Thust blocks and surge
Thust blocks and surge
(OP)
I am busy with the design of a pumped main. Regarding the design of thrust blocks: What is the accepted practice with respect to the pressure considered. I see three alternatives:
1) Normal duty point and flow
2) Pump maximum pressure (potentially zero flow, but not nessarily)
3) Surge pressure.....what flow?
I am sure no 3 wil govern, but may be excessive!
Any comments?
1) Normal duty point and flow
2) Pump maximum pressure (potentially zero flow, but not nessarily)
3) Surge pressure.....what flow?
I am sure no 3 wil govern, but may be excessive!
Any comments?





RE: Thust blocks and surge
don't forget thermal dilatations of pipeline.
RE: Thust blocks and surge
There are no rules and you need to make engineering judgement. Unless there is an absolute guarantee that it cannot happen I would normally design for pump shut in pressure plus a factor of safety and check for surge pressure.
(Unless you have a very long pipeline the short term surge pressure will not mobilise failure in the soil).
But it all depends on factors of safety and pipe length.
Not only the factors of safety you apply to the pressure but also to all other factors you apply to the soils and conditions you take water table levels etc. and the criticality of the situation.
If surge pressure is within the factor of safety then I would be content.
At what flow do you consider surge pressure - normally you would consider a pump trip at maximum flow (negative surge)and the positive reflected wave.
RE: Thust blocks and surge
By maximum flow, I assume you mean duty pont. Yes, this is the surge condition I would be interested in.
With regard to the actual surge pressures: Without actually having done a detailed surge calculation, in the situation I have where I will cross a stream with a steel pipe (1% of total pipe length) and the remainder of the pipeline being PVC (99%), what actually happens to the surge wave in the short section of steel pipe?. My feeling would be that it will not be as high as where the whole pipeline is steel. Something between the two.
RE: Thust blocks and surge
The maximum flow may in fact be higher than the design duty point if you have taken a conservative value for friction loss. It is normal to draw a system envelope using probable minimum and maximum pipe friction.
If you are using PVC pipe you may need to consider whether the specified pressure for the material is the maximum pressure including surge. In which case you need to derate to get the allowable working pressure.
RE: Thust blocks and surge
RE: Thust blocks and surge
Done the system curve calc ( to select pump properly) utilised k=0.03mm, k-0.015, k=0.06 as variables and it does not make much of a difference at the duty point with respect to the above calcs
RE: Thust blocks and surge
I should have added in the first reply that the pipe test pressure is usually the worst case sitaution for the design of thrust blocks.