PIPING DESIGN FOR PUMPS IN PARALLEL 3

[roker]

Hello,

What are the guidlines for the design of the suction and discharge lines for pumps operating in parallel (e.g. symetrical, larger diameters, etc.)?

regards,
roker

 

[pennpiper]

Roker, The following is based on my more than 45 years in the piping engineering and design field. Most refineries and chemical plants install two pumps in each critical process operating point. The reason for this is to maximize operational integrity. What I mean by this is that they do not want a pump failure to shutdown the whole plant. Pumps will fail and for a number of reasons. The owner/operator wants to prevent the big financial loss that results from a shutdown.
Now to answer your question:
A. I recommend that you pipe up the pumps in this situation, first as if there were only one pump. The tie-in for the second pump needs to be a duplicate (routing and fitting wise) of the first pump.
B. The line size for the suction side (from the suction source) should be the full nozzle size all the way to the point the line needs to reduce to match the pump nozzle size. Do not reduce the suction line size when you split for the second pump.
C. The discharge line (from pump #1) should increase in line size from the pump nozzle size to the full line size as soon as possible. This required line size then should continue until there is a valid process or instrument reason to change.
D. The discharge line (from pump #2) should increase in line size from the pump nozzle size to the full line size as soon as possible. This required line size then should continue until it ties in to the other discharge line.
E. As much as possible try to make the two pump suction lines symmetrical from the split to the pump.
F. As much as possible try to make the two pump discharge lines symmetrical from the pumps to the rejoining point.

I hope this helps.

 

[JJPellin]

The piping configuration is very important with all the points made by pennpiper. However, in order to finalize the piping, it is important to understand the process conditions. Specifically, I would be concerned about low flow operation. The pump manufacturer will provde a Minimum Stable Continuous Flow on the datasheet. This flow would not be adequate for parallel operation. In other words you can't assume that one pump can run as low as 100 gpm so two pumps can run as low as 200 gpm. Because the pumps will never be perfectly matched in condition and speed, it is important to be more conservative and provide a higher minimum flow. A properly designed minimum flow spill-back can accomodate lower flow without compromising reliability. Our biggest problem with parallel pumping is because they run two pump when they need higher pressure, but run them at too low a flow rate to be reliable.

 

[abeltio]

avoid air pockets - in horizontal piping do not use concentric reducers... use eccentric reducers with the flat side on top.
if inverted "U"'s are unavoidable... put vents at the top of each inverted U.
air in piping is one of the most common causes of piping problems... and is usually due to poor design practices.


saludos.
a.

 

[cadnutcase]

abeltio,

I used to always pipe my pumps with the eccentrics flat side top, but our R&D guy, who is way smarter than me, convinced me that this is always not the best way to do it.

His philosphy was if you had the flat side on bottom the air would want to naturally go back up the suction line to the header. So I cried BS on him and pulled out my piping guide, you know the blue one that is longer than it is tall. Sure enough if you look at page 104 and 105, you will see that the flat side is down.

So now I want somebody to prove to me that the flat side should be up.

cadnutcase.

 

[pennpiper]

With regards to cadnutcase's comments about the blue “PIPING GUIDE” book. He is right, the book does show the reducer installed in the bottom flat orientation.

I also have a copy of this book in my library. I have used it but only on a limited basis over the years. I will say that the author and the publisher did a fairly good job on this book. Not a great job, just a good job. Over the years as I used this book I found a number of errors that reduced my confidence in the total product.

The major error (eccentric reducer orientation at pump suctions) is just the one cited in this thread. Another (page 93) shows a gate valve installed upside-down. Others are minor typo errors (see page 9, for the reference to reinforced stub-ins 2.11 should be 2.71). Most of these can be discounted as nit-picking and that is true. However, on the subject of eccentric reducer orientation at pump suctions it is another matter.

This one book (“PIPING GUIDE”) is the only source where the bottom flat (BF) orientation is promoted. I have not found any other support for this BF orientation. Every Engineering company I ever saw used the top flat (TF) orientation. Every owner/operator company I ever did work for required the TF orientation. The Hydraulics Institute uses the TF orientation. How can so many people be so wrong? Maybe they are not wrong.

Look at it this way. If one person tells you that the pit viper is not deadly and it is okay to play with it but a hundred people tell you that a pit viper is a deadly snake and that you should not play with it who are you going to believe?

 

[Zapster]

Regarding the installation orientation of an eccentric reducer, what is the consequence of having an air pocket if the fitting is installed flat side down? If there are no pockets in the line from the supply to the suction side of the pump, won’t the air migrate back to it’s origin, header or tank? If there is a pocket in the line that prevents the air migrating up, then you would have a slug of air that the pump would need to digest anyways. If the reducer’s flat side is on the top, and solids/water settles out of solution creating a closed environment that is subject to crevice type corrosion, is this advantageous? Seems like there are a lot of factors here to consider where there is no one simple answer about which is the desired position.

What harm will come to the pump if the eccentric reducer is upside down? In a fuel piping system which is worse a pocket of air on top or a slug of water on the bottom?

 

[svanels]

If you have air pockets in your suction line, you are inviting a "beast" called cavitation. Have you ever seen an impeller erroded by cavitation? It looks like a cheese that was attacked by a bunch of starving mice.

 

[Zapster]

I thought that cavitation damage was caused from the rapid formation and collapse of bubbles. Typically from not having enough suction head and the fluid being reduced to its vapor pressure, vapor bubble can then form and later collapse in a higher pressure region.

If there are no pockets in the line, why won’t the air migrate out before the pump is started. If there are pockets of air in the line, it will be pulled through the pump one time at startup. A gulp of air is not cavitation. If air accumulates in a zone after the system is started, it would indicate that the fluid is saturated with air and that the excess has come out of solution. How would this pocket of air feed a cavitation problem when the fluid is already saturated with air?

 

[cadnutcase]

Pennpiper,

Agreed, just because one guy is telling me something doesn't me I would believe it. This guy is just one of those scary smart guys that I have never seen proved wrong before when the numbers really start to come out.

Other side of the coin, I don't think I have designed a pump arragnment where there was even a little posibility that air pockets could get into the pump.

I really wanted to post up on this just to see what kind of comments I would get back.

The real question are these.
Do I belive him in my particular application he is right.
Yes
Would I try it.
No. (When in Rome)

By the way I looked up page 93, all I know is if I tried to put that valve stem on any type of valve upside down, I would never hear the end of it.

Years ago I took a piping test when applying for a job and I specifically remembering that question being on there.

I got the job.

 

[Zapster]

Cadnutcase,
Piping systems are designed all the time where air pockets are known to exist. Hence, the sell priming pumps come into play. Could this be a pump priming/start-up issue? I don't know, just trying to understand.

 

[katmar]

The eccentric reducer orientation problem is one of those engineering questions that comes up again and again without ever seeming to be totally resolved. I believe the reason that it is never properly resolved is because, except in really extreme cases, it does not matter much.

I have seen pump suctions with the reducer flat side down, or with a concentric reducer, working just fine. The theory, as I learnt it many years ago, is that if there is a possibility of air entrainment then you put the flat side up. And if there are solids or two phase liquids present you put the flat side down.

In my opinion, unless there is a very long perfectly horizontal section feeding straight into the suction of the pump air entrapment will not cause problems. I would love to hear from someone who resolved a pump problem that was definitely linked to a reducer being installed in the wrong orientation. I've never seen or heard of it myself. I agree with Zapster that the air would be pulled through the pump.

Air entrapment is a very different beast from cavitation. The damage that results from cavitation is not due to the bubbles themselves. It is caused by the localised high pressure "implosions" as the bubbles collapse. Air bubbles drawn into a pump would not collapse and would therefore not cause cavitation. The bubbles may cause the pump to run rough, but is not uncommon for pumps in the cane sugar industry to run "on the snore", where they continually draw the suction level down until they pull air into the pump.

I would also like to agree with JJPellin that you should consider the case of the two pumps running simultaneously, even if the idea is that they are actually to be a duty pump and a standby. Operators often believe that if some flow is good, then more flow must be better so if they can run the two pumps together they will.

regards
Katmar

 

[abeltio]

seems like most agree that no matter where the flat is... if properly vented it should not make a big difference...
always learn something...


saludos.
a.

 

[svanels]

And if there is no proper vent, it will make a difference on the opex..

 

[StressGuy]

Alright, here's another issue regarding reducers for you guys to chew on - where to put it? Directly on the pump nozzle, or five diameters back as specified by API?

Seems that most pumps that get installed have typically had the reducer right at the pump nozzle. Yet, API 686 calls for a minimum of 5D of straight run upstream of the nozzle for an even flow and specifically mentions reducers as an item to not have in that 5D section.

Edward L. Klein
Pipe Stress Engineer
Houston, Texas

"All the world is a Spring"

All opinions expressed here are my own and not my company's.

 

[cadnutcase]

StressGuy,

We use a lot of Paco KPV inline pumps, Their recommended piping says 5D's upfront. I had to laugh because we never do it, then I stopped laughing and began to wonder why they did that and if we should be doing it.
Then I got to thinking that if we followed every recommended vendor criteria, our skid packages would be the size of the astrodome and just as hard to ship.
Disclaimer - I am disclaiming I know how to spell. ^

By the way I am in Houston too, we could be neighbors (another good laugh)

 

[Zapster]

where to put it? Directly on the pump nozzle, or five diameters back as specified by API?

Depends if you want the pump flow to match the pump curve.
Depends if you want to keep the efficiency that the manufacturer lists.
Depends if you want to accelerate localized erosion in the pump.
Depends if you have excess NPSHA to afford the reduced NPSHR.
Depends if you want to ensure there are no warrantee issues.
I am sure there are more; these are the ones I can think of off the top of my head.

 

[abba7114]

StressGuy,
My understanding of the 5D of straight run upstream of the suction nozzle is avoiding turn or throthling.The reducer should be right on the nozzle so that the minimum 5D should be down stream the eccentric reducer.

On the issue of reducer orientation some of the postings above did not give any exception for ecc red orientation, if ecc red is not for pump suction is even adviced that the it should be FSD for supportability in group on pipe rack ie. common BOP with other pipes.
And I have seen in some services it recommenden FSU for pump suction reducer in flashing liquid services (pumping liquids at temperature near its boiling point).
Thanks
Abba-Gana A.