Control Valve Leakage Classes IV, V and VI: RFQ Checklist

“Class V shutoff” is not a complete purchase requirement on its own. A control valve leakage class belongs to a specified test procedure, and the test conditions matter as much as the class name. Buyers should distinguish seat leakage from body pressure integrity, stem emissions and the separate isolation arrangements required by a plant. This guide explains how to make a leakage requirement clear enough to quote and verify.

Start with the standard and its edition

Identify the applicable standard, edition and required seat leakage class in the RFQ. The Fluid Controls Institute lists ANSI/FCI 70-2 for control valve seat leakage testing. If the project instead specifies IEC 60534-4 or another document, retain that reference and ask bidders to identify any deviation. Do not assume that a similarly numbered class under another standard has identical procedures.

Classes IV, V and VI use defined acceptance limits and test conditions; they are not simply percentages on one common scale. Class IV is expressed relative to rated capacity under its specified test. Class V involves a leakage limit related to seat or port size and differential pressure. Class VI uses tabulated limits related to port diameter. These descriptions help compare scope, but the agreed standard controls the actual method and acceptance value.

Specify the operating duty separately

State fluid, operating temperature, maximum shutoff differential pressure, flow direction and expected cycling. Identify the proposed seat and trim materials and their service limitations. Do not select a resilient seat solely to obtain a class label without checking temperature, chemical compatibility and wear conditions.

A factory seat leakage test is conducted under defined conditions. It does not establish that the same leakage rate will apply indefinitely at plant temperature or after solids have damaged a seating surface. Emerson's Control Valve Handbook provides background on valve testing and shutoff. Avoid converting a class into an unsupported “zero leakage in all conditions” promise.

Agree the assembled-valve test conditions

Ask the supplier to identify the test medium, temperature range, differential pressure, flow direction, stabilization requirements and leakage measurement method required by the selected procedure. Confirm the relevant port or seat diameter rather than substituting nominal pipeline size. Require the acceptance limit and units to be stated in the test documentation.

Include the offered actuator, minimum available supply pressure, closing action and accessory arrangement in the review. If the test uses a different actuator setup, request the reason and its approval before accepting the result as evidence for the delivered assembly. A larger actuator should not be assumed to solve a seating issue without checking permissible forces and the manufacturer's design limits.

Review a traceable acceptance record

The final report should identify the purchase-order item and valve, the test procedure, the actual conditions, the measured leakage and the acceptance decision. Where witnessing is ordered, agree the inspection point and notice period before testing. Request clarification if a report gives only “passed” while the purchase order requires measured values.

For example, two offers that both state Class V may still differ in actuator supply assumptions, test scope or excluded documentation. Resolve those differences before commercial comparison. Our pneumatic control valve accessory RFQ guide helps define the assembled package, while our valve test report checklist addresses other inspection-record questions.

See our pneumatic control valve product page for the product category. Send the control valve datasheet and leakage acceptance requirements to ZHONGYUE VALVE for a project-specific review. The final technical offer should state the achievable class, governing test procedure and any exceptions explicitly.

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