Piping Layout Considerations | Calgary, AB


Piping Layout Considerations

Project Client and Owner Requirements

Most projects have project-specific requirements imposed by the owner. These usually include additional requirements above the codes and standards which may have direct impact on both pipe layout and equipment location. Most of these requirements derive from operations feedback which the owner contractually invokes on future projects. Owners may not have a thorough understanding of all the levels of detail required to produce a piping design, but they know the finished product. It is very important that all project personnel and designers know and understand these requirements..

Hierarchy of Reference Design Information

To commence the routing and design of any piping system, the designer is referred to Hierarchy of Design Documents. In the ever-expanding electronic engineering environment, the documents identified can and will be replaced with databases, but the flow of required information to design the piping systems will remain the same.

System Piping and Instrumentation Diagrams (P&ID). These are the schematic single line process diagrams which define the sequence of equipment, valves, in-line components, pipeline sizes, and overall system arrangement required for proper system function. Computer-aided P&IDs that link the schematic diagrams to electronic design data are preferred in order to perform computerized P&ID compliance checks.

P&ID Implementation and Physicalization. Piping and instrumentation diagrams are the piping designer’s road map for laying out piping systems. The designer should understand the P&ID and the specific system characteristics. With this knowledge the designer is required to develop the P&ID and arrange connections and branches as required to best suit the process to actual physical design.

Project Piping Specifications. These documents or databases define the following essential information: the system design and operating pressures and temperatures; piping materials; pipe wall thickness or schedules; types of fittings to be used, e.g., butt weld, socket weld, or screwed; and the valve and flange pressure rating and insulation requirements. In addition, the piping specification defines the fabrication, examination, testing, inspection, and installation requirements, including the requirements for seismic installations, where applicable.

Equipment Outlines. These documents can be either imported computer-aided design and drafting (CADD) files or prints of the equipment being piped. They include overall dimensions and the pipe size, wall thickness, flange pressure rating, and locating dimensions of all pipe nozzles and other connections.

General Arrangements or Equipment Location Drawings. These drawings will indicate the location of all major pieces of equipment in the plant which the designer will either verify or relocate, as required, to accommodate the physical pipe routing as designed or redesign the piping to accommodate the particular piece of equipment.

Generally equipment location drawings are developed by senior-level piping designers during the proposal preparation and are taken over by the project team upon award of the contract. From this point on they are revised and updated as part of the normal process of design development. Equipment should be arranged with the piping layout in mind. Equipment locations and relational arrangements should be evaluated during the piping layout design process. Adjustments and occasionally major changes to equipment arrangement are required to solve major piping arrangement problems. Piping system design is dependent on the input from numerous reference sources prior to the start of piping design.

Collection of As-Built Information

CADD and electronic surveying capabilities have changed and are continuously changing. Photogrametry (photographs that are input into three-dimensional CADD models) and laser mapping (laser scanning using a time of flight laser connected to a computer that translates the scanned points to a three-dimensional CADD model) are applications that enable the designer to collect existing conditions which can be imported into the designer’s CADD files. Total Station Surveying is the computerized surveying system which engineering should request for the collection of survey data points with the electronic transfer of information being able to be translated directly into the CADD environment.

Piping Layout Considerations and Planning Studies for Improved Piping Economics

Proper planning is an important activity performed by the piping designer in the early stages of a project. Space conservation and a symmetric piping arrangement are achieved when all the systems are evaluated in the preliminary stages of design. This study will become the final design. It is important to consider the cost of the piping material at this time for the expensive lines. These lines should be kept as short as possible, while maintaining proper piping flexibility even if this requires changing the equipment arrangement.

Detailed design should not start until the planning studies are complete. Ex-pending engineering work hours on details that are subject to change pending thecompletion of the planning study is not recommended.

Piping layout then becomes a matter of designing dimensioned routings from one point to another point with the branches, valves, piping specialties, and instrumentation as indicated on the P&ID. This statement, however, is an over simplification of the process, since many other factors must be considered, such as interference,piping flexibility, material costs, pipe supports, operation and maintenance, and safety and construction requirements.

An example would be moving a pump 3 in (75 mm) to avoid a compound elbow offset in order to connect to the top discharge nozzle. Perhaps the equipment was arranged while planning on a side suction and discharge. Refer to Fig. B3.1.

Nozzle alignment drawing

Pipe Bending

Pipe bending has become increasingly widespread due to a desire for a decrease in fabrication costs. If bending is to be used, the designer should consider special requirements imposed by the process (i.e., tail ends and clamp dimensions are required by the bending machine, and increased distances and space are required because bends have a greater center-to-face dimension than conventional fitting dimensions).

Interferences

One of the most important aspects of piping layout is the avoidance of interferences with other facilities in the plant such as other piping systems; structural steel and concrete; heating, ventilating, and air-conditioning (HVAC) ductwork; and electric cable trays and conduit. For engineering firms using 2D CADD (two-dimensional computer-aided design and drafting) or manual drafting and design, the search for interferences is very tedious and time-consuming since the designer must mentally and visually look for interferences between the systems currently being designed and previously designed or the existing system or facilities, not to mention those systems or facilities in design concurrently. This process is extremely complex at best. Traditionally, this has been accomplished by the use of area composite drawings (see Fig. B3.2) and plastic scale models.

The composite drawings and plastic models show all plant facilities designed to date and are used by the designers to select an interference-free route for the system currently under design; however, the designer still must search out those systems or facilities in design concurrently. Once the designer is satisfied that the current system layout is interference-free, it will be added to the area composite drawing and the plastic model.

An alternative to composite piping drawings and plastic models for interference detection is the use of computer-aided design (CAD). Specifically, three-dimensional (3D) computer modeling can provide an efficient, accurate, and cost-effective alternative to the traditional manual methods for interference detection. This and other CAD applications for piping layout are addressed in the section ‘‘Application of Computer-Aided Design to Piping Layout.’’ Refer to Fig. B3.3.

Piping Flexibility

The effects of the thermal expansion of pipe and fittings as a result of system operating temperature changes cannot be overlooked during the layout and routing of any piping system. The function of piping flexibility or stress analysis has, for the most part, been delegated to the computer particularly in the case of high-temperature, high-pressure piping systems. The piping stress analyst translates and enters the piping design data into the computer, reviews the output data, and if the system is too rigid, may suggest appropriate corrective redesigns. However, it is the piping designer’s responsibility to ensure that the final stress analysis results are incorporated into the final pipe support and pipe routing design.

In the past, a computer stress analysis, including the development of input data and the interpretation of the output, could be expensive and time-consuming if numerous iterations of computer runs were needed to arrive at an acceptable system design. The experienced piping designer, with the knowledge and capability of

FIGURE B3.2 Area piping composite.

A 3D CAD model used for interferences, equipment layout, and pipe routing.

designing piping systems that are inherently flexible, was relied on to keep the number of computer iterations to a minimum. Today, this is much less of a problem with the advent of the personal computer and many computer programs for calculating stresses in piping systems due to thermal expansion and other static and dynamic loads. However, the piping designer must integrate piping flexibility considerations into the piping layout.

The piping designer should route piping with flexibility designed into it, using the minimum amount of pipe, fittings, and expansion loops by considering the following:

  • Avoid the use of a straight run of pipe between two pieces of equipment or between two anchor points.

  • A piping system between two anchor points in a single plane should, as a minimum, be L-shaped, consisting of two runs of pipe and a single elbow. This type of arrangement should be subjected to a ‘‘quick-check’’ analysis to determine if a formal computer stress analysis is required. A preferred solution in this case may be a series of two or more L-shaped runs of pipe.

  • A piping system between two anchor points with the piping in two planes may

  • consist of two L-shaped runs of pipe, e.g., one L-shaped run in the horizontal plane and another in the vertical plane. This arrangement should also be subjected to a quick-check analysis.

  • A three-plane configuration may consist of a series of L-shaped runs and/or U-shaped expansion loops designed into the normal routing of the system.

  • When the expected thermal expansion in any given run of pipe is high, considerthe use of an anchor at or near the center of the run, thereby distributing theexpansion in two directions.

  • For systems consisting of a large-diameter main and numerous smaller branchlines, the designer must ascertain that the branches are flexible enough to withstand the expansion in the main header.

  • Systems which are to be purged by steam or hot gas must be reviewed to ensure that they will be flexible during the purging operation.

  • System or equipment bypass lines may be cold due to lack of flow while the main runs are at operating design temperature, resulting in excessive stresses.

  • Temperatures during initial start-up and testing are often greater than those atoperating conditions.

  • Closed relief valve and hot blowdown systems should be given special attentiondue to rapid transients in temperature.

In addition, the piping designer may use a variety of single- and multiplane piping arrangements, such as the L-shaped, the U-shaped, and the Z-shaped config-urations, in the normal routing of any system, as shown in Figs. B3.4 through B3.9.

multiplane piping arrangements

Valves

The piping designer must be familiar with proper application of all types of valves including gate, globe, plug, butterfly, ball, angle, diaphragm, check, pressure relief, and control valves and their methods of operation including manual, chain, gear, air, hydraulic, or motor. The following general guidelines should be applied when locating valves in any piping system:

  • Valves should be installed with the stems between the vertically upward and horizontal positions with particular attention given to avoiding head and knee knockers, tripping hazards, and valve stems in the horizontal plane at eye level that may be a safety hazard. Large motor-operated valves should be installed in the vertical upright position where possible to facilitate support and maintenance.

  • Valves in acid and caustic services should be located below the plant operator’s eye level or in such a manner as to not present a safety hazard.

  • The location of valves, with consideration for operating accessibility, should be accomplished in the natural routing of the system from point to point, avoiding the use of vertical loops and pockets.

  • Valves in overhead piping with their stems in the horizontal position should be located such that the bottom of the hand wheel is not more than 6.5 ft (2 m) above the floor or platform. Only infrequently operated valves should be located above this elevation, and then the designer should consider the use of a chain operator or a platform for access.

  • Where chain operators are used, the valves should be located such that the chain does not present a safety hazard to the operating personnel.

  • A minimum of 4 in (100 mm) of knuckle clearance should be provided around all valve hand wheels.

  • Valves should not be installed upside down.

  • Space should be provided for the removal of all valve internals.

Improper application and placement of valves in the piping system can be detrimental to system function. This can result in malfunction of the valve and in water-hammer, and this can cause the valves to literally self-destruct. What follows are some specific recommendations and methods of avoiding these problems for some specific types of valves.

Control Valves. All control valve stations should be designed with the valve stem in the vertical upright position and a minimum of three diameters of straight pipe both upstream and downstream of the control valve, in order to reduce the turbulence entering and leaving the valve and to provide space for removal of the flange studs or bolts. Where applicable, this straight pipe will include the usual reduction in pipe size required to match the control valve size. Space must be provided for flange stud bolt removal where control valve bodies are designed for through-bolt installation.

Butterfly Valves. Butterfly valves should be provided with a minimum of five diameters of straight pipe upstream of the valve; and if this requirement has been met, the valve stem and operator may be oriented in the position best suited for operation and maintenance. When a butterfly valve is preceded by an elbow and this straight-pipe requirement cannot be met, the valve stem must be oriented in the same plane as the elbow. That is, if the elbow is in the vertical plane, the valve stem must also be in the vertical plane. This recommendation is based on the fact that the velocity profile of the discharge of an elbow is not symmetric. The result can be fluid dynamic torque that is twice the magnitude of that found for a valve with a straight run of pipe upstream. The resultant eccentric forces applied to valve disk produces excessive vibration and disk flutter which eventually may completely destroy the valve.

Check Valves. The preferred installation of any check valve is in a horizontal, continuously flooded run of pipe with cap up; however, swing check valves will function properly in vertical runs of pipe with the flow up. However, the velocity and the rate of flow must be adequate to move the valve disk away from the seat and to maintain the valve in the open position, as required.

Experience has indicated that check valves are highly susceptible to chattering due to upstream turbulence caused by elbows and branches. Therefore the designer should provide upstream straight pipe in accordance with the valve manufacturer’s recommendations. However, where this information is not available, the preliminary design should include a minimum of five diameters of straight pipe upstream of all check valves. In addition, the designer should be aware that this requirement can be as much as 10 diameters of straight pipe depending on the type of valve and the manufacturer.

Safety Relief Valves. The arrangement for installation of safety and relief valves is very critical and involves the actual location of the valve itself, the design of the vent stack, and the design of any associated drain piping. The designer should adhere to the valve manufacturer’s recommendations and the following guidelines; however, these guidelines relate to the power industry and may be used elsewhere, as applicable.

Valve Location

  • All relief valves must be in the vertical upright position and fitting-bound to the top of a horizontal run of pipe, the pressure source, and must not be located less than one nominal header diameter from any butt weld.

  • A safety valve inlet connection in a high-velocity steam line should be located at least 8 to 10 nominal header diameters downstream of any bend in the header, to minimize the possibility of acoustically induced vibrations. In addition, it should be at least 8 to 10 nominal header diameters either upstream or downstream of any diverging or converging T or Y fitting.

  • No other header branch penetration, for any purpose, should be made in the same circumferential cross section containing the safety valve inlet nozzle.

  • Where more than one safety valve or a service branch is to be installed in the same header run, a minimum distance of 24 in (600 mm) or 3 times the sum of the nozzle inside radii, whichever is greater, shall be provided between the nozzles.

  • Where more than two safety valves are located in the same header run, the spacing between valves should be varied such that the distance between two adjacent valves differs by at least an inlet nozzle diameter.

Open Discharge. Open-discharge safety valve installations (see Fig. B3.10) should be in accordance with the following guidelines:

  • Open-vent stack diameters shall be the calculated minimum flow diameter required for discharge venting without blow-back, except as required to accommodate the movement of the relief valve discharge from the cold to hot position such that the outlet pipe will be centered in the vent stack in the hot position. See Fig. B3.10.

  • The vent stack entry diameter shall be maintained throughout the length of the vent stack; enlarged entry spools, later reduced in size to the calculated minimum flow diameter, are not acceptable.

  • The relief valve outlet shall consist of the mating flange and a fitting-bound short-radius elbow, in order to minimize the moment and forces imposed on the valve body.

  • Vent stacks should be routed, where possible, to provide a straight stack of minimum length. Where offsets or changes in direction are unavoidable, it is desirable to limit the change in direction to 30 or less; however, it could be more. The vent stack should terminate a minimum of 7 ft (2.2 m) or higher above the roof level.

Closed Discharge. Closed-discharge piping systems (see Fig. B3.11) are those piped continuously from the valve discharge flange to a closed receiver, such as a condenser or blow-off tank. This type of system is required for feed water heater shell side relief valves to provide protection against the effects of tube rupture, and may be used in other applications. Other than the normal considerations for designing pipe, there are no specific guidelines for the design of closed systems.

Drains. Relief valve and open-vent stack drains are important, in varying degrees, as discussed in the following:

  • The discharge elbow and above seat body drain points are the most critical for safe valve operation. These drains should be collected into a common, closed drainage system and routed to a point where the drain can safely blow to atmo-sphere. This system must be sloped continuously downward and stress-analyzedto ensure that no strain is imposed on the valve body.

  • Some relief valves now incorporate a relatively large valve top vent connection that is pressurized when the valve blows. This connection may be piped into the combined discharge elbow and valve body drain system and continued, at full-vent pipe size, to the point of the drain discharge.

  • The open-discharge vent stack drip pan drain connection is of the least importance and is only intended to carry away any rainfall entering the stack and the residual condensate from the stack following a steam blow.

Open-discharge safety valve

Open-discharge safety valve

Piping of Centrifugal Pumps

The piping of centrifugal pumps, particularly the suction piping, can seriously affect the operating efficiency and life expectancy of the pump. Poorly designed suction piping can result in the entrainment of air or vapor into the pump and cause cavitation, which displaces liquid from within the pump casing, results in vibrations, and throws the pump out of balance. The cavitation alone can result in severe erosion of the impeller. The out-of-balance condition may result in a slight eccentric shaft rotation, which will eventually wear out the pump bearings and seals, requiring a pump shutdown for overhaul. When routing piping at pumps, the designer should follow the manufacturer’s recommendations, the Hydraulic Institute Standards, and the following guidelines:

  • The suction and discharge piping must be supported independently of the pump such that very little load is transmitted to the pump casing. The designer may consider the use of expansion joints on either the suction or discharge, or both, as necessary. However, expansion joints should be used only when it is unavoidable.

  • The suction of any centrifugal pump must be continuously flooded, and the suction piping shall contain no vertical loops or air pockets.

  • When a reduction in pipe size is required at the pump suction, provide an eccentric reducer flat side up. See Fig. B3.12.

  • The suction side elbow in the piping at horizontal double suction pumps may be fitting-bound and in the vertical plane with the flow from either above or below the pump.

  • When the suction piping is in the horizontal plane, provide a minimum of three to four diameters of straight pipe between the pump suction connection and the first elbow; the eccentric reducer noted above may be included in this straight section.

  • Only long-radius elbows are to be used at or adjacent to any pump suction con-nection.

  • All pump suction lines must be designed to accommodate a conical-type tempo-rary strainer.

  • A pipe anchor must be provided between any expansion joint or nonrigid couplingand the pump nozzle that it is designed to protect.

When pump flanges are cast-iron flat-faced, the mating flanges must also be flat-faced and the joint made up with full-face gaskets and common steel bolts (ASTMA 307, Grade B), not high-strength bolts (ASTM A193, Grade B7). Refer to the Hydraulic Institute Standards for arrangement of pump piping.

Pump suction reducer

Vents and Drains

During the course of physical routing of any system, the designer should provide high-point vent and low-point drain connections for the following purposes:

  • The filling of the piping system with water for hydrostatic testing and operation and the evacuation of entrapped air in