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
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
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.