ERT 312SAFETY & LOSS PREVENTION IN

BIOPROCESS

INTRODUCTION TO RELIEF

Prepared by:Miss Hairul Nazirah Abdul Halim

Objectives

• Define concept of relief

• Describe and Discuss the relief locations, types, scenarios.

• Discuss the relief system in specific unit operations.

Let’s Think…

• Assume that an exothermic reaction is occurring within a reactor. If cooling is lost because of a loss of cooling water supply, failure of a valve, or other scenario, then the reactor temperature will rise.

• As the temperature rises, the reaction rate increases, leading to an increase in heat production. This self-accelerating mechanism results in a runaway reaction.

• The pressure within the reactor increases because of increased vapor pressure of the liquid components and/or gaseous decomposition products resulting from the high temperature.

• Reaction runaways for large commercial reactors can occur in minutes, with temperature and pressure increases of several hundred degrees per minute and several hundred psi per minute, respectively.

• What happen if the reactor has no relief system?

Figure 8-2 Pressure versus time for runaway reactions: (A) relieving vapor, (B) relieving froth (two-phase flow), and (C) closed reaction vessel.

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.

Location of Reliefs

• Pressure relief devices are installed at every point identified as potentially hazardous (potential problems that may result in increased pressures)

• Guidelines for specifying relief positions:

1. All vessels – reactors, 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 coil

4. Vessel steam jackets

Tutorial 1

Specify the location of reliefs in the simple polymerization reactor 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 240°F, (3) adding monomer for a period of 3 hr, and(4) stripping the residual monomer by means of a vacuum

using valve V-15.

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

Figure 8-5 Polymerization reactor without Safety reliefs.

Figure 8-6 Polymerization reactor with safety reliefs.

• 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, in

general, every process vessel needs a relief. This relief is labeled PSV-1 for pressure safety valve 1.b. Positive displacement pump (P-1): Positive displacement pumps are overloaded, overheated, and damaged if they are dead-headed without a pressure-relieving device (PSV-2). This type of relief discharge is usually recycled back to the feed vessel.c. Heat exchanger (E-1): Heat exchanger tubes can rupture from

excessive pressures when water is blocked in (V-b and V-il are closed) and the exchanger is heated (by steam, for example). 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) and the coil is heated with steam or even the sun. Add PSV-5 to

this coil.

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 valves

b) 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.

• 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 toxic chemicals (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.

• 3 subcategory type s of spring-loaded pressure reliefs:

• 1. The relief valve is primarily for liquid service. - The relief valve (liquid only) begins to open at the set

pressure. - This valve reaches full capacity when the pressure

reaches 25% overpressure. - The valve closes as the pressure returns to the set

pressure.

• 2. The safety valve is for gas service. - Safety valves pop open when the pressure exceeds the

set pressure.

• 3. The safety relief valve is used for liquid and gas service. - Safety relief valves function as relief valves for liquids and as safety valves for gases.

Example 8.2Specify the types of relief devices needed for the polymerization reactor in Example 8-1 (see Figure 8-6).

SolutionEach relief is reviewed in relation to the relief system and the

properties of the relieved fluids:

a. PSV-la is a rupture disc to protect PSV-lb from the reactive monomers (plugging from polymerization).

b. PSV-lb is a safety relief valve because a runaway reaction will give two-phase flow, both liquid and vapor.

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

d. PSV-3 is a relief valve because it is for liquid only. A conventional relief device is satisfactory in this service.

e. PSV-4 is a safety relief valve because 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 valve for 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.

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.

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 the hazards 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).

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