14.8  Testing

 

CEGB specifications require that the 100% main boiler feed pump is tested at a load corresponding to turbine MCR to prove conformity with the guarantee, and also at part-load to confirm that the pump performance characteristics are satisfactory. Site tests, however, are of necessity late in the station construction programme, so that any shortfalls in pump performance found at this stage would cause disproportionately large disturbances in the overall construction target. Alternative works testing is therefore required to demonstrate that the pump performance is acceptable before any systematic site testing, which is interrelated with the station as a whole, is commenced.

For steam turbine driven pumps, it is not practical to specify full works testing and instead part-speed interim hydraulic performance tests are carried out at the manufacturer's works to demonstrate that the pumps have reasonable prospects of achieving their specified requirements on site in terms of rated duty and hydraulic characteristics. These interim tests are followed by full-load acceptance tests on site with the plant in service.

Manufacturers works test facilities are generally suitable for full-load acceptance testing of electrically-driven feed pumps. The suction and pressure stage pumps are first tested individually to confirm the head/efficiency guarantees, the 3% head drop cavita-tion performance guarantees, and the shape of their hydraulic characteristics. These are followed by testing of the complete pumpset, including the motor and gearbox/fluid coupling assembly. In each case, hydraulic performance tests are undertaken using the method of measurement laid down in BS5316 [21].

Hydraulic output control tests are included on electrically-driven pumpsets to determine the threshold response, repeatability and input/output characteristics of the fluid coupling arrangement. For starting/standby feed pumps, tests may also be required to demonstrate compliance with run-up time guarantees. These would include the time from test to full-speed and also acceleration from part-speed conditions.

For new pump designs, supporting tests are required to demonstrate that the plant offered will satisfactorily accommodate any possible thermal shock, vapour lock or axial thrust condition that can arise during operation. In addition, a visual cavitation test is carried out on the pressure stage first-stage impeller to demonstrate compliance with NPSH visual cavitation guarantees.

The thermodynamic method of determining pump efficiency has now been successfully applied to CEGB feed pumps. This method compares the isentropic enthalpy rise across the pump with the measured enthalpy rise, and has the advantage over the conventional torquemeter method in that it is suitable for continuous on-line monitoring of plant performance. Accurate measurements of the temperature and pressure parameters at pump suction and discharge are required, and suitable equipment is now installed on new plant. In calculating the thermodynamic efficiency, losses resulting from gland sealing, balance water, bearing friction and heat transfer have all to be taken into account. These secondary losses can be derived at the initial commissioning test by comparing the conventional torquemeter acceptance test results with those obtained from the thermodynamic method.

 

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