Inside Diameter Controlled
For inside-diameter-controlled plastic pipes, the pressure class depends on the hydrostatic design stress and the standard inside dimension ratio (SIDR). This maintains a consistent bore size.
PC = 2 × HDS / (SIDR + 1)
Outside Diameter Controlled
For outside-diameter-controlled pipes, the pressure class uses the standard dimension ratio (SDR). These pipes maintain a consistent external size for fittings and connections.
PC = 2 × HDS / (SDR − 1)
AWWA C900 Standard
The AWWA C900 standard for PVC pressure pipe in municipal water distribution adds a surge pressure allowance to the working pressure when calculating the required SDR.
PC = 2 × HDS / (SDR − 1) − Psurge
Short Term Strength
Short-term strength rating determines the maximum pressure a pipe can withstand for brief durations based on its short-term hoop strength and dimension ratio.
STS = 2 × STHS / (SDR − 1)
Short Term Pressure Rating
The short-term pressure rating divides the short-term strength by a safety factor to provide a usable design pressure for brief loading conditions.
STR = STS / FS
How It Works
Plastic pipe pressure class depends on the hydrostatic design stress (HDS) and the standard dimension ratio (SDR or SIDR). Different formulas apply depending on whether the pipe is controlled by inside diameter, outside diameter, or the AWWA C900 standard. This calculator also covers short-term strength and short-term pressure rating equations.
Example Problem
A PVC pipe has HDS = 2,000 psi and SDR = 21. What is the pressure class (outside diameter controlled)?
- PC = 2 × HDS / (SDR − 1) = 2 × 2,000 / (21 − 1)
- PC = 4,000 / 20 = 200 psi
When to Use Each Variable
- Solve for Inside Diameter PC — when working with inside-diameter-controlled pipes (e.g., some HDPE and PE products), e.g., selecting a pipe for a consistent bore size.
- Solve for Outside Diameter PC — when working with outside-diameter-controlled pipes (e.g., PVC, most fittings match OD), e.g., selecting a pipe for standard fitting compatibility.
- Solve for AWWA C900 Pressure — when designing a municipal PVC water main per AWWA C900, e.g., determining the working pressure class after accounting for surge allowance.
- Solve for Short Term Strength — when evaluating a pipe's resistance to brief pressure spikes, e.g., checking if a pipe can withstand a water hammer event.
- Solve for Short Term Rating — when applying a safety factor to the short-term strength, e.g., determining the usable transient pressure limit for design.
Key Concepts
Plastic pipe pressure ratings depend on the hydrostatic design stress (HDS) — the long-term stress a material can sustain — and the dimension ratio (SDR or SIDR). Lower SDR means a thicker wall and higher pressure class. The AWWA C900 standard adds a surge pressure allowance for municipal water systems where water hammer events are expected.
Applications
- Municipal water distribution: selecting PVC pipe pressure classes per AWWA C900 for city water mains
- Irrigation systems: choosing HDPE or PVC pipe ratings for agricultural pressurized supply lines
- Gas distribution: determining PE pipe pressure ratings for natural gas service lines
- Industrial piping: evaluating short-term strength ratings for transient pressure events in chemical plants
Common Mistakes
- Confusing SDR and SIDR — SDR uses outside diameter and subtracts 1 in the formula; SIDR uses inside diameter and adds 1; mixing them gives the wrong pressure class
- Ignoring surge pressure in water systems — the AWWA C900 formula deducts a surge allowance from the working pressure; omitting it overestimates the pipe's usable pressure rating
- Using short-term strength for sustained operation — STS ratings apply only to brief pressure events; the long-term pressure class (PC) is always lower and is the correct design value
Frequently Asked Questions
What is SDR in plastic pipe?
Standard Dimension Ratio is the outside diameter divided by the wall thickness. Lower SDR means a thicker wall and higher pressure rating. Common SDRs include 11, 17, 21, and 26.
What is the difference between SIDR and SDR?
SIDR (Standard Inside Dimension Ratio) uses inside diameter; SDR uses outside diameter. Inside-diameter-controlled pipes maintain a consistent bore size, while outside-diameter-controlled pipes maintain a consistent external size for fittings.
What is AWWA C900?
AWWA C900 is the standard for PVC pressure pipe used in municipal water distribution. It adds a surge pressure allowance to the working pressure when calculating the required SDR.
Reference: National Resources Conservation Service. National Engineering Handbook. 1995. USDA.
Related Calculators
- Steel Pipe Design Calculator — Barlow's formula for steel pipe.
- Aluminum Pipe Design Calculator — pressure rating for aluminum pipe.
- Buried Plastic Pipe Calculator — wall crushing checks for buried plastic.
- Pipe Expansion Calculator — thermal expansion for restrained and unrestrained pipe.
- Pipe Flow Calculator — compute flow rate and velocity in pipe systems.
- Pressure Unit Converter — convert between psi, kPa, and bar for pipe ratings.
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National Resources Conservation Service. National Engineering Handbook. 1995. USDA.