Design of pressure line orifices for equal distribution 3

[jms336]

I am designing a septic drainfield to recieve pumped effluent.

The drainfield will contain six dead end leaders, each 50-ft long. The estimated flow rate is 40 gpm, or 6.7 gpm per leader. Orifices spaced along the leader will allow the effluent to exit onto the drainfield surface. The regulatory agency requires that the flow from the least effective hole be at least 75% of the flow from the most effective hole.

The leaders are 1" diameter and are installed level. I initially estimated that 1/4" diameter holes at 12" on center would provide a nice even flow. However the contractor advises that he is accustomed to providing fewer holes and smaller holes. He is concerned that the openings closest to the pump will have significantly more flow coming out of them than those near the dead end.

My thoughts are that the pressure at one end of the 1" leader will be almost the same as the pressure at the other end, since a flow rate of 6.7 gpm in 1" pipe would result in 1 psi drop in the 50-ft length (if the flow rate was constant along the full 50-ft length, which it is not).

Please help if you have any experience with this type of situation. I have a feeling that the design may rely more on experience than on calculated values.

 

[bimr]

It doesn’t sound like your Contractor knows what he is talking about. The gravel is what distributes the flow, not the holes. Holes smaller than ½” diameter would surely plug as well.

You need to contact the local government agency that approves the installation and follow the agencies’ guidelines for installation.

For gravity distribution systems, pipes must be at least four inches in diameter, and not subject to corrosion or decay (therefore no metal pipes). Perforated pipes must have at least one row of holes, 1/2- to 3/4- inch in diameter, spaced no more than 36 inches apart. These pipes must have a load bearing capacity of more than 1,000 pounds per linear foot. Open joint tile with tile sections 1/4- inch to 1/2-inch apart may be used. Place half-moon concrete or plastic tile on concrete blocks. Plastic chambers with slotted sidewalls typically used for gravelless systems also may be used, and may be placed directly on the bottom of the trench (Figure 6).

http://www.ianrpubs.unl.edu/epublic/pages/publicationD.jsp?publicationId=277

Trench: Drain field trenches effectively treat liquid flowing from the septic tank. They are the most economical to install and are preferred when possible. A drainfield trench is a level excavation 18 to 36 inches wide and up to 100 feet long. The trench contains a perforated pipe in a bed of 3/4-inch to 2-inch diameter rock covered by natural or synthetic permeable fibers. Some soil treatment systems use large plastic tubing or some other chamber wrapped with fabric in the trench in place of rock. A 6- to 12-inch deep layer of topsoil covers the trench. Sewage flows through the holes in the distribution pipe, to the rock (or tube), through the biomat, and into the soil. Bacteria and fine sewage solids are removed or destroyed in this process.

The trench system may be laid out in one of many configurations to allow for the necessary square feet of surface. There are often inspection pipes on one or both ends of the pipes. These can be cut off at ground level and capped for easier lawn maintenance. The ground surface level of the soil treatment area should always be level or slightly raised above the surrounding ground to avoid excess rainfall flooding the system.

http://www.epa.gov/OW-OWM.html/septic/pubs/homeowner_guide_long.pdf

http://www.sera17.ext.vt.edu/Documents/BMP_septic_drain_field.pdf

http://www.eco-nomic.com/septic.htm

 

[jms336]

Sorry, I was not clear but this is a pressure distribution line inside the void space of an ARC chamber drainfield (Infiltrator systems). This type of system is required in Florida for systems with more than 1,000 gpd of flow. The purpose, aside from the needed lift, is to evenly distribute the effluent within the drainfield.

Thanks for your detailed response and I am sure it will help someone else in the future.

Anyone know about sizing / spacing the orifices on the 1" line?

 

[jms336]

Here is the type of installation we are doing:

http://www.infiltratorsystems.com/files/Q04_PressureDose.pdf

Thanks for any help with the orifice sizing / spacing design.

 

[bimr]

Designing a pressurized distribution system is complex and
involves many variables. The size, and length of the delivery line, manifold line and laterals, the orifice size and spacing, and change in elevation are all taken into consideration. Once these variables are understood then a pump size can be selected. Fortunately many pump manufactures offer programs to design a pressurized system.

The pump moves the effluent through the supply line and manifold to the distribution laterals in the trenches under a low pressure. The laterals are a network of PVC pipes, perforated with small holes (usually 4 mm to 6.5 mm in diameter) and spaced in intervals up to 1.5 m (exact dimensions are determined for each system). The laterals are placed in narrow, gravel filled trenches or in chambers which allow enough storage volume so that the effluent does not exceed 5 to 7.5 cm of the total trench depth during each dosing cycle.

http://cfpub.epa.gov/owm/septic/septic.cfm?page_id=268

You might try contacting the infiltrator systems for guidance.

 

[jms336]

Update - the contractor has built the system using 12" spacing and it was determined that the flow was not being distributed evenly. He reduced the spacing of the openings to 48" and it functioned acceptably.

 

[Spartan5]

jms336:
You're contractor was right.

One of the key aspects of these designs is the residual head at the end of the lateral. You need to know what that target is in order to estimate your flow in the pipe because that residual head is what drives the water through the orifices. The higher the residual head, the greater the flow.

I frequently design these systems. My typical design consists of small diameter laterals (1" to 1-1/2") with 1/8" orifices spaced every 2' to 4'. The laterals are spaced so that the orifice density is no more than 1 orifice per 16 square feet of trench or bed. Ideally, it is no more than 1 orifice per every 12 square feet. The target residual head is 5'. And the standard I use for even distribution is that the flow from the least effective hole be at least 90% of the flow from the most effective hole.

Based on those parameters, a system similar to yours (6 laterals, 1/8" orifices 2' on center, 5' residual) would take a pump rated 64 GPM @ 7' of head (ignoring elevation and dynamic force main losses).

Doing anything with 1/4" orifices really makes it tough to get even distribution unless you space them so far apart you defeat the purpose of even distribution.

In your second example, assuming only 1' of residual pressure, You need a pump capable of 60 GPM at 2' of head. That is similar to the pump in my design. But my distribution is twice as dense as yours, My residual pressure is 5 times as high, and distribution is within 10% from one end to the other.

One thing I would suggest in either design, is a hydromechanical alternating valve that is commonly found in irrigation systems. K-Rain makes one known as a HydroTek Valve. That valve allows you to split the treatment system into several zones. Divide your system into 3 zones of 2 laterals each, and the pump required is 1/3 of the size of the pump in the original system.

Another thing to consider with the use of 1/8" orifices is orifice clogging. In all of my designs using 1/8" orifices, I spec a filtered pump vault with 1/8" filtration drawing from the clear zone of the septic tank or from the clear zone of a second tank following the septic tank. These pump vaults allow me to spec high head, turbine style pumps (submersible well pumps modified for use with wastewater). I can spec a 1/2 HP pump that pushes 30 GPM @ 60' of head with no problems. Everything can be smaller, the pump, the transport line, the laterals, the orifices, etc.

It is important to understand the goal of even distribution. bimr is right and wrong when he says that "The gravel is what distributes the flow, not the holes." In a conventional system, the gravel does distribute the flow. But it only distributes it to the first part of the field that hasn't "failed" (i.e. formed a biomat that impinges the downward flow of the water into the soil below). If you have a 50' long field and dose it by gravity (or uneven pressure flow) you are only using the portions of the field that absorb the water the fastest. Other portions see nothing at all until the portions that were absorbing water no longer do so (and become anaerobic). With even distribution into the stone, the system is able to utilize every square foot of treatment area and has the advantage of enhanced treatment through small, frequent doses into a very aerobic environment.

While I use a spreadsheet I developed in the office for design, I highly recommend the Pump Select program by Orenco:

http://www.orenco.com/directlinks/downloads.cfm

They also supply a lot of the pumps and pumps vaults I specify.

 

[jms336]

thanks, Spartan for you valuable input. I think my error was in assuming that 1 psi drop along the 50-ft lateral would be insignificant. 1 psi is very significant, it turns out, if 5-ft is the target head. Just curious - how did you arrive at 5-ft?

For my project, the contractor was able to modify the drainfield to incorporate 1/4" holes at 48" spacing, and it passed a visual test by the reviewing authority.

In the future I will be looking into the 1/8" holes, and the alternating valve as well, for a more economical design.

 

[rcooper]

http://water-works.wikidot.com/flow-distribution

Something I was working on a while ago and haven't finished, but it will give you an idea on how to size these.

 

[Spartan5]

jms336:
You're welcome.

Regarding the residual head, 5' is sort of a de facto standard as far as I know. It is what is required by a lot of the counties in which I work. Some go as little as 3' and I've never seen more than 5'. I use 5' in all of my design and it has never presented any difficulty from a design standpoint.

I also think, that with 1/8" orifices, the higher residual head is desired to help clear any biological matter that might slough off of the inside of the pipe and plug an orifice.

I'm curious also. With the system that you were involved in, did they measure the residual head? Did they have a requirement?

 

[katmar]

First off let me say that I have no experience of these waste water systems, but I have designed many spargers which employ the same idea. I will explain the logic I use and it may help you.

As you have calculated, if you had 6.7 gpm flowing 50 ft in a 1" line the pressure drop would be close to 1 psi. Because the flow down the pipe decreases at each drain orifice the actual pressure drop would be less than 1 psi. You could calculate it, but for sake of argument let us say the actual pressure drop is 0.5 psi over the 50 ft.

An orifice at the start of the pipe would see a pressure 0.5 psi higher than the last orifice would. If you want a flow variation of 75% between these two orifices then the ratio between the pressures must be (0.75^2) = 0.56. To be safe I would make this ratio 0.7.

Now we know that the differences in pressures that the two orifices see is 0.5 psi, and the ratio of the pressures is 0.7. A bit of math tells us that the pressure at the start of the line must be at least 1.6 psi. I would design for 2 psi at the start of the line, and therefore 1.5 psi at the end. On average each orifice would have a pressure drop of 1.75 psi. If you are going to have 12 orifices then the flow through each one is 0.56 gpm and to give a pressure drop of 1.75 psi at that flow requires an orifice of about 0.15".

Don't take my numbers too seriously because they are probably not typical for a waste water system, but the same principle should apply.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[rcooper]

Just a small note on what katmar is saying, as the flow moves down the pipe, friction will be causing the pressure to reduce as he describes, but the conversion of velocity head to pressure will be making the pressure increase. If the distribution pipe is short, then the pressure at the dead end will be greater than at the feeding end by approximately the dynamic head at the feeding end. If the distribution pipe is long, then the friction losses will dominate.

I guess you could get some idea of how "long" the pipe is by saying fl/d*(v^2)/(2g)=v^2/(2g) so l=d/f. If the pipe longer than this l, then friction is significant. If less than this l, the velocity head will be the dominant factor.

 

[brucedaengineer]

A rule of thumb in orifice distributor systems is to make the head loss through each office say 10 times the head loss along the length of the header - and if it is wastewater - put a filter on feed side that will trap particles that would block your orifices (sorry about that) . So if the head loss along the pipe was 100 mm -(ignore the progressive loss of flow - assume flow is all the way - this is a rule of thumb!) you might size your holes at 1000mm head loss - then decide on how many holes - gives flow per hole - then size hole (orifice formula) - if too small try fewer larger holes - higher flow per hole - same head loss.

That's the (back of envelope) theoretical approach - but you have plenty of practical advice here - which is based on actual experience - try the rule of thumb against these for interest?

 

[tkall]

JMS:
I do not intend to sound abrasive, but many of the replies you have received are incorrect. I have designed thousands of pressurized drainfield systems from small residential to systems with more than a mile of lateral pipe. There is only one software program available that will correctly calculate the hydraulics. squirtonsite.com. The Orenco Pump Select program simply does not give correct results and using a spreadsheet for this task is like using a screwdriver to fillet a fish. I would happy to discuss this with you in depth and help you prepare a correct, reliable design.
Tom

 

[katmar]

Tom, if you have a financial interest in a product you are recommending you should declare it or you could be accused of using Eng-Tips to sell your products, which is not allowed. In any case this software has not been released and appears to have no proven track record (outside your recommendations). Do not be surprised if your post gets deleted.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com