How to Manage Supply Pressure Effect (SPE) Part 2

Welcome to this Swagelok video. Today’s topic is considerations in managing the supply pressure effect of a regulator for lower-flow applications.

An alternative method of regulating supply pressure effect is to use two-stage pressure reduction. This can be accomplished by using two single-stage regulators installed in series, or the two regulators may be combined into one assembly.

To calculate the variability of outlet pressure for a two-stage regulator setup, the inlet pressure difference is multiplied by the SPE of each regulator, as illustrated in the following equation.

The first-stage regulator controls the inlet pressure variation to some degree, while the second-stage regulator controls it further. Together, the two regulators bring the outlet pressure very close to the target.

We will use a KCY model regulator and return to our supply pressure effect example. The cylinder empties from 172 bar to 34 bar, or 2500 psig to 500 psig. Assume that each regulator has a 1% SPE. With a 137 bar, or 2000 psig, inlet pressure drop, the first-stage regulator will experience a 1.3 bar, or 20 psig, increase in outlet pressure. As a result of that increase, the second-stage regulator will experience a 0.01 bar, or 0.20 psig, decrease in outlet pressure.

The net result is a minimal pressure decrease that is only 0.01% of the initial cylinder pressure drop: 0.01 bar, or 0.20 psig, equals 137 bar, or 2000 psig, times 0.1 times 0.1.

Once the cylinder pressure drops below the first-stage setting, in this case 34 bar, or 500 psig, the SPE of this regulator is no longer applicable, and only the SPE of the second-stage regulator is taken into account. Then, the assembly will behave like a single-stage regulator. Notice how the effect on the outlet pressure is lessened from the previous regulator arrangements.

For applications that require a gas cylinder to supply multiple operations, with at least one operation requiring a different pressure, you will need to use two-stage regulation. The first-stage regulator will be located near the gas source, and a second-stage regulator will be located on each of the process lines.

Be sure to avoid the costly mistake of using a two-stage regulator at the gas supply source and a single-stage regulator at the point of use. This setup is equivalent to three-stage regulation, with the regulator controlling outlet pressure from a gas cylinder.

Supply pressure effect is a phenomenon that will always be present. Whenever there is a change in inlet pressure, there will be a corresponding change in outlet pressure.

You can minimize the effects of supply pressure effect for many applications by using a single regulator with a balanced poppet design or by using a two-stage regulator. But if your gas source is servicing multiple operations with different pressure requirements, you may need multiple single-stage regulators: one near the gas source and another on each process line to, in effect, provide two-stage regulation at each point of use.

Refer to swagelok.com and the product literature for additional information.

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Supply pressure effect is defined as the change in outlet pressure due to a change in inlet or supply pressure. If the inlet pressure decreases, there will be a corresponding outlet pressure increase. Conversely, if the inlet pressure increases, the outlet pressure will decrease. Inlet and outlet pressure changes are inversely proportional to each other.

A regulator supply pressure effect value is typically provided by the manufacturer and can be published as either a percentage or as a ratio. Supply pressure effect in a single-stage regulator occurs whenever there is a change in inlet pressure. However, it may only be noticeable when the change is significant. For example, when a gas cylinder has drained and the pressure dropped from 172 bar to 34 bar or 2500 psig to 500 psig. The degree of outlet pressure variation for a regulator can be estimated with this formula. For example, for a regulator with a supply pressure effect, or SPE, of 1%, as a cylinder empties from 172 bar to 34 bar or 2500 psig to 500 psig, the regulator's outlet pressure will increase by 1.3 bar or 20 psig.

One of the most common types of regulators is a spring-loaded pressure reducing regulator. A spring applies force on a sensing element, either a diaphragm or a piston, which moves the poppet closer to or away from the orifice to control the outlet pressure.

In an unbalanced poppet design, the inlet pressure pushes up on the poppet, applying pressure to a portion of the poppet equal to the seat area. As a result, a decrease in inlet pressure means less force is pushing up on the poppet. With the decreasing force, the poppet moves away from the seat, which increases the outlet pressure. The rise in outlet pressure is not strong enough to counterbalance the set spring force to push the diaphragm up to close the poppet at the same outlet pressure. That will close at a higher pressure.

Let's demonstrate that by gradually decreasing the inlet pressure of the spring-loaded pressure reducing regulator. As you can see, at an inlet pressure of 80 bar or 1160 psig, the outlet pressure is 3.0 bar or 43.5 psig. Decreasing the inlet pressure to 70 bar or 1015 psig results in a higher outlet pressure of 3.3 bar or 47.9 psig. Moving down to 60 bar or 870 psig for the inlet pressure causes an outlet pressure of 3.7 bar or 53.7 psig. And finally, an inlet pressure of 50 bar or 725 psig leads to an outlet pressure of 4.0 bar or 58.0 psig, the highest of the process. For many applications, especially high-flow applications, a common method for reducing supply pressure effect is to use a regulator with a balanced poppet design. The objective of this design is to allow the inlet and outlet pressures to control the poppet more evenly. This is accomplished by the O-ring around the lower stem of the poppet and the orifice in the poppet that runs vertically from top to bottom.

Returning to the supply pressure effect scenario, we have the inlet pressure decreasing as the gas cylinder is depleting. The inlet pressure has less effect on the poppet because it is acting on a smaller area. The outlet pressure has sufficient leverage to close the poppet and restrict pressure, correcting much of the rise in pressure downstream.

Let's demonstrate that by repeating the gradual decrease of the inlet pressure performed earlier, now with a balanced poppet regulator. At an inlet pressure of 80 bar or 1160 psig, the outlet pressure is 3.0 bar or 43.5 psig. Decreasing the inlet pressure to 70 bar or 1015 psig results in an outlet pressure of 3.1 bar or 45 psig, still an increase but a smaller one than occurred with the other regulator design. Moving down to 60 bar or 870 psig for the inlet pressure causes an outlet pressure of 3.2 bar or 46.4 psig. And finally, an inlet pressure of 50 bar or 725 psig has an outlet pressure that is also 3.2 bar or 46.4 psig. Notice how the effect on the outlet pressure is lessened from the previous regulator arrangement.