7.1.2 Measurement of air leakage rate
Apart from the air extraction system (see Section 10 of this chapter), air leakage measuring equipment is generally not installed as permanent instrumentation on British power stations. As in locating the air leaks, there are several techniques available to measure the rate of air leakage.
Operational techniques are used to determine whether a reduction in condenser performance is due to condenser fouling, or air inleakage overloading the air extraction equipment. The operations carried out, which only require measurement of condenser back pressure, are:
- Isolation of the air extraction equipment from the condenser on-load, and monitoring the subsequent rate of vacuum decay.
- Changing the number of vacuum pumps in service, and establishing the effect of this on the vacuum.
Typical unacceptable vacuum decay rates are 4 mbar/ min and above, as compared with expected rates of approximately 1 mbar/min.
Where these techniques are not either favoured or practical, other diagnostic techniques are often employed which require measurement of:
- Air temperature in the air extraction line from the condenser; increase in air quantity handled reduces the mixture temperature because of low water vapour partial pressure.
- Undercooling of the condensate outlet from the condenser; high air partial pressures give increased undercooling in some condensers.
- Differential pressure between LP cylinder exhaust and air extraction plant; low differential pressures indicate that the air extraction plant is controlling the vacuum and not the condenser heat transfer performance.
The final technique relies on temporary instrumentation to make direct measurements of air leakage on a periodic basis, or when a condenser performance problem exists. The methods employed include the use of Pilot tubes, anemometers, hot wire anemometers, rotameters and orifice plate assemblies.
Measurements are made both on the air suction pipe and at the air extraction plant discharge. Care is needed at both locations. The assumption made when measuring in the condenser air suction pipe is that the water vapour present is saturated. When measuring at the air extraction plant, if more than one pump is operating, the flow between the pumps is not always evenly distributed.