Mechanical Engineering – Static

Mechanical Engineering is the main stream in which Mechanical Engineering – Static and Mechanical Engineering – Rotary subclassified. In this post we will see more about Mechanical Engineering Static.

Mechanical Engineering – Static discipline calls for the vessels or equipment’s which doesn’t have any rotating or moving element’s to process or store the solid, liquid or gaseous material. However Mechanical Engineering – Static equipment’s working principle depends on temperature, pressure or any static element used inside the vessel or equipment for separation. Most of the static equipment’s are called as pressure vessels.

Mechanical Engineering Static equipment or Pressure Vessels:

Any vessel, which contains either internal or external containment of pressure inside vessel is called pressure vessel.

There are different possibilities for having pressure inside vessel, in which external source or due to direct heat or any other indirect source or on account of, both combined together it is possible. Pressure vessel, itself indicates pressure which is significantly different other than ambient pressure and designed to contain gases or liquids.

Design of any pressure vessel is having major part in mechanical engineering design like maximum operating pressure as well as temperature, corrosion allowance based on process condition and material of construction to be used, safety factor, cryogenic or minimum design temperature which may result in brittle fracture. There are many destructive as well as non-destructive tests covered during construction of pressure vessel, in which mainly involves non-destructive testing, such as ultrasonic testing, radiography, and pressure tests. Pressure tests contains, water, air or gas as a medium to be used for testing.Mostly any pressure vessels are hydro tested considering method is safe and less energy release during any rupture occurs to vessel.

Mechanical Engineering codes, ASME Boiler and Pressure Vessel Code (BPVC) is a formal code in most countries, to built vessels over a certain size and pressure. After design of pressure vessel, third party inspectors are authorized for inspection of each vessel and sign off. It is mandatory for each vessel to have a nameplate with relevantdata about the vessel, likemanufacturer company name, date of manufacture, registration number (National Board registration), U-Stamp (ASME’s official Stamp), maximum allowable working pressure, temperature, even minimum design metal temperature. Whenever someone wish to check vessel design details, name plate is very useful to trace and check ASME code vessel.

Different industrial plants, pressure vessels are mostly used. In general mechanical engineering Static equipment’s or pressure vessel is necessary for any industries or plants to run. Any plant or industry uses mechanical engineering Static equipment’s or pressure vessels like separators, KO drums, boilers, collectors, exchangers, condensers, pipes, etc.Major or widely used pressure vessels is in below sectors.

• Oil & Gas facilities, Crude Oil Refineries, Petrochemical industry, LNG, Power plants, Nuclear or atomic plants, Chemical industries, Fertilizers and Food, Medical sectors with Pharma sectors, etc.

Mechanical Engineering Static equipment’s or Pressure vessels are categorised depends on their various purposes, or different types of shapes or geometries:

• Based on its purpose: KO Drums, Towers or Columns, Reactor, Accumulator, column, Test or Production Separator, Heat Exchangers etc.
• Heating Method: Fired or unfired
• Heat exchangers further classified as Shell & Tube, Plate type, Air Cooled Heat Exchangers, Double Pipe or Multi-tube exchangers, Spiral Heat Exchangers, etc.
• Reactor further having main classification namely, Catalyst type and Agitated type.

Catalyst type: Typically it is cylindrical shell vessel. It is installed in vertical having often two internal catalyst beds. The upper bed is supported by a structural grid and is supported from the inside circumference of cylindrical shell.

Agitated type: This type of rector name itself indicates agitator is required for vessel. In addition to the agitator integral heating cooling system required. Material of construction used generally Steel with internal glass lined, stainless steel, and alloys like glass or epoxy. Top cover of reactor is used for charging Liquids and solids. As a general density of vapours and gases tend to move upwards and discharged through top connections of reactors and liquid tendency from the bottom of the rector.

Other than this, Mechanical Engineering Static equipment’s or pressure vessels are further classified based on their Material of construction, installation strategy, shape or geometry etc.

Reactors:

Rector principle mainly depends on the catalyst effectiveness factor as well as heat and mass transfer effects. In chemical industry, Heterogeneous catalytic reactors are utilized as chemical reactors.

Reactors is a pressure vessel filled with solid catalyst particles. Operating principle of reactor depends on catalyst with heat and temperature effects.

There are three reactor systems

• Fixed bed
• Fluidized bed
• Expanded or ebullating bed

Fixed bed:

In a simple term, fixed bed reactor has fixed cylindrical tube and filled with catalyst pallets and reactants flowing inside bed, and same is converted into final products.

The catalyst configurations like: Single giant bed, multiple horizontal beds, many parallel packed tubes, and multiple beds in their own shells. Looking on the requirement to take care of temperature management at intervals the system, varied configurations is also used. The pure mathematics of pellets is also random form, or spherical, cylindrical, configuration pellets. The flow of a hard and fast bed reactor is generally downward direction.

Fixed bed type mechanical engineering reactors are again classified as,

• Down flow
• Radial flow
• Tubular

Fluidized bed:

Fluidized means this type of rector principle using fluid or Gas catalyst particles suspends by the upward motion to be reacted.

In this type of reactor, fluid flow rate generally gas is important and required high enough for mixing the particles even over flooded out of the reactor. However for mixing in fluid particles are much smaller (Usually on the scale of 10-300 microns) required than other reactor types.

Main benefit to achieve a highly uniform temperature in the reactor is positive key factor of fluidised bed type reactor.

Ebullated bed reactors:

Another sub classification of fluidised bed reactor is Ebullated bed reactor. Principle behind this type of reactor utilizes bubbling or ebullition to achieve proper distribution of reactants and catalysts.

Main advantages of Ebullated bed reactor are as below,

• Processing high level of contaminants like feedstock and exothermic reactions, which are difficult to process using fixed bed or plug flow reactors. This type of reactor technology uses a three-phase reactor (liquid, vapor, and catalyst).
• Continuous and high-quality, mixing of liquid and catalyst particles.
• Excellent temperature control andwith reducing bed plugging and channeling.
• Low and constant pressure drops.
• Characteristics of stirred reactor type operation with a fluidized catalyst.

Ebullated Reactor

Heat Exchanger:

Heat exchanger is broadly used mechanical engineering Static equipment in any industries. As a name indicates as heat exchanger, is a device to exchange heat between two or more fluids. Depends on type of exchangers, fluids may be separated using wall, or may have direct contact with each other. Mostly they are used in petrochemical, refineries, powar plants, space heating, refrigeration, air-conditioning, natural-gas processing, and effluent treatment plants etc. Simple example in day to day life is our vehicles IC (Internal combustion) engines, we are using engine coolant which is flowing through radiator coils and air flows outside the coils, and it cools coolant and heat the air coming in.

Types of Mechanical Engineering exchangers we will be as follows,

Which are the mainly used in the refinery & oil & gas companies.

• Tubular heat exchangers: Shell & Tube heat exchangers
• Plate Type exchangers: Gasketed, Spiral, semi welded type, fully welded type.
• Extended surfaces: Tube fin Air cooled heat exchangers

Shell & Tube heat exchanger

In process industry, shell & tube exchangers are the most widely used mechanical engineering Static exchanger type. First preference for using exchanger in any industry is shell and tube type heat exchangers. These are first choice on account of its well-established procedure for design and manufacture using different types of material of constructions, as well as satisfactory service with high life cycle and availability of codes and standards for its design and manufacture.

TEMA (Tubular Exchanger Manufacturers Association) is the standard used for design and manufacturing of tubular heat exchangers. Leading heat exchanger manufactures developed this standard and provides the design and manufacturing parameters for shell and tube heat exchangers. There are different type of sub classification of shell and tube heat exchanger, based on TEMA standard.

TEMA contained of three different classes to classify designs; TEMA R, TEMA B &TEMA C. In addition to this, TEMA established Recommended Good Practice, for conditions not covered by the above R, B & C classes. All above TEMA Classes fulfilrequirements with ASME Section VIII, Division 1/2.

TEMA R Unfired shell and tube heat exchangers for the generally severe requirements of petroleum and related processing applications.

TEMA B Unfired shell and tube heat exchangers for chemical process service.

TEMA C Unfired shell and tube heat exchangers for the generally moderate requirements of commercial and general process applications.

Operation of Shell and Tube heat exchanger

Shell & Tube Heat Exchanger further classified as per there design are as below,

Fixed tube sheet exchanger

Tube sheet is welded to the shell and the heads are bolted to the tube sheet.

To ease in tube cleaning fixed tube sheet exchanger is designed with removable bonnet or cover plate.

Advantages

• If expansion joint is not used in design, these type of heat exchangers are first choice considering low cost.
• For given diameter of shell, it provides maximum amount of surface.
• Can be single or multiple tube side passes to accommodate thermal design

Disadvantages

• Only clean fluid can be used on shell side, as shell side is not accessible for cleaning and only chemical cleaning possible.

Requirement of expansion joint or bellows may come in picture. As their ends are fixed, on account of this there is no provision for differential expansion between shell & tubes.

Floating head heat exchanger

One tube sheet, called stationary tube sheet is sandwiched between the shell side & channel side flanges.

The floating tube sheet is located internally, bolted construction & floating head is free to move inside longitudinally.

Advantages

• Used even for dirty service for both tube as well as shell side of exchangers. External as well as internal tube bundle can be cleaned mechanically.
• No need any bellows or expansion joints, as floating head design allows for differential thermal expansion between shell & tube bundle.
• For given diameter, higher number of tubes can be provided as compared to AET or BET(Pull through type) type exchangers.

Disadvantage

• To remove tube bundle from shell, shell cover, split ring and floating cover need to be removed.
• More costly as compared to fixed tube sheet & U tube type design.

U – tube exchanger

The tubes are of U construction.Tube sheet is sandwiched between the shell side & channel sideflanges.

Advantages

• Low cost compared to floating head type heat exchangers.
• Capable of withstanding high shock load
• Bundle can be removed out for cleaning on outside
• U-tube allows for differential expansion between steel & tubes.
• Suitable for high pressure application. High pressure on tube side.

Disadvantage

• Because of U bend, tubes can’t be cleaned from inside. Chemical cleaning is only possible.
• No single tube pass or true counter current flow is possible.

According to the exchanger positions in a procedure plant the accompanying general characterization can be made:

1. Exchangers which should be next to other equipment:
2. g. Vertical Reboiler
3. Exchangers which should be close to other equipment:
4. g. Overhead condenser
5. Exchangers located between other process equipment’s:
6. g. Exchanger with process lines connected to both shell & tube side
7. Exchangers located between process equipment and the unit limit:

e.g. Product coolers

Establishing elevations for the exchanger in any mechanical engineering layout design

• Exchangers situated between process gear and as far as possible
• Where process prerequisite came in picture, the height of exchanger generally noted on the PEFS
• For cost considerations, best elevation is at grade.
• However specific NPSH is required and gravity flow is required or connected to pumps exchangers are located on structures. E.g. overhead condensers.

Layout of Shell & Tube heat exchanger in banks

 

Sample exchanger orientation

Tower supported vertical exchanger

Tube bundle removal at grade

Plate Type Heat Exchangers

As its name indicates, plates (corrugated metal plates) are used to achieve required flow arrangement. In addition to plates, gaskets and corner ports also used in design of this exchanger. Each fluid flows through different passages. The plates are clamped together in a frame that includes connections for the fluids. To provide sealing arrangements, each plate is generally provided with peripheral gaskets. The plate heat exchangers are also called as gasketed plate heat exchangers.

 

Air Cooled Heat exchanger

Air cooled heat exchangers are using atmospheric air in the plants to cool the hydrocarbon, utility or process fluids. Main principle behind this is,direct heat transfer from fluid within tube to be cooled by atmospheric air, circulated using forced or induced draft fan.

So as to increase the surface area to have more heat transfer area, fins are also provided at periphery of tubes. These heat exchangers are normally designed, testing and inspection has been done as per EN-ISO 13706.

Types of Air cooled Heat exchanger

There are three types of Air cooled heat exchanger

• Forced Draft
• Induced draft
• Natural Draft (Used for limited use like transformer Oil Cooling)

Different type of Construction Air cooled Heat exchanger

• Single Pass Cooler
• Multipass Cooler
• U Tube Cooler

Consideration from mechanical engineering Equipment layout of View

• Location of air cooled heat exchangers in any plant will be free, from blocking the free air circulation and not to be surrounded by equipment’s.
• Top of the any pipe rack is preferred place to install, to save grade space with compact design requirements and better air flow so that there is no obstruction to reduce air flow.
• Based on the width of the pipe rack, normally tube bundle length is fixed to simplify pipe rack design and can be installed cooler legs on pipe rack columns. Even it is better to adjust pipe rack longitudinal column spacing based on the width of the air cooler bundle, so that legs of air cooler bundle can sit on top of the columns.Sometimes, this may not be practicable considering limited structural feasibility in design and even tube bundle length depends on service conditions and each tube bundle have varied width.
• Between any two sets of air coolers, pathways or access ways are required to be provide. Assume air cooler may comprise of 10 packs and another of 5 packages then walkways ought to be given between, after tenth package and before of next five groups. Considering maintenance requirements and no other place to use tools walkway shall be minimum 1.5 to 2.0 m wide.
• Crane access is also important for air cooler installation and to be installed such a way that at lease accessibility, shall be available from either side of the bundle.
• All around platform for air coolers is better, but if not possible at least operating side platform is mandatory for maintenance and operation.
• In addition to access platforms, underneath platform for air cooler is required as motors hanging at bottom of the cooler, and local access is required during maintenance.
• For all maintenance platforms, permanent staircase to be provided.
• Key point for any air cooler is inlet piping of air cooler and it has to be symmetrical distribution with loops required. Inlet piping to be supported using pipe rack columns extended and same to be taken care during rack design.
• If location of air cooler is at grade then the area below the air cooler shall be paved to avoid the dust or sand flow on tubes.
• Distance between the fans provided is important. If two fans per bay, the height of the below of the fan inlet bell on forced draft units or of the below of the bundle on induced draft units shall be minimum 2 m or one fan diameter whichever is the greater above the ground level or elevated floor or pipe rack. For three or more fans per bay, the height of the below of the bundle shall be agreed with the client or owner or Principal.

Mechanical Engineering Consideration from piping point of View

The air coolers are primarily utilized where vast amount of vapor is required to get dense or substantial amount of gas/fluid is required to be cooled. The application is exceptionally regular in the event of segment overhead vapor buildup.

Following focuses should be taken care while laying air cooler channelling

• Piping distribution to the air cooler should be symmetrical from centre line of complete air cooler assembly.
• On account of supply line having low weight mind will be taken to keep no. of twists to least without giving up utilitarian and stress prerequisite. Proper line sizing to be done at the point of distribution and should be sufficient.
• Length of each branch pipe for all bundles from its header shall be more or less same to keep pressure drop same & equal distribution of fluids to all bundles.
• Inlet side header box will be considered as it settled point for piping interconnection. But the bundle can move in transverse direction of tubes = 6mm or if it is fixed at one edge then it can move by 13 mm in the other direction. For piping header expansion compensation, this movement is required. If air cooler is required to be mounted in eccentric position i.e. to get 13 mm movement in one direction.
• When piping connected to nozzles generates enough force to overcome friction at the bundle support point, the transverse movement of bundle can occur. Hence normally at the support point cooler vendor provides SS plate, PTFE plate or ball bearing to ease the movement.
• The forces due to thermal expansion of piping created on the bundle nozzle shall be less than the limits given by EN-ISO 13706.

Storage Tanks

In day to day life storage tank is used for water storage at home or building, which is also one of the Mechanical Engineering Static equipment. And in industrial terms, storage tanks is a tank that contains or stores or hold compressed gases or liquids. Even reservoirs like artificial lakes and ponds or any manufactured containers also termed as storage tank.

Storage means liquid or gaseous products are stored for time being between production, processing or marketing or any transportation. Storage Tanks are used for the storage of raw material or feed stock, in-process and even finished or semi-finished products in a process facilities. Location of storage tanks are based on the storage product, either feed stock, raw material or final product are located in off sites areas and those for storage of required in-process material are located within the unit limit.

Broadly the whole range of tanks can be divided into three categories.

• Tanks which are open to atmosphere or design pressure up to 2.5Psi (Gauge) are designed as per API650.
• Tanks with design pressure greater than 2.5Psi and up to 15psi can be designed as per API620.
• Tanks which stores product with design pressure of ˃15Psi are designed as per ASME Sec-VIII.

However, tanks designed by using mechanical engineering code ASME VIII are treated as pressure vessel. Below are the types of Tanks generally seen in Oil & refinery fields.

Type of tanks

Storage tanks are classified in different ways.

• Depending on the nature of the product stored (atmospheric &pressurized).
• Depending on the type of construction (aboveground or underground and double wall).

Atmospheric Storage Tanks

Atmospheric tanks are used to store fluids having vapour pressure at storage temperature less than atmospheric pressure. Vapour pressure varies with temperature and increase with increase in temperature. Fluids like water, crude oil, heavy oils etc. are generally stored in atmospheric tanks.

The other type of atmospheric tank is of Floating roof type. The floating rooftanks are designed to prevent loss of product during filling and breathing byreducing the vapour space. The shell & bottom constructions are similar to those of other flat bottom tanks. But, the roof is made to float directly on the surface ofproduct, thereby reducing the vapour space.

Atmospheric Storage Tanks, which are one of the mechanical engineering Static equipment is further classified as,

• Open Top, e.g. Fire Water Tanks
• Fixed (Cone or dome) / Flat Roof
• Floating Roof (Single Deck or Double Deck)
• Internal Floating Roof (Floating roof inside with cone or dome roof at top)

Pressurized Tanks

Low pressure tanks are used for storage of volatile fluids having internal pressure up to 15Psi. Light crude oil, gasoline, naphtha etc. are some of the examples of fluid generally encountered in Oil & gas industries. Tanks with internal pressure up to 2.5 Psi are designed as per appendix-F of API-650. Beyond this & up to 15Psi, tanks are designed as per API-620. In API 620, generally Tanks up to 5Psi are constructed in conical roof type; beyond this pressure spherical roofs are considered.

Pressurized tanks are divided into following categories:

• Fixed Cone Roof
• Fixed Dome Roof

Fixed Roof tanks

Fixed rooftop tanks are by and large used to store items that don’t promptly vaporize at encompassing or put away temperature conditions. For instance, fixed roof tanks can be used to handle non-volatile products such as gas oil, lubricating oil, asphalt and fuel oil. Design of this type of tank consists of a cylindrical steel shell with a permanently fixed roof, as well as from cone or dome shaped to flat. Each type can be further subdivided into as follows:

• Non Pressure fixed roof tanks are used for storage at atmospheric pressure and are provided with open vents.
• Low –Pressure and high pressure fixed roof tanks are used for storage at a low and high internal pressure respectively. They are provided with pressure/vacuum relief valve (breather valve) that should be set to be fully open at the design pressures.

Floating roof tanks

For this type of tank no vapour space is created, as the roof is not fixed to the shell, but floats on the liquid. Because the floating roof rest directly on the liquidsurface, it provides constant pressure above the liquid all time, thus significantlydiminishes stock evaporative losses and reduces the hazards associated withhandling a large, possibly combustible tank vapor volume. The floating roofmoves up & down within the tank, floating always on the varying liquid level dueto pump in and pump out conditions.

Standard floating roofs used in the industries are as follows. Single deck pontoon type, Double deck pontoon, pan type etc. Pan Type is not used in general as puncturing in the deck may cause sinking of the total roof. Based on providing additional fix roof over the floating roof, they are subdivided as follows:

External floating Roof:

This type of tank is designed to work at atmospheric pressure. External floating decks are provided with a rim-seal system which is connected to the deck perimeter and contacts the tank wall. The function of the floating roof and rim seal system is to reduce evaporative loss of the stored liquid.

Internal Floating Roof:

This type of tank is developed to provide protection of the floating roof from Snow loading on a floating roof which may be a problem since snow or water on the floating roof will affect the operating buoyancy, Contamination by rain water of the liquid stored in a floating roof tank is unacceptable, Environmental or vapor loss problem with fixed roof tanks are evident and Contact of the stored liquid with air should be avoided etc.