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As I wrote the section in “Engineering in Emergencies” that made the comment concerning partially full pipes creating problems with air locks I had better respond! The problem is that air will be entrained in the flow so if there are sections where the pipe flows full then air may well collect in high spots creating air locks. But in the case described there should be no problem, assuming the pipeline is designed and built to avoid air pockets, because it would be simplest to fit a gate valve at the beginning of the pipeline to regulate the flow and adjust to ensure the pipe flows full. Given that the pipes have already been ordered then it gives the opportunity to adjust the flow to the optimum and upgrade to higher flows at times when the spring yield is greater. That is a question: the flow might have been estimated at this time of the year but how does it vary throughout the year? If it is found that the pipe cannot take the maximum flow when the yield is highest, then an overflow will need to be designed into the spring box/capture to avoid seepage and erosion of the protected spring.

If there is a drop in height of 150m in 1.6 km then it will be essential to include break pressure tanks in the system to avoid bursting the pipes. Do we know the pressure rating of the pipes that have been ordered? It is not just the pipe pressure rating but that of the joints as well. If the flow has been calculated on the basis of a 150m pressure drop then think again because this will exceed the pressure rating of the HDPE pipe. The system will need to be designed in stages to avoid the high pressures. This will also mean that the flow will be less than originally calculated and, perhaps, match the yield more closely.

I hope this is useful Jan Davies