1. Pressure Vessel Design Safety
SURYAKANT RANDERI
Ex – GM, Marico Group
Hindustan Polyamide & Fibers Ltd.

2. Introduction
The Chemical and metallurgical industries are rapidly expanding both in size and diversity of process.
As a result through attention is given towards safety of the plant design, specially pressure vessels due to its intrinsic safety requirements due to its high pressure operations. It is not possible for all safety members to get exposure for P.V. design with respect to safety aspects, at all execution steps . They normally do regular checking at plant operation and periodic testing. My sincere effort is to give you exposure on the subject matter based on my personnel & professional experience in this field, specially on Loss prevention aspects in process plant design.

3. Contents : DESIGN CRITERIA SELECTION OF DESIGN CODE
SELECTION OF MATERIAL
DESIGN OF PRESSURE VESSEL
FABRICATION & INSPECTION
TEST & TRIAL RUN
OVER PRESSURE PROTECTION‐SAFETY INSTALLATION
OPERATION & RUNNING
REGULAR MAINTENANCE
PRIODIC MAINTENANCE

4. 1. DESIGN CRITERIA‐ CHECK LIST
The use of Pressure Vessels ( Unfired) is governed by
"THE STATIC & MOBILE PRESSURE VESSELS ( UNFIRED) RULES 1981
The Statutory requirements for the safety pertaining to factory Law, Pollution Board,
Enforced by Central and State Governments for the safety of Pressure vessel and
operation decides applicable Code, standards and recommended practice.
If content of pressure vessel is explosive in nature, we will have to follow Explosive Act.
Design includes drawing, calculations,specifications,model code used for the design, and
all other details necessary for the complete description of the P.V. and its construction

5. 1. DESIGN CRITERIA‐ CHECK LIST
Major points to be checked are:
1)Hazard due to failure of material of construction by corrosion or any unusual condition.
It decides material of construction and corrosion allowance to be considered in design
2) Possibility of runaway, strongly exothermic reactions decides High Pressure trip system. It protects Pressure Vessel against over pressure and Vacuum
3) Disposal of gas and/or Liquid during reaction , pressure relieving valves, device
4) Gas Explosion hazards, Installation of rupture disc etc.
5) Blow down and depressing system is to be taken care at upper and lower limit alarms
6) Any design constrain due to availability of space decides Vessel Position
7)Process requirements such as mixing intensity, product discharge from reactor by gravity or pump, vessel diameter to Height/Length ratio etc.
8) Toxicity and flammable properties of material handled, Electrical Area classifications
9) Reliability of available utilities at site and its quality standards suggest
additional Heat Transfer Area requirements at the time of designing pressure vessel

6. 2. SELECTION OF DESIGN CODE & STANDARDS
The design & manufacture of P.V. is always governed by applicable design codes in all countries.
The codes or rules are primarily intended to assure safety in operation and they cover every aspect of the design criteria with the safety of pressure vessel :
1) Selection of fabrication material & its testing
2) Approval of welder and fabrication work shop
3) Fabrication methodology for the set up, welding and fabrication
4) Testing and quality control through design conventions
They form a basis of agreement between the manufacturer and customer , and the customer’s insurance company.
Information and guidance on the P.V. codes can be found on the Internet :
Computer programs to aid in the design of vessels to PD 5500 and the ASME code are available from commercial organizations and can be found on Internet.
Use of program "PVEllite" is most popular in our country that uses ASME code .

7. 2. SELECTION OF DESIGN CODE & STANDARDS
The codes used in our country for the UFPV ( Unfired Pressure Vessels) are:

8. 2. SELECTION OF DESIGN CODE & STANDARDS
While basic aim of the code is same, there are differences in approach arising out of historical back ground, Experience, Raw material availability in the country and the design philosophy adopted in the various countries of origin The choice of code is made by the purchaser and his process Licensor , guided by their experience on the similar plant and strongly influenced by the country in which this experience has been gained.
The difference in codes from the point of design, material selection, welding and fabrication method is dealt with in depth knowledge of design engineering practice. In the United Kingdom all conventional pressure vessels for use in the chemical and allied industries will invariably be designed and fabricated according to the British Standard PD 5500 or the European Standard EN 13445; or an equivalent code such as the American Society of Mechanical Engineers code Section VIII (the ASME code).
Where national codes are not available, the British, European or American codes would be used.

9. 3. SELECTION OF MATERIAL
The choice of material is made considering the various factors like :
1) Environmental under which it is intended to give service , Process Requirements
2) The mechanical strength desired
3) The expected life of the equipment
4) Workability and weldability of the metal selected.
The mechanical strength and process requirements plays the greater part while
selecting optimum design criteria
Material stress value is given here under for the majority of material available and in use for the selection and Fabrication in India :

10. Material Properties: { ENTER TEMPERATURE }
TO Select Material Properties

11. 3. SELECTION OF MATERIAL
CALCULATE DESIGN PRESSURE
Fluid Properties:
Fluid Specific Gravity (g/cm3)
Water, 1.000
Ethanol, 0.789
Alcohol,
Ammonia, .662
Beer, 1.01
Bromine, 2.900
Butane, 0.594
Crude Oil,
Gasoline,
Glucose,
Kerosene,
Milk,
Sulphuric Acid, 1.814

12. 4. Design Pressure Vessels ( Brief):
Based on the selection criteria and check list given earlier :
Select Metric or British system you want to follow for the design & use units uniformly over design
2) Select Design Temperature : It is a process requirements
3) Select Design Pressure
A. Design pressure is 10% or 0.69 to 1.7 bar (10 to 25 psi) above the maximum operating
pressure, whichever is greater. The maximum operating pressure is taken as 1.7 bar (25 psi) above the normal operation pressure.
B. For vacuum operations, design pressures are 1 bar(g) (15 psig) to full vacuum
C. Minimum thicknesses for maintaining vessel/tank structure is to be selected as per code
D. ASME Code:P-design [psi] = P+0.433*SG*H ~~ design pressure including static head
Where P = Operating pressure, SG = Sp. Gr. & H = Liquid Height inside vessel
E. For safe Design Practice use 1.1 times operating pressure(min)

13. 4. Design Pressure Vessels ( Brief):
4) Select Corrosion Allowance
The “corrosion allowance” is the additional thickness of metal added to allow for material
lost by corrosion and erosion, or scaling. The allowance to be used should be agreed between the customer and manufacturer.
Corrosion is a complex phenomenon, and it is not possible to give specific rules for the
estimation of the corrosion allowance required for all circumstances.
The allowance should be based on experience with the service conditions to those for the proposed design.
Guidelines for corrosion allowances are as follows:
1) For carbon and low‐alloy steels, where severe corrosion is not expected,
a minimum allowance of 2.0 mm should be used
2) where more severe conditions are anticipated this should be increased to 4.0 mm.
3) Most design codes and standards specify a minimum allowance of 1.0 mm
4) Select Based on the Service Life predicted by process engineer.

14. 4. Design Pressure Vessels ( Brief):
5) Select Material Properties : To be selected based on Design Temp. & Press
6) Select Design code
The national codes and standards divide vessel construction into different categories, depending on the amount of non‐destructive testing required. The higher categories require 100 per cent radiography of the welds, and allow the use of highest values for the weld joint factors. The lower‐quality categories require less radiography, but allow only lower joint‐efficiency factors, and place restrictions on the plate thickness and type of materials that can be used. The highest category will invariably be specified for process‐plant pressure vessels.
The standard specifies three construction categories:
Category 1: the highest class, requires 100 per cent non‐destructive testing (NDT) of the welds; and allows the use of all materials covered by the standard, with no restriction on the plate thickness.
Category 2: requires less non‐destructive testing but places some limitations on the materials which can be used and the maximum plate thickness.

15. 4. Design Pressure Vessels ( Brief):
Category 3: The lowest class, requires only visual inspection of the welds, but is restricted to carbon and carbon‐manganese steels, and austenitic stainless steel; and limits are placed on the plate thickness and the nominal design stress. For carbon and carbon manganese steels the plate thickness is restricted to less than 13 mm and the design stress is about
half that allowed for categories 1 and 2. For stainless steel the thickness is restricted to less than 25 mm and the allowable design stress is around 80 per cent of that for the other categories.
7) Select Test pressure
Test pressure = Sf*{ Pd*( Fa/Fn) x t/(t‐c) }
Where Pd = design pressure, N/mm2,
fa = design stress at the test temperature, N/mm2,
fn = design stress at the design temperature, N/mm2,
c = corrosion allowance, mm,
t = actual plate thickness, mm.
Sf = Safety factor a) ASME Sec VIII DiV‐1: 1.5 a) ASME Sec VIII DiV‐2: 1.25
2) BS ) IS ) BS1500, Test Pressure is 1.5 times working pressure

16. 4. Design Pressure Vessels ( Brief):
8) Select vessel to be Vertical or Horizontal and ratio of Diameter to height
It is a process and lay out requirements
9) Welded joint efficiency
The value of the joint factor used in design will depend on the type of joint and amount of radiography required by the design code.
Typical values are shown in Table
Taking the factor as 1.0 implies that the joint is equally as strong as the virgin plate; It is achieved by radiographing the complete weld length, and cutting out/remaking any defects. The use of lower joint factors in design, though saving costs on radiography, It will result in a thicker, heavier, vessel, and the designer must balance any cost savings on
inspection and fabrication against the increased cost of materials.

17. 4. Design Pressure Vessels ( Brief):
10) Select Applicable Design Loads :
Design loads
A Vessel must be designed to resist gross plastic deformation and collapse under all
the conditions of loading. The loads to which a process vessel will be subject in service
are listed below. They can be classified as major loads, that must always be considered in
vessel design, and subsidiary loads. Formal stress analysis to determine the effect of the
subsidiary loads is only required in the codes and standards where it is not possible to
Double welded butt or equivalent Single‐weld butt joint with bonding strips
Type of Joint Degree of radiography Maximum Allowable Joint Efficiency demonstrate the adequacy of the proposed design by other means; such as by comparison
with the known behavior of existing
Major loads
1. Maximum weight of the vessel and contents, under operating conditions.
2. Maximum weight of the vessel and contents under the hydraulic test conditions

18. 4. Design Pressure Vessels ( Brief):
3. Dynamic Wind loads ( For Tall Vessels):
For a smooth cylindrical column or stack the following semi‐empirical equation can be
used to estimate the wind pressure:
Pw = 0.05 *u²
where Pw = wind pressure, N/m2,
u = wind speed, km/h.
Note: A wind speed of 160 km/h (100 mph) can be used for preliminary design studies;
equivalent to a wind pressure of 1280 N/m2 (25 lb/ft2).

19. 4. Design Pressure Vessels ( Brief):
4. Earthquake (seismic) loads.
The movement of the earths surface during an earthquake produces horizontal shear
forces on tall self]supported vessels, the magnitude of which increases from the base
upward. The total shear force on the vessel will be given by:
Fs = a * (W/g)
where "a" is the acceleration of the vessel due to the earthquake,
g = the acceleration due to gravity,
W = total weight of the vessel.
The term (a/g) is called the seismic constant Ce, and is a function of the natural period of
vibration of the vessel and the severity of the earthquake. Values of the seismic constant
have been determined empirically from studies of the damage caused by earthquakes,
& are available for those geographical locations which are subject to earthquake activity.

20. 4. Design Pressure Vessels ( Brief):
5.Subsidiary loads
1. Local stresses caused by supports, internal structures and connecting pipes.
2. Shock loads caused by water hammer, or by surging of the vessel contents.
3. Bending moments caused by eccentricity of the center of the working pressure
relative to the neutral axis of the vessel.
4. Stresses due to temperature differences and differences in the coefficient expansion of materials.
5. Loads caused by fluctuations in temperature and pressure.
A vessel will not be subject to all these loads simultaneously. The designer must
determine what combination of possible loads gives the worst situation, and design for that loading condition.

21. 4. Design Pressure Vessels ( Brief):
11) THE DESIGN OF VESSELS UNDER INTERNAL PRESSURE
A Design Cylindrical Shell under internal Pressure
The Cylindrical shell thickness based on pressure and radius is given by:
Minimum practical wall thickness There will be a minimum wall thickness required to ensure that any vessel is sufficiently rigid to withstand its own weight, and any incidental loads. As a general guide the wall thickness of any vessel should not be less than the values given below; the values include a corrosion allowance of 2 mm:

22. 4. Design Pressure Vessels ( Brief):
Comparison of Design Formula for the different codes applicable in India
Let
T or t = Shell thickness
P = Design Pressure
f = Design Stress
E or j or = Weld joint Efficiency, Normally one for Dish heads
do or Do = Outer diameter of shell
di or Di = Internal diameter of shell
R or Ri = Internal radius of shell
C = Corrosion Allowance
L = inside spherical or crown radius of Dish .

23. 4. Design Pressure Vessels ( Brief):
B Design of Head & Closure under internal Pressure
The ends of a cylindrical vessel are closed by heads of various shapes. The principal
types used are:
Flat plates and formed flat heads
Sizes 14 to 300 inches in diameter. From 12 gauge to 1‐1/4 inches thick.
Non‐ASME I.D. or O.D.
Values for the design constant Cp and the nominal plate diameter are given in the
design codes and standards for various arrangements of flat end closures.
The minimum thickness required is given by:t = D* Cp*(Pi/f)^0.5 + Corr
where Cp is a design constant, dependent on the edge constraint,

24. 4. Design Pressure Vessels ( Brief):
2. Hemispherical heads
Hemispherical Heads. When the thickness of a hemispherical head does not exceed L, or P does not exceed 0.665SE, the following formulas shall apply:
Treq [in] = or
P= 2SEt/L+0.2t
L [in] = (Do-2*t)/2 ~~ inside radius with corrosion allowance removed
Volume = 2*pi()*(Do/2‐tf) 3/3 in F³

25. 4. Design Pressure Vessels ( Brief):
3. Ellipsoidal heads
With ts/L ≥ 0.002
Sizes 6‐5/8 to 192” in diameter. From 3/16 to 2” thick
Tolerances comply with ASME EG 32 (d). I.D. or O.D

26. 4. Design Pressure Vessels ( Brief):
4. Torispherical heads
Standard and intermediate sizes 14 to 250 inches. From 3/16 to 1‐3/8 inches thick.
Tolerances comply with ASME requirements. ASME UG‐32 (e) I.D. or O.D.

27. 4. Design Pressure Vessels ( Brief):
5. Conical Heads
From 24 to 240 inches in diameter. From 3/16 to 1‐1/8 inches thick. .
Tolerances comply with ASME requirements. ASME UG‐32 (h). I.D. or O.D
ASME code Requirements:
The required thickness of conical heads or conical
shell sections that have a half apex‐angle α not greater than 30 deg shall be determined by
t = PD/2 cos α (SE − 0.6P) or
P = 2SEt cosα/D+1.2cosα
t = P *Dc/(2fj*P) x 1/cos α
where Dc is the diameter of the cone at the point,
α = half the cone apex angle.

28. 4. Design Pressure Vessels ( Brief):
Choice of closure
1) Flat plates are used as covers for man ways, and as the channel covers of heat exchangers
Flat heads are the cheapest type of formed head to manufacture, but their use is limited to
low‐pressure and small‐diameter vessels.
2) Standard Torispherical heads (dished ends) are the most commonly used end closure for vessels up to operating pressures of 15 bar. They can be used for higher pressures, but above 10 bar their cost should be compared with that of an equivalent ellipsoidal head.
3) Ellipsoidal Vessel head are used extensively Above 15 bar pressure .
Ellipsoidal head will usually prove to be the most economical closure to use.
4) A hemispherical head is the strongest shape; capable of resisting about twice the
pressure of a Torispherical head of the same thickness. The cost of forming a hemispherical
head will, however, be higher than that for a shallow Torispherical head. Hemispherical
heads are used for high pressures.
5) Conical Heads are seldom used for the design of pressure vessel

29. 4. Design Pressure Vessels ( Brief):
12) THE DESIGN OF VESSEL & DISH UNDER EXTERNAL PRESSURE
The required minimum thickness of a cylindrical shell or tube under external
pressure, shall be determined by the following procedure:
Cylinders having Do /t values ≥ 10:
Step 1. Assume a value for thickness "t" and determine the ratios L/Do and Do /t
Take L = Length of the cylindrical portion + 1/3 rd dish height on both side
Step 2. Use Material chart for the external pressure and find out Value of Factor "A"
using value of L/Do( Note: If L/Do greater than 50, enter the chart at a value of line L/Do=" 50"& For values of L/Do less than 0.05, enter the chart at a value of line L/Do= "0.05“
Step 3. Using value of "A" and Do/t find the new value of Factor "B“
Step 4a. Using this value of Factor B, calculate the value of the maximum allowable external working pressure Pa using the following formula:
Pa= ( 4*B)/3*(Do/t)

30. 4. Design Pressure Vessels ( Brief):
Step 4b. For values of A falling to the left of the applicable material /temperature line,
the value of Pa can be calculated using the following formula:
Allowable External pressure Pa =( 2*A*j) /(3*(Do/t)
Step 5. Compare the calculated value of Pa obtained in Steps 4a or 4b with P.
If Pa is smaller than P, select a larger value for t and repeat the design procedure until
a value of Pa is obtained that is equal to or greater than P

31. 4. Design Pressure Vessels ( Brief):
13) Calculation of reinforcement required for the Openings
The “equal area method” is the simplest method used for calculating the amount of
reinforcement required, and is allowed in most design codes and standards. The principle
used is to provide reinforcement local to the opening, equal in cross‐sectional area to the
area removed in forming the opening, Figure below,if the actual thickness of the vessel
14) Note:
1) For other Value of L/Do refer code
2) Use stiffening ring if the calculated thickness of the dish is > t(for internal Press.)+2
3) The thickness for the dish end are calculated in the similar manner using above method and graphs published by code

32. 5. FABRICATION
Follow following specifications and guide lines given in code for the fabrication of vessel :
Welder qualifications welding procedure by Production Test coupons :
The welder is qualified by the competent authority by welding on test pieces using
code specified welding electrodes . The code defines type of electrode, position used
during the welding within well defined specified limits . The welder qualified for the
butt welding joint is qualified for the plate welding but pipe welding requires
separate approval in certain codes.
TEST COUPENS: The production of test coupons are required if the operating temperature
is below ‐20 º F ( - 30 º C ) and only impact testing is required to be cared out
Some code like BS1500 require production coupons for long seam with radiography
2) Cutting Plates and Other Stock
(a) Plates, edges of heads, and other parts may be cut to shape and size by mechanical means such as machining, shearing, grinding, or by oxygen or arc cutting. After oxygen or arc cutting, all slag and detrimental discoloration of material which has been molten shall be removed by mechanical means prior to further fabrication or use.
(b) Ends of nozzles or manhole necks which are to remain unwelded in the completed vessel may be cut by shearing provided sufficient additional material is removed by any other method that produces a smooth finish.
(c) Exposed inside edges shall be chamfered or rounded.

33. 5. FABRICATION 3) Material Identification
The pressure vessel Manufacturer shall maintain traceability of the material to the original
identification markings by one or more of the following methods:
Accurate transfer of the original identification markings to a location where the markings
will be visible on the completed vessel; identification by a coded marking traceable to
the original required marking; or recording the required markings using methods such as
material tabulations or as built sketches which assure identification of each piece of material during fabrication and subsequent identification in the completed vessel
Such transfers of markings shall be made prior to cutting except that the Manufacturer may
transfer markings immediately after cutting provided the control of these transfers is
described in his written Quality Control System
4) Repair of Defects in Materials
Defects in material may be repaired provided acceptance by the Inspector is first obtained
for the method and extent of repairs. Defective material that cannot be satisfactorily
repaired shall be rejected.
5) Forming Shell Sections and Heads
The inner surface of the formed head shall not deviate from the specified shape by more
than 1.25% of ID. Such deviation should not be abrupt and shall be out side of the shape
6) Permissible Out‐of‐Roundness of Cylindrical, Conical, and Spherical shell

34. a) Circumference based on outer diameter of vessel
5. FABRICATION
7) Tolerance for Formed Heads : The limit for the tolerances are well defined for
a) Circumference based on outer diameter of vessel
b) Ovality of vessel under internal pressure
c )Ovality of the vessel under external pressure
d) Alignment of the vessel
8) Lugs and Fitting Attachments
9) Holes for Screw Stays
10) Charpy Impact Tests
11) Heat Treatment : The requirement of heat treatment varies from code to code :
The general criteria is set by d) when vessel is to be used for the Lethal substances
a) Type of material used for the fabrication
b) Plate Material thickness ( say > 38 mm for Carbon steel)
c) Elongation of material fiber more than the specified limit ( 5 to 50 % )
d) Temperature of material exceed specified limit during the fabrication (say 800 º F)
e) If found brittleness or ageing during the fabrication
INSPECTION
1) General 2) The definition of Inspector 3) Access for Inspector 4) Inspection of Materials
5) Marking on Materials 6) Examination of Surfaces During Fabrication
7) Dimensional Check of Component Parts

35. 6. TEST & TRIAL RUN
The test is to be done under presence and guidance of a competent person approved
as a "Inspector" or by a professional organization approved by the factory inspector
PRESSURE TESTS
The pressure vessel codes and standards require that all pressure vessels be subjected to a pressure test to prove the integrity of the finished vessel.
1) A Standard hydraulic test
Hydraulic test is very common and generally carried out as a first choice. Hydraulic tests
are safer because only a small amount of energy is stored in the compressed liquid.
The vessel is tested at a pressure above the design pressure, typically 25 to 30 % higher. The test pressure is adjusted to allow for the difference in strength of the vessel material at the test temperature compared with the design temperature, and for any C.A.
Formulae for determining H.T. Pressure are given in the codes and Std. such as :
Test pressure = 1.3*{ Pd*( Fa/Fn) x t/(t‐c) }
Where
Pd = design pressure, N/mm2,
fa = nominal design strength (design stress) at the test temperature, N/mm2,
fn = nominal design strength (design stress) at the design temperature, N/mm2,
c = corrosion allowance, mm,
t = actual plate thickness, mm.

36. 6. TEST & TRIAL RUN
2) Halogen Leak Inspection of pressurized gas system
After doing Hydraulic testing of the vessel, the Halogen Freon gas is introduced
in the vessel. The vessel is pressurized further by introduction of inert gas or dry air
to approximately 3 Kg/cm2 pressure or a design pressure which ever is less.
The vessel should remain under pressure for a maximum of 0ne and half hour
The presence of leak may be detected with either a diode electrical detector or halide torch
3) Pneumatic Test
A pneumatic test can be substituted where the use of a liquid for testing is not practical
when water is not permitted due to its immediate use and can not be readily dried
The pneumatic test pressure shall not be greater than the design pressure but normally
less than the Hydraulic pressure
Method:
The pressure in the vessel shall be gradually increased to not more than one‐half
of the test pressure. Thereafter, the test pressure shall be increased in steps
of approximately one‐tenth of the test pressure until the required test pressure
has been reached. Then pressure shall be reduce to a value equal to test pressure
divided by 1.1 and held for the sufficient time to permit inspection of the vessel
The metal temperature during pneumatic test shall be maintained at least 30°F (17°C)
above the minimum design metal temperature to minimize the risk of brittle
fracture.

37. 6. TEST & TRIAL RUN 4) Acoustic ‐ Emission Testing
Vessels that are subjected to very high pressure say > 50 bar pressure, are subjected to this test .
During the hydraulic testing of the vessel , a pizo‐electric sensors are installed on the vessel metal plat before taking any pressure inside the vessel.
The stresses developed during the pressure test, causes discontinuities to emit high frequency sound that can be picked up by the sensor.
Sound waves received by the sensor are fed as a electric signal into the analyzer system. The system of testing is capable of indicating over pressure and immediate catastrophic failure. The result from the test can be displayed on a CRT screen and print report
5) Maximum Allowable Working Pressure
The maximum allowable internal or external working pressure for a vessel is the maximum
pressure permissible at the top of the vessel in its normal operating position at the
designated temperature. It includes Static pressure due to liquid inside the vessel
Please refer Section 4.Design for the calculation for P.Max‐ allowable pressure as per code

38. 6. TEST & TRIAL RUN 6) Non‐destructive Testing .
a) Magnetic Particle examination
b) Liquid or dye penetrant examination
c ) Radiographic
PERSONNEL SHOULD BE COMPETANT
(a) He/she has vision, with correction if necessary, to enable him/her to read a
Jaeger Type No. 2 g yp Standard Chart ( Available with Ophthalmologist)at a distance of
not less than 12 in., and is capable of distinguishing and differentiating contrast
between colours used. These requirements shall be checked annually.
(b) He/she is competent in the techniques of the magnetic particle examination
method for which he/she is certified, including making the examination and
interpreting and evaluating the results, except that where the examination method
consist of more than one operation, he/she may be certified as being qualified only for one or more of these operations.

39. 6. TEST & TRIAL RUN
ACCEPTANCE STANDARDS FOR MAGNETIC TEST
All surfaces to be examined shall be free of:
(a) relevant linear indications;
(b) relevant rounded indications greater than 3⁄16 in.(5 mm);
(c) four or more relevant rounded indications in a line separated by 1⁄16 in. (1.5 mm) or less, edge to edge.
ACCEPTANCE STANDARDS FOR LIQUID PENETRANT TEST
(c) four or more relevant rounded indications in a line

40. 7. OVER PRESSURE PROTECTION
Safety by Pressure Relief Devices
A) General
All pressure vessels, irrespective of size or pressure, shall be provided with over pressure
protection in accordance with the requirements as per the code and system design
In addition following shall apply:
(1) It is the user’s responsibility to identify all potential over pressure
and the method of overpressure protection used to mitigate over pressure
(2) It is the responsibility of the user to ensure that the required over pressure protection system is properly installed prior to initial operation.
(3) It is the responsibility of the user or his/her designated agent to size and select
the pressure relief device(s) based on its intended service.
The Intended service considerations shall include the following:
(a) normal operating and upset conditions
(b) fluids
(c) fluid phases
(4) The overpressure protection system need not be supplied by the vessel Manufacturer
(B) SET PRESSURE LIMIT
(a) When a pressure relief device is provided, it shall prevent the pressure from rising more than 10% or 3 psi (20 kPa), ( whichever is greater) above the M.A.W.P.

41. 7. OVER PRESSURE PROTECTION
b) When multiple pressure relief devices are provided they shall prevent the pressure from rising more than 16% or 4 psi ( whichever) is greater above Max Allowable Press
(c) When a pressure vessel can be exposed to fire or other unexpected sources of external heat, the pressure relief device shall be capable of preventing the pressure rise more than 21% above Max Allowable pressure
(C) LOCATION
Pressure relief devices shall be constructed, located, Installed such that they are readily
accessible for testing, inspection, replacement & repair and so that they do not
become inoperative
D) Type of Pressure Relief Valves
All Safety relief valves shall be of the direct spring loaded type.
a) Pressure relief valve
It is a pressure relief device which is designed to re‐close and prevent the further flow of fluid after normal conditions have been restored
b) Non‐reclosing pressure relief device
It is a pressure relief device designed to remain open after operation.
c) Safety valve
It is a pressure relief valve actuated by inlet static pressure and characterized
by rapid opening or pop action ( Example‐Boiler)

42. 7. OVER PRESSURE PROTECTION
d) Relief valve
It is a pressure relief valve actuated by inlet static pressure which opens in proportion to the increase in pressure over the opening pressure.
e) Pilot operated pressure relief valve
It is a pressure relief valve in which the major relieving device that
combined with and controlled by a self‐actuated auxiliary press relief valve
D. Safety Valve Tolerance :
The set pressure tolerances, plus or minus, of pressure relief valves shall not exceed
2 psi (15 kPa) for pressures less than 70 psi (500 kPa) and 3% for pressures above 70 psi (500 kPa)

43. 7. OVER PRESSURE PROTECTION
E. No reclosing Pressure Relief Devices
(a) Rupture Disk Devices
A rupture disk is the pressure sensitive activation device and component that breaks/open up at over pressure
SHAPE : Rupture disks may be designed in several configurations such as plain flat,
prebulged, or reverse buckling.
HOLDER A rupture disk holder is the structure that encloses and clamps the
rupture disk in position.( Flange End, Screw etc.)
TOLERANCE : The burst pressure tolerance at the specified disk temperature shall not exceed ± 2 psi (±15 kPa) for marked burst pressure less than 40 Psi
(300 kPa) and ± 5% for marked burst pressure above 40 psi (300 kPa)

44. 7. OVER PRESSURE PROTECTION
INSTALLATION/Location :
When the rupture disk device discharges directly to the atmosphere then
(a) It is installed within eight pipe diameters from the vessel nozzle entry
(b) Length of discharge pipe not greater than five pipe diameters from the
rupture disk device; and
(c) the nominal diameters of the inlet and discharge piping are equal to or greater
than the stamped value on the device
d) The calculated relieving capacity of a pressure relief system shall not
exceed a value based on the applicable theoretical flow equation
at coefficient of discharge K equal to 0.62
(e) A rupture disk device may be used as the sole pressure relieving device on a vessel.
(f) A rupture disk device may be installed between a pressure relief valve & the vessel
Provided
* the combination of the pressure relief valve and the rupture disk device has enough
capacity to meet the following requirement:
*The marked capacity of a pressure relief valve (nozzle type) shall be 0.9 times rated relieving capacity of the valve alone when installed with a rupture disk between
the inlet of the valve and the vessel
SET PRESSURE/Burst Pressure :
Not more than 105% of the allowable working pressure in any case

45. 7. OVER PRESSURE PROTECTION
b) Pin Devices ( Obsolete and not in use now a days )
(a) A pin device may be used as the sole pressure relieving device on a vessel
* It can be installed between the vessel and pressure relief valve
In a manner same as rupture disc and capacity with all its limitation
Overpressure Protection by System Design ( Using Instrumentation)
We all are aware that the design, installation of the over protection system is a solely
responsibility of the pressure vessel user, they can either design the pressure vessel at higher working capacity or adopt pressure relieving device by proper selection and installation of instruments over pressure protection system. It ma be in combination of both devices
I would like to recommend a High‐Pressure _Trip Systems for the pressure vessel protection when run away reaction occurs .
The oxidation , Hydrogenation , polymerisation etc are few examples
It will be difficult to install relief valves that are large in size and to be open quickly enough to avoid over pressuring the equipment.

46. 7. OVER PRESSURE PROTECTION
THE TRIP CAN ISOLATE THE SUPPLY OF ONE OF THE REACTANT OR HEATING SYSTEM AND OPEN UP COOLING CYCLE TO MINIMISE OVER HEATING DUE TO REACTION FAILURE
It is to be noted here that Fire Relief vent protection is to be kept as it is when replacing the conventional pressure relieving system, by trip system,
BASIC COMPONENTS OF A TRIP SYSTEM
The basic minimum requirements are :
a)Design, Select & Install A High ( rising) pressure sensor & Pressure switch on the overhead Gas/Vapour /Vent line to operate and activate a relay contactor
b) The relay contactor should stop the power supply line connected with the heating device either by hot water pump or Hot oil pump
c) Select and install a solenoid valve associated with trip valve installed on the
steam/Hot water/Hot oil supply line use to heat Pressure Vessel or Reactor