Piping Engineering Deliverables: Piping Design Basis
Objective of this article is to give information about piping design basis document. What is this document and how it is prepared? What are the inputs required to prepare this document? What basic information this document contains? And how information in this document is used for further piping engineering activities. Now, each topic that has been mentioned in this post can be turned into separate post if we write about it in more details. It is my intention to cover key topics that should be covered in piping design basis and give brief explanation about it. If need be, I will make another detailed post regarding that topic.
What is Piping Design Basis document?
The Piping Design Basis Document, or sometimes also called as "Design Criteria Document", is prepared to record technical considerations which are going to be used for piping engineering related activities in projects. At the beginning of project, many technical details are either unclear or unavailable in the "technical requirement document" received from client. Some details have to be assumed based on past similar project experiences. Some details have to be proposed to client if the client does not have any preference. Engineer's experience and judgement plays lot of role in deciding or proposing some of the crucial criteria or method of working in project. All these crucial information, which is required to carry out piping engineering work, have to be clarified and discussed with the customer before engineering activities can be started. After reviewing project technical requirement document, a piping engineer identifies points regarding which more information, clarification or confirmation needed from client. Ways to communicate these concerns are via meetings, emails, comments response sheet, etc... The engineer would write down his concerns and quires on the excel sheet or in email and the client-side engineer would provide his response against each of them. After sufficient exchanges of queries and clarifications, engineers on both sides, come to some commonly agreed upon technical considerations, on which basis all the piping related engineering documents in that project shall be prepared. This technical basis or criteria are recorded in a document called "Design Basis Document"
What are the inputs required to prepare Piping Design Basis Document?
Project's Technical requirement document is the key document to prepare any design basis. This
document could be found among other project contract related documents. When the company usually
receives request for proposal from the client there are many documents mentioning what the project
is about, where it is located, its commercial aspects, contractual terms and so on... Purely
engineering and technical point of view most of the documents you may ignore. But amongst those
documents some documents, usually bundled together, are documents which describe technical
specifications and requirements of the project. These documents contain technical requirements for
all engineering disciplines. And among them there might be few pages or could be few .pdfs related
to piping engineering. These are the documents you must pay your full attention to as an piping
engineer. There might be some other documents that might have been referenced in the document, those
too require your attention. Now, these documents mention most of the technical requirement regarding
project. There are many things... not all of which can be described here, but an engineer is
supposed to read these document and find out the details regarding his work available. For the
things which are not available or not clear enough to understand, engineer need to raise queries and
seek clarifications form client, and the engineer from the client side will respond to these
queries.
ok, what about the details which are neither present in the technical requirement document and nor
does the client have any answer about? In such scenarios engineering organization has its own data
banks, past experience, past project documents, engineer's own experience and judgement, books,
catalogues, standards, vendor's recommendations, and other reliable sources of information. Any of
these can be used to fill the gaps which are left by client. Of course these need to be communicated
with client and get his approval regarding its use. So, all the above mentioned documents and
communication records will provide you the needed inputs to detail out your design basis, which then
will become one of the most important input document for all the downstream engineering related
activities.
What information does Piping Design Basis Document Contain?
piping design basis contains all necessary information that is going to be required for piping engineers and designers to prepare piping related documents. Except Of Course, pipe stress related documents. For pipe stress analysis work separate design basis document is prepared. Information covered in Piping Design Basis Document varies based on project type, project scope, processes involved, companies and stockholders involved. So, in this blog post I shall only cover key items that should be included in any piping design basis document.
Scope of Piping Work.
Scope of piping work is one of the first sections in any piping design criteria document. Also, the section called "Introduction" or "General" can be found in one of the first sections. These sections basically give brief information regarding project such as what the project is, where it is located, who are the parties involved. What are the end goals or products of project, what is the capacity of the project, etc... etc... Now, "scope of piping work" section specifically mentions which piping is involved in the project scope. The actual plant could be very big but only some portion of it could be a part of contract and could be part of piping engineering. That part needs to be identified specifically in these sections. This can be done by identifying piping services or by areas in plot plan. For example, there could be fifty different piping services in the plant but only some of them could be part of the project. These services need to be listed in this section. Or sometimes scope is not decided service wise. It is defined area wise. In a very large plant only piping in certain area is under project scope. key plan identifying these areas need to be given in this section. Sometimes, only piping is in the project scope and not the inline components such as certain type of valves, strainers, specialty items and even some equipment. and in some projects, all these are part of the scope. This clarification needs to be mentioned in this section.
List of piping services
All the piping services to which this piping design criteria document applies should be listed in the document. This list should basically give information regarding service fluids, its design and operating parameters, its base material, whether fluid is toxic, lethal or flammable.
Boundaries and battery limits
They are mentioned clearly in scope of work sections. Scope for some piping could start or end at certain plant boundary or at some equipment nozzle. List of interfaces point, or terminal points (TP points) should also be referred in this section.
Exclusions:
In some cases, it becomes necessary to specifically mention certain piping services or certain areas of the plant which are intentionally excluded from scope of work. Such items are to be mentioned under exclusions.
Deliverable List:
Although, list of piping deliverables is a separate document and is not covered in the piping design basis, its reference should be given in the document. During piping scope finalization, lead piping engineer in consultation with other engineering disciplines and project manager, prepares one deliverable list which only contains piping related documents. This list could be part of the master deliverable list specific to project. Master deliverable list is combined list of all deliverables from all engineering disciplines for particular project. This list is finalized after many discussions and deliberations, internally as well as externally with client. Each project, according to its unique requirements, may include some deliverables in its scope or may exclude some of them. For example, in some very small projects of fifty to hundred lines, client may only request 2D piping GA drawings, they do not need isometrics drawings. Or in some very large projects, such as oil and gas, or power plants, more than one engineering firms can be involved in piping related scope. All most entire piping related scope could be with one organization, but its stress analysis and support detail engineering scope could be with other organization such as Lisega or Anvil, who is expert in piping supports. So, the point is, deliverable list provides lot of clarity regarding the scope of work and estimation of efforts required for the project and should at least be referred in the document.
Reference Documents
Reference documents could be other project specific documents that need to be referenced, or they could be standards that have been used in project. For example, other project documents such as Process design criteria, Service index, Mechanical Data Sheets, etc... could be mentioned in the piping design basis as reference documents. Standards such as ASME B31.3, ASME B16 standards, API standards, and other such standards which have been used for piping design should also be mentioned in the piping design basis as reference documents. In my opinion, some of the following documents should be mentioned in the piping design basis. 1) Process design basis 2) Technical Requirements documents from client. 3) Service Index. 4) Scope of work document 5) CAD plan applicable to project 6) Site survey report for inputs related to site ambient conditions.
Operating and Design Parameters
Operating and design pressures and temperatures are part of process design criteria document. However, piping related some minimum parameters are fixed in design basis. For example, in some organizations and projects it is common practice to fix minimum design pressure for piping as 3.5 bars. Minimum and maximum ambient temperature conditions can be found in the site survey document, but the piping installation temperature should be mentioned in the piping design basis document. This temperature is useful in pipe stress calculations to determine thermal expansions and stresses. This temperature could be an average of maximum and minimum ambient temperature, or it could be the temperature that is occurring during most part of the year and is referred from site environment history data.
Piping Materials Related.
Which piping service will be made of which material? Answer to this question is covered in this section. Although, piping material specification and material classes provides detail information regarding the piping material going to be used for specific piping components, some basic information is provided in the piping design basis. Information such as which material shall be used up to certain temperature range, what shall be minimum thickness to be considered for the certain size of pipe. Under what conditions nonstandard thicknesses can be accepted. Which pipe sizes are to be used, and which are not to be used in project? What kind of piping joints will be used in each of the services?
Material Selection Charts.
In some of the specifications material selection charts can be found. They mention which type of material to be used for certain type of fluids under certain temperature range. which type of pipe can be used? seamless or welded? straight seam or spiral seam. Charts mentioning material compatibility also can be included in the design basis. These charts categorize piping material according to its suitability to fluid by designating them in few categories such as "Acceptable" "Acceptable with caution" "Not Acceptable" or categories such as "Good", "Fair", "Questionable", "Poor", "insufficient data". Later on, piping material engineer has to select or avoid the materials mentioned in these charts according to requirements of fluid while preparing piping material specification. Selection of piping material in the project could also be based on conventional practices. For example, you would never see Inconel material used for cooling water services. They are almost always made of carbon steel or FRP or GRP material. Compressed air services shall almost always use carbon steel. In case of instrument air systems, you may see stainless steel. Material data from previous projects can also be used in current project if it has similar type and nature. These are conventions. Not strict rules. Selection of piping material is always discussed among consultant, client and vendors before finalization.
Connection Details
Which type of connections the piping shall have between pipe, fittings, valves, equipment, inline items, instruments? these connections could be welded, flanged, socket welded or threaded. These are the basic type of connections. There are also connections other than these, which are of temporary type such as Victaulic coupling. Design basis should mention which services shall have which type of connections.
Corrosion allowance
How much corrosion allowance should be considered for piping depends on the piping material being used, life of pipe, corrosion or erosion rate expected over the year or service life and the properties of the fluid itself. Carbon steel pipes with mildly corrosive fluids are usually given up to 1.6 mm corrosion allowance. For more corrosive fluids this limit could be up to 3mm. Highly corrosive services are given 5 to 6mm corrosion allowance too. Corrosion resistant pipe materials such as stainless steel and Inconel are not given any corrosion allowance at all. I shall cover this topic in more detail in another post.
Pipe sizes and Pipe wall thickness
The design basis document mentions some of the basic information regarding certain pipe sizes and their thickness to be used or not in the project. For few examples, in some projects it is mentioned that pipe smaller than half inch shall not be used. Certain odd pipe sizes such as 1.5", 2.5", 5", 7" shall not be used in projects. Pipes smaller than 2 in shall have minimum 40sch or 80sch thickness. Some of the branches such as high point vent or low point drain shall not be smaller than half inch and one inch respectively. Tapings for pressure and temperature instruments shall not be smaller than 3/4 in. Certain services such as sampling systems shall use stainless steel tubing but shall not use tube size smaller than 1/4". Which services shall have short radius elbows, and which will have long radius elbows? Under what pressure temperature and pipe size conditions, miter bends can be used instead of elbows? Although, it is part of process design criteria, sometimes fluid velocity criteria for pipe sizing can also be mentioned in the piping design basis document. Inner surface finish of the pipe is also mentioned in some of the food and pharmaceutical projects. Many such similar examples can be found in piping design basis documents according to project and piping service requirement.
Piping Insulation Related
A separate specification document is prepared for piping insulations. Hence, in design basis only basic information is provided. Such as, which piping services will be insulated and which will not be insulated. Which insulation material shall be used? Mineral wool? Calcium silicate? Glass wool? or any other material? Are there any services handling cold fluids? then for them anti sweat insulation should be considered. Basic categories of insulation with its function is mentioned in the design basis document. Piping insulation usually defined in two categories. One is personal Protections category. In which the function of insulation is to protect person from contact with hot surfaces. And the other is heat conservation category. In which the function of insulation is to conserve the heat of the fluid flowing inside pipe. This type of insulation is very important for the efficient operation of the plant. It is required on all the process related hot piping where losing heat could mean lose in energy and subsequently lose in plant efficiency and ultimately lose in money. Whereas personal protection insulation is not required at all places. Only areas where human movement is expected, such as access ways, walkways, maintenance areas, personal protection insulation is provided to piping. Calculation of applicable insulation thickness depends on many factors such as ambient temperature, wind velocity, temperature of fluid inside piping, the size of the piping, thermal conductivity of insulation material, the surface temperature of the insulation material, whether insulated piping is located indoors or outdoors, purpose of insulation, etc.... Applicable insulation thicknesses are decided based on thickness chart. They are also calculated from some of the known softwares. Insulation specification in itself is a large topic with its rich details. I will cover insulations in separate blog post.
Piping Layout Related.
Piping design basis document must contain piping layout related guidelines. These are the core sections of the design basis document. These sections basically provide guidelines regarding various aspects of piping layout. How will you route your pipe? How much shall be distance between two pipes? pipe slopes, guidelines to ensure accessibility maintainability and operability of valves and equipment. What shall be minimum clearance between wall and equipment, for material and equipment removal and handling, for walkways and man movement, etc…
Access and Clearances
Dimensional information about clearances can be found in design basis documents. Its purpose has already been mentioned in paragraph above. According to requirement of movement for man, material, equipment or equipment parts, dimensions of these clearances are determined. For man movement walkways are provided in plant. conventionally these walkways could have width of about 600 to 900 mm depending on the man movement on the passage is expected. And the clear height of walkway could be about 2.2 meters. Sufficient for average size human to walk comfortably without obstruction.
Empty corridors or removal space can also be provided for material or equipment movement. Some large equipment sometimes needs to be removed from their place and taken to either place of maintenance or out side the plant to be replaced. Sufficient space for crane access to the equipment and then space enough for the equipment to move along the crane and come out of the plant without touching any of the other plant equipment, piping or structure need to be planned. Sometimes the entire equipment does need to come out in one piece. It can be dismantled at place in multiple components and then each component can be brought out one by one. So, the dimension of this space depends on factors such as how large or heavy the component that is being removed? what is the path through which this equipment can be removed? How large is the lifting equipment that is going to be used and its path? After consideration of all those factors the dimensions of these clearance spaces are determined. No piping should ever cross through these passages. If by circumstances it has to, then that piping should be planned removeable type, so that when that equipment has to be removed, the piping that is coming in its path can also be temporarily dismantled.
Pipe racks, pipe sleepers and underground piping culverts have their own space related criteria. When pipe racks are planned between various building or plants they have to cross certain roads, streets, accessways and railway tracks. Criteria mentioning how much clear height should be left below pipe racks at all such crossings can be found in piping design criteria document. According to expected vehicular movement and how large the vehicle is these clear heights are determined. It can vary from 4m to 9m depending on the practice that changes from country to country and project to project. According to type of such crossings expected in plot plan and type of vehicular movement expected at that crossing, the piping design basis mentions specific clear heights at those specific locations.
The information regarding which piping services shall be routed underground and which will be above ground, is also provided in piping design basis. How much earth cover will be provided for buried piping? Will there be any slope to this buried piping? if yes then how much? This information can be found in the sections for underground piping. When underground piping passes below any road or railway track sleeves or culverts are provided.
Expected Plant Design Life.
The years for which the plant is expected to work is sometimes mentioned in the document. sometimes this figure could be 20 to 30 years or could be up to 50 years. Depends on the type of project and its economic viability. During design of piping and all other engineering systems related to project, this expected plant life is taken into consideration. If the piping system is going to work in some sort of batch process, then expected plant life also mentioned in number of operating cycles. Depending on the type of plant and project these operating cycles could be in range of thousands e.g. 7000, 10000, 15000 etc... Number of operating cycles is an important input to determine stress range reduction factors in fatigue stress analysis of piping.
Units used for project
which type of units shall be used for calculations and dimensions on the piping deliverable is mentioned in the design basis documents. It could be SI units or imperial units or any other preferred units. Most of the projects I have worked were in SI units but also have seen European and Canadian projects where FPS units were used. Many factors determine which type units shall be used for the project. It could be customer's preference, could be standards of country where the project is constructed, Could be sometimes preference of key equipment vendors of project, and when there is no preference mentioned by any party then engineering company's own practices determine use of units. In short which type of units shall be used in the project is determined at the very early stages of the project and the same should be mentioned in the design basis documents.
Hydrotest Related:
Whether piping is to be hydrotested or just leak tested is mentioned sometimes in the piping design basis. Piping hydrotest procedures are separate documents with more details but some basic information such as which piping services shall have hydrotest performed on them and which will just be leak tested. What will be the criteria to select the test pressure for the piping? which test fluid shall be used for the hydrotest? will it be water or will it be oil or will it be compressed air?
Plant Co-ordinate System
The co-ordinate system for any project is also selected at during basic engineering. The same information is also mentioned in the CAD plan for the project. During plot plan development the plant co-ordinate system is finalized and is used by all engineering disciplines. Origin of co-ordinate system is marked on the plot plan. This could be corner of some plot, could be centre of some key equipment, particularly significant location of certain building, or if it is some existing plant then the origin of that existing plant can be used for new plant being built adjacent to it. Origin of the co-ordinate system and direction of its axis are chosen such a way that most of the buildings, structures, piping can be planed orthogonally to it. So, that all the dimensions on the engineering drawings can be mentioned in terms of X or Y and not both. The directions of X and Y axis are determine based on the North direction. In case when plant construction is aligned with actual true North, the same can be used as the plant North and the co-ordinates can be mentioned with reference to it. If the plot plan and the construction need to be planned in such a way it is not aligned with the actual north direction then the direction that is aligned with the plant construction is chosen and it is called plant north. This plant north would have some degree of angle with actual north direction but the plant north will be used for creation of all the drawings related to that project. In project it may happen that only one or two large buildings are not in alignment with the over all plot plan. For such buildings building north is defined, and it is applicable to that particular building and drawings related to that building only. So, the design basis documents should give reference to the plant north direction and the co-ordinate system it is going to follow for all the drawings and 3d modelling activities.