Thust blocks and surge

[Camell]

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?

 

[Bjegovic]

during surges, flow may go in both directions, depending on particular case.

don't forget thermal dilatations of pipeline.

 

[BRIS]

I think vertical tanks drain by the force of gravity. Sometimes in order to understand the meaning of a formula it is necessary to return to basics and understand the derivation of the formula. Bernoulli's equation is simply a statement that energy is neither created nor destroyed. It does not contain a heat component and it is not the panacea to resolving all fluid dynamic problems.

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.

 

[Camell]

Thanks. That is the pragmatic approach, and whe i am headed...looking at the individual components of the pipeline on their own merits

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.

 

[BRIS]

Wave celerity in uPVC pipe is lower than steel and the surge pressure will be less. The short length of steel pipe will have no affect on the surge pressure calculation and can be ignored.

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.

 

[BRIS]

PS sorry for the first paragraph in my first reply - it was intended to be a reply to a different thread !!

 

[Camell]

I though as much, so it will be reasonable to design the ancor blocks for the pipe bridge for the surge condition in the plastic pipe?

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

 

[BRIS]

The pressure in the steel pipe will be the same as in the adjacent plastic.

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.