Pump Suction Straight Run

This question has been asked before. Check the subject listings.

Bob,
You wrote:
In my post, I asked two questions... not one:

1) Is there a requirement in a particular code or recommended practices document that requires a certain length of straight piping on centrifugal pump suction lines?

2) Often, there is a reducer located just ahead of the pump suction nozzle... doesn't this add to the turbulence prior to a liquid entering the pump?

Yes, I agree, a question has been asked before, but I'm not able to locate the answers to my questions. I really value your piping wisdom and I'm hoping for your expert guidance on this one.

Let me try to address them better than I did before.
1. No this is not a requirement in any recognized piping "Code." An organization called the "Hydraulics Institute" has put out some recommendations about the "10D" issue. I have always seen this as an issuee of theory vs. practical application.

2. The practice of a reducer up-close to the suction of a pump is not a mystery to me. The pump manufacturer and their pump designers have designed a pump to perform to a certain curve. The process engineer making use of that pump needs to insure that the pump is not starved. To do this the suction line is almost always one or more sizes larger than the pump nozzle. In theory this makes the "larger" suction an extension of the source retention vessel. This has not been a problem to pumps installed over the last fifty years that I know of and it has been done for maybe fifty years before that.

I also went looking for the previous postings on this subject. I went through over 2000 on this Forum and others and I did not find it. Sorry!

Below I have included something I saved. I hope it helps.

The question:
“OK I am having a major verbal match with my piping designer. I need a hard cite/reference to a reference book for the required number of straight pipe diameters upstream of the suction flange/suction reducer. I checked all my references (Karassik, Marks, Dufour, Rip Weaver, etc.) but no hard numbers. Everybody just says "smooth flow into the suction. So I did a search here and read all the posts re: straight suction lengths (all 60-plus posts) but nobody gave any cites.
The Pumps are horizontal split-case multistage, BFW service, 4x6-10, 3600 rpm, 3900' TDH, 1160 gpm, pumping temp 180° F, 12" suction line size to get velocity down to 2+ ft/s.”

My answer:
Pete, I commend you for the way you asked your question. You are one of the very few (maybe the only one) who has given us some specific information about your pump. (Pumps are horizontal split-case multistage, BFW service, 4x6-10, 3600 rpm, 3900' TDH, 1160 gpm, pumping temp 180° F, 12" suction line size to get velocity down to 2+ ft/s). Most of the people who ask questions do not give us the information (tools) we need to give qualified answers.
Following the average question, almost all of the people who give answers, give answers that are too general and too vague. The answer to most questions like the question really should be very specific. The hazard here is not so much with the original questioner but with all the novice pipers and others out there who read an inappropriate general answer to a vague question and then accept it as gospel and apply it to all similar sounding situations.
The problem here is that there are so many different types, styles and configurations for pumps that the “popular rule” (10 diameter before the suction flange) makes sense for maybe only 5% of the total family of pumps and no sense for the other 95%. We could also bring up all the different types of pumps that we know about now and we would still miss the new latest state-of-the-art pump.
So, in my opinion the “10D” rule does not apply to all pumps.
I can understand the basic logic for using the “10D” rule on the typical straight in, end suction API style pump. However, even with this type of pump we have to ask if an “inducer” is included on the pump impeller is the desired smooth flow destroyed?
I do not believe it applies to any of the many types of positive displacement pumps. It is just not an issue.
I also do not believe it applies to the vertical “inline” centrifugal pumps or the typical “API”, top suction, top discharge pump. I also do not believe that it applies to your specific pump.
My reasoning, based on my more than 45 years experience in the piping field is this: Why go to all the trouble and extra cost to provide the “10D” configuration on one side of the pump flange gasket and then on the other side the pump case contorts into a complex reverse curve before the liquid comes into contact with the impeller.
You gave us the specifics about your pump but not all that is important. I suspect that it is a side suction pump. This suction nozzle is well below the centerline of the pump. A cut-away of the pump would no doubt show that the inlet path is a very complex shape.
You also told us that the commodity is Boiler Feed Water (BFW) which is a tough fluid to handle do to the potential to boil and create bubble cavitation problems. I would guess that the Deaireator Drum is located directly or near directly 25 to 30 feet above the BFW pumps. The pump suction line should drop down to the pumps in the most direct and minimum length possible with the following exception. The suction line will be operating at a very high temperature and therefore needs to have proper flexibility built into the configuration. The last drop should be minimum distance from the suction nozzle. There are two potential options in my book. There should only be the elbow the reducer and then the pipe flange (fitting-to-fitting). The alternate is a reducing elbow then the pipe flange. Under the elbow (same in either case) there should be a properly designed (secondary) pipe support.