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Nitrogen Control Unit / Control Pressure Adjustment
Date: August 2005
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This information bulletin affects - NCM, NCU2 & NCU4 Version 2
The following information describes methods of adjusting the control pressure setting of the NCU V2 systems.
Overview
The Nitrogen Control Unit utilizes a low pressure control system. The low pressure gas is supplied by a control pressure regulator that recieves its gas supply from the high pressure nitrogen inlet of the system so no external low pressure supply is needed. The control pressure regulator then supplies gas to the individual channel proportional valves that will in turn load and unload the top control piston of the channel dome load regulators. Since the dome ratio of the main channel regulators is 44:1 (the top control piston is 44 times the area of the venting seat), the ideal control pressure is the maximum system outlet pressure of 5000 psi divided by 44 or 114 psi. The system will operate with a lower control pressure as the programmable logic controller will automatically adjust the electrical signal to the proportional valves to maintain the desired pressure setpoints however the system will not be able to reach its maximum operating pressure if the control pressure is not at least 114 psi.
Pressure Adjustment
It should be noted prior to the adjustment of any gas pressure regulator that the output pressure, although set by the force exerted by the compression of a spring against a load piston, is also inversely affected by the inlet pressure supplied to the device. Since the inlet pressure acts against the valve seat in a direction opposite that of the setpoint spring force, we find that the higher the inlet pressure the lower the regulator setpoint. For this reason, the setpoints listed in this procedure are valid only when the control unit is being supplied 5000 psi. One will note that during the depressurization phase the control pressure will increase to a maximum of 145 psi. Since the control pressure system is equipped with a 150 psi relief valve, setting the control pressure higher that allowed with 5000 psi inlet pressure will result in the relief valve lifting during pressurization and depressurization on the gas controller.
1) Depressurize the controller.
Remove all inlet pressure from the NCU device prior to beginning any work on the pneumatic system.
2) Insert a test gage
Remove the control pressure tubing from the outlet of the control pressure regulator and insert a Tee and pressure test gage between the regulator and the proportional valve inlets.
3) Repressurize the controller inlet to 5000 psig
. Caution: when the system is repressurized, the control pressure regulator will overshoot while the inlet pressure is low. Do not adjust the regulator at this time. The controller should be operated for at least a couple of pressure cycles to allow the control pressure to stabilize. If this procedure is being performed with the controller disconnected from a test stand or a molding unit, one should install a bleed valve in the test gage line to vent off some gas between adjustments to assure a proper setpoint reading.
Measure and adjust the control pressure
If the control pressure is not set at approximately 115 to 120 psi, loosen the locknut on the bottom of the control pressure regulator and insert a 3/16ths Allen wrench in the end of the adjustment screw. Rotate the adjustment screw clockwise to increase the setpoint and counterclockwise to decrease the setpoint. Be sure to run a pressure cycle (or vent a small amount of control pressure through a bleed valve) between each adjustment to assure proper reading.
Depressurize the controller and remove test gage
One should note that the system will have control pressure all the time while the inlet is pressurized. Prior to removing the test gage and re-attach the low pressure tubing to the control regulator, completely depressurize the system inlet.
Field adjustment
Experienced technicians only. While the above procedure uses a test pressure gage to properly adjust the system control pressure, it is possible to perform a quick setpoint without attaching the test gage when the operator is complaining that the system will not reach full operating pressure. After verifying the system is being supplied at least 5100 psig, set a pressure profile that requires 5000 psi for a period of about 60 second (or more)and 1000 psi/sec ramp. With the channel being used dead-headed against a closed tube (outlet plugged), start a cycle and wait until the pressure stabilizes. If the controller does not reach 5000 psi, increase the control pressure setting slightly by turning the adjustment screw clockwise no more that 1/8 turn at a time. Note the outlet pressure. If the outlet pressure does not increase as the control pressure is increased, STOP. Low control pressure is not the problem. If the outlet pressure does increase slightly, continue slowly until the outlet is reading 5000 psi and tighten the set crew locking nut. DO NOT CONTINUE INCREASING THE SETPOINT. Make note of the above section on pressure adjustment. If the regulator is set to high, then the control pressure relief valve will lift when the system is connected and disconnected from a pressure source.