Differential pressure cells, installation

Fio. 6.20. Installation or differential pressure cells (transmitters) (a) arrangement for non-corrosive dry gas and non-corrosive liquid (b) piping for wet gas (non-condensing) and liquids with solids in suspension (c) arrangement for steam and condensable vapours... 

Consequently, our first attempt is to install measuring devices that will monitor T and V directly. For the present system this is simple by using a thermocouple (for T) and a differential pressure cell (for V). 

Maintenance of a PSVE system is expected to cost 2% of the installed capital cost of the system per year. Operation and waste disposal costs are a function of concentration of contaminants and the airflow rate and will therefore vary widely (D14489S, p. 26). Once a system is installed, no utilities are generally required. If a valve and differential pressure control system are used, these could be run by solar-cell-powered batteries (D18119L, p. 384). 

Differential pressure sensors are used to measure the bed pressure, leaving one leg open to the atmosphere to compensate for barometric pressure variations. Care must be taken in the installation to minimize the plugging with solids. This is normally done by tilting the tank nozzle on which the DP cell is mounted downward from the sensor, so the solids tend to settle away from the sensor surface. A shutoff valve and washer flush tap are recommended to allow easy maintenance. 

Another early effort to compensate for pressure variations in binary distillations resulted in the development of the differential vapor pressure cell. The version made by Foxboro is known as the DVP Cell. As shown in Figure 10.1, a bulb filled with liquid whose composition is the same as that desired on a particular tray is installed on that tray. It is connected to one side of a differential-pressure transmitter. The other side of the AP transmitter is connected direcdy to the same tray. When the liquid on the tray has the same composition as the liquid in the bulb, pressure in the bulb will be the same as pressure on the tray. For that condition the AP transmitter is normally set to read midscale. Deviations in the tray composition from that in the bulb are then reflected by AP transmitter output signals above or below midscale. 

The basic rangeability of this meter is the same as that of an orifice plate (3 1), but if two (a high span and a low span) transmitters are used, and the flow element is accurately calibrated over the complete flow range, it can be increased to 10 1. This performance can be obtained from all properly calibrated d/p flow elements, not just from the V-cone design. The V-cone flowmeter should be installed horizontally so that the two pressure taps are at the same elevation. This guarantees that the d/p cell will detect a zero pressure differential when there is no flow. 

The most common and useful detector used in SEC is the differential RI. These are simple to use and largely maintenance-free. Unfortunately, they are temperature-sensitive, and should be installed in a room with a reasonably constant ambient temperature and away from draughts and direct sunlight. Some of the more expensive, modern instruments have built-in thermostatic control. If an additional detector is incorporated into the system then this should be installed between the column and RI detector. The flow cells in RI detectors cannot withstand very high pressures and must therefore be connected to the end of the system to avoid excessive back pressure. All tube connections should be made with 0.01" bore s/s tubing, the length of which should be kept to a minimum. 

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