Pressure Detector Functions
Although the pressures monitored vary slightly depending on the design details of the facility, all pressure detectors are used to provide up to three main functions: indication, alarm and control. Since the fluid system may operate in both saturated and undersaturated conditions, an accurate pressure indicator must be available to maintain proper cooling. Some pressure detectors have audible and visual alarms when a preset limit is exceeded. Some pressure detector applications are used as input to protective features and control functions.
Detector Failure
If a pressure tool fails, spare detector elements should be installed. If spare gauges are not installed, the pressure may be read on an independent local mechanical gauge if available, or an accurate pressure gauge may be installed at a suitable point in the system. If the detector is functional, pressure readings can be obtained by measuring the voltage or current values across the detector probe and comparing these readings to the calibration curves.
Environmental Concerns
Pressure instruments are sensitive to atmospheric pressure changes around the detector. This is especially evident when the detector is located in an enclosed space. A change in the pressure around the detector causes a change in the indicator pressure from the detector. This greatly reduces the accuracy of the pressure device and must be taken into account when installing and maintaining these tools. Ambient temperature changes affect the accuracy and reliability of the pressure detection tool. Changes in ambient temperature can directly affect the resistance of components in the instrumentation circuit and thus affect the calibration of electrical/electronic equipment. The effects of temperature changes are reduced by circuit design and by keeping the pressure sensing device in the proper environment. Moisture will also affect most electrical equipment, especially electronic equipment. High humidity causes moisture to accumulate on the equipment. This moisture can cause short circuits, grounds, and corrosion, which in turn may damage components. The effects of humidity are controlled by keeping the equipment in a suitable environment.
Summary
The three functions of the pressure control tool and the alternating pressure methods for control are summarized below.
Pressure detectors perform the following basic functions:
- the sign
- alarm
- Control
If a pressure detector is disabled:
- A spare detector element may be used (if installed).
- Local mechanical pressure can be used (if available).
- A precision pressure gauge may be installed in the system.
Environmental concerns:
- atmospheric pressure
- environment temperature
- humidity