Basic Piping Questions
A few days ago I received an E-mail from Newton Newton Raj in India. He asked “Can you please answer the following questions?” I suggested that the best place for these types of questions would be the http://www.pipingdesigners.com website under “Technical Forum.” My thought was that everyone could get the benefit of seeing both the question and the answer. There is also the opportunity for others to offer supportive or conflicting answers and opinions.
His questions were]www.pipingdesigners.com[/url]. There you will find an article titled “Thoughts on Job Descriptions.” In this article you should read the duties and responsibilities of both the piping designer and the piping material engineer. There is an organization chart included in that article that shows the way a typical piping group was organized on a project where I worked. Everybody had a role to play and it was normally one that they were properly trained to do. Thus, you must realize that my answer is based on my training, my experience and maybe my knowledge of a few of the other global engineering companies. There may be companies that do not operate the same as my company did. You and others will need to make whatever adjustment that is required.

Now for your questions:
#1 (How do we select the pipe material?) I am not sure what you mean by the term “we” in this question. You have stated in some of your past posts that you are a piping designer. You are currently on an assignment as a field engineer. So, “you” as a piping designer (or a field engineer) are not authorized to “select” the pipe material. The selection of the piping material is the responsibility of the Lead Piping Material Engineer assigned to the project. The Lead Piping Material Engineer produces a document called the Piping Material Specifications for the specific project. This document would normally include some general notes about the piping material and then it would also include the specific material charts for each “line class” broken down by commodity, pressure, temperature and metallic (or non-metallic) groupings. The Piping Material Engineer works with the Process Engineer during the development of the P&ID’s and is (or should be) the only person authorizes to assign (or select) a line class to a specific line.

#2 (When do we take branch connection in the side of a line?) This is basic piping and is the responsibility of a piping designer. I would say that to attach a branch to the side of a line would only be done underground for a gravity drain situation. For normal above ground refinery, power plant or chemical plant pipe rack pressure piping, I would not do it. The reason I would not do it is that a branch off the side of a primary line would block the lane (along side that first pipe) where I may want to run another line in the future.

#3 (When do we take branch connection from the top/bottom of a header?) This is also the piping designer’s responsibility and should be founded in good training and experience. The answer to this question follows the first two basic rules of piping. Rule #1 – There is a rule for everything. Rule #2 – For every rule there will be an exception. So with this in mind let us consider the following. Rule #1 in this case says if the other end of the branch is higher than the header then take the branch from the top of the header. If the other end of the branch is lower than the header then take the branch from the bottom of the header. Rule #2 in this case says if the commodity is steam, instrument air or other commodity that has a lot of condensables or particles then take the branch from the top.

#4 (When do we use a long stub & short stub?) This is not normally the piping designer’s choice. This is or should be a function of the piping material specifications. The goal is to buy material in bulk and not have too many options that may increase the project costs.

#5 (When do we use slip-on flange, a weld neck flange or a lap joint flange?) The answer to this question is mixed. First, the piping material engineer should have made it very clear which type of flanges are included in (and/or allowed in) each line class specification. The piping material specifications should not include every flange type in every line class. If lap joint flanges are the flanges called for by the line class then you may only use lap joint flanges. For other line classes you would normally use only weld neck flanges for all flange situations for 3” (NPS) and larger. The use of a slip-on flange could be an exception here only when there is a space limitation problem.

#6 (Is an eccentric reducer normally only used at pump suction?) No! Eccentric reducers are also used for line size reductions in pipeways in order to keep the bottom of pipe (BOP) the same and in proper contact with the pipe supports. (It is ever used for any other cases? If so, give the details.) Yes! I have also used eccentric reducers on both the inlet and outlet piping to some equipment. The equipment in question had connections that were very close together and the flange sizes for the equipment nozzles were one or more sizes smaller than the inlet and outlet line sizes and I needed to use the eccentric reducer to off-set the centerline of the two pipes to provide proper line spacing as they increased in size.