Ask Alicat: How do you control pressure in a closed system?

Alicat PCD family of dual-valve pressure controllers

Alicat PCD family of dual-valve pressure controllers

Alicat Scientific’s PCD-Series dual-valve pressure controllers are unique among the hundreds of competing pressure controllers on the market today. Designed primarily for dead-ended, closed volume pressure control applications, the PCD allows you to control the pressure efficiently without continuously wasting costly gas through a bleed valve.

The video below shows you how efficient the PCD is!

Single-Valve Pressure Controllers in Closed Systems


Alicat PC-Series single-valve pressure controller

PC-Series single-valve pressure controller, with optional sensor connector

When you supply gas to the inlet of a single-valve pressure controllers, the proportional control valve adjusts to ensure that the pressure in your process remains constant. If process pressure decreases, the valve opens to restore the target pressure. If the pressure increases, the valve closes to allow the remaining components of the system to relieve the excess pressure.

This is very good in an open system, but what happens in a dead-ended application, or a closed system? If the pressure within the closed system becomes too great, there is no place for the excess gas to go. To solve this problem, a simple needle valve connected to the closed volume allows venting of excess pressure. With gas continuously venting through the needle valve, the pressure controller can close its valve to allow the system pressure to decline.

This resolves the application issue but generates a new financial problem: Gases can be very expensive. The continuous venting of costly gases like helium is a waste of resources and money. The flow totalizer in the video shows how quickly gas is vented through just a small continuous bleed.

PCD-Series Dual-Valve Pressure Controllers

Alicat PCD-Series dual-valve pressure controller

PCD-Series dual-valve pressure controller

Alicat’s PCD-Series dual-valve pressure controller incorporates a second proportional control valve at its outlet to vent excess pressure. The PCD maintains a stable pressure through the central process port to the closed volume under test. This valve opens only when needed, so almost no gas is wasted.

If the system pressure becomes too great, the outlet valve opens to return the pressure to the chosen setpoint. Because this process happens a thousand times every second, the amount of gas lost while maintaining a constant pressure in a closed volume is exceedingly little. In this way, choosing a PCD for applications in closed volumes will save you a lot of money.

Ordering a PCD

When Alicat builds a PCD, we have a number of valves to choose from for both the inlet valve and the outlet valve. Correct valve sizing for PCD’s depends upon the following three parameters:

  1. Inlet pressure. (P1) Lower inlet pressures generally require larger inlet valve openings.
  2. Exhaust pressure. (P2) Customers typically vent either to atmosphere or to vacuum. Generally, a vacuum will generate greater exhaust flow rates than venting to atmosphere.
  3. Process Volume. (Vol.) We also ask for the volume of the closed process under test because greater volumes require larger flows to fill and vent in a timely manner. You should note when the customer needs to fill or evacuate the process volume quickly.

Once you have provided these three parameters, Alicat engineers will select the valves that will allow the most efficient filling and evacuation of your process volume while maintaining excellent control stability.

Alicat PCRD-Series dual-valve pressure controller for high flows

PCRD-Series dual-valve pressure controller for high flows

For those who require high flow rates into or out of the process volume at low pressures, we may recommend our PCRD Series. This series uses the same Rolamite valve that we use in our larger mass flow controllers for larger flows.

See the application notes below for processes that can benefit from the use of dual-valve pressure controllers.

If you are ever in doubt about what to choose for your application, please do not hesitate to call one of our Alicat sales engineers at 520-290-6060.