6 replies to this topic

[janson_kaniaz]

Does anyone know the reason for 3% pressure drop criteria limit?

[fallah]

Does anyone know the reason for 3% pressure drop criteria limit?

In brief,providing stability for PSV opening (by preventing chatter occurance).

[sheiko]

Does anyone know the reason for 3% pressure drop criteria limit?

The inlet line losses (non-recoverable pressure drop) are kept at 3% which is about half of the difference between blowdown and set pressure.
Typically most (not all) of the blowdown is 90-94% which is 6-10% below set pressure.
With low values of blowdown, the valve could chatter (or open and close at rapid intervals).

[star@25]

Based on API RP 520, The pressure drop above 5 % will create chattering in the PSV. Hence it should be limited to 5 % based on the API standard. But in the normal practice it shall be limited to 3 % for safer side to avoid chattering in the PSV.

[Vimalesh Agnihotri]

Yes. Dela P in inlet line should be below 3% to avoid PSV chattering & PSV seating damage.

[chemsac2]

Refer to discussion in following link:

http://www.cheresour...ultiple-valves/

It has to do with blowdown limit. You want valve to fully open within overpressure limit. To achieve this, huddling chamber is provided in relief valve construction. This huddling chamber helps in meeting overpressure limit, but on flip side valve now does not close at set pressure during closing, but at a lower pressure called blowdown.

Typically blowdown is 5-7% of set pressure. If set pressure is 100 barg, blowdown is typically 93 barg. Thus, during relief event, valve will start opening at 100 barg. Will be fully open at 110 barg. During closing, valve does not reseat at 100 barg, but at 93 barg. Due to tolerance, blowdown may be anywhere between +/-3% i.e. between 90 to 96 barg.

During initial nonflowing condition, vessel pressure and PSV inlet pressure is same. As vessel pressure approaches set pressure, PSV lifts and flow starts.

If pressure drop in inlet line is more than 3%, vessel may see 100 barg pressure, but due to line loss, PSV inlet may see 96-97 barg. This is blowdown value and hence valve may close. But relief scenario is still active and again vessel pressure increases. This leads valve to reopen as due to no flow, pressure in vessel and at PSV inlet is same.

This cycle of rapidly opening and closing of PSV is called chattering and may lead to valve damage.

Hope it helps.

Regards,

Sachin

[fallah]

chemsac2,

Just to modify the definition of blowdown:

Typically blowdown is 5-7% of set pressure. If set pressure is 100 barg, blowdown is typically 93 barg.
Blowdown is the difference between set pressure and the closing pressure of the PSV,thus in your case blowdown would be 7 bar (or 7% of set pressure) and 93 barg is closing pressure.

Fallah