The terms used to describe pressure relief valves, as well as the concepts behind them, can be confusing. Here you will find some of the more common terms and a brief explanation of their meaning.
Differential pressure is simply the difference in pressure on either side of a point of measurement. For a pressure relief valve, the differential pressure is the difference between the pressure on one side of the valve, compared to the other. In most cases, our pressure relief valves are mounted to a sealed system and exposed to the ambient air. Therefore, the differential pressure would be the difference between the pressure of the sealed system and ambient pressure. If the pressure inside the sealed system is higher than ambient, it is referred to as a positive pressure or simply “pressure”. If the pressure inside the sealed system is lower than ambient, it is referred to as a negative pressure or a “vacuum”.
The cracking pressure of a pressure relief valve is the differential pressure at which the valve begins to open. One way to measure the cracking pressure, and an easy way to visualize the concept, is to attach the valve to an air supply and submerge it in water, just below the surface. As the pressure inside the valve is increased, the cracking pressure is the point at which the first air bubble is observed to escape from the valve. It is important to note that the flow rate of the valve is minimal at this pressure. The pressure must be further increased to obtain a flow rate that is useful for pressure equalization.
The variation in cracking pressure of a particular valve is described as a percentage above and below the nominal value. For example, our BV2000 valve has a standard cracking pressure of 0.25 +/- 20% psi in both the outward and inward directions. Each valve is tested to verify that its cracking pressure falls within this range. A reliable cracking pressure is critical to ensure that the pressure in the system is tightly controlled. If there is excessive variation in the cracking pressure, the pressure experienced by the system where it is used will also vary.
The reseal pressure of a pressure relief valve is the differential pressure at which the valve returns to a closed position, after it has cracked. The cracking pressure is always slightly higher than the reseal pressure, due to the fact that static friction of the seal must be overcome for it to open.
Reseal pressure is always stated as a minimum value. This is because a higher reseal pressure is advantageous over a lower reseal pressure and can never be higher than the cracking pressure. Each valve is tested to ensure that its reseal pressure is above the minimum required for that configuration.
Flow rate is the volume of liquid or gas that flows through the valve in a given amount of time and is a way to quantify the valve’s ability to equalize a build-up of pressure or vacuum. In general, the flow rate of a valve is correlated to its size. A 4-inch valve, like our BV4000, will have a higher flow rate than a 2-inch valve, like the BV2000.
The flow rate of a valve is dependent on differential pressure. As the pressure or vacuum is increased, the flow rate will also increase. Due to this relationship, the flow rate always has to be measured at a particular pressure. Using our various in-house flow benches, we have the ability to generate flow curves for all of our valves. An example of a flow curve generated here at Stratotech of one of our products is provided below.
When selecting a valve for a particular application, we will usually start with the required flow rate to narrow down the search. For shipping containers that will be shipped by military cargo aircraft, customers can use our Flow Rate Calculator to determine the flow rate needed. The output of this calculator is sometimes used as a worst-case scenario for any enclosure, because the rate of pressure change during aircraft ascent and descent is relatively high compared to other causes of pressure build-up.
Most Common Units of Measure Used by Stratotech
The units of measure most commonly used here at Stratotech for pressure are psi (pounds per square inch) and inH2O (inches of water). 1 psi is equal to about 27 inH2O so inH2O tends to be used for lower pressure valves. A range of 0.2 to 2.0 inH2O is common as well as 0.25 psi to 10.0 psi for the higher pressure range. The most common unit of measure used at Stratotech for flow rate is cfm (standard cubic feet per minute). We can (and do) use other units depending on the customer’s needs.