Automating dome loaded pressure controllers

The process pressure of dome loaded back pressure regulators is set by applying the required pressure to the reference side of the controller at a one-to-one ratio. Traditionally they use springs to set the upstream pressure. However, The change in the spring force as it is compressed is a major source of error or overpressure.

Pressure Control Solutions offers the Equilibar back pressure regulator which is designed to maintain upstream pressure at its own inlet to enable high precision and stability in controlling the pressure in chemical processes. It is said to be particularly suitable for low flow, extreme temperature, mixed phase fluids, corrosive media and sanitary process conditions.

“Until recently we  used single-valve electronic controllers to get the right pressure levels for our dome loaded back pressure controllers, then to reduce pressure, we added artificial leaks. Although this solution worked very well in practice, the downside was that it required us to continuously use inert, high-pressure gas,” said Armand Bergsma, owner of Pressure Control Solutions.

“This configuration is fine for low-pressure applications using a compressed air network, or high-pressure applications where the customer provides a centralised high-pressure network,” continued  Bergsma. “However, it can be a problem if such facilities are not available. 

“If there is no centralised high-pressure network, it is possible to use 200 or 300 bar gas cylinders. The reactors used to test catalysts usually only need a pressure of 150 to 180 bar, so no more than 50 to 20 bar respectively should be used from a 200-bar nitrogen cylinder in order to maintain maximum process pressure. For extended periods another another way to limit the consumption of dome gas will be required.”

Together with Bronkhorst Nederland the company looked for a solution to reduce the consumption of dome gas. The solution was found by combining Pressure Control Solution’s Equilibar dome loaded back pressure controller with the new Bronkhorst EL-PRESS Process Pressure Controller (PPC). This device features an integrated PID controller, which makes it possible to regulate two control valves, greatly reducing the consumption of dome gas.

The pressure controller opens the control valve located upstream of the pressure sensor. Meanwhile, the downstream valve remains closed, which causes the reference pressure to increase. Gas consumption during the pressure build-up is negligible. The control valve located downstream of the pressure sensor is only opened once the pressure is lowered. “Naturally, the upstream valve remains closed when this happens,” continued Bergsma. “By choosing the right valve (orifice size) and PID settings, it is possible to either quickly or slowly increase and/or reduce the dome pressure, depending on our requirements.”

The Bronkhorst PPC does not just reduce gas consumption, it also offers stability. Because it employs clean pure gases, processes that combine an Equilibar precision dome loaded pressure controller and the PPC are able to control pressure stability at a rate better than 0.1%.

The process side of the dome loaded pressure controller is often used to process aggressive reactants and by-products at high temperatures. For these severe applications, the Equilibar can be manufactured from chemically inert materials such as SS316, Hastelloy, Zirconium and Monel. The multiple-orifice design is capable of controlling gases and liquids as well as multi-phase flows.