Introduction to overpressure protection

1.

INTRODUCTION
TO
OVERPRESSURE PROTECTION

2.

Why Needed
• PROTECTION OF PERSONNEL AND
PROPERTY
• SATISFY REQUIREMENTS OF
APPLICABLE
CODES AND REGULATIONS

3.

Most Common Causes
Of Overpressure
• BLOCKED DISCHARGE
• EXPOSURE TO EXTERNAL
FLAMES
• THERMAL EXPANSION

4.

Blocked Discharge
PRD
Pressure
Vessel
FULL
FULL INPUT
INPUT
FLOW
(FROM
(FROM
COMPRESSOR
COMPRESSOR
OR
OR PUMP)
PUMP)
OUTLET BLOCK
VALVE CLOSED

5.

External Fire
PRD
STORAGE OR
PROCESS VESSEL

6.

Thermal Expansion
PRD
LIQUID FULL PIPE OR
PRESSURE VESSEL

7.

Must Consider The One
Worst Case
BLOCKED
DISCHARGE
EXTERNAL FIRE
THERMAL
EXPANSION

8.

Overpressure Relief Devices
• PRESSURE RELIEF
VALVE
• RUPTURE DISC
• RUPTURE DISC UNDER
PRV

9.

Pressure Relief Valve
(PRV)
• OPENS AT SET PRESSURE
• CLOSES AT RESEAT PRESSURE

10.

Rupture Disc
(RD)
• A PREDETERMINED WEAK POINT
IN A PRESSURIZED SYSTEM
• OPENS AT BURST PRESSURE
• DOES NOT RE-CLOSE

11.

Pressure Relief Valve &
Rupture Disc
• RD BURSTS OPEN
• PRV OPENS IMMEDIATELY
• PRV RE-CLOSES AT A
PRESSURE BELOW RD BURST

12.

Maximum Allowable
Working Pressure (MAWP)
• “DESIGN PRESSURE”
• NORMALLY USED WITH
PROCESS
VESSELS AND ASSOCIATED
PIPING
• NORMALLY PRV SET PRESSURE
OR
RD BURST PRESSURE

13.

Maximum Operating
Working Pressure (MOWP)
• “DESIGN PRESSURE”
• NORMALLY USED IN NATURAL
GAS TRANSMISSION AND
DISTRIBUTION
SYSTEMS
• DEPENDENT ON PIPE
MATERIAL , TEMPERATURE OF
AMBIENT & GAS,
LOCATION OF PIPE (SPARSELY
OR

14.

PRESSURE %
WHERE PRDS
NORMALLY SIZED
(EXCEPT FIRE CASE)
110
SET PRESSURE
TOLERANCE
ALLOWABLE
ALLOWABLE
OVERPRESSURE
OVERPRESSURE
100
BLOWDOWN,
% OF SET
TYPICAL
RESEAT
PRESSURE
95
90
MAWP
MAWP
TYPICAL SET
PRESSURE
TYPICAL
OPERATING
PRESSURE
(“THE MONEY
MAKER”)

15.

% MAWP
USUALLY 110%
MAWP
ALLOWABLE
ACCUMULATION
(SYSTEM)
ALLOWABLE
OVERPRESSURE
(PRD)
100
SET PRESSURE
MAWP

16.

% MAWP
USUALLY 110%
MAWP
ALLOWABLE
ACCUMULATION
(SYSTEM)
ALLOWABLE
OVERPRESSURE
(PRD)
100
MAWP
SET PRESSURE
(ALMOST NEVER SHOULD IT BE <
MAWP )

17.

Back Pressure
Pb (back pressure)
Inlet
THE PRESSURE AT THE OUTLET
OF A PRESSURE RELIEF DEVICE.

18.

Built-up Back Pressure
DISCHARGE
PRV IS OPEN
AND IS FLOWING
In

19.

Possible Pressure Source
Possible Pressure Source
TO FLARE
AND/OR
BURNER
STACK
TO ATMOS.
CLOSED
DISCHARGE
HEADER SYSTEM
PRV
(Closed)
Possible Pressure Source
Purge?
SUPERIMPOSED BACK
PRESSURE
Protected System

20.

Lift
LIFT

21.

Orifice Area
AN = AREA OF DN
(NOZZLE DIAMETER),
OR
AC = CIRCUM. OF DN X
LIFT,
[“CURTAIN
AREA”]
DN
WHICHEVER IS LARGER
LIFT

22.

Safety Valve
• A PRV ACTUATED BY STATIC
PRESSURE
• RAPID OPENING (POP ACTION)
FULL LIFT
LIFT
SET
ALLOWABLE
PRESSURE OVERPRESSURE
• USED ON COMPRESSIBLE MEDIA
(GAS, VAPOR, STEAM)

23.

Relief Valve
• A PRV ACTUATED BY STATIC PRESSURE
• OPENS GENERALLY IN PROPORTION
TO OVERPRESSURE
FULL LIFT
LIFT
SET
PRESSURE
ALLOWABLE
OVERPRESSURE
• NORMALLY USED ON NONCOMPRESSIBLE MEDIA (LIQUIDS)

24.

Safety Relief Valve
(SRV)
• A PRV THAT MAY FUNCTION AS
EITHER A SAFETY OR RELIEF
VALVE
WITH SATISFACTORY
PERFORMANCE

25.

Pressure Relief Valve
(PRV)
A GENERIC TERM WHICH INCLUDES
SAFETY VALVES, RELIEF VALVES,
AND
SAFETY RELIEF VALVES (A “SAFE
& ALL-ENCOMPASSING TERM”)

26.

The Ideal PRD
PRV
Protected System
LEAK-FREE TO SET PRESSURE
OPENS AT SET PRESSURE
RELIEVES ONLY THE INPUT INTO
THE PROTECTED SYSTEM
STABLE DURING RELIEF CYCLE
RE-CLOSES AT SET PRESSURE
(0% BLOWDOWN)