, frequency and amplitude
Amplitude, Fig 4, in the case of pressure pulsations is the magnitude of the pressure pulsations. The measuring unit for amplitude can be expressed in several ways. Commonly used are: Pa (pascal), mbar (millibar) psi (pounds per square inch) and mWc (meter water column).
Fig.4 shows graphically and mathematically the relationship between these terms, rms, peak or peak-to-peak.
Resonance is the effect observed when a device sends a certain vibration (pulsation) in a frequency which is the same as the natural frequency of another aggregate or pipe. When the second aggregate receives this vibration it will also start to vibrate. It is possible that the resonance vibration will have a much bigger magnitude than the initial vibration or pulsation.
3.4 Overlapping frequencies
Overlapping frequencies refer to a situation where two or more components, like the headbox feed pump and the headbox screen, initiate pressure pulsations which are on the same frequency level or very near to each other. The effect of overlapping frequencies is a higher amplitude of the pressure pulsation.
4. Pressure pulsations initiated by a centrifugal pump
Only periodic pressure pulsations at frequencies which are in harmony with the rotational speed (n) of a pump are caused by the pump.
4.1 Pressure pulsations at impeller vane frequencies
a) Pressure pulsations at impeller vane frequencies (fz) are initiated by each impeller vane, when passing the volute cutwater, Fig.5. The effect is about the same as closing a valve very quickly and thereby causing a pressure shock.
z = 7 vanes
n = 600 rpm
fz = z x n/60 = Hz
fz = 7 x 600/60 = 70 Hz
The distance between the impeller and the volute cutwater affects the amplitude of the pressure pulsation. To obtain a low pressure pulsation, a certain distance between the impeller and the volute cutwater is required. This distance is determined by the hydraulic characteristics of the pump.
Figure 5. Pressure pulsations at impeller vane frequencies
b) Design of impeller vanes
Fig. 6 shows different designs of impeller vanes. In the case of Fig.6a, where the tips of the straight, not staggered vanes are passing the volute cutwater with the whole width at once , the magnitude of the pressure pulsation is considerably bigger than in the other case, when the impeller vanes are skewed and staggered, Fig.6c.
In this case, the width of the vane tips passing the volute cutwater is considerably smaller, and therefore the magnitude of the pressure pulsation is smaller. This effect is even bigger if an impeller with unsplit vanes is used.
Fig.7 shows the difference of the pressure pulsation Dp in relation to the capacity between straight vanes split, not staggered versus straight vanes split and staggered in a test run. The diagram may only be seen as a tendency showing that one type of impeller causes higher pulsations that another type of impeller.
Figure 7. Effect of impeller vane design on pressure pulsation at different flows
4.2 Pressure pulsations at rotational frequencies
Pressure pulsations at rotational frequencies are initiated by:
n = 1 000 rpm
fn = n/60 = Hz
fn = 1 000/60 = 16.6 Hz
The result of unacceptable pressure pulsations at rotational frequencies on paper basis weight in MD (Machine Direction) can be seen in Fig.9.
Figure 8. Pressure pulsations at rotational frequencies
Figure 9. Variation of paper basis weight in MD at rotational frequencies
4.3 Effect of suction conditions
If the difference between NPSHa (Net Positive Suction Head), available from the system, and NPSHr, required by the pump, is too small, it may influence the pulsation level. NPSHa must be higher than NPSHr.
4.4 Other aspects of pressure pulsations initiated by a centrifugal pump
a) Pressure pulsations, initiated by a centrifugal pump, can be found at several multiples (1 ... x times) of the rotational speed. The highest amplitudes of pressure pulsations are initiated at one and two times of the rotational frequency (fn) and one and two times of the vane frequency (fz).
Pressure pulsations at two times rotational frequency (fn) can originate from misalignment between drive and pump.They can also occur, if there is a clearance in the coupling, or if a flexible coupling or an incorrectly assembled universal shaft is used.
b) Pressure pulsation level is influenced by the position of the duty point to BEP (Best Efficiency Point). Fig.10 shows that the lowest pulsation is reached, if the duty point is near to BEP and to the left of it.
Figure 10. Effect of position of duty point on pressure pulsation
5. Impeller design and quality
As the impeller design and manufacturing quality are by far the most important factors in respect of pressure pulsations, special attention must be paid to them.
Fig.11 shows the most important design details and manufacturing steps which are necessary to obtain a high-precision impeller initiating only low-pressure pulsations.
Figure 11. Design and manufacturing quality of the impeller
6. Minimizing pressure pulsations
This chapter summarizes the conditions under which it is possible to get the lowest pressure pulsations from a centrifugal pump used to feed the headbox. It also describes other aspects which should be taken into consideration, when planning a paper machine approach system.
6.1 Pressure pulsations at impeller vane frequencies
Pressure pulsations at impeller vane frequencies (fz) are:
Based on field experience, the frequencies of pressure pulsations at impeller vane frequencies are in most cases above 60 Hz, and as they are dampened by the system, their effects on the paper quality in general are not harmful. It is therefore an advantage to use a pump with higher speed, because the higher the frequencies of the pressure pulsations the more effectively they are dampened.
6.2 Pressure pulsations at rotational frequencies
Pressure pulsations at rotational frequencies (fn) are:
6.3 Other aspects
a) Overlapping operating frequencies; pump and screen. As the screen, which is placed between the headbox feed pump and headbox, also initiates pressure pulsations, its frequencies should not be near or at the pump frequencies. Otherwise the pressure pulsations initiated by the headbox feed pump can be reinforced by the pressure pulsations initiated by the screen.
b) As variable speed drive is frequently used for new headbox feed pumps, the speed of the motor should not vary.
There are attenuators available on the market. But first of all, when planning a paper machine approach system, attention should be paid to build and install only components which assure low pressure pulsations without any adverse effects on the paper quality.
7. Testing of pressure pulsations
Pressure pulsation tests can be carried out at the test station in the factory. An example of a pulsation test report is shown in Fig.12. The graphic presents the pressure pulsations amplitude versus frequency.
The vertical scale for the amplitude is divided into 500 mmbar (0.5 mbar) between the dotted lines. The total length is 2.500 mbar. The values are expressed in rms.The horizontal scale for the frequency is divided into 500 cpm (cycles per minute) between the dotted lines. The total length is 3000 cpm (50 Hz). The pressure pulsation at rotational speed is 1.75 mbar, in this example. These scales can be changed e.g. the frequency band up to 200 Hz or higher. Other explanation of the graphic, see Fig.12.
With the total of worldwide almost 4000 installed headbox feed pumps and 1st stage cleaner pumps, Sulzer Pumps has an outstanding high number of references for different applications in respect of papergrade or paper machine supplier.
This, together with our basic research, modern designing methods and manufacturing facilities, enables us to supply these pumps according to the high demands of today's fast and modern paper machines producing paper in high quality.