Valve Magazine - Spring 2009

Figure 4. Examples of small passage drilled-hole and tortuous-path control valve trim for liquid service. These would reliably prevent cavitation in clean service applications, but could become clogged if exposed to a flow stream with entrained debris.

hole cages or a series of stacked plates that force the flow through a tortuous flow path. Figure 4 shows typical examples of each design. Both designs are regularly used in clean service applications on offshore platforms, where they deliver effective and reliable protection against cavitation.

However, such traditional designs can be unreliable in offshore applications where debris will be entrained in the flow stream, as their small flow passages make them susceptible to clogging and damage. In an offshore platform, the most common debris would be sand, though other trash associated with industrial construction, such as weld slag, pipe scale and fasteners, can also be present. Most traditional anti-cavitation control valves would easily clog and become damaged if forced to pass such debris.

In critical or severe service applications, damage to the control valve can have a ripple effect on the surrounding system. In the case of a produced water pump recirculation valve, for example, the control valve provides flow from the discharge side of the pump back to the suction side. This typically occurs during start-up or shut-down of the pump, when recirculation flow is required. If the valve cannot pass sufficient flow because it is clogged, the pump could be starved of flow, leading to interstage cavitation in the pump and, potentially, to total pump failure. The resulting cost to repair or replace the pump would far exceed the cost to repair or replace the offending valve, which illustrates why investing in a control system appropriately designed to meet the needs of the

specific application is a wise decision.

A SOLUTION TO REDUCE
VALVE FAILURE

Those not experienced in offshore applications may feel they face a no-win situation: fail to apply anti-cavitation techniques and cavitation could cause equipment to fail; specify anti-cavitation trim with small passages, and clogging may result in equipment failure.

There are solutions to this dilemma. Among them is a set of unique control valve trim designs in which small flow passages are replaced by a series of specially engineered, large-area expansion and contraction regions. Figure 5 shows such a control valve trim. This design delivers a high level of cavitation protection by safely staging the pressure drop, while also providing large flow passage areas that can pass entrained solids with no troubles. These control valves also offer a high level of reliability because they eliminate clogging and damage by debris as a failure mechanism.

While most debris problems in valves are associated with control valves on

Figure 5. Shown here is an example of multi-stage, debris-tolerant trim for liquid service.

liquid service, the same types of considerations apply to control valves used in severe service gas applications. Figure 6 shows two examples of specialty control valve trims commonly used in high-pressure gas letdown applications. Similar to anti-cavitation liquid control valve trims for clean service applications, the technology uses small drilled holes or tortuous flow passages. The purpose, however, is to minimize noise and vibration levels associated with the high-pressure reduction of a compressible gas.

While the risk of clogging and damage is less in gas service than in liquid

Figure 6. Depicted are small-passage drilled-hole and tortuous-path control valve trim for gas service. These are reliable in clean service applications, but could become clogged if exposed to a flow stream with entrained debris.