Passing valves: why a closed valve still leaks

What passing means, and what it does not

Passing is an internal leak. The valve is closed, but flow continues through the seat.

It is not an external leak. A stem or body leak releases product into the air and needs different work. Our live leak sealing guides cover that case.

A passing block valve is most serious during an emergency. If isolation fails, more product escapes until another isolation point works.

Five warning signs worth acting on

These signs are screening evidence, not proof. Pressure can rise from heating or equalising. Meters can misread. Treat each sign as a reason to test the seat properly.

Prefer a guided version? Run the four-question passing valve check.

1. Downstream pressure rises after the valve is closed.
2. Flow or metering continues when it should read zero.
3. A hissing or whistling sound comes from the seat of a closed valve.
4. A bubble or pressure test fails during routine checks.
5. The valve became harder to operate in recent months. Fouling can restrict movement and damage seats.

Why seats stop sealing

Common causes include these four. Other causes exist, such as incomplete travel, wrong stop settings or damage beyond the seat faces.

1. Debris. Pipe scale, weld slag and dirt collect at the seat. Pressure then wears small scratches into leak paths.
2. Scoring and galling. Metal surfaces rub without lubricant and cut grooves across the sealing faces.
3. Dried or moved sealant. In valves designed with seat sealant systems, the sealant washes away or dries over years of service.
4. Hard seals in dry gas. Dry gas service removes lubricant over time. Seal behaviour then depends on the seal material, temperature and pressure history.

Industry data shows this fault is common. A valve integrity study covered upstream and midstream sites. Passing was the largest fault group at 61 percent in that study population. Seized valves were next at 28 percent. Your own network may differ, which is why surveys matter. See the full breakdown in our interactive failure mode chart.

The decision: three repair routes

Once passing is confirmed by a proper seat test, there are three options. The right choice depends on seat condition, the valve design and site rules. Check three gates first: the valve has injection fittings in good condition, the design and service suit injection, and operations permit the work.

1. Cycle and flush. If loose debris caused the leak, cycling and cleaner injection may clear the seat. This is usually the least intrusive first step, and it works only for light fouling.
2. Online sealant injection. Cleaner softens residue, then non-hardening sealant fills damaged sealing surfaces. Injection uses the valve's injection fittings under live pressure, with no shutdown. In suitable valves, this can restore isolation for galled, scored or lightly cut seats.
3. Repair or replacement. Sealant cannot repair badly damaged seats, cracked balls or broken trims. This route needs isolation and often a shutdown. Teams therefore try routes 1 and 2 first.

An injected seal is not permanent. Valve cycling washes some sealant downstream. Sealed valves therefore need planned top-ups. Our field services set that plan for your valves.

What an online repair looks like

1. Survey the valve type, size, fittings, line pressure and fault history.
2. Inject valve cleaner and allow it to work into the seat area.
3. Cycle the valve where operations allow, so the cleaner spreads.
4. Inject non-hardening sealant in controlled amounts, within the fitting and equipment ratings, watching the pressure response.
5. Test the seat again, record the result, and set a stop-work rule if the response is wrong.

Product choice matters. Organic sealants can harden in dry gas and seize the valve. We make synthetic cleaners and non-hardening sealants and the high-pressure injection equipment used for safe injection.