4.1 Pressure accumulator as a pressure damper
5.1.3 Operating of inline suppressor with the rock drill
The inline suppressor was installed in the hydraulic supply line near the rock drill. Installation was as shown in Figure 4-10, except that the accumulator was replaced with the 1 ¼ “ inline suppressor. Figures 5-3 and 5-4 show the influence of the inline suppressor. It damps high frequency oscillation quite well.
Pressure [bar]
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 Time [s]
300 250 200 150 100 50 0
Pressure [bar]
250 200 150 100 50 0
0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 Time [s]
Figure 5-3. Pressure oscillation without damper (above) and with inline suppressor (down).
Power density [W / Hz]
Figure 5-4. Power density of pressure oscillation of the rock drill with inline suppressor.
5.1.4 Discussion of inline suppressor
The inline suppressor is mainly a low pass filter. The suppressor damps well high frequencies but passes low frequency oscillation through, which can be seen in the Figure 5-3. Probably the theoretical nominal size of the suppressor, like the nominal size of the accumulator, is too small, because it did not damp low frequency oscillation well. In practice this kind of suppressor is easy in install to the system. As the name indicates, it is installed inline and this means that the suppressor is easy to support. The cooling of the inline suppressor is better than that in an accumulator because of the oil flow through the suppressor.
6 DISCUSSION
The aim of this thesis was to study properties of pressure dampers and this way improves the reliability of the supply line of a pulsating actuator. For example, the reliability of the hydraulics of a rock drill is not good, but users have become accustomed to it. The reason behind the problems is the very demanding actuator. The rock drill causes high pressure oscillation in the system. The hydraulic power required is tens of kilowatts and the length of the hose between the pump and the rock drill is about 20 m.
Because the rock drill is installed at the tip of the boom, the demands placed on the dampers are high. The size and the weight of a damper are limited. Reliability of dampers is essential and a damper should not decrease the efficiency of the system.
Different kinds of dampers were tested with a servo valve and with a real rock drill. The T-pipe seems to be the most promising damper for low frequency. The accumulator damps well and it is worth considering if problems of reliability can be solved. The accumulator needs a very good supporting rack which supports the accumulator and conducts heat to the environment.
The inline suppressor damps high frequency oscillation well.
The hydraulic motor, running without a load, damps pressure oscillation. The hydraulic motor has a certain inertia, which is increased by a flywheel. Then inertia of the hydraulic motor absorbs the pressure oscillation. This idea was tested by installing the hydraulic motor in the pipeline and by running the test sequence in the reference [Ijas -00b]. The good damping was achieved on a wide frequency band but the total efficiency and the price were insufficient. The motor was mainly an interface between an unstable and a stable line.
Table 1. Comparison of studied dampers.
Damper Maintenance Damping low frequencies (30-60Hz)
Problems when damping low frequencies
Price
Accumulator Needs Excellent Need maintenance Medium
Inline suppressor Needs Weak Price, weak damping Expensive
T-pipe No Good Long hose Very cheap
Helmholtz resonator
No Good Big size Cheap
It is possible to damp the supply line of the rock drill sufficiently if the price and the size of the damper are not considered. Figures 6-1 and 6-2 give the pressure oscillation when a 0.6 L accumulator and 1 ¼” inline suppressor are used at the same time. Damping is good, but the construction is complicated. The space required is large and there are two membranes, which decreases the reliability of the system.
Figure 6-1. Pressure oscillation of the rock drill without damper (above) and with the accumulator and the inline suppressor (down).
Power density [W / Hz]
Figure 6-2. Power density of pressure oscillation of the rock drill with the accumulator and the inline suppressor.
The pressure oscillation shakes hoses. Obviously this decreases reliability, but also the possible outer abrasion of the hose raises the temperature. Abrasion can be more dangerous for the hose, depending on the supporting of the hose. The main reason for the warming is abrasion of the hose. If the pressure oscillation cannot be damped, the hose must be supported well. A special abrasion-resistant hose can be used or the hose can be installed inside a hose protector or a sleeve. Usually, when it is desired to increase the hydraulic power, pressure is a factor which is increased, because this permits smaller components to be used. This gives better power density.
Unfortunately, the price of hose does not support this trend. Especially, a big (over 1”) high-pressure hose is very expensive. Another undesired feature is stiffness in high-high-pressure hoses.
Also, the inner diameter of the fitting in high-pressure hose can be very tight.
Also, the hydraulic power can be increased by increasing the flow rate. The power density does not become better, but puts less load on hydraulic hoses. Especially if the hydraulic line is composed of several parallel hoses, the load for one hose is lower. The “parallel hose” concept was theoretically studied in the reference Ijas 2002b and it appears that parallel hoses become competitive at power levels over 100 kW.
7 CONCLUSIONS AND FURTHER WORK
Properties of pressure dampers were simulated and experimentally tested. The pulsating actuator was the servo valve or the rock drill.
The main results of the thesis can be summarised in the following statements:
x Dimension equations and simulation models of pressure dampers work quite well even for low frequencies.
x When the pressure oscillation is not of regular sine form the “natural frequency method” is not necessarily the best way to tune the accumulator
x The T-pipe damps low frequencies well but the length of the T-pipe (or the T-hose) is long. The long hose is not necessary a problem.
x The T-pipe damps the vibrations slightly due to step excitation.
x The inline suppressor damps high frequencies well (1000-2000 Hz).
x The best solution for a rock drill machine is a well supported accumulator installed near the mainline or the T-pipe
Parts of the tests were done with the real rock drill, but the operating point was not the usual one. In the future, the best of these hydraulic line concepts should be tested in real drilling conditions. The damper used should be tuned at a frequency of about 50 Hz.
Another point for study is the efficiency of the rock drill when the hydraulic line is damped.
Does the damper disturb the operation of the rock drill? This can be determined best by measuring the total efficiency of the rock drill from the hydraulic pump to the button bit.
How the reliability changes when the pressure oscillation is damped and when a special abrasion-resistant hose is used can only be determined by long test runs in real conditions. All in all, long test runs are needed where the effect of changes on reliability and efficiency are studied.
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Appendix A Transducers used in the test systems.
Appendix B Simulation models
Appendix A Transducers used in the test systems.
Appendix B Simulation models
Kajaste [-99b] stated that the pipe model of Mäkinen works excellent when the viscocity factor is low enough, for example 0.37 1/s. In this case the viscosity factor was
> @
According to Kajaste [Kajaste -99b] the hose models of Mäkinen should operate well.
Flow rate through the hose was about 1.3*10-3 m3/s (80 l/min). In that case the Reynolds number
Flow is probably laminar in this operating point.
Figure 1. Simulation model of test environment.
“Valve” was modelled using a turbulent orifice equation. The discharge coefficient was 0.6 and the density of the oil 890 kg/m3. The steady-state opening of the valve was 4.5 mm (circle area) and the amplitude of the excitation was 0.2 mm. The frequency was increased from 0 to 100 Hz.
Figure 2. Simulation model of hose arrangement.
Figure 3. Parameters used of pipe models.
Figure 4. Simulation model when the accumulator was studied.
The accumulator was modelled using Equation 7. The damper was modelled using Equations 7, 13 and 14 in the Helmholtz resonator simulation. The PQ model [Mäkinen -00] with zero flow at the end of the hose operated as the T-pipe damper.
Figure 5. P-C8 model [Mäkinen -00].
The steel pipe is modelled using the P-pipe model (P-C8) in the Figure 2. The P-pipe model means that the inward oil flow is known and the other end of the pipe is connected to the volume, which was now the pressure filter. The speed of the sound was 1100 m/s and there were eight modes in use in the model.
Q-models (Q-C16, Q-C17) are for situations where oil flow at the both side of the hose are knows. In this case there were 16 modes in use (Figure 6).
Figure 6. Q-C16 model [Mäkinen -00].
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