Posted by: jensapardi | November 23, 2009

Mechanical Equipments

 

 Storage Tank.

Volume of a small tank may be half or  1 cubic meter, but in the tank farm for petroleum  it can be thousands cubic meter. Geometrically, storage tank designed as cylindrical, spherical or cubical. To prevent liquid inside tank from rain water, tank need roof. There are 2 type of large cylindrical tank roof. Fix roof as conical or dome roof  and floating type roof. Floating roof will eliminate or minimize volume of vapor of liquid to be stored such as gasoline. Floating roof is equipped with flexible hose to prevent accumulated rain water on the roof and flows it to ground through nozzle. But sometime, fix roof also combined with floating deck. The purpose of floating deck is similar with floating roof to minimize volume of vapor. This fixed roof shall have adequate ventilation to prevent accumulation of vapor in between floating deck to roof in case of any leakage from floating roof, even small.

In medium capacity, storage tank made from nonmetal such as GRP or steel depend on the services. Large capacity storage tank made from steel and painted except stainless steel. For hydrocarbon services, storage tank designed as per standard such as API. API-650 Welded Tanks for Oil Storage and API 620 Design and Construction of Large, Welded, Low Pressure Storage Tanks.

A lot of variable shall be used in mechanical design for tank, such as liquid specific gravity, maximum liquid level, tank high, tank diameter, material specification to be used, fabrication and inspection method, roof type, effect of wind and effect of earthquake. Those variables are covered in API 650. If we have familiar and well known to use each variable on excel sheet, than we can also use software such as E-tank to make work easier.

Large storage tank as per API 650 constructed at field. This tank made from prefabricated steel plates. Prefabrication of steel plate may be at shop or at field. One by one of prefabricated plates to be lifted by crane and then to be fitted and welded until to the top of tank shell. Tank roof also made and constructed similar with the tank shell, but need temporary support before roof is completed.

VESSEL

Vessel is general term as closed space and quite large volume, such as ship or cargo box. But in this case, vessel is a pressurized closed space with steel wall or other strong materials. If there is nothing inside except fluid to be serviced itself without any reaction when operate, this process equipment named drum. If there are any reaction when operate, named reactor or exchanger. If there are exchanger tubes inside and will be any heat exchange, named heat exchanger or cooler and if there is catalyst inside and will be any chemical reaction, named reactor, reformer or converter. Name of equipment based on it service or function of process equipment in the plant.   

Process equipment wall that made from cylindrical vessel will contain shell and head or cover at both end of shell. Head or cover may be designed in flat, spherical or elliptical shape. Usually, elliptical head is most preferable because of cost reason. In access to other equipment, vessel is equipped with holes and pipes inserted to wall that named nozzle and for accessing human into vessel, there is manhole with its cover. 

Due to there effect for human and environment, vessel or pressure vessel must be designed, fabricated and inspected follow the codes such as ASME VIII. Design pressure of vessel varies from a kg/cm2 until hundreds kg/cm2. Quite a lot of variables must be use to determine shell and head thickness such as pressure, temperature, material specification, diameter, joining method, inspection method that has covered in the codes.

Vessels need support and fixed to the concrete foundation by anchors bolts or bolted to steel structure. Vertical vessel that installed on the ground usually supported by skirt and for light vessel supported by steel legs and where vessel installed on steel structure supported by lugs or wings. Skirt is made from steel plate with diameter same as vessel diameter. Wind and earthquake factor and any holes on the tall vessel skirt shall be taken account in the design. Buckling effect shall be considered for steel leg support design. Horizontal vessel usually supported by saddles that made from steel plates.

 Piping

In process plant, fluid flows through pipes from one equipment to each other, except flows through open channel such as drainage or sewer. Pipe is connected to equipment and piping system components by flanges, welded or threaded. There are some piping system components; pipe itself, pipe fittings, flanges, valves, strainers, pipe supports, spring support or spring hanger.

Piping design shall be accordance with codes and standards such as ANSI/ASME. For example ANSI/ASME B31.1 Power Piping, ANSI/ASME B31.3 Process Piping, ANSI/ASME B31.8 Gas Transmission and Distribution Piping System and other standard or code for other purpose piping system. Pipe material specification, diameter and thickness of pipe, pipe fittings and valves are has standardized such as in ANSI standards.

Similar with vessel design, there are variables that must be considered in thickness calculation such as pressure, temperature, selected diameter, material specification, joining method, inspection method, piping system purpose that has been covered in the codes. If there are high differences between operating temperature and ambient temperature, pipes will expand and will produce external force and moment in the pipe itself . This additional force and moment will add more stress to pipes if pipe route, pipe support type and arrangement are too rigid. Weight of liquid flows in pipe and weight of pipe itself will also produce bending moment, that mean there is additional stress to pipe. We have to consider those additional stress by selecting and arrange pipe support so that pipe stress shall not higher than calculated stress by codes. For complex piping route, using software will be helpful such as Caesar II. When necessary, pipe hanger or spring support is a choice as flexible support mainly in vertical movement of pipe.

Pump

The purpose of pump to deliver liquid from one place to other place or from one equipment to other equipment in different elevation or in different pressure. There are some type of pump mostly used : reciprocating pump, gear pump, screw pump, diaphragm pump and centrifugal pump.

Reciprocating and gear pump are categorized as positive displacement pump. In positive displacement pump, delivered liquid can not be stopped. When delivered liquid is stopped, in very short period there will be any damages either part of driver, part of pump itself or part of equipment at downstream of the pump whichever is weak. To protect equipment parts from damages, system shall be completed with trip control or pressure releasing equipment such as relieve valve. The advantage of positive displacement pump is almost unlimited discharge pressure, but capacity is limited.

Centrifugal pump is most popular. From home as water pump, fire fighting system to industries. Centrifugal pump is divided into several name based on how impeller handle the liquid. When the liquid flows in axial direction through impeller, named axial pump. When the liquid flows in radial direction through impeller, named centrifugal pump. When liquid flows in between axial and radial direction, named mixed flow pump. To lift up head as specified by process requirement, number of impeller can be more than one in one casing, named multistage centrifugal pump.

Pump design has been standardized by national standards, such as API 610 Centrifugal Pump for General Refinery Services, API 674 Positive Displacement Pumps-Reciprocating, API 675 Positive Displacement Pumps-Controlled Volume and ANSI standards.

 Which appropriate pump type shall be use? It will depend on process requirements. Main variable to be used for selecting pump type are capacity and differential pressure or head. Following chart presents operating range of centrifugal pump. This chart is only for single pump and does not for high speed pump. When necessary, parallel operation is commonly used in process plant.

 

Pump powered by driver such as electric motor, turbine or engine. Electric motor rotate at constant speed, but turbine and engine have adjustable speed. Selection of driver is depend to total power balance in the plant. If there is a lot of steam as side product, we can use steam turbine. If there is no steam or very limited steam, we can use electric motor. And if there is no steam and electric power we can use diesel engine or gas engine which ever is available.

Pump rotated by driver through coupling. But for small pump, impeller may be directly mounted to motor shaft and pump casing mounted to motor casing. If operating speed of pump should less than driver speed, reducing gear shall be installed between them.

The purpose of compressor is compress gas, air or refrigerant from lower pressure to higher pressure and deliver gas from one equipment or place to other equipment. Almost similar with pump, there are some type of compressor such as centrifugal compressor, reciprocating compressor, and screw compressor. The differences between pump and compressor, pump handles liquid as incompressible fluid and compressor handles gas as compressible fluid. Ratio of specific gravity (SG) between gas and liquid at atmospheric pressure is around 1/1000.

In the reciprocating compressor, gas is compressed by piston inside cylinder from large volume to smaller volume. Pistons sealed with piston rings. Gas into cylinder and gas out from cylinder controlled by one way valves. In screw compressor, gas is compressed continuously by two precision screw with opposite rotation. Screws sealed with oil for steel screws. In the centrifugal compressor, gas as a mass rotated through impeller and compressed due to centrifugal force to discharge side. Between impeller to impeller and between impeller to ambient sealed with labyrinth, carbon ring, oil seal or gas seal.  

In practical, centrifugal compressor is multistage impeller arranged in one casing or two casing. There is also centrifugal compressor that has some impeller with difference speed mounted in one large gear casing, named Integrally Geared Centrifugal Compressor. Reciprocating compressor also divided into more than one piston or stages. Power from driver or between stages is delivered through coupling with or without reduction gear.

Compressors also have been standardized as minimum requirement such as API 617 Centrifugal Compressor for General Refinery Service, API 618 Reciprocating Compressor for General Refinery Service , API 672 Packaged, Integrally Centrifugal Plant and Instrument Air Compressor for General Refinery Services, API 619 Rotary-Type Positive Displacement Compressor for General Refinery Services.

The purpose of steam turbine is drive rotating equipment such as pumps, compressors, blowers. Power of steam turbine come from steam where flows through turbine blades and convert the thermal energy to mechanical energy. Decreasing steam pressure or expanding steam volume will decrease the enthalpy or release thermal energy become kinetic energy through nozzle or stationary blades. Kinetic energy to be transfer to turbine blades become mechanical energy through rotor. If steam expanded at nozzle in the front of moving blades and no pressure drop in moving blades, named impulse turbine or Rateau turbine. If steam expanded at both stationary blades and moving blades, named Reaction turbine. And if one nozzle for two Rateau stage which split by stationary blades as flow direction without any pressure drop across stationary  blades, named Curtis turbine.

 In practical, single Curtis turbine applies up to about 2000 kW of power and multistage steam turbine which is combination between Curtis+Rateau stages or Curtis+Rateau stages+Reaction stages or Rateau stages+Reaction stages are available from about 1000 kW up to more than 50 MW for Industrial and up to several hundred MW for Power Plant.

Steam turbines also have been standardized by national standard such as API 611 General Purpose Steam Turbine for Refinery Services and API 612 Special Purpose Steam Turbine for Refinery Services.

 


Responses

  1. Thanks for sharing this great article! That is very interesting I love reading and I am always searching for informative information like this.

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  2. Its been nice visiting your site. Keep updating.


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