Swing check valves, 1/4" to 6"

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.

• ⚠ It fails silently, and that is the whole problem. 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; it is the single most effective thing you can do for a check valve.
• Orientation is not optional. 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. The full rules.
• It needs a minimum forward flow to stay open. 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.
• ⚠ It slams. 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. When to use something else.
• Five variants, one closing principle. 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. The comparison.

Five variants, one photograph each. All photographs of the stocked item, not renders.

• JIS 10K dimensions are quoted per order for most sizes. The catalogue publishes dimensions for the 3" and 4" sizes and part numbers for all eight. For the other sizes, tell us the size and we measure a stocked unit.
• ANSI 150 SS316 face‑to‑face is quoted as the catalogue prints it: 152 mm at 1/2", 178 mm at 3/4", then 127 to 292 mm from 1" to 4", in line with ASME B16.10. On a tight replacement, measure the gap in your run before you order and we confirm the length against a stocked unit.
• The cast steel and SS316 tables each carry their own face‑to‑face figures — the two columns differ by a few millimetres at some sizes, so 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 rating is carried in the part number. Seven of the nine sizes read ‑300‑ and two read ‑200‑ — the 1/2", which has “200 WOG” cast into the body (see the photograph), and the 2½". State the size on your enquiry and the rating is confirmed on the quotation.
• ⚠ Three of the five publish no working temperature. 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. All five test tables.
• Stated size ranges understate themselves. 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.
• ANSI 150 and JIS 10K do not bolt to one another, whatever the pressure rating says. Check the bolt circle on the mating flange.

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.

• The risk rises with head and with line length. 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.
• It is worse on pump trip than on normal shutdown. A controlled stop lets flow decay gently. A power cut does not, and that is exactly when the check valve is most needed.
• → If slam is a concern, use a dual plate check valve. 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.
• → Or a spring check valve. 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.
• Do not try to fix slam by fitting the swing check loosely or partly closing a valve near it. Both make the flow into the disc worse, not better.
• Tell us the line length, the head and whether the pump can trip. 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.

A swing check has one moving part — the disc, on its hinge. Everything else is a casting, a pin, a gasket and the bolting that holds the bonnet on. That is why they last, and why the two things that actually fail are the seat face and the hinge pin.

• Only the cast steel valve has a separate hardfaced seat. Item 2 is A105 + F6 — a 13% chrome facing on the seating surface, with the same facing on the disc. That is what lets it work at 425 °C and take repeated closure without the seat washing out. The stainless variants seal 316 on 316, which is softer.
• Graphite on cast steel, PTFE on everything else. That single difference is why the cast steel valve is rated to 425 °C and the threaded stainless one stops at 180 °C. It is the gasket, not the body, that sets the ceiling.
• ⚠ The threaded SS316 fasteners are 304, not 316. Items 5, 6 and 7 — pin, plain washer and bolt — are SS304 in a valve whose body and disc are 316. The pin is in the flow path, so on chloride‑bearing service it will pit before the body does. The flanged SS316 variants use SS316 pins.
• The cast steel bonnet fastening is a B7 bolt with a 2H nut — listed in the catalogue as “bonnet bolt, ASTM A193 B7” and “bonnet bolt nut, Gr.2H”, the standard high‑strength combination.
• Item 7 of the ANSI 150 SS316 list is printed “S316” in the catalogue — the material is SS316, the same as the rest of the trim.
• The JIS variant uses Japanese grades. SCS14A is the JIS cast stainless equivalent of CF8M. It is the only variant here specified in JIS material designations, which is at least internally consistent with its flange standard.
• Brass all the way through, including the pin. Body, disc, cap, pin, washer and bolt are all brass on that variant, with PTFE gaskets. Nothing to seize, nothing dissimilar — and no corrosion resistance beyond what brass gives you.