The valve that only works in one direction — and the one that fails without telling you. Five variants in cast steel, SS316 and brass, flanged or threaded, plus how it goes in and why it slams.
DN8 to DN150 across the range.
ANSI 150 · JIS 10K · NPT · BSPT.
Working pressure by variant.
Body material.
Each has its own page, with the full parts list, dimensions and catalogue PDF.
Nothing operates it. Flow does.
A swing check valve is a disc on a hinge. Forward flow pushes it open and holds it there; when flow stops or reverses, the disc swings shut under its own weight and the backpressure behind it, and seals against the seat. There is no handle, no stem and no actuator — the fluid does all of it, which is why a check valve is the cheapest way to stop reverse flow and also the least visible when it stops working.
What it protects against is usually a pump. Stop a pump on a filled line and the column of fluid above it tries to run back down through the impeller, spinning it backwards. A check valve at the discharge stops that. It also keeps two pumps on a common header from pushing each other backwards, and keeps a drain line from siphoning the wrong way.
A piece of grit or scale caught on the seat holds the disc fractionally open. From outside the valve looks completely normal — and it is passing reverse flow. There is no handle position to check. Fit a Y-strainer upstream; it is the single most effective thing you can do for a check valve.
Flow must follow the arrow cast into the body, and on a horizontal run the bonnet must point up so the disc closes with gravity rather than against it. On a vertical run, flow must be upward.
Below that the disc floats near the seat and chatters, wearing the hinge pin and the seat face. A check valve that is too big for the duty wears out faster than one that is the right size.
A swing disc closes fast on flow reversal, and a fast closure in a full line is water hammer. On long lines and high-head pumps that shock can be worse than the backflow you were preventing.
The manufacturer's catalogue is the reference for every figure below; anything it does not state — a dimension, a temperature, a test figure — we obtain from the factory or measure on a stocked unit.
A swing check is the default. It is not always the answer. All six are on this site and they do the same job in different ways. The differences that matter are how fast they close, how much pressure they cost you when open, and how much room they take in the run. Pick on those, not on price.
| Type | How it closes | Pressure drop | Length in the run | Right when |
|---|---|---|---|---|
| Swing check | Disc swings on a hinge, closed by gravity and backpressure | Low — the disc lifts clear of the bore | Long | The general case. Cheapest, least restriction, widest size range. |
| Spring check | A spring pushes the closure member shut, so it does not need gravity | Higher — the spring resists flow | Short | Any orientation, including downward flow. Also closes sooner and more gently, so less slam. |
| Dual plate check | Two spring-loaded half discs fold together in the middle | Low for its length | Very short — wafer body | Space is tight, or weight matters, or you want much less slam than a swing disc gives. |
| Single plate check | One spring-loaded disc on a hinge | Low | Very short — wafer body | Same short-body case in the smaller sizes. |
| SAE flange check | Spring-loaded poppet | Moderate | Short | You are on SAE flanged hydraulic pipework, not process pipework. |
| High pressure check | Spring-loaded poppet or ball | Moderate to high | Short | Pressure is well beyond 300 PSI — hydraulics rather than 150# process lines. |
Because a fully open disc sits up out of the flow path. That is its real advantage over every spring type: nothing pushes back against forward flow once it is open.
A swing check needs gravity to help it shut. If you need downward flow, you need a spring check, not a swing check fitted upside down.
Two light half-discs travelling a short distance close far more gently than one heavy disc swinging through 90°.
A dual plate body fits between two flanges. A swing check at 6″ needs 356 mm of pipe run to itself.
A check valve is not a shut-off. If you need to stop flow deliberately, you need a gate valve or a ball valve as well.
End connection first, then material. The flags are real and they are explained below.
| Variant | Ends | Size | Body | Pressure | Temperature | Choose it when |
|---|---|---|---|---|---|---|
| ANSI 150 cast steel | ANSI 150# flange | 1½″–6″ | A216 WCB | 150# | −29 to 425 °C | Hot duty, or the largest sizes. The only variant to 6″, and the only one rated above 180 °C. |
| ANSI 150 SS316 | ANSI 150# flange | 1/2″–4″ | SS316 / CF8M | 150# | not published | Corrosive duty on ANSI pipework. Widest flanged range at the small end. |
| JIS 10K SS316 | JIS 10K flange | 1/2″–4″ | SCS14A / SS316 | 10K | not published | Japanese-standard pipework. Most dimensions on request — see its page. |
| Threaded brass | NPT | 1/2″–4″ | Brass | 200 / 300 PSI | not published | Water and general service on threaded pipework, where cost matters and the fluid is not aggressive. |
| Threaded SS316 | BSPT or NPT | 1/4″–2″ | SS316 / CF8M | 200 PSI | −29 to 180 °C | Small bore in stainless, or anything below 1/2″. Two thread forms, 16 part numbers. |
Only the cast steel (−29 to 425 °C) and the threaded SS316 (−29 to 180 °C) state one. The two flanged stainless variants and the brass one state none at all. If temperature matters, say so and we will get the figure from the factory.
The catalogue publishes dimensions for the 3″ and 4″ sizes only, and part numbers for all eight. For the other sizes, tell us the size and we measure a stocked unit.
Seven of the nine sizes read -300- and two read -200-: the 1/2″, which has “200 WOG” cast into the body, and the 2½″. State the size on your enquiry and the rating is confirmed on the quotation.
The cast steel and SS316 columns differ by a few millimetres at some sizes — quote from the table of the variant you are buying. On a replacement, measure the valve you are taking out and we confirm against a stocked unit.
The brass page says “1/2″ to 4″ (DN40–100)” where 1/2″ is DN15, and the threaded SS316 page says “1/4″ to 2″ (DN40–100)” where 1/4″ is DN8 and 2″ is DN50. The inch figures are right in both cases; the DN figures are not.
Whatever the pressure rating says. Check the bolt circle on the mating flange.
A swing check valve has no handle, so there is nothing to get wrong at the point of use — everything that can go wrong goes wrong at installation, and then stays wrong quietly. The three orientation rules below are not preferences: a swing disc needs gravity working with it, and if you deny it that, the valve will either not close or not stay closed.
Every one of these bodies has a flow arrow cast into it — you can see it in the brass and threaded stainless photographs. Fitted backwards a check valve does not restrict flow a little, it blocks the line completely, which at least announces itself immediately.
The hinge pin must be horizontal and above the disc, so the disc falls shut. Rotate the valve onto its side and the disc has to close sideways with no help from gravity; turn it fully over and gravity holds it open.
Then gravity closes the disc when flow stops. A swing check must never be used on downward flow — for that you need a spring check, where a spring does the job gravity cannot.
One flake of scale between disc and seat holds the disc a fraction open. The valve then passes reverse flow while looking perfectly normal.
No stem position, no handle, no indicator. Unlike a gate valve you cannot even tell by feel.
It is the single most effective thing you can do for a check valve, and it costs a fraction of the pump the check valve is protecting. Y-strainers.
Fitting one immediately downstream of a pump discharge, an elbow or a reducer puts turbulent flow onto the disc, which makes it flutter and wear. A few diameters of straight pipe upstream is worth having.
Oversized is worse than tight: below the minimum flow the disc floats and chatters, wearing the pin and the seat. If your flow varies a lot, tell us the low figure as well as the high one.
All three flanged variants have a bolted bonnet and the two threaded ones a screw-on cap. Either way the disc, hinge and pin come out through the top — so leave headroom above the valve, not just length in the run.
A flanged check valve at 6″ is not light, and it must not hang on the pipework or be used to pull a misaligned flange together.
If you need to isolate the line, fit a gate valve or a ball valve as well. A check valve stops reverse flow; it does not stop you.
The one place a swing check is the wrong choice.
When forward flow stops, the disc has to travel from wherever it was sitting all the way back to the seat. On a swing check that is a heavy disc swinging through most of 90°, and by the time it arrives the fluid behind it is already moving backwards. It stops that reverse column almost instantly, and the energy has to go somewhere: into a pressure spike travelling back up the line. That is water hammer, and you hear it as a bang.
A short low-head line barely notices. A long line off a high-head pump can generate a spike well above the working pressure — enough to damage the pipework, the pump, or the check valve itself.
A controlled stop lets flow decay gently. A power cut does not, and that is exactly when the check valve is most needed.
Two light spring-loaded half-discs travel a short distance and are already closing before reverse flow builds. It is the standard answer to this problem, and the wafer body takes far less room in the run. Dual plate check valves.
The spring starts closing the moment forward flow falls off, rather than waiting for reversal, so the closure is earlier and gentler. It also works in any orientation. Spring check valves.
Fitting the swing check loosely, or partly closing a valve near it, makes the flow into the disc worse, not better.
That is enough for us to say whether a swing check is right, and it is a five-minute conversation against a very expensive bang.
It is a disc on a hinge. Forward flow pushes it open and holds it there; when flow stops or reverses, the disc swings shut under its own weight and the backpressure behind it, and seals against the seat. There is no handle, no stem and no actuator — the fluid does all of it.
A piece of grit or scale caught on the seat holds the disc fractionally open. From outside the valve looks completely normal — correct size, correct orientation, no leak — and it is passing reverse flow. There is no handle position to check and no indicator to read. Fit a Y-strainer upstream.
Three rules. Flow follows the arrow cast into the body. On a horizontal run the bonnet must point up so the disc falls shut. On a vertical run, flow must be upward. A swing check must never be used on downward flow — for that you need a spring check.
A swing disc closes fast on flow reversal, and a fast closure in a full line is water hammer. The risk rises with head and line length, and it is worse on pump trip than on normal shutdown. If slam is a concern, use a dual plate or a spring check.
Five. ANSI 150 cast steel (1½″–6″, the only one above 4″ and the only one rated to 425 °C), ANSI 150 SS316 (1/2″–4″), JIS 10K SS316, threaded brass NPT (1/2″–4″) and threaded SS316 BSPT/NPT (1/4″–2″).
No. A check valve stops reverse flow; it does not stop you. If you need to shut the line deliberately, fit a gate valve or a ball valve as well.
Because the catalogue does not print one. Only the cast steel (−29 to 425 °C) and the threaded SS316 (−29 to 180 °C) state a temperature. The two flanged stainless variants and the brass one state none at all. If temperature matters, say so and we will get the figure from the factory.
No, whatever the pressure rating says. The bolt circle and hole pattern differ. Check the mating flange before ordering.
6″ (DN150), and only in the ANSI 150 cast steel variant. The other four stop at 4″ or 2″. Note the length: at 6″ this valve needs 356 mm of pipe run to itself.
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Related: spring check for any orientation · dual plate check if slam is a concern · Y-strainers to fit upstream · gate valves for isolation · all low pressure valves.
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