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Title:
METHOD FOR REGULATION OF A DIFFERENTIAL PRESSURE ACROSS A SEAL AND ASSOCIATED SYSTEM
Document Type and Number:
WIPO Patent Application WO/2016/083196
Kind Code:
A1
Abstract:
A fluid pressure control system and method for regulation of a differential pressure across at least one seal by lowering, raising or keeping constant the pressure in a first volume of fluid in relation to the pressure in a second volume of fluid, which is separated from the first volume of fluid by the at least one seal, is disclosed. The system comprises pressure sensors (12, 13) arranged for sensing the pressure in the first volume of fluid and in the second volume of fluid, a controller (14) arranged to generate a motor control signal based on detected difference in pressure between the first and second fluid volumes, and a hydraulic displacement unit (15, 16) operative in response to the control signal for regulation of pressure in the first volume of fluid by moving fluid to or from the first fluid volume.

Inventors:
KJONIGSEN, Tom (Holmsasveien 19, Sande, N-3070, NO)
VESTBOSTAD, Gaute Yddal (Bondibraten 69, Askar, N-1387, NO)
TOMTER, Ole Petter (Eyvind Lyches vei 10, Sandvika, N-1338, NO)
Application Number:
EP2015/076843
Publication Date:
June 02, 2016
Filing Date:
November 17, 2015
Export Citation:
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Assignee:
VETCO GRAY SCANDINAVIA AS (Eyvind Lyches vei 10, Sandvika, N-1338, NO)
International Classes:
F04D29/10; F04D29/12; F16J15/00; F16J15/40; F04D25/06
Domestic Patent References:
WO2008033162A22008-03-20
WO2011161517A12011-12-29
Foreign References:
US20120027564A12012-02-02
US4606652A1986-08-19
US5746435A1998-05-05
US20120027564A12012-02-02
Attorney, Agent or Firm:
LEE, Brenda (The Ark, 201 Talgarth Road, Hammersmith London W6 8BJ, W6 8BJ, GB)
Download PDF:
Claims:
CLAIMS:

1. A method for regulation of a differential pressure across at least one seal by reducing, raising or keeping constant the pressure in a first volume of fluid in relation to the pressure in a second volume of fluid which is separated from the first volume of fluid by the at least one seal, the method comprising sensing (12, 13) the pressure in the first volume of fluid and in the second volume of fluid, generating, in a controller (14), a motor control signal based on detected difference in pressure between the first and second fluid volumes, and driving a hydraulic displacement unit (15, 16) in response to said control signal for regulation of pressure in the first volume of fluid by moving fluid to or from the first fluid volume.

2. The method of claim 1, wherein the first volume is barrier fluid (BF) supplied to at least one shaft seal (10) in a motor/pump or in a motor/compressor assembly (1, 2), and the second volume is process fluid (PF) at the suction or discharge side (5) of the pump or compressor (1).

3. The method of claim 1 or 2, wherein controlling the differential pressure across the seal comprises maintaining a higher pressure in the first fluid volume than in the second fluid volume.

4. The method of claim 3, wherein the pressure difference between the first and second fluid volumes is kept constant.

5. The method of any previous claim, wherein controlling the differential pressure across the seal comprises controlling rotational speed and rotational direction of a hydraulic displacement unit comprising a bidirectional pump (16) controlling the flow of fluid to and from the first fluid volume (BF).

6. The method of claim 5, comprising generation of a speed control signal delivered by the controller (14) to a variable speed drive (VSD) (17) which controls an electric motor (15) drivingly connected to the bidirectional pump (16).

7. The method of claim 6, further comprising generation of a four quadrant motor control signal for an electric motor (15) operating in four quadrants.

8. The method of claim 7, wherein electrical power is recovered by the four quadrants motor (15) operating as generator in braking mode.

9. A fluid pressure control system for regulation of a differential pressure across at least one seal by lowering, raising or keeping constant the pressure in a first volume of fluid in relation to the pressure in a second volume of fluid which is separated from the first volume of fluid by the at least one seal, the system comprising pressure sensors (12, 13) arranged for sensing the pressure in the first volume of fluid and in the second volume of fluid, a controller (14) arranged to generate a motor control signal based on detected difference in pressure between the first and second fluid volumes, and a hydraulic displacement unit (15, 16) operable in response to the motor control signal for regulation of pressure in the first volume of fluid, by moving fluid to or from the first fluid volume.

10. The system of claim 9, wherein at least one pressure sensor (13) is arranged for sensing the pressure in barrier fluid (BF) supplied to at least one shaft seal (10) in a motor/pump or in a motor/compressor assembly (1, 2), and at least one pressure sensor (12) is arranged for sensing the pressure in process fluid (PF) at the suction or discharge side (5) of the pump or compressor (2).

11. The system of claim 9 or 10, wherein the hydraulic displacement unit (15, 16) comprises a bidirectional pump (16) also operable as a hydraulic motor.

12. The system of claim 9 or 10, wherein operation of the bidirectional pump (16) is controlled by the controller (14) over a VSD (17) and an electric motor (15) also operable as a generator.

13. The system of claim 12, wherein the bidirectional pump (16) is driven by an electric motor (15) operating in four quadrants.

14. The system of claim 13, wherein in braking mode the four quadrants motor (15) is operable as an electrical power generator.

15. The system of any of claims 9-14, wherein the hydraulic displacement unit (15, 16) is in flow communication with the pressure controlled first fluid volume (BF) and with a container (8).

16. The system of any of claims 12-15, wherein the bidirectional pump (16) and the electric motor/generator (15) are placed in separate containers (20; 24) and drivingly interconnected via a magnetic coupling (25).

17. The system of any of claims 9-16, wherein a drain port of the hydraulic displacement unit (15, 16) is open to a container (20) containing the unit, the container drained via check valves (22, 23) to a fluid line (9, 9') connected to the hydraulic displacement unit (15, 16).

Description:
METHOD FOR REGULATION OF A DIFFERENTIAL PRESSURE ACROSS A SEAL AND ASSOCIATED SYSTEM

TECHNICAL FIELD OF THE INVENTION

The present invention refers to a fluid regulation method and system.

In one aspect of the invention, a method is provided for regulation of a differential pressure across a seal by lowering, raising, maintaining or keeping constant the pressure in a first volume of fluid in relation to the pressure in a second volume of fluid which is separated from the first volume of fluid by the seal.

In a second aspect the invention refers to a corresponding system and its components by which is achieved the regulation of a differential pressure across a seal that separates a first volume of fluid from a second volume of fluid.

Both aspects of the invention are useful in subsea and land based implementations for regulation of barrier, lubrication or cooling fluid pressures in rotating machinery, such as pumps, compressors or generator assemblies.

One implementation of the present invention is the regulation of barrier fluid pressures during startup, steady state running and shutdown of a subsea pump or compressor, etc. However, the invention can also be used in other implementations both subsea and on land. The method and system of the present invention may for example be applied to motor/generator or turbine/generator assemblies or other rotating machinery wherein a fluid for lubrication, cooling or separation purpose needs to be separated from a fluid processed by the rotating machine.

The group of seals referred to in this context comprises, but is not limited to, seals that are applied around rotating shafts (shaft seals) or seals applied around pipes and cables passing a wall to an apparatus housing or container, etc. The seals may also include seals at a passage or opening that provides communication between two adjacent containers, and seals at an opening that provides communication between an encased volume and the ambient environment. BACKGROUND AND PRIOR ART

In pumps and compressors which are operated in subsea implementations, such as where a pump or compressor is used in the transport of well fluid or for water injection into well bores or hydrocarbon formations, e.g., the pressure in a barrier fluid needs to be controlled and maintained within defined limits relative to the pressure of the process fluid in the pump or compressor in order to avoid intrusion of process fluid into seals, bearings and motor parts.

During startup of a subsea pump or compressor used for boosting the pressure in process fluid, the barrier fluid supplied to the pump or compressor motor will expand due to increasing temperature. The expanded volume must be allowed to escape in a controlled manner to avoid excessive overpressure in the barrier fluid relative to pump or compressor pressure at any time. During start-up the process fluid pressure may also change, depending on the use, and the barrier fluid pressure has to be adjusted accordingly. In a pump connected to a subsea well, e.g., if the pressure at the process side of the seal is the pump inlet pressure, the barrier fluid system will have to relieve fluid to reduce the barrier fluid pressure, in order to maintain a stable differential pressure over the seal.

During steady state conditions barrier fluid needs to be supplied to the pump or compressor motor to compensate for leakage through the motor and shaft seals, in particular with respect to leakage via the shaft seals at the suction side of the pump/compressor where the differential pressure is at the largest.

During shutdown the suction pressure will increase and barrier fluid needs to be supplied to the pump or compressor motor to maintain overpressure relative to the pressure in the process fluid at the suction side of the pump or compressor. After shutdown the barrier fluid temperature will also slowly decrease, and fluid will have to be supplied to the system in order to maintain the differential pressure in spite of a decrease in the barrier fluid temperature.

The above-mentioned operational modes briefly explain the critical conditions under which barrier fluid and pressure must be regulated in a subsea pump or compressor implementation. However, corresponding conditions and operation characteristics and demands may be expected also in pump or compressor implementations on topside or land and in connection with other rotating machinery, such as in motor/generator or turbine/generator assemblies and other rotating arrangements on land or subsea. In prior art solutions different hydraulic component configurations have been used and tested, in most cases based on pressure and flow control valves, with the aim to control the barrier fluid pressure during startup, steady state and shutdown of the pump or compressor.

A barrier fluid pressure regulation system for a subsea motor and pump module is previously disclosed in WO 2011/161517 Al . Pressure controlled flow control valves are arranged in a barrier fluid supply system and actuated in response to detected pressures in barrier fluid on either side of a flow restriction to refill the high pressure side with barrier fluid which is supplied from a pressurized accumulator. If required, such as in a startup sequence e.g., a relief valve in a by-pass line is controllable for dumping barrier fluid from the high pressure side to the lower pressure side of the flow restriction.

A method for the pressure regulation of a barrier fluid in a pumping device is previously known from US 2012/0027564 Al . In this method, a pressure reducing valve for the differential pressure over a seal in the pumping device is opened, and a feed line for barrier fluid is connected to a source for barrier fluid via the open valve, the pressure of which lies above the pressure of pumping medium by a predetermined value.

Field experience indicates however that problems in terms of contamination, corrosion and erosion often hamper the operation of valves and may reduce the reliability and operational lifetime of older systems relying on valves to regulate the pressure in barrier fluid in pumping or pressure boosting devices. SUMMARY OF THE INVENTION

An object of the present invention is to provide a solution that addresses the above- mentioned problems, providing increased reliability and operational lifetime of pumps, compressors or other rotating machinery. To meet the object the present invention teaches a method for regulation of a differential pressure across at least one seal by lowering, raising or keeping constant the pressure in a first volume of fluid in relation to the pressure in a second volume of fluid which is separated from the first volume of fluid by the at least one seal.

The method comprises: sensing the pressure in the first volume of fluid and in the second volume of fluid, generating, in a controller, a motor control signal based on detected difference in pressure between the first and second fluid volumes, and driving a hydraulic displacement unit in response to said control signal for regulation of pressure in the first volume of fluid by moving fluid to or from the first fluid volume.

In a pump or compressor implementation the first fluid volume is barrier fluid that is supplied to a shaft seal in a motor/pump or in a motor/compressor assembly, and the second fluid volume is process fluid at the suction or discharge side of the pump or compressor. In this context the first fluid volume comprises fluid which acts as a barrier (barrier fluid) to prevent intrusion of process fluid from a pump or compressor chamber into a motor compartment, a coupling chamber, seals and bearings etc., or fluid for lubrication (lubrication fluid) of rotating parts in rotating machines such as pumps or compressors, or fluid for cooling purposes (cooling fluid). The process fluid of interest is typically oil, gas, water, or mixtures thereof.

In embodiments of the invention, such as in a pump or compressor implementation, controlling the differential pressure across the seal usually means maintaining a higher pressure in the first fluid volume than in the second fluid volume. One embodiment foresees that the pressure difference between the first and second fluid volumes is kept constant.

In a preferred embodiment, regulation of the volume of the first fluid is accomplished by controlling the rotational speed and rotational direction of a bidirectional pump which is operated in either direction in response to the sensed differential pressure for moving fluid to and from the first fluid volume.

In the above preferred embodiment a motor control signal is delivered by the controller to a VSD (Variable Speed Drive) which controls an electric motor that is drivingly connected to the bidirectional pump.

The motor control signal may be composed as a four quadrant motor control signal for an electric motor that operates in four quadrants. In this embodiment electrical power may be recovered from the four quadrants motor operating as generator in braking mode.

In analogy herewith the object is met also through a fluid pressure control system for regulation of the differential pressure across at least one seal by lowering, raising or keeping constant the pressure in a first volume of fluid in relation to the pressure in a second volume of fluid which is separated from the first volume of fluid by the at least one seal. The system comprises pressure sensors arranged for sensing the pressure in the first volume of fluid and in the second volume of fluid, a controller arranged to generate a motor control signal based on detected difference in pressure between the first and second fluid volumes, and a hydraulic displacement unit operative in response to the motor control signal for regulation of pressure in the first volume of fluid by moving fluid to or from the first fluid volume. In a pump or compressor implementation at least one pressure sensor is arranged for sensing the pressure in barrier fluid supplied to a shaft seal in a motor/pump or in a motor/compressor assembly, and at least one pressure sensor is arranged for sensing the pressure in process fluid at the suction or discharge side of the pump or compressor.

In a preferred embodiment the hydraulic displacement unit comprises a bidirectional pump which is operable also as a hydraulic motor. In other words, the pump can be driven by a motor for displacement of hydraulic fluid from a supply of fluid at lower pressure to a recipient fluid volume which is to be maintained at a relatively higher pressure. In a reverse mode of operation the pump can be driven by hydraulic fluid moving from the high pressure side to the lower pressure side while powering the motor from which work or energy can be recovered.

The bidirectional pump is drivingly connected to an electric motor which is controlled by the control unit over a VSD. Advantageously, the electric motor is operable also as a generator from which electric power can be recovered in the reverse mode of operation. In such embodiment the motor that drives the bidirectional pump is advantageously an electric motor operating in four quadrants. Thus, in braking mode the four quadrants motor can be operated as an electrical power generator.

The fluid pressure control system comprises a pipe arrangement by which the hydraulic displacement unit is in flow communication with the pressure controlled fluid volume on one hand and with a container holding a supply of fluid on the other hand. In a subsea implementation the supply of fluid may be held at a pressure substantially corresponding to the pressure of seawater outside the container.

Alternatively, the hydraulic displacement unit may be arranged in flow communication with the pressure controlled fluid volume and with an accumulator containing fluid supplied from an external source.

In one embodiment, the pump and the electric motor, or the motor/generator if appropriate, are placed in separate containers and drivingly interconnected via a magnetic coupling. In embodiments of the invention a drain port of the hydraulic displacement unit may be arranged to open into the container that houses the hydraulic displacement unit, wherein the container is drained via check valves to a fluid line connected to the hydraulic displacement unit. BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further explained below with reference to the accompanying drawings, schematically illustrating embodiments of the invention by way of examples. In the drawings,

Fig. 1 shows implementation of the fluid pressure regulation system in a motor/pump or motor/ compressor assembly,

Fig. 2 shows an installation of a hydraulic displacement unit in a fluid filled container or vessel,

Fig. 3 shows an installation of the pump and a motor in a common fluid filled container, Fig. 4 shows an alternative installation of the pump and motor in separate containers or vessels,

Fig. 5 shows a fluid pressure control system setup for electrical redundancy, and Fig. 6 shows a fluid pressure control system setup for enhanced redundancy. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Fig. 1 illustrates schematically a pump or compressor 1 that is drivingly connected to a motor 2 via a drive shaft 3. The pump/compressor 1 is driven by the motor for acceleration of flow or for boosting pressure in a process fluid PF which enters a pump/compressor chamber 4 via a pump/compressor inlet 5 at a first flow or pressure, on the suction side of the pump/compressor 1. On the pressure side of the pump/compressor 1 the process fluid is discharged via a pump/compressor outlet 6 at a second flow or pressure which is raised above the flow/pressure on the suction side of the pump or compressor 1.

The motor 2 is installed in a housing 7 to which barrier fluid BF is supplied from a fluid container 8, via barrier fluid supply line 9. The barrier fluid in the motor housing is maintained at a pressure that is higher than the internal pressure of the process fluid in the pump/compressor chamber 4 in order to avoid intrusion of process fluid into the motor housing via shaft seal(s) 10, sealing about the drive shaft 3. The barrier fluid pressure may depending on the specific application and the type of seal also be lower than the internal pressure of the process fluid in the pump/compressor chamber 4. In result of the pressure differential across the seal or seal(s) 10, barrier fluid will escape or leak via the interface between the two surfaces of the seal. It is a general aim to minimize an unavoidable leak flow at the shaft seal(s) 10, without risking intrusion of process fluid into the motor housing. To reach that goal the differential pressure across the seals needs to be monitored and maintained at all times. Since at transfer between different operational modes variations in pressures can arise suddenly, such as at startup or shutdown, a system for regulation of the barrier fluid needs to react fast and accurate.

In order that a barrier fluid regulation system responds according to variations in pressure in the process fluid, the pressures in the process fluid and in the barrier fluid are monitored by pressure sensors. The readings from the pressure sensors are processed in a controller that generates the accurate command for regulation of the barrier fluid in response to variations in pressure in the process fluid. The regulation of the barrier fluid is carried out in a way which will be explained in detail further below. In the embodiment of a barrier fluid regulation system as implemented in the motor/pump or motor/compressor assembly 1 of Fig. 1, pressure sensors 12 and 13 are arranged to register changes in pressure in the process fluid and in the barrier fluid, respectively. The readings of the pressure sensors 12 and 13 are sent to a controller 14 for processing according to control logic installed in the controller 14. The pressure sensors 12 and 13 can be realized as differential pressure sensors reporting a differential pressure value to the controller 14. Alternatively, the controller can be arranged to calculate a differential pressure based on the readings of the pressure sensors 12, 13.

Pressure sensors suitable for the purpose are, e.g., capacitive transducers or piezoelectric sensors, delivering an output signal in the range of 4-20 mA.

The controller 14 may be a device that is integrated in the pump or compressor control system, or a stand-alone programmable logic controller (PLC). Basic function of the controller comprises: read the signals from the pressure sensors; convert signals to pressure readings; compare the converted pressure readings (and raise an alarm if required); calculate true pressure (average); calculate barrier fluid pump speed reference (using a PID algorithm, e.g.); and generate a rotational speed and rotational direction reference signal.

In a subsea implementation the controller 14 can be arranged to communicate with a topside operator station, and will require supply of electric power from an uninterruptable power supply (UPS).

Based on readings from the pressure sensors 12 and 13, i.e. based on detected or calculated differential pressure over the seal or seals 10, the controller 14 thus generates a command to request operation of a motor 15 which drives a pump 16 that is installed on the barrier fluid supply line 9. The command generated in the controller 14 is a motor control signal that controls the operation of an electric motor 15 via a variable speed drive or VSD 17 that powers the motor 15.

The pump 16 is a bidirectional pump which can be driven in a first or forward direction to raise the pressure in barrier fluid in the motor housing 7 by supplying barrier fluid from the fluid container 8, or in a second or reverse direction to lower the barrier fluid pressure in the motor housing 7 by dumping barrier fluid to the fluid container 8.

The fluid container 8 can be a tank with a pressure equal to the surrounding pressure, i.e. equal to seawater pressure in a subsea application, or it can be an accumulator bank. The fluid container 8 may be equipped with a fluid level sensor 18 that communicates with the controller 14.

The direction of rotation and the rotational speed of the pump 16 and the motor 15 are controlled from the controller 14 via the VSD 17 and determined by the content of the motor control signal which is generated in the controller and based on readings from the pressure sensors. The motor/pump assembly 15, 16 can thus be controlled from the controller 14 via the VSD 17 to maintain a desired differential pressure between the barrier fluid, on one side of the seal(s) 10, and the process fluid on the opposite side of the seal(s) 10, by transferring fluid between the fluid container 8 and the motor housing 7 as required.

In a wider sense the motor and pump assembly 15, 16 is a hydraulic displacement unit which is operable, preferably, either as a pump when driven by the motor 15 for raising barrier fluid pressure in the motor housing 7, or operable as a motor when driven by a return flow of barrier fluid from the motor housing 7 to the fluid container 8 upon relief of barrier fluid pressure. Pump types suitable for operation in a hydraulic displacement unit in the barrier fluid regulation system are gear pumps or axial piston pumps or radial piston pumps, etc. The pumps need to be designed for full system pressure on both pump ports A and B (see Fig. 1).

In order to take advantage of the work and energy produced in the pump 16 during reversed mode of operation, i.e. when the pump acts as a motor powered by return flow of barrier fluid, the pump motor may advantageously be arranged for operation also as a generator. To this purpose a four quadrants motor can be chosen to drive the pump 16. In braking mode the four quadrants motor operates as a generator feeding electric power from the barrier fluid regulation system or to power consumers or batteries included in the system. In this embodiment the VSD 17 and the controller 14 are correspondingly arranged with control logic and electronics adapted for generation of motor control signals that shift the motor between the four modes/quadrants of operation.

Different types of brushless motors can be considered for the application as pump motor in this context. Suitable motor alternatives are three phase induction motors with squirrel cage rotor windings, or three phase motors with permanent magnet rotor. The first preferred alternative may be the three phase squirrel cage induction motor which has a simple and robust design that can be designed for running submerged in fluid. As VSD 17 a standard four quadrant drive with resistor load can be used. The four quadrant drive is needed for the motor 15 to operate either as a motor or as a generator depending on the barrier fluid flow direction and differential pressure conditions. The VSD receives a rotational speed reference signal from the controller 14 and supplies a 3 -phase variable frequency power to the motor which drives the pump at variable speed in both rotational directions.

Power from the motor/generator 15 to the VSD can be sent back to the grid or to a topside UPS battery bank. Alternatively the recovered energy can be dissipated in a local subsea resistor bank, or the recovered power can be supplied to a local subsea UPS. A contactor circuit may alternatively be used instead of the VSD 17. This is however a less preferred design since the motor will then run at full speed for short time intervals which is less favorable with respect to wear and life expectancy for the motor and the pump. Also, the four quadrant motor drive would be more difficult to control via the contactor circuit, in particular when a squirrel cage motor is used. Further it should be pointed out that the barrier fluid system in Fig. 1 may comprise a connector 19 for refilling the barrier fluid system from an external fluid supply.

Although the invention has so far been explained with reference to a motor and pump or compressor assembly it should be realized that the barrier fluid regulation system comprising the hydraulic displacement unit 15, 16, the controller 14 and pressure sensors 12, 13 can be used in other applications and in connection with other rotating machines wherein a differential pressure is required across a seal that separates fluids in the rotating machine. In a more general sense the barrier fluid introduced in the motor housing 7 can be seen as a first volume of fluid supplied to one side of at least one seal 10 that separates the first fluid from a second volume of fluid represented by the process fluid in the pump chamber 4 on the opposite side of the at least one seal.

In this context the first fluid volume shall be understood to comprise not only fluid supplied to the motor compartment interior in general but also fluid filling other structures in the motor housing such as coupling chamber, seal boxes, bearings and bearing or lubrication boxes etc., as well as other passages or channels in the motor and motor housing structure that communicate with the barrier fluid container via the hydraulic displacement unit 15, 16.

In this connection it shall also be pointed out that number and location of pressure sensors 12, 13 can be different from the location and number depicted in the schematic drawing of Fig. 1.

It shall further be pointed out that the number and location of seals 10 in Fig. 1 is merely for illustration purposes. The barrier fluid regulation system may be implemented to generate, restore and maintain a differential pressure across shaft seals at other locations in rotary machines or across other types of seals in a motor housing, a coupling chamber, a pump or compressor housing, or in a generator or turbine housing, etc., whenever a differential pressure across a seal that separates fluids is required.

It shall also again be pointed out that although barrier fluid is widely used in the description of embodiments, the expression shall be understood in a wider sense to include a fluid which, alternatively or in combination, can function also for the purpose of lubrication and/or cooling. The barrier fluid can be liquid or gas, and may or may not have dielectric properties.

Fig. 2 illustrates the arrangement of a hydraulic displacement unit comprising the barrier fluid pump 16 in a pressure or pressure compensated vessel 20 that is filled with barrier fluid. Inside the hydraulic displacement unit fluid will leak from high pressure areas to low pressure areas. The fluid leakage needs to be removed since pressure buildup inside the pump housing would otherwise risk damaging the shaft seals in the hydraulic displacement unit. Conventionally this leakage fluid is sent to a drain port connected to a tank, or alternatively the drain port is connected to the pump or compressor suction side. In the embodiment of Fig. 2 the drain outlet is open to the inside of the vessel 20, permitting leakage fluid 21 into the fluid filled interior of the vessel. Differential pressure across shaft seals and housing walls will then be zero. Excessive fluid inside the vessel 20 can escape to the side with lowest pressure side of the hydraulic unit via one-way valves 22 and 23.

It is preferred that the electronic components of the barrier fluid regulation system, comprising the controller 14, the VSD 17 and the electric motor 15, are housed inside an atmospheric container filled with nitrogen. However, the motor 15 may alternatively be installed in a pressurized fluid filled environment.

Fig. 3 illustrates an arrangement wherein the electronic components 14 and 17 are installed at atmospheric conditions in a container 24, whereas the motor 15 and the pump 16 share a common fluid filled vessel 20.

Fig. 4 illustrates the arrangement of the barrier fluid pump 16 in the vessel 20 filled with barrier fluid, whereas the electronic components including the controller 14, the VSD 17 and the electric motor 15 are installed in an atmospheric container 24. The motor 15 and pump 16 are drivingly connected via a magnetic coupling 25 installed on a drive shaft 26 between motor and pump.

The barrier fluid system components are advantageously located inside a retrievable unit. Two redundant retrievable units may be considered enabling uninterrupted operation during replacement of the failing unit. In particular, system reliability can be increased by redundancy of critical components.

Fig. 5 illustrates an embodiment of the barrier fluid regulation system that provides electrical redundancy. The embodiment of Fig. 5 comprises dual motors 15 and 15' installed in a fluid filled vessel 20. The motors 15, 15' are drivingly connected to a singular pump 16 via a common drive shaft 27. Each motor 15 and 15' is individually controlled via separate controllers 14 and 14' and associated VSDs 17 and 17' installed in an atmospheric container 24. Each motor control is separately powered via power and communications connections 28, 29 and 28', 29', respectively. Fig. 6 illustrates an embodiment of the barrier fluid regulation system that provides complete redundancy. The embodiment of Fig. 6 comprises dual pumps 16 and 16' installed on parallel fluid supply lines 9' and 9" in a fluid filled vessel 20. Each pump is drivingly connected to a separate motor 15 or 15 ' via individual drive shafts 30 and 30'. Each motor 15 and 15' is individually controlled via separate controllers 14 and 14' and associated VSDs 17 and 17' installed in an atmospheric container 24. Each motor control is separately powered via power and communications connections 28, 29 and 28', 29', respectively.

The configuration of Fig. 6 is shown with redundant motors connected to a common supply line 9 from the barrier fluid container 8. Solenoid valves 31 and 31 ' are installed on the parallel supply lines 9' and 9". Sensors and solenoid control circuits are not shown in Fig. 6, however, each of the two redundant controllers 14 and 14' will control its own solenoid valve. Switching off power to one system will close the corresponding solenoid valve and disable the associated circuit. The barrier fluid regulation system disclosed provides simple monitoring of component wear. More precisely, by monitoring the operation of the barrier fluid pump 16 during steady state operation it is possible to estimate the internal leakage rate in the barrier fluid pump. If the VSD drives the barrier fluid pump to supply barrier fluid to the container 8, the internal leakage rate in the pump is higher than the leakage rate through the seal(s) of the pump or compressor, and vice versa. This information can be used for preventive maintenance of the barrier fluid pump 16. Also, the leakage over the seal(s) in the pump or compressor can be estimated by monitoring the level in the barrier fluid container 8 during steady state operation.

The present barrier fluid regulation system, using a hydraulic displacement unit for barrier fluid pressure control, provides several advantages compared to valve based barrier fluid regulation systems:

• No barrier fluid lost during start-up

• Enables redundancy without increasing system complexity

• Simple software monitoring of component wear • More tolerant to fluid particle contamination than valve based systems.

From the above, a skilled person will realize that modifications of the illustrated embodiments are possible without departing from the teaching of this invention which is reflected in the appended claims.