Underground Piping Plans

By: Mark Layport

Underground Piping Plan

One of easiest piping plans to be generated, but also one of the trickiest!

Basically you are documenting ALL piping that is to be buried in a given area.

This "area" is normally the whole facility! This is because this U/G (underground) piping is normally done at a smaller scale covering a larger area. Also it's a continually changing design with a lot of variables, and trying to get more then one designer to coincide (like in above ground piping) is far more difficult, so it's normally left up to a single designer to deal with it all.

Because this designer has to take in all the controlling parameters and correctly deal with them, this is an assignment for a Sr. level designer!

Information required

You'll need the following information:

  • The "approved" equipment layout drawing (equipment location plan, or foundation location
    plan …which ever drawing is used to locate the equipment exactly). I say "approved"
    plans because in locations the under ground piping can & will be routed very closely to
    the equipment in locations, and having the engineers still moving things around can cause
    extensive re-designs causing lost of mans-hours of design work!
  • All the foundation drawings/details for that equipment & support/piperack locations.
  • You'll need a "cut sheet" (manufactures equipment drawing) to be able to locate all drains or
    commodity connections (where the piping to that connection will be routed under ground).

Under Ground Piping

Basically the underground piping falls into one of two categories, process lines & drain lines.

Normally all buried piping is buried with a minimum cover requirement (18" to 36"approx.) over the top of pipe (T.O.P.) …this is both for stress requirements & minimum installation/labor requirement. The weight of vehicles going over buried piping, a minimum required depth is needed to be able to distribute the load so not to damage the buried pipe …basically "the deeper the better"! …But why spend the man-hours ($) burying pipe deep into the ground to avoid damage when the same thing can be accomplish at a shallower depth and less effort? This is a balance of the two requirements. However "IF" the underground piping is located in an area that it is impossible to have heavy crossing traffic going over it, then the burial depth can be far less.

Process piping

Process piping is probably the easiest routed …enters the ground at point "A" and follows a routing that should be half way agreed on by the powers that be on the project by this time, and exits the ground at point "B" …only issues are line spacing, burial depth, & corrosion protection on the pipe (if any).

Drain piping

Drain piping is more complicated, this normally falls in two categories … pressure drains, and gravity drains.

Pressure drains

Pressure drains are a "closed systems" that utilize pressure to push the commodity through the piping system. This piping is normally routed in the same trench with gravity drains (I'll explain further during that discussion). This is a "closed system" cleans out are not required in these lines, and this piping does not need to be sloped.

Gravity Drains

Gravity drains are an "open system" that utilizes a sloped piping system so the commodity can get from point "A" to point "B" on its own utilizing the gravitational force acting on it.

This "open system" means at points along this piping system there are branches to this piping that is open to the atmosphere to allow two things to happen, first and most important, this is where the waste commodity is introduced into the drain header. Secondly this is where the system is "vented" …this allows the commodity to run down the system (kind of like putting a straw down into a container of liquid and putting your thumb over the end of the straw, with drawing the straw, the liquid will remain in the straw until you remove your thumb and the liquid is allowed to flow out of the straw, because the system (straw) was allowed to "vent", same thing on a gravity drain).

These open branches, where the drains on the equipment introduce the waste commodity into the drain system, are commonly called "drain funnels" or "drain hubs". The term "funnel" or "hub" is from the reducer placed at the opening of the branches in an inverted position so the large end is up …to act as a "funnel" to this opening. This also allows multiple small drains (1/2" to 1" normally) to be centered onto one drain funnel … (I personally witnessed 5 each ½" drains routed to a single 6" drain funnel).

A common "funnel size is 6" so a 6"x 4" or a 6"x 3" reducer is used. The 3" & 4" size are a common branch size, but don't get too hung-up if the size is a bit different (note - don't use any size below 2" ...good piping designer practice is not to bury lines smaller then 2", For a couple of reasons, lines below 2" are normally threaded, so a potential leak at any threaded fittings. Also they are too easy to mangle with a backhoe causing major damage).

Another "trick" with these funnels, they are normally positioned where the end is protruding approximately 3" out of the finishes surface (compacted dirt, gravel, or asphalt). This helps keep other "stuff" from getting into the drain system that doesn't need to be there (I have also seen screens cut out of perforated expanded metal that set just inside the "funnel" to further help filter the system).

Sloping drain headers 

A gravity drain system is where the piping is sloped to utilize gravitational forces action upon it to get the waste commodity to where it can be properly taken care of or stored (as I had indicated earlier). In my experience 3 slope angles are commonly used, ¼" per foot, 1/8" per foot, & 1/16" per foot. Now ¼" per foot is common slope in residential piping, it's a "fast" slope and can deal with most household "chunky" waste, however dropping a 1" deeper into the ground every 4 feet, this drain piping can get deep fairly quickly.

As I discussed earlier, a minimum cover is usually required over the buried piping system, but now you are getting even deeper, now the man-hour installation factor or cost comes in to play. So as engineering compromise the 1/8" per foot slope is used quite often and it can deal with most industrial waste fluids. Lastly 1/16" per foot is used mostly where you have to cover longer distances only dropping 1" every 16 feet. This is alittle easier installation, but usually some "sagging" does occurs in the header.

However this installation normally is capable of draining, even with this sagging, before the header is "slugged" full.

Drain headers are normally run in their own trench with other drain headers.
This is for a few reasons, first it is easier to slope the system if that has to occur with other sloped lines.
Now pressure drains are routinely run with sloped gravity drains, normally because they end up in the same place (or close to it). Pressures drains don't require sloped piping (as I indicated earlier), BUT it doesn't hurt that it is!

Trying to slope some lines and not others in the same trench becomes a major construction headache …so sloping the drains becomes the determining factor in the same trench, not only for engineering reasons, but only for a construction factor. This also dictates that any other buried lines that are routed with gravity drain will be configured using the 45 deg. ells. & a short spool piece between them, for any 90 Deg. change in direction (see this discussion in Clean-Outs) …again because it is routed with gravity drains that do require this "unique" configuration.

As I mention earlier sloped drain systems can get pretty deep over a long run, so it is advantageous to centrally locate the buried drain tank(s), ponds, or waste disposal area where the waste commodity eventually ends up, where-by shortening the drain system piping that enter this area. This does not mean dead in the middle of the facility! …But off-set to the side, waste disposal is not normally the most important "system" in a facility, so it wouldn't occupy a prime spot in that facility.


These are the other branch connections on the main drain header.
They are usually a 45 deg. lateral off the main header, & a 45 deg ell. in the vertical, and a threaded cap or a flange with a blind at the end. These clean-outs (C/O) are spaced approx. every 100 foot to provide a "port" so if the drain header becoming plugged maintenance people can go to this point, open up the port and introduces a "roto-router" (rotating blade at the end of a flexible cable) to be able to clear out the plug.
This tool can normally reach about 100 ft., so positioning clean-outs ever 100 ft on the header is critical in case plugging does occurs deep in the drain header. This tool is very flexible and "could" make it through a couple 90 Deg ell., but the drain header needs to be designed to utilize this roto-router tool's flexible and not impede it, so all changes in direction on the drain header are done using 45 deg ell.s and not 90 Deg. ell.s (this helps the flow also).
This configuration is also used in positioning/routing "drain funnels or drain hubs", this is because these points "could be" used to introduce this rooter-router tool as well, especially if the plug has occurred in that branch before the main header.
A few design notes, as stated before, spacing for C/O's are approx. 100 ft., but there is another controlling issue, you don't want to try to push a roto-rooter through more then 5 fittings (this is a rule of thumb only) so a C/O would be required closer then the 100 ft. in this situation.

One last clean-out location that needs to be discussed is the one on the very beginning of the main drain header.
This is a configuration of 2 each 45 deg. ell.s in the vertical and a flange & blind (flg'd because of the larger size of the main header) …could be a threaded cap on smaller drain headers. This is probably the most important one because it's the first entrance into the main drain header. It should be located where it can be easily accessed for maintenance
equipment (all C/O's should be readily accessible!). An issue that can happen with all C/O's is "if" they could occur where there is vehicular traffic. Having a directly connected piping system exposed to traffic would cause stress/damage issues to that piping system.
To eliminate this issue "covers" are fabricated to cover these C/O's. These covers have lids on them to be able to readily access these clean outs.
These covers are basically a larger size section of pipe so you have 1" to 3" inches of clearance of any C/O flanges, this larger pipe would have 3 to 4 "lugs" welded to the inside and spaced roughly an 1" from the top edge so as a circular plate steel lid could be fashioned to just fit the inside bore of this "cover".
A 1" diameter hole would be positioned into the center of this lid so the maintenance people could use a tool to remove this lid.

Lid thickness to be able to with stand the traffic …say around 3/8" to ½" thick. Design note - With the clean-out flange just fitting inside this cover, hex. headed bolts will have to be tack- welded to the underneath side of the flange so nuts are accessible from the top.


This is usually done at a smaller scale then normal 3/8" scale piping plans …say ¼" down to 1/8" scale. This is done for a couple reasons, the U/G piping system is normally a less "complicated" system (visibly) then the above ground piping. Also the area of responsibility controlled by the single designer is larger (as stated earlier). So for expedience and simplicity of documentation these drawing use the smaller scales.

SO what should be documented on a U/G piping plan?

"Simplicity" is the key word here! It would start with the equipment location drawing …showing all "control points", roads, battery limits, all support locations, and finally equipment locations. Now when we show supports and equipment locations we want to use the simplest depiction of this image! Centerlines, an outline of the foundation, and an outline of the equipment …THAT IS ALL! Back on the board day (drafting by hand) this was done primarily to reduce time and effort required to make the drawing, but in the computer world that is not so much of an issue. However drawings that are over "decorated" (nozzles, bolt holes in flanges, skid structure, or trying to Xref in the entire U/G electrical system) the drawing now become visibly "busier" …thus harder to read by everybody, but more importantly construction personnel (people that actually have to use this drawing to build from) and can easily lead to construction errors, so the drawings still needs to be as simple as possible!
This simplicity can still help cut down on "drafting time" too, even in the computer world!

Now the other "issue" is this, U/G piping is usually the first thing into the ground (not counting foundations) …because this is normally the deepest item in grade. SO why would we show anything else that happens above it? Normally electrical conduit runs are at shallower depths, so above the U/G piping. Trying to document items that have no bearing on the U/G piping is a waste of time and effort, and gets back into the "over decoration issue" mentioned earlier!
NOW if the electrical designer has "plans" for a particular area or the rare occasion that their electrical routing is as deep as your U/G piping, and you as the piping designer can avoid or take special note of that area or documenting a minimal representation of that conflict only …that's all part of being a good designer and touching bases with the other disciplines to resolve issues before they start …the backhoe operator & welders are not the people to be trying to resolve engineering &/or design problems!
Documentation of this system can be tricky, but not impossible. Remember the contractor is going to need to know where every ell. is located and what the "invert elevation" is at that point.

Invert elevation is the bottom of the inside bore of the pipe …basically the "wet" part of the piping system. This can sound like a formable task, but just remember it's just the normal B.O.P. elevation (bottom of pipe) plus the wall thickness. This is called out in this manner: INV. EL. 97'-6 ½" …with a leader pointing to the intersection of the centerlines on the fitting (remember we are trying to maintain this as a common elevation on all U/G piping at the point!). Another thing to remember, if your calculations come up with a value in the 16's or 8's of an inch document it to the closest ¼". Remember this piping system is not being built in a machine shop, and trying to control a pipe to that degree of accuracy in a ditch is next to impossible (and normally cause the construction people to laugh at the engineer/designer that called it out!) …SO again, keep it simple as possible! This points to the last issue, all points of intersection …change of direction, drain connections, & clean-outs is clearly documented with either coordinates or dimensions & an invert elevation call out!

Leaving this information out and putting notes like … "Where the piping is not dimensionally tied down or located by coordinates, construction shall install this piping as close as practical to the indicated routing." …notes like this are a "red flag" that the designer and/or engineering company does not full know what they are doing! This now requires the construction people to do the engineering, but doesn't relieve the original engineering firm of the responsibly for the system, or any errors made by the construction people.

When are U/G piping drawings not required? 

"IF" you have a facility that doesn't have any piping that is routed U/G is one obvious reason. However if you only have a few lines that are run underground and they don't run in the same trench then just documenting them on the above ground piping plans would be acceptable. But any time you have more then one line that runs in the same trench and has to comply with the design requirements of any other pipe in that trench (such as sloped piping), then a separate "Under Ground Piping Plan" would be advisable.
Remember a few design requirements, main U/G headers are not to be routed under equipment, tanks, or buildings foundations (however branches can start from these locations). This is because that "IF" a line becomes plugged, and maintenance people can't remove the plug, then the line will have to be excavated and replaced (plugged portion only). Try not to route line closer then 18" to foundations (edge to edge, & 12" in a pinch). Again line excavation should not disturb set equipment foundations! Use a smaller scale to cover large areas on the drawing, keeping things
simple (do not over decorating the drawing), run all sloped piping in one trench (if possible), position all drain funnels (hubs) and C/O's for maintenance, fully documenting location, slope, and depth.

Don't bury piping smaller then 2".
Following these guidelines will help produce a professional drawing that is easily read and constructed from,culminating in a key piping system in any facility.

About the Author


{cb:Mark is formally trained as a Piping Designer, he has over 39 years of work experience from the North Slope of Alaska to the oil fields in central California, and has worked on projects world wide. He is presently employed as the Design/Drafting Supervisor at a consulting engineering firm and is presently working in California.}

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