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?

 

[PUMPDESIGNER]

BobPE
I just gotta love the way that you straighten us all out.
What a hoot. Let us do it, but if you will not let us do it correctly then read our books and take your best shot.

I can just see a defence attorney trying to make headway with you on the stand.

LAWYER - So Mr. Engineer Bob, you say that the system blew up because no engineers were used to design the system. How do you know that?

BobPE - How do I know that your not a lawyer?
LAWYER - But I am a lawyer.
BobPE - Prove it right now, in front of us all.
LAWYER - I can't, you will just have to take my word for it.
BobPE - Same here. PUMPDESIGNER

 

[BobPE]

LOL pumpdesigner!!!!

How did you know lawyers love me??? LOL

My favorite answers on the stand are YES and NO though....LOL

Take care....

BobPE

 

[d23]

PUMPDESIGNER:

My family sometimes looks at this post. Please watch your language. That "L" word is very very bad!!!

 

[shup0739]

Thank you to everyone that replied to my post. Yes the system is being designed to last for years if possible. The chemistry of the water is an interesting point. I don't really have much experience with piping design, i'm actually a young ME out of MSU so the rule of thum isn't too attractive. I just wanted to get an idea of what others felt about this rule. Moreover, I belive that if I'm redesigning a system that I would like to last for a few years than a low velocity would be ideal, as well as affordable. After modeling the current piping scheme it turns out that the current pipes are not as efficient as they ought to be and that might suggest build-up in the pipes or material that is reducing the flow.

I will definately continue to work on this and I hope to have a better understanding.

thank you all

sam

 

[jmw]

I don't know about water flow, but in hydrocarbons, static electricity build up is the big risk but even so i think the flow velocity limit here is a bit higher!

 

[209larry]

Sometimes it is better to run an orifice sizing program to get a beta ratio between 0.4-.5 at 25kpa differential pressure by varying the size of the pipe although metering is not required. It is mostly like the right pipe size!!!

Viscosity factor is not taken into consideration in 5ft/sec rule.

Rgds,

209larry

 

[HEC]

I have worked in the food and pharmaceutical industries and the magic number of 1.5m/sec is used for minimum "flow" rate for effective CIP or Clean In Place. This is for effective scouring and to prevent bacterial growth in the pipe. I might suggest that this is the origin of the "Magic Number".

 

[sloquick]

I don't know too much and didn't read all the responses, but I did pick up on the word "steep". I recall a story about the closing of a penstock with a valve unfortunately located at the top of the hill. All the momentum and gravity pulled a vacuum on the line which promptly collapsed.

 

[ChasBean1]

The bigger the better. Snails have rights too!

 

[aberta]

The key statement is "steep pipe". It does not say piping is pressurized or a drain pipe.Water coming down a steep pipe that is not under pressure cauases "cavitation". Hence 5 ft/sec is as good as any low number. If the piping is under substantial pressure, velocity could be a lot higher.

 

[BobPE]

aberta:

read on in the postings and the steep pipe is defined as pressure flow.

BobPE

 

[umreng]

Keeping the flow velocity below 5 ft./sec. will prevent siphoning from occuring. The elevations and piping geometry may also preclude siphon action, but if all other conditions are correct, the low flow velocity will be a virtual guarantee that siphon action will not initiate when the pump is de-energized.

 

[davefitz]

For the case where you are branching out to 5 different branches, the velcoity of 5 fps is sufficitnly low that you will minimize flow unbalances due to changes in static head in the manifold. Also, 5 fps is high enough to prevent accumulation of sediment and may prevent the attachment of some biological growth.

If it is a filterd water and not containing multiple branches, then 10 fps is a typical economic pipe size. An absolute max of 27 fps to prevent erosion/corrosion in carbon steel pipes is rarely justified.

 

[Metalguy]

Steep pipe? And here I thought he made a typo and it's really steel pipe. Hmmm, wonder if it's a steep copper pipe? <g>

 

[BobPE]

Metalguy:

You never know??? LOL I thought so too, but then I thought what ASSUME means to me as I was tought by other engineers!! LOL Typo's have brought down many a good design...

BobPE

 

[TrevorP]

It does say in a later post that he's using &quot;schd 40 stl piping&quot;, so maybe it's steep steel? Or maybe it was a typo, and it was meant to be &quot;sheep&quot; piping - something to eat the snails perhaps?

 

[jmw]

What's the penalty for steep steeling (or is it called rusting?) is it still a hanging offence? Besides, isn't Typo innocent until proven guilty? He seems to be the scape goat for everything that goes wrong.

 

[belinealianto]

it's safer to keep its velocity below 5 fps in the aspect of maintenance and economics. the higher velocity can cause the erosion in pipe. the more friction head will occur because of high velocity. i have researched that water velocity below 5 fps cause no erosion on the innerside wall of pipe after operating it for three years.
if you keep the velocity higher enough more cavitation may occur because of increasing the velocity head that cause the drop in pressure head.

 

[3Pipes]

Anyone blindly applying rules of thumb, especially ultra conservative values with flow velocities down to 4 fps for clean water, deserves to be shot. Line sizes should be what is required for reasonable pressure drop, reasonable fabrication costs AND to meet pump nozzle load requirements without optimistic and highly theoretical assumptions regarding temperature and support deflection.

Process Engineers are the single biggest cause for stress hours over runs. (Oops - the cat is out of the bag). Even at ambient temperature it is physically impossible to maintain nozzle loads connecting pipe of 2 nominal sizes or more greater than the pump nozzle loads.

10&quot; on 6&quot; is definitely gold for a pipe stress engineer.

 

[Paperpete]

OK since it appears to be OK to dog down on the process guys let me continue. Many times have I come in contact with a basic system where a standard pump (Goulds 3196 or similar) is used to pump thru a pipe and then a control valve will control the flow. Often the pipe is sized for a reasonable pressure drop at the full flow of the pump. The first problem is that the fluid will have a pressure drop over the valve alone (even when fully open) that will prevent the pump from ever delivering the max flow (look at pump curve). The second problem is that the pressure drop over the valve when it is not fully open will be greater and therefore the energy savings that you thought you were getting with the huge pipesizes goes out the window. I have found that a pump driven by a VFD is in most cases the best solution. For longer pipe runs you can almost pay for the VFD if you do not buy a control valve and run a smaller size pipe. I hate to hand out rule of thumbs but I have had great luck with 9-14 fps. One job I did with a VFD the installation cost was 70% of a conventional design (most modern VFDs have the ability to accept a 4-20mA so the need for compressed air and a I to P as needed with pneumatic control valves dissapear). In this installation the same amount of fluid were pumped as with the old system using half the kWhrs/month being more quiet and more reliable with less moving parts. The design of systems like this does require someone with a solid understanding of the flows affect on the pressure drop and the pumpspeeds affect on the flow and pressure (affinity laws).