LOGOS Learning · 12
Inherent vs installed characteristic: control valve authority
The inherent characteristic is the valve flow versus travel at constant dP; once installed in a pumped line the flow follows q = 1 / sqrt(1 - N + N / f^2), where N is the valve authority. With low authority a linear valve turns quick-opening and an equal-percentage valve becomes nearly linear.
Change the travel, the characteristic and the authority. The same valve behaves differently depending on how much of the ΔP it keeps.
Quick test
Three questions about this lesson. Got one wrong? The explanation shows right away.
1. An oversized valve (Cv too large) tends to…
2. With low authority, an installed linear valve behaves like…
3. In an equal-percentage valve, equal increments of travel…
Why this happens
Arrows compare with the same valve and travel at the highest authority in the lesson (N = 0.50).
Inherent characteristic
Each equal step of travel multiplies the flow by the same factor. Little flow at first, a lot at the end.
Valve authority
In a pumped line, it compares the ΔP the fully open valve holds at maximum flow with the ΔP it would hold at zero flow (pump at shut-off, no losses). The lower it is, the more the pump and piping dictate the flow.
Installed characteristic
With low authority, an equal-percentage valve becomes almost linear once installed: the gain is more uniform along the travel. That is why it is the usual choice in process plants.
Capacity (IEC 60534)
In the simplified form for non-vaporizing liquids, Kv is the water flow in m³/h that passes with a 1 bar drop. The engine applies the full IEC 60534-2-1 (FR, FP and choking), with the real line P1 and P2, to reach the required Kv.
Formulas in plain text
- Inherent characteristic
f(x) = R^(x - 1) (equal percentage); f(x) = x (linear)- f = relative capacity, fraction of maximum Kv (-) · x = relative travel, 0 to 1 (-) · R = rangeability (50 in the lesson)
- Valve authority
N = dPv_100 / dPv_0- dPv_100 = valve dP fully open at maximum flow (bar) · dPv_0 = valve dP at zero flow, pump at shut-off (bar)
- Installed characteristic
q = 1 / sqrt(1 - N + N / f^2)- q = relative flow, fraction of maximum flow (-) · N = valve authority (-)
- Flow coefficient (IEC 60534, simplified liquid)
Kv = Q * sqrt((rho / rho0) / dP)- Kv = flow coefficient (m3/h at 1 bar) · Q = flow (m3/h) · rho / rho0 = specific gravity relative to water (-) · dP = pressure drop across the valve (bar)
Frequently asked questions
What is the difference between inherent and installed characteristic?
The inherent characteristic is measured by the manufacturer with constant dP across the valve. The installed characteristic is what you get in the real line, where the pump and piping take part of the dP as flow rises, so the curve is distorted by the valve authority.
What is a good control valve authority?
A common rule of thumb is N of about 0.25 to 0.5. Below that the pump and piping dominate the flow and the installed curve drifts far from the inherent one, so the loop gain becomes uneven.
When should I choose equal percentage instead of linear?
In pumped process lines with low authority, where the equal-percentage valve installs almost linear and keeps the gain uniform. A linear valve in the same line behaves like quick-opening and makes the loop nervous at low flow.
How do you convert Kv to Cv?
Cv = 1.156 * Kv. Kv is the water flow in m3/h with a 1 bar drop and Cv is the water flow in US gpm with a 1 psi drop.