PSI vs. Bar vs. Class vs. lbs. vs. CWP
Valves and Pressure Classes
When it comes to pressure ratings there are many different classifications, both imperial and metric; and while some are easily interchangeable, some are not. Generally, pressure can be defined as the force that is applied on a certain unit of surface area. Regarding valves, pressure classification is the maximum allowable pressure at a specified temperature that can be applied to the body of a valve.
This article has been purposed to provide you with the most up-to-date and accurate information regarding these classifications with respect to valves and pipelines in North America. For starters, we use ASME B16.34 as the North American standard for gauging safe pressure specifications. While this is our standard, many valve manufacturers, distributors, resellers, and end-users see valves that have been imported from various locations around the world. This, along with inadvertent mistakes made when requesting or classifying the valves based on Maximum Allowable Working Pressure (MAWB) has caused a lot of extremely detrimental consequences – for both worker safety and profitability.
Some of the ratings you will see used to describe MAWB:
PSI: Pounds per Square Inch – psi measures the pressure as a one-pound force applied on an area of one square inch. It is important to note that psi should be written as either psig (psi gauge) or psia (psi absolute). When dealing with valves, you can assume you are using psig.
PSIG: Pounds per Square Inch Gauge – They take away the atmospheric pressure (wherever you are) and calibrate the pressure to only represent what your measuring
PSIG = PSIA – 1 atm (where atm is atmospheric pressure)
PSIA: Pounds per Square Inch Absolute – it includes both the atmospheric pressure as well as the pressure of what you are measuring. Atmospheric pressure at sea level is 14.70psi.
PSIA = PSIG + 1 atm
Bar: Metric unit of pressure used to measure the force applied perpendicularly on a unit area of a surface. While it is its own measurement (bar) it is often calculated and remembered in terms of kilopascals kPa (for lower pressures) or megapascals MPa (for higher pressures). It is legally recognized and used in the European Union, and commonly used in Canada as well.
1 bar = 100 kPa = 0.1 MPa
Lbs. or pounds: This measurement is a unit of weight, it sometimes gets used interchangeably with psig or Class (which is a label, not a value), which it should not. If you see this being used, you should double-check the actual working pressure needed. As a general rule of thumb, pounds should not be used as a measurement of pressure.
Cold Working Pressure (CWP) – CWP dictates the MAWP for valves (and other such instruments) that do not meet or comply with ASME B16.34. Many valves do not need to have a pressure rating as high as those recommended in this standard for the applications they will be used for. When this is the case they typically just value the MAWP as psig. For instance, you may have a small, threaded valve that has a MAWP rating of 1500 or 2500 psig, it would be easy to confuse this valve as being Class 1500 or 2500, which would actually need to have a psig of 3705 or 6170 psig*, respectively.
Class: The valve class is a label** based on the material of the valve and temperature of the application. The MAWP is then presented as a function of this as psig for each temperature range. Materials in this standard are rated as either Standard, Special or Limited. You can find all information on valve classes in ASME B16.34, North American Standard here. The original version of this standard uses imperial units since the US was a large influence during its creation, but there is a second iteration that has been converted to metric.
It’s important to know the difference between all of these ratings and measurements as many distributors and industrial plants source equipment from locations all over the world and the wrong attribution of them can cause costly ramifications.
Testing and Safety
A common test for valves is the Hydrostatic Test Standard, this tests valves at 150% of their MAWP, to ensure they have a safety margin when in operation. The test involves filling the vessel with a liquid – usually water with a coloured dye (to help spot leaks) and bringing the valve to the specified test pressure. When valves go over their MAWP in operation, they can fail causing leakages to atmosphere or worse yet, explode – a huge factor in worker safety.
*For the purpose of examples in this article we have referenced the psig values for B16.34 Group 1.1 Materials (A – Standard Class)
**Valve Classes are labels not “numbers” they don’t have a numerical value in relation to the maximum allowable pressure – ie. Psig.