How do pressure ratings work?
On its face, a pressure rating seems like a simple enough statistic. The pressure rating is the maximum pressure a pipe, fitting or valve should be capable of withstanding in a normal conditions.
Also known as maximum allowable working pressure (MAWP), a pressure rating lets an engineer or plumber determine which part will work best with the system being built. The higher the pressure rating, the less likely the part is to fail due to structural issues during heavy use.
HOW IS PRESSURE MEASURED?
There are a number of ways pressure can be expressed. We will focus on Pounds per Square Inch (PSI), since that is the most common term used in the United States, where we do the majority of our business.
You may also see WOG, which stands for “Water-Oil-Gas.” That term denotes the PSI that the part can handle at ambient temperatures with any of those media.
WHAT TO CONSIDER
Here are a few things to consider when determining the pressure rating to go with.
Higher temperatures adversely affect the working pressure of the part. That is, a stainless steel ball valve rated at 1,000 psi can hold that pressure at up to 100 degrees. But it may have a maximum pressure of only 600 psi at 650 degrees.
And just because the part has a higher pressure rating, though, doesn’t mean that it is the better buy. If it was, we would only stock parts rated for the highest pressure and the choices would be a lot simpler.
However, parts with higher pressure ratings generally cost more. A valve with a 3,600 psi pressure rating will usually have thicker walls than one with a 1,000 psi rating. The cost of the extra material alone will increase the price, as will the difference in materials used and the cost of the part’s development.
Brass, while less expensive, is not as strong as the more costly steel. So, if a part will work at a lower pressure rating and there are no other factors to consider, like material composition or connection type, buying a part at a lower cost is a no-brainer.