GE PACE5000 User Manual page 56

6 Reference and Specification
Supply contamination
Some supplies may need the removal of water, oil or particulate contamination. Any water in the
compressed gas supply will be in vapour form, i.e. non-condensing and must be removed using a
mist filter. Oil must be completely removed as this causes a rapid deterioration of the control valve
performance. The compressed gas supply must not contain particulates and must be removed
using a particulate filter. Do not use a compressed gas supply containing corrosive material.
Systems without a negative supply
Without a negative supply (vacuum pump), release the positive pressure from the system to
atmosphere through the negative supply port.
The release from the negative port may be piped away to a place where this discharge causes no
disturbance or hazard. Alternatively, a diffuser may be fitted to the negative supply port to diffuse
air flow.
The need for a negative supply (for optimum performance)
General
Supply pressures (at least 110% of range but less than the MWP) there must be a difference of 10%
of full-scale between the supply pressure and the maximum output pressure. When operating at
positive or negative full-scale, there must be a pressure difference between supply and output to
cause a gas flow.
Operating near atmospheric pressure or below
Any controller operating near atmospheric pressure or below requires a vacuum pump or other
negative supply connected to the negative supply port for optimum performance. Without a
vacuum supply, as the output pressure approaches atmospheric pressure, the differential pressure
approaches zero resulting in a reduced flow to the output.
Reduced flow causes an increase in the time to control to atmosphere, especially with large user
volumes, and an increased overshoot at low pressures, see figures 2-3, 2-4 and 2-5.
Vacuum Pump
Each PACE Control Module has a vacuum sensor so it is normal practice to connect a vacuum
pump to the -ve supply port. The flow rate of the vacuum pump does not matter but generally
higher pump flow improves PACE control performance. However, low pressure ranges <700mbar
require vacuum regulation or the use of the negative gauge pressure generator IO-NEG-G-GEN-1
option.
Conclusion
Use a vacuum supply for:
• Absolute ranges
• Negative gauge ranges
A vacuum supply improves:
• Time to reduce system pressure at pressures below 2 bar (30 psi), full-scale.
• Control stability near atmospheric pressure.
• Overshoot at low pressures.
• To improve performance at or near gauge zero.
K0443 Issue No. 3 6 - 2