Читать книгу Continuous Emission Monitoring - James A. Jahnke - Страница 91

The sample pump is an important element of the extractive system and is used to transport the sample from the stack to the analyzer. A pump should be sized appropriately to meet the demands of gas analyzers and be designed so that no air in‐leakage occurs (i.e. around a rotary shaft seal) and that no contamination is introduced from pump lubricating oils. Two types of pumps meeting these criteria are (i) diaphragm and (ii) ejector pumps. These pumps are commonly used in source monitoring applications.

Diaphragm pumps operate by mechanically stroking a piston or a connecting rod to move a flexible diaphragm (Figure 3‐13). The diaphragm is circular and can be made of a flexible metal plate, Teflon, polyurethane, or other types of elastomer. The reciprocating action of the diaphragm moves the gas in short bursts. As the diaphragm is raised, the gas is drawn through the suction valve into the pump cavity. When the diaphragm is pushed down, the suction valve closes and the discharge valve opens. The gas is then displaced out into the sample line. Because only the chamber, diaphragm, and valves come into contact with the gas, contamination of the gas is minimized.

Figure 3‐13 A diaphragm pump.

Diaphragm pumps can be used before the sample gas conditioning system and can even be operated hot. However, particulate matter and condensed acid can weaken the pump head by particle abrasion or chemical attack. Unless the gas is properly filtered and the pump head adequately heated, it is better to locate the pump after the conditioning system. Eventually, the continual, rapid flexing action of the pump will cause the diaphragm to split or tear.

The flow rate of a diaphragm pump can be controlled by placing a throttle valve in the line on the discharge side of the pump or by installing a by‐pass line and valve from the suction side to the discharge side. If the pump is operating at less than its full capacity, controlling flow with the throttle valve will cause the pump to work against a high discharge pressure and pump life will be reduced. For this reason, it is better to control the flow using a bypass valve.

The ejector pump (also called an eductor or air aspirator) takes advantage of the Bernoulli effect to draw a vacuum through a sampling system (Figure 3‐14). In the Bernoulli effect, the moving jet of air reduces the air pressure normal to its flow. This is a common effect that is familiar in venturi flowmeters and jet carburetors. This reduced pressure pulls the sample gas through the sample line; if the jet velocity increases, the vacuum increases. Typically, the filtered plant air or compressed cylinder gas is used for the high‐velocity gas stream. The ejector pump is simple in its design and can be incorporated as part of an inertial filter probe system as shown in Figure 3‐7. Here, the pump draws in the sample through an inertial filter that would be installed in a heated cabinet located outside the stack. Another pump, usually a diaphragm pump, is used to pull the filtered sample to the analyzer. In the ejector pumps applied where the sample gas is not filtered, particulate matter can build up and dry out in the annular space of the pump. In such cases, it may be necessary to use steam instead of compressed air to provide the motive force for the pump action.

Figure 3‐14 The ejector pump or eductor.