Exam 5

Question 1

What are the advantages and disadvantages of using a conventional spring-operated relief valve compared to a balanced bellows relief valve?

Answer 1

SPRING-OPERATED
ADVANTAGES: reliable, versatile, reseats 4% below set pressure.
DISADVANTAGES: affected by back pressure; can chatter with high backpressures.
BALANCED BELLOWS
ADVANTAGES: relief pressure not affected by back pressures, handles higher buildup back pressures, protects spring from corrosion
DISADVANTAGES: bellows may fatigue/rupture, flow is a function of backpressure, may release flammables/ toxics to the atmosphere

Question 2

maximum allowable accumulated pressure (MAAP)

Answer 2

sum of MAWP and allowable accumulation

Question 3

Explain and give an example of the difference between revealed and unrevealed failures. Explain the importance of inspection interval on unrevealed failures.

Answer 3

no.. snake in my boot

Question 4

Explain why a pressure gauge is often put between a rupture disk and a relief valve where the rupture disk is exposed to process conditions.

Answer 4

pressure gauge allows you to determine whether the rupture disk has ruptured, exposing process directly to the relief valve. If pressure gauge shows an increase in pressure, the disk has ruptured and needs to be replaced.

Question 5

Describe important aspects of conventional spring operated valves, balanced bellows valves, pilot operated valves, buckling pin valves, and rupture disk.

Answer 5

SPRING-OPERATED: opens based on pressure drop across the valve seat, reusable, back pressure can prevent relief at set pressure.
BALANCED BELLOW: bellows on the backside of the valve seat ensures that the pressure on that side of the seat is always atmospheric. back pressure can still decrease flow rate, but valve will always open at the desired set pressure. more expensive than spring-operated.
PILOT-OPERATED: more complex and expensive, used for accuracy. when pilot valve reaches set pressure, it opens and releases the pressures above the main valve. Large valve piston then opens and exhausts the system fluid.
BUCKLING PIN: rupture pin set to pressure, pin buckles at precise pressure. single-use valve, internals not exposed, full open where other valves can have whisping.
RUPTURE DISK: single-use, low cost, premature rupture can occur due to flexing, can be installed upside down. once open always open.

Question 6

What are the main considerations which would typically indicate a rupture disk should be used as opposed to a spring-operated valve only?

Answer 6

rupture disks are specially designed to rupture at a specified set pressure;
are available in much larger sizes than spring-operated relief systems;
typically cost less than equivalently-sized spring-operated relief valves;
typically used in corrosive service;
can provide absolute isolation when handling extremely toxic materials or flammable gasses, where spring-operated reliefs may weep;
used to relieve slurries that may plug spring-loaded devices

Question 7

What is the difference between sterilization, disinfection, decontamination, and sanitization?

Answer 7

STERILIZATION: destroys all viable, or living, forms of
microorganisms.
DISINFECTION: destroys microorganisms in their vegetative state. spores may not be killed by disinfection
DECONTAMINATION: reduces number of disease-producing microorganisms and renders an object safe for handling
SANITIZATION: reduces number of microorganisms on inanimate objects to a level considered safe by public health standards

Question 8

accumulation

Answer 8

pressure increase over the MAWP of a vessel during the relief process. Expressed as a % of the MAWP

Question 9

Name five guidelines which are used to identify the need for a relief device.

Answer 9

all vessels need reliefs;
blocked-in sections of cool liquid-filled lines that are exposed to heat or refrigeration need reliefs;
positive displacement pumps, compressors, turbines need reliefs on discharge side;
storage vessels need pressure and vacuum reliefs to protect against pumping in or out of blocked-in vessel or against the generation of a vacuum by condensation;
vessel steam jackets are often rated for low pressure steam. Install reliefs in jackets to prevent excessive stream pressures due to operator error or regulator failures

Question 10

Give a brief overview of what is involved in a HAZOP study.

Answer 10

Hazard and Operability study: multi-person, multidisciplinary; PFD & P&ID, process specialties, identify streams, identify items and scenarios, apply guide words and move through PFD, leading indicators, write/followup action items, document HAZOP

Question 11

Explain what is meant by the “bathtub” failure rate.

Answer 11

failure rate that is high at startup, high at shutdown/end of life, but low/constant during operation. example: human lifespan.

Question 12

In the Dow F&EI, name three of the most important areas which act to prevent loss in the area of process control (excluding explosion control on dusts).

Answer 12

• operating instructions/procedures
• reactive chemical review
• other process hazard analysis

Question 13

which government agency supports CAMEO Chemicals? What does CAMEO Chemicals do?

Answer 13

NOAA: national oceanic atmospheric administration; database containing thousands of SDSs for chemicals that are transported, used, and store in the US.

Question 14

operating pressure

Answer 14

gauge pressure during normal service, usually 10% below the MAWP

Question 15

Set pressure

Answer 15

pressure at which relief device actives

Question 16

Explain the process involved in classifying operations in a risk matrix.

Answer 16

x

Question 17

overpressure

Answer 17

pressure increase in the vessel over the set pressure during the relieving process. Expressed as % of set pressure.

Question 18

Why is the potential formation of aerosols important for biohazards?

Answer 18

can remain suspended and can spread through ventilation systems; causes vast majority of infections; can be generated by many actions

Question 19

What distinguishes each of the four Risk Group (RG) classifications?

Answer 19

RG1: no or low individual/community threat
RG2: moderate individual, but low community threat
RG3: high individual but low community threat
RG4: high individual and community threat

Question 20

back pressure

Answer 20

pressure at the outlet of the relief device during relief process from pressure in the discharge system

Question 21

blowdown

Answer 21

pressure difference between relief set pressure and the relief reseating pressure. expressed as % of set pressure

Question 22

What is a BSC? What is the difference between a Class I and Class II BSC?

Answer 22

biological safety cabinet.
Class 1: draws room air from the front and exhausts through a HEPA filter; no product protection; potential for cross contamination
Class 2 (includes A1,A2,B1,B2): product protection; supply air is HEPA-filtered (sterile), but airflow can also be recirculated; can be used for Risk Groups 1,2,3.

Question 23

What is LOPA? What is PFD as related to LOPA? What is typically assumed to be true for each LOPA layer?

Answer 23

Layer of Protection Analysis; analyzes probability of failures of independent protection layers (IPLs) in the event of a scenario previously studied in a quantitative hazard evaluation like HAZOP;
PFD = probability of failure on demand

Question 24

What is common mode failure? Give an example of when a common mode failure could be encountered in a chemical processing facility?

Answer 24

domino effect of process issues after one failure;
single-point failure that affects several pieces of equipment simultaneously;
power outage leads to loss of cooling system, leads to process upset/relief

Question 25

What distinguishes a hybrid or gassy system when considering pressure relief systems?

Answer 25

gassy: non-tempered
hybrid: either tempered or non-tempered
gassy and hybrid: both produce non-condensable vapors

Question 26

What is the difference between a Risk Group and Biosafety Level (BSL) classification?

Answer 26

Agents are classified into four Risk Groups (RGs) according to their relative pathogenicity for healthy adult humans;
a BSL is a set of biocontainment precautions required to isolate dangerous biological agents in an enclosed laboratory facility

Question 27

Give an example of when a checklist was critically important from The New Yorker article by Gawande.

Answer 27

procedures don’t rely on memory, specify minimum steps in procedures, example: Pronovost was in ICU and put in a checklist for putting in patient lines–> dramatic decrease in line infection rate from 11% to 0%

Question 28

what is the difference between a tempered and non-tempered system when considering pressure relief system?

Answer 28

in a tempered system, vaporization removes energy via heat of vaporization and therefore tempers the rate of temperature rise resulting from an exothermic reaction.
non-tempered system is not cooled by vapor evolution.

Question 29

MAWP

Answer 29

maximum allowable working pressure; max gauge pressure permissible at the top of a vessel for a designated temperature.