Pipe Design General Considerations

What is Piping Design?

Piping design is the process of designing and laying out a system of pipes to transport fluids (liquids and gases) from one location to another. This can include pipelines for oil and gas, water distribution systems, and industrial process piping for a wide variety of applications. 

Piping design general consideration involves determining the most efficient and effective layout of pipes, selecting appropriate materials and fittings, calculating pressure drops and flow rates, and ensuring the system meets safety and regulatory requirements. It also includes consideration of factors such as temperature, pressure, corrosion, and mechanical stress.

The goal of piping design is to create a system that is reliable, cost-effective, and easy to maintain, while meeting the specific needs of the application for which it is designed.

General Consideration for Piping Design

There are many general and engineering considerations while designing a piping layout of any plant. Some of them are as follows:

  • Pipe Spacing
  • Flange Installation
  • Valve Installation and Operation Consideration
  • Accessibility
  • Piping Arrangement and Pipe Routing
  • Slope Piping, Free Drain Piping, No Pockets Piping
  • Symmetrical Piping

Pipe Spacing

The pipe space refers to the center-to-center distance between the two adjacent pipes. Whenever, a piping engineer or designer places pipes side by side, there has to be minimum distance between two adjacent pipes to avoid clash during installation of valves or flanges, to have enough space in case one pipe is in expansion at the bends, to optimize rack or sleeper width, to make space for pipe supports like guides etc. This is achieved by considering the basic pipe spacing formula described below:

When two adjacent lines are of the same size, the line whose flange rating (Class 150, Class 300, and so on) is higher shall be regarded as having the larger pipe size. If they are of the same flange rating, either line may be regarded as having the larger pipe size.

Basic pipe spaces (A), (B), and (C) shall be properly applied according to the following conditions regarding pipe space.

Basic Pipe Space (A)

This is the most general case of pipe space determination. As shown in the figure below, the clearance between the smaller pipe outer surface and the larger pipe flange outer surface is 25 mm minimum. However, the clearance between the pipe outer surfaces is at least 75 mm for 3 in. or larger pipes, and at least 60 mm even for 1 in. pipes.

Basic Pipe Space (B)

This is a case of pipe space determination in piping design similar to Basic Pipe Space (A). When two adjacent lines are different in pipe size by only one size or when the line of the smaller size has the higher flange rating, Basic Pipe Size (B) sometimes becomes larger than Basic Pipe Size (A). In this case, the pipe space is determined based on Basic Pipe Space (B).

Basic Pipe Space (C)

This is a case of pipe space determination in piping design which keeps a minimum clearance of 75 mm between the pipe outer surfaces or the pipe insulation outer surfaces. If the pipe space for insulated lines is determined based on Basic Pipe Space (A) or (B), the actual clearance between the pipe outer surfaces will be insufficient, resulting in interfering with insulation or painting work. Basic Pipe Space (C) is therefore sometimes adopted to keep the minimum required space. This pipe space also applies to intersecting lines.

Flange Installation

Flanges shall be installed at the following locations:

(1) Connection of piping with equipment nozzles (flange type )
(2) Installation locations of piping and instruments (flange type)
(3) A location in piping where removal of a piping part is required.
(4) A location in piping at which the material changes.
(5) End point in piping where a blind flange is to be installed.
(6) Tie-in point of piping
(7) A location in a piping (having special lining ) where requiring “flange cut-in” for fabrication purposes.
(8) A location in a piping where requiring “flange cut-in” because of fabrication in an area where the use of fire is prohibited.

Valve Installation/Operation

Valves shall be classified in accordance with importance and frequency of the operation. In this connection, the installation of an operating platform or the method of operation shall be reviewed so that the valve may be operated complying with the purpose of the installation and no obstacle should be given to the normal operation of the related equipment and additionally, the piping arrangement shall be considered to minimize the required cost as low as possible.

(a) Classification of Valves by the Frequency of Operation
Valves shall be classified in the manner described below according to the frequency of their operation.
(a-1) Operating Valve
Operating Valves refer to those which are, or may possibly be, operated several times or more during
operation of the related equipment.
(a-2) Non-Operating Valves
Non-Operating Valves refer to those valves which are, or may possibly be, operated during maintenance
or for the special operation of the related equipment.

(a-1) Operating Valve
2in. and larger operating valves shall be operable from the grade or from a platform. It is ideal to be
operable on the ground if possible, however, it might be required to operate them on a platform according to the process requirements or to their locational relationship with other valves or instruments.
For example, if the valve shown below is an operating valve (2in. and larger), following two cases may be

CASE a-1-1
Where there is no requirement in process or operational conditions (e.g. locational relationship with instruments, etc.), the piping shall be arranged “operable on the ground”. However, a platform shall be selected to be installed if the piping arrangement on the ground is more expensive tha providing a platform and does not make any interference to the operability, accessibility, maintainability, etc. in the surrounding of the platform.

CASE a-1-2
If the process requirement (line to be ’ free drain’) to provide a drain pocket is impossible, a platform shall be installed to enable the operation of the piping from the platform. In this case, it is required to review the location of the platform so it would not cause any interference to the operability, accessibility or maintainability, etc. of other valves prior to determining the installation locations.

(a-2) Non-operating Valve
Non-operating valves, which are to be operated very seldom in the normal operation of the related equipment and are mainly for special operation or for maintenance, may be operable either from a movable rack or temporary scaffoldings, however, desirably they should be operable from the ground or from a platform. Accordingly, where the piping arrangement operable from the ground cause any difficulty, consideration shall be given to utilize any existing platform which has been provided for operation of other valves, etc.

(b) Use of Chain Wheel
A chain wheel shall be used for 2in. and larger operating valves with the center elevation of the hand wheel located at 1850 mm or higher from the floor of operating platform.

(c) Use of an Extension Stem

Where the installation location of the valve is too low and not suitable to provide the operating platform for such a low elevation, operation of the valve with an extension stem is recommended. Refer Example below:

(d) Installation Elevation of Valves

The installation elevation of valves in piping design shall be as shown in the figure below, and shall be selected for the elevation ranges of “first, second, third and fourth” with reviewing easiness and safety in operation.

(e) Horizontal Distance from Platform

The location of the valve designed to be installed outside the platform, and operated from the platform shall be as follows:


A sufficient review shall be made for operation of valves, instruments, etc., when preparing piping arrangement to enable operation of the unit without any difficulty, and at the same time access ways for inspection of the related units shall be secured in the arrangement. The following access spaces shall be secured while considering piping design:

(1) Access Space Above-Ground

(2) Access space for Platform

Piping Arrangement and Pipe Routing

Piping arrangement refers to the determination of a piping configuration (route) extending from a certain point (starting point) to another point (ending point), and to lay (position) those piping materials and component (miscellaneous parts) accordingly. A sufficient review of the pipe routing and general piping arrangements shall be carried out so that various terms of the following items are thoroughly checked and satisfied without missing any other items by satisfying one particular item only.

(1) Arrangement shall present no obstacle to the functioning (process requirements) of equipment.
(2) Arrangement shall cause no problem in respect of the strength of the piping system.
(3) Arrangement shall be made with due consideration to the operability and accessibility of equipment and piping.
(4) Arrangement shall have due consideration to the maintainability of equipment.
(5) Arrangement shall have due consideration to the location of piping supports.
(6) Arrangement shall be selected for the shortest possible route and the minimum possible materials for piping system.

Most of pipelines what we plan to make routing (determine the route) is normally of the type running aboveground. However some of them is of the type running underground in trench or direct buried. Furthermore, the aboveground piping is laid at an elevation of 10 m or less in most cases. The shortest possible course shall be routed with due consideration to the above stated points.

Criteria in Routing

The shortest piping route shall be determined in accordance with the criteria described in the following when planning piping route in horizontal and vertical directions.

(a) Horizontal Routing:

In principle, horizontal routing shall be restricted to the directions of east and west or north and south. Under the following conditions, however, piping shall be routed in other directions (horizontally diagram directions ) wherever possible.

(a-1) Where typically diagonal piping is applied horizontally around equipment. Refer Piping Arrangement around Heat Exchanger or Piping Arrangement around Centrifugal Pumps.

(a-2) Where piping runs around equipment

When there is no possibility of interference with other piping. Even, if there are a few of few lines of piping in the same vicinity, piping especially, of large diameter or high grade material shall be routed in a horizontally diagonal direction whenever possible.

(a-4) Where supports may be saved by routing diagonally.

(b) Vertical Direction

In principle , vertical routing shall be restricted to the completely vertical direction. Under the following conditions, however, piping shall be routed in vertically diagonal directions whenever possible.

(b-1) Where a diagonal piping has to be applied as typical (flare piping, etc.):

(b-2) Where piping is vertically laid along side equipment with following the configuration of the equipment:

(b-3) Where a diagonal piping is required to obtain an economical piping arrangement:

(b-4) Where piping length becomes excessively too long even if fittings are connected directly


Pipelines shall be assembled in groups as far as possible when laying in arrangement. Grouping might require a certain sacrifice in respect of the shortest piping route, however, great advantages are obtainable for installation of supports and improvements in operability and accessibility, and grouping may present a good appearance.


In many cases, those requiring operation such as valves, instruments and other miscellaneous components are installed in piping. Some of these are designed to be operated from grade, or those which may be operated on the platforms or from the ladders. Furthermore, some items may be specified by the process requirements of the location for operation. Accordingly, the shortest piping route shall be determined by satisfying these requirements in operation.


It might be happened many times that the actual piping route somewhat deviates from the shortest route because of the location or the method of installation of supports although the most suitable location of support installations is desirable.
On the other hand, however, it might be advantageous in respects of cost and work execution to provide supports with pipelines collected in a group rather than installing supports per each line.

Slope, Free Drain, No Pockets Piping

What is Slope Piping?

The Slope in piping is provided to avoid accumulation of liquid in the line. The application of slope will force the liquid to go to the next low point of the line (it could be a piece of equipment).
Slope could be either upstream or downstream. slope 1:100 means that every 100 m of horizontal length, the line will be sloped by 1 m. The most typical application of slope is relevant to the PSV inlet and outlet lines.

The gradient in the sloped piping may be or may not be given in P & I Diagram depending on cases. The sloped piping is indicated as in the sketch below, which indicates that the line slopes down from the left to the right and the line shall be laid in the gradient of 1/500 or more.

As for the method of providing slope, that which attaches a ”pipe shoe” whose height being adjusted according to the slope (even when having no thermal insulation) is general, and a continuous slope may be provided by combining with other type of pipe support when the pipe shoe appears too high. Slope and support span shall be determined so as to avoid drain pockets due to free deflection of the piping.

Free drain and No pockets Line

All of pipelines indicated as follows in P & I Diagram shall be arranged to form free-draining piping to avoid drain pockets.

(a) No Pocket (Do not make pocket)
(b) Free Draining
(c) Gravity Flow

(1) Horizontal pipelines may be arranged with assuming them as free-draining lines. However, care shall be paid to that it might not form a free-draining piping depending on the location of the reducer.

(2) Where an orifice plate is located at the side where the drain is discharged by the gravity flow, no free-drain piping is formed.

In this case, the piping arrangement shall be planned to place the orifice in the vertical run of piping, or in a piping configuration having the orifice at a higher elevation so that drain is discharged to both sides of the orifice.

Symmetrical Piping in Piping Design

Sometimes, there is requirements of equally laying piping arrangements in order to distribute the pressure drop, flow rates, or condensate formation in pipelines connected to different equipment. Examples include piping arrangement for AFC (Air Fin Coolers), Exchangers etc.

As an example, see below snapshot where in two pipeline systems running between Point (A) and Point (B) shall be completely same or symmetrical. In this case, care shall be taken that these two systems may not form symmetrical shape depending on the pipeline configuration before Point (A), or after Point (B).

Final Notes

There are many more general considerations related to Piping Design and Layout that has to be taken care during conceptual and detail design layout development. Some of them were discussed in the above post. The details of piping layout developments for various equipment and piping systems are described further in our blog. Reader is advised to refer these for further details. At the end, any suggestions or comments are welcome.

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