Q1. What is a stainless steel swing type check valve used for, and where is it used?
A1. A check valve prevents reverse flow and does nothing else — it is not an isolating valve and cannot be used to shut a line down. It is fitted wherever backflow would damage equipment, contaminate a line or drain a header: at the discharge of every pump, on compressor and blower outlets, on the suction of a pump fed from a pressurised source, downstream of a meter, at the outlet of a boiler feed line, and on any branch that must not drain back. It works automatically, with no operator and no external signal, so it has to be chosen so that it neither chatters at low flow nor slams when the flow reverses. Because it is a passive item with no adjustment once it is in the line, its closing behaviour is what decides whether it protects the line or damages it. In stainless construction it is used on corrosive and clean services: acids and alkalis, food and pharmaceutical product lines, and demineralised or high-purity water.
Q2. What design variants are available, and how do I tell them apart?
A2. The type is chosen from how much space is available and how fast the flow reverses. A swing check has a hinged disc and is the most common on water and steam mains, but it needs a long installation length and its disc can slam. A dual-plate (wafer) check puts two spring-loaded half discs in a short body, which suits tight spaces and reduces slam. A lift check guides the disc vertically and is used on high-pressure small-bore lines and where the flow is always in one direction. A tilting-disc check is the answer on large compressor and pump discharges, because the disc closes before the flow has fully reversed. Nozzle or axial checks use a spring-loaded ring around the axis and are chosen where the response has to be very fast.
Q3. How do I choose between austenitic and duplex stainless grades?
A3. Start from the medium rather than from a grade list. CF8 (304 type) is adequate for water, steam and mildly aggressive chemical service; CF8M (316 type) is the usual minimum where chlorides, dilute acids or urea are present. Above roughly 60 °C, chlorides become a stress-corrosion cracking risk for austenitic grades whatever the molybdenum content, and duplex stainless is the standard answer because it resists both cracking and pitting better. Put the medium, its concentration, the chloride level and the temperature into the enquiry — those four figures decide the grade, and a specification that says only ‘stainless steel’ leaves the choice open to whoever quotes it.
Q4. What should I watch during installation, operation and maintenance?
A4. Orientation rules differ by type and must be followed: a swing check needs a horizontal line with the hinge above the flow, a lift check needs the flow to lift the disc against gravity, and a dual-plate check can go in vertical or horizontal pipe but needs a straight run either side. A check valve is not an isolation valve — it does not replace a gate or ball valve for maintenance, and it should never be the only barrier against reverse flow on a line where backflow would be dangerous. On pumps, fit the check downstream of the isolation valve, not between it and the pump.
Q5. What are the common failure modes, and how are they avoided?
A5. A valve that fails early usually fails for one of four reasons: it was selected on the normal condition and then met the extreme one, it was installed against the flow arrow, it was used for a duty it was never designed for such as throttling or as the sole isolation point, or it was commissioned without the pipework being cleaned. Only the first of those is a selection issue; the other three are found on site, which is why the installation checks matter as much as the specification.
Q6. How do I select the right one for my line?
A6. Selection starts from the service, not from the catalogue. Fix the medium and its concentration first, because that decides the body material and the seating. Then the design pressure and the design temperature together, since the allowable pressure at temperature is lower than the cold rating and it is the hot figure that governs. Then the size, the connection, and how often the valve will be operated, which is what decides between a handwheel, a gearbox and an actuator. Bring those five answers to the enquiry and the rest of the selection follows from them.
Q7. How do I order, and what information is needed for a quotation?
A7. Give us the service conditions rather than only a model number: size, pressure rating, medium, temperature and connection. Where a standard applies, name it and the revision. Custom work is normal in this range — end connections, operator type, trim material and surface treatment are all specified per order.
