A check valve is the only valve on a plant that nobody operates. It has no handwheel, no actuator and no control signal. It opens because the fluid pushes it open, and it closes because something, gravity, a spring or the reversing flow itself, pushes it shut. It looks like the simplest component in the piping system. In practice it is the one most often blamed for water hammer, pump damage and unexplained banging in the night, and most of those complaints trace back to a selection made on line size and price alone.
What actually closes the valve
Every check valve has a closure member, a disc, plate, ball or piston, that sits on a seat. In forward flow the dynamic pressure of the fluid lifts the closure member off the seat and holds it open against whatever is trying to close it. When forward flow decays, that holding force falls away and the closing force takes over.
The closing force comes from one or more of three sources. Gravity acts on the disc weight in a swing or lift check. A spring acts in a dual plate, nozzle or spring loaded lift check. Reverse flow acts on any design, and in a badly chosen valve it is the main closing force, which is exactly the problem. If the disc is still travelling when the fluid column reverses, the reverse flow accelerates the disc onto the seat and the moving column is then stopped dead. The pressure surge that follows is what people call check valve slam.
The aim of good selection is therefore easy to state. The disc should reach the seat at the moment forward velocity reaches zero, or as close to it as possible, so that there is almost no reverse velocity to arrest.
Sealing is a separate question from closing. Once seated, a check valve relies on the reverse differential pressure to load the disc against the seat. At high back pressure that gives a good seal. At very low back pressure a metal seated check can pass a steady trickle, which is why a check valve is never accepted as positive isolation for maintenance. The closure test for check valves under API 598 is applied in the reverse direction, the direction that tends to close the disc, for exactly this reason.
Swing, lift, dual plate and nozzle designs
The swing check has a disc hinged above the seat. It gives low head loss when fully open and tolerates moderate solids, which keeps it common in water and wastewater work. Its weakness is a long disc travel and a heavy disc with a lot of inertia. On a system where flow decelerates fast, such as a pump discharging against a high static head, the disc lags behind the flow and slams. Lever and weight or lever and spring attachments help, as do oil dashpot dampers on large valves, but they do not change the basic geometry.
The lift check uses a disc or piston guided vertically above the seat, rather like a globe valve with no stem. Travel is short and the seating is good, so it suits small bore steam, condensate and high pressure service. Head loss is high, since the flow path turns twice, and the guides foul in dirty service.
The dual plate check, covered for oil and gas use by API 594, splits the disc into two half plates hinged on a central pin and closed by a torsion spring. The plates are light and travel a short arc, so they close quickly. Wafer and lug versions are short and cheap to fit between flanges. The penalties are a hinge pin sitting in the flow stream, spring fatigue in pulsating service and plate wear if the valve sits half open for long periods.
The nozzle check, often sold as non slam or axial flow, uses an annular disc moving a very short distance along the pipe axis against a spring, inside a venturi shaped body. It is the fastest closing design and the one I specify on compressor discharge and on pump stations with several pumps on a common header. It is also the most expensive and the least tolerant of solids.
Slam and why closure speed matters
Slam is not mainly a noise problem. The noise is the symptom of a pressure transient, and that transient can split a cast iron body, crack a flange, shift pipe supports or damage pump seals and bearings. The magnitude of the surge depends on the reverse velocity at the instant of closure and on the wave speed in the pipe and fluid, which is the classic water hammer relationship.
How fast the column decelerates is set by the system, not by the valve. A pump stopping against a high static head through a short discharge pipe decelerates violently. A long gravity main with little static lift decelerates slowly. Good manufacturers publish dynamic characteristic curves that plot reverse velocity at closure against system deceleration, and those curves are the only rational basis for choosing a check valve on a surge critical duty. If a supplier cannot provide one, treat that as a warning.
In general, light discs with short travel and spring assistance give the lowest reverse velocity. That ranks the common designs roughly as nozzle, then dual plate, then swing, with lift checks depending heavily on the spring. On parallel pump installations, where one pump tripping leaves the others holding full header pressure against its check valve, a fast closing design is not optional.
Sizing for velocity rather than line size
The commonest check valve fault I find on site is a valve that is too big. A check valve sized to the line on a pump that spends most of its life at reduced flow never opens fully. The disc floats in the stream, chattering against its stop or against the seat, and wears out hinge pins, plate bearings and seat faces.
Each design needs a minimum velocity to hold it fully and stably open. Manufacturers publish this figure, and it depends on disc weight, spring rate and orientation. Compare it with the velocity at the minimum continuous flow you expect, not the design flow. If the valve is marginal, reduce its size and fit concentric reducers either side. The extra pressure loss is usually trivial against the cost of replacing a disc every year.
Pressure class normally follows the piping class, with ASME B16.34 governing most steel valves, but remember that the surge at closure can exceed the steady operating pressure. A surge analysis on a long pumped main will sometimes push the pump station valves up a class.
Installation details that decide service life
- Respect orientation. Swing checks work horizontally and in vertical upward flow only if the disc is designed for it. Unsprung lift checks are usually horizontal only. Dual plate valves in horizontal lines should have the hinge pin vertical so the two plates load evenly.
- Keep disturbances away. An elbow, tee or pump outlet directly upstream skews the velocity profile and makes one plate or one side of the disc flutter. Ask the supplier for the straight length they need and honour it.
- Check disc clearance. Wafer check plates swing beyond the body face, and in lined or heavy wall pipe, or bolted next to a butterfly valve, they can hit the adjacent component.
A check valve chosen for its closing behaviour, sized for the velocity it will actually see and installed with clean upstream flow will run for decades without attention. One picked from a price list by nominal size is likely to be the loudest component in the pump house within a year.