Technical guides
Which valve to use: ball, butterfly, knife gate, gate or check
Almost every valve problem starts the same way: someone asked for “a 4-inch valve”. Diameter is the last thing that matters. The first is what it has to do.
Isolating a line, regulating a flow and stopping reverse flow are three different functions. A valve that does one of them well usually does the other two badly, and that is behind most premature replacements.
Ball valve: fast isolation
A quarter turn opens or closes it completely. It seals tightly, operates fast, and in the open position leaves the bore essentially clear, so pressure drop is minimal.
It is the isolation valve par excellence: isolating equipment, closing a branch, shutting off for maintenance. It is not a regulating valve: running it partly open erodes the seat exactly where the flow passes, and before long it no longer seals. That is the most common misuse and the most expensive.
Butterfly valve: large diameter, tight space
The disc rotates inside the body, so the valve takes up very little length along the line and weighs far less than a gate valve of the same diameter. On large-bore lines the difference in price and weight is substantial.
It regulates reasonably across a mid-opening range and serves for isolation where absolute tightness is not required. Its drawback is that the disc stays in the flow even when open, which introduces pressure drop and makes it a poor candidate for fluids carrying solids.
Knife gate valve: slurry and solids
Designed for what jams everything else. The blade cuts through the material and the body, with no cavities where solids collect, passes slurry, fibre and sludge without clogging.
It is the valve of mining and of process plants with suspended solids. It is not a high-pressure or gas-tight valve: it is a dirty-service valve.
Gate valve: full isolation
A gate rises and falls perpendicular to the flow. Fully open it leaves the bore intact with almost no pressure drop, which makes it ideal for lines that stay open and are only closed to isolate.
Like the ball valve, it does not regulate: partly open it chatters and erodes the seat. And it is slow to operate, because the handwheel takes many turns. That last point is an advantage when you want to avoid the water hammer of an abrupt closure.
Check valve: it stops backflow
Nobody operates it: the flow opens it and gravity or a spring closes it when flow stops or tries to reverse. Its job is to keep the pump from spinning backwards and stop the liquid column returning.
It goes immediately downstream of the pump, and its orientation matters: fitted the wrong way round it protects nothing and blocks the line. Check the arrow on the body before torquing the flanges.
If the job is to regulate, none of these is the answer
For real regulation — holding a flow or a pressure — the right valve is a globe or control valve, designed to work stably at intermediate positions. Using a ball or gate valve for that ruins the seat and wastes the money spent on a good valve.
The data that defines the valve
- Function: isolate, regulate or prevent backflow.
- Fluid, with solids if any and their size.
- Working pressure and temperature.
- Connection type: flanged, threaded or wafer, and flange class.
- Operation: manual, pneumatic or electric actuator.
- Frequency: is it operated daily, or an isolation valve touched once a year?
With those six data points you get quoted the valve the service calls for. With diameter alone, you get quoted whatever is in stock.
Frequently asked questions
Not advisable. Partly open, the flow erodes the seat exactly where it passes, and over time the valve stops sealing. For regulation, a globe or control valve is what applies.
The butterfly weighs and costs considerably less and takes up far less length along the line. The gate wins when you need a fully clear bore and zero pressure drop with the valve open.
Most likely because it is not a knife gate valve. Ball, gate and butterfly valves have cavities or a disc in the flow where solids build up. For slurry and suspended solids, knife gate is what applies.
