Gravity drainage pipe size question 1

[rs1600]

Hi,

i am hoping someone will be able to offer me some help with the following query in as simplistic fashion as possible.

Take a typical 25m swimming pool with an overflow channel around the perimeter of the pool. Assumiing the flow into this channel is 200m3/hr. In the base of the channel (typically at one end of the pool) will be 1.5m long vertical drainage pipe/s into a tank (say 6" max. upvc).

The channel can be assumed to be 400mm deep x 200mm wide. The water depth must not exceed 100mm

My query is how many 6" pipes would be required to drain away this 200m3/hr? how is this calculated and how can i work this out for different scenarios?

any help would be really appreciated.

thankyou

 

[Lan123]

What is the perimeter length of typical 25 m pool?

 

[racookpe1978]

What is a typical 25 mm pool?

Round? Elliptical? Heart-shaped? Rectangular? Figure 8? 8<)

 

[rs1600]

Appologies,

I was refering to a 'typical' 25m long swimming pool at your local leisure centre. Assume 25m long x 12.5m wide swimming pool with a 200mm wide x 400mm deep channel around the perimeter.

However, if i can deturmine what m3/hr. a 6" pipe can flow with 100mm water depth, will this allow me to deturmine the number of 6" pipes required to drain the full 200m3/hr.

i am basically looking for a 'guide' to help deturmine the number and size of drain pipes which would be required, not nessisarily exact science if that helps?

thanksyou and hope you can help me out.

 

[Lan123]

Q total = 0.056 Cum/s
assuming that the the Downstream end of the 6" pipe has a freefall, and, if the 6" pipe has a 1:200 slope, the wQtotal ahould be able to flow through a single pipe at a half full.

If the permieter channel is at 1:200, one half of the Q total should be able flow through each leg of the channel at 90 mm water depth.

 

[katmar]

You can find the theory, and some tables giving the flowrate as a function of pipe slope and fraction filled at

http://www.cispi.org/handbook/chapter8.pdf

If the 6" pipe is 1.5 m long, straight and vertical it will be able to take about 275 m³/h before it floods. Most of the head losses are in the entrance to and exit from the 6" pipe, rather than along the pipe. If you are feeding a bit less than 275 m³/h you may find that the pipe gradually fills up as it tends towards flooding, and then suddenly syphons empty. To avoid this cycling you would have to limit the flow to less than 25 m³/h per 6" pipe and to distribute the flow evenly amongst the 6" pipes. (This is termed "self venting" flow).

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[ione]

Good pointer by Katmar (as always)
You might want to use the service in the link below to evaluate the size of the pipe.

http://www.freecalc.com/selffram.htm

"Designing Piping for Gravity Flow"; P.D. Hill would also deserve a read

 

[rs1600]

Thanks all for the input.

Lan132,
have you suggested 1:200 slope because there is no chart/info. for a completely vertical pipe?
Also is it possible to explain how you worked out the 90mm water depth? I would guess that the depth would increase moving away from the drain point?


Katmar,
I will have a read through the info. you kindly provided, and try to get an understanding.


To add to my understanding, you mention 'self venting' am i corrct to say - The 6" pipe the water falls down into the tank is also able to act as the vent i.e. a separate vent would therefore not be required in principle?

Is the 25m3/hr. mentioned a typo, i.e. 250m3/hr? If not, then this would mean i require 8 No. 6" pipes to drain away 200m3 without the water depth exceeding 100mm (correct?). If the water depth was say 200mm how would this affect the flow capacity through a 6" pipe?

Finally if the water level was high in the tank? i.e. The bottom part of the 1.5m vetical pipe was submerged within 1m of water. This would mean that the water falls 0.5m down the pipe before hitting the water level in the tank. how would this impact on the drainage capacity?

sorry for all the questions, i am just trying to get some basic understanding on the way different variables will affect the level of draiange possible, i hope you can offer help.

Thanks.

 

[ione]

rs1600,

As katmar will confirm 25m3/hr (110 usgpm) is not a typo, it is approx the flow that can be handled by a 6" pipe without "pulsation"

 

[rs1600]

ok thankyou ione,

25m3/hr. would not be enough, therefore i assume if the tank is vented separatly then 275m3/hr is the max. flow down the 6" as mentioned in katmar's original post or am i completly off track here?

this would mean 1 No. 6" pipe would be ok, instead of 8 No.

 

[katmar]

In self venting flow the liquid tends to run down the walls of the pipe leaving an air gap up the middle, so yes - in principle a separate vent should not be required. But it would be impossible to have self venting flow in a 6" pipe if the water level in the channel was 100 mm above the pipe opening. You would need an elaborate distribution system to feed the 8 No. 6" pipes and I think in this instance a self venting design would be impractical, unless you could use a single 14" pipe.

The level in the lower tank does affect the flowrate in that the difference in levels between the water in the channel and the water in the lower tank is the driving force to get the water to flow down the pipe. The smaller this difference the smaller the driving force to overcome an essentially constant resistance. If the lower level was raised to be only 0.5 m below the channel level the flow would decrease to about 160 m³/h.

The problem with having a single 6" drain from the lowest point of the channel is that when it goes into syphon mode it will draw a lot of water from the channel down before air is sucked in and the syphon breaks. This could put a large pulsating load on downstream equipment. A way around this would be to put a weir at the lowest point in the channel so that the water flows from the channel over the weir and into a separate box with the 6" drain going from the bottom of this box to the lower tank. With this setup the surge on syphoning would be limited to the contents of the pipe (about 25 litres). I think this would be safer too because in syphon mode the velocity in the 6" pipe would be over 4 m/s and if the opening of the pipe was accessible at the bottom of the channel to people in the pool you could start sucking small children into your filter. OK - that's an exageration, but it could be scary if someone put their hand into the pipe.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[rs1600]

Thanks Katmar,

I have read through the attachement you posted earlier which makes a bit more sence (except the fractional figures table, which im not sure how to apply yet).

most of the equations and tables i have looked at relate to sloping pipework, rather than vetical? could you tell me what equations etc. are most appropriate to research and understand for what i am asking, i can then try and educate myself a bit better?

To recap typical senario - There is 'x'm3/hr. weiring over the pool edge into a perimeter channel. the channel drains to a balance tank below via vertical drainage pipes. The pipes are 1.5m long but the bottom 1m is submerged below the balance tank water level. I wish to be able to deturmine how many pipes of a given diameter will be required for various applications.

If you could point me in the right direction i will try to leave you guys alone until i have done a bit more research.

many thanks again.

 

[katmar]

You can use the sister page to the one already given by ione.

http://www.freecalc.com/fric.htm

Unfortunately it only works in US units, and it is set up to calculate the pressure drop from the flow rate, so you have to do a bit of trial and error. Remember to set 1 entrance effect and 1 exit effect. For this type of calculation the orientation of the pipe is irrelevant, but the calculated pressure drop in feet should match the difference in water levels in your situation.

This site does not give any background theory, but if you Google Darcy-Weisbach you will find plenty of references. I suspect Wikipedia will be a good place to start.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[ione]

You can find valid stuff in the attachment (Norsok Standard). Take a look at page 11 (vertical lines).
The Nikuradse formula for the friction factor allows you to avoid bothering iterations (the Colebrook formulation for the Darcy friction factor, usually used for turbulent flow, asks for Reynolds number which depends on flow and pipe diameter via velocity). As Katmar pointed out in your specific scenario the available head is mainly dissipated in entrance/exit to/from drainage pipe, so the assumption of setting the 50 % of the available head equal to the piping friction loss is not necessarily the most precise one.

 

[KernOily]

Ione - Do you have an ISBN for that book ("Designing Piping for Gravity Flow"; P.D. Hill)? I can't find it using a web search.

You guys need to be careful about hydraulic radius. The D-W equation assumes the pipe is running full. In my experience this is almost never the case in real life esp. in gravity flow systems. Best to use the Manning formula and design in plenty of fudge to make sure you are not running up against the ragged edge. Gravity flow, open-channel systems tend to get into trouble.

 

[cvg]

it is unlikely that mannings equation will apply in this situation. First of all, mannings assumes relatively mild slopes. It does not apply to vertical pipes. Secondly, inlet conditions will control the amount of water that can enter the pipe. It is more likely that you will have a weir condition as the flow goes through critical depth at the inlet of the pipe and then free falls until it hits the tailwater at the bottom of the pipe. In addition, if you are not careful, a vortex will form (think flushing toilet) which will further reduce the efficiency of the pipe. An anti-vortex plate or cover can prevent that.

 

[ione]

KernOily,

It is an article appeared in Chemical Engineering Magazine, September 5, 1983.

 

[katmar]

It is correct that D-W assumes full pipe flow. But this is still a worthwhile calculation to do because it gives the best indication of the maximum flow that can be obtained - which occurs when the pipe is flooded and therefore full of water. cvg makes a good point that a minimum head above the entrance is required to ensure flooded flow. According to Perry 7th Ed Fig 6-30 one and a half pipe diameters of head would ensure flooded flow.

This puts one peg in the ground in determining the range of feasible flows. The other important peg is to calculate the self venting flow rate. Between these two points the flow is likely to be unsteady, or even pulsating, but there are many instances of drains working this way. If the variation in flow does not affect the operation or safety then it may be acceptable.

Manning assumes sloped pipes with the liquid flowing in the lower part of the pipe and the vapor space at the top. In a vertical pipe flowing at less than the flood point the liquid tends to flow down the walls with the vapor in the middle of the pipe. I don't think Manning would give a good indication under these conditions. In fact, a friction pressure drop calculation is meaningless in vertical down flow at rates below the flood point because the static head gain is greater than the frictional head loss.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[cvg]

flow depth is 4 inches into a 6 inch diameter hole. This will clearly not produce full flow, estimate the flow using a weir equation. With much greater depths of flow, the controlling flow can be calculated using the orifice equation.

 

[KernOily]

Good point on the vertical - I missed it in the original post. Manning is not a good choice, in this case. Oops! Thanks for the correction.