PSM Instrumentation has used its expertise in differential pressure measurement to develop an innovative submersible sensor for the Ministry of Defence (MoD). After the conclusion of successful trials, Royal Navy Frigates will be equipped with the devices to measure the content of fuel oil tanks. Other Naval vessels are being considered for the future and the system can also be applied to land-based tank systems throughout industry.
For trim stability the ships’ fuel tanks are maintained full, achieved by replacing fuel with seawater as it is used. Several tanks are linked and a seawater header tank exerts a pressure on the whole chain ensuring that oil, followed by seawater, is pushed from one tank to the next. The principle depends on the reluctance of oil and seawater to mix.
The PSM DP transmitter is installed in the base of the tank. One side of the DP measurement is connected to a reference leg, which is pre-filled with seawater. As this is a captive leg the pressure it exerts is constant. The other side of the DP monitors the hydrostatic head in the tank. This varies according to the relative content of oil or seawater in the tank and so a differential pressure develops which is a function of the difference in specific gravity between seawater in the reference leg and the specific gravity of the oil in the tank. The header tank pressure is equally applied to both sides and so acts as a static offset and does not effect calibration.
The shallow depth of the tanks and small specific gravity differential demanded an instrument with high sensitivity, and yet the arduous installed conditions and shock immunity required called for an extremely rugged device.
In service access to tanks is not possible so reliability, long-term stability and routine maintenance intervals of four years minimum were included in the design brief. Corrosion protection was also a particular concern for the MoD and much of the sensors’ construction employs stainless steel – Hastelloy C276.
The tanks are classified hazardous so the units are IS approved to Eex 1a 11c T6 and provide an industry standard two-wire 4-20mA output.