Tuesday, February 7, 2017

How does a pressure reducing valve work? Pressure regulator and vaporizers principles. Example of a residencial and industrial uses.

During the installation of LP gas to a residence or industry, it is important the use of pressure regulators and vaporizers when is required, to ensure the efficient use of the gas.
Installation of an industrial LP tank with vaporizer, pressure regulators and safety shut off valve

To read this article in Spanish, please visit: www.guiaautomatizacionindustrial.blogspot.com

What is the purpose of a pressure reducing valve or pressure regulator?

The typical use of the pressure regulators is located in the residences that uses gas to cook or heat. Normally a residence oven or furnace or heater require low gas pressure compared  o the input gas pressure. The nominal value is between 0.36 – 0.4 psi. The pressure of a residential tank is around 50-200 psi, that is why we need pressure reducing valves or pressure regulators.
But the pressure regulators have limitations and sometimes it is required to install multiple valves in order to get the desire pressure.
For example, a residential gas tank has a pressure of 100 psi, and it is required to reduce the pressure around 0.4 psi, the firs pressure regulator reduces the pressure to 4 psi and a second pressure regulator reduces the pressure to 0.4 psi. Sometimes these 2 pressure
regulators are embedded in one device.

But the pressure regulators not only reduce the pressure, they also maintain the pressure constant.

How does the pressure regulator maintain the output pressure constant?

Lateral view of a pressure regulator (pressure reducing valve)

The output pressure can be adjusted manually by the screw adjuster or pressure adjuster, this is located on the top of the valve and push an internal spring. The function of the spring is to counteract the force of the gas inlet pressure on the valve. In this way, the pressure is regulated by the effect of the opposing forces between the spring and the inlet pressure.

When the inlet pressure increases, the impulsion force of the fluid overcomes the spring force, causing the contraction of the spring and closing the valve connected to the spring through the stem. The valve closes in order to maintain the outlet pressure on the set value.
When the inlet pressure decreases, the impulsion force of the spring overcomes the inlet pressure force, causing the expansion of the spring and opening the valve connected to the spring through the stem. The valve opens in order to maintain the outlet pressure on the set value.
There are different types of pressure regulators, but the operation principle is the same, and their main parts are:
  • A mechanism the regulate the flow that could be a butterfly valve or globe, etc.
  • A load opposite to the force of the inlet pressure, like a spring, diaphragm or both.

How does pressure reducing valve work?

How does the vaporizers work?

Let’s analyze the case where we have an industrial LP tank of 13000 liters.

The LP in the tanks is on liquid state, when the LP is exposed to an atmosphere with a temperature greater than 0 C, the LP boils absorbing heat, changing to gas state and cooling the material that is in contact to it.

It is important to mention that the recommendation for LP tanks is keep a 15% free of its total capacity, because the temperature outside the tank boils the liquid LP inside, and the volume of the vapor is 250 time greater than the liquid, generating a high pressure into the tank.

The vaporization in the tank depends of the wet surface, or the quantity of liquid LP in the tank and the ambient temperature. If the wet surface is less or if the temperature is less, then the vaporization is less.

Normally the vaporization in residential tanks takes place inside the tank, the ambient temperature that is in contact to the tank is enough to vaporize the LP, increasing the pressure in the tanks, this pressure is required for the distribution of the gas around the residence.

When the ambient temperature around the tank is not enough to produce the vaporization of the LP, the LP liquid starts cooling and reducing the output pressure, causing a reduction of the output flow. To increment the output flow, the LP is vaporized through external vaporizers.

The change between the liquid state to gas state is handle by the vaporizers. The main function of a vaporizer is to increment the LP temperature, to reach the boiling point.
Because the vaporization of the LP liquid causes cooling of the material in contact, it is necessary that the vaporizer stay warm to avoid a frozen pipe or device.
In this specific case, the vaporizer maintains the LP gas to a temperature of 5 °C at 15 psi.
While the gas goes downstream, its temperature increases until the ambient temperature. The pressure regulators are located downstream in order to reduce the pressure to the nominal pressure required for the furnace 0.8 psi.
After the LP gas reaches the nominal pressure required by the furnace, the following stage is the safety system using shut off valves.