Pipe for a Sewage Force Main: Why a Gravity Sewer Spec Will Not Survive a Pump

A pump station goes in at the low end of a subdivision, and the sewer leaving it is drawn on the same sheet as the sewer arriving. The arriving line is ASTM D3034 SDR 35. So, the drawing says, is the one leaving.

That single carry-over is what this page exists to catch. A force main is a sewer that a pump has turned into a pressure pipe, so it is bought against a pressure standard: ANSI/AWWA C900-22 for PVC, AWWA C906-21 for HDPE. It needs a pressure class, a separate surge check, restrained joints and a minimum flow velocity. The gravity specification already in your hands assigns no pressure class at all. That number was never in the standard.

A fused joint restrains itself; a gasketed one does not. This 0:30 clip shows the fusion.

A force main is a sewer line that has been turned into a pressure pipe

The sewage is identical on both sides of the pump. The loading is not. A gravity sewer flows partly full at atmospheric pressure, so the governing load comes from outside: earth cover, traffic, and the ring deflection those produce. Downstream of a pump the same fluid sits under continuous internal pressure, and the governing stress moves into the pipe wall.

Two design problems, two families of standards. Get the separation right and the specification follows. Miss it and every number downstream, from the 4 in minimum diameter to the restraint detail, is read off the wrong document.

The gravity specification measures ring stiffness and never assigns a pressure class

Read the scope of ASTM D3034-24E01 and the absence is plain. It covers flattening resistance, impact resistance, pipe stiffness, extrusion quality, joining systems and marking. No pressure class appears in it. The installation practice it points to settles the question: ASTM D2321-26 is titled Underground Installation of Thermoplastic Pipe for Sewers and Other Gravity-Flow Applications.

So what does SDR 35 actually buy? A minimum pipe stiffness of 46 psi tested to ASTM D2412, with SDR 26 at 115 psi. Both are ring values: resistance to squashing under earth load. Neither describes holding pressure from the inside, which is why no supplier can hand you a psi rating for SDR 35 sewer pipe. For the gravity system feeding the wet well, a PVC drainage pipe and fittings specification is the right document, and our uPVC drainage guide covers how those sizes are called out.

Working municipal specifications already keep the two apart, buying D3034 SDR 35 gravity pipe in one section and AWWA C900 force main pipe in another.

Gravity sewer versus force main: one fluid, two different standards.

Criterion	Gravity sewer (ASTM D3034)	Force main (AWWA C900)	Verdict
Governing number	Pipe stiffness, 46 psi at SDR 35	Pressure class, 165 to 305 psi by DR	Not interchangeable
What it resists	Earth load and ring deflection	Internal hoop stress plus surge	Different failure mode
Pressure rating	None assigned by the standard	DR 18 = 235 psi; DR 25 = 165 psi	The absence is the finding
Installation practice	ASTM D2321, gravity-flow applications	Pressure bedding plus joint restraint	Restraint is additional
Joint duty	Gasketed, unrestrained, no thrust	Restrained, fused or thrust blocked	Check every bend

Source: ASTM D3034-24E01 and ASTM D2321-26 scope statements; Uni-Bell PVC pressure pipe standards for sewer force mains; municipal sewer specification Section 1016, Rev. 11/2020. Compiled 22 September 2026.

For the small-diameter pressure lines around a pumped system, distributors and procurement teams can work from our HDPE pipe and fittings catalogue.

Pressure class is where the force main spec starts, and surge is a separate check

PVC pressure pipe to ANSI/AWWA C900 runs 4 to 60 in across a ladder of dimension ratios: PC 165 psi at DR 25, PC 235 psi at DR 18, PC 305 psi at DR 14, all at a design safety factor of 2.0. The current edition is C900-22. Picking a DR off that ladder is the first decision, and on its own incomplete.

AWWA C900 pressure class by DR. Source and method: Transcribed from the Uni-Bell C900 class table.

Dimension ratio	Pressure class (psi)
DR 51	80
DR 41	100
DR 32.5	125
DR 27.5	150
DR 26	160
DR 25	165
DR 21	200
DR 18	235
DR 17	250
DR 14	305

Here is the trap for anyone working from older habits. Before 2007, C900 used a 2.5 safety factor with surge overhead built into the class, so “the class covers water hammer” was roughly true. The 2007 revision moved to a 2.0 factor and pulled surge into its own calculation. Quoting a class and stopping there leaves the surge check undone.

The Uni-Bell force main design guide sets three checks: working pressure against the class, recurring surge against the class, and occasional surge against 1.6 times the class. Those multipliers belong to PVC. HDPE to AWWA C906-21 is rated on a different ladder, which is where a number copied from the wrong document does real damage.

Surge allowances are material-specific: a PVC multiplier does not transfer to HDPE.

Material and standard	Pressure class (psi)	Recurring surge limit	Occasional surge limit	Published as
PVC, AWWA C900, DR 18	235	Working plus surge within class	Within 1.6 times the class	Formula, Uni-Bell guide
PVC, AWWA C900, DR 14	305	Working plus surge within class	Within 1.6 times the class	Formula, Uni-Bell guide
HDPE PE4710, AWWA C906-21, DR 11	200	300 psi, 1.5 times the class	400 psi, 2.0 times the class	Stated values, Charlotte Water

Source: Uni-Bell Force Main Design Guide for PVC Pipe; Charlotte Water Design Manual Section 33 31 20. PVC rows state the published formula rather than a derived figure. Compiled 22 September 2026.

Four checks that only exist once there is a pump on the line

Velocity runs opposite to the usual instinct: oversizing a gravity sewer costs money, while oversizing a force main costs maintenance. Two independent jurisdictions, WSSC and Iowa DNR, land on the same floor of 2 ft/s, about 0.61 m/s, to hold solids in suspension. Fall below it and solids drop out; re-suspending them takes 3 to 3.5 ft/s, which the station may not produce. The practical ceiling is near 6 ft/s and the minimum diameter is 4 in.

Air is second. Every high point on a pressurised line collects it, and trapped air throttles flow while sharpening surge. The strongest guidance is geometric: profile the line so intermediate high points do not exist. Where terrain refuses, valve selection goes to AWWA M51 rather than a rule of thumb.

Thrust is third, created by the pressure itself at changes in direction, at reducers and at dead ends. A gravity line never sees it, so gravity fittings carry no restraint. On a force main every bend, tee and closed valve needs a restrained joint, an external restraint device or a thrust block, because an unrestrained gasketed joint can pull apart under pressure. Fused HDPE handles it inherently; our note on HDPE pipe joint types shows how that differs from a gasketed joint.

Hydrogen sulfide is fourth, and the budget for it usually lands on the wrong target. It attacks cast iron, ductile iron, steel, copper, asbestos cement and concrete, and is not a concern for PVC pipe. Damage concentrates at the discharge structure.

What to put on the specification line, the numbers to ask a supplier for, and where our range stops

A force main line item a reviewer will accept carries five things. Leave one out and the bid returns the cheapest reading.

• Standard and edition, such as ANSI/AWWA C900-22 or AWWA C906-21
• DR or pressure class, chosen against working pressure and surge
• Material designation, PVC or PE4710
• Joint and restraint type at every bend, tee and dead end
• Test regime, with the method named not implied

Testing is material-specific too. Fused HDPE is proved hydrostatically, commonly at 150 psi held four hours and steady within 5 percent over the final hour; compressed-air leakage testing of HDPE pressure piping is prohibited on safety grounds. Fusion is a procedure specification, ASTM F2620 or PPI TR-33.

On lead time we publish no figure, because we hold no verified one for force-main pipe and a plausible-looking number is worse than silence. What drives it is knowable, so ask directly. Is this DR and diameter stock or a production run? What is the extrusion slot, in weeks? Is the resin grade held or bought against the order? What is the lead time on fusion and restraint hardware, which often runs longer than the pipe itself? Ask for the date the pipe leaves the plant, not a duration.

One boundary, stated plainly. IFANNova’s pressure systems stop at OD 110 mm, while force main diameters begin at 4 in. This is not pipe we supply, and we would rather say so than route the enquiry onward. Our range serves the small-diameter pressure and drainage work around a pumped system: the pipe material selection matrix shows where each system sits, and pressure testing and creep in plastic pipe covers how sustained pressure is verified.

Conclusion

Before the force main line item leaves your desk:

• Does the pipe standard name a pressure class? If it names a stiffness, it is the gravity document.
• Is the DR checked against working pressure and against surge, separately?
• Does the surge multiplier belong to the material actually quoted?
• Does the flow clear 2 ft/s at the smallest pumping rate, not only at peak?
• Is restraint drawn at every bend, tee, reducer and dead end?
• Does corrosion protection sit on the discharge structure, not the pipe?

Frequently Asked Questions