A check valve matched to the line size is usually oversized
Check valves are the one item in a piping system that should be sized on velocity rather than on the nominal pipe bore. The disc of a swing or lift check needs a minimum flow velocity to hold it firmly against its full-open stop. Below that threshold the disc floats somewhere in the stream, chattering against the stop and the seat with every pressure fluctuation, which wears the hinge pin, rounds the seating face and eventually drops debris into the line. Because the valve is normally selected to match the adjacent flange, and because the line itself was sized for peak duty, a check valve running at part load spends most of its life in exactly that condition. The practical fix is to establish the minimum sustained flow first, not the design flow. Where the turndown is wide, either fit a smaller valve between reducers, or move to a spring-assisted type — a dual-plate or nozzle design holds its discs in a defined position regardless of velocity and is unbothered by low flow.
Slam is a closing-speed problem, not a sealing problem
When forward flow stops, the column of fluid begins to reverse before the disc has finished travelling to its seat. The later the disc lands, the higher the reverse velocity it has to arrest, and the resulting pressure surge is what is felt as slam and water hammer. A conventional swing check has the longest disc travel and therefore the slowest closure, which is acceptable on a gravity line but not behind a pump with high static head, and not where pumps run in parallel and one can trip while the others keep delivering. A tilting-disc design shortens that travel by pivoting the disc closer to its centre of pressure. A spring-loaded dual-plate closes faster still because the springs, not gravity or reverse flow, drive the plates shut. A nozzle check goes furthest: a venturi flow path and a short, spring-loaded axial stroke close the valve essentially before flow reverses, at the lowest pressure loss of any type in the range. The added cost is normally recovered the first time it prevents a surge event on a pump discharge header.
Orientation and upstream geometry are part of the specification
A check valve responds to the flow profile it is given, so where it sits matters as much as what it is. A disc immediately downstream of a pump, an elbow or a partly open valve sees an asymmetric, swirling profile that loads it unevenly and provokes flutter; allow a straight run of several pipe diameters ahead of the valve wherever the layout permits. Orientation is equally constraining and is easy to get wrong on a drawing: swing checks work horizontally or in vertical upward flow, lift and piston checks are generally horizontal unless supplied specifically for vertical service, and dual-plate valves accept most orientations provided the pin axis is set so gravity does not favour one plate. State the intended orientation on the enquiry — it changes the spring selection and sometimes the type. Where the valve will be maintained rather than replaced, the renewable seat rings available on the cast designs are worth specifying at the outset.
What to include on the enquiry
- Minimum, normal and maximum flow rate — the minimum is the number that decides the size
- Intended orientation: horizontal, vertical up, or vertical down
- Pump curve and static head where the valve protects a pump discharge
- Whether non-slam closure is required, and whether pumps operate in parallel
- Type preference if any: swing, lift, tilting-disc, dual-plate wafer or nozzle
- Pressure class and body material against the design temperature
- Bolted-cover or pressure-seal construction for high-pressure and high-temperature lines
- End connection: RF, RTJ, BW, wafer or lug
- Cryogenic duty to BS 6364, sour service to NACE MR0175, and EN 10204 3.1 certification if required
Design guidance and standard references on this page are general engineering guidance for valve selection. Confirm the final specification against your line list and the applicable design code for the project.
