INTRODUCTION TO INTRODUCTION TO PRESSURE PRESSURE VESSELSVESSELS

PRESSURE VESSELSPRESSURE VESSELS

Pressure vessels are the containers for fluids Pressure vessels are the containers for fluids under high pressure.under high pressure.

They are used in a variety of industries likeThey are used in a variety of industries like

Petroleum refiningPetroleum refining Chemical Chemical PowerPower Food & beverageFood & beverage PharmaceuticalPharmaceutical

TYPES OF PRESSURE VESSELSTYPES OF PRESSURE VESSELS

There are three main types of pressure vesselsThere are three main types of pressure vessels

in generalin general

Horizontal Pressure Vessels Horizontal Pressure Vessels Vertical Pressure VesselsVertical Pressure Vessels Spherical Pressure vesselsSpherical Pressure vessels

However there are some special types of Vessels like However there are some special types of Vessels like

Regeneration Tower, Reactors but these names are given Regeneration Tower, Reactors but these names are given

according to their use only.according to their use only.

HORIZONTAL PRESSURE VESSEL HORIZONTAL PRESSURE VESSEL

VERTICAL PRESSURE VESSELVERTICAL PRESSURE VESSEL

The max. Shell length The max. Shell length to diameter ratio for a to diameter ratio for a small vertical drum is small vertical drum is about 5 : 1about 5 : 1

TALL VERTICAL TOWER TALL VERTICAL TOWER

Constructed in a wider Constructed in a wider range of shell diameter range of shell diameter and height.and height.

They can be relatively They can be relatively small in dia. and very large small in dia. and very large (e.g. 4 ft dia. And 200 ft (e.g. 4 ft dia. And 200 ft tall distillation column.tall distillation column.

They can be very large in They can be very large in dia. and moderately tall dia. and moderately tall (e.g. 3 ft dia. And 150 ft (e.g. 3 ft dia. And 150 ft tall tower).tall tower).

Internal trays are needed Internal trays are needed for flow distribution.for flow distribution.

VERTICAL REACTORVERTICAL REACTOR

Figure shows a typical Figure shows a typical reactor vessel with a reactor vessel with a cylindrical shell.cylindrical shell.

The process fluid The process fluid undergoes a chemical undergoes a chemical reaction inside a reactor.reaction inside a reactor.

This reaction is normally This reaction is normally facilitated by the presence facilitated by the presence of a catalyst which is held of a catalyst which is held in one or more catalyst in one or more catalyst beds.beds.

SPHERICAL PRESSURIZED SPHERICAL PRESSURIZED STORAGE VESSELSTORAGE VESSEL

MAIN COMPONENTS OF MAIN COMPONENTS OF PRESSURE VESSELPRESSURE VESSEL

Following are the main components of pressure Following are the main components of pressure

Vessels in generalVessels in general

Shell Shell HeadHead NozzleNozzle SupportSupport

SHELL SHELL

It is the primary component that contains the It is the primary component that contains the pressure.pressure.

Pressure vessel shells in the form of different Pressure vessel shells in the form of different plates are welded together to form a structure plates are welded together to form a structure that has a common rotational axis.that has a common rotational axis.

Shells are either cylindrical, spherical or conical Shells are either cylindrical, spherical or conical in shape.in shape.

SHELLSHELL

Horizontal drums have cylindrical shells and are Horizontal drums have cylindrical shells and are constructed in a wide range of diameter and constructed in a wide range of diameter and length.length.

The shell sections of a tall tower may be The shell sections of a tall tower may be constructed of different materials, thickness and constructed of different materials, thickness and diameters due to process and phase change of diameters due to process and phase change of process fluid.process fluid.

Shell of a spherical pressure vessel is spherical as Shell of a spherical pressure vessel is spherical as well.well.

HEADHEAD

All the pressure vessels must be closed at the All the pressure vessels must be closed at the ends by heads (or another shell section).ends by heads (or another shell section).

Heads are typically curved rather than flat.Heads are typically curved rather than flat. The reason is that curved configurations are The reason is that curved configurations are

stronger and allow the heads to be thinner, stronger and allow the heads to be thinner, lighter and less expensive than flat heads.lighter and less expensive than flat heads.

Heads can also be used inside a vessel and are Heads can also be used inside a vessel and are known as intermediate heads.known as intermediate heads.

These intermediate heads are separate sections These intermediate heads are separate sections of the pressure vessels to permit different design of the pressure vessels to permit different design conditions.conditions.

NOZZLENOZZLE

A nozzle is a cylindrical component that A nozzle is a cylindrical component that penetrates into the shell or head of pressure penetrates into the shell or head of pressure vessel.vessel.

They are used for the following applications.They are used for the following applications.

Attach piping for flow into or out of the vessel.Attach piping for flow into or out of the vessel. Attach instrument connection (level gauges, Attach instrument connection (level gauges,

Thermowells, pressure gauges).Thermowells, pressure gauges). Provide access to the vessel interior at MANWAY.Provide access to the vessel interior at MANWAY. Provide for direct attachment of other equipment Provide for direct attachment of other equipment

items (e.g. heat exchangers).items (e.g. heat exchangers).

SUPPORTSUPPORT

Support is used to bear all the load of pressure Support is used to bear all the load of pressure vessel, earthquake and wind loads.vessel, earthquake and wind loads.

There are different types of supports which are There are different types of supports which are used depending upon the size and orientation of used depending upon the size and orientation of the pressure vessel.the pressure vessel.

It is considered to be the non-pressurized part of It is considered to be the non-pressurized part of the vessel.the vessel.

TYPES OF SUPPORTSTYPES OF SUPPORTS

SADDLE SUPPORT:SADDLE SUPPORT:

Horizontal drums are typically supported at two locations Horizontal drums are typically supported at two locations by saddle support. by saddle support.

It spreads over a large area of the shell to prevent an It spreads over a large area of the shell to prevent an excessive local stress in the shell at support point. excessive local stress in the shell at support point.

One saddle support is anchored whereas the other is free to One saddle support is anchored whereas the other is free to permit unstrained longitudinal thermal expansion of the permit unstrained longitudinal thermal expansion of the drum.drum.

TYPES OF SUPPORTSTYPES OF SUPPORTS

LEG SUPPORT:LEG SUPPORT:

Small vertical drums are typically supported on legs that Small vertical drums are typically supported on legs that are welded to the lower portion of the shell.are welded to the lower portion of the shell.

The max. ratio of support leg length to drum diameter is The max. ratio of support leg length to drum diameter is typically 2 : 1typically 2 : 1

Reinforcing pads are welded to the shell first to provide Reinforcing pads are welded to the shell first to provide additional local reinforcement and load distribution.additional local reinforcement and load distribution.

The number of legs depends on the drum size and loads to The number of legs depends on the drum size and loads to be carried. be carried.

Support legs are also used for Spherical pressurized storage Support legs are also used for Spherical pressurized storage vessels.vessels.

Cross bracing between the legs is used to absorb wind or Cross bracing between the legs is used to absorb wind or earth quake loads.earth quake loads.

TYPES OF SUPPORTSTYPES OF SUPPORTS

LUG SUPPORT:LUG SUPPORT: Vertical pressure vessels may also Vertical pressure vessels may also

be supported by lugs.be supported by lugs.

The use of lugs is typically limited The use of lugs is typically limited to pressure vessels of small and to pressure vessels of small and medium diameter (1 to 10 ft) medium diameter (1 to 10 ft)

Also moderate height to diameter Also moderate height to diameter ratios in the range of 2:1 to 5:1ratios in the range of 2:1 to 5:1

The lugs are typically bolted to The lugs are typically bolted to horizontal structural members in horizontal structural members in order to provide stability against order to provide stability against overturning loads.overturning loads.

TYPES OF SUPPORTSTYPES OF SUPPORTS

SKIRT SUPPORT:SKIRT SUPPORT:

Tall vertical cylindrical pressure vessels are typically Tall vertical cylindrical pressure vessels are typically supported by skirts.supported by skirts.

A support skirt is a cylindrical shell section that is welded A support skirt is a cylindrical shell section that is welded either to the lower portion of the vessel shell or to the either to the lower portion of the vessel shell or to the bottom head (for cylindrical vessels).bottom head (for cylindrical vessels).

The skirt is normally long enough to provide enough The skirt is normally long enough to provide enough flexibility so that radial thermal expansion of the shell does flexibility so that radial thermal expansion of the shell does not cause high thermal stresses at its junction with the not cause high thermal stresses at its junction with the skirt.skirt.

THIN WALLED PRESSURETHIN WALLED PRESSURE VESSELS VESSELS

Thin wall refers to a vessel having an inner-radius-to-wall-Thin wall refers to a vessel having an inner-radius-to-wall-thickness ratio of “10” or more (r / t ≥ 10).thickness ratio of “10” or more (r / t ≥ 10).

When the vessel wall is thin, the stress distribution throughout its When the vessel wall is thin, the stress distribution throughout its thickness will not vary significantly, and so we will assume that it thickness will not vary significantly, and so we will assume that it is uniform or constant.is uniform or constant.

Following this assumption, the analysis of thin walled cylindrical Following this assumption, the analysis of thin walled cylindrical and spherical pressure vessel will be carried out. and spherical pressure vessel will be carried out.

In both cases, the pressure in the vessel will be considered to be In both cases, the pressure in the vessel will be considered to be the gauge pressure, since it measure the pressure above the gauge pressure, since it measure the pressure above atmospheric pressure existing at inside and outside the vessel’s atmospheric pressure existing at inside and outside the vessel’s walls.walls.

THIN WALLED PRESSURETHIN WALLED PRESSURE VESSELS VESSELS

The above analysis indicates that an element of material taken The above analysis indicates that an element of material taken from either cylindrical or spherical pressure vessel is subjected to from either cylindrical or spherical pressure vessel is subjected to biaxial stress, i.e. normal stress existing in only two directions.biaxial stress, i.e. normal stress existing in only two directions.

Actually material of the vessel is also subjected to a radial stress, Actually material of the vessel is also subjected to a radial stress, σ3,σ3, which acts along a radial line. This stress has a max. value which acts along a radial line. This stress has a max. value equal to the pressure p at the interior wall and decreases through equal to the pressure p at the interior wall and decreases through the wall to zero at the exterior surface of the vessel, since the the wall to zero at the exterior surface of the vessel, since the gauge pressure there is zero.gauge pressure there is zero.

For thin walled vessels, however, the redial stress components are For thin walled vessels, however, the redial stress components are ignored because r / t = 10 results in σ1 & σ2 being, respectively, 5 ignored because r / t = 10 results in σ1 & σ2 being, respectively, 5 & 10 times higher than the max. radial stress, (& 10 times higher than the max. radial stress, (σ3)max = pσ3)max = p

THIN WALLED PRESSURETHIN WALLED PRESSURE VESSELS VESSELS

It must be emphasized that the formula derived for thin It must be emphasized that the formula derived for thin walled pressure vessels should be used only for cases of walled pressure vessels should be used only for cases of internal pressure. internal pressure.

If a vessel is to be designed for external pressure as in the If a vessel is to be designed for external pressure as in the case of vacuum tank, or submarine, instability (buckling) of case of vacuum tank, or submarine, instability (buckling) of the wall may occur & stress calculations based on the the wall may occur & stress calculations based on the formulae derived can be meaningless.formulae derived can be meaningless.