Last Wednesday afternoon, a call came in from the site of a waterworks phase II project around Suzhou: several newly installed gate valves failed to seal tight during pressure testing, before the line was even commissioned. We went on site and stripped one valve down — the problem wasn't the valve body. The pipeline flushing sequence had been done wrong, and weld slag and sand had lodged on the sealing surfaces. We've run into this several times in recent years.

A Gate Valve Leaking Before It's Even Commissioned: Weld Slag Is Usually the Culprit

A gate valve seals tight through the compression between the two sealing rings of the disc and seats. Weld slag, mill scale, and sand from welding are all hard particles that lodge precisely between those two sealing rings. With the gate valve fully open, the disc retracts into the bonnet and the particles settle in the cavity at the bottom of the body; once the valve closes, the disc presses the debris into the sealing surfaces — at best scoring them, at worst leaving the valve unable to close fully.

Put another way: when a gate valve on a new pipeline fails, eight times out of ten it isn't the valve quality — the pipeline wasn't flushed clean.

Keep the Gate Valve Fully Open During Flushing — Never Use It as a Throttle

The most common shortcut on site is flushing with the gate valve partly closed, hoping the flow through the gap carries the debris out. The reality is the opposite. A gate valve is not a control valve: high-velocity, sand-laden water across a half-open disc erodes the disc and sealing surfaces faster than any scratch from debris.

Another bad practice: half-opening a gate valve as a temporary dam. Weld slag jams the disc guides, the handwheel gets heavier with every turn, and eventually it seizes altogether — no on-site cleaning will fix that; the valve has to be stripped down.

Flush, Reinstall, Pressure Test — Out of Order, All for Nothing

There is only one correct sequence. Before welding and pressure testing, remove the gate valves from large-diameter lines and substitute temporary spools; flush and clean the pipeline first, then reinstall. If removal isn't practical, the gate valve must be fully open during flushing so the media flows straight through the body cavity.

After flushing and reinstallation, stroke each gate valve several full turns — full travel, smooth feel — before the hydrostatic test. During the test, close the gate valve fully, pressurize the line, and check the bonnet and outlet end for weeping. If operation feels gritty, don't force it; check whether debris is jammed in the guides first, and if in doubt, send us your service conditions for review.

At Inquiry, Spell Out the Flushing and Testing Plans Too

Water treatment projects often use resilient seated gate valves, whose rubber-encapsulated sealing surfaces seal well — but once damaged by debris they generally must be replaced. Metal-seated gate valves withstand erosion better, yet sand still isn't good for them. Which type to choose depends on the debris in the media and the pipeline's flushing conditions.

So when inquiring, don't just state size and pressure — describe the media, debris content, and the flushing and testing arrangements, so the selection and shipping documents actually line up. You can start with the gate valve series parameters in the product center, or contact us directly to review your conditions. Selection starts from service conditions — a product catalog can't answer these questions.

Your next new pipeline — flushed first, or gate valves installed first? Get it backwards, and on pressure test day you'll most likely find a crowd circling a gate valve.

Suzhou Akameng Automation Equipment Co., Ltd. · Officially Authorized SUFA Distributor (Authorization No. SFSQ-0036) · Tel 0512-66673578 (WeChat 15851489497)