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Trunnion Ball Valves Operating Dynamics Analysis

November 26, 2017

For a time, the idea has been around that a trunnion mounted ball valve could be opened under complete differential pressure if the piping system can withstand the forces as well as the increase in pressure. As dimensions and operating pressures grow, however, if that valve is opening with very little if any pressure downstream, several factors need to be considered.

Operation Property

Manufacturers have long known about the dynamic vaning effect that occurs when launching a ball valve in most scenarios in which a flow rate goes through the valve driven by the pressure differential. During the initial opening, the forces try to shut the valve again. When the valve is open, these vaning forces reverse and try to push the ball to a full open position. For this reason, trunnion mounted ball valves that were manual have been fitted using worm-type, self-locking gearboxes. When a spur gear is used (that is not self-locking), the vaning forces may back-drive the ball, the equipment and handwheel, which, consequently, can tear the handwheel in the operator’s grip. The operating features are like all ball valves. 

For years, field technicians commissioning pipelines reported that before moving to a full open position actuated ball valves will open to about 20 to 25 degrees then cease. This typically happened when filling a pipeline that was new, and it had been brushed off as something which happened because the actuator was either undersized or the supply pressure was not set properly. The issue is manifesting itself in factories where gas tests are performed as pressures and valve sizes have grown.

Testing

Testing valves for operability in a factory is different from studying how valves will perform in the specialty. This is because the factory testing has just the growth of the fluid trapped from the seat to generate a small amount of flow once the pressure is released. The test fluid consists from what might be experienced in the area of viscosity and a mass. Nonetheless, in high-pressure or huge valves, the amount of gas flow seems to be sufficient to make an opening effect similar to what happens. This is not likely to take place during seat hydro tests because the volume of the water displaced when opening a valve at a test rack is very small. The operating characteristics are similar for all trunnion mounted ball valves

For decades, field technicians commissioning pipelines reported that actuated trunnion mounted ball valves will open to about 20 to 25 levels then stop before proceeding to a full open position. This happened when filling a new pipeline, and it had been brushed off as something that happened because the actuator was either undersized or the source pressure was not set. 

The issue is manifesting itself in factories where gasoline tests are performed as valve dimensions and working pressures have increased. Manufacturers have known about the lively vaning impact that occurs when opening a ball valve in most scenarios where there goes a flow rate through the valve driven by the pressure differential. Throughout the launching, the compels attempt to close the valve again. After the valve is almost open, these vaning forces reverse and attempt to push the ball to a full open position. Because of this, trunnion-mounted ball valves that were manual are fitted with worm-type gearboxes. If a spur gear is used (which isn’t self-locking), the vaning forces may back-drive the ball, the equipment and handwheel, which, in turn, may rip the handwheel from the operator’s grip.

Research

The strain in the ports behind the upstream chair followed the upstream pressure within milliseconds and emphasized the fact that the seat remains pushed onto the ball with substantial force during a large part of the opening cycle; this in turn contributes to greater torque than seasoned in a normal hydro test, where water expansion is slight or nonexistent. 

The rate of corrosion of the upstream pressure (shown as pressure fall) is driven by several things including speed of opening the valve, pressure differential, fluid viscosity, and upstream, downstream and cavity volume. Test conditions are comparable with natural gas as the medium, but amounts upstream and downstream of the valve have been assumed to be infinite. Although this is the strategy that is safer, to analyzing a valve inside a system that is controlled limits exist, including the fact that the volume on the upstream side of the valve closure is tiny. 

Although this scenario is intense, it does mirror filling an empty strand with gas. Depending upon the valve geometry and configuration of elements, the diagnosis can take days or even weeks due to the memory necessary to run iterations. To ease this, the valve has been set at selected angles of starting compared to every degree. Choked flow requirements existed up to 40 levels of the travel with a pressure differential maintained and behind the seat. Figure 3 shows the flow condition at a ball valve has been opened with an infinite quantity of pressure upstream under pressure. 

The angle where the seat face mentioned earlier’s damage happened was translated to a time depending on the simple fact as plotted in Figure 1 that the actuator was a speed type. The graph demonstrates that at 200 barg, the upstream chair is driven into the open ball. Since a low-flow volume is during mill gas testing, a study has been conducted to learn what’s going on to the pressure in a variety of locations of the valve. Millisecond pressure recorders were set up onto a test valve to measure pressure downstream and also. The results showed that in 23 degrees out of open, the upstream strain had only dropped about 47 percent of the starting pressure to 200 gauge pressure (barg) (Figure 1). This point was selected because it is the angle where slight indenting of this soft seat face material happened (Figure 2). 

Transient Analysis

The analysis work was confirmed by gas testing where valves were subjected using a high-pressure differential supplied from a volume tank to an opening cycle. Although this arrangement did not provide volumes, the results mirrored the CFD investigation. This intensification of the load varies depending on fluid viscosity, the upstream volume, and rate of opening. By indenting when performing factory gas tests the seat face at 10 and 8 o’clock it manifests. The 8 and 10 o’clock seat locations translate into an opening angle of approximately 20 to 25 levels based upon the layout (Figures 1 and 2). 

This intensified load could be substantial, and given the incorrect operating conditions, may be good enough to harm even metal sealing seat face surfaces (Figure 4). Knowing that key factors affect valve functionality, it might look supplying a design solution could be easy; however, that is not the case because so many variables have to be considered. The study also identified the effects of the load onto the seat face when the seat is supported on a ball that was half-open. Confront contact load is studied in the closed position to evaluate sealing capability. However, if the valve is opened, the chair remains loaded with pressure against the chunk–so that the segment that is supported is exposed to a load that was higher part of the seat face is supported. There is an intensification of the touch load in which the seat transitions onto the ball out of the section that is unsupported. 

After numerous runs, there conducted from break-open to 60 degrees was a analysis. The preceding study was performed under conditions that were static–the ball has been analyzed at angles during specified intervals. For cases, the analysis examines the effects of the valve though it were always opening, which delivers a more realistic scenario. The trade-off with this analysis is the amount of memory necessary to compute the case. Using a power of 200 cores, the transient analysis took to run because remeshing of the model needed to be done in second fractions of a degree to achieve convergence of the results. 

This kind of analysis cannot be run for every condition, however, it did affirm that the conditions run at various angles. The results clearly showed the seat continues to load onto the ball before the pressure is equalized between side and the valve pit of the chunk. Hence, the torque associated with this condition can be credited not only to the lively vaning effect, but also to frictional drag.

Variable to be considered

Pressure differential is a massive factor in this evaluation. Many questions Have to Be considered, such as: Will the valve be opened against complete differential pressure? 

§ At what temperature does the stroking occur? (This also affects the soft seals and the galling propensity of both metal-to-metal sealing.) Conditions, variables and the queries are seemingly infinite. A valve designed to meet every operating condition that is sensed could be for that which is required, overdesigned. Additionally, every functioning condition can’t be analyzed to ascertain performance boundaries. 

§ What is the type of actuation? Is it hydraulic, electrical or pneumatic? (The latter takes more energetic loading on the valve.) 

§ Is your actuator fitted with full stroke dampening? 

§ Is your actuator over- or undersized? (The chair load in a large bore valve is substantial.) 

§ What quantity downstream has been pressurized? It is imperative to see that these characteristics are typical in most ball valves irrespective of design or manufacturer. When the valve gets seats the consequences can be reduced by design, but cannot be removed. It’s also important to realize opening valves beneath total differential pressure with little or no pressure downstream also affects other valve types (like gate and butterfly valves) in another method. What is the rate of actuation? Once the valve has been cracked open, 

§ Can the actuator accelerate/jump quickly and pressure is discharged as seen in valve testing? The variable is the medium–liquid versus gasoline. While the liquid isn’t highly compressible, it is propelled by the strain energy at the pipeline upstream of the valve through an opening valve in a way. The issue becomes more complicated because the liquid may provide some lubricity, reducing the seat drag. The pipeline downstream might not be empty since fluid acts before the threat angles identified previously are reached. The viscosity, differential pressure and temperature can also be factors. Multiphase flow then takes the problem’s complexity . 

§ Are there any speed controls? If the speed controls are adjusted in the area, what happens? (in case the valve is opened too slow, the chair confront is subject to erosion. If the valve has been opened too fast, the threat angle associated with high load is attained prior to any pressure equalization.) 

§ Can the actuation neglect as is, neglect fail or close open?

§ Manufacturers should set charts demonstrating the valve bore dimensions, chair substance and pressures to which the valve could be opened without possible damage of the seat sealing surface (Figure 5). The following summarizes recommendations for system, operational and actuation considerations for trunnion-mounted ball valve Software: Operating speeds will need to be controlled or scrutinized. Pneumatic actuators should have from leaping to the angle associated with the high-seat load after the initial launch of 30, stroke dampening to inhibit the actuator. At least 15% should reduce the recommended opening pressure if the pneumatic actuator isn’t dampened. End users and builders designing systems where high pressures are present or volumes will probably be filled, or in which large valves are used must think about embracing a bypass system a small valve is used to prefill fluids.

What could be done

§ Manufacturers should set charts demonstrating the valve bore dimensions, chair substance and pressures to which the valve could be opened without possible damage to the seat sealing surface (Figure 5). 

The following summarizes recommendations for system, operational and actuation considerations for trunnion-mounted ball valve Software: Operating speeds will need to be controlled or scrutinized. 

Pneumatic actuators should have from leaping to the angle associated with the high-seat load after the initial launch of 30, stroke dampening to inhibit the actuator. 

At least 15% should reduce the recommended opening pressure if the pneumatic actuator isn’t dampened. End users and builders designing systems where high pressures are present or volumes will probably be filled, or in which large valves are used must think about embracing a bypass system a small valve is used to prefill fluids.

Conclusion

Testing ball valves opening under full differential pressure Demonstrates that, knowing what that pressure is to Get a given diameter can, the life of the valve increased and Also the design of piping systems Enhanced

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