How much water thro a 4inch pipe
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 Lemon Shark
 Posts: 37
 Joined: Sun Oct 10, 2010 7:22 am
 Location: thailand
How much water thro a 4inch pipe
I know I,v asked this question before somewhere, but I,v built a second set of filter bays and connected it to the original one by 2 4 inch pipes and its pumping dry when I switch the pump on , Its a 25000 ltr per hour pump with a 2.5 inch outlet . Iv got a large Venturi on it now which is slowing it down so it's working ok but I don't want it slowed down so the question is how many 4 inch pipes am I going to have to fit . All the other bays have 40 by 20 cm weirs and the filter is fed by 4 3inch pipes (I know but there's a reason ) so I think it must be the 2 4 inch pipes that's the problem . Thoughts please . Regards John

 Site Admin
 Posts: 554
 Joined: Sun Apr 16, 2006 10:24 am
Re: How much water thro a 4inch pipe
Without a picture I'm not completely sure how your system is plumbed and what level drop it will tolerate before the flow into the last chamber becomes less than the rate at which water is being pumped out but these are some general principles that may help.
It is frequently said that the flow you can get by gravity through a 4 inch pipe is 2,000 (Imperial) gallons per hour but this depends greatly on how much head you can afford to lose because the flow rate depends on the difference in height between the surface level in the pond or tank where the water is coming from and the surface level of the tank it's flowing into. If anyone reading this doesn't find that clear then consider two extreme cases:
1. If you had a very deep tank at ground level and there was a 4 inch pipe leading out directly onto the ground then you would get a flow through the pipe of almost 7,000 (Imp) gallons per hour because there would be a great difference between the tank water surface and the open end of the pipe.
2. If you could only allow a small difference (e.g. less than 1 mm) in head loss between the pond or tank where the water is flowing from and the tank where it's going to then the maximum flow rate would be very small, maybe only a few gallons per minute.
In a practical situation, more like what we have in a gravity fed filter system, we could normally allow a centimetre or so loss but the flow rate will greatly depend on how much this head loss can be without causing a problem. A further limiting factor if two tanks are connected by a pipe near the surfaces of the tanks is this: As long as the connecting pipe stays fully under water at both ends the flow rate will be at its maximum for any particular head loss but, if the levels drop in operation so that part of the pipe is above water, then you won't have the full flow rate through the pipe. The flow rate through a half full pipe will be half of the flow rate you would get if it was fully submerged.
Although there isn't a fixed flow rate that you will always get through a 4 inch pipe it's possible to estimate whether a single new pipe will be sufficient or not. Turn off the pump until the tank is full then turn it on and time how long it takes to empty then turn off the pump and time how long the tank takes to refill. At the moment the time to fill will be longer than the time taken to empty so, in normal operation, this will mean that the tank won't refill as quickly as the pump is trying to empty it so it will keep running dry. What you need is for the time to fill to be less than the rate at which it empties. One extra pipe will decrease that time by one third but, if that isn't enough, then a second extra pipe will halve the fill time.
It is frequently said that the flow you can get by gravity through a 4 inch pipe is 2,000 (Imperial) gallons per hour but this depends greatly on how much head you can afford to lose because the flow rate depends on the difference in height between the surface level in the pond or tank where the water is coming from and the surface level of the tank it's flowing into. If anyone reading this doesn't find that clear then consider two extreme cases:
1. If you had a very deep tank at ground level and there was a 4 inch pipe leading out directly onto the ground then you would get a flow through the pipe of almost 7,000 (Imp) gallons per hour because there would be a great difference between the tank water surface and the open end of the pipe.
2. If you could only allow a small difference (e.g. less than 1 mm) in head loss between the pond or tank where the water is flowing from and the tank where it's going to then the maximum flow rate would be very small, maybe only a few gallons per minute.
In a practical situation, more like what we have in a gravity fed filter system, we could normally allow a centimetre or so loss but the flow rate will greatly depend on how much this head loss can be without causing a problem. A further limiting factor if two tanks are connected by a pipe near the surfaces of the tanks is this: As long as the connecting pipe stays fully under water at both ends the flow rate will be at its maximum for any particular head loss but, if the levels drop in operation so that part of the pipe is above water, then you won't have the full flow rate through the pipe. The flow rate through a half full pipe will be half of the flow rate you would get if it was fully submerged.
Although there isn't a fixed flow rate that you will always get through a 4 inch pipe it's possible to estimate whether a single new pipe will be sufficient or not. Turn off the pump until the tank is full then turn it on and time how long it takes to empty then turn off the pump and time how long the tank takes to refill. At the moment the time to fill will be longer than the time taken to empty so, in normal operation, this will mean that the tank won't refill as quickly as the pump is trying to empty it so it will keep running dry. What you need is for the time to fill to be less than the rate at which it empties. One extra pipe will decrease that time by one third but, if that isn't enough, then a second extra pipe will halve the fill time.

 Lemon Shark
 Posts: 37
 Joined: Sun Oct 10, 2010 7:22 am
 Location: thailand
Re: How much water thro a 4inch pipe
Thanks for the detailed reply , the two 4 inch pipes are 20 cm (top of pipe ) below the water level and drop 50 cm into the bottom of next set of bays , I,v replaced an old vortex that wasn't big enough , and that had 1. 4inch pipe going into the filter bays , I,v replaced it with a large 3.6 metre set of bays with discharge and 2m settlement chamber with izeki nets , I thought putting the extra 4 inch pipe would have been enough,but obviously not . The pump is 25000 lph . I will use it as is for now b,cos my builders on another job now but I,ll change it when he comes back probably go ott . Thanks again