In process application, it is important that control valve completely shutoff flowing fluid when placed in a closed position, this is a fundament requirement of the valve.
But in many case valve does not use for only shutoff purpose instead of for on/off purpose. Many control valves pass their lives in a partially open or partially close state.
Some of the valve design like two-port valve never has that ability to shutoff the flowing fluid, However, for some application tight shutoff is a mandatory requirement.
For this type of condition of the valve, we have some classification of the control valve. This class six type is useful in determine leakage using the bubble test method.
Let's have a look into the type of class and allowable leakage drop :
(1). CLASS 1
- Maximum allowable leakage rate: No.
- Pressure drop: No.
(2). CLASS 2
- Maximum allowable leakage rate: 0.5 % of rated flow capacity (Air or Water)
- Pressure drop: 45-60 PSI or maximum operating pressure.
(3). CLASS 3
- Maximum allowable leakage rate: 0.1 % of rated flow capacity
- Pressure drop: As per class 2.
(4). CLASS 4
- Maximum allowable leakage rate: 0.01 % of rated flow capacity
- Pressure drop: As per class 2
(5). CLASS 5
- Maximum allowable leakage rate: 0.0005 ml/min per inch orifice size per PSI.
- Pressure drop: Maximum operating.
(6). CLASS 6
- Maximum allowable leakage rate: Bubble test (Air or Nitrogen).
- Pressure drop: 50 PSI or Maximum operating.
BUBBLE TEST :
Bubble test is used for class 6 seat leakage is based on leakage rate of the air or nitrogen gas pass the closed valve seat.
Seat leakage is measured by counting the rate of the gas or air bubble escaping a bubble tube submerged underwater.
For 6" of the valve maximum bubble rate is 27 bubble per minute.
Do the setup as shown in the below figure to check valve seat leakage.
From this method of seat leakage, a term bubble-tight shutoff was originated.
To achieve tight shutoff we have to use soft seat material like Teflon instead of hard seat metal to metal contact.
To perform this test limited operating temperature range is required.