8.1.1  Test codes and practices - part 3

 

Several methods are available for measuring the rate of air leakage into the condenser, and reference should be made to Section 7 of this chapter which covered these aspects. Generally, the preferred position for measurement is at the air pump discharge (Fig 4.33), because measurements at the air pump suction require determination of the steam/air ratio, and are made difficult by the low density of the steam/air mixture.

The measurement of condensate flowrate is described in CEGB Site Test Code No. 2 [17].

(d) Condenser heat load determination Site Test Code No. 3 recommends that condenser heat load should be derived from a calculation of the heat rejected by the turbine-generator, preferably using heat rate data obtained concurrently with the condenser test.

(e) Tube cleanliness and air blanketing Both on-load and off-load methods for the determination of condenser tube cleanliness are required, and also the occurrence and location of air blanketing need to be identified.

Figure 4.35 illustrates the on-load method for determining condenser tube cleanliness. Briefly, sample tubes in groups of three or four are selected such that operating conditions throughout the condenser are represented. Separate supplies of CW are provided for each sample tube by means of insulated pipes passing through the waterboxes, and one tube in each group is selected as a clean tube which is acid-cleaned before commencement of the test.

On-load determination of cleanliness factor

Cooling water is pumped through the clean and corresponding fouled tubes at a flowrate similar to that on the rest of the condenser. The common inlet temperature and individual tube outlet temperatures and flowrates are measured, from which the cleanliness factor of each sample fouled tube is calculated according to Site Test Code No. 3 [15], Appendices A and B.

The off-load method requires sample tubes to be withdrawn from the condenser. In a laboratory, the cleanliness factor is determined by measuring the thermal resistance of the fouled tubes and comparing this with the thermal resistance of some of the tubes after acid cleaning.

Air leaking into an inadequately vented condenser will blanket the tubes, impede condensation and increase the condenser pressure. The position of air blanketing can often be inferred from measurement of the CW outlet temperature distribution at the tubeplate.

An abnormally low temperature rise indicates the probability of air blanketing somewhere along the condenser. A more precise indication of the position of the air blanketing can be made by inserting a small probe into the steam space to measure the temperature and pressure within the tubenests, as shown in Fig 4.36.

Air blanketing surveys by inserting a probe into the condenser tubenest

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