Control valve Cv & flow characterisation. 550 m³/hr.

Why Cv measurement matters

The flow coefficient Cv (or Kv in metric) is the single most important parameter for sizing a control valve. It describes how much flow passes through the valve per unit of pressure drop at a given opening position and fluid viscosity. Get Cv wrong and the consequence is either a dramatically undersized valve that saturates at low flow demand, or an oversized valve that operates at 5–10% open — far outside its controllable range, prone to cavitation, and unable to achieve stable closed-loop control.

Accurate Cv measurement requires simultaneous, calibrated measurement of differential pressure across the valve and the volumetric flow rate through it — at multiple flow setpoints and multiple valve openings — with sufficient resolution to detect the subtle non-linearities that distinguish a well-machined trim from an out-of-spec casting.

What this rig is designed to characterise

Proportional control valves

The primary design target. Proportional valves modulate flow continuously in response to a 4–20 mA or 0–10 V command signal. Characterising them requires sweeping both the command signal (opening position) and the flow rate independently to build a complete Cv surface — not just a single design-point spot check. The rig's VFD-controlled cascade enables precise, stepless flow control at every point in the matrix.

Industrial butterfly and globe control valves

Wafer and lug butterfly valves, single-seat globe valves, cage-guided globe valves, and rotary plug valves from DN65 to DN350. The three instrumented pipeline sizes accommodate the full range of industrial valve body sizes through standard ANSI B16.5 flanged connections. Changeover between line sizes is a matter of closing automated isolation valves — no hydraulic disconnection required.

Check valves and pressure-reducing valves

Flow-through verification of check valves (cracking pressure, full-open Cv), pressure-reducing valves (outlet pressure stability vs. upstream flow), and backpressure regulators can all be accommodated on the same pipework with suitable instrumentation routing.

System architecture explained

1. Five-pump VFD cascade

Five centrifugal pumps, each independently driven by a Mitsubishi variable frequency drive, are arranged in a parallel cascade. For low-flow testing (FM·01 range, 1.6–16 m³/hr) only the two 30 kW machines run. Stepping up to medium flow adds one or two 75 kW units. Full-flow testing engages all five pumps for a combined output capacity of up to 550 m³/hr. At no point does any single pump operate below its minimum stable flow or near its shut-off head — a common cause of vibration and instrumentation noise in under-designed rigs.

2. Three electromagnetic flow meters

Electromagnetic flow meters (Coriolis-class accuracy, 4–20 mA and frequency outputs, ±0.5% of reading) are permanently installed on the DN65, DN100, and DN250 instrumented lines. Water is the standard test fluid, so no viscosity correction is needed for most measurement campaigns. For non-water fluids, the LabVIEW software applies user-entered fluid properties to generate corrected Cv values.

3. Differential pressure instrumentation

Three Siemens differential pressure transmitters (0–5 bar range, SS316 wetted parts, 4–20 mA output) are mounted immediately upstream and downstream of the valve under test connection points. Stainless steel sensing lines, manual root valves, and isolation / equalising valves ensure accurate, repeatable ΔP measurement without air pockets or static head errors.

4. 10,000-litre reservoir system

The cylindrical mild-steel tank with anti-corrosive internal lining acts as the hydraulic accumulator, de-aeration vessel, and debris trap in one. An independent Forbes Marshall air eliminator removes entrained air before it reaches the pumps or the instrumentation planes. The water accumulator downstream of the pump station suppresses pressure pulsations that would otherwise appear as noise on the ΔP transmitters.

5. Automated valve architecture

All pump suction valves and flow-line selection valves are actuated (motor- or pneumatic-actuated) with limit switches reporting open/closed status to the DAQ. The LabVIEW sequence engine can therefore switch between flow meter ranges and start/stop pump stages automatically — without the operator leaving the DAQ workstation. Safety interlocks prevent a pump from starting unless its suction valve confirms open; a safety relief valve on each supply line prevents over-pressure events.

Data acquisition and reporting

National Instruments PCI card-based hardware interfaces all 4–20 mA sensors simultaneously. The LabVIEW application runs a test script that steps through a user-defined matrix of flow setpoints and valve opening commands, waits for each condition to stabilise (configurable settling time), then captures a burst of samples and writes the average to the test record. Every measurement includes: date/time, valve serial number, operator ID, inlet fluid temperature, flow rate on the active meter, ΔP on all three transmitters, line pressures, and the computed Cv value.

Test results are displayed live as a Cv-versus-opening chart with the manufacturer's reference curve overlaid. Pass/fail markers appear in real time at each point. At the end of the sequence, the application generates a PDF test report automatically, attaches the live chart, and saves both to an archive folder named by valve serial number — ready for quality sign-off without any manual data entry.

Installation and commissioning

The rig is delivered as a collection of modular sub-assemblies: pump skid, piping array, reservoir, control cabinet, and DAQ workstation. Neometrix provides site plans, piping and instrumentation diagrams (P&IDs), and complete mechanical and electrical commissioning services. Operator training covers hardware operation, the LabVIEW test interface, routine maintenance procedures, and first-level calibration checks.