Piping Engineering : Piping Layout: Heat Exchanger And Condenser Location In Plant And Piping Arrangement

INTRODUCTION

Heat Exchangers are widely used equipments in the chemical, petrochemical and refinery type of plant. The control of heat within a plant operation is done by direct heat application in a furnace, or by heat exchange in a shell and tube exchanger / plate heat exchanger. Types of heat exchanger may be Shell and Tube, Aircooled (Fin-Fan), Plate type or Coil type(spiral).

Various process duties of heat exchanger have an effect on the layout aspects viz.

a. Exchange of sensible heat between two process fluids.

b. Cooling with air or water.

c. Chilling with refrigerated liquid.

d. Heating with steam or hot water.

e. Condense by water or a cold process stream.

f. Vapourising by steam (e.g. column reboiler) or any heat transfer liquid.

Fixed tube exchangers are used when the temperature differences between shell side and tube side

fluid are small.

Floating head or U-type exchangers are used where there is a significant temperature difference.

Kettle -type reboilers are used for evaporation in case of limiting pressure drop, otherwise vertical reboilers are used for evaporation.

Plate heat exchangers are generally used in low-pressure, low temperature applications. The plate exchanger occupies less space than shell and tube exchanger for equivalent heat exchanger surface.

Aircoolers are used for overhead condensers of column and consist of fin-tube bundles with a header box to each end, having inlet on top of header-box at one end and outlet on bottom of header box at the other end.

For single pass arrangement of air-cooler exchangers, inlet nozzles are mounted on the top of the header box and the outlet nozzles are located at the opposite end and mounted on the bottom of the header box.For double pass arrangement, the inlet and outlet nozzles are located at the same end.

LOCATION

Exchangers should be located close to the major equipment with which it is associated in PFD / P&ID. Reboilers are placed next to their respective towers and condensers are placed over reflux drums. Exchangers between two distant pieces of process equipment should be placed at optimal points in relationship to piperacks.

Most exchangers are to be located at grade level with elevations to have a clearance of 1m above Finished Ground Level (FGL). Elevated exchangers may be necessary to fulfill the NPSH requirement of a downstream centrifugal pump.

In case of large numbers of heat exchangers, they are grouped in one or more category to save pipe work, structural work, provision of lifting and maintenance facilities, platform requirement etc. Paired or grouped exchnagers shall be spaced to allow minimum 450mm preferrably 600mm between the outside of adjacent channel or bonnet flanges to facilitate access to flange bolts during maintenance. Adequate space shall be provided on either side of paired exchanger and at both ends of grouped exchanger for control and operator access as illustrated.

Groups of exchangers should be generally located by the alignment of channel nozzles in a vertical plane so as to present an aesthetic appearance.

The support saddle with oblong holes for provision of thermal expansion are normally located on the saddle farthest from the channel end but the final location depends on the plant layout and the stress analysis of the connected piping.

Exchanger piping should be as direct and simple as possible by considering alternatives such as arranging exchangers side by side / stacking them for reversing flows.

Exchangers are sometimes mounted on structures, process columns and other equipment. Special arrangements for maintenance and tube cleaning should be provided in such cases.

ACCESS FOR OPERATION & MAINTENANCE

Internals of heat exchanger require periodic cleaning and repair. It is important that exchangers and the surrounding piping are arranged to facilitate access to the internal parts.

Horizontal clearance of at least 900mm should be left between exchangers flange to flange or exchanger flanges to piping. Where space is limited, clearance may be reduced between alternate exchangers but in no case clearance over insulation between channel flanges shall be less than 600mm.

The channel ends of exchangers should face the local access road for tube bundle removal and the shell cover should face the piperack. Pulled out bundles should not extend over main access road.

A typical exchanger arrangement with clearance for access, operation and maintenance is shown.

Access for tube bundle removal is usually 500mm more than the bundle length.

Mobile equipment should be used for handling tube bundles and covers at grade level. Expensive built-in facilities e.g. lifting beams, monorails to be kept minimum.

The use of tube-bundle extractor eliminates the need for permanent tube bundle removal structures. These mechanisms weigh around seven tons and are capable of pull forces about 500,000lbs. The tube-bundle is held in position by crane and balanced by the extractor's leveling cradle and pulled out of its shell with pull rod attachments that use hydraulic force.

For air-cooled exchangers, platform arrangements must suit maintenance access requirements. Considerations must be given to fin-tube bundle removal, tube rodding out at header boxes, motor and fan access.

Provision of easily removable spool pieces, flanged elbows, break flanges or short pipe runs will be helpful for maintenance of exchangers.

Illustrations for exchanger location in unit plot plan alongwith the provisions of operating and maintenance space, accessibility are shown.

STEPS TO DO HEAT EXCHANGER PIPING

Collect the mech. spec. of the heat exchanger.

Study the P&ID around the heat exchanger and identify the relationship with the connected equipment viz. column or reactor.

Note the special process (functional) requirement along with the specific arrangement details with respect to level or height difference from other related equipment like pumps.

Study the nozzle schedule, its position on the equipment and verify with P&ID.

Make a sketch for the relative location and level. Find out the tube cleaning space in case shell &

tube Heat Exchanger as well as the maintenance space of tube bundle length.

Locate the control valve stations both for shell side and tube side fluids.

Decide the flanged joints location for replacement of Heat Exchanger / maintenance facility.

Check, in general, that cooled streams flow downwards and heated streams flow upwards. This arrangement is mandatory when there is a change of phase, desirable when the streams are liquid but not important when the streams are gas or vapour or there is no super heating, super cooling.

Check, piping should not foul with exchanger removal either in the horizontal direction for bundle-pulling nor in the vertical for whole exchanger removal.

Check, locally mounted PI, TI on equipment or process lines, sight glasses and level indicators are visible from access aisles and valves are accessible from the aisles.

Mark the supports / support arrangement on the piping study.

Ensure accessibility / operation facility for all valves as well as the clearance kept are not less than the recommended minimum requirement

HEAT EXCHANGER PIPING

SHELL & TUB

After all the required information has been collected for the piping design of a heat exchanger, the first step is to outline clearance and working space in front and around both ends of the exchanger. These working spaces should be kept clear of any piping and accessories to facilitate channel, shell-cover and tube-bundle removal as well as maintenance and cleaning.

The free space at the side of horizontal shell can be used for placement of control stations.

The piping is elevated from grade to have a clearance of 300mm above the grade level. The piping connected to channel head nozzles should be furnished with break flanges to facilitate the removal of the channel head.

Steam lines connecting to a header in the yard can be arranged on either side of the exchanger centerline without increasing the pipe length.

Cooling-water lines, if under ground, should run right under the lined-up channel nozzles of all coolers.

Access to valve hand wheels and instruments will influence the piping arrangement around heat exchangers. Valve handwheels should be accessible from grade and from a convenient accessway. These accessways should be used for arranging manifolds, control valves and instruments.

PLATE & SPIRAL EXCHANGERS

Piping at spiral and plate exchangers is also arranged to allow opening of covers and removal of plates.

Controls at the spiral exchanger are located on the ends of the unit, clear of the cover plate swing area, piping attachment to cover plate nozzles of spiral exchanger will have break flanges.

Controls for plate exchanger are located at the front and one side of the exchanger. The piping is elevated to have clearance from grade as well as convenience for operation of valves.

AIR COOLERS