what is a control valve seat leakage classification?

When the application requires a control valve to modulate clean fluid, there are many designs that will provide both tight shut-off and excellent control. However, when the media contains solids (i.e. a slurry), there is no perfect valve. These standard control valves will typically provide shut-off when brand new, however erosion from abrasive fluid often results in the valve leaking shortly after being in service. Relying on a control valve to provide both control and isolation in an abrasive application is not standard industry practice. Control valve manufactures recommend installing a dedicated on/off valve (in line with the control valve) when isolation is required. 

Generally, metal-seated slurry control valves allow a certain amount of leakage past the seat when the valve is fully closed. This leakage rate can range from Class I to Class VI (as shown in the below table). Note that Class V and VI shut-off ratings are typically reserved for isolation valves with resilient seats (e.g. rubber, urethane, etc). Slurry control valves that claim to have Class V shut-off often degrade to Class IV (or lower) fairly quickly.

CONTROL VALVE LEAKAGE CLASSIFICATION

Class I

Maximum Seat Leakage: 
As agreed between customer and supplier

Test Media: 
None Required

Class II

Maximum Seat Leakage: 
0.5% of rated capacity

Test Media:
Clean air/water @ ambient temperature, 45-60 psi-g

Class III

Maximum Seat Leakage: 
0.1% of rated capacity

Test Media: 
Clean air/water @ ambient temperature, 45-60 psi-g

Class IV

Maximum Seat Leakage: 
0.01% of rated capacity

Test Media:
Clean air/water @ ambient temperature, 45-60 psi-g

Class V

Maximum Seat Leakage: 
0.0005ml per minute, per inch of seat diameter, per PSI differential

Test Media: 
Clean water at ambient temperature, maximum service pressure, above 100 psi-g

Class VI

Maximum Seat Leakage: 
Variable, eg. 3 bubbles/min for 2" valve, 6 bubbles/min for 3" valve. Refer to ANSI/CI 70-2 for complete values.

Test Media: 
Air/nitrogen at ambient temperature, 50 psi-g

Bubble-Tight

Isolation valves are generally marketed as bubble tight. It is widely accepted that"bubble-tight" refers to a valve that, when fully closed, does not pass any bubbles when one side of the valve seat is pressurized with air/nitrogen, and the other side of the seat is submerged in water. Actual test procedures can vary from manufacturer to manufacturer. There is no formal standard to define bubble-tight.

CONTROL VALVE LEAKAGE CLASSIFICATION

LEAKAGE CLASS

MAXIMUM SEAT LEAKAGE

TEST MEDIA

Class I
As agreed between customer and supplier
None Required
Class II
0.5% of rated capacity
Clean air/water @ ambient temperature, 45-60 psi-g
Class III
0.1% of rated capacity
Clean air/water @ ambient temperature, 45-60 psi-g
Class IV
0.01% of rated capacity
Clean air/water @ ambient temperature, 45-60 psi-g
Class V
(Soft seat)
0.0005ml per minute, per inch of seat diameter, per PSI differential
Clean water at ambient temperature, maximum service pressure, above 100 psi-g
Class VI
(soft seat)
Variable, eg. 3 bubbles/min for 2" valve, 6 bubbles/min for 3" valve. Refer to ANSI/CI 70-2 for complete values.
Air/nitrogen at ambient temperature, 50 psi-g
bubble-tight
(soft seat)
* See Note Below
* See Note Below
* Isolation Valves: Isolation valves are generally marketed as bubble tight. It is widely accepted that "bubble-tight" refers to a valve that, when fully closed, does not pass any bubbles when one side of the valve seat is pressurized with air/nitrogen, and the other side of the seat is submerged in water. Actual test procedures can vary from manufacturer to manufacturer. There is no formal standard to define bubble-tight.

slurryflo control valves with class IV shut-off

The patented SlurryFlo control valve design incorporates a metal-to-metal seating surface, which provides with Class IV shut-off, even after substantial wear. Class IV shut-off isolates 99.99% of the flow, however SlurryFlo valves typically exceed this value during factory hydro testing.
SlurryFlo Control Valves have a Class IV Leakage Rate and isolate 99.99% of flow.
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