As design engineers, we normally design systems that require pressure regulation for proper operation. But do we really understand regulators?
According to the ASPE Plumbing Dictionary, a regulator is “A device intended to reduce a variable inlet pressure to a constant outlet pressure under variable flow conditions.” ASPE goes on further to define gas pressure regulators as, “A device used to control and maintain a uniform gas pressure that is lower than the supply pressure at the inlet of a regulator.” This fits with my own definition of a pressure regulator, “A device that controls a variable upstream pressure to a consistent downstream pressure within a narrow bandwidth.”
One must also remember that gaseous fluid, such as natural gas, is a compressible fluid, while a liquid fluid, such as water, is not compressible. This compressible verse non-compressible will influence the operations of a regulator due to the density of the fluid.
A July 2024 article in the ASHRAE (American Society of Heating, Refrigeration, and Air-Conditioning Engineers) Journal titled “Natural Gas, Old Buildings, Assumptions and No Good Deed Goes Unpunished,” by Michael Gallagher, PE reminded me that there is more to a regulator than what we think we know.
In my opinion, we expect a gas pressure regulator to reduce a variable inlet pressure to a regulated and reasonably consistent downstream pressure that is maintained within an acceptable operating pressure bandwidth for the equipment being served. That is what most of us in the design community expect when we select a regulator. But that is not always what is provided on our packaged equipment or what someone may have installed in the field, as Mr. Gallagher pointed out.
First we must understand how a regulator operates, the internals of the regulator, in which a lockup device shuts off flow when the set point has been reached. This setpoint is not a single point but a bandwidth of a pressure point, plus or minus of the setpoint in which the connected equipment can operate. Not all regulators are of the lockup type. A non-lockup regulator is simply a flow control device, in which the flow/pressure is reduced downstream only when flow is occurring. In the static condition, no flow occurs, the pressure is equal on both sides of the control. These are for specific operations and should only be used when there is a lockup regulator controlling the pressure to the non-locking regulator, such that the pressure can never exceed the desired setpoint.
So, let us look more closely at the lockup type regulator, a device that works hard to ensure the gas flowing through the lines into the equipment does so at the desired pressure. Not having sufficient gas pressure will render the equipment useless. However, having too much pressure can have catastrophic results that may lead to explosions that could harm others and damage or destroy property.
The regulator concept was developed in 1835, and has had a long-lasting impact on the installation of gas systems. There are various types of regulators, but function the same: to use a valve system to control natural gas, propane or other gas flows. Common appliances that use regulators include gas stoves, propane grills, boilers, furnaces, or oxy-fuel bottles for welding. Each type of regulator involves a set spring attached to a rod that runs down from a set screw through a diaphragm into the valve.
There are three primary operating components working together to regulate the pressure within the valve. The loading mechanism (spring and screw) determines the delivery pressure. The sensing element, or diaphragm, senses the force against the spring. Finally, the control element accomplishes the reduction of the inlet pressure through to the outlet pressure.
So how does the regulator work? Gas enters the regulator’s chamber, putting pressure on the diaphragm. The diaphragm then move upward as controlled by the spring. This allows a specific flow of fuel from the source to the appliance or device. Turning the screw clockwise will push the diaphragm down and allow more gas to flow into the regulator chamber. Turning the screw counter-clockwise reduces the amount of fuel and pressure within the regulator.
The mechanics of the gas regulator function well together. However, there is another component that comes into play called the surrounding air. Atmospheric pressure, based on elevation above sea level will affect gas pressure. The inner parts work by sensing the pressure both upstream and downstream. The atmospheric pressure affects the way the regulator senses downstream pressure.
There is a difference between pressure reduction and pressure regulation. The application between two types, reduction or regulation, is the main difference. A pressure reduction regulator is used to reduce the inlet pressure of the gas, so that it is at the ideal operating pressure on the outlet. It is a normally open valve installed upstream of the pressure-sensitive equipment it needs to regulate, as it controls downstream pressure. Basically, this is a flow control device that, when in a static condition, has equal pressure on both sides of the device.
Pressure regulation is often called a back-pressure regulator or back-pressure valve. Its purpose is to maintain a set pressure at the outlet while dealing with a variable upstream pressure. This type has an internal lockup device that shuts down the flow when the downstream pressure exceeds the setpoint. This is not to say that the downstream pressure is a discreet set point. After all, we are dealing with a compressible fluid (gas). Hence, the setpoint will have a bandwidth in which it operates.
This bandwidth will have a maximum pressure and a minimum pressure, with the set point being the medium between the high and low. And as we are dealing with a compressible fluid, the distance between the regulator and the appliance control valve will play a part in the variations of pressure within the line.
This is the result of an appliance control valve that can terminate flow instantly causing a shock wave within the gas delivery line between the regulator and the control valve. The greater the distance between the regulator and control valve the greater the potential for the shock wave to exceed the desired setpoint.
The pressure regulator is a “black box” in which the manufacture has created the design to assure a reasonably constant downstream pressure against the variable upstream pressure. This is considered a lockup style regulator in which the internal control elements assure that the downstream pressure remains within the desired bandwidth, much like the sine wave used on the electrical side; peak, medium, and minimum. This style regulator never allows the pressure to exceed the maximum value, regardless of the variable upstream pressure.
So, when you are selecting a pressure regulator, be sure that your selection is a lockup type and not just a flow control.