Pump head 1

[khan644]

I am in the middle of the actual pump head calculation. I found that the designer has considered the diversity factor of 75%.

i.e. The actual flow required for all the equipment is added together is about 240 L/s where as the pump’s total duty flow is 180 L/s

Carrying on while doing pump head for index route @ my first branch, the diverging flow is 110 L/s and the index route flow is (240 – 110 = 130l/s)

Which will mean again my pumps have to give a 240 L/s which is not the case. My basic question is how to incorporate diversity factor of the pumps in the head calculation.

Note:

1. I'll be using Darcy Weisbach eqn for pipe pressure drop.

 

[quark]

I am not sure about whay you mean by index route but I presume you split the main flow into two branch flows. 75% is a good diversity factor if you are dealing with HVAC systems. The cooling/heating loads calculation with 10% redundancy + rounding off redundancy + experienced based redundancy + estimation based on peak load conditions all together can keep you in safe mode. Further, your calculated head will always be higher than the actual head.

When you say 75% diversity factor then the design is based upon 82.5l/s in one branch and 97.5l/s in the other branch.

Having said that, with the advent of variable speed technology, the safe bet will be to design the pump for full capacity and manipulate the pump speed as per flow requirement.

 

[texanman]

Is the index route your farthest hydraulic unit? This should be the path with the highest pressure drop.

As for the diversity, you want to know if at any time, all the flow is needed for the building load.

I think 75% is too low. Better to design for above 90% or even 100%, and as quark suggests, rely on the variable speed pumping system. That is if you have that type.
hope this helps,

 

[khan644]

Thanks, Quark and Texanman.

By index route I meant the farthest route or say "The worst pressure drop loop"

Moreover, I am in a hard core contracting firm, the deisgn is already there, I am here to look whether the specified Head meets the Actual head based on shop DWGS

The pumps are VFD Driven plus 75% diversity factor. Now with the new acquired understanding, I feels its just not right. Thanks to quark.

What I understand I must do ...

Consider that the LOAD on my index route is the highest and else were the least ( The worst scenario) So my calculation should have highest flow along the index route.

Is I am right

 

[imok2]

Of course if you recommend a bigger pump then the size and cost will increase (good luck)and if you accept the designers diversity factor you have to make sure that all the control valves are the proper size. I go along with quark and texanman and would recommend a larger diversity factor and let the variable flo pump handle the required flow

 

[SAK9]

What is the pipe sized for?240 l/s or 180 l/s.

It looks like 180 l/s is based on block load(coincident peak loads)and this is what the pipe should be sized for.For eg a building with exposure N,S,E W facades could have a peak load of 100,120,130,150 Kw for each facade.The maximum load is not the sum of these loads because they will peak at different times.

However branches shall be sized for their maximum flows so that they their maximum demand is met when they peak.So while doing the head calculation,you need to consider a flow of 180 l/s in the mains and maximum flow rate for your index run.

From a practical perspective, I would suggest you a prepare an isometric sketch of the index run with all flow rates,distances and fittings marked up.Prepare a calculation sheet in excel identifying each segment by numbers and submit your calculation to the consultants for approval.As a contractor that will cover you up pretty well.You would not need to worry about how the consultant arrived at 180 l/s.

 

[khan644]

Thanks all,

I'll go along with SAK9. So the conclusion could be.

"maximum flow rate for your index run"