PRESSURE SAFETY VALVE
Protec�on from extra generated pressure in the system.
PRESSURE SAFETY VALVE & PRESSURE RELIEF VALVE
1. PRESSURE SAFETY VALVE (PSV):
When system pressure exceeds the set pressure, the safety valve opens suddenly.
� Used for compressible fluids
Example: Gases
2. PRESSURE RELIEF VALVE (PRV):
When system pressure exceeds the set pressure, the relief valve opens gradually.
� Used for non-compressible fluids
Example: Water, Oil
STEP | PRESSURE CONDITION | VALVE STATUS
STEP
PRESSURE CONDITION
VALVE STATUS
1
< Set Pressure
Closed
2
= Set Pressure
Starts opening
3
> Set Pressure (Overpressure)
Fully open
4
Pressure reducing
S�ll open
5
= Reseat Pressure
Closes
TERM | MEANING
TERM
MEANING
Set Pressure
Pressure at which valve starts to open
Overpressure
% pressure above set point during discharge
Relieving Pressure
Set pressure + overpressure
Blowdown
Pressure difference between opening and closing
Back Pressure
Pressure on outlet side (can affect opera�on)
Reseat Pressure
Pressure at which valve closes a�er opening
APPLICATION | ALLOWABLE OVERPRESSURE
APPLICATION
ALLOWABLE OVERPRESSURE
Normal case
10%
Mul�ple PSVs
16%
Fire case (API 521)
21%
� Non-Fire Cases
Installa�on Type
Max Set Pressure
Max Accumulated Pressure
Single Valve
100%
110%
Mul�ple Valves
100% / 105%
116%
� Fire Case
Installa�on Type
Max Set Pressure
Max Accumulated Pressure
Single Valve
100%
121%
Mul�ple Valves
100% / 105% / 110%
121%
� Minimum Blowdown – 3% Rule
As per API 520 and ASME Sec�on VIII guidelines:
•
Minimum blowdown is typically 2–3% of the set pressure.
Example (3% Blowdown):
•
Set Pressure = 100 bar
•
Blowdown = 3%
•
Valve will reseat at:
→ 100 - (3% of 100) = 97 bar
� Why You Can’t Have 0% Blowdown:
•
The valve would close too soon, system pressure may not be low enough
•
It can cause rapid open-close cycling → damage to seat, spring, and vibra�ons