Overpressure, Protection & Relief System

Question 1

Pressure Relief Device

Answer 1

Actuated by inlet static pressure and designed to open during
emergency or abnormal conditions to prevent a rise of internal fluid
pressure in excess of a specified design value.

Question 2

Examples of Pressure Relief Device

Answer 2

• pressure relief valve
• non-reclosing pressure relief device
• vacuum relief valve

Question 3

Safety Valve

Answer 3

An automatic pressure relieving device
actuated by the static pressure upstream of the valve.
Characterized by rapid full opening “pop action”. It is
used for steam, gas or vapor service.

Question 4

Relief Valve

Answer 4

An automatic pressure relieving device
actuated by the static pressure upstream of the valve,
which opens in proportion to the increase in pressure
over the opening pressure. Typically it is used for
incompressible fluid service.

Question 5

Safety Relief Valve

Answer 5

An automatic pressure actuating
relieving device, suitable for use as either a safety or relief
valve, dependent on application. Majority of pressure
relief valve designs are safety relief valves

Question 6

Design Pressure (DP)

Answer 6

the
gauge pressure at the top of the equipment in its operating position that is
used as the basis to determine the minimum thickness of equipment parts
at the DT (Design Temperature).

Question 7

Maximum Allowable Working Pressure (MAWP)

Answer 7

s the maximum gauge pressure
permissible at the top of the equipment in installed operating position and
at a designated temperature

Question 8

Operating Pressure (OP)

Answer 8

The OP is the gauge pressure which prevails inside equipment and piping
during any intended operation. The OP is determined by the process
engineer.

Question 9

Set Pressure (SP)

Answer 9

SP is the inlet pressure in bar (g) at which a pressure relief valve is set
to open under service conditions.

Question 10

Back-pressure (BP)

Answer 10

BP is the pressure at the outlet of a pressure relief device as a result of the
pressure in the system downstream of the pressure relief device.

Question 10

Built-up Back Pressure (BBP

Answer 10

The increase in pressure in the discharge header that develops as a result
of flow after the pressure relief device or devices open.

Question 11

Superimposed Back-pressure (SBP)

Answer 11

the static pressure against the outlet of a safety/relief device at
the time the device is required to operate. SBP in the discharge system is
the result of pressure caused by releases from other sources

Question 11

Phases of Relief Study

Answer 11

– Individual relief
– Global relief

Question 12

Steps in Study Relief

Answer 12

• Step 1 : Identify overpressure scenarios
• Step 2 : Calculate required relief rates
• Step 3 : Size all pressure relief devices
• Step 4 : Calculate all Global relief case
• Step 5 : Analyze the KO Drums, Flare and Radiations

Question 13

Documents Required

Answer 13

• Obtain latest and updated PFD and P&ID
• Heat and Material Balance (HMB) at study case
• Simulation model for the unit
• Vendor Rotating equipment data (performance curves,
  etc.)
• Pressure Relief Devices, vessel and heat exchanger data
  (detail drawings)
• Operating/Emergency procedures
• Review current projects and existing PHA’s (as support to
  scenario identifications).

Question 14

API 521

Scenario to be considered for overpressure
scenario

Answer 14

1. Closed Outlet (block valve or control valve)
2. Inadvertent valve opening / source of high pressure
3. Check valve malfunction
4. Utility Failure – Total / Partial
5. Equipment Failure / Loss of cooling
6. Loss of heat in series fractionating – Light material carry over
   to next equipment
7. Overheating – Reboiling / HE Failure
8. External Fires
9. Reaction
10. Thermal expansion
11. Tube rupture

Question 15

Rupture disks (also known as “bursting disks”) shall be considered:

Answer 15

1) to accomplish a fast response time, which cannot be achieved with a PRV.
This could be required to cope with a sudden gas breakthrough due to a heat exchanger tube burst or malfunctioning of a level control valve into a liquid-filled system;
2) to prevent PRVs in vacuum service from drawing gas or air back into the
process;
3) to protect PRVs from being in continuous contact with a corrosive, solidifying, or polymerizing process fluid;
4) to protect PRVs from accumulation of solids and dusts;
5) to prevent leakage of very toxic substances through the PRV.

Question 16

Global Relief Case

Answer 16

A single event that can result in multiple relief devices relieving
simultaneously to a common relief flare header.

Question 17

Typical Global Scenarios:

Answer 17

– Total power failure
– Partial power failure
– Cooling failure
– Instrument air failure
– Steam failure
– Fire

Question 18

Purpose

Flare Knockout Drum

Answer 18

To separate liquid from vapor
so as to ensure that no liquid enters the flare tip

Question 19

Adequacy of drum

Answer 19

(1) liquid separation
(2) liquid retention time

Question 20

Overpressure Scenario Identification

Answer 20

1. Start with a system sketch (for existing , system from P&IDs and PFDs).
2. Analyze on equipment-by-equipment basis, or Pressure Relief Device(PRD)-by-PRD basis.
3. Typically start at front end of process.
4. Identify maximum pressure of all potential feeds.
5. Identify any heat input including external fire.
6. No credit for favorable instrument or operator response.
7. Document in checklist format.
8. Include comments to explain rationale.