5.1 Plant Maintenance: Static Equipment and typical maintenance jobs

What are Static Equipment?
Basically these are the equipment that do not have a prime mover driven rotating component will be considered a static equipment, and here's some example with what sort of Offline/On-line preventive maintenance which are usually carried out on the static equipment. Corrective maintenance will be discussed at the end of this post, as it is what we wish to avoid.

Piping & Typical PM jobs: 
- Thickness monitoring(5y or less, depending on code used API570:Piping or API580:RBI)
- Visual Inspection (1y or less)
- Insulation reconditioning (When necessary)
- Painting (1y or more, depending on visual inspection)
- Pressure testing (Usually 10y or less, if critical or national code requires)

Source: Wikipedia


Pressure Vessel & Typical PM jobs: 
- Thickness monitoring(10y or less depending on code used API510:PV or API580:RBI)
- External Visual Inspection (1y or less)
- Internal Visual Inspection (half est. remaining life of the vessel, if remaining life is less than 4y internal inspection shall be every 2y)
- Insulation reconditioning (When necessary)
- Painting (1y or more, depending on visual inspection)
- Pressure testing (Usually 10y or less, if critical or national code requires)
- Types of inspection may vary if the vessel operates in potentially damaging susceptible services

Source: Wikipedia


Above ground tanks & Typical PM jobs: 
- Thickness monitoring(10y or less depending on code used API653:AGT or API580:RBI)
- External Visual Inspection (1y or less)
- Internal Visual Inspection (12y or less, unless with internal lining or leak detection installed)
- Insulation reconditioning (When necessary)
- Painting (1y or more, depending on visual inspection)
- Types of inspection may vary if the vessel operates in potentially damaging susceptible services

Source: Wikipedia


Heat exchangers & Typical PM jobs: 
- Thickness monitoring(10y or less depending on code used API510:PV or API580:RBI)
- External Visual Inspection (1y or less)
- Internal Visual Inspection (half est. remaining life of the vessel, if remaining life is less than 4y internal inspection shall be every 2y)
- NDT Inspection, WFMPI, eddy current, ultrasonic leak test (when leaks are suspected, or when service is critical, in corrosive media)
- Insulation reconditioning (When necessary)
- Painting (1y or more, depending on visual inspection)
- Pressure testing (Usually 10y or less, if critical or national code requires)
- Types of inspection may vary if the vessel operates in potentially damaging susceptible services

Source: Wikipedia


Distillation/Absorption columns & Typical PM Jobs
- Thickness monitoring(10y or less depending on code used API510:PV or API580:RBI)
- External Visual Inspection (1y or less)
- Internal Visual Inspection (half est. remaining life of the vessel, if remaining life is less than 4y internal inspection shall be every 2y)
- Insulation reconditioning (When necessary)
- Painting (1y or more, depending on visual inspection)
- Pressure testing (Usually 10y or less, if critical or national code requires)
- Tray/Packing replacement (Usually 2y or more, depending on manufacturer design or efficiency)
- Types of inspection may vary if the vessel operates in potentially damaging susceptible services

Source: Wikipedia


Reactors & Typical PM Jobs
- Thickness monitoring(10y or less depending on code used API510:PV or API580:RBI)
- External Visual Inspection (1y or less)
- Internal Visual Inspection (half est. remaining life of the vessel, if remaining life is less than 4y internal inspection shall be every 2y)
- Insulation reconditioning (When necessary)
- Painting (1y or more, depending on visual inspection)
- Pressure testing (Usually 10y or less, if critical or national code requires)
- Catalyst replacement (Usually 2y or more, depending on manufacturer design or efficiency)
- Types of inspection may vary if the vessel operates in potentially damaging susceptible services

Source: Wikipedia

Separators vessels & Typical PM Jobs
- See Pressure vessel above.

Source: Wikipedia


Steam traps & Typical PM Jobs
- Downstream steam check (1y or less, Live steam (faulty), lazy steam(working))
- Ultrasonic testing (1y or less, functionality check)

Source: Spirax Sarco

Manual Valves/Pressure relief valves & Typical PM Jobs
- These are usually done offline during shutdowns, unless the valves can be positively isolated for removal.
- Refacing of valve seat/trim
- Repacking of seal
- Replacement of corroded/ parts
- Overhaul of soft kit/spring/actuator/etc
- Overpressure test (MV), Set pressure test (PRV)

 

Source: Wikipedia

Note: About positive isolation - Positive isolation is done for safety reasons, usually either by closing two valves both up/downstream OR closing one valve with a blind installed both up/downsteam, to prevent any passing process media from escaping when maintenance works are carried out.

Valves are usually neglected by operators, they operate these valves using pipe wrench of various sizes. The huge torque applied with this leverage tool usually will speed up damage of the seats and as a result we experience passing of process media. Therefore, positive isolation is good practice to protect maintenance workers.


Filters/Strainers
- Differential pressure monitoring (Frequently)
- Backflushing or cartridge replacement(See manufacturer's recommended interval and action)

Source: Spirax Sarco


What sort of Plant maintenance should be carried out?
Preventive Maintenance
These type of maintenance are pre-scheduled on the plant's CMMS (Computerized Maintenance Management Systems).  The benefits of having a preventive maintenance program is that, there will always be some work for the contractors to carry out (be it to clock time sheet or earn earn extra cash??) Okay, the real benefit is that the manufacturer/code/national recommended maintenance intervals can be practised and followed up, so as to prolong the equipment life.

Types of preventive maintenance for Static equipment
- As discussed above at the top of this post.
- This type of maintenance are carried out offline or on-line, depending on the type of plant operations adopted or code requirements which may or may not permit carrying it out on-line.



Corrective Maintenance
These type of maintenance are usually carried out because preventive maintenance is not well practised, due to natural causes, design flaws, operating not according to manufacturer's prescribed conditions and many more reasons.

Types of corrective maintenance for Static equipment
- Box-in/Leak-sealing of pinhole leakage on pipe/vessel/weldments
- Pipe Spool replacement of piping due to the above
- Vessel/tank repair by cladding, shell course replacement, lap patch, plate lining due to thinning
- Steam trap replacement due to malfunction
- Heat exchange tube plugging due to tube leakage
- Unplugging of catalyst/packing/trays/filters by manual intervention


*Source: All images are from Wikipedia.org, except steam traps from Spirax Sarco. All photos are taken from open sources educational sites which allows for their material to be shared. Credits are given to the rightful image owner in this statement. Please drop me a comment should you be the rightful owner of the image and would like to request compliance for credit to be given to you directly.

4.2 Welding rod, filler wires and materials

Different welding process uses different types of filler material to aid the mechanism. A brief description to the types of filler rod/wire they use, to have a visualization of how they are fitted for the actual welding process, watch the videos from post 4.1 Welding terminologies.

SMAW - Uses a generally thicker rod with flux. Flux help produce shielding gas, adds alloying element to strengthen weld, reduces cooling rates when doing each pass. Manual feeding of weld stick is required.

Electrodes are usually classified in 4 numbers E-XXXX or E-6015
First two numbers refers to the yield strength in kpsi, 60 refers to 60k psi.
Third number refers to the welding position --> 0, All position. 1, Horizontal & flat
Fourth number refers to the electrode type

 There are 3 main types of electrodes based on the Fourth number:
a) Cellulose - No. 0, 1. High moisture content, good for joining thick sheet metal
b) Rutile - No. 2,3,4. Good for thin sheet metal.
c) Basic  - No. 5,6,8. Low hydrogen type, requires oven or hot box. Good for eliminated hydrogen embrittle cracking, when moisture is broken down by the high temperatures.

FCAW - Uses a hollow filler wire, filled with flux at the core. Basically same properties as above, however it comes in a wire coil, filler wire is automatically dispensed, no need for manual feeding.

GMAW - Same automatic feeding of filler wire like FCAW, but uses a shielding gas and no flux wires are required.

GTAW - Uses a thin and long filler rod without flux. The stick is hand held and melted with the tungsten torch.

Electrode sizes

These rods & wires comes in different sizes as well, usually depending on the application of weld:

1) Weld build up, usually use a thicker rod to speed up the process

2) Welding small pipes, use a small rod for better control 

3) Welding larger pipes, use a thicker rod to reduce weld time.

Benefits of thick filler rod

- Improve speed of material deposition

- Reduce number of passes (improves speed, good for weld build up jobs)

Benefits of thin filler rod

- Better control of welding process

- Increased number of passes 

Filler Rod Materials

Filler wire should preferably match the material of the base metal, in cases where dissimilar metals needs to be joint there are special application filler wires available. Here are some examples but not all listed for common stainless steels:

Base Metal A	Base Metal B	Recommended Filler material
SS301/302/303/304/305	SS301/302/303/304/305	SS308L
SS304/SS316/L	A106/A53, CS	SS309L
SS301/302/303/304/305	SS409	SS309L
SS301/302/303/304/305	SS316L	SS309L
SS316L	SS316L	SS316L

SS309L, a very versatile filler material, good for joining dissimilar stainless steels. However it contains no molybdenum which SS316L requires.

Stainless steel 316 has high carbon content and is not recommended as SS316L for welding. Due to the high carbon content, carbon tends to combine with chromium at grain boundary and the depletion of elemental chromium leads to localized corrosion. Strongly recommend SS316L, if the component needs to be welded and 316 material needs to be used.

Hardfacing, whenever repairs are required to a surface which requires increased hardness for erosion resistance, for example metal valve trims or injector tips, hardfacing electrodes such as Stellite is recommended. Cobalt is the main ingredient to give the extra hardness.

4.1 Welding Terminologies

Welding is very important in the industry. It's process is often taken for granted, as not many people know them well, especially for those who need to work with pipes but not on the fabrication floor. By extensively understanding these concepts, it improves communication between the various teams which in turn improves weld quality. Welding is the most commonly used joint in all process piping connections around the world. It can join similar materials, ferrous or non ferrous(brazing) or even dissimilar materials.

In this post, welding terminologies would be run through to familiarize with welding before proceeding further and more in dept into welding processes, discontinuities and other information.

Why weld pipes?
- Superior strength compared to other
- Proven and documented methods of joining which controls welder quality, process control and material quality.
- Welding is required to move forward to using other joints such as union joint and flange joint.

Different ways of specifying welds by different trades (Design Engineer vs Welding engineer)
Types of pipe weld normally specified by Design Engineers on Weld Map drawings
Butt weld - direct butt to butt join
Socket weld - fillet weld to a pipe connected to a recessed fitting
Field Weld - pre tack welded, allowance for field modification usually based on tolerance usually of about -+100mm
Tack Weld - pre weld, to join pipes before fully welded on the field.

Types of pipe weld normally specified by Welding Engineers on Welding Procedure Specification (WPS)
Bevel-groove weld
V-groove weld
J-groove weld
U-groove weld
Square groove weld
Fillet weld



Welding terminologies

Discontinuities
A flaw on the weld not desired, may be acceptable or unacceptable by inspectors depending on welding code specifications. Discontinuities are allowed, as long as they meet code requirements. The types of discontinuities described in AWS codes will be described in another post.

Defects
A flaw on the weld unaccepted by welding code specifications. Defects shall be rejected.

Base metal
The material which the welder is expected to join.

Filler metal
The material which the welder applies to the base metal.

Weld metal
- A new metal formed by melting of base + filler metal.
- The reason why PQR is required to verify strength. Different material batch produces different weld strength.

Flux
- Provides the following to the weld process: shielding gas, interpass cooling temperature control by slag, reinforcement alloy to weld metal
- Present in SMAW & FCAW.
- Can cause inclusion discontinuity

Slag
By-product of burning flux, needs to be remove before adding additional weld passes.

Weld Passes
The number of times the weld stick needs to pass between the gap to be welded. One who has never welded a pipe would think that one weld would close the gap, but it all depends on pipe size and thickness. For thicker pipes, more passes would be required usually in this manner root pass --> hot pass --> fill pass --> cover pass. On smaller pipes, less passes would be required.

Bevel size & angle
Size of slope for bevel, V, J, U groove welds and its angle. The lesser the angle, the less filler material is consumed.

Steeper angles for Bevel/V, usually around 37deg (Usually on process pipes)
Less Steep angles for J/U, usually around 20deg. (Usually on long distribution pipelines to save time and cost)

Groove face
Face of the bevel.

Root face
Face of the root.

Root gap
The gap between the base metal. Root gap depends on thickness of weld metal.

Weld toe
The top side of weld where filler metal and base metal touches.

Weld root
The bottom side of weld where filler metal and base metal touches.

Weld reinforcement
The height of the weld protrusion on top side of weld. Excessive reinforcement is a discontinuity, as it weakens the joint at the weld toe, as sharp corner induced stress.

Weld Face
The face of the weld looking from top side of the weld.

Root reinforcement
The protrusion on bottom side of weld. Excessive reinforcement is a discontinuity, as it weakens the joint at the weld root, as sharp corner induced stress.

Weld interface
The surface on base metal where filler metal are deposited.

Heat Affected Zone
A section on the base metal parallel to the fusion zone. This section is slightly harder which increases likelihood of embrittement cracking, the reason why pre-heating and PWHT are required, to slow down the annealing(cool down) which reduces hardness.

Fusion Zone
The area where new metal is situated after welding.

Fusion Face/Fusion Line
Same as weld interface, but a name used for when welding has complete.

Source: Weldguru.com

Hydrogen induced cracking
When moisture content are present during welding, they break down under high temperature of approximately 3000 - 3500degC at the arc, hydrogen is produced and entrapped in the weldment. This will induce porosity and subsequently cracking.

Filler rod/Electrode
Commonly known as welding stick, consumable and many more, it transmit electricity to heat up the tip of electrode which melts it along with the base metal.

Weld cup
Commonly for gas welding, purpose of the cup is to reduce welder handshake during weld and is a nozzle for shielding gas. Filler wire is fed from the cup for GMAW/FCAW, no filler wire for GTAW.

SMAW
Shielded Metal Arc Welding or Stick welding, a type of manual welding which uses electrical arc current to melt the filler stick(electrode) and base metal.

Here's a clear visual explanation from ChuckE2009 of how it is done.

Source: ChuckE2009


GMAW
Gas Metal Arc Welding/Metal Inert Gas welding is a type of automatic fed welding process which uses a flexible wire electrode. During welding, electrode is fed from the feeder along with shielding gas.

Here's a clear visual explanation from Weldingtipsandtricks of how it is done.

Source: Weldingtipsandtricks


FCAW
Flux Core Arc Welding is a type of automatic fed welding process which uses a flexible tubular electrode with flux within the tube core. It is similar to GMAW, except with a flux core and that that shielding gas may or may not be used.

SAW
Submerged Arc Welding is a type of automatic fed welding process, where flux in powder form is laid before filler rod passes through the joint.

Source: Redrockautomation

GTAW/TIG
Gas Tungsten Arc Welding or Tungsten Inert Gas welding is a type of automatic fed welding process which uses a tungsten non-consumable electrode on the cup. A separate filler rod (without flux) is manually fed to the joint together while the tungsten electrode is fired.

Here's a clear visual explanation from Weldingtipsandtricks of how it is done.

Source: Weldingtipsandtricks



X-RAY TEST
X-ray tests may or may not be conducted after welding to verify weld size for integrity and soundness, it usually depends on design parameter Ew (Welding efficiency) specified by design engineer.

PQR
Procedure Qualification Record. This is a record of the welders ability to perform certain type of welding process and the type of materials joined. Various tests are performed on the test coupon to verify the integrity and soundness of the joint
- Bend test
- Tensile test
- Ductility
- Hardness (Rockwell)
- Toughness (Charpy V-notch)

WPS
Welding Procedure Specification describes how the welding process will be performed. This specification needs to be backed up and referenced from the PQR. Welding inspectors will refer to these two documents to verify the weld preparation, interpass preparation and post welding inspection activities.