Valve Magazine - Summer 2009

CONTROLLING CAVITATION

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cavity formation to the potential for causing cavity formation. This cavitation index is defined as follows:

(P1 - PV)

; ; σ = ____________

(P1 - P2)

Where all pressures are in consistent absolute pressure units, and:

P1 = Upstream absolute pressure, measured two pipe diameters upstream from the valve,

Know Your Options

Although globe valves are the first to spring to mind when many engineers think of control valves, rotary control valves are also a viable technology. Just as we have come to expect with globe control valves, rotary control valves are provided as an assembly with valve, actuator and positioner coming from the same manufacturer.

When selecting valves, remember many choices are available and every

P2 = Downstream absolute pressure, measured six pipe diameters downstream from the valve,

PV = Vapor pressure of the liquid at flowing temperature.

Through laboratory and field testing, manufacturers can establish recommended operating Sigma limits for various valve designs. In actual applications, additional sizing and scaling factors need to be considered. For example:

A manufacturer determines by application will have a best choice. There also, however, may be a lowest-cost choice that is perfectly acceptable for many applications. As an operator or a specifying engineer, it is important you know the full spectrum of products available to meet your requirements. VM

JIM CASEY is marketing manager, Rotary Control Products, for Flowserve Corporation.

( www.flowserve.com). Reach Casey at jccasey@flowserve.com.

testing that water flowing over-the-plug through a fully open, single-seated three-inch globe valve at 100.5 psia and 80° F (vapor pressure = 0.5 psia), starts to cause cavitation damage at a downstream pressure of 61 psia. The sigma damage (σdamage) or sigma incipient damage index (σid) for this case is:

(100.5 - 0.5)

; σid = __________ = 2.53

(100.5 - 61)

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Around The Clock Valve Safety

The Babbitt Safety Wheel provides reliable valve actuation 24 hours a day in refining, processing and power plants around the globe.

The Manufacturer’s Recommended Sigma (σmr) for this particular style of valve in continuous operation at these reference conditions is generally set equal to a value slightly higher than σid.

The pressure drop at which cavitation first occurs, sigma incipient (σincipient), is much lower than for incipient damage. σincipient can be determined from vibration tests described in ISA-RP75.23.01 and the value of σincipient will be significantly greater than σmr or σid. At the other end of the spectrum is choked flow, which is caused by an increased volume of vapor due to cavitation. Choked flow is determined where the flow rate reaches and remains at a maximum value regardless of further increases in pressure drop from a constant upstream pressure. This same test valve will choke at a sigma value (σchoked) somewhat smaller than σmr.

If this same type of valve in a 10-inch size operates wide open at an upstream pressure (P1) of 500 psia and a downstream pressure (P2) of 310 psia, and with a water temperature of 180° F (vapor pressure = 7. 5 psia), the operating sigma is: