INTRODUCTION INTRODUCTION API 510 STUDY MATERIAL
API 510 STUDY MATERIAL HOW TO USE THESE BOOKS HOW TO USE THESE BOOKS
These books can be used in a self-study or instructor led format. There are two volumes, the Text and the Questions and Answers.
TEXT BOOK TEXT BOOK
The Text book's table of contents follows the API 510 Body of Knowledge that was in effect at the time of its writing. Each area can be studied as a stand alone module for those who do not intend to sit for the API 510 exam, but want to obtain a better understanding on a given Code subject.
The process found to most effective for general use is to study each subject of interest and complete the quizzes at the end of that module. As regards to calculations, after mastering the given material, refer to the Advanced Material section to increase the depth of understanding. The Advanced Material covers the calculations required for some actual circumstances that might be encountered in the field.
For those intending to sit for the API 510 examination, some helpful suggestions are contained in the back of the Text book. These include such things as what paragraphs to tab within the ASME Code books, and cross over subjects from the API to the ASME Codes. At this writing the exam candidate is allowed to use the ASME Code books and the API books on the first portion of the test only. No reference material is allowed for the second half of the test!
QUESTIONS AND ANSWERS BOOK QUESTIONS AND ANSWERS BOOK The Questions and Answers are divided into
The Questions and Answers are divided into two types.two types.
The first portion covers the ASME Codes, Sections VIII Div. 1 Unfired Pressure Vessels, Section IX Welding, and Section V Nondestructive Testing. These questions are typical of previous National Board Authorized Inspector exams. These should be used to obtain a feel
for the nature of the ASME Code questions. They are not for memorization.
The second portion contains questions from the API 510 Code and the Recommended Practices, titled RPI 572 Inspection of Pressure Vessels, RPI 576 Pressure Relieving Devices and Chapter II -Conditions Causing Deterioration of Failures. These questions are for memorization if the examination will be taken!
API 510 Module
Table of Contents Table of Contents API CODES
API 510 Corrosion Rates and
API 510 Corrosion Rates and Inspection IntervalsInspection Intervals
Scope 6
Inspection Interval 10
Records and Test 11
Metal loss including corrosion averaging 15
Corrosion rates 15
Remaining Corrosion Allowance 16
Remaining Service Life 16
API 576 Pressure Relieving Devices API 576 Pressure Relieving Devices
Scope 19
Types of pressure relieving devices 19
Reasons for Inspection 22
Causes of Improper Performance 23
Frequency and Time of Inspection 23
API 572 Inspection of
ASME Section VIII Div. 1 ASME Section VIII Div. 1
Joint Efficiencies Joint Efficiencies
UW-3 Weld Categories 48
UW-51 RT Examination of Welded Joints 58
UW-52 Spot Examination of Welded Joints 59
UW- 11 RT and UT Examinations 61
UW-12 Maximum Allowable Joint Efficiencies 69
Postweld Heat Treatment Postweld Heat Treatment
UW-40 Procedures for Postweld Heat Treatment 93
UCS-56 Requirements for Postweld Heat Treatment 94
Vessels Under Internal Pressure Vessels Under Internal Pressure
UG-27 Thickness of Shells Under Internal Pressure 96
UG-32 Formulas and Rules for Using Formed Heads 107
UG-34 Unstayed Flat Heads and Covers (Circular) 113
Cylinder Under External Cylinder Under External PressurePressure
UG-28 Thickness of Shells and Tubes (External Pressure) 120 Pressure Testing
Pressure Testing
UG-20 Design Temperature 127
UG-22 Loadings 129
UG-25 Corrosion 130
UG-98 Maximum Allowable Working Pressure 131
UG-99 Hydrostatic Test Pressure and Procedure 132
UG-100 Pneumatic Test Pressure and Procedure 135
UG-102 Test Gages 138
Minimum Requirements for Attachment Welds at Openings Minimum Requirements for Attachment Welds at Openings
Reinforcement for Openings in Shells and Reinforcement for Openings in Shells and HeadsHeads
UG-36 Openings in Vessels 146
UG-37 Reinforcement of Openings 147
UG-40 Limits of Reinforcement 147
UG-41 Requirements for Strength of Reinforcement 147
UG-42 Reinforcement of Multiple Openings 148
Minimum Design Metal Temperature and Exemptions from Impact Testing Minimum Design Metal Temperature and Exemptions from Impact Testing
UG-84 Charpy Impact Test Requirements 161
UCS-66 Materials 164
UCS-67 Impact Testing of Welding Procedures 164
UCS-68 Design 164
Practical Knowledge Practical Knowledge
UG-77 Material Identification 170
UG-93 Inspection of Materials 171
UG- 116 Name Plate Markings 172
UG-119 Name Plates 174
UG- 120 Data Reports 175
Section IX Section IX
Welding on Pressure Vessels (Section IX
Welding on Pressure Vessels (Section IX Overview)Overview)
Article I General Requirements 176
Article II Welding Procedure Qualifications 177
Article III Welding Performance Qualifications 179
Section V (NDE Subsection A) Section V (NDE Subsection A)
Article 2 Radiography 195
Article 5 Ultrasonics 198
Article 6 Liquid Penetrant 199
Article 7 Magnetic Particle 201
Article 9 Visual Inspection 202
Advanced Material Example Problems Advanced Material Example Problems
Static Head of Water 204
Corrosion 217
Cylinders Under Internal Pressure 220
Heads Under Internal Pressure 222
Charpy Impact Test Evaluation WPS/PQR 226
Advanced Exercise Problems Advanced Exercise Problems
Internal Pressure Shell Calculations 228
Internal Pressure Head Calculations 229
Solutions for Advanced Exercises 230
Appendix Appendix
Helpful information for the API Exam Helpful information for the API Exam
Listing of where to find answers to API questions in Section VIII ASME 236
Instructions for the proper tabbing of ASME Code books 237
Practice WPS and PQR forms 240
API 510 Module
PRESSURE VESSEL INSPECTION CODE
Overview Section 1 Section 1 General General Scope: Scope:
The API 510 applies to pressure vessels in the petrochemical and refining industries after they have entered service. The ASME Code applies to the new construction of vessels. While it applies only to new construction it is often the Code to which a vessel is repaired. There are other construction Codes to which a vessel can be constructed, for instance the Department of Transportation (DOT) provides rules for the construction of and shipping of compressed gas cylinders. The Code for the construction of storage tanks is API 653 and so forth.
The API 510 exempts certain vessels such as: a. Vessels on moveable structures tank cars. etc..
b. All vessels exempted by Section VIII DIV. 1 of the ASME Code. c. Vessels that do not exceed given volumes and pressures. Section 6 Alternative Rules for Natural Resource Vessels. Glossary of Terms:
Glossary of Terms:
In this section the terms used in the API 510 Code are defined such as Alteration, ASME Code, API Authorized Inspector, Construction Code, Maximum Allowable Working Pressure, Maximum Allowable Shell Thickness and On-Stream Inspections just to mention a few. Study this section carefully as many questions on the Exam often come from here.
Section 2 Section 2
Owner-User Inspection Organization Owner-User Inspection Organization
The main thing of interest in this section is the qualifications required for an API 510 inspector. Here the experience and educational requirements are listed in detail. Questions
Section 3 Section 3 Inspection Practices Inspection Practices Preparatory Work: Preparatory Work:
Often questions are asked about what must be done before entry into a vessel. draining, cleaning, purging and gas testing also the warning of personnel in the area, both inside and outside the vessel, etc.. Checking of safety equipment is necessary as well as inspection tools.
Modes of Deterioration and Failure: Modes of Deterioration and Failure:
Some of the listed modes of deterioration are fatigue, creep, brittle fracture, general corrosion stress corrosion cracking, hydrogen attack, carburization, graphitization, and erosion. A general question may be asked such as; list six modes of deterioration or a more specific question such as; what is creep dependent upon.
Corrosion-Ra
Corrosion-Rate te Determination:Determination:
One important aspect of vessel maintenance and operation is the determination of how frequently a vessel needs to be inspected. This can be largely driven, by the rate at which a vessel is corroding. There are three methods recognized by API 510 for this determination. a. A corrosion rate may be calculated from data collected by the owner or user on vessel providing the same or similar service.
b. Corrosion rate may be estimated from published data or from the owner user's experience. c. After 1,000 hours of service using corrosion tabs or on-stream NDE measurements. If the estimated rates are in error they must be adjusted to determine the next inspection date. Maximum Allowable
Maximum Allowable Working Pressure Determination:Working Pressure Determination:
The continued use of a pressure vessel must be based on calculations using the current edition of the ASME Code or the edition the vessel was constructed to. A vessels MAWP may not be raised unless a full rerating has been performed in accordance with section 5.3. In corrosive service the wall thickness used in the calculations must be the actual thickness as determined by the inspection. but must not be thicker than srcinal thickness on the vessel's srcinal material test report or Manufacturer's Data Report minus twice the estimated corrosion loss before the next inspection.
Defect Inspection: Defect Inspection:
Careful visual examination is the most important and most universally accepted method of inspection. Other methods that may be used to supplement visual inspection are magnetic
Inspection of Parts: Inspection of Parts:
a. The surfaces of shells and heads should be checked for cracks, blistering, bulges, or other signs of deterioration. With particular attention paid to knuckle regions of heads and support attachments.
b. Inspect welded joints and their heat affected zones for cracks or other defects. Rivets in vessels shall be inspected for general corrosion, shank corrosion. If shank corrosion is suspected hammer testing or angle radiography can be used.
c. Examine sealing surfaces of manways, nozzles and other openings for distortion, cracks and other defects. Pay close attention to the welding used to make these attachments. Corrosion and Minimum Thickness Evaluation:
Corrosion and Minimum Thickness Evaluation:
Corrosion occurs in two ways, general (a fairly uniform wasting away of a surface area) or pitting(the surface may have isolated or numerous pits, or may have a washboard like appearance in severe cases). Uniform wasting may be difficult to detect visually and ultrasonic thickness measurements are normally done for that reason. A pit may be deeper than it appears and should be investigated thoroughly to determine its depth. The minimum actual thickness and maximum corrosion rate may be adjusted at any inspection for any part of a vessel. When there is a doubt about the extent of corrosion the following should be considered for adjusting the corrosion rates.
a. Nondestructive examination such as ultrasonics or radiography. If after these examinations considerable uncertainty still exists the drilling of test holes may be required.
b. If suitable openings exist readings may be taken through them.
c. The depth of corrosion can be gauged from uncorroded surfaces adjacent to the area of interest.
d. For an area of considerable size where circumferential stress governs the least thickness may along the most critical element of the area may be averaged over a length not exceeding the following:
1. For vessels with an inside diameter of 60 inches or less one half the vessel diameter or 20 inches whichever is less.
f. As an alternative to the above the thinning components may be evaluated using the rules of Section VIII Division 2 Appendix 4 of the ASME Code. If this approach is used consulting with an engineer experienced in pressure vessel design is required. g. When corrosion is located at a weld with a joint efficiency less than 1.0 and also in the
area adjacent to the weld special consideration must be given to the calculations for minimum thickness. Two sets of calculations must be performed to determine the maximum allowable working pressure; one for the weld using its joint efficiency and one for the remote area using E equals 1.0. For purposes of these calculations the surface at the weld includes one (1) inch on either side of the weld or twice the minimum thickness whichever is greater.
h. When measuring a ellipsoidal or torispherical head the governing thickness may be as follows:
1. The thickness of the knuckle region with the head rating calculated using the appropriate head formula.
2. The thickness of the central portion of the dished region, in which case the dished region may be considered a spherical segment whose allowable pressure is calculated using the Code formula for spherical shells.
The spherical segment of both ellipsoidal and torispherical heads shall be considered to be in an area located entirely in with a circle whose center coincides with the center of the head and whose diameter is equal to 80 percent of the shell diameter. The radius of the dish of torispherical heads is to be used as the radius of the spherical segment. The radius of the spherical segment of ellipsoidal heads shall be considered to be the equivalent spherical radius K1D, where D is the shell diameter (equal to the major axis) and KI is as given in Table 1.
Section 4 Section 4 Inspection and Testing or
Inspection and Testing or Pressure VesselsPressure Vessels and Pressure-Relieving Devices and Pressure-Relieving Devices General:
General:
Section 4 requires that pressure vessels be inspected at the time of installation unless a Manufacturer's Data Report is available. Further all pressure vessels must be inspected at frequencies provided in Section 4. These inspections way be internal or external and may require any number of nondestructive techniques.
The inspection may be made while the vessel is in operation as long as all the necessary information can be provided using that method.
b. Supports
c. Allowance for expansion d. General alignment e. Signs of leakage
Buried vessels shall be monitored to determine their surrounding environmental condition. The frequency of inspection must be based on corrosion rate information obtained on surrounding piping or vessels in similar service.
Vessels known to have a remaining life in excess of 10 years or have a very tight insulation systems against external corrosion do not need to have the insulation removed for inspection however, the insulation should be inspected for its condition at least every 5 years.
Inspection Intervals: Inspection Intervals:
The period between internal or on-stream inspections shall not exceed 10 years or one-half the estimated remaining corrosion-rate life whichever is less. In cases where the remaining safe operating life is estimated at less than 4 years the inspection may be the full remaining safe operating life up to a maximum of 2 years. Internal inspection is the preferred method On Stream may be substituted if all of the following are true.
When the corrosion rate is known to be less than 0.005 inch per year and the estimated remaining life is greater than 10 years internal inspection of the vessel is unnecessary as long as the vessel remains in the same service, complete external inspections are formed and all of the following are true:
The non-corrosive character of the contents have been proven over a five year period. Nothing serious is found during the externals. The operating temperature of the vessel does not exceed the lower temperature limits for the creep-rupture range of the vessel metal. The vessel cannot be subject to accidental exposure to corrosives. Size and configuration make internal inspection impossible. The vessel is not subject to cracking or hydrogen damage. The vessel is not plate-lined or strip-lined.
The remaining life calculation formula is given in Section 4 and will be demonstrated in a latter example problem along with the other formulas required for pressure vessels in accordance with API 510.
Pressure Test: Pressure Test:
Pressure
Pressure-Relieving -Relieving Devices:Devices:
One of the major concerns for pressure relief devices is their repair. Pressure relief devices must be repaired by qualified organizations having a fully documented written quality control system and repair training program for repair personnel. No hard and fast rule is given for the testing of relief devices the interval between tests is dependent on the service conditions of the device. There are minimum of 15 items that should be addressed in the written quality control documentation. Such as a Title page, Revision log, Contents Page, Statement of Authority, Organizational Chart, etc. . Previous Exams have required naming 6 of these 1 5 items.
Records: Records:
Pressure vessel owners and users must maintain permanent and progressive records on their pressure vessels. Items that should be included are Manufacturer's Data Reports, vessel identification numbers, RV information, results of inspection and any repairs or alterations performed.
Section 5 Section 5 Repairs, Alterations and Rerating of
Repairs, Alterations and Rerating of Pressure VesselsPressure Vessels General:
General:
Section 5 covers repairs and alterations to pressure vessels by welding and the requirements that must be met when performing such work. These repairs and alterations must be performed to the edition of the ASME Code that the vessel was built to.
Authorization: Authorization:
Prior to starting any repairs or alterations the approval of the API 510 Inspector and in some cases an engineer experienced in pressure vessels must be obtained. The API 510 Inspector may give approval to any routine repairs if the Inspector has satisfied himself that the repairs will not require pressure tests.
Approval: Approval:
The API Inspector must approve all repairs after inspection and after witnessing any required pressure tests.
Defect Repairs: Defect Repairs:
No crack may be repaired without prior approval of the API Inspector. If such repairs are required in a weld or plate they may be performed using a U- or V-shaped grove to the full depth and length of the crack. The U or V is then filled with weld metal. If the repair will be
Procedur
Procedure and e and Qualifications:Qualifications:
The repair organizations must use qualified welders and welding procedures in accordance with applicable- requirements of Section IX of the ASME Code.
Qualification Records.. Qualification Records..
Qualifications Records must be maintained for all welding operations and must be available for review by the API Inspector prior to all welding operations.
Heat
Heat Treatment-PreheTreatment-Preheating:ating:
Alterations and repairs can be performed on vessels that were srcinally postweld heat treated by using only preheating within specific limitations. Postweld heat treatment in these cases would not then be required. This alternative applies to only P-Nos. 1 and P-Nos. 3 materials of the ASME Code and should be used only after considering the srcinal intent of the postweld heat treatment. In some services the heat treatment was required due to the corrosive nature of the contents of the vessel. In such cases this type of procedure may not restore the metallurgical condition needed to combat corrosion. For this reason consulting with an engineer experienced with pressure vessels is required. Two techniques for these types of repairs or alterations are described in Section 5.2.3 and are very similar to those found in paragraph UCS-56 of Section VIII Division 1 of the ASME Code. The major differences are the minimum preheat temperature and the holding time and temperature after the completion of the welded repair or alteration. Details and applicability of these procedures will be discussed in detail during the coverage of paragraph UCS-56 of the
ASME Code.
Local Postweld Heat Treatment: Local Postweld Heat Treatment:
The API 510 Code permits postweld heat treatment to be applied locally, this means that the entire vessel circumference may not be required to be included in the heat treatment. Just as in the alternative to postweld heat treatment above consideration to applying this local treatment must be made with regards to service. It does not apply to all situations the following four steps must be applied prior to using this type of heat treatment.
a. The application must be reviewed by a qualified engineer.
Repairs to Stainless Steel Weld Overlay and Cladding: Repairs to Stainless Steel Weld Overlay and Cladding:
Prior to the repair or replacement of corroded or missing clad material a repair procedure must written. Some of the concerns that must be addressed are as follows; out gassing of the base metals, hardening of the base metal during repairs, preheating and interpass
temperatures and postweld heat treatment. Design:
Design:
The design of welded joints included in the API 510 are in compliance with those of the ASME Code. All butt joints shall be full penetration and must have complete fusion. Fillet weld patches may be allowed as temporary repairs and can be applied to the inside or outside of vessels but require special considerations. The jurisdiction where the vessel is operating may for instance prohibit their use. Patches to the overlay in vessels must have rounded corners; this is also true of flush (insert) patches.
Material: Material:
All materials for repairs must conform to the ASME Code. Carbon or alloy steels with a carbon content which exceeds 0.35 percent may not be used in welded construction.
Inspection: Inspection:
The acceptance of welded repairs or alterations should include NDE that is in agreement with the ASME Codes that apply. If the ASME Code methods are not possible or practical, alternative NDE may be used.
Testing: Testing:
After repairs a pressure test must be applied if the API Inspector believes one is needed. Normally pressure tests are required after an alteration. If jurisdictional approval is required and it has been obtained NDE may be substituted for a pressure test. If an alteration has been performed a pressure vessel engineer must be consulted prior to using NDE in place of pressure test.
Rerating: Rerating:
Rerating a pressure vessel by changing its temperature ratings or its maximum allowable working pressure may be done only after meeting the requirements of API 510 given in Section 5.3. Calculations, compliance to the current construction code, current inspection records indicating fitness, pressure testing at some time for the proposed rerating and approval by the API Inspector are required. The rerating is only complete when the Inspector has overseen the attachment of an additional nameplate with the required information given in Section 5.3.
API 510 Module
CORROSION RATES AND INSPECTION INTERVAL
Examples
Metal loss equals the previous thickness minus the present thickness. Problem #1
Problem #1
Determine the metal loss for a tower shell course which measured .600" in during its last internal inspection in March of 1989. The present reading is .570" March 1993.
Metal loss = Previous thickness minus the present thickness. .600" Previous
-.570" Present .030"
Answer: Metal Loss = .030 inch
Corrosion rate equals the metal loss per given unit of time, i.e., per year. Problem #2
Problem #2
Using the data of Problem #1 calculate the corrosion rate of the tower. Corrosion Rate = Metal Loss
Time Therefore:
March 1993-March 1989 = 4 years
Corrosion Rate = .030” = 0.0075 in./per year 4 Yrs.
Corrosion allowance equals the actual
Corrosion allowance equals the actual thickness minus the required thickness.thickness minus the required thickness. Problem #3
Problem #4 Problem #4
Calculate the remaining service life of the tower of problem #1. .070" corrosion allowance from Problem #3
.0075" corrosion rate from Problem #2 .070 " = 9.33 Yrs.
.0075”
Internal inspection equals half of the remaining service life, but not greater than ten (10) years.
9.33 Yrs. = 4.6 Yrs. 2
API 510 Module SECTIONS 1, 2, and 3
Find the answers to these questions by using the stated API 510 paragraph at the end of the question.
Quiz #1
1. What code covers maintenance inspection of petrochemical industry vessels? (1. 1. 1)
2. Define MAWP according to the API 510 Code.(1.2.8) [1997 3.8]
3. Define rerating. (1.2.14) [1997 3.11]
4. What is a pressure vessel?(1.2.11) Sect VIII U-1(a) [1996 3.11]
5. Under what circumstances must an API 510 inspector be re-certified? (App. B Paragraph B. 6) [1996 B4.1 App. B]
6. In terms of creep, what must be considered? (3.2) [1996 5.2]
7. What is the most valuable method of vessel inspection? (3.5) [1997 5.5]
8. Describe the correct way to clean a vessel for inspection. (3.5) [1997 5.2]
9. What metals might be subject to brittle fracture even at room temperature? (3.2)[1997 5 2]
10. Name five methods other than visual that might be used to inspect a vessel.(3.5) 11. When a new Code vessel is installed, must a first internal inspection be performed?(4.1)
ANSWERS TO QUIZ #1 1. answer: API-510
2. answer: is the maximum gauge pressure permitted at the top of a pressure vessel in its operating position for a designated temperature.
3. answer: A change in either temperature rating or maximum allowable pressure of a vessel or both.
4. answer: A container designed to withstand internal or external pressure by an exterior source by the application of heat direct or indirect or both.
5. answer: Inspector who has not been actively engaged in an API inspection within the previous 3 years. Re-certify by written examination.
6. answer: Time, Temperature & Stress. 7. answer: Careful visual examination
8. answer: wire brushing, blasting, chipping, grinding(or combination) 9. answer: At ambient temperature, carbon, low alloy, and other Ferritic Steels. 10. answer: 1. Magnetic Particle 2. Dye Penetrant 3. Radiography 4. Ultrasonic
Thickness measurement. 5. Metallographic Examination 6. Acoustic Emission Testing 7. Hammer Test.
11. answer: No as long as manufacture report(Data) assures that the vessel is satisfactory for the intended use is available.
API 510 Module
RP 576 INSPECTION OF PRESSURE RELIEVING DEVICES Overview
Overview Scope: Scope:
This recommended practice covers automatic pressure relieving devices commonly used in the petrochemical and oil refining industries. The recommendations found in RP-576 are not intended to replace and regulations that may exist in a jurisdiction.
Types of Pressure Relief Valves: Types of Pressure Relief Valves:
The three major types of pressure relief valves are the safety valve, relief valve and the safety relief valve. Pressure relief valves are classed based on their construction, operation and applications.
Safety Valves Safety Valves
A safety valve is a spring-loaded device containing a seat and disk arrangement. It also has a part just above the disk referred to as a huddling chamber. When the static pressure beneath the disk has risen to a point where the force exerted on the disk begins to overcome the springs downward force the disk slowly opens. When this has occurred the pressure beneath the disk is exposed to the huddling chamber. The huddling chamber adds a much greater area exposed to pressure than the disk alone. This results in a sudden rapid opening to the venting systems releasing the pressure to safe point at which time the valve will close. Safety valves have an open spring and usually have a lifting lever.
Safety valves are used for steam boiler drums and superheaters. They may also be used for general air and steam services. The discharge piping may contain vented drip pan elbow or a short piping stack vented to the atmosphere.
Safety valves are not fit for service in corrosive service, where vent piping runs are long, in any back pressure service or any service where loss of the fluid cannot be tolerated. They should not be used as a pressure control or bypass valve and are not suited for liquid service.
Relief Valve Relief Valve
A relief valve is a spring-loaded device that is intended for liquid service. This type of valve begins opening when the pressure beneath its seat and disk reaches the set pressure of the
Safety Relief Valves Safety Relief Valves
A safety relief valve is a spring-loaded valve that is capable as functioning as a relief valve in liquid service or as safety valve in gas or vapor service. Safety relief valves may be of the conventional, balanced or pilot operated types.
Conventional SRV Conventional SRV
A conventional SRV has its spring housing vented to the discharge side. Its opening pressure, closing pressure and relieving capacity are directly affected by changes in back pressure.
Conventional SRVs are used in flammable, hot and toxic services. Usually they are piped to safe remote points of discharge such as a flare stack. Conventional SRVs are found in service for gas, vapor, steam, air or liquids. Conventional SRVs are also used in corrosive service. Conventional SRVs may not be used in services where any backpressure is constant or where any built-up backpressure exceeds 10% of its set pressure. They are not to be used on steam boilers, superheaters or as pressure control or bypass valves.
Balanced Safety Relief Valves Balanced Safety Relief Valves
A balanced SRV has a pressure-balancing bellows, piston or both. This arrangement is provided to minimize the effect of any backpressure on the operation of the balanced SRV. Whether it is pressure tight downstream depends on its design. It may have a lifting lever as an option.
Balanced SRVs are used in flammable, hot and toxic services. Usually they are piped to safe remote points of discharge such as a flare stack. Balanced SRVs are found in service for gas, vapor, steam, air or liquids. Balanced SRVs are also utilized in corrosive service. They are not to be used on steam boilers, superheaters or as pressure control or bypass valves. Because balanced-type valves have vented bonnets and the vent may need to piped to a safe point. In the event that a bellows fails in such a valve the fluid will be discharged to the bonnet and out its vent.
Pilot-Operated Safety Relief Valves Pilot-Operated Safety Relief Valves
A pilot operated safety relief valve (POSRV) is a pressure relief valve whose main relieving valve is controlled by a small spring loaded (self-actuated) pressure relief valve. It is a control for the larger valve and may be mounted with the main valve or remote from the main valve. The ASME Code requires that the main valve be capable of operating at the set pressure and capacity even if the smaller fails.
Pilot operated relief valves are used under conditions where any of the following are true: a large relief valve is required, low differential exists between the normal operating pressure
Pressure and/or Vacuum Vent Valves Pressure and/or Vacuum Vent Valves
Pressure and/or vacuum vent valves are used for the protection of storage tanks and are categorized into three kinds; weight loaded, pilot operated or spring and weight loaded. These valves protect against an excessive differential in the outside pressure (atmospheric) and the inside pressure or vacuum. If while drawing down (draining) a storage tank where to develop a vacuum the tank might be crushed by atmospheric pressure. In the case where internal pressure where to exceed design pressure the tank might bulge or rupture. In cases where the tank might operate alternating between pressure and vacuum a breather type valve is used, this valve will both vent gas pressure and break any vacuum, which might develop during operations of the storage tank.
Rupture Disks Rupture Disks
A rupture disk (RD) is a thin plate (usually in the shape of a bulge) that may be made of various metals or of combinations or metals in thin layers. RDs may also be made of plastic-metal combinations or coated plastic-metals. Non-plastic-metallic RDs are manufactured from impervious graphite (usually flat) and other non-metallic materials. The rupture disks are held between specially made flanges and designed to rupture at predetermined pressure and are of course not capable of reclosing. Most rupture disks are designed to have the inside of the bulge facing pressure although some are made to have the outside of the bulge facing pressure, these are called reverse buckling RDs They may be used to protect against excessive internal pressure. If the service involves a vacuum, the rupture disk normally will use a vacuum support. A rupture disk in this service is designed to protect against an excessive internal pressure should it occur due to a failure of the system. Each type of RD has special considerations based on its design. A RD can be used alone or in combination with a pressure relief valve.
Normal uses of RDs include all of the following; protections for the upstream side of PRVs against corrosion, protect RVs against plugging or clogging, in place of PRVs if nonreclosing is permitted, as additional backup over pressure protection, in outlets of vent piping to protect the PRV from corrosion and to minimize leakage of a PRV.
Special handling for, storage, applications and the installation of RDs is required and the manufacturer's recommendations directions should be followed. A special consideration in the ASME Code is the relieving capacity rating of the safety relief valve if the RD is installed between the SRV and the vessel.
Variations with Resilient Valve Seats Variations with Resilient Valve Seats
When tighter sealing of PRVs is desired the valves are manufactured with 0 rings in the seating parts. The valves are similar to PRVs with metal to metal seating only but with soft parts to increase the seal tightness against leaking. The applications for these types of valves are numerous but fall into the following categories; corrosive service, toxic/flammable/expensive products, operating pressure very close to the set pressure, in vibrating minor pressure surges, hard foreign particles in fluid and in pulsating pressure or vibrating service.
Care should taken when choosing the material that the soft parts, such as O-Rings, are made from. They must resist the chemicals and pressures they are exposed to in the intended service. Comparable service should serve as a guide when choosing materials, failing this information the valve manufacturers can be consulted.
Reasons for Inspections Reasons for Inspections
If a pressure relief valve fails to open overpressure could occur and cause serious damage and even loss of life. Protection of personnel and equipment may finally depend on the proper functioning of the safety relief device. For these reasons the general condition of the
devices and the frequency of inspection must be established. Causes of Improper Performance Causes of Improper Performance
The primary causes of failure or improper performance fall into categories as listed in RP 576. They can be classified as follows; corrosion, damaged seating surfaces, failed springs, improper setting/adjustment, plugging/sticking, wrong materials for the service, installation in the wrong service or location. Rough handling during service and shipping or installation. Improper hydrostatic tests of discharge piping can cause damage to springs or to bellows of balanced relief valves.
Frequency and Time of
Frequency and Time of InspectionInspection
Definite time intervals are required for the inspection, testing and repair of relief devices. Some services require more frequent inspection than others but the basic frequency must be based on safety not economics. API 510 establishes the maximum frequency to be 10 years but actual service may require a shorter interval between inspections. The ideal time for
API 510 Module
RP 576 SECTIONS 1 AND 2
Find the answers to these questions by using the stated API 576 paragraph at the end of the question.
Quiz #2
1. How often should a safety relief valve be tested"? (4.5)
2. A vessel made of P-1 material one inch thick is being repaired by welding. The vessel was srcinally postweld heat-treated. Is there any method to avoid PWMT of the repair? (5.2.3)
3. Why are relief devices installed on pressure vessels? (RP 576 21.)
4. How many types of pressure relief valves are there? (RP 576 2.2.1.1 Section VIII UG-126)
5. You notice that a pressure relief device has a closed bonnet. What type of valve is it? (2.2.1.3.1)
6. While reviewing maintenance records you notice that bulged rupture disks in a unit are three years old. Is this okay? (2.2.3.3)
7. A pilot operated safety valve has been installed in heavy crude service. Is this okay? (2.2.1.5.3)
1. During s/d’s or 10 years. (5.1.1) 2. yes
3. to protect personnel and plant equipment.
4. safety valve, relief valve, safety relief valve, pilot operated safety relief valve. 5. relief valve.
API 510 Module
RP 576 SECTIONS 3, 4, 5, 6, 7, and 8
Find the answers to these questions by using the stated API 576 paragraph at the end of the question.
Quiz #3
1. Describe a shop inspection of a relief device. (3.2)
2. Name three causes of improper performance of a pressure relieving device. (Titles of Section 4 paragraphs)
3. The spring of a relief valve broke. What probably caused it to break? (4.3) 4. The valve shop is setting safety relief valves using water is this acceptable? (4.4) 5. You are ask to set a schedule for the inspection of relief devices; what will determine
the time between the setting of valves? (5.1.1 the max. is 10 years per API 510) 6. You notice workers opening RV. discharge lines to the atmosphere. What precautions
should be taken? (6.1.1)
7. What should the operating history of a pressure valve include? (6.1.3)
8. You are asked to visually inspect an RV before it is taken to the shop. What is the purpose of this and why is it important? (7.1.1)
9. What is the purpose of a pressure/vacuum vent valve on an atmospheric tank? (7.3.2) 10. Why are records kept for pressure relieving devices? (8.1)
Answers Quiz#3
1. Check pop pressures, extend check for external conditions, and conform to specifications. 2. Corrosion, damage seat surfaces, and improper length of piping? (4.2)
3. Surface corrosion, stress corrosion. 4. No.
API 510 Module
API RP 572 INSPECTION OF PRE SSURE VESSELS OVERVIEW Section 1 Section 1 General General Scope: Scope:
This recommended practice addresses the following items; description of types of vessels, construction, maintenance, reason for and method of inspection, causes of deterioration, repair methods and records/reports.
Section 2 Section 2 Types of
Types of Pressure VesselsPressure Vessels
The definition of a pressure vessel per API 572 is a container that falls within the scope of the ASME Code Section VIII Division 1 and is subjected to an external or internal design pressure greater than 15 psi. Section VIII Division 1 should be consulted for the exact definition and exemptions. The definition of a pressure vessel is found in the ASME Code Section VIII Division 1, page 1 in the first paragraph.
Pressure vessels can have many different shapes, they may be: spheres (balls), cylinders with various heads attached such as flat or hemispherical and may consist of inner and outer shells (jacketed). Many methods of construction are used. The most common is the cylindrical shell made of rolled plate and welded with heads that are attached by welding. Riveting was used prior to the development of welding. Vessels are no longer made by riveting, but some riveted vessels are still in service today. Vessels are also made of the hot forging and multi-layer (cylinders inside of cylinders) techniques. Multi-multi-layer vessels are found primarily in high pressure service.
The vast majority of vessels are made of carbon steels. For special services the carbon steel may be lined, clad or weld metal surfaced with corrosion resistant materials such as stainless steels. Some vessels are constructed entirely of various metals such as monel, nickel titanium, or stainless steel. The material chosen will be determined by the required service conditions. Temperature, pressure and the fluids to be contained are the primary concerns in material selection. For reasons of economy different parts of a vessel may be made of different materials using only the most expensive where needed. Many pressure vessels are
Section 3 Section 3 Construction Standards Construction Standards
The first unfired pressure vessels were constructed to the design of the user or manufacturer. This was true until about 1930 after that time the API/ASME Code or the American Society of Mechanical Engineers Code (ASME) was used. In 1956 the API/ASME Code was discontinued and the ASME Code was adopted as the standard for the construction pressure vessels within its scope. Section VII Divisions 1 and 2 of the ASME Code are the unfired pressure vessel Codes. Section VII Division 1 is the Code the vast majority of vessels are built to; Section VII Division 2 used for vessels in high pressure service or where lower factors of safety are desired. Division 2 has more restrictions on construction, materials, inspection and nondestructive examination than Division 1. These restrictions usually result in a vessel that would be thinner than that required by Division 1 and the resulting cost savings could be significant is some instances.
Heat exchangers are built using both the ASME Code and the Standards of Tubular Exchanger Manufacturers Association (TEMA).
Section 4 Section 4 Maintenance Inspection Maintenance Inspection
The basic rule for the maintenance of a vessel in service is to maintain it to the srcinal design and the edition of the Code it was constructed under. If the vessel is re-rated this is may done using the srcinal or latest edition of the Code. This implies that persons responsible should be familiar with the srcinal construction edition of the Code and the latest edition of the Code if a vessel has been re-rated. In addition personnel responsible for these vessels must be familiar with any nations state, county or city regulations. The ASME has minimum requirements for construction, inspection and testing of pressure vessels that will be stamped with the Code Symbol however jurisdictions may have more restrictive requirements. Compliance with ASME Code may not be enough to satisfy a jurisdiction's requirement.
Section 5 Section 5 Reasons for Inspection Reasons for Inspection
The main reason for inspection is to determine the physical condition of a vessel. With this information the causes and rate of deterioration can be established and safe operations between shutdowns can be determined. Correcting conditions causing deterioration and planning for repairs and replacement of equipment can also be done using the inspection information. Scheduled shutdowns and internal inspections can prevent emergency shutdowns and vessel failures. Periodic inspection allows the for the forming of a well planned maintenance program by using data such as corrosion rates to determine replacement and repair needs. External visual inspections along with the thorough use of various nondestructive examination techniques can reveal leaks, cracks, local thinning and unusual
Section 6 Section 6 Causes of Deterioration Causes of Deterioration
The causes of deterioration are many but fall into several general categories as follows: inorganic and organic compounds. steam or contaminated water, atmospheric corrosion. These types of corrosive agents fall into the class of chemical and electrochemical attack. Attack is also possible from erosion and, or impingement. The attack could come from any combination of the above examples.
Corrosion is the prime cause of wear in pressure vessels. The most common internal corrodents are sulfur and chloride compounds. Caustic, inorganic acids, organic acids and low pH water can also cause corrosive attack in vessels.
Erosion is the wearing away of a surface that is being hit by solid particles or drops of liquid. It is similar to sandblasting and is usually found where changes in direction or high-speed flow is present. It occurs in such places as inlet nozzles and the vessel wall opposite the nozzle. Outlet nozzles are likely spots when fast flowing products are in use. In some instances corrosion and erosion are found together.
Metallurgical and physical changes can occur when a vessel material is exposed to fluids the vessel contains. Elevated operating temperatures also contribute to these problems. The changes that take place may be severe enough to result in cracking, graphitization, hydrogen attack, carbide precipitation, intergrannular corrosion, embrittlement and other changes. Mechanical forces such as thermal shock, cyclic temperature changes (high to low temps on a frequent basis), vibrations, pressure surges, and external loads can cause sudden failures. Cracks, bulges and torn internal components are often a result of mechanical forces.
Faulty materials can build in failure into a pressure vessel or one of its components. Bad materials can result in leakage, blockage, cracks and even speed up corrosion in some. The selection of an improper material for new construction of or for a repair to a vessel will often result in the same type of failures as will proper materials that have manufacturing or fabrication defects.
Faulty fabrication includes poor welding, improper or lack of heat treatment, tolerances outside those permitted by Codes and improper installation of internal equipment such as trays and the like. Any of these types of faulty fabrications may result in failures due to cracks or high stress concentrations, etc., in vessels.
Opportunities for inspections will require the input of all groups involved; process, mechanical and inspection personnel. The opportunity may have to be made if any laws require a frequency or the insurance company has a requirement for it in the policy written on the equipment. A convenient time for inspections, of course, is any time equipment is removed from service for cleaning. Also if a vessel or exchanger was removed for operational reasons, an inspection might then become needed to insure the integrity of the equipment before returning it to service.
Another consideration for the inspection of vessels is the review of the in service operational records to look for pressure drops and out of the ordinary conditions that might indicate a problem.
Section 8 Section 8
Methods of Inspection and Limits Methods of Inspection and Limits
To perform a proper inspection it is important to know the history of the vessels to be inspected. Knowing what repairs have been required in the past and inspecting the repair after it has been in service may help to develop better repair methods. It may also help to locate similar problems. In every case, careful visual inspection is a requirement. Knowing the service conditions of a vessel allows the concentration of efforts in areas known to have problems in a particular service.
Safety precautions before entering a vessel are of the utmost importance. Vessels have small openings and often many internal obstructions that make getting out of one quickly nearly impossible. The bottom line is: make sure it is safe to enter a vessel. Such things as isolation of lines by blinding, purging and cleaning along with gas testing prior to entry cannot be overlooked. In some cases protective clothing and air supply systems are called for if entry is desired before cleaning to look at the vessel's existing conditions for indications of problems. Always inform personnel inside and outside a vessel that inspection personnel are entering the vessel. Loud noises made by inspection or maintenance might scare others, causing injury.
Preparatory work needed for vessel inspection should include checking in advance to make sure all equipment is present and is in usable condition.
External inspections should start with ladders, stairways, platforms and walkways connected to the vessel. Loose nuts, broken parts and corroded materials must be searched for by visual inspection and hammer testing for tightness. Since corrosion is most likely to occur where water can collect, these areas should be inspected carefully, using a pick or similar object. Slipping hazards such as slick treads should be looked for and noted on the inspection report. Foundations and supports must be inspected for the condition of the fireproofing. The settling of foundations, spalling (flaking) and cracking of the fireproofing are always a concern.
Concrete supports are inspected with same concerns as concrete foundations. Close attention to any seals and the possibility of trapping moisture because of faulty seals should be investigated.
Steel supports should be examined for corrosion, distortior4 and cracking. If corrosion is severe, actual measurements of the remaining thickness should be performed and a corrosion rate established just as in a vessel. Wire brushing, picking and tapping with a hammer is frequently used inspection techniques. Most of the time corrosion can be slowed or prevented by proper. painting alone. Sometimes protective barriers such as galvanizing are required. As part of steel support inspection, vessel lugs should be examined using the same methods of wire brushing, etc., described above. Welds used to attach lugs can develop cracks and some cracks can then run into the vessel's walls. If a vessel's steel supports are 'insulated and an indication of leakage is present, the insulation must be removed to determine if corrosion under insulation has occurred.
Guy wires are cables that stretch from different points of a vessel to the ground where they are anchored to underground concrete piers (deadmen). Inspection of these guy wires must include checking the connections for tightness and the cables for the correct tensions. The connections consist of turnbuckles used for tightening and U bolt clips for securing. An connectors must be checked for proper installation and the presence of corrosion- The cable must be checked for corrosion and for broken strands.
Nozzles and adjacent areas are subject to distortion if the vessel foundation has moved due to settling. Excessive thermal expansion, internal explosions, earthquakes, and fires can cause damage to piping connections. Flange faces should be checked for squareness to reveal any distortion, If evidence of distortion is found cracks should be inspected for, using non-destructive examination. All inspections should be external and internal whenever possible. Visible gasket seating surfaces must be inspected for distortion and cuts in the metal seating surfaces. Wall thickness readings must also be taken on nozzles and internal or external corrosion monitored.
Grounding connections must be inspected for proper electrical contact. The cable connections should be tight and properly connected to the equipment and the grounding system. All grounding systems should be checked for continuity (no breaks) and resistance to electrical flow, Continuity checks are usually made using electrical test
equipment such as an Ohm meter. lie resistance readings are recommended to be between 5 and 25 Ohms.
External surface corrosion appears in forms other than rust. Caustic embrittlement, hydrogen blistering and soil corrosion are also found on the external surfaces of equipment. Area of a vessel that need special attention often depends on its contents. When caustic is stored or used in a vessel, the areas around connections for internal heaters should be checked for caustic embrittlement. In caustic service, deposits of white salts often are indications of leaks though cracks. Hydrogen blistering is normally found on the inside of vessels, but can appear on the outside if a void in the vessels material is close to the outer surface. Unless readily visible, leaks in a vessel are best detected by pressure testing. Cracks in vessels are normally associated with welding and can he found using close visual inspection. In some services nondestructive tests to check for cracks is justified and should be performed. Other concerns when performing external inspection are bulges, gouges, and blistering. Hot spots when found in service should be monitored and thoroughly evaluated by an engineer experienced in pressure vessels.
Internal inspections should be prepared for by assembling all necessary inspection equipment such as tools, ladders, and lights.
Surface preparation will depend on the type of problems that a vessel may have in a given service. Ordinarily the cleanliness required by operations is all that is needed for many inspections. If better cleaning is required, the inspector can scrape or wire brush a small area. If serious conditions are suspected, water washing and solvent cleaning may not be enough to reveal problems. In these instances, power wire brushing, abrasive grit blasting, etc., may be required.
Preliminary visual inspection should be preceded by a review of reports of previous inspections. Preliminary inspection usually involves seeking out known problem areas based on inspection experience and service. Many vessels are subject to a specific type of attack such as cracking in areas such as upper shell and heads. Preliminary inspection may reveal a need for additional cleaning for a proper detailed inspection.
Detailed internal inspections should start at one end of a vessel and progress to the other end. A systematic approach such as an item check list will help to prevent overlooking hidden but important areas. All parts of vessel should be inspected for corrosion. hydrogen blistering, deformation, and cracking. In areas where metal loss is serious, detailed thickness readings should be taken and recorded. If only general metal loss is present, one thickness reading on each head and shell may be enough. Larger vessels require more measurements.
Pitting corrosion will require local examination by first scraping the surface and then and measuring the pit depth. Pit gauges allow for measuring pit depth if an uncorroded area adjacent to the pit is available to gauge from In the case of large pits or grooves, a straight edge and steel rule often will allow measurement by spanning the large area and lowering the steel rule into the pit and measuring the depth.
Hammer testing is often a good method of finding thin areas. Experience is needed to interpret the sounds made by hammering. Usually a dull thud will indicate a loss of metal or
Nozzles should be checked for corrosion and their welds for cracking at the time of the vessels internal inspection. Normally ultrasonic thickness readings will reveal any loss of metal in nozzles and other openings in a vessel. Internal equipment such as trays and their supports are visually inspected accompanied by light tapping with a hammer to expose thin areas or loose attachments. Conditions of trays must be determined to check for excessive leakage caused by poor gasket surfaces or holes from corrosion. Excessive leakage can cause operational problems and may lead to poor performance of a vessel or unscheduled shut downs.
Inspection of metallic linings must determine if the lining has been subjected to service corrosive attack, that linings are properly installed, and that no cracks or holes are present in the lining. Most problems with linings are found by careful visual inspections. Tapping the lining lightly with a hammer can reveal loose lining or corrosion. Welds around nozzles deserve special attention due to cracks or holes that are often found in these areas. If the surfaces of the lining are smooth, thickness measurements using ultrasonic techniques may be performed. If required, small sections of lining can be cut out and measured for thickness. A very useful method of tracking the corrosion rate of linings, is by the welding of small tabs at right angles to the lining when the lining is first installed. These tabs are made of the same material and thickness as the lining and can be easily measured at the time of installation and at the next inspection to determine the rate of corrosion taking place in the vessel. Remember that both sides of the tab are exposed to the corrosion and the lining's loss must be determined by dividing the tab's loss by two. A bulge in a liner can be caused by a leak in the liner permitting a pressure or a product build tip between the liner and the protected base metal.
Nonmetallic liners are made of many different materials such as glass, plastic, rubber. ceramic, concrete, refractory, and carbon block or brick liners. The primary purpose when inspecting these types of linings is to insure that no breaks in the lining are present. These breaks are referred to as holidays. Bulging, breaking, and chipping are all signs that a break is present in the lining. The spark tester method if very effective in finding breaks in such nonmetallic linings as plastic, rubber, glass, and paint. The device uses a high voltage with a low current to find openings in linings. The electrical circuit is grounded to the shell and the positive lead is attached to a brush. As the brush is swept over the lining, if a break is present, electricity is conducted and an alarm is sounded. A little warning: this is obviously not a device to be used in a flammable or explosive atmosphere nor should the device have such a high voltage value that it can penetrate through a sound lining. The spark tester is not useful for brick concrete, tile, or refractory linings. Remember linings can be damaged during a careless inspection; often just by dropping a tool.
In thickness measurements using radiographs, the placement of a device such as step gage (a device of a known material and thickness) in the radiographic image is compared to the image of the piping or vessel wall and the thickness determined by measurement.
Corrosion buttons are made of a material that are not expected to corrode in a given service and then installed in pairs at specific locations in the vessel. Measurements are taken by placing a straight edge across the two buttons and then gauging the depth with a steel rule or some other measuring device. When corroded surfaces are very rough, test holes through the vessel may be used to measure the wall thickness. A variation on test holes is depth drilling. In this technique, small holes are drilled to a known depth (not all the way through) in the new vessel wall, then plugged with corrosion resistant plugs to protect the bottom of the hole from corrosion. During internal inspections the plugs are removed and depth readings are taken. Any wall loss that has occurred is detected by the hole depth becoming more shallow than the srcinal reading.
Special methods of detecting mechanical changes include nondestructive techniques, acid etching small areas to find cracks, and sample removal. Acid etching requires abrasive cleaning and the application of an appropriate (for the metal) chemical usually acid. The etching approach allows fine cracks to stand out in contrast to the base metal. Sample involves the removal by mechanical cutting out a small portion of the area of interest and then analyzing it under a microscope. Often the filings created during the removal can be cleaned and then subjected to a chemical analysis. A weld repair to the site of sample removal will be required and should be made as carefully as any welded repair.
Metallurgical change tests can be made using many of the same techniques described in mechanical changes. Additional tests include hardness chemical spot, and magnetic tests. Portable harness testers such as the Brinell will detect poor heat treatment, carburization and other problems that involve a change in hardness. Chemical tests to a small portion of a metal will reveal the type of metal to determine if the wrong metal has been installed possibly during a pervious repair. Magnetic tests are used to determine if a material such as austenetic stainless steel; normally not magnetic, have become carburized, which will allow the austenetic stainless to become attracted to a magnet.
Testing Testing
Hammer testing used during visual inspection will reveal conditions such as; thin sections. tightness of bolts and rivets, cracks in linings, lack of bond in refractory and concrete linings. The hammer is also used to remove scale for spot inspection. Hammer testing is an art learned from experience and caution is warranted whenever using this method. It is not smart to hammer on anything under pressure and hammering on some piping systems can dislodge scale or debris and plug up a portion of the system such as a catalyst bed.
vessel may be revealed. If the vessel's supports can not hold the weight of the fluid or the vessel cannot tolerate contamination by the testing fluid, a gas test (pneumatic) may be used. Pneumatic testing, by its nature, can be more dangerous than hydrostatic testing. Caution is always advisable during a pneumatic test, and it is normally the last choice of types. The reason for this is that gas that has been compressed has a great deal of stored energy, and if failure occurs, it will likely be explosive. Have you ever blown out a car tire? During a pneumatic test, a soap solution is often applied to weld seams and fittings and then, looking for bubbles, leaks can be revealed. Another method, sound detection, uses special listening devices to bear and locate the leaks. Another sound based device is Acoustic Emissions. As a vessel is pressurized, it emits sounds from any flaws present in the metal. By using several listening devices attached to different parts of the vessel, the location of a serious flaw is found by using triangulation. Some vacuum vessels can be tested with internal pressure rather than a vacuum. If a vacuum vessel can be pressure tested, it is the preferred method because it is easier to detect leaks with internal pressure.
Vacuum tests are conducted by creating a vacuum inside the vessel and observing the vacuum gage for any loss of vacuum that might occur. If the vacuum remains unchanged the assumption is made that no leak exists.
Testing temperature can be very important with some pressure vessel materials due to the brittle characteristics of these metals at low temperatures. The ASME recommends that the test temperature be at least 30°F above the minimum design metal temperature to prevent the risk of brittle fracture. A brittle fracture can be compared to glass breaking and shattering. For that reason every effort must be made to prevent it. In combination with a pneumatic test and its stored energy; a brittle failure would be a devastating bomb. For all materials the general recommendation for test temperature is 70°F minimum and 120°F maximum for safety when conducting a pressure test, no unnecessary personnel should be allowed in the area until the test is complete. Pneumatic tests must follow a procedure described in the ASME Code that raises the pressure in small steps with short stops at each step.
Pressure testing of exchanges can be performed when they are first shut down and before bundle removal in order detect any leaks that might have been present during recent service. If leaks are detected during the initial test, partial disassembly can be performed and the test pressure reapplied to locate the source of the leaks. Heat exchangers may also be disassembled and cleaned, inspected, repaired if needed, then reassembled and tested. If a leak is detected in the exchanger after re-assembly, disassembly will again be required to repair the leak. The method of testing an exchanger will depend on its design. Some can be tested with their channel covers removed if of the fixed tube sheet design with the pressure applied to the shell side. If a tube in the bundle is discovered to be leaking at other than the
thicknesses above what are required by the Codes they were built to. Extra thickness can be required by the design as sacrificial metal (corrosion allowance) in the vessel parts.
Extra thickness can be due to the nominal plate thickness as opposed to the actual thickness required by calculation, i.e., the shell has a required thickness of .435 " and .500” plate is used because .435" is not manufactured. Owners, Users or Codes may require that the metal cannot be less than a certain thickness in a particular service. Sometimes a reduction in pressure or temperature for a vessel will allow its continued service with thinner metal.
Methods of repair to vessels should be reviewed to insure that they comply with any Codes or standards that may apply. Several jurisdictions recognize the minimum repair techniques of the API. Other jurisdictions require that the repairs be made to the National Board of Boiler and Pressure Vessel Inspectors (NBBPVI), National Board Inspection Code-23 (NBIC) and that the repair concern holds a valid R (Repair) Stamp from the NBBPVI. In addition to using a concern holding the R Stamp an NBBPVI Repair form R1 may also be required. In some instances, Insurance Carriers will require that the NBIC be followed and that an NBIC Authorized Inspector in their employ approves the repair. Repairs made to vessels by welding will require visual inspection as a minimum and may also involve various nondestructive examinations (NDE) methods based on the severity of the repair and the srcinal NDE used in the construction Code. Unless the Inspector can accept a sound technical argument against requiring a pressure test after a major repair, one should be applied. If the repair to a vessel involves cracks special preparation of repair area is required. The major concern in crack repairs is the complete removal of the crack. Cracks may be removed by chipping, flame, arc, or mechanical gouging. Any crack removal technique that uses high heat input to the affected area can cause the crack to grow, so caution must be used with those techniques. In cases where many cracks are present it is normally better to replace the entire section of the material. Shallow cracks may be removed by grinding using a blending method if the final thickness does not fall below the minimum required.
Inspection records and reports are important and are required by most Codes and jurisdictions such as the State, API, and the NBBPVI NB-23. These reports are of three types: Basic Data, Field Notes, and Continuous File. The basic data includes srcinal manufacturer's drawings and data reports as well as design information. Field notes are notes about and measurements of the equipment and may be written or entered into a computer data base. Usually field notes are in the form of rough records inspections and repairs required. Continuous files include all information about a vessel's operating history, previous inspection reports, corrosion rate tables (if any) and records of repairs and replacements. Copies of reports containing the location, extent, and reasons for any repairs should be sent to all management groups such as Engineering, Operations, and Maintenance departments.
Heat Exchangers are used to transfer heat from one gas or liquid to another gas or liquid without the two fluids mixing. Heat exchangers fall into classes: condensers and coolers. A condenser has the effect of changing a gas fluid to a liquid or partial liquid fluid and ordinarily use water as the coolant. Coolers lower the temperature of a fluid and may use water or another process fluid of a lower temperature as the coolant. Sometimes air is used
