Water temp rise in pump at deadhead 1

[badennis]

I need to get information on determining the temperature rise in a municipal water supply system when city water is boosted through a pump that is running near shut-off at low/no demand periods. Automatic controls on the system will shut the pump off at high pressure but what is potential highest temp achievable on the discharge water. Trying to prevent pump damage. Thank you for your assistance.

BAD

 

[TD2K]

Any pump book has this formula.

tr = h(1/e - 1)/(778*Cp)

tr is the temp rise in deg F
h is the pump head in feet
e is the efficiency as a fraction
Cp is the heat capacity of the fluid

You can also calculate it from basic principles.

At shut-off from the pump curve, read off the efficiency. Calculate the required BHP for the pump and the hydraulic Hp, the difference between the two (pump inefficiency) goes into the fluid as heat. Use the flow rate and the heat capacity to calculate the temperature rise.

 

[PUMPDESIGNER]

You may come to the conclusion that the calculations are a waste of time unless you just want to know.

Any pressure boost system must by design deal with that temperature rise. An expert in pressure boost systems can do this or you can purchase manufactured stations.

Design considerations for boost systems deal with temperature rise by alternating pumps, purge systems that dump warm water down the drain to cool the pump, proper pump selection (low Ns and Nss pumps heat the water much less), and VFD.

Individual site requirements also dictate design. Tall buildings must have a high static head held constantly on the line so a pump must run even at no flow. Buildings with 5 stories or less with good incoming supply can have pumps turned off and VFDs are good for that too.

PUMPDESIGNER

 

[briand2]

Take care...I have seen plastic pipe failing in such a situation due to excessive temperatures. Take advice from the suppliers of the booster pump set.

Brian

 

[badennis]

Thank you all for your help. Advice is helpful and appreciated.

BAD

 

[TD2K]

When you say at deadhead, I'm assuming (bad thing I know) that you still have flow through the pump. If so, the formula I gave you is correct.

If you are talking about no flow, then it's a matter of how much energy you are putting into the fluid trapped (in effect) in the pump case and the temperature rise per unit time taking into account, if you want, heat losses from the pump's surface. You usually can not run this way very long before you start vaporizing the fluid as well as other problems.

You would have to talk to your pump vendor to get information on the pump energy requirements and efficiency at a total no-flow condition.

 

[SandeepRaheja]

Every pump design has a minimum continuous flow(MCF) on thermal considerations which is called as the thermal MCF. This information should be obtained from the pump vendor.

While low energy pumps can even run almost continuosly at shutoff, this is generally not possible for high energy pumps. Running below this critical flow limit (thermal MCF) may lead to the damage of the pump due to excessive temperature build up & consequent vaporization as already stated by others. Frequently bypass arrangements are put in place to ensure that the pump runs above the Minimum Continuous Flow Rates. Can bypass arrangements be provided around Booster Pumps ? The bypass line may recycle this flow (thermal MCF) back to the suction line of the pump. Since the thermal MCF of the pump is a small fraction of the pump BEP, the bypass line size should be small enough compared to the main line size. Choosing a sufficiently long recycle line which joins the main line sufficiently upstreams may help to prevent the rate of heat build up. This may help to protect the pump even if the pump has been improperly selected and the solution would be cheaper compared to the use of VFDs.

I would request others to brainstorm on this, as i am myself not 100% confident about the provenness of this solution.


regards
Sandeep Raheja