A weir-type diaphragm valve shuts a passage by pressing a flexible membrane onto a raised seat in the body. Lifting the membrane opens a path over that weir. The diaphragm separates the wetted passage from the stem and compressor above it. This page shows that structure and calculates the flow coefficient required by a specified liquid duty.
Diaphragm Valve Interactive Calculator
Set a liquid-flow duty to calculate the required operating Cv and equivalent Kv. Move the lift slider to inspect how the membrane clears or seals the weir. Lift is a visual position setting: actual capacity at that position requires the chosen valve’s measured flow characteristic.
Equation Used
- Target flow is in US gallons per minute, not Imperial gallons.
- Pressure drop is the difference across the valve, not absolute inlet pressure.
- Specific gravity is relative to water.
- Lift changes the schematic gap and selected-lift readout, not the required duty Cv.
- Body, membrane and drive proportions are schematic; no actual valve size, flow curve or actuator force is inferred.
Required operating Cv is fixed by the flow duty. It is not the full-open rating of an unspecified valve. No universal Cv-versus-lift curve or throttling-risk percentage is assumed.
The membrane closes onto the weir
The diaphragm rim stays clamped to the body. A compressor attached to the dry-side stem moves its central region toward or away from the weir. At zero lift, the centre contacts the raised seat and interrupts the passage. At positive lift, liquid can pass below the membrane and over the body weir.
The animation holds the selected lift while moving flow markers through the open passage. Changing the lift slider moves the compressor and membrane together. The dots show direction only; they are not a simulated velocity field. At zero lift the dots disappear because the passage is closed.
Only the body and diaphragm bound the wetted passage in this simplified section. The drive stays on the other side of the membrane. It is a weir-type illustration, not the different straight-through or full-bore construction.
Geometry and flow sizing answer different questions
The cutaway explains how this valve opens and seals. The calculator asks what operating flow coefficient is needed for a target liquid flow, density and pressure drop. It does not choose a nominal valve size or predict a given product’s capacity from the drawing.
The manufacturer’s tested flow characteristic connects actual travel to actual capacity. Material compatibility, operating temperature, pressure limits and diaphragm construction are additional product-specific decisions.
Required Cv for a liquid duty
For Q in US gallons per minute, pressure drop ΔP in psi, and liquid specific gravity SG relative to water, the simple liquid-flow relation is Q = Cv √(ΔP/SG). Rearranging gives Cv = Q √(SG/ΔP).
The equivalent metric coefficient uses flow in m³/h and pressure drop in bar: Kv = Q √(SG/ΔP). The calculator converts the units before applying that equation. Both coefficients describe the same required operating capacity.
A percentage of stem travel is not automatically the same percentage of Cv. Without a measured product curve, a full-open rating cannot be deduced from the preview lift. A numerical risk percentage cannot be established from travel alone.
The relation is a liquid-duty estimate. It does not include compressible gas behavior, cavitation, flashing or viscosity corrections, and the drawing does not establish whether a particular operating point satisfies those assumptions.
100 US gpm of water across 4 psi
With Q = 100 US gpm, SG = 1 and ΔP = 4 psi, required Cv is 100 √(1/4) = 50. The same flow is about 22.71 m³/h and the pressure drop is about 0.276 bar, giving an equivalent Kv of approximately 43.25.
Selecting 50% lift shows a half-lift geometric preview. It does not prove that a particular valve has Cv 50 at that opening. Consult its measured travel curve to connect the duty to an operating position.
What changes the required coefficient
Doubling target flow doubles required Cv at fixed pressure drop and density. Quadrupling the available pressure drop halves required Cv at fixed flow and density. Increasing specific gravity increases required Cv through its square root.
Lift changes the visible opening, but it cannot alter those duty requirements. A real valve must provide the required coefficient at an acceptable position. Use the chosen valve’s measured flow-versus-travel curve to find the operating position that supplies the required coefficient.
Diaphragm valve questions
Why does the Cv requirement stay unchanged when I move lift?
The duty inputs set required Cv. The lift control shows the mechanism and updates the selected-lift result. Capacity at that position depends on the actual valve’s tested curve.
Why does required Cv remain visible at zero lift?
It is the coefficient required to deliver the entered target flow, not a claim that the closed valve delivers that flow. The closed preview shows no through-flow.
Can lift alone establish throttling risk?
No. Pressure recovery, fluid state and the valve’s measured flow characteristic also affect throttling behavior. Check the manufacturer’s operating guidance for the required duty.
Does the drive rod touch the liquid?
No. The membrane separates the wet passage from the compressor and stem.
Can I use this for air or steam?
No. This calculator uses the incompressible-liquid relation, not a gas or steam sizing model.
References
GEMÜ, Industrial Diaphragm Valves, page 4: weir-type cross-sections and wet/dry separation. Swagelok, Valve Sizing Technical Bulletin MS-06-84: liquid flow-coefficient equations and unit conventions.
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