Why is it important to remain around 5 ft/s flow? 6

[shup0739]

For flowing water in steep piping...why is it important to keep the flow at or below 5 ft/s?

Is it this a critical verlocity in which errosion begins?

 

[TD2K]

5 ft/sec for erosion, even if you had some sand in the water, is low IMO. Velocity limits for water are typically in the low teens and if the water is clean, those can be increased significantly.

At higher velocities, especially in small lines, pressure loss becomes greater and greater. If this is a gravity system, this might be the reason they recommend using 5 ft/sec as a maximum velocity. For water in 4" pipe as an example, this is just over 1 psi per 100 feet of pipe.

 

[MJCronin]

shup0739,

Design flow velocities for various liquid are simply "rules of thumb" for piping system sizing. At 5 ft/second, erosion is not an issue for metallic pipe in non-slurry service.

For water systems, designing to velocities of about 4-9 ft per second provides a system with a reasonable balance between high pumping costs and capital installation costs........There are many exceptions to this 4-9 ft/second guideline.

Some of these are:

Pump Suctions...... 3- 5 ft/second
Acid Systems........2-4 ft/second
Slurry Systems...... varies considerably
Feedwater Systems....10-15 ft/second

This topic is discussed in the "Piping Handbook" by Nayyar

Anyone else have any thoughts on this......

MJC

 

[hacksaw]

Are you dealing with conduit or open-channel flow? gravity or pumped discharges?

 

[25362]

MJCronin is absolutely right in that the published values are generally indicative to be used as a guide, the final selection for any given pipe may be governed by pressure drop and system environment, mostly economic factors. One rule of thumb speaks of drain lines with linear velocities in the range 1.5-4 fps; for liquid to reboiler (without pumps) 2-7 fps. For gravity separators literature's R-O-T gives 0.5-1.5 fps.

In addition to Hacksaw's query, is the water free of solid particles or suspended liquid droplets ?

 

[shup0739]

Well i'm basically pumping water from a tank to a vessel through branch piping and I have it going into 8 different nozzles.

There is no solid particles.

errosion i guess isn't a factor if i'm using schd 40 stl piping. But wouldn't it have an effect on the vessel as it discharges through the nozzle at even higher velocity because of the nozzle geometry. I would assume that the vessel surface will be affected by the high speed water hitting it and cause errosion.

any inputs on this is appreciated ...

 

[PUMPDESIGNER]

I personally do not find that rule of thumb useful.
Perhaps I take any possible benefits for granted, but I find the rule not useful except for one thing ...
When you cannot win an argument based on logic, or someone just seems to be unable to comprehend what you are saying, you just whip out that old 5ft/sec rule and then everyone just agrees.
The rule seems to cause more problems than if it did not exist.

The rule also has caused me problems.
Devotees of the rule question situations where we intentionally increase velocity above THE RULE. I hate that, cannot seem to satisfy them with my answers and I feel that they are now suspicious of the design.

Is there any history on this rule? Is there any study that was made? The rule may be useful, and in fact in my early years perhaps I did benefit without realizing it. PUMPDESIGNER

 

[hacksaw]

the main issues in a clean fluid (falls in-line with what everyone is saying) is that 5 fps is neither too high nor too low. Reasonable pressure drop, economic pipe size (more or less), etc. I wouldn't be surprised if its origins date to the first attempts to pipe water into cities in lead pipe.

Sedimentatation can be a problem at really low velocities, pressure drop and erosion a problem above 10 fps.

There are always exceptions.

One area that has not been mentioned is that during line filling, you want to keep the velocities moderate to avoid water hammer.

 

[PUMPDESIGNER]

hacksaw
That was a pretty good answer. Your answer brings up an interesting issue. By my experience most people view that 5/sec rule as an UPPER limit. Economics usually drives people towards the upper limit.
There are problems as you point out with low velocity. We are lately becoming involved in problems with snails, which is a low velocity problem. PUMPDESIGNER

 

[ccfowler]

Rules of thumb are just that and very importantly NOTHING MORE! They provide some reasonable reference points from which to start for estimating purposes, but that is ALL! Perfectly good systems can operate at much higher and lower flow rates depending upon their individual circumstances and needs.

All engineering work involves judgements, trade-offs, and compromises to accomplish desired or needed results and functionalities. Some systems need to be designed to function for decades and longer while others may need to work well for only hours or even only minutes. Is there any reason that some rule of thumb should be expected to apply uniformly to all?

A design that relies on rules of thumb is a design that runs an excessive risk of failure, needless costs, or other similarly welcome design "features."

For your system, shup0739, how well do you know the water's chemistry? Just how corrosive is it? Water is never "just water." You may need to consider corrosion allowances in your design for your system's water chemistry.

As an anecdotal example, I know of a community where the municipal utility supplied water is very "hard" due to its very high mineral content. After passing through conventional water softenters, this water is sufficiently corrosive that many residential plumbing fixtures routinely fail become cosmetic wrecks due to corrosion after only a few years of service (some cheap fixtures may not last a year).

Stars to MJCronin, PUMPDESIGNER, and hacksaw for your sensible and well considered replies.

 

[hacksaw]

Snails!

Two issues: what friction factor are you using? and does it help if they are all crawling in the direction of flow?

 

[PUMPDESIGNER]

That is pretty good hacksaw, got a smile on me.
Before I explain I will head off BobPE so that I do not hear it from him.
No, there was no engineer involved on the project, and I agree that was their true and deeper problem.

A certain design build company installed some systems.
They greatly oversized the pipe, my guess is fear of the unkown. Velocity ranges from .2 ft/sec up to a maximum of around 2.2 ft/sec. Some sections of the pipe never see more than .75 ft/sec.

We know of about 30 jobs this guy did, and many of them have the snail problem. Snails enter microscopic, then grow up to the size of golf ball. PUMPDESIGNER

 

[d23]

shup0739

My opinion:

Any rule of thumb is a relative term that needs to be qualified. The 5 FPS assumes some balance between HP and the tubing cost or fatigue. Rules of thumb offer a starting point only. The best velocity calculations weigh the additional HP required against the pipe cost and fatigue. If you have a project that will only last a few weeks velocities in the low twenties may ultimately be a reasonable design. If you want the tubing to last twenty years velocities of 2 to 3 FPS may be more appropriate. In the end velocities will require a certain amount of design tradeoffs.

As everyone has stated a qualified engineer needs all the facts to make a good design.

 

[25362]

The problem with this ROT is that, according to shup0739, it limits the velocity to a maximum of 5 fps, and doesn't say "about" that would leave room for interpretation and variation.

 

[smckennz]

The reason is that it is an "average" velocity. No magic. No hassles. Water hammer wont go too high, no erosion problems, no sedimentation problems, normally no distribution pressure problems. And there is ample room for future expansion. Use it, within the nearest pipe size, and you wont go far wrong. Design for 10 fps and all hell will break loose when the design demand (by others) increases by 20%. You will claim its not your fault, but it is; you have not made provision for uncertainty. These numbers have been offered by others after they have made made many mistakes. Conservative? Yes. They are trying to help you. By all means depart from them IF you know what you are doing. Rule of Thumb numbers are what you use when you dont know exactly what you are doing.

Cheers

Steve McKenzie

 

[PUMPDESIGNER]

smckennz & d23
I like smckennz's statement "Rule of Thumb numbers are what you use when you dont know exactly what you are doing." And d23's statement that a good system requires specific analyses with the facts.

I must be honest and admit that I gave too much credence to that rule in my early years. But I think that attitude was picked up from others who relied completely on the 5/sec rule. But I have it all together now and know everything.

And if you do work like d23, where his systems only last a few hours or weeks, I suppose d23 just gets all giddy just thinking about what a rebel he is. Fastest I ever go is about 12 maybe 15 ft/sec. to create a highly accurate flow meter run. PUMPDESIGNER

 

[DLANDISSR]

The only application of the 5fps rule of thumb is for pump suction piping. See the "Pump Handbook" by Krassik, etc. This does not apply to pump discharge piping.

This is for flow from a basin to a pump suction. Since the flow is by gravity in this case velocities and resulting friction losses must be kept down so as not to cavitate the pump. Calculations should be made to determine NPSH for the specific pump and system.

The normal flow in a pipe I have used is 10fps. For larger lines and clean fluid this could be increased to 12-15fps.

Another issue which has not been mentioned is sound. As the velocity increases there increased sound levels. If the piping is, say hot or cold water distrubution for heating and cooling in an office building, velocites should be kept to 6 to 8 fps to minimize noise.

In hydraulic systems velocities are as high as 20fps. Beyond this erosion would become a problem.

 

[Duck50]

Well, I have seen every possible response to the old 5 fps rule, except for the one that I learned. I did the piping design for a hotel and the architect told us to keep the velocity to 5 fps because one cannot hear the sound of the water rushing through the lines, which, if you think about it, would not be a nice thing in a 5 star hotel!

GTD

 

[hacksaw]

the mention of hydraulic lines is interesting.

in the literature you find peak (not sustained) velocities in high pressure systems reaching above 35 ft/s

 

[BobPE]

hacksaw hit it right on the head in his first post if you are dealing with wastewater and potable water. There are definate velocities indicated in many texts that should for the most part be followed if you do not want your design scrutenized, but I too have seen this carried to the extreme and had people tell my that I was wrong for using another velocity because they had a book that told them differently, similar to what pipedesigner went through. They are rules of thumb for engineers, but they are now in the hands of lay people and so far as they are concerned, they are written in stone because they have a book that tells them this. The lay people forget that we engineers wrote the books LOL....

As long as you know whay you are doing and why, then by all means do it. If you don't know what you are doing and have no clue why you are doing it, then follow whats in our books....

BobPE