12 Circulating water pumps
The structural design of main circulating water pumps in the CEGB has changed over the last 20 years from horizontal split casing double-entry designs to the vertical spindle bottom inlet single-entry type. The horizontal pumps, while easy to maintain, require a large floor space, and the weight problems on the larger units necessitated the use of multiple casing castings.
The alternative vertical spindle pumps, now generally specified, have been developed for both metal casing and concrete volute applications, with the choice depending on the overall economics of the installation. They have significant advantages compared with the horizontal designs in terms of the compactness and cost of mechanical, electrical and civil engineering work.
CEGB experience has been generally based on hydraulic designs where the specific speed has been limited to 1.6. This reduces the risk of both head/ flow characteristic instability and high powers at low flows. It also gives a good efficiency spread over the range of operating flowrates. The head/flowrate and absorbed power characteristics are now controlled by the following specification:
- Pumps are designed so that the discharge head falls continuously as the flowrate is increased from zero to rated capacity by a minimum of 1% for any increase in discharge flow.
- The pumps are capable of sustained running at any point on their head/flow curve from 25% to the maximum runout condition.
- The input power rises continuously as the flow is increased from zero to the rated flowrate.
Circulating water pump speeds are normally in the range 150-300 r/min. Low speed motors, suitable for direct coupling to the pumps, are expensive and it is usually economically justifiable to include a step-down gearbox between the pump and motor. The combined efficiency of a 1000 r/min high speed motor and associated gearbox can be similar to the low speed multi-pole motor. When comparing capital costs of the two options, the costs of extra cranage equipment and the additional civil work to support the heavier motor have to be added to the direct drive pumpset costs. The two main advantages of the direct drive option are its simplicity, which can lead to higher availability, and the elimination of any noise control requirements arising from the high speed motor/ gearbox combination.
For flowrates over 10 mVs, main cooling water pumps are now based on the concrete volute type. Below around 6 mVs, it is impractical to consider concrete volutes because of physical access limitations in the mechanical seal area, and metal casing designs have been used. Between 6 and 10 mVs the CEGB have so far installed only metal casing pumps, al¬though concrete volutes have been successfully used elsewhere.