FIELD OF THE INVENTION

The present invention relates to a pressure exchanger system with vessel elongation absorber, particularly but not exclusively a pressure exchanger system for use with a reverse osmosis system or plant.

BACKGROUND OF THE INVENTION

Many industrial processes operate at high pressures and produce high pressure waste streams. It is desirable to recover this energy and pressure exchanger systems are often employed to transfer pressure energy from a high pressure fluid stream to a low pressure fluid stream. For example, a reverse osmosis system for carrying out desalination of sea water for producing fresh water needs high pressure for delivering the sea water through the reverse osmosis membranes. Energy recovery devices exist which enable some of the membrane rejected stream to be reused thereby enabling a large proportion of the energy in the reject stream to be recovered.

Reverse osmosis systems pump a feed saline solution into a membrane array at high pressure and this is divided by the membrane array into a super saline solution (brine) at high pressure and potable water at low pressure. The high pressure brine is no longer useful in this process as a fluid but its pressure energy has a high value. A pressure exchanger serves to recover pressure energy from the brine to a feed saline solution. Low pressure brine is also formed which can be expelled to drain. The pressure exchanger saves energy by reducing the load on the high pressure pump because a large proportion of the membrane feed flow is pressurized with out significant energy input to the pump.

A number of pressure exchanger energy recovery systems are available on the market place.

Generally, the pressure exchanger comprises at least two pressure vessels each provided with a piston which dynamically divides the vessel into a brine and a feed sides and valve systems that serve to recover pressure from a high pressurized brine stream (membrane rejected stream) into a low pressure feed stream. The piston in a pressure vessel is configured to move from one end of the pressure vessel to the other end while keeping the two sections isolated so that there is no fluid communication between the sections. A commonly used isobaric recovery device is the Dual Work Exchange Energy Recovery (DWEER). On their brine side, each vessel has a single port which may act as an inlet or outlet port with a switch valve. A switch valve, or a set of valves, are configured to switch the vessel's brine port from a "pressurized state" where high pressurized brine flows into the vessel, to an "exhaustion state", in which low pressure brine flows out of the vessel. A linear spool device for DWEER is discussed in US Patent No. 5797429 (DesalCo, Ltd) and sold by Calder.

On their feed side, each vessel has a single port which may act as an inlet port, configured to feed the vessel with low pressurized sea water and an outlet port, configured to deliver high pressurized sea water feed into a reverse osmosis (RO) system. Check valves are provided on each pressure vessel to direct fluid into and out of the pressure vessel at predetermined pressures. Different types of energy recovery systems operate using this basic concept.

The outlet port of the pressure vessel is connected through check valves and piping to a circulation pump. The circulation pump compensates for pressure loss in the system and further pumps the pressurized feed water into the membrane.

During pressurization cycle of a pressure vessel, in which pressure from pressurized brine stream is exchanged into feed water stream, the overall length of the pressure vessel is increased relative to its original length. Contrary, during feeling cycle, in which low pressure feed water enters and feels the pressure vessel, the overall length of the pressure vessel is decreased to an about the original length. This pressure recovery cycles repeats itself 3 -4 times per minute. As a result the pressure vessel experiences length changes with the same frequency. Such length changes create additional dynamic load and stress on the piping material. The piping system which connects the check valve outlet to the circulation pump is unable to compensate the pressure vessel's length changes. One known solution in the market today providing length changes compensation to the piping is, for example, a Victaulic™ type coupling. However, this type of coupling is unable to provide a sufficient solution in a high frequency application. Using this solution in a high frequency application ends up with a fatigue failure of the coupling. It is one aim of present invention to provide an improved reverse osmosis system using at least one pressure vessel that aims to overcome or at least alleviate, the abovementioned drawbacks.

A further aim of the present invention is to provide an improved pressure exchanger energy recovery system that aims to overcome, or at least alleviate, the abovementioned drawbacks.

Yet a further aim of the present invention is to provide an improved coupling that aims to overcome, or at least alleviate, the abovementioned drawbacks.

SUMMARY OF THE INVENTION

A first aspect of the present invention provides a pressure exchanger system comprising at least one pair of pressure vessels, each vessel having a brine side and a feed side and preferably a piston for dynamically dividing each vessel into the brine and feed sides, each side having an inlet port and an outlet port or a single port which may act as an inlet and as an outlet ports for directing fluid into and out of the pressure vessel, the pressure exchanger system further comprising a valve system adapted to direct brine and feed fluids into and out of the pressure vessels at predetermined pressures, preferably by a switch valve and a low pressure inlet check valve and a high pressure outlet check valve respectively. The pressure exchanger further comprises a vessel elongation absorber device comprising an inner element for connection to a pressure vessel, preferably a high pressure feed check valve outlet, the inner element being mounted to a housing element configured for connection to a pressure vessel stand/support, wherein the inner element is slidable with respect to the housing element in response to length changes of the pressure vessel. The outer element may be further configured for connection to a circulation pump piping wherein the inner element is slidable with respect to the housing element in response to a pressure vessel length changes.

Preferably, the inner element of the elongation absorber device comprises an annular element defining an internal cavity having an internal cross section So, the annular element having a leading and a trailing edge, the annular element further comprising a flange extending outwardly for receipt in a recess formed in the housing element. The leading edge of the inner element defines a ring having an area S 1. Preferably, the flange has a smaller cross section than the cross section of the recess in which it relies allowing sliding of the inner element with respect to the housing element. The flange divides the recess, which is wider, into two cavities reside on each side of the flange.

A first gap or cavity may be provided on one side of the flange having a fluid communication with the internal cavity provided by the inner element. A spacer element is preferably provided between the inner and housing element to maintain the first gap or cavity open. A second gap or cavity may be provided on the opposite side of the flange having a fluid communication to the atmospheric pressure. According to another embodiment a draining hole may be provided with this second gap or cavity in order to drain some liquid leakages from the absorber device.

One aspect of the present invention is to reduce the dynamical load on the both the pressure vessel stand and the circulation pump piping. Such a dynamical load reduction may be reached by balancing forces based on the following formula:

P x (So+ Sl) = (P-l) x S2.

In this formula P is defined by the recovery brine fluid pressure and S2 is defined by the flange surface facing the first gap or cavity.

The relative movement between the inner element and the housing element occurs, especially in a high frequency process cycles, in exchange of exposing the system to dynamical loads.

It is to be appreciated that there should be no fluid leakage between the inner and housing elements of the elongation compensation device. In this respect, appropriate seals, such as sealing rings, should be provided along the slideable surfaces of the inner and outer elements to prevent fluid leakage from the device. As mentioned above, in case of some negligible liquid leakage into the second gap or cavity, a draining hole is configured to remove such leaked liquid from the absorber device.

The pressure exchanger according to the first aspect of the present invention is preferably in the form of a dual exchange energy recovery system. To this end, a second aspect of

the present invention provides an energy recovery system comprising at least one pressure exchanger system according the first aspect of the present invention.

The pressure exchanger of the present invention is preferably incorporated into a reverse

osmosis plant, for example, in a desalination plant. To this end, a third aspect of the present invention provides a reverse osmosis plant incorporating a pressure exchanger

energy recovery system according to the second aspect of the present invention.

It is to be appreciated that the elongation absorber device may be installed retrospectively as a coupling to an existing pressure exchanger system. To this end, a fourth aspect of the present invention provides a coupling comprising an housing element configured for connection to first pipe and an inner element configured for connection to a second pipe the inner element is configured to be slidable with respect to the housing element in response to an axial length change in the first and/or second pipes.

The inner element of the elongation absorber may comprise an annular element defining an internal cavity, the annular element having a leading and a trailing edge, the annular element further comprising a flange extending outwardly for receipt in a recess formed in the housing element, the flange being of a smaller cross section than the cross section of the recess to allow sliding of the inner element with respect to the outer element. A first cavity is provided on one side of the flange configured to have a fluid communication with the internal cavity provided by the inner element. A second cavity which is configured to be open to the ambient pressure is provided on the opposing side of the flange.

Preferably, a gap is provided between the leading edge of the inner element and the housing element, the gap being in fluid communication with the internal cavity provided by the inner element. The inner element may define a first surface and the housing having a recess defines a second surface and the ratio between the first surface and the second surface is about 1.

A spacer element may be provided between the inner and housing element to maintain the first cavity.

BRIEF DESCRIPTION OF THE DRAWINGS.

The invention will now be described in further detail with reference to the following

examples and accompanying figures in which:

Figure 1 shows a prior art energy recovery system. Figure 2 shows an energy recovery system incorporating a elongation absorber device.

Figure 3 is an embodiment of an axial vessel elongation absorber device according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION.

The present invention is concerned with an elongation absorber device for pressure exchanger systems that recover energy from high pressure fluid processes. An example of one energy recovery system that may be adapted according to the present invention is a DWEER™ energy recovery system.

Referring now to Fig. 1. Fig. 1 shows an exemplary pressure recovery system 100 having pressure vessels 101a and 101b. Brine connection 103 is configured to feed high pressurized brine into switch valves 102. Rejected brine ports 104a and 104b are configured to drain exhausted brine from pressure vessels 101a and 101b respectively. Pressure vessels are installed on base stand 105. Vessels support elements 106 and 107 are rigidly connected to stand 105. Vessel support element 106 is rigidly connected to pressure vessel. Pressure vessels lay on vessels supporting element 107 in such a way that it allows relative movements between the pressure vessel and the supporting element. Low pressure feed inlet check valve 109 is installed on the feed side of pressure vessel 101. High pressure feed outlet check valve 110 is also connected on the feed side of the pressure vessel 101. Pressure recovery system 100 shown in Fig. l may be a part of a reverse osmosis (RO) plant.

Referring now to Fig. 2. Elongation absorber 108 has an inlet port which is connected downstream the high pressure feed outlet check valve 110. Elongation absorber 108 has an outlet port which is connected to piping system and circulation pump (not shown). Two rods 111 are rigidly connecting the elongation absorber housing 70 as defined in Fig. 3, to the stand 105.

Turning now to Fig. 3. Elongation absorber device 50 comprises a high pressure feed inlet port 30 and a high pressure feed outlet port 31. Elongation absorber device 50 further comprises an inner element 60 and a housing element 70. Surface 70a is part of the housing element 70 defining holes 72 and 74 configured to connect the housing element of the elongation absorber device to a pressure vessel stand (not shown in Figure 3). Rigid rods, such as rods 111 shown in Fig. 2, may be used to connect holes 72 and 74 to a pressure vessel stand. Inlet port 30 of inner element 60 is configured to be connected to a high pressure feed check valve 110 outlet. The inner element 60 is an annular part defining an internal cavity 62 having an internal cross section So and has a leading and a trailing parts, 63, 61 with edges having surfaces 63a and 61a respectively. Edge surface 63a defines a donut- shape ring having an area Si. The total area S equals to the sum of So and Si. The inner element 60 is configured to move along the housing element 70 by the provision of parts 61 and 63 and flange 64 extending outwardly from the inner element for receipt in a recess 76 in the housing element, the recess being wider in cross section than the flange to allow limited movement of the flange within the recess. Flange 64 divides recess 76 into a first cavity 84 and a second cavity 88. Flange 64 has a first surface 91 facing the first cavity and a second surface 92 facing the second cavity. Recess 76 has a first surface 93 facing the first cavity 84 and a second surface 94 facing the second cavity 88. A ring surface S ₂ is defined by flange surface 91. Sealing rings 80 are positioned along slideable surfaces between the housing and internal elements in such a way that there is no fluid leakage from the elongation absorber device. Spacers 82 are configured to maintain the cavity 84 open and channels 86 are configured to create fluid communication between the internal cavity 62 within the internal element and the first cavity 84. Spacers 82 assure that first cavity 84 does not get closed and fluid will easily flow from space 62 to first cavity 84 at different positions or installations of the device. Second cavity 88 provides an empty space at an ambient pressure into which internal element 60 may slide during axial elongation of the pressure vessel. Draining holes in the second cavity 88 may drain negligible amount of leaked fluids which may leak from the absorber device into the cavity (not shown). Gap 90, which is in fluid communication with space 62, also provides sliding possibility of internal element 60 during axial elongation of the pressure vessel. Force balance between the force exerted on surfaces S and S2 is reached based on the following formula:

P x S = (P-l) x S ₂ .

P is the pressure of the high pressure feed fluid. In this manner, by allowing a sealed sliding movements between the internal element and the housing element the elongation absorber device of the present invention absorbs axial length changes of a pressure vessel with high frequency cycles while avoiding mechanical loads and stresses within its piping system and hence avoiding associated fatigue failures.