Key takeaways
Flow control is one of those phrases that means two entirely different engineering problems depending on what is moving through the pipe. In a liquid or gas line, a flow control valve works by creating a restriction: change the opening, change the pressure drop, change the flow. In a line carrying powder or granules, that relationship does not exist. Bulk solids do not behave like fluids, and a valve that throttles them by partially closing will bridge, pack and eventually stop.
Most articles on flow control valves cover only the first case. This one covers both, because in process plants handling powders the valves that control flow are a distinct family with their own working principles, and choosing from the wrong family is a common and expensive error.
Control Versus Isolation
Before types, a distinction worth being strict about. An isolation valve has two jobs, open and shut, and is designed to seal well in the closed position. A control valve is designed to sit at intermediate positions and hold a defined flow rate there.
Using one for the other causes predictable damage. A gate valve held partly open erodes its seat until it no longer seals. A modulating control valve used purely for shut-off is expensive equipment doing a job a simpler valve would do better. Where both functions are needed, either specify a valve rated for both or install two valves.
Flow Control in Liquids and Gases
In fluid systems the valve is a variable restriction. Flow through it depends on the pressure difference across it and on the flow area the valve presents, which is expressed as a flow coefficient. Sizing a control valve means selecting one that will operate in the middle of its travel at normal conditions, because a valve running nearly shut or nearly open has almost no useful control authority.
How flow changes as the valve opens is called its characteristic. A linear characteristic gives flow proportional to travel. An equal percentage characteristic gives progressively larger increments as it opens, which suits systems where line pressure drop varies with flow. A quick opening characteristic delivers most of its flow early in the stroke and suits on-off service. The characteristic is chosen to match the system, not the fluid.
| Valve type | How it controls flow | Typical use |
|---|---|---|
| Globe and needle | A plug moves against a seat, changing the restriction gradually | Fine throttling where accuracy matters more than pressure loss |
| Ball with V-port | A rotating ball with a shaped opening varies the flow area | Modulating control with tight shut-off in the same valve |
| Butterfly | A disc rotates in the flow path, changing the effective aperture | Large diameter lines where cost and space favour a compact valve |
| Diaphragm and pinch | A flexible element is compressed to close the passage | Slurries, abrasives and hygienic duty with no cavities |
| Gate | A wedge lifts clear of the bore | Isolation only; throttling a gate valve damages the seat |
Why Bulk Solids Are Different
Powders and granules do not flow because of a pressure gradient. They flow because gravity or a mechanical device moves them, and they resist that movement in ways fluids do not.
The consequence is that solids flow control valves do not throttle. They meter. Flow rate is set by how fast a rotor turns or how a dosing element is sequenced, not by how far the valve is open.
Valve Types for Bulk Solids Flow Control
| Valve type | How it controls flow | Typical use |
|---|---|---|
| Rotary airlock valve | A vaned rotor turns in a housing, carrying a fixed volume per pocket | Feeding into or out of pneumatic conveying while maintaining a pressure seal |
| Metering or CT valve | A rotor driven through a variable frequency drive sets discharge rate directly | Controlled feeding onto weigh hoppers and into mixers |
| Combination valve | Separate coarse and fine elements, each with its own actuator | Batch weighing, where bulk fill and final dribble need different rates |
| Slide gate | A plate withdraws across the outlet | Open or shut duty rather than modulating control |
| Butterfly for powder | A disc rotates within the passage | Isolation between vessels and sections, not accurate metering |
Rotary airlock valves
A vaned rotor turns inside a close-fitting housing. Each pocket carries a fixed volume from inlet to outlet, so discharge rate is set by rotor speed. Because the rotor always presents a seal between inlet and outlet, the valve also maintains a pressure differential, which is what allows it to feed into or out of a pneumatic conveying line without losing conveying air. Leak-proof, non-sparking construction handles flow rates up to 6,000 kg/hr in Cybernetik’s range.
Metering valves
Where the requirement is an accurate discharge rate rather than a pressure seal, a metering valve driven through a variable frequency drive gives direct control of flow. Cybernetik’s CT valve handles up to 6,000 kg/hr with zero leakage and vacuum compatibility, in a hygienic GMP build with either a multi-blade or butterfly rotor depending on the material.
Combination coarse and fine valves
Batch weighing needs two flow rates: a fast bulk fill to get close to target, then a slow dribble feed to land on it accurately. A combo valve provides both from one unit, using a butterfly rotor with a central feeder and separate actuators for coarse and fine flow, at up to 6,000 kg/hr. Doing this with a single valve means either slow filling or poor accuracy.
Specification Decisions for Solids Flow Control
“When a material handling requirement can be solved with a flat belt, the right design delivers a simple, economical, gentle, and highly maintainable conveying solution.”
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Industrial Applications
Flow Control Valves from Cybernetik
Cybernetik builds powder and bulk solids flow control valves as part of complete material handling and processing lines, engineered around the product being handled and the equipment on either side.
| Parameter | CT Valve (REFE-CTVA) | Combo Valve (REFE-COVA) |
|---|---|---|
| Capacity | 1000 to 6000 kg/hr, customisable | 1000 to 6000 kg/hr, customisable |
| Inlet and outlet diameters | 100 to 300 NB | 100 to 300 NB |
| Materials of construction | SS304 or SS316 | SS casing, SS or aluminium rotor |
| Flow rate control | Variable frequency drive | Variable frequency drive |
| Rotor options | Multi-blade or butterfly rotor | Butterfly rotor with central feeder for fine feeding |
| Actuation | Electric, optional pneumatic turning the rotor 90 or 180 degrees | Separate actuators for coarse and fine flow, optional pneumatic or electro-pneumatic |
| Leakage | Zero leakage, vacuum compatible | Leak controlled by rotor and seat design |
| Hazardous area | Explosion resistant ATEX construction available | Explosion resistant ATEX construction available |
| Build | Hygienic, GMP built, easy maintenance | Hygienic, GMP built |
Products handled across the range include chocolates, sugar and sugar powder, wheat flour, rice flour, lentil flour, chickpea flour, corn starch, green peas flour, spices, tea and coffee powder, whey powder and active pharmaceutical ingredients, across food, pharmaceutical and chemical industries.
Cybernetik has operated for more than three decades, is headquartered in Pune with offices in the United States and the UAE, and has installed over 6,000 systems across 30 plus countries, including more than 400 custom automation solutions. With over 600 employees and divisions spanning Process Automation, Packaging Automation, CleanTech, Extraction, Labs and Defence, a valve is specified with knowledge of everything it connects to. Further background sits on the Cybernetik about page.