Pressure Vessel Basics (Part 2)

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This is the continuation and the final installment of our Pressure Vessel Basics.

 

In this part, we will talk about the ff:

– Different types of flanges used in a pressure vessel

– General types of vessel heads

– Definition and uses of different pressure vessel internals including the difference of trays vs packings.

– Internals and Externals Parts of a Pressure Vessel

 

There are various types of flanges used in the oil and gas industry. For this particular topic, listed below are the common types installed on pressure vessels.

 

a. Welding Neck – it consists of a circular fitting with tapered hub flange to match pipe bore where it is then butt-welded. These are normally made from forged materials.

welding neck
b. Slip-On Flange – much like the welding neck except for absence of the tapered hub, and has a larger bore to fitthe pipe. This flange is fitted over the pipe and then fillet welded on outside and inside of the flange.

slip-on welding


c. Socket-weld Flange – This flange is counter-bored, fitted to accept the pipe and fillet welded at the outside of the flange only. However, before welding sufficient space must be created between the pipe and the flange. One disadvantage of this is the gap which might pose a problem for some service.

socket welding

 

d. Lap-Joint Flange – this flange is always used with a stub end. The flangeis not welded to the stub which makes it easier to assemble and aligned. Normally used in low pressure and non-critical applications.


lapped joint

e. Threaded Flange – this type does not require any welding to be performed. However, in some cases, a seal weld is also performed afterwards. Normally used in low pressure and non-critical applications.

 

threaded

f. Blind Flange – used to blank off pipeline ends, valves and vessel nozzle openings.

blind

 

 

Manhole/Manway – this is an opening in the vessel which is used to access the internals of a vessel. Normally 24” diameter in size, manways manwayare almost always required to be present in every vessel where frequent maintenance inside is envisaged. Manways are covered with a blind, gasket, bolts and nuts. The cover is supported by a hinge or davit for ease of handling.

 

Inspection Openings or Hand holes – where manways/manholes are not physically feasible due to size or process constraint, hand holes or inspection openings are the second choice. This is an opening smaller than the manway, normally 12” in diameter and used for visually checking the inside of a vessel.

 

3) Vessel Shell

This is the cylindricvessel shellal portion of the vessel. This is normally made from rolled steel plates. Thickness of the shell is calculated based on design process parameters to hold the pressure inside the vessel. Tangent to tangent length and diameter size also varies on the design requirements. For small diameter vessels, seamless pipes can also be utilized.

 

 

4) Vessel Heads

This serves as the end caps or enclosures joined at both ends of the shell. Ellipsoidal head, hemispherical head and torispherical head are the most common ASME pressure vessel dished heads used in the industry.

 

a) Ellipsoidal Head – also called as 2:1 elliptical head. This resembles an ellipse with varying radius that results to a smooth transition from the head to the cylindrical part of the vessel. The height of the head is equal to the quarter of its diameter. This is the most common type used in the industry and is touted as the most economical.

Semi Ellipsoidal head

b) Torispherical Head – this has a dish with a fixed radius and a transition section to the cylindrical part called “knuckle radius”.

torispherical head

 

 

c) Hemispherical Head – This type is used for high pressure rating. The pressure acting on the head is equally distributed, hence, this makes it the ideal shape but is more expensive than the ellipsoidal head. The radius of the head is equal to the radius of the cylindrical shell.

hemispherical head

 

5) Pressure Vessel Internals

Vessel internal varies differently based on its intended service.  While some vessels require a lot to perform certain processes, others have the minor and basic ones only. Listed below are some of the most common pressure vessel internals used in the industry.

a) Demister – is a device used on a vapor-liquid separator vessels to enhance the separation of liquid droplets entrained in the vapor steam. The demister removes micron-size droplets entrained in the vapor. This works by coalescing smaller droplets by obstructing their path when they enter the demister which is normally made of wire mesh. Unlike vapor, droplets gets entrained through the demister and causes collision with other entrained smaller droplets to form bigger droplets. Bigger droplets becomes too heavy and large to rise with the vapor and pass thru the demister so it drops down the vessel and gets collected.

demister

 

b) Inlet distributor – a device used to equally distribute liquid/gas mixtures into a separator vessel or column. On packed columns, liquid distribution is crucial for its performance and so these are normally designed by specialist vendor to achieve optimum output.

 

c) Trays/Packings – packings sometimes can be randomly filled materials like Raschig rings or structured material that is used to improve contact between two phases in a chemical process. This is usually used in a chemical reactor, a scrubber or distillation column.

Trays are used for the same purpose which is to have a contact between two phases to separate them. Given for example is a Glycol Contactor which is used to remove any water content in the natural gas by introducing lean glycol in the system.  Lean glycol is pumped on the top of the trays which floods the tray and flows down through the downcomer sections of each tray. While this is happening, the natural gas is introduced at the bottom of the column and rises through the bubble caps and disperses as bubbles through the lean glycol on the trays. This is where the contact and separation between the two phases happens. The lean glycol is highly hygroscopic which absorbs the water vapor in the gas.

 

GDU

tray

 

d) Vortex breakers – a device used to stop formation of a vortex. These are normally located at the outlet nozzle of the vessel leading to a pump. Formation of vortices can entrain vapor in the liquid stream which can lead to cavitation of the pumps. These are typically made of crossed plates welded to the inside of the vessel at the outlet nozzle.

 

vortex breakervortex breakers

packing vs tray

packed column

 

6) Pressure Vessel Externals

These are external attachments welded to the vessel for to do its intended purpose. Below are some of the most common external attachments:

nameplate and brackets

a) Nameplate & brackets – this shows the identification of the vessel indicating various parameters and information. Nameplate is either welded or riveted on the bracket which is permanently fixed on the vessel. Normally located near the manways where it is conspicuous. Nameplates are marked and stamped by manufacturers in accordance with the design code used such as ASME. Nameplates are made of corrosion resistant material such as 316SS while the bracket is made of same material as the vessel where it is attached.

 

b)Lifting & Tailing lugs – these are pieces of metal welded on the vessel to facilitate erection / lifting of the equipment. Normally designed with lifting factor of 1.5 or 2 x the lifting weight.

linfitng and trailing lugs

 

c) Platform and Ladders – Ladders are steps consisting of two parallel members connected by rungs. Used for vertical access to the platforms. Usually with cage to prevent the man from falling; Platform is an elevated flat surface or stage for maintenance and access. Platform is fabricated in sections.

platform and ladders

 

 

d) Stiffener rings – these are rings made of flat bar, plate and various structural shapes welded around the circumference of the vessel shell. These rings are installed on vessels in such cases where the resulting thickness due to internal/external pressure is too thick. Placing a stiffener ring reduces the effective length of the vessel shell which in turn reduces the shell thickness and makes it economical / practical. For tall columns, it is a practice to put multiple stiffener rings. This will also help to prevent the collapse of the vessel during vacuum conditions.

 

e) Pipe Support and guides supports and guides for attached piping that are bolted to clips, which are welded to the vessel.

 

f) Column Davit – a hoisting device attached to the top most part of the vessel. Used for handling internals and piping during installation.

 

 g) Earthing Lugs – a piece of metal that connects a grounding conductor to a grounding electrode. Usually made of 304 SS material.

 

 h) Insulation and Fireproofing Rings – Fireproofing rings are pieces of metal bolts welded on skirt or saddle to facilitate application of fireproofing; Insulation rings are made of flat or angle bars attached around the circumference of the vessels. Used to support the weight of the insulation.

 

SUMMARY:

We do hope you have enjoyed what we have prepared for you.  Just took us quite a while to release the final cut of this article but we’re quite happy that we have finished it. Please leave a comment and tell us what you think of it or if we miss something. Also, please do us a small favor and share it to other Filipinos that are in this industry who can benefit from it.

Everyone might have been very busy the whole week, and so let us end this week with a quote. Thank you for reading!

“Patience, persistence and perspiration make an unbeatable combination for success.” ~ Napoleon Hill

 

Photo credit to Supakitmod of freedigitalphotos.net

 

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