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PPCH - Air in system, explanation of behavior

Purpose
This document describes the behavior of a PPCH hydraulic pressure controller/calibrator with air in it, or with a large volume attached (especially if the volume is full of air).

This procedure is intended for users or other authorized personnel trained on operation of PPCH pressure controllers

Notes
Refer to Figure 9 in the PPCH Operation and Maintenance Manual for a schematic of the PPCH.

Assume that a gauge with an internal volume of 30 cc has been connected to the TEST port of the PPCH but the gauge has not been prefilled with liquid (so it is full of air). A pressure setpoint has been entered and the PPCH is starting to control to it.
PPCH opens its INLET (Up) valve to try and increase the pressure. Oil is flowing from the PPCH into the gauge and is compressing the air. Because the air is being compressed the pressure rises very slowly. If oil was being added to a gauge that did not have air in it then the pressure would go up as expected because oil is pretty much not compressible (especially when compared to air).

Because the pressure is going up slowly (or not at all) and the PPCH doesn’t know that it is compressing air, the PPCH opens the INLET valve more causing more oil to flow into the gauge.

The above continues until the air is just about fully compressed around 300 psi (2 MPa). Now it is like there is no air in the gauge, but the INLET valve is open much too far and the pressure goes up very fast and overshoots the target. The PPCH closes the INLET valve as fast as it can but it is too late and the pressure has overshot the target.

Note that the above behavior will occur regardless of the setpoint. However if the setpoint is above 5000 psi (35 MPa) most people don’t notice the strange control behavior - with very slow, then very fast pressure increase, because the PPCH can oftentimes recover if the setpoint is high enough where it can recover from the initial problems caused by air or large volume.

From the above you can see that it is very important to remove as much air as possible from the gauge before connecting it to the PPCH, and to ensure that any air is purged from the PPCH.

Also note that behavior similar to the above can occur (usually just slow and not such a pronounced fast pressure increase) when the test volume is more than 200 to 300 cc even if there is no air in the system.

If you have air in the system refer to Section 5.10.1 in the PPCH Manual "PRIMING THE PUMP".

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