PetroStreet

What are the on-line PSV calibration techniques?
What are the safety aspects we need to take care of while doing Online PSV calibration?
Can I online calibrate all type of PSVs or not? If not what are the limitations?

Imran,
These are the in-service / online techniques to verify set pressure of a safety device.
1. Testing with system pressure - raising steam pressure to the set pressure, get PSV popped and verify.
2. Testing with pressure from other sources - For POP valves mainly, an accessory is used where raising the system pressure is not desirable.
3. Testing with Auxiliary Lift / assist devices - Instead of raising the steam pressure, a special equipment is used which hydraulically lifts the valve and then a balancing force is applied externally. That balanced force is then interpreted to evaluate the set pressure of the PSV.

Safety Aspects
- Safety aspects of (1) are obviously high as the whole equipment / system is over-pressurized and other PSVs are gagged. But this is the basic test to be used for ASME Power Boilers conforming to ASME Sec. 1.
- (3) is now preferred due to lower risk especially to mechanical integrity of the whole equipment. ASME has accepted it as an alternate to (1) causing steam over-pressurization in boiler.

Limitations
- If you can raise the system pressure and do not have any problem then, no limitations in going for (1).
- (3) have certain limitations regarding the lifting equipment being used.

Go through ASME PTC-25 for more information.

Hope this would help.

Imran,
Do search for Trevitest method which falls under the category of testing with auxiliary lift and is being widely used across all the petrochemical plants. For older boilers, Trevitest is always preferred due to risk associated with system pressure testing.