Swing vs Silent (Axial) Check Valves

Three end-suction pumps on concrete plinths, each discharge pipe rising through flanged valves — Pumps in parallel. Each discharge needs a check valve that shuts before the flow turns back.

Key takeaways

A swing check valve needs reverse flow to finish closing. Its disc meets the seat with a moving column behind it.
An axial (silent) check valve uses a spring and a short stroke. Its disc is designed to seat as forward flow fades.
Slam depends on how fast the system decelerates after a pump stops. Val-Matic's slam study puts single-pump, low-head systems under about 6 m/s². High-head or multi-pump systems can reach about 12 m/s².
Milwaukee Valve cites a rule of thumb for water: about 2.3 m/s holds a swing check fully open. Milwaukee's silent checks open fully at about 1.2 m/s. A part-open disc can chatter, wear early and lose extra head.
Swing checks still belong on large, low-head, slow-decelerating and piggable lines.

01How do swing and silent (axial) check valves close?

Swing and axial check valves both stop reverse flow, but they finish closing at different moments. A swing check hangs its disc from a hinge; gravity and returning flow swing it shut. An axial check slides a spring-loaded disc a short way along the pipe axis. The swing check finishes closing on reverse flow, while the axial check is designed to be seated as forward flow fades.

DFT's water hammer page makes the same point: a swing check needs reversal under way before it shuts. DFT says its spring seats the disc as forward velocity reaches zero, so no reverse flow builds.

John Ballun's 2003 Water Technology article reports slam tests by Val-Matic, a check valve maker. It cites studies naming three routes to fast closure: a low-inertia disc, a short stroke, or a spring. An axial check combines a short stroke with a spring, as AVK describes its nozzle check.

Check valves are also called non-return valves, as Spirax Sarco notes. DFT's silent check page gives two more names for the axial pattern: non-slam and spring-assisted. The name silent comes from the slam it avoids.

Swing vs axial silent check valves, criterion by criterion
Criterion	Swing check	Axial silent check
Closing action	Hinged disc; gravity, then reverse flow	Short-stroke disc; a spring as flow decays
Slam risk	Among the highest when the system decelerates fast	Among the lowest; designed to seat as forward flow stops (DFT)
Velocity to open fully (water, one maker's rule)	About 2.3 m/s	About 1.2 m/s
Head loss when fully open	Low, through a full bore	Slightly higher; the disc sits in the stream
Orientation	Horizontal; vertical up-flow only where the maker allows	Any orientation; down-flow needs a modified valve (DFT)
Straight run after a reciprocating pump, elbow or tee (one maker's guide)	Ten to twelve diameters	Four to five diameters
Purchase price	Lower	Higher, as DFT's own comparison concedes
Maintenance	Top-entry designs can be repaired in the line (SLB)	Check the maker's service method; DFT supplies internal repair kits
Belongs on	Large bore, low head, slow deceleration, piggable lines, water and treated wastewater	Pump and compressor discharge, high head, parallel pumps

02Why does closing speed decide whether a check valve slams?

Check valve slam happens when flow turns back toward a stopped pump before the disc is shut. The returning flow snaps the disc shut and is halted at once: a water hammer. Slam depends on how fast the liquid column decelerates, set against how fast the disc can close.

In Ballun's article, a single-pump, low-head system decelerates at under 20 ft/s², about 6 m/s². High-head or multiple-pump systems can reach 40 ft/s², about 12 m/s². Engineers usually get the deceleration figure from a transient analysis of the system. It depends on pump inertia, the length of the liquid column, pipe friction and static head.

Ballun cites a basic rule for water lines. Each 1 ft/s of reverse velocity gives about 100 ft of surge head, or 43 psi. In metric terms, about 0.3 m/s of reverse flow adds about 3 bar. Ballun advises avoiding reverse velocities above that 1 ft/s mark, because the slam is extremely loud.

The bang is not the disc hitting its seat. Ballun traces it to the pipe stretching under the surge. A soft-seated valve can sound just as metallic. In Val-Matic's tests, spring-assisted valves resisted slam best, while long-stroke swing and ball checks slammed most.

In their words
“The best check valve is not necessarily the one with the least potential to slam.”
John V. Ballun, P.E., on Val-Matic's slam tests, Water Technology (2003)

Match the closing behaviour to the system, then weigh head loss, cost and access. The surge itself is explained in our guide to water hammer.

03How do head loss and flow velocity change the swing vs axial choice?

Head loss and velocity change the choice because a check valve only performs as designed when fully open. On Milwaukee Valve's summary of the usual rule for water, a swing check needs about 2.3 m/s (7.5 ft/s) to open fully. Milwaukee's own spring-loaded silent checks need about 1.2 m/s (4 ft/s). Size either pattern for the real flow range, not the pipe bore.

AVK's selection guide credits the swing check's full bore with minimal loss, the nozzle check slightly more. Spirax Sarco's tutorial says a swing disc floating in the stream typically loses more than other types. Both hold true: a swing check is low-loss only while its disc is pinned fully open.

A Valve Magazine article warns that a part-open valve can chatter and wear early. It also loses more pressure than its flow coefficient, Cv, suggests. DFT traces most oversizing to picking the valve by pipe size alone. On duties with a wide flow range, the right check valve is often smaller than the line.

For fluids other than water, Milwaukee's bulletin scales the minimum velocity with the square root of specific volume. On a near-flat gravity line, check the crack pressure first. DFT's silent checks start opening at about 0.5 psi, roughly 0.35 m of water. Full opening comes at about 1.0 psi differential.

Rule of thumb

Check the velocity at your lowest steady flow, not just at design flow. A swing check below its maker's minimum there can chatter for much of its life. A smaller valve, or a spring-assisted check sized for the low flow, fixes that.

04How do orientation, straight pipe and maintenance differ for swing and axial check valves?

Orientation, straight pipe and maintenance access often settle the choice before slam does. A gravity-closed swing check has few mounting options, while a spring-closed axial check suits almost any position. Both need steady flow upstream, and the swing check needs more straight pipe.

DFT's guidance keeps swing checks to horizontal runs. Some pipeline swing checks are rated for vertical up-flow, as SLB's Tom Wheatley valve page lists. Ballun adds that gravity-closed discs in vertical pipe may slam more. A swing check never belongs in vertical down-flow, where gravity holds its disc open.

DFT states its in-line checks work in any orientation, provided flow follows the arrow on the casting. For downward flow, DFT modifies the valve to carry the disc weight and any static head. Write the orientation on the enquiry, because a down-flow valve is built differently.

DeZURIK summarises MSS SP-92 as ten straight diameters after pumps, tees or fittings, and five after elbows. Confirm that figure against the current edition and your purchase order. Milwaukee Valve asks for less with silent checks: four to five diameters, against ten to twelve for swing. Its figures apply after a reciprocating pump, elbow or tee.

SLB notes that its top-entry swing check can be repaired in the line. Ask whether the axial check can be serviced in place. If not, put isolation on both sides, or plan to drain the line.

Run this list before the valve goes into the line:

Orientation stated: horizontal, vertical up-flow or down-flow, so the valve build or pattern matches.
Straight run measured: the distance from the pump, tee or elbow upstream, checked against the maker's figure.
Spares route agreed: DFT supplies repair kits of internal parts for the valves it makes.

05Where does a swing check belong, and where does an axial check belong?

A swing check valve suits big bores, low head, gentle deceleration and pipelines that must pass a pig. An axial check valve suits fast-decelerating systems: high-head pump discharge, parallel pumps and compressors. Choose by the system's deceleration first, then by head loss, cost and access.

Spirax Sarco notes swing checks are used mainly above DN125, where their light construction saves cost. The same tutorial warns that large discs are heavy and slam hard when they do close. On a large, low-head line that slows gently, a swing check is the right choice.

AVK sees swing checks as ideal where surge risk is low, and common in water and wastewater. With raw sewage, AVK warns that solids can affect performance, so design and materials need care. Piggable pipelines favour full-opening swing checks outright, since an axial disc sits in the bore. SLB lists the ability to pass pigs and spheres for its full-bore pipeline swing check.

Ballun notes a check valve can be fitted with an oil dashpot, a hydraulic damper, so it closes slowly. The pump must then tolerate some backspin, he adds. AVK notes that variable speed drives can ramp pumps down gently. Slowing the system is sometimes cheaper than changing the valve.

Axial checks belong wherever the column decelerates fast. Think of pump discharge into a tall riser, or parallel pumps sharing one header. DFT says its spring-assisted checks suit liquid, gas and steam, in horizontal or vertical lines.

Optimus Distributor is an authorised distributor for DFT Inc. of Exton, Pennsylvania. DFT makes engineered silent and axial check valves. Our DFT ranges are Basic-Check and GLC NAB silent check valves. The ranges also include SCV threaded in-line check valves and PDC flanged check valves.

DFT describes the GLC NAB for salt and brackish water, and the PDC for reciprocating compressor discharge. Optimus Distributor, in Subang Jaya, Selangor, supplies across Malaysia, including Sabah and Sarawak. Range details are on our DFT check valves page.

DFT does not publish flow curves; it offers custom sizing from application data. We sit down with your engineers to make sure you get the right solution. The selection order is in our guide to choosing the right industrial valve. Quoting steps are in how ordering and technical support work.

06Questions buyers ask about swing and silent check valves

Can a swing check valve be installed vertically?

Only with upward flow, and only where the manufacturer rates it for that. DFT's guidance limits swing checks to horizontal runs. Never fit one in down-flow, where gravity holds the disc open.

Why does my check valve chatter?

Chatter usually means the disc never reaches full open, because the valve is oversized for the flow. The disc hunts and wears its hinge or guide. A smaller valve or a lighter spring usually cures it.

Is a dual-plate check valve the same as an axial check valve?

A dual-plate check is different: two half discs on a central hinge, closed by a torsion spring. The two plates swing open rather than slide along the axis. Val-Matic's tests ranked its spring-assisted dual disc among the best non-slam types.

What information is needed to size a check valve?

Send the flow range from minimum to maximum, the fluid, pressure, temperature, pipe size and orientation. Say what sits upstream, such as a pump or elbow. DFT sizes from application data, not published flow curves.

Swing vs silent (axial) check valves: where each one belongs