Valve Magazine - Spring 2009

range, changes in the corrosion level of the process fluid, hydrate formation and external damage to the valve. Some of these potential problems can be avoided by following proper operating procedures, using advanced diagnostic tools and selecting more robust valves.

DEFINE THE SOURCE OF
THE VALVE ISSUES

An additional factor beyond those valve issues listed above, which can defeat even the best operating procedures and most advanced diagnostic monitoring equipment, is cleanliness of the process fluid. In nearly all cases, valve specification sheets assume that the process fluid flowing through the valve—whether it is a gas, a liquid or a mix—is clean and free of foreign debris. In the “real world,” this often is not the case, and the presence of foreign debris should be expected in many severe service or critical applications on offshore platforms. Such debris can quickly damage or clog a traditional control valve, which can ultimately cause a critical system to fail.

The first step in preventing this is determining which valves are critical or severe service, a step that can sometimes be challenging. Often such applications are only identified as “a control valve with a high-pressure rating,” and this over-simplification can lead to poor valve selection for critical or severe service applications. A better approach is to fully understand each specific

application and the difficulties that particular process may present. This approach makes it easier to put greater focus on the critical or severe service control valves.

When designing control systems for offshore oil and gas platforms, engineers can target applications that should be considered critical or severe service and where debris may be entrained in the flow stream. Some examples include the high-pressure separator letdown valve, overboard dump valve and produced water reinjection pump recirculation valve.

While each of these is a unique application, all three have the common thread of being on liquid service with a high-pressure drop. In such applications, the valve supplier’s initial direction is to provide a control valve with suitable trim to safely drop the pressure without creating cavitation damage.

UNDERSTAND MECHANICAL CONSIDERATIONS Cavitation occurs in a control valve when the process fluid pressure drops below vapor pressure and then recovers back above that vapor pressure. As the process fluid pressure drops below the vapor pressure, the fluid tries to turn into a gas. Small bubbles begin forming, but before the liquid can completely change phases, the sudden increase in pressure during the recovery phase of the valve forces the bubbles to collapse.

A pressure imbalance on the surface of the bubbles causes them to take on a toroidal shape. At the final point of collapse, a small jet of very high pressure is formed in the center of the torus. If located near a solid surface, these high-pressure jets can cause erosion.

In severely cavitating applications, damage to the control valve can occur very rapidly, leading to equipment failure. Figure 1 shows an example of cavitation damage. Figure 2 illustrates an example of the pressure drop in a single-stage control valve that would exhibit cavitation.

In a control valve, pressure is dropped by forcing the fluid to flow through geometry that changes flow direction and/or passage size and by separating or combining flow streams. In high-pressure drop applications, it is common to do this in series, creating a control valve that uses multiple stages to safely drop the pressure. The ultimate goal is to provide a sufficient number of stages to always keep the fluid pressure above the vapor pressure. Figure 3 shows an example of the pressure drop in a multi-stage control valve that would prevent cavitation.

COMPARE EFFECTIVE AND INEFFECTIVE DESIGN The most common types of control valve trim used to provide multi-stage pressure reduction incorporate a flow element with a series of concentric drilled-