Introduction to Relief

8/11/2019 Introduction to Relief

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ERT 312

SAFETY & LOSS PREVENTION IN

BIOPROCESS

INTRODUCTION TO RELIEF

Prepared by:

Miss Hairul Nazirah Abdul Halim


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Objectives

Define concept of relief Describe and Discuss the relief locations, types, scenarios.

Discuss the relief system in specific unit operations.


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Lets Think

Assume that an exothermic reactionis occurring within areactor. If cooling is lost because of a loss of cooling watersupply, failure of a valve, or other scenario, then the reactortemperature will rise.

As the temperature rises, the reaction rate increases, leadingto an increase in heat production. This self-accelerating

mechanism results in a runaway reaction. Thepressure within the reactor increasesbecause of

increased vapor pressure of the liquid components and/orgaseous decomposition products resulting from the hightemperature.

Reaction runaways for large commercial reactors can occurin minutes, with temperature and pressure increases ofseveral hundred degrees per minute and several hundred psi

per minute, respectively.

What happen if the reactor has no relief system?


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Figure 8-2 Pressure versus time for runaway reactions:

(A) relieving vapor, (B) relieving froth (two-phase

flow), and (C) closed reaction vessel.


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Relief Concept

Pressure relief systems are required for the following reasons:

to protect personnel from the dangers of overpressurizing

equipment,

to minimize chemical losses during pressure upsets, to prevent damage to equipment,

to prevent damage to adjoining property,

to reduce insurance premiums, and

to comply with governmental regulations.


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Location of Reliefs

Pressure relief devices are installed at every point identified aspotentially hazardous (potential problems that may result in

increased pressures)

Guidelines for specifying relief positions:

1. All vesselsreactors, storage tanks, towers and drums.2. Positive displacement pumps, compressors and turbines need

reliefs on the discharged side.

3. Blocked-in section of cool liquid-filled lines that exposed to

heat such as heat exchanger & cooling coil4. Vessel steam jackets


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Tutorial 1

Specify the location of reliefs in the simple polymerizationreactor system illustrated in Figure 8-5. The major steps in this

polymerization process include:

(1) pumping 100 lb of initiator into reactor R-1,

(2) heating to the reaction temperature of 240F,

(3) adding monomer for a period of 3 hr, and

(4) stripping the residual monomer by means of a vacuumusing valve V-15.

Because the reaction is exothermic, cooling during monomeraddition with cooling water is necessary.


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Figure 8-5 Polymerization reactor without Safety reliefs.


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Figure 8-6 Polymerization reactor with safety reliefs.


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Refer to Figures 8-5 and 8-6 and Table 8-1 for relief locations.

a. Reactor (R-1):A relief is installed on this reactor because, ingeneral, every process vessel needs a relief. This relief islabeled PSV-1 for pressure safety valve 1.

b. Positive displacement pump (P-1):Positive displacementpumps are overloaded, overheated, and damaged if they aredead-headed without a pressure-relieving device (PSV-2).This type of relief discharge is usually recycled back to thefeed vessel.

c. Heat exchanger (E-1):Heat exchanger tubes can rupture fromexcessive pressures when water is blocked in (V-b andV-il are closed) and the exchanger is heated (by steam, forexample). This hazard is eliminated by adding PSV-3.

d. Drum (D-1):Again, all process vessels need relief valves,

PSV-4.e. Reactor coil:This reactor coil can be pressure-ruptured when

water is blocked in (V-4, V-5,V-6, and V-7 are closed) andthe coil is heated with steam or even the sun. Add PSV-5 to

this coil.


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Relief Types

Specific types of relief devices are chosen for specific applications:- for liquid, gases, liquid & gases, solid and corrosive materials

Vented to atmosphere or vented to containment system

2 categories of relief devices:

a) spring-operated valvesb) rupture discs

2 types of spring-operated valves:

a) conventional

b) balanced-bellows

Rupture discs are specially designed to rupture at a specified relief

set pressure.


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Rupture discs are frequently installed in series to a spring-loaded

relief

(1) to protect an expensive spring-loaded device from a corrosive

environment,

(2) to give absolute isolation when handling extremely toxicchemicals (spring-loaded reliefs may weep),

(3) to give absolute isolation when handling flammable gases,

(4) to protect the relatively complex parts of a spring- loaded

device from reactive monomers that could cause plugging,(5) to relieve slurries that may plug spring-loaded devices.


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3 subcategory type s of spring-loaded pressure reliefs:

1. The relief valveis primarily for liquidservice.

- The relief valve (liquid only) begins to open at the setpressure.

- This valve reaches full capacity when the pressurereaches 25% overpressure.

- The valve closes as the pressure returns to the set

pressure.

2. The safety valveis for gasservice.

- Safety valves pop open when the pressure exceeds the setpressure.

3. The safety relief valveis used for liquid and gasservice.- Safety relief valves function as relief valves for

liquids and as safety valves for gases.


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Example 8.2

Specify the types of relief devices needed for the polymerizationreactor in Example 8-1 (see Figure 8-6).

Solution

Each relief is reviewed in relation to the relief system and theproperties of the relieved fluids:

a. PSV-la is a rupture discto protect PSV-lb from the reactivemonomers (plugging from polymerization).

b. PSV-lb is a safety relief valvebecause a runaway reaction willgive two-phase flow, both liquid and vapor.

c. PSV-2 is a relief valvebecause this relief is in a liquid service line.A conventional valve is satisfactory.


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d. PSV-3 is a relief valvebecause it is for liquid only. A

conventional relief device is satisfactory in this service.

e. PSV-4 is a safety relief valvebecause liquid or vapor service is

possible. Because this vent will go to a scrubber with possibly

large backpressures, a balanced bellows is specified.

f. PSV-5 is a relief valvefor liquid service only. This relief

provides protection for the following scenario: The liquid is

blocked in by closing all valves; the heat of reaction increases

the temperature of the surrounding reactor fluid; and pressures

are increased inside the coil be cause of thermal expansion.


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Relief Scenarios A relief scenario is a description of one specific relief event.

Usually each relief has more than one relief event, and the worst-

case scenario is the scenario or event that requires the largest

relief vent area.


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Relief Systems

A relief system rarely vented to the atmosphere.

A relief is discharged to a knockout system to separate the liquid

from the vapor.

The liquid is collected

The vapor is discharged to another treatment unit depends on thehazards of the vapor

The vapor treatment unit: condenser, scrubber, incinerator, flare

or combination of them.

This system is called a total containment system (Fig 8-12).


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Figure 8-12 Relief containment system with blowdown drum.The blowdown drum separates the vapor from the

liquid.

Documents

Introduction to Relief