FIELD OF THE INVENTION

The present invention relates to a flow measuring device.

BACKGROUND OF THE INVENTION

A positive displacement measurement of a flow is basically about measuring the position of a piston while being displaced by the flow. A linear variable differential transformer LVDT, which also are referred to as a linear variable displacement transformer, linear variable displacement transducer etc., is used to measure the position of the piston. By measuring the time it takes for the piston to move a certain distance and by knowing the diameter of the cylinder, the flow can be calculated. The advantage of the positive displacement flow measurement is the accuracy. The disadvantage is the complicity since there must be a mechanism that makes the piston change its direction of motion when it reaches the end of the cylinder. Generally, there are two alternatives for changing the pistons direction of motion.

The first alternative is known from the article“Hydraulic Four-Way Valves” dated 13.01.2011 (retrieved 19.04.2018 from http://www.valvehydraulic.info/valve- 2/hydraulic-four-way-valves.html). Here, it is shown a four-way valve (also referred to as a 4/2 valve) having a pressurized flow source (P), a return to tank (T) and two ports connected to the hydraulic cylinder (A and B). The position of the valve determines the movement direction of the piston.

The second alternative is shown in fig. 1. Here, it is shown a two-way valve (also referred to as a 2/2 valve) connected to a single acting cylinder with spring return. Gravity can also be used to return the piston. When the two-way valve closes, the piston is forced to move against the direction of the spring force (or gravity). When the two-way valve opens the spring (or gravity) returns the piston to its starting point.

Both alternatives have disadvantages. A four- way valve has four times more sealing surfaces than a two-way valve. Hence, it has four times more potential leakage paths and it is approximately four times harder to actuate because of the friction from the sliding seals. From a subsea installation point of view that is

disadvantageous because the electric power available to actuate the valve is limited. The disadvantage of a gravity or spring returned piston is that the flow measurement can only take place when the piston is moving in the direction where it is displaced by the flow. When the spring or gravity returns it to the starting point, no flow measurement can be made. This is a dead stroke and a continuous measurement of the flow is therefore not possible. If a spring is used to return the piston, the gradually increasing compression of the spring can influence on the flow

measurement since a higher pressure is needed to move the piston at the end of the stroke compared to the beginning of the stroke.

A limiting factor to all flow measurements principles is the flow range. Ideally a flow measuring device should be able to handle a wide range of flows to reduce the need for various sizes of the device. For subsea installations this is particularly important because the project phase is usually so long that the target flow ranges estimated at the beginning of the project quite often turns out to be something else at the end of the project. Hence the flow measuring device selected at the beginning of the project does not fit at the end of the project. A flow measurement device that could have handled a much wider flow range than current flow meters would have solved this problem.

Hence, one object of the invention is to provide a flow measurement device which can handle a large variation in flow range.

Another object of the invention is to provide a flow measurement device where flow can be measured continuously with a reduced number of seals, i.e. few potential leak paths.

Another object of the invention is to provide a flow measurement device having a relatively simple mechanism to change the direction of motion of the piston or pistons.

Another object of the invention is to provide a flow measurement device with a low electric power consumption.

Another object of the invention is to provide a flow measurement device where it is relatively easy for the linear variable differential transformer to sense the position of the piston.

Another object of the invention is to provide a flow measurement device where erosion due to particles in the fluid is reduced. Such erosion may cause damages and wear to the components of the flow measurement device.

SUMMARY OF THE INVENTION

The present invention relates to a flow measuring device for measuring a parameter representative of the fluid flow rate between a fluid inlet and a fluid outlet, comprising:

- a housing and a piston axially displaceable within the housing;

- the piston comprises a first piston section arranged within a complementary first housing section and a second piston section arranged within a complementary second housing section; where the first and second piston sections are different from each other,

- the first piston section comprises a first piston surface area forming a first compartment with the first housing section;

- the second piston section comprises a second piston surface area and a third piston surface area forming respectively a second compartment and a third compartment with the second housing section; where there are provided:

- a first fluid line connecting the first compartment with the fluid inlet;

- a second fluid line connecting the second compartment with the fluid outlet;

- a third fluid line connecting the third compartment with the fluid outlet;

- a fourth fluid line connecting the first compartment with the second compartment, where:

- a first valve is provided in the second fluid line;

- a second valve is provided in the fourth fluid line; and

- a position sensor for measuring an axial displacement of the piston within the housing, the axial displacement being representative of the fluid flow rate;

- the flow measuring device further comprises a control system for controlling the first valve.

Preferably, the first valve is a two-way valve, which, when controlled to be open, is allowing fluid flow from the second compartment to the fluid outlet.

Preferably, the second valve is a check valve. Here, the second valve is allowing fluid to flow from the first compartment to the second compartment when the pressure is higher in the first compartment than in the second compartment.

In the present description, the term“two-way valve” is defined to be a valve with one inlet and one outlet, where fluid communication between the inlet and the outlet is controlled to be either open or closed.

In the present description, the term“check valve” is defined to be a valve with one inlet and one outlet, where fluid communication between the inlet and the outlet is closed when the fluid pressure at the inlet is equal to the fluid pressure at the outlet and where fluid communication between the inlet and the outlet is open when a predetermined pressure difference between the inlet and the outlet occurs.

The control system is preferably controlling the first valve based on signals from the position sensor. Alternatively, the control system may control the first valve based on signals from other sensors capable of sensing parameters representative of the end position of the piston.

The fluid flow rate can be determined by the axial displacement of the piston within the housing per unit of time. In one aspect, the function of the timer unit and the calculation unit may be located elsewhere, such as topside, either as separate units or implemented as part of other monitoring software and/or hardware.

In one aspect, the control system of the flow measuring device may comprise a timer unit and a calculation unit, where the flow measuring device is configured to output axial displacement per unit of time, which is also a parameter representative of the fluid flow rate between the fluid inlet and the fluid outlet.

Alternatively, the fluid flow rate may be calculated by means of the axial

displacement per unit of time and physical parameters of the flow measuring device. Alternatively, the flow measuring device can be calibrated before use by means of a calibration process. In these alternatives, the control system may calculate the fluid flow rate as a fluid volume per time unit based on the axial displacement, the time and physical parameters or based on the axial displacement, the time and calibration data from the calibration process. Here, the flow measuring device is configured to output fluid volume per time unit, which is also a parameter representative of the fluid flow rate between the fluid inlet and the fluid outlet.

In one aspect, the position sensor is located radially outside of the first piston section for measuring the position of the first piston section relative to the housing.

Alternatively, the piston is connected to a piston rod protruding out from the piston housing. In such an embodiment, the position sensor may measure the displacement of the piston rod as a parameter representative of the displacement of the piston within the housing.

In one aspect, the first and second piston sections are different from each other in that the first piston section has a first diameter and the second piston section has a second diameter different from the first diameter, where the first diameter of the first piston section is smaller than the second diameter of the second piston section.

In one aspect, the fourth fluid line and the second valve are integrated in the piston.

The fourth fluid line may be provided as an axial through bore within the piston, where the second valve is provided in the axial through bore.

Alternatively, the fourth fluid line and the second valve are located radially outside of the piston. In such an alternative, the fourth fluid line is at least partially integrated in the housing, while the second valve may be connected to or provided outside of the housing.

In one aspect, the first piston surface area and the second piston surface area are facing in opposite directions. Preferably, the first piston surface area and the second piston surface area are provided in opposite ends of the piston.

In one aspect, the flow measuring device is configured to be in a resting mode, in which the piston is positioned distal from the first fluid line and the first valve is open.

In this resting position of the piston, fluid is allowed to flow through the flow measuring device from the inlet to the outlet.

In one aspect, the flow measuring device is configured to be in a first flow measuring mode in which the first valve is closed and in which fluid flow into the fluid inlet will cause the piston to move in a first direction towards the first fluid line.

During the piston movement in the first direction, the position sensor is measuring the axial displacement of the piston.

In one aspect, the flow measuring device is configured to be in a second flow measuring mode in which the first valve is open and in which fluid flow into the fluid inlet will cause the piston to move in a second direction away from the first fluid line.

During the piston movement in the second direction, the position sensor is measuring the axial displacement of the piston.

In one aspect, the second valve is a check valve allowing fluid flow through the fourth fluid line from the first compartment to the second compartment when a predetermined pressure difference between the first compartment and the second compartment occurs.

Alternatively, the second valve is an active valve controlled by the control system, where the second valve is controlled to be:

- open in the resting mode;

- open in the first flow measuring mode;

- closed in the second flow measuring mode.

In one aspect, the flow measuring device further comprises a first movement preventing fluid line providing fluid communication between the first compartment and the second compartment when the piston is moved a predetermined distance in the second direction.

In one aspect, the flow measuring device further comprises a second movement preventing fluid line providing fluid communication between the second

compartment and the third fluid line when the piston is moved a predetermined distance in the first direction. The flow measuring device preferably comprises a connection interface for outputting the flow measurement signal. The connection interface typically comprises communication bus connector. Preferably, the connection interface also comprises a power supply connector for connection to a power source, as the position sensor, the first valve, optionally the second valve and the control system are operated by means of electric power.

As discussed above, it should be noted that the signals from the position sensor could be outputted from the flow measuring device via the connection interface directly or via the control system. The signals can be outputted as un-processed signals as a parameter representative of the axial of the piston. Alternatively, the control system can be configured to determine the fluid flow rate by means of the signals from the position sensor and then output the fluid flow rate itself via the connection interface. In the latter case, the control system comprises a timer.

In one aspect, the first piston section and the second piston section are provided as one single piston body having a mainly T-shaped cross section.

Alternatively, the first piston section and the second piston section are provided as separate bodies being connected to each other.

Preferably, the position sensor is a linear variable differential transformer. DETAILED DESCRIPTION

Embodiments of the invention will now be described with reference to the enclosed drawings, where:

Fig. 1 illustrates a prior art two-way electrically operated control valve;

Fig. 2 illustrates a first embodiment of flow measuring device;

Fig. 3 illustrates an enlarged view of the piston and the piston cylinder housing of fig· 2;

Fig. 4 illustrates an alternative embodiment to the flow measuring device of fig. 2; Fig. 5 illustrates yet an alternative embodiment to the flow measuring device of fig. 2;

Fig. 6 illustrates yet an alternative embodiment to the flow measuring device of fig.

2;

Fig. 7 illustrates the initial mode;

Fig. 8 illustrates the first mode;

Fig. 9 illustrates the second mode;

Fig. lOa-c illustrates the piston and piston cylinder housing device of yet an alternative embodiment of the flow measuring device of fig. 2, where the piston is shown in different positions. It is now referred to fig. 2 and 3, where a first embodiment of a flow measuring device is shown. The flow measuring device 1 is indicated by a dashed rectangle 1 and has a fluid inlet 2 and a fluid outlet 3. The purpose of the flow measuring device 1 is to measure a parameter representative of the fluid flow rate between the fluid inlet 2 and the fluid outlet 3.

The flow measuring device comprises a housing 10 and a piston 20 axially displaceable within the housing 10. In fig. 3, the longitudinal center axis of the piston 20 is shown as dashed line II, which line is also indicating the axial direction of the piston movement.

The piston 20 comprises a first piston section 20a and a second piston section 20b, where the first and second piston sections 20a, 20b are different from each other. In the present embodiment, the piston sections have a circular shape when viewed in a plane perpendicular to the dashed line II, i.e. the piston sections are cylindrical. However, the piston sections can also have an oval, triangular, rectangular or polygonal cross section.

As shown in fig. 3, the first and second piston sections 20a, 20b are different from each other in that the first piston section 20a has a first diameter D20A and the second piston section 20b has a second diameter D20B different from the first diameter D20A, where the first diameter D20A of the first piston section 20a is smaller than the second diameter D20B of the second piston section 20b.

There piston in fig. 2 and 3 has three piston surfaces. The first piston section 20a comprises a first piston surface area A20A provided in a first end of the piston 20 and the second piston section 20b comprises a second piston surface area A20B provided in a second end of the piston 20. Hence, the first and second surface areas A20A and A20B are provided in opposite ends of the piston 20.

A dashed separation line SL indicates the separation area between the first and second piston sections 20a, 20b. At this separation line SL, the second piston section 20b protrudes radially from the first piston section 20a, forming a third piston surface area A20C. The second and third piston surface areas A20B, A20C are provided on opposite sides of the second piston section 20b.

The difference between the diameter D20B and the diameter D20A is indicated in fig. 3 as a difference D20C. Hence, the second piston surface area A20B is equal to the sum of the first piston surface area A20A and the third piston surface area A20C.

As apparent from the cross-sectional view of the piston 20 of fig. 2 and 3, the piston 20 is substantially T-shaped in cross section. The piston sections 20a, 20b can be made as separate bodies connected to or fixed to each other, or the piston 20 can be made as one body.

The housing 10 is adapted or complementary to the piston 20 and comprises a first housing section lOa adapted or complementary to the reciprocating first piston section 20a and a second housing section lOb adapted or complementary to the reciprocating second piston section 20b. Hence, the length (in the axial direction) of the first housing section lOa is e equal to the length of the first piston section 20a and the length of the second housing section lOb is equal to the length of the second piston section 20b plus the effective stroke length. The length of lOa and 20a must also be at least as long as the effective stroke length.

According to the above, the first piston surface area A20A is forming a first compartment 1 la with the first housing section lOa. The second piston surface area A20B is forming a second compartment 1 lbl with the second housing section lOb. The first and second compartments 1 la, 1 lbl are provided in opposite ends of the housing 10. The third piston surface area A20C is forming a third compartment 11 b2 within the second housing section lOb. The third compartment 1 lb2 is located axially between the first and second compartments 1 la, 1 lbl .

As shown in fig. 3, a first sealing device l2a is provided between the first piston section 20a and the first housing section lOa, separating the first compartment 1 la from the third compartment 1 lb2. A second sealing device l2b is provided between the second piston section 20b and the second housing section lOb, separating the second compartment 1 lbl from the third compartment 1 lb2. The first and second sealing devices l2a, l2b are preferably o-rings or other well known types of sealing elements. It should be noted that in the present embodiment, the first and second sealing devices l2a, l2b are provided in recesses in the piston 20, i.e. they follow the axial movement of the piston 20 within the housing 10.

The fluid in these compartments 11 a, 1 lbl, 11 b2 will exert a force to the piston, where the forces applied by the fluids in the respective compartments are dependent on both the fluid pressure and the areas A20A, A20B and A20C.

In fig. 2, it is shown that the fluid measuring device 1 comprises four fluid lines 41 , 42, 43, 44, where a first fluid line 41 is connecting the first compartment 1 la with the fluid inlet 2, a second fluid line 42 is connecting the second compartment 1 lbl with the fluid outlet 3, a third fluid line 43 is connecting the third compartment 11 b2 with the fluid outlet 3 and a fourth fluid line 44 is connecting the first compartment 1 la with the second compartment 1 lbl . Hence, fluid may flow from the inlet 2 to the outlet 3 via the compartments 1 la, 1 lbl and fluid may also flow between the outlet 3 and the third compartment 1 lb2. The fluid measuring device 1 further comprises a first valve 32 provided in the second fluid line 42 and a second valve 34 provided in the fourth fluid line 44.

In fig. 2, it should be noted that the second fluid line 42 is separated by the first valve 32 into a first fluid line section 42a between the second fluid compartment 1 lbl and the first valve 32 and a second fluid line section 42b between the first valve 32 and the outlet 3.

The first valve 32 is a controllable two-way valve, which, when controlled to be open, is allowing fluid flow from the second compartment 1 lbl to the fluid outlet 3. When the first valve 32 is controlled to be closed, fluid is prevented from flowing from compartment 1 lbl to the fluid outlet 3.

In the present embodiment, the second valve 34 is a passive check valve. If the fluid pressure in first compartment 1 la is lower than or equal to the fluid pressure in the second fluid compartment 1 lbl, the second valve 34 closes, and fluid flow through the fourth fluid line 44 is prevented. If the fluid pressure in first compartment 1 la is higher than the fluid pressure in the second fluid compartment 1 lbl , the second valve 34 opens, and fluid flow through the fourth fluid line 44 is allowed. Typically, a fluid pressure difference between the fluid pressure in the first compartment 11a and fluid pressure in the second compartment 1 lbl must be above a certain fluid pressure difference threshold value Drί34 in order to open the second valve. The second valve 34 is passive in that no electrical power is needed to operate the valve.

The fluid measuring device 1 further comprises a position sensor 30 for measuring an axial displacement of the piston 20 within the housing 10. The position sensor 30 is in the present embodiment a linear variable differential transformer LVDT. This type of position sensor is considered known for a person skilled in the art and will not be described in detail here. Such sensors convert a position or linear

displacement of the piston from a mechanical reference (zero, or null position) into a proportional electrical signal containing phase and amplitude for position information. As the piston will be moved by the fluid flow, as will become apparent from the description below, the electric signal outputted from the position sensor 30 will be a parameter representative of the fluid flow rate (fluid flow per unit of time).

In the present embodiment, the position sensor 30 is located radially outside of the first piston section 20a for measuring the position of the first piston section 20a relative to the housing. As the diameter of the first piston section 20a is relatively smaller than the diameter of the second piston section 20b, a wall thickness WTlOa of the first housing section lOa can be thinner than a wall thickness WTlOb of the second housing section lOb due to a relatively lower cylinder circumference in the first compartment 1 la. This allows the position sensor 30 to be positioned closer to the piston section 20a, with more accurate measurements as a result. The most important dimensioning factor for the wall thickness is the overall working pressure of the system. For subsea injection purposes this can involve pressures up to 20.000 psi (1380 bar). The effect of the pressure difference between the compartments is negligible. It is typically less than 10 bar. Under such high pressure conditions the smaller diameter of the housing section lOa can retain the pressure with a significantly thinner wall thickness than housing section lOb.

In an alternative embodiment, the position sensor 30 could be located radially outside of the second piston section 20b for measuring the position of the second piston section 20b relative to the housing 10. In yet an alternative embodiment, the piston can be mechanically connected to a piston rod, where the position sensor 30 is located radially outside of the piston rod for measuring the relative position of the piston rod.

The flow measuring device 1 further comprises a control system 36 connected to the first valve 32, to the position sensor 30 and to a connection interface 5.

In the present embodiment, the control system 36 takes care of communication via the connection interface 5, which typically will be a standard or proprietary communication bus. The control system 36 outputs information representative of the fluid flow rate between the fluid inlet 2 and the fluid outlet 3 via the communication interface 5, based on signals received from the position sensor 30. The control system 36 also receives information via the communication interface 5. The received information may typically be a request to start a flow rate measurement.

The control system 36 will typically be a digital signal processor, a microcontroller etc. programmed to communicate with the position sensor 30, the first valve 32 and the communication interface 5. In some alternative embodiments, the control system 36 may perform additional operations, which will be described in the alternative embodiments below.

It should be noted that on the present embodiment, the control system 36 also receives electrical power via the connection interface 5.

In fig. 2 it is shown that the fourth fluid line 44 is provided as a through bore within the piston 20 and that the second valve 34 is provided as a section of the bore.

Operation of flow measuring device

The operation of the flow measuring device 1 will now be described with reference to the figs. 7 - 9.

Initial mode

In fig. 7, an initial mode or resting mode SO is shown. Here, the piston 20 is resting in a position distal from the first fluid line 41 , i.e. with a relatively small volume of fluid in the second fluid compartment 1 lbl . Here, the first valve 32 is controlled to be open. Preferably, no or little power is required to keep the first valve 32 open.

In this resting mode, fluid is allowed to flow through the flow measuring device 1 from the inlet 2 to the outlet 3 via the first fluid line 41, the first fluid compartment 1 lbl, the fourth fluid line 44, the second fluid compartment 1 lbl and the second fluid line 42. It should be noted that the fluid flow through the fourth fluid line 44 will stop if the fluid pressure over the second valve 34 is not above the fluid pressure difference threshold value mentioned above.

If a fluid rate measurement is desired, an instruction signal is sent by the control system 36 to close the first valve 32. The control system 36 will typically do this based on a signal received by the control system 36 via the interface 5.

Alternatively, the closing signal may be sent by the control system 36 to the first valve 32 in a predetermined manner, for example periodically (every hour, every week etc).

First flow measuring mode

When the first valve 32 is closed, the flow measuring device 1 is considered to be in a first flow measuring mode SI , illustrated in fig. 8.

Now, fluid will not exit from the second compartment 1 lbl to the outlet 3 via the first valve 32. Hence, fluid pressure will increase in the first fluid compartment 11a and also in the second fluid compartment 1 lbl , as the second valve 34 will open when the fluid pressure difference threshold value Apt34 is reached.

As mentioned above, the fluid pressure in the third compartment 11 b2 is always equal to the pressure of the fluid outlet 3, which here is assumed to be constant and lower than the fluid pressure in the fluid inlet 2.

As the fluid pressures in the first and second compartments 11a, 1 lbl increases and as the second piston area A20B is larger than the first piston area A20A, the piston will move in a first direction illustrated by arrow A in fig. 8 towards the first fluid line 41. The third compartment 1 lb2 will be emptied via the third fluid line 43 to the outlet 3. During the piston movement in the first direction A, the position sensor 30 is measuring the axial displacement of the piston 20.

Mathematically expressed, the piston 20 will move to the left when:

(area A20B x pressure in second compartment 1 lbl) > (area A20A x pressure in first compartment 1 la) + (area A20C x pressure in third compartment 1 lb2)

Since the pressure in the second compartment 1 lbl can be expressed as the pressure in the first compartment 1 la minus the threshold value Apt34 of the second valve 34, the equation can be reformulated as: (area A20B x (pressure in the first compartment 1 la minus the threshold value Apt34)) > (area A20A x pressure in first compartment 1 la) + (area A20C x pressure in third compartment 1 lb2)

It is now assumed that area A20A is lcm ² , area A20B is lOcm ² and area A20C is 9cm ² (as mentioned above, the second piston surface area A20B is equal to the sum of the first piston surface area A20A and the third piston surface area A20C).

It is further assumed that the outlet pressure is 100 bar, that the threshold value Apt34 is 2 bar and that the piston moves without friction.

The above equation can be solved, with the result that the pressure in the first compartment l la > 102.2 bar.

Hence, the piston will move to the left when the pressure in the first compartment l la exceeds the threshold value Apt34 of the second valve 34 with only 0.2 bar.

If the fluid rate is high, the piston 20 will move faster towards the first fluid line 41 than if the fluid rate is low. During the piston movement, the position sensor 30 will send signals to the control system 36. As will be described below, these signals can be transferred directly or unprocessed to the interface 5. Alternatively, the control system 36 may process the signals received from the position sensor 30 and then transfer the result of this processing to the interface 5.

The fluid flow rate can here be theoretically calculated based on the second piston area A20B minus the first piston area A20A, the length of the piston displacement and time of the piston displacement.

Second flow measuring mode

When the piston 20 has moved in the direction A to its other end position, i.e. the piston 20 being positioned proximal to the first fluid line 41 , the control system 36 is configured to send an opening instruction to the first valve 32 in order to open the first valve 32. Preferably, the opening instruction is sent based on information from the position sensor 30 indicating that the movement in the direction A is close to reaching its end position.

When the first valve 32 has been opened again, the flow measuring device 1 is considered to be in a second flow measuring mode S2, illustrated in fig. 9.

Fluid is now allowed to flow out from the second fluid compartment 1 lbl via the first valve 32 and the fluid pressure in the second fluid compartment 1 lbl is equal to the fluid pressure of the outlet 3. Consequently, the piston 20 will move in the direction B, i.e. away from the inlet 2. During the piston movement in the second direction B, the position sensor 30 is measuring the axial displacement of the piston 20 as described above.

The fluid flow rate can here be theoretically calculated based on the first piston area A20A, the length of the piston displacement and time of the piston displacement.

It should be noted that in this second flow measuring mode S2, the second valve 34 will be closed, as the threshold value Apt34 required to open the second valve 34 multiplied with area A20A will typically be equal to or larger than the frictional forces needed to move the piston and to drive the fluid out through flow channel 42 and 43. However, if an abrupt fluid pressure surge is arriving at the inlet 2, the second valve 34 may open in a short period of time, as the second valve 34 may open faster than the piston is able to be displaced.

Second embodiment

It is now referred to fig. 4. In most aspects, the embodiment in fig. 4 is similar to fig. 2 and 3. Only differences between fig. 4 and the above embodiments will be described here.

First, the fourth fluid line 44 is not provided as a bore through the piston 20.

Instead, the fourth fluid line 44 is provided as a fluid line located radially outside of the piston 20. In such an alternative, the fourth fluid line 44 could be integrated in the housing 10, while the second valve 34 may be connected to or provided outside of the housing 10. In fig. 4, it is also shown that the second valve 34 is connected to the control system 36, indicating that the second valve 34 can be controlled by the control system 36.

In such an embodiment, the second valve 34 will be controlled to be open in the resting mode SO, open in the first flow measuring mode Sl and closed in the second flow measuring mode S2.

Alternately, the second valve 34 may also here be a passive check valve as described above with respect to the first embodiment.

Third embodiment

It is now referred to fig. 5.

In most aspects, also the embodiment in fig. 5 is similar to fig. 2 and 3. Only differences between fig. 5 and the above embodiments will be described here. Here, a fifth fluid line 45 is provided between the inner surface of the first housing section lOa and the first fluid line section 42a of the second fluid line 42. When the piston 20 moves to the right in fig. 5, i.e. the flow measuring device 1 is in its second flow measuring mode S2, the piston 20 will stop when the o-ring l2a passes the inlet of the fifth fluid line 45, due to a direct fluid communication between the first fluid compartment 1 1a and the second compartment 1 lbl via the fifth fluid line 45. By closing the first valve 32, the piston will move in the direction A again, as described above with respect to the first embodiment. This fifth fluid line 45 can therefore be referred to as a first movement preventing fluid line 45.

Moreover, a sixth fluid line 46 is provided between the inner surface of the second housing section lOb and the outlet 3, for example via the third fluid line 43 as shown in fig. 5. When the piston 20 moves to the left in fig. 5, i.e. the flow measuring device 1 is in its first flow measuring mode S 1 , the piston 20 will stop when the o-ring l2b passes the inlet of the sixth fluid line 46, due to a direct fluid communication between the second fluid compartment 1 lb2 and the third fluid line 43 via the sixth fluid line 46. By opening the first valve 32, the piston will move in the direction B again, as described above with respect to the first embodiment. This sixth fluid line 46 can therefore be referred to as a second movement preventing fluid line 46.

Fourth embodiment

It is now referred to fig. 6.

As is known for a person skilled in the art, fluid flow rate is expressed as a volume unit per time unit. It is also known in the prior art mentioned in the introduction below, that axial displacement of a piston, i.e. a length unit, together with the time of the axial displacement of the piston, can be used to calculate the fluid flow rate either by means of geometries of the flow measuring device itself (surface areas, volumes etc) or by means of a pre-calibration process.

The control system 36 of the other embodiments of the flow measuring device 1 described above may therefore comprise a timer unit and a calculation unit, where the control system 36 is measuring the axial displacement per time unit and is calculating the fluid flow rate as a volume unit per time unit which are outputted via the interface 5.

However, the flow measuring device 1 may also output other parameters which are representative of this fluid flow rate via the interface 5. Here, it is preferred that the calculation of the fluid flow rate is performed outside of the flow measuring device 1 itself. In this case, the flow measuring device 1 may be configured to output axial displacement, which is also a parameter representative of the fluid flow rate between the fluid inlet and the fluid outlet. In fig. 6, a further control system 136 is shown, connected to the control system 36 of the flow measuring device 1 via the interface 5. The further control system 136 comprises a timer unit and a calculation unit. The further control system 136 may be configured to start the timer unit when an instruction signal is sent to the control system 136 to start a flow measurement, i.e. to close the first valve 32. The further control system 136 is configured to receive information about the axial

displacement of the piston from the control system 36 during the piston movement in the first direction A and/or in the second direction B and to calculate the fluid flow rate as a volume unit per time unit based on the information about the axial displacement and information from its timer unit.

This further control system 136 can be located topside, either as separate units or implemented as part of other control and/or monitoring systems for the subsea installation.

If the position sensor is a linear variable differential transformer, axial displacement may be outputted as a digital or analog voltage value.

It is now referred to fig. lOa, lOb and lOc, showing an alternative embodiment of the flow measuring device 1. It should be noted that the flow lines 41, 42, 43, the inlet 2 and the outlet 3 are not shown in these drawings, as they correspond to the above drawings.

In fig. lOa, the second valve 34 is provided within the piston 20 and comprises an elongated member 34a, a fluid sealing member 34b connected via a spring 34c to a first end 34k (left side in fig. lOa) of the elongated member 34a. The fourth fluid line 44 is provided axially through the piston 20, radially outside of the elongated member 34a. The fluid sealing member 34b is provided for closing the fluid line 44 through the piston 20, by sealingly engaging towards the first piston section 20a of the piston 20.

The elongated member 34a is axially displaceable within the piston 20. This axial displacement may be prevented by a valve locking mechanism 34d provided at the second piston section 20b of the piston 20, which locks a second end 341 of the elongated member 34b to the second piston section 20b. In fig. lOa, the valve locking mechanism 34d is locked, and the fluid sealing member 34b will only allow fluid through the fluid line 44 if the force provided by the fluid difference between compartments 11a and 1 lbl exceeds the biasing force of the spring 34c.

In fig. lOb, it is shown that the piston 20 has moved to the right to a position where the valve locking mechanism 34d is released by releasing members 34f provided in the compartment 1 lbl . Here the fourth fluid line 44 is open, i.e. fluid flows through the piston 20 without any fluid difference between compartments 11a and 1 lbl exceeding the biasing force of the spring 34c. This corresponds to the resting mode described above.

In addition, the axial displacement of the elongated member 34b to the right with respect to the piston 20 is limited by an pivoting lever arm 34h and the axial displacement of the elongated member 34b to the left with respect to the piston 20 is limited by a stop 34i (fig. lOc) of the piston 20 engaging the second end 341 of the elongated member 34a.

In fig. lOc, it is shown that the piston 20 has moved to the left to a position where the valve locking mechanism 34d is locked again, by means of the pivoting lever arm 34h forcing the 341 of the elongated member 34b into locking engagement with the valve locking mechanism 34d. The pivoting lever arm 34h is actuated by an actuating arm protruding in an axial direction into the compartment 1 lbl as shown in fig. lOc.

Other alternative embodiments

It should be noted that the closing instruction sent from the control system 36 causing the flow measuring device 1 to go from it first flow measuring mode to its second flow measuring mode can be based on other information than information indicating that the movement of the piston 20 in the direction A is about to reach its end position. The closing instruction can be sent from the control system 36 after a flow rate measurement has been performed, or based on information from a further sensor capable of sensing parameters indicating that an end position for the piston movement in the direction A has been reached.