- Products
- Severe Service Valves
- Anti-Surge - Compressor Recycle
- Pipeline Surge Relief Control
- Centrifugal Pump Minimum Flow Recirculation
- Fire Water Deluge System
- Separator Level Control
- Vector Velocity Control Trims
- Turbotrol and Multi-Spline Trims
- Choke Valves
- Globe Control Valves
- Aftermarket
- Rotary Control Valves
- Actuators and Instruments
- Product Enquiries
- Industry Sectors
Roll Over Images To Expand
-
- Figure 3. Cage-guided trim with severe cavitation damage.
-
- Figure 4. HFL-9 stage anti-cavitation trim.
-
- Figure 5. Comparison between low and high recovery trim designs.
SEVERE SERVICE
VALVES
SEVERE SERVICE
VALVES
Centrifugal Pump Minimum Flow
Recirculation
The trim shown in Figure 3 on this page illustrates the extreme damage that high levels of cavitation can cause on pump minimum flow recirculation applications - it can literally tear a trim apart in minutes.
The factors causing this high level of damage are either the incorrect specification of the operating conditions or incorrect design of the trim. In this instance, the outlet pressure specified at 6 bar was in fact atmospheric pressure, resulting in this extreme erosion. The valve operated for only a few hours before it had to be taken out of service.
The original trim design incorporated five stages (ten turns) of pressure let-down, but the pressure drop apportionment did not eliminate cavitation due to the much lower downstream pressure. To handle this pressure drop and eliminate cavitation, the trim design needed an additional four stages of let-down, giving a total of 18 turns within the existing valve body, see Figure 4.
The area increase through the trim resulted in lower pressure drops in the outlet stages of the trim, thereby eliminating cavitation, even though the outlet pressure was close to atmospheric. This design is a hybrid of the KKI Turbotrol valve that is used on similar applications.
What is cavitation?
Cavitation occurs on liquid applications when the fluid static pressure firstly reduces below the fluid vapour pressure (Pv) and subsequently rises (recovers) above the fluid vapour pressure. This results firstly in the creation of small vapour bubbles (cavities), which subsequently collapse producing localised shock wave micro-jets. If these impact on the metallic surface of the valve then severe pitting and erosion damage can occur.
In its most severe form, cavitation can destroy a control valve trim in a matter of hours. It is therefore important to control the level of cavitation when selecting a control valve.
