Speed Limit

[Bernie2]

This is intended to try and help new Plumbers who are interested in system design and those who are not, to help them realize that there are governing factors in Plumbing.

BS 6700 states the fastest velocity you want water to go in a cold water pipe is about 3 meters per second, with a gradual velocity reduction if hot water is used.

The copper pipe makers quote BS EN 805 as between 0.5 meters per second and 2 meters per second, with a maximum of 3.5 meters per second maximum.

Does it make sense?

To get some idea. 3 meters per second is about 6.7 miles per hour or running pace? Is that why they call it running water

Anyway speeds faster than these can cause the side of the pipes to wear away both in copper and plastic pipe. Speeds slower than these can mean sediments are left behind in the pipe.

Where did they come from?

Well potable water is full of all kinds its not the sparkling clean stuff we think it is. Most of the stuff will not harm you of course, but over time it will build up in the pipes. I once had shrimps coming out of our taps, very common it is. They are a bit small to eat and being honest I did not fancy them swimming around in my mouth too much.

There is also the increased chance of water hammer, something we seem to hear a lot about on this forum. The more velocity the more the backlash, so Mr Newton reckons.

So how do you govern water velocities?

To big a topic for me in this mail.

Perhaps some of the lads know some simple ways to pipe size and work out velocities?

 

[Plouasne]

From some of the replies to questions raised on this forum, by guess and by god, would be a good bet

 

[TGor]

snip....

So how do you govern water velocities?

To big a topic for me in this mail.

Perhaps some of the lads know some simple ways to pipe size and work out velocities?

Great subject...

The velocity or speed of the water in a pipe system is a function of the actual flow rate and the internal diameter of the pipe ... V= Q/A so for a given flow rate the velocity in the pipe will vary dependant on the actual internal diameter of the pipe itself. The velocity will affect the pressure lost due to friction higher velocity .. more friction!

Example:

BS/EN1057 table X Copper ID = 13.6mm @ flow rate of.218litres/sec = 1.5 metres per/sec Friction loss 2.36 kPa/Metre length with the velocity at 3 metres per sec. the flow rate increases to .436 i.e. doubling the flow rate doubles the velocity. but increase the friction loss significantly to 8.06 kPa/metre length

plastics pipe have thicker walls so that a 15 mm equivalent plastic pipe would have a smaller ID thus a higher velocity and friction loss than the equivalent copper pipe for the same flow rate.. that is to say that all 1/2" pipes do not give the same results.

Example

15mm plastic pipe with ID of 11mm (Guess) at velocity of 1.5 M/Sec gives a flow rate of .143 L/Sec and friction loss of 3.06 kPa/Metre length

At 3.0 M/Sec velocity flow rate of .285 L/Sec friction loss of 10.46 kPa/Metre length

which is quite a difference between the two materials.

to answer your question simply Bernie, velocity is governed by flow rate!

hope this helps a bit.

 

[david williams]

dont they teach this on your reactfast 6 week masters plumbing course

 

[TGor]

dont they teach this on your reactfast 6 week masters plumbing course

hehehehehehehe yeah I'll bet!!!

 

[Bernie2]

Yes, I was trying to introduce the idea of equivalent length and a few other things to the new guys.

Its just to show how involved Plumbing really is.

I don't think anybody new on the forum will bother much with Darcy Weisbech equations or Moody graphs, when they can probably just number crunch it on a bit of free software.

But at least if we can get them thinking a bit about what they are doing, instead of just plonking pipes in and stepping up the pump to make the water whizz around, it may help them.

 

[stevetheplumber]

Yes, I was trying to introduce the idea of equivalent length and a few other things to the new guys.

Its just to show how involved Plumbing really is.

I don't think anybody new on the forum will bother much with Darcy Weisbech equations or Moody graphs, when they can probably just number crunch it on a bit of free software.

But at least if we can get them thinking a bit about what they are doing, instead of just plonking pipes in and stepping up the pump to make the water whizz around, it may help them.

all very well if your designing large systems for the average jobbing plumber totally over the top
usually if the jobs big enough to need pipe sizing its going to be carried out by some one whose earning much more than us at the design stage

 

[bobpape]

Is it not a fact that no matter how fast or slow the water passes through the pipe it has to rub on the pipe walls therefore friction occures at any speed,

 

[secret squirrel]

Very interesting read, gives me even more to think about and a little more research required.

To answer your question david it is only covered briefly and was mentioned when covering distances from the hot water cylinder 22mm not being adequate over a certain distance. I can't remember this distance but if I go to a job which rings alarm bells I will look it up. Please let me at this point thank you for your very informative point relating to this subject.

 

[migoplumber]

forgotits a matter of how much friction!
think of a mega power jet washer on your car, times that by 10, imagine now water passing through a pipe doing the same, the friction would be extreme, and the pipe would wear out as quickly as the power jet would eat a hole in your car.

extreme example.

send the water through your pipe at 1.5 metres/second and life is dandy

shaun

p.s. i still have an old roca wheel dial, align your pressure, tap type, and flow requirements, gives you a k factor for deciding pipe sizes. dont use it much these days, but a nice simple peice of kit.

 

[bobpape]

Would not the friction increase with the speed of flow

 

[Phil]

Great post Bernie2, very interesting reading keep them coming.

Talking of pressure washers, how are they allowed to pump mains water?

I've asked this before somwhere but didn't get a satisfactory answer.

 

[TGor]

Would not the friction increase with the speed of flow

Exactly the pressure lost due to friction is a function of the velocity, pipe I.D. and roughness ! Calculated using various formulae such as Darcy-Weisbach, Hazen-Williams etc. BS6700 uses Lamonts smooth pipe formula because the roughness of copper and plastics is not significantly different Gal pipe is though.

Pipe sizing is essentially 4 steps:

1 estimate the flow rate required by the pipe(s).
2 estimate a size and calculate the velocity
3 determine the pressure losses in pipes and fittinings (due to friction & static head)
4 determine the residual pressure available at fixtures

proper pipe sizing is a good skill to have and plumbers can find it useful even on smaller jobs

The risks associated with pipe sizing are as follows..


Undersizing (too small)

§ Excessive velocities
§ Excessive noise levels
§ Lack of adequate supply at fixtures
§ Excessive Variation in pressures at fixtures particularly showers
§ Excessive Variation in temperatures at mixing valves (Cold Water failure shut off)
§ Increased risk of velocity related corrosion (impingement attack, erosion corrosion etc)
§ Increased potential for damaging water hammer effects.
§ Unnecessary pump duty
§ Lack of available pressure for fire protection (high pressure losses due to friction)

Over sizing (too large)

§ Excessive and unnecessary installation cost.
§ Time lag for availability of hot water at fixtures (larger volume of water to displace)
§ Consequent waste of energy and water
§ Higher heat losses from HW pipework
§ Low flow rates causing depositing
§ Elevated risk of pitting corrosion (low flow rates)
§ Deterioration of water quality (low flow rate)
§ Wastage of water if drained down (higher than necessary amount of water in the system)

As can be seen the more serious performance risks occur if pipework is under sized but the risks due to oversizing can be just as significant .. it is important to get the right balance. Design "engineers" tend to oversize in some cases significantly! ​

 

[justlead1]

Hi In a effort to demonstrate the effect head, pipe size, pipe length has on delivery. I found Box Formula a good tool as a mathematical guide

Q = L/s D = Dia of pipe in mm H = Head in Metres L = pipe length


Q = the square root of
D to the power of 5 x H Divided by L x 2 5x 10 to power of 5
Once written in maths speak it can be transposed to find any 1 from 4

Written up on excel, punch the figures in and get immediate results a lot quicker than log tables

 

[david williams]

all comes down to proper training in plumbing and plumbing design,the latter seems not to be a consideration these days.

 

[justlead1]

Hi I came across erosion by friction of pipes on the cooling water inlet at an oil refinery near Southampton some years ago. The plumbing work shop had a planned maintenance scheme. They had three identical pipe arrangements that were on a rota. The effected pipes were removed and replaced by the stand by set, those removed were relined and so on. The pipes and fittings were steel flanged and 24" diameter. To reduce the massive cost of new they were lined with lead sheet approx 6 mm thick, fitting tee's, bends etc were cleaned and tinned inside and lead welding was built up over the whole internal area to a thickness of 6 mm acting as sacrificial. I can't remember the exact time scale but i think it was every 3 months. You start to see why petroleum products are expensive.