A passing Class IV seat test comes from converting the valve's rated Cv into a leakage-flow limit at the specified test conditions, then measuring that flow in compatible units. The common shortcut—multiplying Cv by 0.01% and labeling the result mL/min—fails because Cv is a flow coefficient, not a volume rate.
Reject the quick calculations
Start by removing three numbers that regularly produce a false pass or failure:
- Do not use normal operating flow, design flow, or maximum plant flow as valve rated capacity.
- Do not treat
0.01% × Cvas gpm, mL/min, or bubbles per minute. - Do not turn bubble counts into volume flow until the counter's bubble volume and measurement conditions are known.
For a Class IV criterion of 0.01% of rated valve capacity, convert the percentage first:
0.01% = 0.01 / 100 = 0.0001
Allowable leakage = 0.0001 × rated capacityThe multiplication is straightforward; defining rated capacity in the same flow basis as the leakage test is the real job. Get it running, then fix it properly: if production needs an immediate check, compare a calibrated flow reading with the calculated limit, but retain the full test record for the permanent acceptance decision.
Check before continuing: the worksheet must show 0.0001, not 0.01, as the multiplier.
Set the governing test basis
Record the standard edition, leakage class, valve configuration, test medium, applied differential pressure, flow reference conditions, allowed flow units, valve closing method, and permitted measurement method. These items control the conversion from capacity coefficient to actual test flow.
The test differential pressure is not automatically the process differential pressure. Likewise, gas flow at actual test conditions is not interchangeable with a flow stated at reference conditions. Absolute pressure, temperature, density, and the reporting basis matter for gas measurements.
Confirm that Class IV is the purchase or datasheet requirement. A seat test cannot prove compliance when the specified class, test method, or acceptance basis is missing. Stop here if the governing standard edition or required test conditions cannot be identified; obtain the purchase specification or the manufacturer's approved test procedure.
Check before continuing: one test sheet must define the medium, differential pressure, closure condition, and reporting units that will be used for both calculation and measurement.
Read the correct rated Cv
Use the valve's rated Cv for the installed valve and trim. Look for fields such as rated Cv, selected Cv, or valve capacity coefficient on the valve datasheet, sizing sheet, certified drawing, or manufacturer documentation.
Match the number to the actual body size, trim size, characteristic, flow direction, and installed internals. A sizing calculation may list both a required Cv and a selected or rated Cv. Required Cv describes the process duty; rated Cv describes the valve's capacity at its rated opening. The Class IV percentage applies to rated capacity, not the smaller process requirement.
| Candidate value | What it represents | Use for the Class IV basis? |
|---|---|---|
Required Cv
|
Capacity needed at a process operating point | No |
| Normal or maximum process flow | Plant operating duty | No |
Rated or selected Cv
|
Capacity of the supplied valve and trim | Yes, after matching the installed configuration |
Unidentified catalog Cv
|
Possible value for another trim or flow direction | No, until configuration is matched |
Check before continuing: the rated Cv on the worksheet must trace to the installed trim, not merely to the nominal body size.
Convert capacity into a leakage limit
First calculate the coefficient-level limit:
Cv_leak,max = 0.0001 × Cv_ratedThis result remains a capacity coefficient. Convert it to flow with the equation and conditions applicable to the test medium.
For an incompressible liquid using the conventional Cv relationship in compatible US customary units:
Q_rated,test = Cv_rated × sqrt(ΔP / SG)
Q_leak,max = 0.0001 × Q_rated,test
Where:
Q = volumetric flow in the units associated with the Cv equation
ΔP = test differential pressure
SG = liquid specific gravity at test conditionsFor gas, use the applicable compressible-flow equation or the manufacturer's calculation method. Supply the actual upstream and downstream absolute pressures, gas temperature, gas properties, and declared reference conditions. A liquid square-root equation cannot produce a valid gas leakage limit.
A bubble counter does not define the acceptance criterion; it only measures leakage. Convert counts to volume flow only when its calibrated volume per bubble is known:
Measured volume rate = bubble count × calibrated bubble volume / elapsed timeUse the pressure and temperature at the counter's measurement location. If the instrument reports mL/min directly, verify whether that is actual or referenced volume flow before comparing it with the calculated limit.
Check before continuing: the allowable limit and the test instrument output must have identical flow units and reference conditions.
Connect and run the seat test
- Confirm the valve identity, installed trim, rated
Cv, leakage class, and required flow direction. - Isolate the outlet measurement path from body, fitting, tubing, and instrument leaks. Perform a blank or zero-leak check on the rig.
- Place the valve in the closure condition required by the governing procedure. Use the specified actuator action or closing force; extra force can hide seat damage and invalidate the result.
- Apply the required test medium and differential pressure. Read differential pressure across the valve rather than relying only on a supply regulator setting.
- Allow pressure and indicated leakage to stabilize. Use a measurement interval long enough for the chosen instrument to resolve the limit without inventing a fixed timing requirement.
- Record measured leakage, units, reference conditions, differential pressure, medium, closure method, and instrument identification.
If leakage is high, check the rig, flow direction, actuator travel, seating force, positioner zero, and debris before dismantling the valve. Tightening linkage, increasing actuator force, or lapping the seat without diagnosing the cause may change calibration, damage the trim, or conceal an incorrect test setup.
Check before continuing: hold the specified differential pressure with stable readings and no detectable bypass leakage from the test apparatus.
Prove the result end to end
Repeat the closure and measurement cycle after reopening the valve. A repeatable result separates seat performance from trapped pressure, instrument lag, or a temporary particle on the seating surface. Compare the largest valid measured value with the calculated maximum:
Pass when measured leakage ≤ allowable leakage
Fail when measured leakage > allowable leakageReview the full chain: installed trim to rated Cv, rated Cv to test-condition rated flow, 0.0001 multiplier to allowable leakage, and instrument indication to the same units and reference basis. Document any correction applied by the instrument rather than comparing corrected and uncorrected flows.
Final check: another technician must be able to reproduce the limit and the pass/fail decision from the recorded valve data, test conditions, formulas, and instrument readings.
FAQ
Why does 0.01% of Cv not equal mL/min?
Cv is a capacity coefficient, not a flow unit. Multiply rated Cv by 0.0001, then convert that coefficient to flow using the specified medium, differential pressure, properties, and reference conditions.
Why does the Class IV calculation use rated Cv instead of required Cv?
Required Cv represents a process operating point. Rated Cv represents the supplied valve and trim capacity, which is the basis for the stated 0.01% limit.
Why does my bubble count not match the calculated leakage?
Bubble count is not a volume rate until the bubble volume and elapsed time are known. Confirm the counter calibration and whether its volume is reported at actual or reference pressure and temperature.
Why does the valve pass at one test pressure and fail at another?
The allowable flow conversion and the physical seat leakage both depend on differential pressure and fluid conditions. Test at the differential pressure required by the governing procedure and measure that pressure across the valve.
When should I stop the FCI 70-2 Class IV test and escalate?
Stop when the standard edition, rated Cv, installed trim, test conditions, or instrument reference basis cannot be verified. Do not force the actuator or modify the trim to obtain a pass. Send the valve identification, datasheet, test setup, pressure readings, and leakage records to the valve manufacturer's official support channel.