4.4 Welding discontinuties

Why you should be very careful with the choice of words:

Welding discontinuities are conditions that exist in the weldment which may or may not be accepted. Meaning these little "defects" on the welds may still be accepted, if it falls beyond the code acceptance criterias, so be very careful with using the terms discontinuities and defects.

There are also many other words in use on the market such as Flaws, Imperfection, Indications.

Welding defects are the discontinuities which are not acceptable by the inspector in reference to Company design requirements, ASME code requirements or AWS code requirements, whichever takes precedence.

For pressure vessels, you could find the defect acceptance criteria on latest edition of ASME Section VIII Div 1 in the Appendixes and for pipes in ASME Section B31.3 in the Appendixes. Most of these defects will be measured based on the result of NDE testing such as RT, UT, MT and PT. However an inspector may reject a weld based on visual inspection if it obviously contains reject-able discontinuities.

According to API Inspection codes, NDE shall be carried out by the Examiner which is basically the certified NDE technician. Results of the NDE shall be reviewed by the Inspector for acceptance at his discretion.

Types of discontinuities

Excessive Overlap

- When weld bead extends too much beyond the weld toe. 

Excessive Undercut

- When weld bead do not fully cover the beads

Excessive Weld Reinforcement

- Height of weld bead is too high, presents unnecessary stresses to the join. 

Inclusion 

- Inclusion of impurities

Porosity

- Inclusion of porous air sacks/bubbles

Arc Strike

- Spatter like effect caused by electrode scratching when it does not starts due to cold weather.

Cracking 

- Usually caused by presence of water when welding, water breaks down to its elemental form causing causes hydrogen induced cracking. 

Crater crack

- Always pull the weld away from direction of weld when completing, this prevents crater cracks from forming. 

Lack of fusion

- Temperature is too low during passes, the binding between the metals is incomplete. Can occur between passes, or between base/filler metal.

Lack of penetration

- Too fast welding,  and low temperature resulting in inability to penetrate into the root gap,as such not meeting desired weld size. 

5.5 Plant Maintenance: Refractory Repair

Refractory Repairs

Refractory are similar to lining and coatings, for which the purpose is to protect the metal enclosure from the effects of heat and chemicals.

Refractories are found in fired spaces such as ovens, boilers and reactors

What operators fear during operations is hot gas pass due to refractory failure, this can potentially heat up the metal surface beyond its design temperature reducing the stresses of material. What do I meant by this statement?

Metal properties, in reference to ASME II-D Material Properties, it can be observed from the several charts for ferrous steels that the Maximum Allowable Stress values remain uniform up to approximately 600F before it begins losing it, therefore the ability of the equipment to hold pressure & stress reduces. This issue is especially important for reactors as they normally operate at higher pressures as compared to ovens and boilers.

As a result of the overheating of metal, we not only have a problem holding the pressure, but we also face many other problems if refractory damage is allowed to be left in place. To summarize the ill effects:

• Exposure of metal surface to harsh process media
• Exposure to acidic components such as H2S, polythionic acid, naphenthenic acid, ammonia bisulfide, chlorides, hydrogen and many more can result in significant increase in corrosion rates or Stress Corrosion Cracking.

• Damage to insulating bricks
• Refractories are normally lined in several layers especially for high temperature service involving harsh chemicals ie. reactors. The penetration of gas through fire bricks can lead to insulating bricks failure.

• Hot spots
• Overheating of metal results in red hot spots and eventually metal damage due to creep which causes material to loss its strength over time.

General Acronyms 

• Hot face bricks - High alumina content to resist high temperatures
• Insulating bricks/castables - These are usually lined between the hot face bricks and metal wall.
• Mortar - Adhesive to join bricks, don't use them as castable, they are meant to be thinly spread.
• Pour castable - Similar to refractory bricks, however for difficult to lay areas to reduce work time. 
• Ramming Castable - Also known as plastic castable, used for filling voids in castable and refractory brick spaces by injection.

Damage Evaluation and monitoring

Monitoring:

Basic monitoring for refractory condition can be made by online condition assessment from exterior by utilising the following methods to have a glimpse of the metal condition which can reveal whether if refractory have deteriorated.

• Live Infrared thermography 
• Monitors hot spots, loss of refractory material by localized or overall temperature increases.
• Sectional insulation removal 
• Inspect for distortion and change in OD which are signs of creep, 
• Inspect for surface glazing which are signs of hot spots

Damage Evaluation
After shutdown of unit, thorough inspection of the equipment can be performed to identify suspected damaged observed during monitoring.

Even if no damage are suspected, brief inspection of the below can also be carried out to identify minor indications especially if operating temperature is much much higher above ambient, as thermal expansion can cause unexpected damage from minor cracks and nicks. Inspect if you can, this is likely your one and only chance in a long time, as plant equipment are expected to be running 24/7 and have high availability!

• Loss of mortar material or strength
• Loss of mortar strength between bricks can be caused by improper mixture and application during brick laying.  The physical condition can only be identified offline, tell tale signs are loose bricks, broken castable or cracked refractory. Therefore important properties to watch during repairs and construction are: 

• Crush strength - Resist compression
• Adhesion strength - Resist loss of bonding
• Water content - Gives mason more time for application
• Air content - More air, more porosity, but good mortar closes up this issue.
• Time after mixture - Mortar is an adhesive, it loses adhesiveness in air over time.
•  Quality of brick - Good bricks should have slightly rough surface to improve adhesion with mortar, they should also have low porosity and high crush strength

• Erosion of refractory or mortar material 
• In high flow rate furnaces such as gas reactors or those containing 2 phase flow, there will be loss of refractory material. 
• Mortar having less crush strength than bricks is much easily eroded or damaged by thermal expansion. Watch out for loss of mortar which can increase build up of material between bricks and over time even penetration to insulating layer behind the bricks.

• Build up of carbon
• Observed this in a gasifier, which is an incomplete combustion furnace with a feedstock containing high metal content.
• One of the main product carbon monoxide breaks down into carbon under high temperature with metal as the catalyst with the refractory material acting as an catalyst.
• Along with erosion of mortar, the carbon built up had penetrated into the area between the bricks and insulating castable as a result pushed the bricks inwards into the furnace and caused several cracks and rise in metal temperature.

• Crack on bricks
• Inspect for damaged bricks in suspected areas. 
• Refractory may contain layers of bricks, should suspected area shows perfection on the hot face, but shows a hot spot on the thermography, hot leak could come from another location within close proximity. 
• Avoid denial of defects due to good hot face condition, it could be underlying.  
• Quality of bricks
• Always ensure quality source or experience with the process. This is beyond quality documents, as documents are usually doctored these days.
• Perform factory visits, request for sample fabrication and witnessed testing before purchase in placed.
• Example of tests 
• Crush strength
• Porosity (Cutting it up)
• Rung test (Not effective)

• Areas with gaps exposing metal surface
• Inspect metal surface for
• Corrosion - This contagious issue could spread beyond and behind the refractory
• Glazing - This is signs of hotspot formation and may cause catastrophic failures.

Repairs and Installation

• General Information for Refractory Repair
• Only damaged section needs to be repaired, there is no need to spend excessively
• For custom shape bricks that do not fit, they can be cut by the mason to fit, but in no case cut it down to less than 1/2 of original size.
• MDB will state maximum mortar thickness, avoid large joints to minimise failures.
• For Pour castable refractory, watch out for its preparation process, as there are also factors to monitor such as mixing temperature, mixing time, water content material still within shelf life. 
• Fill all gaps possible with plastic castable.
• A frame or form work is normally used for laying bricks/casting on difficult positions such as overhead domes. 

• Loss of mortar material or strength
• For such failures, the reasons and what to look out for has been listed above. For the repair be prepared to inspect quality of raw material, quality of workmanship which includes mortar preparation to brick laying process. 
• Use quality manuals from Manufacturer Data Book or use a Specialist. Workmanship is very important for performance.

• Crack on bricks
• For such failures, it usually points to quality of bricks from factory and has been listed above.

• Areas with gaps exposing metal surface
• Consult the inspector, repair organization & specialist for repair procedures as it varies from case to case basis. Refer to Pressure Vessel repair section.

5.3 Plant Maintenance: MOM Statutory Inspections (Singapore)

This post will cover only maintenance aspects of MOM Statutory inspections but not design and construction.

A little story about MOM and its jurisdiction control
MOM is the Ministry of Manpower in Singapore. Under this ministry, a Workplace Safety and Health department takes care of all industrial health, safety, environment related issues.

Pressure vessels, boilers, steam related equipment and more all fall under jurisdiction control. They pay attention to equipment carrying steam, air and refrigerant. Nothing else.

The lack of jurisdiction over equipment carrying other hazardous fluids probably because those are usually taken care of by ASME or CE inspections where most equipment makers needs to comply to.

We also know that steam, air and refrigerant is the most common utilities fluid used in industrial installations since the era of industrialization in 1900s. The rule book was passed down from British Law and developed then on, and little probably changed.

Inspection Intervals
These vessels are required to be inspected during fabrication by an accredited Authorized Examiner(AE) This is similar to the Authorized Inspector(AI) in ASME context. What needs to be done here will not be explained here, as we are more focused on maintenance and operations.

How about during Operations? What requires inspection on an interval basis by an AE.

• Air/Steam Receivers (PV & exchangers)
• Piping
• Steam equipment
• Safety valves connected to the above devices

Interval and details of inspection
Steam Boilers fired and unfired - 1 year

Steam/Air receivers - 2 year

Refrigerant receivers - During Fabrication and After Repairs
Piping - During Fabrication and After Repairs (For MP and HP steam systems)
Hydrotesting - After repairs or every 10 year

Thickness Measurement - 10 year
Safety valves - Calibrated during the inspection interval

Corporate Level Assessment
New initiatives: Corporate Level Assessment. What is it?
With this intitiative, delayed Statutory inspection intervals could be applied by plant owner. However this is subjected to approval after submission of the relevant maintenance procedures, inspection data and reports, audit reports and etc,

This initiative is designed to be somewhat ambiguous this is so that approval can be made on a case by case basis. However rest assured safety related corruption cases will be weeded out due to high level of interviews on MOM Authorized Examiners and corporations during the submissions.

Under this initiative boilers can be granted extension of 2 years and pressure vessels 5 years. Depending on condition of equipment and documentation submitted. Requiremets are as such

- Competent Person (CP) to be engaged to scrutinize the CLA reports to be submitted ( this is on top of AE and cannot be the same person)

- Yearly Internal Audit to ensure that the below are in place to be done internally. 

- 2 Yearly Audit by an External Agency. (Similar to the internal audit, except that its called a Technical Audit. Requirements for the audit is a long list of reports, as you can see below)

- All related Visual Inspections and NDT shall be carried out by an SAC accredited Inspection Body(IB)
- All related Calibrations shall be carried out by an SAC accredited laboratory

- Scheme For Guaranteed Safe Use (SGSU) to be developed, this is basically the equipment inspection plan, company safety procedure, plant process description, brief maintenance and operation history of equipment to be captured, equipment datasheets & drawings, organization chart of key parties, 

- Produce to auditors yearly eqpt remaining life calc, BFW pump condition monitoring logs, BFW treatment logs, steam blowdown logs, PSV test summary and certs, documents pertaining to RBI if any, past CLA audit reports if any, past CP report if any.

- The above requirement may vary, depending on what is found in the approved SGSU submitted and approved. So do submit only what is within your means and resources. Just follow the SGSU and you will be fine, if not you may be fined or removed from CLA Scheme!

Repairs

All Repairs needs to be informed by writing/emailing to MOM directly for approval by commissioner before appointing an AE for the endorsement of repair. 

Method statement of fabrication, inspection and testing plans will need to be submitted for endorsement by AE and review by MOM. Repair requirements per API 510, NBIC or PCC-2 has so far been accepted, mostly subjective to decision of the AE. 

3.2 Valve Spec & Purchase

What Are Valve Specifications?

Similar to piping specification, but provides you the guide to what those valve commodity codes on the piping specification means.

Typical information includes:
1) Commodity code
 Commodity codes are usually a long string of text for example the below I have seen:

*VOEACCM

*VAACE6PM

*CVV1137YY

These codes is how Company or Contractor label their valves in a systematic manner based on the specifications below.

2) Type of service
The service fluid the valve is recommended for.

3) Materials of construction

 Material of construction of the body, stem, handwheel, disc, seat. 

Type of body constructions for example forged, casted, machined, etc.. (Body commonly casted)

3.1)Type of Trim
This is the material specification of the disc and seat. Read API 600/602 for a list of trims typically carried by vendors and their applications. They range from a number from 1 to 12 and this numbering is not in order of anything.

Trims can come in hard or soft material. Where Hard here refers to hard facing material such as stellite and martensite stainless steels (400 series) and soft refers to the usual 316. There are other classes of trims non-API for example resilient soft seats used in harsh chemical service in typical resistant materials such as PTFE, PEEK, VITON and many more, theses resilient soft seats are usually much susceptible to damages and would require increased frequency and cost of maintenance.

3.2 ) Packing construction

*Packing compression type: with or without Bellville springs 

Teflon or other Polymer packing

Steel wire reinforced graphite yarn packing 

Or a mixture of graphite yarns with pure graphite rings (Harsh applications)

4) Type of Valve construction for eg.
Ball - 3 piece bolted, 2 piece screwed
Globe/Gate - bolted bonnet, screwed bonnet, welded bonnet, pressure seal
Butterfly - wafer type, lug type
Check - Piston, swing disc, lift disc, butterfly double disc, hoerbiger type

5) Flow Pattern
High/Low CV
Full bore / Reduced bore.
2 way, 3 way, 4 way flow.

6) Valve Testing and Inspection
Most common testing and inspection standards for new and repair of valves will comply to API 598.
Refer to it for exactly what needs to be tested and inspected to verify that a valve is fit for service. It fulfils the basic requirement to ensure valves qualities.

Purchasing Issues

Some thing you might ponder about during purchases:

Why are certain brands so significantly cheap / expensive? 

Let us break down a valve into its individual components to analyze. When you breakdown to this level you would realize each valve is intricate when you perform the installation and maintenance. 

However from an inexperienced purchaser/piping designer point of view, they will consider a valve as "it's only a valve." without realizing valves are part of a major problem in a plant should it be of inferior quality, Potential problems we might face will be mentioned in  Section 3.7 Valve Repairs.

Potential areas observed that manufacturers saves on that are less investigated by purchasers/piping designers:

* Packing design and material (Potentially results in low MTBF, frequent leaks)

* Body material (Composition of ingot material from inferior source not meeting composition req'd)

* Casting method (Casting method/flow plays great importance in strength of material)

* Quality Control (Despite documentation, hidden lapses and doctored inspections would still be present from some overseas manufacturers especially in China/India where QC seems haphazardly done. Nevertheless, I have still seen quality products from China and India, of course at a higher price. So nationality of these valves shouldn't be stereotyped, but rather the quality control process and how the purchaser would like to manage these issues should these lower cost destinations needs to be specified.

Point at the end of the day... You usually pay for what you get.

5.4 Plant Maintenance: Valve Repair

Valve Repair

Repair of valve is a pretty straightforward activity to perform.

Valve repairs are very common especially on systems handling steam and corrosive fluids. Beware of steam cuts, it can grow from a condensate droplets to a huge mist cloud in few days. The longer a leak is withheld without repair, the higher the maintenance activity.

1. Various forms of valves problem:

a) Leaking packing gland
Major cause for this problem is corrosion, aging and steam. (Top & Bottom Left Photo)

b) Leaking flange
Major causes of this problem is corrosion, aging, steam, misalignmemt and unsuitable gasket.

c) Leaking bonnet
Major cause of this problem is same as leaking flange, as this item is a flange! (Top & Bottom Right Photo)

d) Passing
Major cause of this problem is unsuitable trim design, aging, improper usage caused by operators with a torque wrench.

e) Knocking and vibration
Major cause of this problem is a
* Design flaw, most likely a stop valve with check valve function is installed and flow rate is at natural frequency of the system.  Or..
* Unsuitable valve for draining/venting/gage installednear a reciprocating equipment which operates at its natural frequency.

Contractor design manual usually covers this, where to locate the drain/vent valves for which size and pipe thickness. Fatigue cracking will come after those who do not review designers work!

f) Corrosion
Major cause of this.. Process spills nearby, leaking flanges, leaking fitting on valve body, located near the sea or a corrosive area. (Top & Bottom Right Photo)

2. And now how to repair ? 

a) Packing gland leak

1) First thing to do will always be to tighten it, however there is always a limit to tightening, eventually there will be no packing left. Sometimes the packing tight bolts are jammed or corroded, so If all fails.. Isolate the line, put in PTW and the usual stuff.

Chesterton 1600 Series Packing Yarn (2016)

Source: Chesterton 1600 Valve Packing Webpage

2) Remove packing with a gland extractor, Replace the gland with original spares or typical repair "yarn" available in most gland material. Use compatible material ie. Teflon for corrosive or cryogenic lines, use graphite with inconel braiding type for hot systems.
* Note that some valves especially pressure seal valves uses a special sandwich packing system comprising of graphite yarn + pure graphite configuration for improved sealing and operatibility.

3) Another way to repair a valve with leaking gland is to tap a hole on the bonnet and inject a sealant. The sealant usually lasts between 2 to 5 years. A more expensive option compared to packing change, if you cant stop the system and leaking fluid from the gland is a huge serious issue in your plant.

4) Some valves have different type of sealing ie. Belleville spring, pressure seal. Refer to IOM for proper repair procedures, original spares are usually recommended for repairing such valves.
I observed a shoddy pressure seal valve repair by lapping the steam cut area and boxup. This clearly doesn't work. Do it good, do it right.

5) If it still keeps leaking upgrade the packing to Chesterton Valve Sealing System 5800 and equivalent. This system basically upgrades your packing to the sandwich type mentioned above and adds live loading spring washers to packing bolts for thermal stress compensation. See picture.

Chesterton 5800 Series Packing Kit (2016)

Source: Chesterton Valve Packing Repair Kit Webpage


b) Leaking flange
1) Same as above, first remedy is usually to tighten. But wait.. Is the bolt head already corroded and rounded? Is the flange still leaking despite tightening, If yes..

2) Isolate the line and break the flange. Inspect the flange and gaskets.

3) Is it a gasket problem ie. Wrong size, material type? Pls install the right gasket!

4) Is it a flange problem? Wrong rating, corroded flange face.

4.1) Corroded flange face can be machined down with onsite machining to achieve required finishing to B16.5 about 250um rms. However do check thickness of flange to ensure u have suffice material.

4.2) What if you cant machine? Use gasket sealant paste, these usually works well if you use good brand like Permatex form A.  Not endorsing any brands here but this is what we have.

4.3) Rarely I face the problem of a incorrectly rated flange. But it is still a possibility. Check the design, operating pressure/temperature vs the flanges. Corrosion of flange face and gaskets are more of a harassing issue which leads to leaking flanges.

c) Leaking bonnet
Refer to leaking flange.

d) Passing
1)Simple.. Change the valve

2) what if its welded or your on a budget constraint ?
a) inspect it and if its in trouble
b) perform a weld build up repair
c) change out the entire trim/bonnet assembly.
These usually fixes the problem, however if they keep returning let say every year, you know there is a problem with the trim material, valve brand or even body design. I have seen some design which promotes crevice corrosion and some design which fails so frequently but others that hold up so well. Do accept a brand change sometimes, its good.

e) Knocking/Vibration
Unfortunately there is no quick fix to this. Redesign the pipe connections or system

d) Corrosion
- Brush it and paint it

- If it keeps pestering you, the material is likely unsuitable, talk to management for an upgrade, usually they would prefer if you brush and paint it though it keeps you busy and may cost more in the long run.

- For the equipment near process spills, my plant had successfully protected them by building mini shelters over where corrosive fluids tends to attack valves, control valves, junction boxes. We build them with insulation cladding, they are extremely cost effective.

- Some valves comes with relief vent ports or hinge pins providing an escape way for process fluid, do expect them to leak overtime and suggest to perform Corrosion Under Insulation checks by creating a window on the insulation at where these points locate, and of course not forgetting to create the Inspection schedule.