Determining the operability of a fluid driven safety valve

Abstract

For determining the operability of a fluid driven safety valve, a method comprising the following steps is described: A partial stroke test is performed on the safety valve, resulting in a stroke-pressure curve. The stroke pressure curve is extrapolated (330, 340) beyond the measured range (360) up to the safety closing position (350). From the extrapolated stroke-pressure curve, the closing pressure reserve (320) can be determined. In this way, the functionality of the safety valve can be checked during operation.

Claims

1. A method for determining the operability of a fluid driven safety valve with a valve member (120) and a spring return, wherein the valve member is to be moved to a safety position by the spring return in case of a complete pressure drop of the driving fluid; wherein the safety valve has means for determining the pressure of the driving fluid; wherein the pressure of the driving fluid acts against the spring (160); wherein the safety valve has means (180) for determining the position of the valve member; wherein the method comprises the following steps: a partial stroke test is performed on the safety valve by varying the pressure in the driving fluid; the position of the valve member (230) in dependence of the pressure (220) in the driving fluid is recorded during the partial stroke test, whereby a stroke-pressure curve is obtained; the stroke-pressure curve is mathematically modeled beyond the measured range; the mathematically determined stroke-pressure curve is extrapolated up to the safety position of the valve member (340, 350); the pressure (320; 420; 520) corresponding to this safety position in the mathematically determined stroke-pressure curve is determined.

2. The method according to claim 1, wherein, for the extrapolation of the stroke-pressure curve, the stroke-pressure curve is modelled in a relevant range by a linear equation; and the linear equation for the stroke-pressure curve is determined from at least two stroke-pressure values (220, 230; 330) measured during the partial stroke test.

3. The method according to claim 2, wherein, the at least two stroke-pressure values (220, 230; 330) measured during the partial stroke test are measured after the static friction of the valve member has been overcome.

4. The method according to claim 1, wherein, during commissioning of the safety valve, a stroke-pressure characteristic curve for the safety valve is recorded; and the stroke-pressure curve is mathematically modelled beyond the measured range by shifting the stroke-pressure characteristic curve recorded during commissioning of the safety valve to a stroke-pressure value measured during the partial stroke test after the static friction of the valve member has been overcome.

5. The method according to claim 4, wherein, the stroke-pressure characteristic curve for the safety valve recorded during commissioning of the safety valve is approximated by a linear equation in a range after the static friction of the valve member has been overcome.

6. A fluid driven safety valve having a valve member (120) and a spring return; wherein the valve member is to be moved to a safety position by the spring return in case of a complete pressure drop of the driving fluid; means for determining the pressure of the driving fluid; wherein the pressure of the driving fluid acts against the spring (160); means (180) for determining the position of the valve member; a control having means to carry out a sequence of steps according to claim 1, wherein the control performs a partial stroke test on the safety valve by varying the pressure in the driving fluid; and wherein during the partial stroke test, the control records the position of the valve member (230) dependent on the pressure (220) of the driving fluid during the partial stroke test, whereby a stroke-pressure curve is obtained; the control calculates the stroke-pressure curve beyond the measured range; the control further calculates and extrapolates (340, 350) the stroke-pressure curve to the safety position of the valve member; and determines the pressure (320; 420; 520) corresponding to this safety position in the calculated stroke-pressure curve.

7. A processor coupled to a memory programmed with executable instructions which control a safety valve with a valve member driven by a fluid, wherein the executable instructions carry out steps which perform a partial stroke test on the safety valve by varying a pressure in the fluid; and during the partial stroke test, record the position of the valve member dependent on the pressure of the fluid during the partial stroke test, whereby a stroke-pressure curve is obtained, calculate the stroke-pressure curve beyond the measured range, further calculate and extrapolate the stroke-pressure curve to a safety position of the valve member; and determine the pressure corresponding to the safety position in the stroke-pressure curve.

Description

(1) An embodiment is shown schematically in the figures. Identical reference numbers in the individual figures denote identical or functionally identical elements or elements corresponding to each other with respect to their functions. Specifically,

(2) FIG. 1A shows the typical arrangement of a fluid-driven safety valve in the open position;

(3) FIG. 1B shows the typical arrangement of a fluid-driven safety valve in the closed position;

(4) FIG. 2 shows stroke and pressure over time during a partial stroke test (PST) on a safety valve;

(5) FIG. 3 shows a stroke-pressure curve;

(6) FIG. 4 shows another example of the break-away reserve and the closing pressure reserve; and

(7) FIG. 5 shows an example of a negative closing pressure reserve.

(8) FIG. 1A shows a typical arrangement of a fluid-driven, single-acting process valve 100 with a valve housing 105. Between the inflow side 110 and the outflow side 115 there is a valve member 120 which can be pressed into the valve seat 125 to throttle the flow of a process fluid flowing from inflow side 110 to outflow side 115. The valve rod or drive rod 130 is used for this purpose. The passage of the valve rod 130 through the fluid-tight valve housing 105 is sealed by a seal or stuffing box 140.

(9) At the upper end of the valve rod 130 there is a fluidic drive 145, wherein the driving fluid is typically gas. The drive 145 has two chambers, a lower compressed air chamber 150 and an upper chamber 155, in which two springs 160 act on the valve rod 130 via a plate 165. The two chambers 150 and 155 are separated by a membrane 170, wherein the membrane 170 is impermeable to the driving fluid, typically compressed air. Such a design is referred to as a single-acting pneumatic drive because compressed air is introduced into only one chamber, the compressed air chamber 150, and not both. The valve rod 130 must pass through the housing of the drive 145 and be sealed against the driving fluid. For this purpose, this passage is sealed by a drive housing seal 175.

(10) Typically, the valve rod 130 also has a signal receiver 180 to determine the position of the valve member 120.

(11) In FIG. 1A there is sufficient compressed air in the compressed air chamber 150 so that the process valve 100 is open.

(12) In FIG. 1B the compressed air chamber 150 is vented such that the springs 160 could close the process valve 100.

(13) Such a process valve is basically suitable for use as a safety valve, as it closes automatically in the event of uncontrolled venting.

(14) Two types of forces or torques can occur in an open/close valve which counteract the movement of the drive 145. These are on the one hand the static friction and on the other hand the sliding friction. An increased static friction can be recognized by a change in the breakaway pressure. The sliding friction leads to a parallel displacement of the stroke-pressure curve. In the worst case, both effects can lead to the valve no longer being able to close.

(15) In order to detect this in time, the pressure or force or torque reserve can be determined with a PST (conversion of the pressure reserve by means of the torque characteristic curve of the drive to the torque reserve). If, for example, the process pressure of the process medium is known, the minimum pressure or force or torque reserve required for safe closing of the process valve can be determined from the pressure surface of the valve element to which the process pressure can be applied.

(16) FIG. 2 shows the relative stroke of the valve member 120 and the pressure over time during a Partial Stroke Test (PST) on a safety valve 100. It is a single-acting pneumatic drive with spring return. The safety position is closed without electrical power.

(17) The setpoint 210 shows the ideal frictionless stroke when closing the valve with constant venting. The pressure is reduced and the spring forces of the drive 145 are released and move the valve member 120 in the direction of the closed position.

(18) The actual pressure curve 220 first shows a significant reduction in pressure before the stroke (actual stroke) 230 changes. This shows the breaking away of the valve member 120 from its open position, in which it was possibly already stuck, i.e. subject to static friction. The difference between the initial, maximum pressure and the pressure at the moment of breakaway is called breakaway pressure 240. The breakaway pressure is the pressure or force that is sufficient to overcome the static friction and disengage the valve.

(19) The pressure in the driving fluid at the moment of breakaway remains as reserve 250 if it is greater than 0 bar. If not, the safety valve 100 can no longer fulfil its function.

(20) The pressure is readjusted after the breakaway, i.e. increased again, so that an overshoot is avoided or reduced.

(21) After overcoming the static friction and after the accelerated stroke movement, uniform venting of the drive 145 takes place (slow pressure reduction) until 90% of the stroke is reached. The actual stroke curve 230 is approximately linear in the area of sliding friction and parallel to the nominal stroke curve 210, shifted parallel by the amount of the sliding friction.

(22) After reaching 90%, the pressure in the drive 145 is increased again so that the valve member 120 fully opens again against the spring forces. The PST has been completed.

(23) By eliminating the time and plotting the pressure 220 over the stroke 230, the stroke-pressure curve of FIG. 3 can be generated from FIG. 2.

(24) The closing pressure reserve 320 is the pressure reserve that remains after the safety valve has been closed. The closing pressure reserve 320, which occurs with constant movement (sliding friction), can be determined from FIG. 3. For this purpose, the stroke-pressure curve is extrapolated according to FIG. 3 up to the closed position (stroke=0).

(25) Thereby there are various possibilities to determine the progression of the curve. Either one determines a linear equation during the movement of the PST (after overcoming the static friction), which is used for the extrapolation. Or one can take a reference curve (characteristic curve) which was recorded during initialization or commissioning. Furthermore, the theoretically determined characteristic curve of the spring could also be used.

(26) FIG. 3 shows the determination of the closing pressure reserve 320 under the assumption of a constant linear movement beyond the range determined by the PST. The interpolation curve 340 was determined using two measured pressure points 330. The closing pressure reserve 320 corresponds to the pressure value 350 at stroke=0 on the interpolation line 340.

(27) During the PST, a reversal of motion 360 occurs (here at 90% of the stroke). The hysteresis resulting from the reversal can also be taken into account when determining the linear equation (not shown in the diagram). If several measured stroke-pressure points 330 from the PST are available, the less favourable points for the closing pressure reserve 320 are used to determine the linear equation.

(28) In FIG. 3, the reserve for the breakaway pressure 310 is greater than the reserve for complete closing 320.

(29) In FIG. 4, the breakaway pressure reserve 410 is less than the complete closing reserve 420.

(30) In FIG. 5 even a negative closing pressure reserve 520 occurs, so that a safe closing of the safety valve 100 is no longer possible. This case can occur with very high sliding friction, e.g. at the valve rod. Furthermore FIG. 5 shows the breakaway reserve 510 for this example.

Glossary

(31) Fluidic Drive of a Valve

(32) One refers to a fluid driven valve, if the drive rod of the valve is moved by a membrane which is pressurized by a fluid, typically compressed air, and thereby positioned.

(33) Stroke-Pressure Curve

(34) The stroke-pressure curve of a valve or a partial stroke test gives the position of the valve member, in other words: the stroke, in dependence of the pressure in the drive fluid of the fluidic driven valve.

(35) PST: see Partial Stroke Test

(36) Partial Stroke Test

(37) In order to ensure the safe operation of a fitting, it is cyclically tested whether the actuator moves. During these tests it is not desired that the fitting moves completely to the safety position in order not to disturb the running process. In a partial stroke test, the actuator is only moved as far as is necessary to ensure that the actuator moves a part of its travel without significantly influencing the process of the plant. Thereby it is also tested whether the actuator detaches or breaks loose from its position at all. The actuator returns to its initial position after the partial stroke test. With this test the basic movability of the actuator can be tested.

(38) Valve Member

(39) The valve member is the element that closes the valve when it is pressed onto the valve seat.

REFERENCE NUMERALS

(40) 100 process valve 105 valve housing 110 inflow side 115 downstream side 120 valve member 125 valve seat 130 valve rod or actuator rod 140 stuffing box or seal 145 fluidic drive 150 compressed air chamber 155 upper chamber 160 spring 165 plate 170 membrane 175 Drive housing sealing 180 signal receiver 210 nominal value of the stroke 220 actual value of the pressure 230 actual value of the stroke 240 breakaway pressure 250 breakaway pressure reserve 310 breakaway reserve 320 closing pressure reserve 330 two points for the determination of the interpolation line 340 interpolation line 350 pressure value at stroke=0 360 90% of stroke, end of PST 410 breakaway reserve 420 closing pressure reserve 510 breakaway reserve 520 negative closing pressure reserve