A common practice: installing a needle valve for throttling, plus a globe valve upstream for shut-off. The idea is “double isolation” – but improper series connection introduces extra risks.The main Needle Valve product names of China Needle Valve Network include:GMJ14F/H-250 High Sealing Sampling Valve,GMJ11F/H-25 High Sealing Sampling Valve,J21W/H Internal And External Pressure Gage Needle Valve,J43W/H Flange Needle Valve,JJ•BY1 Single Flange Pressure-taking Needle Valve,J61Y/W High Temperature and Pressure Needle Valve,JX49W/H Level Gauge Valve,JX29W/H Thread Level Gauge Valve,J13 Internal Thread Needle Valve

Problem 1: More leak points

Each threaded connection is a potential leak path. Adding a globe valve adds two more connections. In high-pressure gas or hydrogen systems, leak probability rises exponentially with each joint.

Problem 2: Trapped pressure in the cavity

When both valves are closed, medium can be trapped between them. Thermal expansion of trapped liquid can create “hydraulic lock” pressure – several times system pressure – potentially bursting the spool piece or instrument fittings.

Problem 3: Wrong operating sequence

If the globe valve is closed first, then the needle valve, medium is trapped between them. Correct sequence: close globe valve first, then needle valve (so the needle valve takes the seal); open needle valve first, then globe valve.

Best practice

• Non-critical systems: a single needle valve is sufficient.

• High-pressure or toxic media: add a globe valve upstream, but install a vent between the two valves.

• Frequent maintenance needs: use an integrated manifold (3-valve or 5-valve), combining needle, block, and vent valves to minimise threaded joints.

Bottom line: Avoid unnecessary series connection. If double isolation is required, use an integrated manifold with a vent path.

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