CHAPTER VII

PIPING LAYOUT

7.1 INTRODUCTION

A piping layout or piping general arrangement drawing is the most significant drawing

developed by a piping designer. The piping arrangement drawing evolves from the

foundation location and equipment location drawings. It shows all mechanical equipment

and vessels in the unit and the pipes connecting them, including manholes, ladders,

platforms, and davits. It identifies all structural supports such as pipe racks, equipment

structures, columns, braces and any fireproofing they may have. Once locations for

foundations and equipment have been established, piping configurations are added to the

drawing with the aid of symbols that represent fittings, flanges, and valves. Written

information placed on the arrangement drawing includes equipment coordinates,

identification numbers, elevation callouts, line numbers, flow arrows, and dimensions

establishing pipe locations. Instrumentation symbols are included to indicate type,

position, and orientation for accessibility by plant personnel. Ladders and platforms are

also shown on equipment and structures that have them. A nozzle schedule is included

that contains detailed information about all piping and instrument connections for every

piece of equipment. Information such as nozzle number, size and pound rating,

orientation, elevation, and projection is also included. With so much required information

on a drawing, it is easy to understand why the piping arrangement drawing must be neat,

accurate, and legible.

Piping is a major expenditure in the design and construction of industrial, refinery,

petrochemical, or power-generating plants when one considers engineering costs,

material costs, and fabrication and field labor costs. Proper planning and execution of the

design and routing of pipe can have a major impact on controlling the total installed cost.

The design department designing a refinery or petrochemical complex consists of four

main functions:

• Piping .

• Structural/ civil.

• Electrical & instrumentation.

• Equipments.

An experienced piping designer should have thorough knowledge of functions of the

piping department activities. In addition to this, he should have a broad knowledge of the

other sections also.

A piping designer must also know about the following:

• Details and specific requirements of various equipments which are connected by

his piping.

• Materials necessary for various services

• Piping flexibility basics.

• Field construction practices and constraints.

• General knowledge of plant operation and maintenance.

Piping design and equipment arrangement are interrelated subjects that cannot be well

taught in the classroom. Most good designers throughout history have learned their

profession by a combination of academic and practical work. Field and design office plus

a little shop experience is good preparation for designing.

The piping layout design is developed through three major sets of drawings. They are

• Equipment layout / plot plan.

• Piping General Arrangement drawing (piping plan).

• Piping isometrics.

To develop the above drawings the piping designer needs lot of information and

documents from various disciplines. Some of the major inputs required to start the piping

design are:

• Piping and Instrumentation Diagram (P&ID) from process department .

• Line list from process department.

• Piping material specification.

• Overall site plan from civil department.

• Major equipment sizes and details.

• Drawings from other sources .

7.2 PROCESS FLOW DIAGRAM

Flow diagrams describe in a schematic drawing format the flow of fluids and gases

through a unit or an entire plant. By using symbols to represent various pieces of

equipment, the flow diagram provides the piping designer with an overall view of the

operation of a facility. The flow diagram is used by the piping group to develop and lay

out the plot plan. When developing the plot plan, the arrangement of the equipment in the

facility reflects, in part, the logical sequence of flow depicted on the flow diagram.

However, many other factors such as code requirements, client standards and preferences,

worker safety, and cost also influence the positioning of equipment.

Once the plot plan is finalized, the piping designer routes the pipe between two vessels as

indicated by the

flow diagram using piping specifications and accepted design practices. The flow

diagram is usually “yellowed out” as each line is completed and incorporated into the

design. Process engineers are responsible for developing flow diagrams. In many large

engineering firms, an entire department is dedicated to the development of flow

diagrams. The process flow diagram is the first flow diagram

developed by the flow diagram department. It includes the following:

• major equipment

• main piping

• direction of flow

• operating pressure and temperature

• major instrumentation

SCHEMATIC DIAGRAM OF A UNIT:

PROCESS FLOW DIAGRAM:

7.3 PIPING AND INSTRUMENTATION DIAGRAM (P&ID)

P&ID is the main input required for piping design. It indicates the design process

equipment and interconnecting piping required to perform the function for which the

system is intended. It also indicates the instrumentation and control requirements. It

defines the sequence of equipment, valves, inline components, pipeline sizes, and overall

system arrangement required for proper system function. Piping and instrumentation

diagrams are the piping designer’s roadmap for laying out piping systems. The designer

should understand the P&ID and the specific system characteristics. P&ID are

representative schematics and are not drawn to scale.

The following information are taken from the P&ID by piping designer:

• Flow scheme.

• Piping material for each piping sections.

• Valve requirements and types of valves.

• Line size.

• Insulation and heat tracing requirements.

• Equipment connection details.

• Process vents and drain requirements.

7.4 EQUIPMENT LAYOUT / PLOT PLAN

Plot plan is derived from two drawings, site plan and equipment arrangement drawing.

The piping group produces a site plan into a small scale. It shows whole site including

boundaries, roads, buildings, railroad spurs, pavement, process plant areas, large

structures, storage areas, effluent ponds, waste disposal, shipping and loading areas. True

or geographic and assumed or plant north are marked and their angular separation is also

shown. Then a key plan is produced by adapting the site plan and dividing the area of the

site into smaller areas identified by key letters or key numbers. Equipments are arranged

by a piping group. The piping group usually makes several viable arrangements of

equipment, seeking an optimal design that satisfies process requirements. When the

equipment arrangement drawings are approved, they are developed into plot plans by the

addition of dimensions and coordinates to locate all major items of equipment and

structures. A pot plan shows the following:

• All equipments.

• Major structures.

• Main and sub pipe racks.

• Access ways.

• Control room.

• Space for future expansion.

• North and east extremities of building, center lines of steel work or other

architectural constructions.

• Plant north and true north

• Any other items of important.

Equipment layouts are drawn to scale of 1:50, 1:100, 1:250 etc depending on the size of

the plant and the number of equipments. Updated copy of the above drawings are sent to

civil, structural, electrical or other groups involved in design, to inform them about

requirements as the design develops.

For the correct placing of all the above items in an equipment layout, following inputs are

required:

• Available plot area.

• Process flow diagram.

• P&ID.

• Line list for sizing rack.

• Dimension of equipments / data sheets of equipments.

• Type of building, structures and foundations.

• Wind directions.

• Equipment erection, maintenance and operation requirements.

7.5 DRWAINGS FROM OTHER SOURCES

Piping drawings should be correlated with the following from design group and from

vendors. Points to be checked are listed:

7.5.1 ARCHITECTURAL DRWAING

• Outlines of walls or sidings, indicating thickness.

• Floor penetrations for stairways, lifts, elevators, ducts, drains etc.

• Positions of doors and windows.

7.5.2 CIVIL ENGINEERING DRWAING

• Foundation, underground piping, drains etc.

7.5.3 STRUCTURAL STEEL DRWAING

• Position of steel columns supporting next higher floor level.

• Supporting structures such as overhead cranes, monorails, platforms or beams.

• Wall bracing where pipes may be taken through walls.

7.5.4 HEAT VENTILATING & AIR CONDITIONING (HVAC) DRWING

• Paths of drawing and rising ducts, fan room, space heater etc.

7.5.5 ELECTRICAL DRWAING

• Positions of motor control centers, junction boxes& control panels.

• Major conduits or wiring runs(including buried runs)

• Positioning of light.

7.5.6 INSTRUMENTATION DRWAING

• Instrumentation panel& console locations.

7.5.7 VENDOR DRWAING

• Dimension of equipment.

• Position of nozzle, flange type& pressure rating etc.

7.5.7 MECHANICAL DRAWING

• Position and dimension of mechanical equipment.

• Piped service needed for mechanical equipment.

7.6 LINE LIST

Line list is a document prepared by process engineers. Each line in a unit is listed in the

list.

Line list (line designation sheet or table) include:

• The number of the line

• Line size

• Material of construction

• Conveyed fluid

• Pressure, temperature, flow rate

• Test pressure

• Insulation and jacketing

• Connected line (which will usually branch)

A line list will look like as follows:

7.7 PIPING GENERAL ARRANGEMENT DRAWING

Once, the equipment layout in finalized, the next step is to introduce the piping network

connecting the various equipments. This is done in another set of drawings called the

piping plan or piping general arrangement drawing.

The entire plant will be divided into numerous units or sections and each piping plan

shows the piping arrangement of a section.

Piping plans are normally drawn to scale of 1: 33.33. it shows the plan view of the plant

with detailed dimensions of the piping arrangement. Where required, sectional views and

elevations are also shown for more clarity.

7.8 CONSIDERATION FOR PLANT AND PIPING LAYOUT

As we have seen, developing a plant layout involves locating various equipments like

pumps, compressors, vessels, towers, furnaces etc. and arranging the piping network

connecting these equipments as per process schematic requirements given in the P&ID.

A good plant layout takes into account, the most important features of the plant such as

plant economy, appearance and arrangement of the equipment from aesthetic point of

view, proper maintenance facilities, safety considerations and to facilitate movement of

erection machinery like crane etc during the construction activities.

All these featured have direct bearing on the economy of the plant and a plant layout

designer has to consider carefully, all the above aspects to arrive at an optimum plant

layout.

Plant Economy:-

Second biggest cost factor for a plant like refinery, petrochemical or other process plant

is the piping cost. There is a good scope to achieve an economy by way of saving in

piping cost by improving up on plant layout. Basically, plant economy means installing a

plant in a smallest possible space, consistent operability , safety and law of maintenance

and using the smallest possible amount of piping material, structural steel or concrete.

Erection / Construction equipments:-

Erection scheme of all equipment must be considered during equipment layout. While

developing the equipment layout, the construction dept must be consulted for erection of

equipment. Adequate clearance and open access should be provided for erection of tall

columns, and heavy equipments.

Safety Requirements:-

Various international regulations, guidelines and safe practices set rules for minimum

distance between different types of equipments. The distance between fired heaters, Fuel

oil dry tank, control room, fire water hydrant /monitors, blow down facilities, water

spray deluge valve etc are mainly detected by safety consideration.

For e.g.:-Fired heaters shall be located minimum 15 m away from other hydrocarbon

units, to avoid a fire incase of a gas leak etc.

Similarly fire hydrant/monitors should be so located that incase of a fire, the operator

should be able to safety operate the Hydrant / Monitor.

Operation and maintenance requirements:-

Any plant is run by operators and adequate access for the operation to perform various

activities shall be provided. All the manually operated valves, instruments etc should

either be located at grade level or if at a higher level, suitable platforms, ladders etc shall

be provided. Similarly maintenance of equipments need adequate space. Sufficient

overhead and horizontal clearances shall be provided around each equipment for crane

access, removal of parts etc. It is also essential to have road access to equipments

facilitate access for cranes, trucks etc.

7,9 LAYOUT CONSIDERATION FOR PIPE RACK

Pipe rack is the structure with tiers at generally 3 to 4 levels and pipe laid on these tires.

One of the important steps in preparing an equipment layout is the arrangement of the

pipe rack with relation to the equipment. The simplest arrangement is the pipe rack in the

middle and the row of equipment on either side and access roads parallel to the row of

equipment on the both sides. But these type of arrangement require a long plot area and

in several cases, it may not be feasible. Hence “L” Shape, “H” shaped racks may be

considered.

The total width of the rack can be 6m, 8m and 10m for single bay and 12m and 16m for

double bay having three tires maximum.

• The spacing between pipe rack portals is generally taken as 3m.

• Clearance beneath pipe rack shall be 3m.

• Road clearance beneath pipe rack should be 7m for main road and 5m for

secondary road.

• A head room clearance of minimum 2.2 m is provided. For all lines to clear man

height.

• Locate the largest bore and the heaviest piping as close to stanchions as possible.

• Lines requiring a constant fall (relief headers) can be run on cantilevers from

pipe-rack stanchions or on vertical extensions to pipe-track stanchions.

• Run the hot line requiring expansion loops on the outside edge of pipe way to

permit loops to have greatest width over the pipe way and facilitate nesting.

• Takeoff elevations from pipe ways should be at a constant elevation, consistent

with the range of pipe sizes involved.

• Change elevation whenever banks of pipes, either on pipe ways at grade or at

higher elevations on pipe racks, change direction.

• Elevations to the underside of pipe racks should be the minimum for operation

and mobile maintenance equipment and consistent with allowable clearances.

• Open pipe trenches may be used between plants where there is no risk of

flammable vapors collecting.

• It sometimes is convenient to run open trenches alongside roadways. (Soil from

the trench can be used to build up the road.)

• Where a pipe way or road changes from a parallel direction, the pipe generally is

run beneath the road.

7.10 PIPE RACK SPACING

Arrangement and positioning are important factors in the layout of a piping facility.

Space is limited. Area and boundary limits force conservation of space. Arranging

equipment throughout the unit in an orderly and sequential fashion is a necessity.

Therefore, proper spacing and arrangement of pipe in the pipe rack requires special

attention. A pipe rack has a defined width; therefore, working within the allotted space

makes spacing crucial. Not only must pipe be arranged to take up a minimum amount of

space, but allowances should be made for any pipe that might be added in the future. Line

spacing dimensions are based on a clearance of 1” between the outside diameter of the

largest flange and the outside diameter of the adjacent pipe. The minimum spacing

between any two lines is 4”. If either of the lines is insulated, the thickness of the

insulation must be added. When lines are placed adjacent to a wall, column, building, or

other structure, a minimum clearance of 2’-0” is required from the outside diameter of a

flange. Pipes having orifice flanges also require a minimum clearance of 2’-0” on either

side of the pipe. Figure shows the requirements for establishing the minimum clearances

for line spacing. The line spacing chart shown in Table provides the minimum clearances

for pipe without insulation.

DRAWING PIPE IN THE RACK

When representing pipe in a pipe rack, the careful arrangement and organization of

names, dimensions, and line numbers will make the drawing easier to read. Figure shows

a pipe rack that has been well organized. Notice how the alignment of notes, dimensions,

and other callouts makes the drawing easy to read. The following guidelines will help

you organize your drawing:

• Keep flow arrows the same size and aligned.

• Line numbers should be left justified when possible.

• Pipe commodity should be identified on utility lines only.

• Line spacing dimensions should align across the pipe rack from one pipe support

column to the other. This allows coordinates for each pipe to be calculated since

each pipe support column is positioned using a coordinate.

7.11 LAYOUT CONSIDERATION FOR EQUIPMENTS

7.11.1 Layout consideration for pumps

• Locate pumps close to equipment from which they take suction.

• Consideration should be made to locate pumps under structures or with their

motor ends under a pipe rack.

• Pump suction lines are generally larger than discharge line to avoid problems

arising from a low net positive suction head (NPSH).

• End suction top discharge is the preferable option for pumps, when taking suction

directly from tanks or vessels located at grade.

• Pumps should be arranged in rows with center line of discharge is on a common

line.

• Clearance between pumps or pumps and pipes are a minimum of 900mm.

7.11.2 Layout consideration for compressors

• It is important to locate reciprocating compressors, anchors, and restraints for

pipes belonging to the compressor system on foundations that are independent of

any building, structure, or pipe track or rack. This independence gives the

associated piping stability and minimizes unnecessary fatigue and possible failure.

• Spacing between compressors and other equipment varies with the type of

machine and its duty.

• Particular attention must be paid to withdrawal of engine and compressor pistons,

cam shaft, crank shaft, and lube oil cooler bundle; cylinder valve maintenance

clearance with the least possible obstruction from piping supports.

• Compressors generally are provided a degree of shelter, that is, a sheets building.

Keep the sides up to 8 feet above grade and open and vent the ridge to allow for

escape of flammable gas, which might leak from the machines.

• Certain types of compressors, owing to the height of the mass foundation above

grade level, require a mezzanine floor of a grid construction to avoid trapping any

gas and for operation and maintenance.

7.11.3 Layout consideration for exchangers

• Tubular exchangers usually have standard length tubes of 2.5, 4, 5, and 6 m.

• Whenever possible locate exchangers at grade to facilitate maintenance and tube

withdrawal.

• Two or more shells forming one unit can be stacked or otherwise arranged as

indicated on the exchanger specification sheet, which is delineated by the

manufacturer.

• Exchangers with dissimilar service can be stacked, but rarely more than three

high, except for fin-tube-type units.

• Horizontal clearance of at least 900 mm should be left between exchangers or

between exchangers and piping.

• Where space is limited, clearance may be reduced between alternate exchangers,

providing sufficient space is left for maintenance and inspection access.

• Where a rear shell cover is provided with a davit, allow clearance for the full

swing of the head. Set overhead vapor exchangers or condensers at such elevation

that the exchanger is self-draining.

• Arrange outlets to a liquid hold pot or trap, so that the underside of the exchanger

tubes is above the liquid level in the trap.

• Arrange exchangers so that the fixed end is at the channel end.

• Vertical exchangers should be set td allow lifting or lowering of the tube bundle.

• Consult the Vessel Department as to the feasibility of supporting vertical

exchangers from associated towers.

• Space should be left free for tube or bundle withdrawal, with the exchanger

channels preferably pointing toward an access area or road.

• If an exchanger is situated well within the plot, leave a free area and approach for

mobile lifting equipment.

• Air fin exchangers, preferably, should be located in a separate row outside the

main equipment row, remote from the central pipe way.

• Consider locating air fin exchangers over the central pipe way if plot space if very

limited.

7.11.4 Layout consideration for fired heaters

• Fired heaters should be located at least 15 m away from other equipment that

could be a source of liquid spillage or gas leakage.

• To avoid accumulation of flammable liquids, no pits or trenches should be

permitted to extend under furnaces or any fired equipment, and if possible, they

are to be avoided in furnace areas.

• Ensure ample room at the firing front of the fired heater for operation and removal

of the burners and for the burner control panel, if required.

• Bottom-floor fired furnaces require adequate headroom underneath the furnace.

Wall fired furnaces require an adequate platform width with escape routes at each

end of the furnace.

• Apart from adequate platform and access to the firing front, other structural

attachments and platforms around furnaces should be kept minimum. Access by

means of stepladder is sufficient.

• Arrange fired heaters on a common center line, wherever possible.

• Provide unobstructed space for withdrawal.

• Operation and maintenance platforms should be wide enough to permit a 1-m

clear walkway.

• Escape ladders should be provided on large heaters.

• Vertical heaters usually are supplied with stub supporting feet; ensure drawings

show adequate supports elevated to the required height.

• Headroom elevation from the floor level to the underside of heater should be 2.3

m, to provide good firing control operation.

7.11.5 Layout consideration for columns

• Columns usually are self-supporting with no external structures.

• Circular or segmental platforms with ladders are supported from the shell.

• The maximum allowable straight run of a ladder before a break platform should

not exceed 9 m.

• The factors influencing column elevation are the provision of a gravity flow

system and installation of thermosyphon reboilers.

• Depending on the plant arrangement, columns may have to be elevated to a height

in excess of the normal requirements to allow for headroom clearance from lower-

level piping off-takes.

• The skirt height of all columns or vessels providing suction to pumps, particularly

if handling hot or boiling liquids should be adequate for the pump NPSH

requirements.

• Access platforms should be provided on columns for all valves 3" and above,

instrument controllers and transmitters, relief valves, manholes and blinds or

spades, and other components that require periodic attention.

• Platforms for access to level gauges and controllers should not be provided if

underside of supporting steelwork is less than normal headroom clearance from

grade.

• Adjacent columns should be checked, so that platforms do not overlap. For

layout, 2.0-2.5 m between shells, depending on insulation, should suffice.

• Allow a 900 mm minimum clearance between column foundation and the

adjacent plinth.

• Provide clearance for the removal of internal parts and attachments and for davits

at top of columns, if relevant.

• The center line of manholes should be 900 mm above any platform.

• Horizontal vessels should be located at grade, with the longitudinal axis at a right

angle to the pipe way, if possible.

• Consider saving plot space by changing vessels from the horizontal to the vertical,

if possible, and combining vessels together with an internal head (subject to

project or process approval).

• The size and number of access platforms on horizontal vessels should be kept to a

minimum and are not to be provided on horizontal vessels or drums when the top

of the vessel is 2.5 m or less from the grade.

• The channel end of vessels provided with internal tubular heaters should face

toward an open space. The withdrawal area must be indicated on studies, general

arrangements (GAs), and plot plans.

• Internal agitators or mixers are to be provided with adequate clearance for

removal. Removal area must be indicated on studies, GAs, and plot plans.

(COLUMN PIPING)

7.11.1 Layout consideration for tanks

• The layout of tanks, as distinct from their spacing should always take into

consideration the accessibility needed for firefighting and the potential value of a

storage tank in providing a buffer area between process plant and for example,

public roads and houses, for safety and environmental reasons.

• The location of tanks relative to process units must be such as to ensure maximum

safety from possible incidents.

7.12 LAYOUT CONSIDERATIONS TO ENSURE PIPING FLEXIBILITY AND

FACILITATE SUPPORTING

While routing the piping every effort has to be made to provide adequate flexibility in the

piping using a minimum number of fittings, loops at appropriate locations and flexible

joints. When a quick check the determines that the system is not flexible enough, the

designer reviews the system to determine whether or not he can re design, may be by

adding an elbow or two to increase flexibility.

Two cardinal principles in routing lines for economic support , restraint are:-

• Group the pipe lines so as to minimize the number of structures needed solely for

pipe supports and restrains.

• Keep lines located close to possible points of supports. i.e.:- Either to grade or

two structures which are to be provided for other purposes.

EQUIPMENT PIPING

While routing the piping to the equipments, care has to be taken to ensure that any special

requirements and constraints of the equipments operation, maintenance etc are taken in to

consideration.

For e.g. :- Pump suction piping shall be arranged with a particular care to avoid various

pockets or un necessary pressure drip and piping shall be as short as possible, to avoid

cavitations and ensured the required NPSH at the pump suction.

While routing compressor piping, adequate supports and clamps need to be provided to

contain vibration

Suction piping to centrifugal compressors should be designed to allow sufficient straight

length. i.e. 5D minimum of pipe (D= Diameter of pipe) immediately ahead of suction

nozzle to allow dissipation of un desirable flow distortion causes by elbows, valves or

other fittings At heat exchangers the piping shall not run in the way of the handling

facilities for tube-bundle removal.

The basic rule for the piping at the heat exchangers is- fluid being heated should flow up

and fluid being cooled will flow down. However if no considering or vaporizing can

occur during heat transfer, flow can be routed in any manner. In any case for best heat

transfer, reverse flow must be maintained. i.e. the flow should be in opposite directions.

Hence, designing the piping layout requires through understanding of the process

conditions and requirements, mechanical design, operation and maintenance features of

the connected equipments, Special requirements of the instruments, valves etc in the line,

to name a few.

Mastery in piping design can be achieved only by vast experience, continuous learning

process and interaction with various departments.

7.13 PIPING ISOMETRICS

Piping isometric drawings shows the pipe routing in the isometric view with detailed

dimensions, which can be used for fabrication of the piping spools. Isometrics are not

drawn to scale.

Other information shown in the isometric are

• Plant north

• Line number and specification

• Bill of material

• Stress analysis requirements

• Design temperature and pressure.

• Hydro test pressure.

• NDT requirements