# Water Pressure in CPVC Plumbing System

Pressure is the force that pushes water through pipes, or else (Pressure = Force/ Unit Area)

Water pressure determines the flow of water from the tap. It’s a measure of the force that gets the water through our mains and into the pipes. The amount of pressure at your tap can depend on how high the service reservoir or water tank is above your home, or on how much water you are using.

Units of Pressure

The units of pressure are KG/cm², Bar & PSI (pounds per square inch)

Understanding Elementary Calculations of Water Pressure

What does 1 KG/cm² water pressure mean? To understand this let’s look at a building where we have to pump water to 10 meters height. So, for this we shall have to use a pump to push up water. What are we fighting against is GRAVITY. The gravitational pull towards the earth surface is 9.8kg/cm² say 10kg/cm². Pressure is independent of pipe size so irrespective of the size, pressure is always a function of height and as a thumb rule, we can say 10 meters height means: - You shall require 1kg/cm² pressure to pump up water. If a water tank is placed on the top of the terrace (10Mts height) the pressure on the ground floor shall be 1kg/cm².

So now if we say there is a 40 storied building and can CPVC pipe be used? Let’s look at the building height. Height of Each floor shall be 10ft say 3.3, meters so 40 floors mean 40x 3.3m = 132 meters the pressure required to pump up water from the ground floor of the terrace be 132/10= 13.2kg/cm².

History Behind Developing Standardized Thermoplastic Pressure Ratings

Designing piping systems with thermoplastics was relatively new in the 1950s.There were no standardized methods to consistently determine thermoplastic performance based on different factors (e.g. heat and pressure). Needless to say, establishing a means to compare materials and define pressure-bearing capabilities was imperative.

In 1958, the now-called Plastics Pipe Institute established the Working Stress Subcommittee (now called the Hydrostatic Stress Board). The goal of the board was to develop a hydrostatic design stress test for thermoplastic compounds.

After evolving the testing method 15 times, ASTM D2837 was published in 1969. ASTM D2837 is the methodology still in use today to gauge the long-term hydrostatic strength of thermoplastic compounds, such as CPVC.

**Standard Thermoplastic Pressure Rating Methodologies**

It is important to understand how the CPVC piping systems will perform when exposed to constant stress in presence of water at various temperatures. A study of the long-term hydrostatic strength (LTHS) should be conducted therein. There are two primary methods to determine a piping material’s long-term hydrostatic strength. The ASTM D2837, “Standard Test Method for Obtaining Hydrostatic Design Basis for Thermoplastic Pipe Materials or Pressure Design Basis for Thermoplastic Pipe Products” is the most commonly used. The other is ISO 9080.

The ASTM methodology involves the following steps:

- Define the material’s Long-Term Hydrostatic Stress (LTHS) value.
- Use its LTHS value to identify the material’s Hydrostatic Design Basis (HDB) category.
- Multiply the HDB by a design factor to come up with the material’s Hydrostatic Design Stress, or maximum allowable stress.

**Understanding the calculations of CPVC Pressure Ratings**

Calculating the pressure rating is not as easy as increasing the pressure in a pipe till it bursts. Short-term tensile strength tests are not good indicators of a thermoplastic’s long-term strength, compared to metals. For this reason, LTHS in plastic piping system is determined by analyzing stress versus time-to-rupture (that is, stress-rupture) test data. This data covers a testing period of no less than 10,000 hours and is derived from sustained pressure testing of pipe made from the subject material.

**LTHS** is determined by analyzing stress versus time-to-rupture (that is, stress-rupture) test data.

To determine CPVC’s LTHS value, pipe samples are tested at various stress levels, which will cause the pipe to rupture ranging from 10 hours to more than 10,000 hours. For pressure rating at 180°F (82°C) test times as per PPI’s requirements should exceed 16,000 hours.

The stress versus time-to-failure data is then plotted on graph where it should form a straight line which can be fit by linear regression analysis. The line is then extrapolated to determine the stress level which would result in a 100,000-hour rupture time. In most cases, the extrapolated stress level at 100,000 hours is used as the material’s Long Term Hydrostatic Strength (LTHS) value.

Once the LTHS value has been determined, the materials **Hydrostatic Design Basis (HDB)** also needs to be defined by categorizing the LTHS value as underlined in ASTM D2837. Hydrostatic design basis (HDB) can be defined as a measure of the material's ultimate strength performance when subjected to certain end-use conditions. However, internal stress won’t be the only variable that can affect long-term performance.

For this reason, the Hydrostatic Design Board decided that a material’s HDB value should be multiplied by a design factor to determine the material’s maximum allowable pressure stress, or **Hydrostatic design stress (HDS)**. For water applications, it was determined that a design factor of 0.5 should be applied to CPVC piping system.

Finally, to define a **pipe’s pressure rating**, its diameter and wall thickness also need to be factored. HDS is plugged into the following equation to define a pipe’s pressure rating:

S = P (D-t) / (2t) Or S = P (d+t) / (2t)

Where:

- S = Stress
- P = Pressure
- D = Average outside diameter
- d = Average inside diameter
- t = Minimum wall thickness

The value derived from this formula is the pressure rating you’ll see printed on the side of a pipe. It is based on the pipe’s performance at 23°C in potable water application.

This comprehensive process, involving rigorous testing, industry oversight, and a conservative safety factor, guarantees the reliability and performance of CPVC piping in potable water systems. As a result, professionals can confidently use these pipes, as these are designed to withstand specific operating pressures.

### Prasenjit Misra

Prasenjit Misra, an accomplished professional, having extensive exposure in plumbing field, currently holds the position of Lead Demand Creation & Business Development of TempRite South Asia division at Lubrizol India.