Specifying industrial valves: from line conditions to purchase order.

Every good specification begins with three facts the plant already knows: what flows in the line, at what pressure, and at what temperature. Write them down first, because every later decision is a function of them — the pattern, the class, the materials, even which certificates are worth paying for. A specification that starts from a valve catalogue instead tends to work backwards, bending the duty to fit a part number.

Add the operating context while you are there: is the duty open-shut or regulating, how often will the valve cycle, is the medium clean or dirty, and does the line ever see pigging? Five minutes of honesty here saves two rounds of quotation questions later.

A pressure class is an envelope, not a number: Class 150 permits one pressure at ambient and a lower one at every step of temperature above it, per material group, as tabulated in ASME B16.34. So the class is chosen from design pressure AND design temperature together — the classic error is rating a hot line from its gauge pressure alone. Work the check in the pressure class reference, then confirm against the current edition of the standard: the table on your purchase order governs, not a website.

Ends must match the line: flanged to the flange standard your piping uses, butt-weld for high-integrity welded systems, threaded or socket-weld at small bore. The operator is the last mechanical choice — handwheel for occasional stroking, gearbox where the torque climbs with size and class, actuator where the valve answers a control system. An actuated line needs its mounting pad and signal stated on the enquiry, or the quotation comes back as questions.

The finished purchase order reads like the enquiry that produced it: size and class, end connection, body and trim material, service conditions, operator, and the testing and certification required — six lines per item, nothing implied. Send that list to the counter and the quotation states what is from stock and what is on indent; the whole route is on how ordering works. A specification written this way survives contact with any supplier — which is exactly the test it should pass.

Each decision above has a deeper memo behind it. These five carry the detail this guide deliberately compresses — read the one your specification is currently stuck on.