This article was printed in CONTROL's November 2009 edition.
By Gerald Liu, PE
Instrument technicians are often called by their instrument engineers to look at repeated control valve failure problems during plant shutdowns. From a maintenance point of view, the definition of "severe service" in control valves can be based on how long the valves last.
If the same or similar damage is observed in a control valve for two consecutive shutdowns, the service conditions experienced by that valve can be classified as severe service, and so the valve can be defined as a severe-service control valve. Of course, other definitions of severe service exist. The responding instrument engineer has two ways to tackle this problem. In general, the process conditions could be questionable, and/or the valve could be unsuitable for handling the process conditions, i.e. not sized correctly.
For any plant operating for many years while production was maintained without change, process conditions after reconfirmation can be considered valid and good. In such cases, the instrument engineers should take a closer look at how the damaged valve serving the process conditions was sized, rather than just ordering the same valve using control valve data sheets from when the plant was built. This enables instrument engineers maintaining the control valves to rewrite specifications for procuring and engineering new control valves best suited for the application. This is the best practice for eliminating repeated failures.
However, it's not unusual for the supplier of the damaged valve to try every means to discredit the instrument engineers by approaching their management to negotiate for a solution to monitor the valve problems without eliminating them. Using an asset management system to monitor control valves plant-wide while continuing to supply the same parts for the damaged valves is an unwise approach.
The relationship between the process conditions and the velocity (in the control valve in this case) is governed by this derivation of the Darcy's Equation:
P1 – P2 = f (L/D) pv2 / 2 gc
in which P1 and P2 are, respectively, the upstream and downstream process fluid pressures of the control valve, while f (L/D) is the loss coefficient or resistance (the k-factor) of the control valve trim; v and p are respectively the fluid velocity and the fluid density at the exit of the control valve trim; and gc is the universal gravitational constant of 32.2 lbmft/lbfs2.
Assuming incompressible flow, if the k-factor of the control valve trim is small, then the trim exit fluid velocity will be high because the trim can't dissipate much of the fluid energy. With kinetic energy increasing quickly according to the second order of velocity, it causes internal damage by cavitation, vibrations and/or unacceptable control valve noise. If the trim exit kinetic energy is excessive, piping and downstream equipment can be damaged (Figure 1).
Conversely, if the k-factor of the control valve is large enough to reduce the exit fluid kinetic energy to a safe value, damage to the control valve or downstream components can be eliminated. With compressible flows, for a given trim exit fluid kinetic energy, the density of the fluid at the trim outlet becomes smaller when P2 gets lower, and thus the outlet velocity becomes higher. This can cause objectionable noise problems when vent valves with very low pressure outlets are concerned. This explains why a good low-noise vent valve requires a high k-factor, such as 10, has many stages (often 30), and is expensive. In actual practice for design of gas vent valves, the trim exit fluid kinetic energy should often be less than 1 psi to meet EPA noise standards.
However, while trying to reduce cavitation in valves, a challenging question arises—why rewrite the control valve specification for in-house use when ISA already has so many established control valve specifications and standards readily available? The answer is simple—ISA standards on control valves do not consider or calculate the k-factor/loss coefficient of control valve trims. In ANSI/ISA S75.01 standard, "Flow Equations for Sizing Control Valves," paragraph 4.3, "Piping geometry factor Fp," it's stated that the factor ∑k is the algebraic sum of the four effective velocity head coefficients of all fittings attached to, but not including the valve. ANSI/ISA S75.01 calculates the flow capacity and the impact of flow choking (cavitation, flashing and sonic flow) on the Cv of control valves. It doesn't calculate the capabilities of the control valves to resist the damage from flow choking.
To calculate the capability of a control valve, one has to use a method called trim analysis. That is, one must consider the trim flow geometry, then evaluate the total k-factor/loss coefficient based on trim inlet area or trim outlet area, substitute into the Darcy's equation above for a given set of process conditions, and use 70 psi as the criteria for the trim exit outlet fluid kinetic energy. If both sides of the equation are satisfied, then the control valve with the selected trim is capable of handling the process conditions.
Other methods of calculating the k-factor of a control valve trim exist. For complicated trim geometries, or when manufacturers won't provide information about their trims, one can still do iterative calculations by assuming a value for the k-factor of the trim, calculating the Cv's to see how close the calculated Cv's will match with the Cv's published (at various valve openings) by the specific valve manufacturer. If the calculated Cv's match with the published Cv's to within 5% at various valve openings, then the assumed k-factor will become the k-factor of the trim in question.