GB1576620A - Fluid pressure attenuator - Google Patents

Description

(54) FLUID PRESSURE ATTENUATOR (71) We, INTERNATIONAL RESEARCH & DEVELOPMENT COMPANY LIMITED, a British Company, of Fossway, Newcastle upon Tyne, NE6 2YD, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to apparatus for the reduction of pressure ripples in hydraulic pipe lines and is particularly although not exclusively applicable to outlet pipe lines from positive displacement hydraulic pumps.

Pressure ripples in pipe lines, especially those connected with positive displacement hydraulic pumps, cause vibrations and noise. The suppression of such pressure ripples is desirable both from the environmental point of view and for economic reasons because components and pipes in the system will be less susceptible to wear.

A number of solutions to this problem have already been proposed and usually involve the employment of gas filled accumulators which act to attenuate the pressure fluctuations in order to suppress the pressure ripples. Such devices employ elastomer seals or moving parts such as pistons between the hydraulic fluid and the volume of compressible gas. Such arrangements are not entirely satisfactory from the economic and reliability viewpoint.

The object of this invention is to overcome the problems of previous solutions and produce a simple device which is economic to manufacture and has a high degree of reliability.

According to the present invention, a hydraulic pressure ripple attenuator comprises a plain cylindrical pipe devoid of movable parts having an open inlet end and an open outlet end for connection into a hydraulic line, the pipe extending through a sealed pressure vessel, and the interior of the pipe communicating with the interior of the vessel through openings through the wall of the pipe whereby the liquid in the vessel has a pressure-absorbing effect and attenuates pressure pulsation in the hydraulic fluid line.

The arrangement makes use of the compressibility of the hydraulic fluid employed and is analogous to an electric smoothing circuit wherein the fluid within the outer pipe provides a "capacitance" effect which is required in a ripple "smoothing" device. "Inductance" is provided by the cyclic acceleration of the fluid in the inner pipe.

Said openings in the pipe are preferably located near the inlet end of the pipe.

The pressure vessel walls may have a degree of compliance and this augments the capacitance effect due to the compressibility of the fluid.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a longitudinal cross-section of a hydraulic pressure attenuator in accordance with the invention; and Figure 2 is a cross-sectional view on the line X-X of Figure 1.

In the arrangement shown in Figures 1 and 2 the attenuator comprises a cylindrical pressure vessel consisting of a mild steel tube 10 by welding to end caps 11 and 12, the end caps having through bores 13 and 14 which form an inlet and an outlet, respectively, for connection to a hydraulic pipe line, not shown. Within bores 13 and 14 there is fixed in a sealed manner an inner pipe 15 which forms a connection between the inlet bore 13 and outlet bore 14. At the left hand end of the inner pipe 15 near the inlet bore 13 the wall is pierced to produce three openings 16 which allow communication between the inside of the pipe 15 and the annulus 17 defined between the pipe 15 and the surrounding pressure vessel tube 10. A bleed valve 18 is screwed into a bleed hole ila in the end cap 11, said bleed hole providing access to annulus 17 for the purpose of bleeding off any air which may become trapped during installation or operation.

The annulus is normally pre-filled with hydraulic fluid prior to connection to the hydraulic pipe line and the bleed valve is employed to prevent entrapment of air.

In operation the attenuator described above is connected into a hydraulic pipe line and the flow of hydraulic fluid is directed through the attenuator. The flow path is essentially through the inner pipe 15 from inlet 13 to outlet 14 but there is communication through openings 16 in inner pipe 15 with the more or less static reservoir of fluid in annulus 17 such that when pressure ripples occur in the hydraulic circuit connected to the inlet 13 the fluid within the annulus 17 is compressed and so attenuates the pressure ripple.

The device thus described is analogous to an electric smoothing circuit in that the hydraulic fluid within the annulus 17 provides a "capacitance" which suppresses the pressure ripple; "inductance" is provided by the cyclic acceleration of the fluid in pipe 15 and there is effectively formed a low-pass filter network analogous to those used in electrical smoothing circuits. Thus ripple frequencies below the network resonance are transmitted without attenuation whilse those sufficiently above are progressively attenuated.

In its simplest form as described with respet to the specific embodiment the resonant frequency Jo can be represented by

where M is the hydraulic inductance (or inertiance) and C is the hydraulic capacitance (or compliance). This inertiance M is defined by the expression P, - P2 = M dQ dt where P, and P2 are. respectively, the upstream and downstream pressures and Q is the flow rate. It can bc shown that, for fluid of density p flowing though a pipe of length e and uniform cross-section A, the inertiance M is given by M = pt A Thc compliance CH is defined by the expression Q = CH dP dt and it can be shown that in the device described, if V is the volume of the enclosed fluid, K is the adiabatic bulk modulus of the fluid, Je is the tensile stress in the vessel walls. and E is Youngs Modulus for the vessel walls, then CH = V l 3fe 1 K ' PE In this expression the first term Kvis that due to the compressibility of the fluid itself while the second term is that due to the compliance of the walls of the pressure vessel.

A typical attenuator in accordance with the invention having a flow rating of 30 gallons per minute would comprise a pressure vessel 18 inches long and having a diameter of 5 inches with a pressure rating of 4,500 lbs/in2. It progressively attenuates above 100 Hz and would normally be used on frequencies above 300 Hz.

The invention is not restricted to the form shown, and more complex networks may be formed involving a number of these devices connected in series or parallel, but contained within a single pressure vessel.

WHAT WE CLAIM IS: l. A hydraulic pressure ripple attenuator comprising a plain cylindrical pipe devoid of movable parts having an open inlet end and an open outlet end for connection into a hydraulic line. the pipe extending through a sealed pressure vessel, and the interior of the pipe communicating with the interior of the vessel through openings through the wall of the pipe whereby the liquid in the vessel has a pressure-absorbing effect and attenuates pressure pulsation in the hydraulic fluid line.

2. A hydraulic pressure ripple attenuator as claimed in claim 1 in which the openings in the wall of the pipe are close to the inlet end.

3. A hydraulic pressure ripple attenuator as claimed in claim 1 or 2 in which the pressure vessel has a bleed valve for release of trapped air to ensure complete filling of the vessel with the hydraulic fluid.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   providing access to annulus 17 for the purpose of bleeding off any air which may become trapped during installation or operation.

   The annulus is normally pre-filled with hydraulic fluid prior to connection to the hydraulic pipe line and the bleed valve is employed to prevent entrapment of air.

   In operation the attenuator described above is connected into a hydraulic pipe line and the flow of hydraulic fluid is directed through the attenuator. The flow path is essentially through the inner pipe 15 from inlet 13 to outlet 14 but there is communication through openings 16 in inner pipe 15 with the more or less static reservoir of fluid in annulus 17 such that when pressure ripples occur in the hydraulic circuit connected to the inlet 13 the fluid within the annulus 17 is compressed and so attenuates the pressure ripple.

   The device thus described is analogous to an electric smoothing circuit in that the hydraulic fluid within the annulus 17 provides a "capacitance" which suppresses the pressure ripple; "inductance" is provided by the cyclic acceleration of the fluid in pipe 15 and there is effectively formed a low-pass filter network analogous to those used in electrical smoothing circuits. Thus ripple frequencies below the network resonance are transmitted without attenuation whilse those sufficiently above are progressively attenuated.

   In its simplest form as described with respet to the specific embodiment the resonant frequency Jo can be represented by

   where M is the hydraulic inductance (or inertiance) and C is the hydraulic capacitance (or compliance). This inertiance M is defined by the expression P, - P2 = M dQ dt where P, and P2 are. respectively, the upstream and downstream pressures and Q is the flow rate. It can bc shown that, for fluid of density p flowing though a pipe of length e and uniform cross-section A, the inertiance M is given by M = pt A Thc compliance CH is defined by the expression Q = CH dP dt and it can be shown that in the device described, if V is the volume of the enclosed fluid, K is the adiabatic bulk modulus of the fluid, Je is the tensile stress in the vessel walls. and E is Youngs Modulus for the vessel walls, then CH = V l 3fe 1 K ' PE In this expression the first term Kvis that due to the compressibility of the fluid itself while the second term is that due to the compliance of the walls of the pressure vessel.

   A typical attenuator in accordance with the invention having a flow rating of 30 gallons per minute would comprise a pressure vessel 18 inches long and having a diameter of 5 inches with a pressure rating of 4,500 lbs/in2. It progressively attenuates above 100 Hz and would normally be used on frequencies above 300 Hz.

   The invention is not restricted to the form shown, and more complex networks may be formed involving a number of these devices connected in series or parallel, but contained within a single pressure vessel.

   WHAT WE CLAIM IS: l. A hydraulic pressure ripple attenuator comprising a plain cylindrical pipe devoid of movable parts having an open inlet end and an open outlet end for connection into a hydraulic line. the pipe extending through a sealed pressure vessel, and the interior of the pipe communicating with the interior of the vessel through openings through the wall of the pipe whereby the liquid in the vessel has a pressure-absorbing effect and attenuates pressure pulsation in the hydraulic fluid line.
2. 2. A hydraulic pressure ripple attenuator as claimed in claim 1 in which the openings in the wall of the pipe are close to the inlet end.
3. 3. A hydraulic pressure ripple attenuator as claimed in claim 1 or 2 in which the pressure vessel has a bleed valve for release of trapped air to ensure complete filling of the vessel with the hydraulic fluid.
4. 4. A hydraulic pressure ripple attenuator substantially as described with reference to the

   accompanying drawing.
5. 5. A hydraulic supply system comprising a positive-displacement hydraulic pump having connected in its output line a pressure ripple attenuator as claimed in claim 1, 2 or 3.